Tectonic Evolution of Southeast Asia
Geological Society Special Publications Series Editor
A. J. FLEET
GEOLOGICAL SOCIETY SPECIAL PUBLICATION NO. 106
Tectonic Evolution of Southeast Asia EDITED BY
ROBERT HALL SE Asia Research Group Department of Geology Royal Holloway London University Surrey, UK AND
DEREK BLUNDELL SE Asia Research Group Department of Geology Royal Holloway London University Surrey, UK
1996
Published by The Geological Society London
THE GEOLOGICAL SOCIETY The Society was founded in 1807 as the Geological Society of London and is the oldest geological society in the world. It received its Royal Charter in 1825 for the purpose of 'investigating the mineral structure of the Earth'. The Society is Britain's national society for geology with a membership of 8000. It has countrywide coverage and approximately 1000 members reside overseas. The Society is responsible for all aspects of the geological sciences including professional matters. The Society has its own publishing house, which produces the Society's international journals, books and maps, and which acts as the European distributor for publications of the American Association of Petroleum Geologists, SEPM and the Geological Society of America. Fellowship is open to those holding a recognized honours degree in geology or cognate subject and who have at least two years' relevant postgraduate experience, or who have not less than six years' experience in geology or a cognate subject. A Fellow who has not less than five years' relevant postgraduate experience in the practice of geology may apply for validation and, subject to approval, may be able to use the designatory letters C Geol (Chartered Geologist). Further information about the Society is available from the Membership Manager, The Geological Society, Burlington House, Piccadilly, London W1V 0JU, UK. The Society is a Registered Charity, No. 210161
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Contents HALL, R. & BLUNDELL,D. J. Tectonic evolution of SE Asia: introduction
vii
Part 1: Present-day tectonics MCCAFFREY, R. Slip partitioning at convergent plate boundaries of SE Asia
3
MALOD, J. A. & KEMAL, B. M. The Sumatra margin: oblique subduction and lateral displacement of the accretionary prism
19
RANGIN, C., DAHRIN, D., QUEBRAL,R. & THE MODEC SCIENTIFICPARTY. Collision and strike-slip faulting in the northern Molucca Sea (Philippines and Indonesia): preliminary results of a morphotectonic study
29
RICHARDSON,A. N. & BLUNDELL,D. J. Continental collision in the Banda arc
47
SNYDER,D. B., MILSOM,J. & PRASETYO,H. Geophysical evidence for local indentor tectonics in the Banda arc east of Timor
61
HUGHES, B. D., BAXTER, K., CLARK,R. A. & SNYDER,D. B. Detailed processing of seismic reflection data from the frontal part of the Timor trough accretionary wedge, eastern Indonesia
75
MILSOM, J., KAYE,S. & SARDJONO.Extension, collision and curvature in the eastern Banda arc
85
Part 2: Tectonic development of Southeast Asia METCALFE, I. Pre-Cretaceous evolution of SE Asian terranes
97
PACrd-IAM, G. Cenozoic SE Asia: reconstructing its aggregation and reorganization
123
HALL, R. Reconstructing Cenozoic SE Asia
153
SIMANDJUNTAK,T. O. & BARBER,A. J. Contrasting tectonic styles in the Neogene orogenic belts of Indonesia
185
RICHTER, B. & FULLER, M. Palaeomagnetism of the Sibumasu and Indochina blocks: implications for the extrusion tectonic model
203
STOKES, R. B., LOVATTSMITH,P. E & SOUMPHONPHAKDY,K. Timing of the Shan-Thai-Indochina collision: new evidence from the Pak Lay Foldbelt of the Lao PDR
225
LOVATr SMITH,P. F., STOKES,R. B., BRISTOW,C. & CARTER,A. Mid-Cretaceous inversion in the Northern Khorat Plateau of Lao PDR and Thailand
233
HtrrCHISON, C. S. The 'Rajang accretionary prism' and 'Lupar Line' problem of Borneo
247
OMANG, S. A. K. • BARBER,m. J. Origin and tectonic significance of the metamorphic rocks associated with the Darvel Bay Ophiolite, Sabah, Malaysia
263
NGAH, K., MADON,M. & TJIA, H. D. Role of pre-Tertiary fracture in formation and development of the Malay and Penyu basins
281
TJIA, H. D. & LIEW, K. K. Changes in tectonic stress field in northern Sunda Shelf basins
291
CLENYELL, B. Far-field and gravity tectonics in Miocene basins of Sabah, Malaysia
307
McCOURT, W. J., CROW, M. J., COBBING,E. J. & AMIN, T. C. Mesozoic and Cenozoic plutonic evolution of SE Asia: evidence from Sumatra, Indonesia
321
SAMUEL, M. A. & HARBURV,N. A. The Mentawai fault zone and deformation of the Sumatran Forearc in the Nias area
337
WAKITA,K., SOPAHELUWAKAN,J., MIYAZAKI,K., ZULKARNAIN,I., & MUNASRI.Tectonic evolution of the Bantimala Complex, South Sulawesi, Indonesia
353
vi
CONTENTS
WILSON, M. E. J. & BOSENCE, D. W. J. The Tertiary evolution of South Sulawesi: a record in redeposited carbonates of the Tonasa Limestone Formation
365
BERGMAN, S. C., COFFIELD,D. Q., TALBOT, J. P. • GARRARD,R. A. Tertiary tectonic and magmatic evolution of western Sulawesi and the Makassar Strait, Indonesia: evidence for a Miocene continent-continent collision
391
ALI, J. R., MILSOM, J., FINCH, E. M. & MUBROTO,B. SE Sundaland accretion: palaeomagnetic evidence of large Plio-Pleistocene thin-skin rotations in Buton
431
VROON, P. Z., VAN BERGEN, M. J. & FORDE, E. J. Pb and Nd isotope constraints on the provenance of tectonically dispersed continental fragments in east Indonesia
445
LINTHOUT, K., HELMERS, H., WIJBRANS,J. R. & VAN WEES, J. D. A. M. 4°Ar/a9Ar constraints on obduction of the Seram ultramafic complex: consequences for the evolution of the southern Banda Sea
455
CHARLTON, T. R. Correlation of the Salawati and Tomori Basins, eastern Indonesia: a constraint on left-lateral displacements of the Sorong fault zone
465
MALAIHOLLO,J. E A. & HALL, R. The geology and tectonic evolution of the Bacan region, east Indonesia
483
BAKER, S. & MALAIHOLLO,J. Dating of Neogene igneous rocks in the Halmahera region: arc initiation and development
499
PUBELLIER, M., QUEBRAL,R., AURELIO, M. & RANGIN, C. Docking and post-docking escape tectonics in the southern Philippines
511
CROWHURST, P. V., HILL, K. C., FOSTER, D. A. & BENNETT, A. R Thermochronological and geochemical constraints on the tectonic evolution of northern Papua New Guinea
525
WOPFNER, H. Gondwana origin of the Baoshan and Tengchong terranes of west Yunnan
539
ZI-IOU, Z., LAO, Q., CHEN, H., DING, S. & LIAO, Z. Early Mesozoic orogeny in Fujian, southeast China
549
Index
557
Tectonic evolution of SE Asia: introduction R O B E R T H A L L 1 & D. J. B L U N D E L L
SE Asia Research Group, Department of Geology Royal Holloway, University of London, Egham, Surrey TW20 OEX, UK 1 Fax no. (44) 01784 471780 Email
[email protected] SE Asia is probably the finest natural geological laboratory in the world yet is still not geologically well known. It is a spectacular region in which the manifestations and processes of plate collision can be observed at present and in which their history is recorded. It is a region that must be understood if we are to understand mountain belts, arc development, marginal basin evolution and, more generally, the behaviour of the lithosphere in collision settings. Furthermore, the region is developing rapidly on the economic front, and a major part of this rapid development is built on natural resources. The geological reasons for the distribution of these resources are therefore of major importance for the inhabitants of the region and for attempts to discover and exploit them. These were some of the thoughts which stimulated the collection of papers (Fig. 1) in this volume. In order to understand the development of this complex region an essential first step is to identify the key features of the active tectonics and determine how plates and sub-plate lithospheric fragments are moving. How successfully can rigid plate tectonics be applied in describing present tectonics? Where are the boundaries between plates? What are the rates at which different parts of the region are moving? The first part of this volume includes a number of papers which deal with these questions, based upon the application of GPS (Global Positioning System) measurements to determining the nature and rates of plate movements and plate boundary zone deformation, results from the BIRPS deep seismic reflection experiment in the Banda arc, the first to cross a modern active margin, and other geophysical and geological studies. MeCaffrey provides a regional overview of recent GPS results and earthquake data bearing on the present plate tectonics. It is ironic that in such an active region the identification of several plates and determination of some important relative plate motions, critical to a full kinematic description, are still very uncertain. McCaffrey discusses the way in which motion is partitioned in obliquely convergent settings, almost the rule in SE Asia. Oblique convergence is commonly inferred in the past for SE Asia (as shown in many of the later papers in the
volume) and is often used as an explanatory tool in orogenic belts elsewhere in the world. He draws attention to the deformation of the upper plate in these convergent settings and emphasizes the importance of a three-dimensional understanding of the process. In this rapidly evolving region, information from slip vectors and geodetic measurements will allow a fourth dimension of time to be included as data accumulate, which will raise the important problem of whether and for how long the present motions can be assumed to extend back into the past. Two well-known areas of oblique convergence, the Sumatra and Philippine Sea plate margins, illustrate the realities of present tectonics, first in identifying small plates or smaller tectonic elements, and second in providing a kinematic description. The increase in obliquity of convergence between Java and north Sumatra has long been considered to result in thrusting normal to the subduction trench and arc-parallel movement on the Sumatran fault. However, as McCaffrey points out, this simple model does not predict some important features, such as subduction west of the Andaman Sea and differences in amounts of extension between north and south Sumatra. Malod & Kemal discuss evidence from marine surveys off Sumatra and propose that this area can be understood as a number of plate slivers between the trench and the Sumatran fault, with variations in the partitioning of movement in different parts of the Sunda forearc. This is explained as the result of differences in coupling between the subducting and overriding plates, possibly reflecting the presence of major structures on the subducting slab, notably an extinct spreading centre on the Indian plate. Further east, the Philippine fault and trench are also considered as the joint expression of partitioning of oblique convergence, but once again applying this simple model presents problems: how does this paired trench-fault system link southwards into the arc-arc collision of the Molucca Sea? Rangin et al. tackle the first important problem of what is actually present in the zone of transition by mapping structures from the south Philippines into Indonesia and determining which features belong
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolution of SoutheastAsia, Geological Society Special Publication No. 106, pp. vii-xiii.
vii
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Fig. 1. Areas discussed in the papers in this volume. 1: McCaffrey; 2: Malod & Kemal; 3: Rangin et al.; 4: Richardson & Blundell; 5: Snyder et al.; 6: Hughes; 7: Milsom et al.; 8: Metcalfe; 9: Packham; 10: Hall; 11: Simandjuntak & Barber; 12: Richter & Fuller; 13: Stokes et al.; 14: Lovatt Smith et al.; 15: Hutchison; 16: Omang & Barber; 17: Ngah et al.; 18: Tjia & Liew; 19: Clennell; 20: Mccourt et al.; 21: Samuel & Harbury; 22: Wakita et al.; 23: Wilson & Bosence; 24: Bergman et al.; 25: Ali et al.; 26: Vroon et al.; 27: Linthout et al.; 28: Charlton; 29: Malaihollo & Hall; 30: Baker & Malaihollo; 31: Pubellier et al.; 32: Crowhurst et al.; 33: Wopfner; 34: Zhou et al.
to which plate, a demanding task. Their use of the technique of multibeam mapping, combined with geophysical evidence, is producing highly detailed maps, in many cases of much higher quality than those available for land areas nearby, and revealing unsuspected structures. The arguments surrounding the tectonics of the Banda arc, and some of the complexities of the collision zone, for example its curvature, age of the Banda Sea, and significance of deep basins such as the Weber trough, also illustrate the difficulties of linking simple kinematic models to reality. Very recently two BIRPS deep seismic reflection profiles have crossed the Banda arc and imaged the deep structure of the central part of the AustraliaSE Asia collision zone. Richardson & Blundell discuss how deep reflectors can be linked to recent seismicity, and their connection to structures on land. They identify two sets of divergent structures in the collision zone: a southern set dipping in the same direction as subduction is related to the subducted Australian margin, whereas a northern
set, in the upper plate, is antithetic, and more recent. They argue that these structures changed orientation and migrated north because buoyant continental crust blocked the subduction zone, although Australia continued to move north. Using the BIRPS images to estimate the volume of material in the collision zone they infer that an earlier Neogene collision event, involving a separate microcontinental fragment or outer margin high, must have preceded the present phase of collision which began at about 2.4 Ma. Snyder e t al. draw attention to unusual features of the BIRPS 'Timor' sector of the Banda orogenic zone, including a Bouguer anomaly and thin sediment cover implying a thicker continental crust than normal beneath the accretionary complex, a very narrow forearc, and presence of the volcano Gunung Api in the backarc. Like Richardson & Blundell, they infer the presence of additional Australian margin crust but suggest it formed either a promontory of the Australian margin or a prebreakup extensional basin within the former pas-
INTRODUCTION sive margin, now inverted and thickened. Also like Richardson and Blundell they deduce that the deformation has advanced across the collision zone, causing crustal fracturing in the backarc region and allowing magmatic uprise and underplating beneath Gunung Api. Hughes et al. have reprocessed part of the Timor section to bring out the details of the Timor trough and the accretionary wedge developed on its northern flank. Australian shelf sediments are clearly imaged, relatively undisturbed, and beneath a low-angle decollement above which are highly deformed sediments stacked in a complex set of thrust sheets indicative of the northward underthrusting of the Australian continent. However, a covering drape of sediment indicates that deformation in this area has been inactive for about 1 Ma. One feature that has bedevilled discussions of the Banda arc is the significance of the deep troughs which parallel the arc, which in a superficial way have characteristics of trenches, but when studied more carefully seem to be in the wrong position. Milsom et al. present evidence that the deformation front between Australia and the Banda arc extends between the Kai islands, in the region of maximum curvature of the arc, rather than following the apparent subduction-related feature of the Aru trough. They conclude that the arc must have been a continuous feature before collision with a curvature acquired recently. The present setting of the region is both the start and the end point in attempts to comprehend its development. The start, because an understanding of what is present provides the first clues of what has happened in the past few millions or tens of millions of years, leaving traces in the slabs beneath the arcs, the arcs themselves, and in the stratigraphic record of the arc regions as well as the Sunda continental shelf further north. The end, because the present is the result of what has happened in the past, and many of the present features of the region may be explicable only in terms of what has now disappeared and can only be inferred. Thus, our understanding of the region and its development is an iterative synthesis, each new step forward requiring a reconsideration of present and past. The second section of the volume therefore begins with three overview papers intended to synthesize earlier observations which may provoke new research improving and extending our present regional understanding. Metealfe reviews the pre-Cretaceous development of SE Asia and east Asia and shows that the region has grown by addition of allochthonous terranes which separated from different parts of Gondwana. Their northwards movement was accompanied by the opening and closing of three successive oceans: the Palaeo-, Meso- and
ix
Ceno-Tethys. Assembly began with the formation of Cathaysia in the Late Devonian-Early Carboniferous and its growth continued within the Palaeo-Tethys in the later Palaeozoic. As the Meso-Tethys opened by Late Carboniferous-Early Permian rifting of the northern margin of Gondwanaland so the Palaeo-Tethys began to close, and so subsequently did the opening of the Ceno-Tethys result in elimination of the MesoTethys. The final stage of the reassembly of Gondwana in Asia is not yet complete but the present complexity of the region, as well as the comprehensive grasp of so many disciplines in earth science required to attempt the task of describing its development, give an indication of Metcalfe's achievement in producing a comprehensible synthesis. A description is an essential step in identifying the driving processes and modelling the development of the region. Reconstructing SE Asia in the Cenozoic requires a description of India-Eurasia collision, the motion of the Philippine Sea plate and the present collision of Australia with eastern Indonesia. Eurasian extrusion models have proved popular with many workers in SE Asia, partly because of the striking similarities of plasticine models to tectonic maps, and partly because they offer a means to explain and link different events. As with many major advances, the extrusion model has also been effective in provoking other new ideas and the search for new evidence. Paekham considers the relationships between the observed geology and different models of the region, reviewing the timing of different events, the evidence from crustal volumes for extrusion, and the relationship between predicted and observed palaeomagnetic measurements. In east Asia and western SE Asia the estimates of rates and amounts of movements predicted by the extrusion model, and those that can be determined, are becoming much closer. The inadequacies of the fit cannot just be explained as weaknesses of the model but also emphasize the variable quality of data and their uncertainties. Packham concludes that a regional understanding of SE Asia requires a better understanding of the timing of deformation, especially uplift, in the Himalayas and central Asia, a clearer picture of whether rotations detected using palaeomagnetic data are regional or local, and much new stratigraphic information, particularly in the eastern part of the region. The key role of palaeomagnetic data in developing regional models is a theme discussed by Hall in an attempt to produce a kinematic model for the whole of Cenozoic SE Asia. At the centre of the region is the island of Borneo from which rotations recorded appear to be in conflict with those predicted by an indentation model. Further east, new
X
R. HALL • D. J. BLUNDELL
data from the Philippine Sea plate confirm longterm clockwise rotations of the whole plate suggested by many earlier studies. The attempt to synthesise these results suggests that, even if Cenozoic extrusion of continental fragments from east Asia is accepted, this has not been the most important driving force in the development of the marginal basins of eastern SE Asia. According to this model, development of marginal basins was linked physically and temporally, and opening appears to be mainly subduction-related rather than indentor-driven. This new model does suggest some possible configurations for the region which are different from those previously accepted and may provoke reconsideration of some evidence, especially the habit of looking for local explanations of tectonic phenomena. The animation which accompanies the paper reminds us that regional events may have causes outside the immediate area, and the manifestation of plate movement changes may propagate gradually across the whole region, as plate boundary changes in one area cause changes in others. Several aspects of the model, such as the proposed ages of some basins, for example the Banda Sea, remain to be tested by ocean drilling, palaeomagnetic work, and stratigraphic and structural studies. The remaining papers in the volume are arranged broadly in geographical order. Simandjuntak & Barber illustrate the variation in orogenic styles from different parts of Indonesia. The present diversity and complexity of tectonic processes in SE Asia may provide keys to the interpretation of other orogenic belts. The differences in the history of deformation within the region may leave traces other then geological structures and Richter & Fuller discuss the still thorny question of the implications of palaeomagnetic data from Sundaland and Indochina. They conclude that different parts of Sundaland and Indochina have deformed in different ways; some parts as small blocks with rotations indicating local deformation, some extrusion-driven rotation, principally in Indochina, and some parts recording the results of deformation dominated by the oblique subduction of the Indian plate. Distinguishing between such areas for the whole SE Asia region is an important task for geologists and palaeomagnetists for the future. Even the most carefully constructed regional kinematic models are totally dependent on basic data revealing the timing of tectonic events and the evidence for them. The papers concerned with areas around the South China Sea suggest the need for revisions of models as well as new interpretations of relationships between effects and supposed causes. In Laos, Stokes et al. argue that suturing of the Shan-Thai and Indochina blocks occurred in
the Late Jurassic and that the Indosinian Orogeny, currently assumed to be of Permian or Triassic age is significantly younger than commonly assumed. Based largely on seismic data, Lovatt Smith et al. suggest regional tilting, compressive folding, reverse faulting and basin inversion in Thailand record important phases of structural development which pre-date the currently assumed Tertiary age of structuring. If correct, these interpretations have implications for hydrocarbon exploration and potential. Hutchison draws attention to the contrast between tectonic models of the South China Sea region and the geology of Borneo and proposes that the term Rajang Group should be more carefully applied. An older turbidite sequence, assigned to the Rajang Group proper, represents an accretionary prism compressed and uplifted between the Schwaner Mountains volcano-plutonic arc and a South China Sea microcontinent during an Eocene orogeny. Similar but younger turbiditic rocks deformed by a Miocene orogeny are interpreted not as deposits of a forearc, but as derived from the eroding and uplifted Rajang Group, and should be separated from it. A further record of the late Mesozoic or early Tertiary subduction setting of the NE Borneo margin is to be found in the large Darvel Bay Ophiolite Complex of Sabah. Mineralogical and geochemical studies by Omang & B a r b e r suggest its formation in a suprasubduction zone environment, but with complexities due to high T-low P deformation along a transform fault. High P-T garnet pyroxenites and amphibolites found as clasts in Miocene rocks were derived from a metamorphic sole underlying the complex, formed during subduction and emplacement of the ophiolite. Within the Sunda shelf are sedimentary basins of the South China Sea and adjacent areas which record a link between east Asian tectonics and the plates beyond the subduction zones bounding SE Asia. The importance of pre-existing structures in controlling tectonic development is often forgotten and Ngah et al. suggest that the Malay, Penyu and West Natuna basins originated in the Late Cretaceous as three rift arms that developed during doming of continental crust above a mantle plume. The hydrocarbon potential of these basins was subsequently influenced by changes in stress patterns which Tjia & Liew argue resulted from the interplay of Eurasian extrusion driven by Indian indentation, and changes in directions and rates of motion of the plates in the Pacific and Indian Oceans. Borneo is situated in the middle of this region at the south side of the South China Sea and ought to record the effects of these changes. The geology of Sabah is therefore of considerable regional interest since, if Borneo has rotated, it is an area where the consequences should be most
INTRODUCTION obvious. Clennell discusses the interplay between large-scale regional plate motions (his 'far-field tectonics') and pre-existing structures and local tectonic influences for the development of the unusual circular basins of Sabah. These papers on sedimentary basins show that we are still some way from clearly linking local and regional tectonics. Tjia and colleagues show that there were reversals in the sense of movement on important faults, that the effects of fault movements differ from area to area, and that there is still uncertainty in the timing of fault movements. Clennell infers that basins in Sabah appear not to record some tectonic events because they were decoupled due to the thicknesses of underlying muds and m61anges. Sumatra is an area where a long history of subduction should be recognizable since the island is usually considered to have been situated above the northward-subducting Indian plate from at least the Mesozoic. Despite this there appear to be distinct periods characterized by igneous activity, separated by intervals with little or none. McCourt et al. use isotopic dating and geochemistry to identify plutonic episodes and their character, which they link to plutonism elsewhere in Sunda margin. Understanding the tectonic significance of the igneous episodes needs to be the next step forward. McCourt et al. speculate that variations in the obliquity of convergence, and collision of allochthonous terranes are implicated, although Neogene and younger strike-slip faulting complicates the picture. It would be useful to consider the Cenozoic history of this margin in the light of known plate motions, although the major uncertainty here is not the motion of the Indian plate but the orientation and position of Sumatra; different regional models show very different configurations for the early Tertiary. The Cenozoic history of this margin is clearly complex as indicated by other papers in the volume and Samuel & H a r b u r y show that this complexity is still far from understood. In their paper, based on detailed studies on land in the Sumatran forearc islands, principally Nias, they interpret the Mentawai fault system not as a strike-slip fault, but as an extensional structure with late contractional reactivation. If correct, at least one of the plate slivers of the forearc proposed by Malod & Kemal either does not exist or is of very recent origin. Samuel & Harbury's work also illustrates the importance of field-based studies in providing a firm stratigrapbic basis for the interpretation of other evidence, such as seismic and marine geophysical evidence, and they infer a long extensional history for the Sumatran forearc, with major extensional structures later reactivated, again with possible links to changes in plate motions such as the angle of convergence.
xi
A good stratigraphic base is fundamental to attempts to interpret the tectonic evolution of the region and Wakita et aL provide an example of how this is achieved by detailed radiolarian studies. The Bantimala Complex and Balangbaru Formation of south Sulawesi record critical events in the accretionary growth of the SE Sunda margin and dating based on micropalaeontological evidence is difficult to obtain and interpret since the turbidite sequences yield few fossils, and these may be reworked. Radiolaria, which can often be assigned to narrow zones, provide an additional means of comparing the ages of different lithological units and suggest the Bantimala Complex and Balangbaru Formation are contemporaneous, requiring a modification of previous tectonic interpretations. From the same region Wilson & Bosenee show how redeposited limestones of the Tonasa Limestone Formation can be used as indicators of tectonic activity. Detailed measured sections, well dated by fossils, illustrate how a very clear palaeogeographic picture can be deduced and linked to larger-scale tectonics. Their results provide a basis for regional interpretation and will be of considerable interest to those exploring for hydrocarbon in the frontier regions of east Indonesia. Sulawesi is currently much less wellknown than it deserves to be, especially considering its large size and critical position at the Eurasia-Australia-Pacific junction, and recent results have shown that simple tectonic models for its development need reconsideration. Bergman et al. present data from west Sulawesi which will need to be incorporated in new models and speculate on possible solutions. Of considerable interest is the evidence, based on isotopic studies, for a magmatic contribution from old Australian-type continental crust to the Tertiary plutonic rocks of west Sulawesi. Bergman et al. also focus attention on the structures around the Makassar Strait. This has previously been widely accepted to be an extensional basin but they interpret it as a foreland basin bounded by converging Neogene thrust belts, with the late Miocene western Sulawesi magmatic arc recording continent-continent collision. The collapse of the orogenic belt is seen as the cause of young extension in the region. The rapid changes predicted by the model are certainly consistent with the variety and speed of tectonic processes currently observed in SE Asia. The Neogene collisions of continental fragments in Sulawesi are a principal cause of its geological complexity and Ali et al, provide some insights into how tectonic models can be tested using palaeomagnetic data. In Buton, large rotations are recorded, but are apparently very local, and were very rapid. This work reinforces the value of, and need for, many more palaeomagnetic studies in SE
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R. HALL • D. J. BLUNDELL
Asia in order to separate local from regional motions. Buton is one of several continental fragments which are now being reassembled in SE Asia. These include Australian and Sundaland material but their origin can often only be inferred from indirect arguments, commonly controversial. Vroon et al. suggest that isotopic evidence can contribute to solving this problem and show how different types of continental crust can be characterized by analysis of igneous and sedimentary rocks. They suggest that different parts of east Indonesia have provenances in southern New Guinea, north Australia, Pacific New Guinea and Sundaland, leaving the tectonicians with an additional tool but, in this area, some additional problems to solve. In the midst of the continental fragments of east Indonesia are the deep basins of the Banda Sea, as yet unsampled by the ocean drilling programme, and of uncertain age. Linthout et al. report new isotopic ages from Seram implying Neogene spreading in the southern Banda Sea before ophiolite obduction on Seram in the Late Miocene. These ages are broadly consistent with ages of rocks recovered during recent dredging in the Banda Sea and with the tectonic reconstructions of Hall, although the great depths and low heatflow measurements remain apparent inconsistencies. On the north side of the Banda Sea the Sorong fault system separates Australia from the Philippine Sea and Molucca Sea plates and terminates in the continental fragments of east Sulawesi. The timing of movement on the strike-slip faults has never been clear, although recent work suggests this plate boundary zone became a strike-slip system in the early Miocene. The timing of movements and the distribution of continental crust in this region is of major interest, not least in the search for hydrocarbons, since this is an area of established production, recent discoveries, as well as active exploration, all linked to Australian crust. Based on stratigraphic arguments, Charlton argues that two of the basins, the Salawati basin of western New Guinea and the Tomori basin of eastern Sulawesi, were originally a single sedimentary basin, now separated by latest Miocene to Quaternary movements on the fault, implying a left-lateral displacement of about 900 km. Movement on the fault system is one of the latest complications in the development of east Indonesia; on the south side of the fault system is Australian crust while on the north side are the arc-arc collision of the Molucca Sea and the clockwise-rotating Philippine Sea plate. In the fault zone, which includes several major splays, are fragments of both Philippine Sea and Australian origin and Bacan is one of the islands which includes rocks of both provenances. Bacan therefore offers the possibility of elucidating
some of the history of the plate boundary zone, and establishing the timing of tectonic events. Malaihollo & Hall report new stratigraphic data from Bacan which record the early arc history of the Philippine Sea plate and the arrival of continental crust, providing a basis for distinguishing different tectonic models. Baker & Malaihollo discuss the timing of volcanism in the islands of Halmahera immediately north of Bacan, which records the initiation of subduction of the Molucca Sea plate beneath Halmahera and the development of the present-day arc-arc collision. Volcanism began in the middle to late Miocene and migrated northwards implying that the double subduction system was established between about 15-12 Ma. This evidence still remains to be incorporated in models linking east Indonesia to the Philippines. Pubellier et al. provide further evidence from the southern Philippines critical to linking the two areas and understanding the development of the present tectonic setting described earlier in the volume by Rangin et al. Once again, the theme of partitioning of oblique convergence is emphasized. In addition, there are complications reflecting Neogene changes in plate motions and the complexities of intra-arc deformation. Strain has been partitioned between several orientations of faults, reactivated at different stages as thrusts and wrench faults, as well as subduction zones. Of particular interest is the way in which this development has resulted in intra-arc extension and fragmentation within the Philippines. The interplay of subduction and strike-slip faulting is a theme which appears in many of the papers in the volume. Ancient strike-slip motion is often difficult to demonstrate and quantify and is consequently often neglected. However, there is evidence, traditionally linked to the convergent component of collision, which may be differently interpreted. Crowhurst et al. show that fission track data suggest that the Papuan metamorphic rocks may be interpreted as representing early Neogene extension after arc collision, rather than contraction-related metamorphism. These arguments may be applicable in other parts of SE Asia where fission track and isotopic ages are revealing unsuspected events, very young ages, and short time periods for the very complex tectonic evolution of many parts of the region. The volume concludes with two papers from south China accompanied by interpretations of the timing and significance of events. Wopfner suggests that the Baoshan and Tengchong Blocks in western Yunnan have a Gondwana origin, supported by the presence of Upper Palaeozoic glaciomarine deposits, cold-water faunas and Glossopteris. The terranes separated from Gondwana in the Early Permian and docked with
. ° .
INTRODUCTION Cathaysia in the Late Triassic, although Tertiary strike-slip faulting on the Nujiang Line has juxtaposed the two terranes. Z h o u et al. reinterpret part of the history of SE China, diverging in particular from the traditional practice of relating major unconformities to separate orogenies, and suggesting instead that they record different stages in the evolution of a single orogeny following early Mesozoic collision between the South China and the South China Sea blocks. SE Asia is one of the most exciting regions of the globe for any earth scientist. The size, difficulties and practicalities of the region demand a long-term investment of effort but the rewards are illustrated by the papers in the volume. These give an insight into how the history of the region will be uncovered, as well as providing an overview of present regional tectonics and its development. Because of the rapid rates of movements in many parts of the region new geodetic tools and increasingly refined methods of examining earthquake data mean that realistic and rapid tests of regional and global plate models are possible. The challenge for the future is to examine how far interpretations of these data can be pushed back into the past, to improve kinematic descriptions and models, and to identify the processes which have led to the complexity of the region and which can be applied to older orogenic belts. In these tasks, there remains an important role for the field geologist as well as those developing and applying new technologies. An improvement of our understanding will have benefits for knowledge as well
Xlll
as for the many inhabitants of the region, in helping to develop its resources and mitigate its hazards. This Special Publication arose from a conference on the Tectonic Evolution of SE Asia held at the Geological Society in London in December 1994. In addition to the reasons outlined above for the conference, the London University SE Asia Research Group wished to mark the retirement of Dr A. J. Barber in 1994. Tony Barber initiated several of the studies which are presented as publications in this volume and was instrumental in developing the programme of the SE Asia Research Group over many years, particularly by fieldwork throughout the region. We wish him a long and happy retirement. We thank all of the following who provided reviews of manuscripts: J. R. Ali, M. Allen, M. G. Audley-Charles, A. J. Barber, H. Bellon, S. C. Bergman, J. C. Briden, C. S. Bristow, T. R. Charlton, J. Charvet, B. Clennell, D. Q. Coffield, M. C. Daly, J. E. Dixon, C. Elders, R. Ellam, A. Fortuin, M. Fuller, R. J. Garrard, N. S. Haile, N. A. Harbury, K. C. Hill, A. J. Hurford, C. S. Hutchison, S. J. Kelly, J. Malod, S. J. Matthews, R. McCaffrey, M. Menzies, I. Metcalfe, J. S. Milsom, A. H. G. Mitchell, G. Moore, S. J. Moss, R. J. Murphy, G. J. Nichols, G. Packham, C. D. Parkinson, S. Polachan, M. Pubellier, A. J. Racey, C. Rangin, J-P. Rehault, M. A. Samuel, D. Snyder, P. Styles, R. E. Swarbrick, M. E Thirlwall, E Tongkul, J. J. Veevers, R. von Huene, G. K. Westbrook, H. J. Wensink. We are especially grateful to Diane Cameron who carried out many of the major tasks related to organizing the conference, in addition to her assistance in reviewing and editing the manuscripts. Simon Baker, Steve Moss and Moyra Wilson also provided considerable assistance during the preparation of the volume, for which we are extremely grateful.
Slip partitioning at convergent plate boundaries of SE Asia ROBERT McCAFFREY
Department of Earth & Environmental Sciences, Rensselaer Polytechnic Institute, Troy, N e w York 12180, USA Abstract: The active tectonics of SE Asia can be characterized by the interactions of large, rigid plates separated by broad zones of deformation. The relative motion of these plates across their boundaries is often partitioned in the sense that the normal and shear components occur on different structures. Earthquake slip vectors and geological and geodetic measurements are used to infer the degree to which oblique convergence, which is ubiquitous in SE Asia, is partitioned. The active tectonics of Sumatra, the Himalayan thrust, the Philippines, the New Guinea fold-andthrust belt, the Huon-Finisterre collision, and the San Cristobal trench can be understood in terms of upper plate deformation associated with oblique convergence. Western Java may also exhibit partitioning of oblique subduction. Structures accommodating normal and shear components of the motion are often very close. Arc-parallel strain rates are estimated for forearcs of the region. The arc-parallel deformation of forearcs of the SE Asia region demonstrates that plate convergence, whether normal to structure or not, is a three-dimensionalprocess.
When convergence between two plates is not perpendicular to their boundary, shear stress parallel to the plate boundary results in marginparallel shear strain within both plates. In a collision belt or subduction zone, if one of the plates is weak in shear, then the total slip may be partitioned into shear and thrust components (defined by the plate boundary orientation). Often these shear and normal components of slip across the boundary are accommodated on different geological structures. Oblique convergence is globally much more common than trench-normal convergence and often the obliquity varies along the margin. The deformation in many convergence zones may be understood in terms of such slip partitioning. A widely cited example of these parallel structures is along the San Andreas system, which comprises a strike-slip fault and a parallel fold-and-thrust belt (Mount & Suppe 1987) even though the relative motion between the Pacific and North American plates is only a few degrees from the trend of the San Andreas fault. Throughout SE Asia and the SW Pacific, we see several examples of oblique convergence (Fig. 1), both in subduction and collision settings. Studies of Sumatra led Fitch (1972) to first propose that slip was sometimes accommodated by parallel thrust and strike-slip faults. Since that study, it has been found that partitioning of oblique subduction is globally quite common although it is rarely completely partitioned (Jarrard 1986; McCaffrey 1994). Similar geometries, in which a major strike-slip fault accommodates the boundary-parallel shear
strain, exist in the Philippines (Barrier et al. 1991) and possibly in Irian Jaya (Abers & McCaffrey 1988). Other convergence zones that have clear slip partitioning are the Himalayan thrust (Molnar & Lyon-Caen 1989), the Aegean (Gilbert et al. 1994), New Zealand (Anderson et al. 1993) and the Aleutians (Ekstr(Sm & Engdahl 1989; McCaffrey 1992). McKenzie & Jackson (1983) argued that, if plate motions are driven by ductile shear from below, oblique convergence in continental settings should be completely partitioned, that is, slip will occur only on strike-slip and pure thrust faults. This theory provides a test of crustal dynamics in oblique collision zones but requires detailed knowledge of the deformation. SE Asia is a good region to test such ideas because oblique convergence zones are plentiful and fast. In the past few years much has been done to improve our knowledge of the pattems of crustal deformation in SE Asia. In particular, systematic studies of large earthquakes, geological studies of active faults, and geodetic measurements using the Global Positioning System (GPS), that commenced in the SW Pacific region in 1988, are revealing important results, and surprises. The goal of this paper is to review our knowledge of the crustal deformation in SE Asia with attention to the partitioning of slip between thrust and shear faults in the broad deforming boundaries. First, constraints on the motions of the major plates are discussed in order to put bounds on the convergence geometries and then the deformation within these zones is described.
From Hall, R. & Blundell, D. (eds), 1996, Tectonic Evolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 3-18.
4
R. McCAFFREY
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49
normal moveout curves for reflections at two-way travel times longer than around 10 s, equivalent to depths of around 25 km. For greater depths, indirect evidence from refraction experiments in the Timor trough and Banda Sea (Bowin et al. 1980), and from a general knowledge of p-wave velocities in the upper mantle had to be used for the depth conversion.
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seabed multiples, which partly obscure primary reflections from deeper levels, and a high level of random noise. The multiples were so easily recognized on the brute stack that they could be distinguished from primary reflections passing through them. Attempts to remove these multiples for the final stack sections were largely successful, but at the same time probably degraded the primary reflections. The brute stacks displayed the clearest images of primary reflections, multiples and diffraction patterns, and so were used more than the final stacks in preparing line drawing interpretations. Following the interpretation method of many other deep seismic experiments, discontinuous linear segments of reflections identified as primary on the seismic sections were traced on to a transparent film overlay. These line drawings more clearly delineate the overall structure and are treated as an interpretation of the geometries of the main reflections (Fig. 3). The principal reflections interpreted on the line drawings were depthcorrected and migrated in the plane of the section to give their depth geometry (Fig. 4). The only direct velocity inforhaation available was provided from the moveout analysis of reflected data from the 4.6 km long streamer. This offset was insufficient to discriminate between
The upper 2.1 s TWT beneath the seabed of the continental shelf portions of both sections consist of fairly parallel reflectors interpreted as sedimentary bedding. This is consistent with the tie of the TIMOR line with the Troubadour No. 1 oil exploration well (Fig. 1) which penetrated Recent to Triassic marine sediments lying unconformably on granite encountered at a depth of 3315m, equivalent to 2.16 s TWT. Extensional faults are evident in the upper 3 s TWT of both sections and appear to penetrate the crystalline basement as well as cut the entire sedimentary cover. These faults were probably initiated in the Jurassic when the Australian Northwest Shelf was last stretched (Bradshaw et al. 1988; Powell 1982). In the region of Sumba Island, where subduction is continuing at present, many faults that were originally related to rifting and spreading are still in active extension and are giving rise to earthquakes with extensional mechanisms. Some strong, north-dipping reflectors were also identified in the otherwise transparent upper crust (Fig. 3). These reflectors appear to sole out of the Jurassic faults to cut through the entire upper crust and continue into a reflective zone in the lower crust. The upper continental crust consists of crystalline rocks so it is most likely that these reflectors represent faults, following the interpretation of other deep seismic reflection surveys elsewhere (e.g. Matthews et al. 1990). Between 8 and 12 s TWT at the southern end of the TIMOR section and 7 and 14 s TWT at the southern end of the DAMAR section lies a zone of intense, sub-horizontal, laterally extensive, layered reflections. The base of this zone is consistent with an increase in interval velocity from 6.3-7.3 to over 8 km s-1 at this level on both profiles (Fig. 5) and corresponds well with the normal jump in p-wave velocity across the Moho to 8.1 km s-1 in the upper mantle (Fowler 1990). The velocity structure and character of reflection patterns in this zone is therefore much like that of the lower continental crust imaged by a number of
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other deep seismic reflection surveys from other areas (e.g. Mooney & Brocher 1987). The base of this zone of high reflectivity has been identified as the Moho (originally by Fuchs 1969) and it would be reasonable to apply the same interpretation here. This reflectivity within the lower continental crust has been associated with crustal extension, involving basaltic sill emplacement (Serpa & Voogd 1987; Warner 1990) and/or ductile shear zones (Reston 1987). Since this part of the Australian continent became a rifted margin in the Jurassic (Bradshaw et al. 1988; Powell 1982), it would be reasonable to assign the sub-horizontal reflectors in the lower crust to this age. On this basis the crustal thickness at the southern end of both sections is 35-40 km, consistent with a crustal thickness of 30-40 km determined in this area using seismic refraction data (Bowin et al. 1980).
The other observed deformation of the crust is a gentle flexure as it is bowed down beneath the Timor trough, bringing the Moho down to a depth there of 45-50 km.
Subduction Trench (TIMOR SP 6400-6450, D A M A R SP 700-750) Undisturbed, horizontally bedded sediments of 544 m thickness (0.48 s TWT) are present in the Timor Trough at the base of the trench on the TIMOR profile. Using a sedimentation rate of 480 m/Ma in the trough axis obtained from analysis of DSDP-262 (Johnston & Bowin 1981), situated SW of Timor, these sediments accumulated in a period of c. 1.1 Ma during which little convergence
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CONTINENTAL COLLISION IN THE BANDA ARC has taken place. However, immediately south of the horizontal sediments, a small, north-dipping reflector observed on TIMOR at SP 6500 is interpreted as a thrust which has created a hanging wall anticline and delaminated the upper 0.6 s TWT of sediments, sliding them 200-400 m to the south. Other thrusts have formed beneath. A similar situation is observed on the DAMAR profile. Thus a stick-slip system of convergence appears to be operating SE of Timor in much the same way as has been observed in the Timor trough by Karig et al. (1987) SW of Timor. Interval velocities in excess of 5 km s-1 are encountered beneath the trench at 3 s and 2 s TWT below the seabed on the TIMOR and DAMAR profiles respectively (Fig. 5). This is in approximate agreement with refraction data (Bowin et al. 1980). Seismic velocities appropriate for granitic basement are thus encountered at about the same depth below the seabed both at the trench and on the continental shelf at the southern end of the two profiles. The crust, measured from the base of the highly reflective region, appears somewhat thicker at the trench than at the southern end of both sections (Fig. 4). Thickening could imply that the crust has shortened on, or just prior, to its arrival at the subduction trench. However, upper crustal shortening through inversion of extensional faults offsetting sedimentary layers is not evident, except within the trench. The zone of high reflectivity, interpreted as representing the lower crust, appears to terminate abruptly just north of the subduction trench in the DAMAR section. If the high reflectivity of the lower crust is interpreted as a structural fabric of Jurassic age, this would be destroyed if it were folded and sheared by compression. In this case, reflectivity would be drastically reduced, which could explain its abrupt termination.
Accretionary Prism (TIMOR SP 5800--6400, D A M A R SP 750-1100) A few kilometres north of the trench, reflectors within 4 s TWT below the seabed are less coherent. This is probably because of the disrupted nature of the sedimentary package. Deeper reflectors tend to be swamped by strong seabed multiples on the brute stack and noise on the final stack. Nevertheless, a series of southward-verging duplex structures is visible on both sections within what appears to be an accretionary prism (Fig. 3). This interpretation is consistent with the results of seismic reflection surveys to the west and southwest of Timor and south of Sumba (Karig et al. 1987; Van tier Werff et al. 1994) and southeast of
53
the Tanimbar Islands (Jongsma et al. 1989; Richardson 1994). Thus there is evidence that a proportion of the sedimentary sequence is accreted to the underside of the developing accretionary prism within a few kilometres of the trench. In this portion of both TIMOR and DAMAR profiles there is a general increase in seismic velocity from south to noah within 4 s TWT below the seabed (Fig. 5). This is probably the result of compaction and de-watering of the sedimentary package as it is broken up and accreted to the upper plate. North-dipping reflectors co-linear with sedimentary reflectors dipping beneath the trench appear to extend only c. 30 km north of the subduction trench on both profiles. North of this, regions of reflectors parallel to sedimentary reflectors dipping beneath the trench appear to occur in discrete 'packages' offset by prominent south-dipping reflectors (Fig. 4). Shallower packages appear to be offset to the north of deeper packages. The upper surface of these packages can be followed downwards in a relatively straight line to the base of the profiles and extrapolated to c. 140 km depth to join the northern (top) surface of the plate defined by earthquake hypocentres which has been defined in this area by McCaffrey (1989). There are three alternative interpretations for the actual physical nature of the reflectors within the packages. The north-dipping reflectors may represent: (1) remnant sedimentary material attached to the lower plate that has escaped accretion to the collision complex and has been carried to great depth; (2) faults or shear zones representing the zone of decoupling between the upper and lower plates; (3) accreted (underplated) sedimentary rocks. This last interpretation was adopted by Clowes et al. (1987) for similar deep reflectors dipping parallel to the subduction zone under Vancouver Island and by Moore et al. (1991) for the eastern Aleutian Islands, but the other two possibilities appear equally plausible. Whichever is preferred, the packages of reflectors may be interpreted as broadly representing the upper surface of the Australian plate beneath the collision complex. Therefore, it does not appear that the slab has detached above about 140 km depth.
Central Collision Complex (TIMOR SP 3200-5800, D A M A R SP 1100-2350) North of the accretionary prism, where the reflectors dip mainly to the north, both profiles pass into a region of predominantly south-dipping reflections. This geometry is similar to that seen on
54
A. N. RICHARDSON & D. J. BLUNDELL
high quality oil industry seismic reflection profiles in the Tanimbar Islands region made available by Union Texas (SE Asia) Inc. (Richardson 1994) and in the forearc region around Sumba, along strike from westem Timor (van der Werff et al. 1994). The transition zone between north-dipping and south-dipping reflectors appears complex in most cases. The south-dipping reflectors represent either sedimentary layers or low-angle faults because they do not have the characteristic shape of diffraction tails. They are unlikely to represent sedimentary layering because they appear fairly continuous to considerable depth whereas there is no sign of wellbedded sediments just below the seabed. On the TIMOR line (SP 32004400, 4700-5100), the patterns of the south-dipping reflectors are reminiscent of large-scale thrust sheets (Fig. 4). Similar features are observed on the seismic reflection profiles around the Tanimbar Islands where south-dipping reflectors terminate upwards in antiform folds. On the DAMAR profile, major south-dipping reflectors appear to offset northdipping packages of reflectors to the north (Fig. 3), consistent with north-vergent thrust faulting involving the subducted plate. In both the BANDA and Union Texas profiles, interval velocity inversions coincide with these south-dipping reflectors (Fig. 5). If a region with a normal velocity gradient were deformed into a stack of thrust sheets, velocity inversions could be expected to develop at the base of some thrust sheets where high velocity material overrides low velocity material. Shallow earthquakes occur in this area and McCaffrey (1988) has calculated hypocentres and focal mechanisms for a number of them (Figs 1 & 4). Because they have been projected up to 60 km onto the profile it is impossible to correlate particular earthquakes with individual reflectors. However, it is clear that there is a reasonable association and the dominantly thrust mechanisms of the earthquakes support the other evidence that the reflectors represent thrusts. The reflectors' appearance, their coincidence with interval velocity inversions and association with thrust mechanism earthquakes all suggest that they represent north-vergent thrusts which appear to reach depths of tens of kilometres, possibly penetrating into the mantle. Similar reflectors observed elsewhere have been interpreted as shear zones and ancient subduction zone traces (Matthews et al. 1990). The crest of the upper thrust sheet at SP 3900 on the TIMOR profile (Fig. 3) is a topographic feature of the seabed and forms the eastward, submarine extension of the island of Kisar. Kisar is offset from the trend of the main archipelago and consists of tightly folded felsic schists and mylonites which
can be correlated with the Aileu Formation of northern Timor (Richardson 1994). The presence of the thrust on the TIMOR profile in alignment supports the idea that Kisar may have been uplifted on an out-of-sequence thrust. However, the structural grain in Kisar runs approximately N-S (Richardson 1994), at right angles to the structural trend of the other islands. It is possible that Kisar has rotated by 90 ° during thrusting, or alternatively that it forms a lateral ramp. The seabed high at SP 3400 on the TIMOR section (Fig. 3) is mid-way between Kisar and the volcanic island of Romang. South-dipping reflectors with velocity inversions (Figs 3a & 5a) visible at SP 3200-3400 are interpreted as thrust faults. Interval velocities below the topographic high at TIMOR SP 3400 are similar to those below SP 3900 but are generally lower than the next topographic high to the north at SP 2900. The crustal thickness appears to be 18-23 km. The topographic high at SP 3400 on the TIMOR section is therefore interpreted to be similar in composition to Kisar and so may be the northernmost occurrence of continental rocks at the surface along this profile. Dating features identified on deep seismic reflection profiles is one of the most intractable problems within the field of deep seismics (Klemperer et al. 1990) unless a reflection can be traced to outcrop or there is other independent evidence. Since none of the south-dipping structures on TIMOR and DAMAR have been identified before, let alone recognised at the surface, dating them must rely on more speculative geometrical observations. Consequently, the cross-cutting geometric relationship of the south-dipping reflectors with the packages of north-dipping reflectors is used as evidence for their age. The packages of north-dipping reflectors are interpreted as representing the upper surface of the Australian plate beneath the collision complex. The amount of subduction is thought to have been extremely limited south of western Timor for the last 0.6 Ma (Karig et al. 1987). Horizontal sediments at the trench axis on the TIMOR profile imply that subduction has been inactive there for the last 1 Ma apart from a few hundred metres of south-vergent thrusting. The shallowest cross-cutting, southdipping structure on the TIMOR and DAMAR profiles cuts the boundary of north-dipping reflectors about 30 km north of the trench axis. Assuming the rate of convergence to be decreasing (Johnston & Bowin 1981), it would have taken this part of the lower plate c. 1.4 Ma to have reached its present location. Hence, this thrust cannot be older than c. 2.4 Ma which is approximately the age of onset of the current collision derived from evidence in the Timor trough south of western Timor (Johnston & Bowin 1981).
CONTINENTAL COLLISION IN THE BANDA ARC The top of the north-dipping lower plate and the deepest of the south-dipping thrusts define the collision orogen as a triangular-shaped wedge (Fig. 4). Sedimentary material is being added to the southern end of the wedge by underplating on north-dipping thrusts. However, the northern portion of the collision orogen is deformed on south-dipping, north-vergent thrust sheets. The exact depth of the Moho is difficult to constrain for this portion of the profiles because the poor resolution of the velocity picks leads to significant variations between adjacent values of interval velocity. However, assigning the base of the crust to the boundary between interval velocities less than and greater than 8000 ms -l, the crust appears to be thickest directly under the topographic highs: SP 4600 (> 46 km), SP 4000 (c. 30 km) and SP 3400 (> 36 km) on the TIMOR section and SP 1600 (c. 50km) and SP 2150 (c. 20 km) on the DAMAR section (Fig. 5). A 2D density model was produced to match a gravity profile along the TIMOR line recorded in a previous survey by Woodside et al. (1989). Although poorly constrained, the model serves to delimit the approximate thickness of the crust in this region. Two end-member versions of the model were produced. In one (Fig. 6a), which defines a minimum value for crustal thickness, the crust of the central collision complex was made 30 km thick, the same as the continental crust of the Australian Northwest Shelf. A reasonable fit
55
between calculated and observed gravity was achieved. In the second (Fig. 6b), crust with 2 6 7 0 k g m -3 density representing the central collision complex was enlarged at the expense of an upper mantle region with 3 1 0 0 k g m -3 density, whilst maintaining the fit of calculated with observed gravity. The second model gives an upper limit for which a reasonable match can be made with the observed gravity data. In this, crust of the central collision complex attains a maximum thickness of nearly 60 km.
Volcanic Arc (TIMOR SP 1700-3200, DAMAR SP 2350-2900) At the northern end of both seismic sections, submarine volcanoes associated with the subductionrelated volcanic islands of the Banda arc form prominent topographic highs. The volcanoes are said to be built on oceanic crust on the basis of seismic refraction data (Bowin et al. 1980). Long sub-horizontal reflectors just below the seabed are interpreted as delineating lava flows (D. Snyder, pers. comm., 1995). The TIMOR profile crosses the main volcanic ridge at SP 2950. Volcanoes at SP 2000 and SP 2350 on the TIMOR profile lie to the north of the main volcanic ridge and have a steepsided, narrow trough between them at SP 2130 (Fig. 3a). This sharp trough lies along strike from the surface expression of the Wetar Thrust visible
Gravityo (mgal) /
•
t: 1::o°o °
"1°°I -200 2670
~
~
i
o
2800
i
2800
50
3300 \,........ lOO
(km) 150 2o0
/ ?
// /.s
3300 150
3
3200
2OO
/ 2~o
2~6
S 1~o Distance
~o
.=b
from Trench(km)
~
-~o
-lOO
loo Distance
3200 ~o
~
fromTrench(km)
-~o
250
Fig. 6. Gravity models to constrain estimates of crustal thickness in the central part of the Banda orogen. Densities of polygons are in kg m-3.
56
A. N. RICHARDSON 8Z; D. J. BLUNDELL
on sonar imagery (Masson et al. 1991). A southdipping reflector reaches the surface at the base of this trough and can be traced 50 km southwards to a depth of c. 20 km. Two earthquakes with thrust mechanisms project on to the TIMOR section within the horizontal range of this reflector (Fig. 4a) although with significantly greater hypocentral depths. It may be significant, however, that the south-dipping nodal planes of both these events dip at an angle close to the migrated dip of the reflector. This reflector is therefore interpreted as evidence for the along-strike extension of the Wetar Thrust, associated with current movement. A number of major, south-dipping reflectors are also present in this portion of the DAMAR section which extend close to the base of the seismic record (Fig. 3b). These structures therefore appear to be the dominant fabric of the upper plate on both profiles. Some shorter north-dipping reflectors are present on both profiles but none are as long or as continuous as the south-dipping reflectors (Fig. 4). The volcanic arc portion of the DAMAR profile includes the greater proportion of earthquakes less than 100 km deep along the transect (Fig. 4b). Most of these earthquakes have thrust mechanisms, with one nodal plane which has a south-dipping component, and plot close to the major reflectors. It is not possible to correlate specific earthquakes with individual reflectors because of the large envelope of uncertainty in hypocentral locations and the distance that they have been projected on to the section. However, the fact that earthquakes with thrust mechanisms are occurring in this region does suggest that the south-dipping reflectors may represent active thrust faults which penetrate the crust and upper mantle to depths of up to 100 km. On the TIMOR profile, the crustal thickness (defined where Win t > 8000 m s-1) varies from c. 12 km to over 20 km beneath the volcanoes. This is abnormally thick for oceanic crust, even including the volcanic edifices. Interval velocities > 8000 m s-1 are attained at depths of 8-19 km below sea-level on the DAMAR profile (Fig. 5b) which implies that the crust is thinner than on the TIMOR profile but still thicker than normal oceanic crust.
The backarc region (TIMOR SP 1-1700, D A M A R SP 2900-3200) A number of marine geophysical surveys have been conducted in the backarc region. From Lombok to Maopora, the backarc region is dominated by south-dipping reflectors beneath the volcanoes, sometimes overlain by an accretionary prism (Silver et al. 1983). Likewise, the south-dipping reflectors in this part of the BANDA profiles are
interpreted as thrusts because of their appearance on seismic profiles. The two most northerly volcanoes on the TIMOR section, at SP 400 and SP 1200, are isolated seamounts built on the oceanic crust of the Banda Sea. The northernmost edifice rises above sea-level to form the active volcano, Gunung Api. It is not clear why this volcano is active so far to the north of the subduction zone. The volcanoes traversed by the TIMOR profile appear to line up with Gunung Api and the seamount in between them (Fig. 1). The NNW-SSE lineation of seamounts suggests that they may have formed over a 'leaky' transform fault (D. Snyder, pers. comm. 1992). The DAMAR profile passes close to one of the seamounts at SP 2600 and directly over another at SP 2800, a few kilometres to the north. The basin areas in between the volcanoes show clear sedimentary layering near the surface and long, gently dipping reflectors down to the depth of the first multiple (Fig. 3). The velocity pick at TIMOR SP 1599 in the centre of one of these basins is interpreted as representing 1840 m of sediments (cf. oceanic lithosphere Layer 1, Vint = 1900-2400m s-1) and 5860 m of volcanics and intrusives (oceanic lithosphere Layers 2 and 3, Vin t - - 4 2 1 7 m s-1) overlying upper mantle material (oceanic lithosphere Layer 4, Vin t -- 8058 m s-1) at 7.7 km below the sea-bed. Most velocity picks at the extreme northern end of the DAMAR profile have velocities typical of the upper mantle (Vin t > 8000 m s-1) occurring 3 . 7 4 km below the seabed. All these velocity structures broadly agree with seismic refraction data (Bowin et al. 1980) and are therefore interpreted as representing normal oceanic crust. The northern part of the DAMAR line appears to traverse rather rough sea floor. However, one of the major south-dipping thrusts reaches the surface in this region and the rough topography may represent a small accretionary prism.
Summary The southern end of both profiles, which traverse the Australian Continental Shelf, is interpreted in a similar manner to deep seismic reflection profiles from other passive continental shelf areas. Wellstratified sediments overlie a largely reflectionfree upper crustal crystalline basement and a highly reflective lower crust. This arrangement is common in continental regions that have undergone extension. Structures at shallow depths under the northern slope of the trench (or the southern part of the collision zone) dip exclusively northwards and are interpreted to represent thrusts and tilted sediments in an accretionary prism. Deeper, north-dipping
CONTINENTAL COLLISION IN THE BANDA ARC structures are interpreted to be related to the interface between the lower and upper plates. This is supported by the extrapolation of this surface to greater depths where it corresponds well with the upper surface of the Australian plate as defined by earthquakes. The northern part of the collision zone is dominated by predominantly south-dipping structures which extend to sub-crustal depth. These are interpreted as thrusts along which the lateral shortening and crustal thickening have taken place. The thrusts are estimated to be no older than 2.4 Ma, the age of initiation of the collision of the Australian margin with the arc, and seem to be active at the present day at the northern end of the collision zone. A maximum crustal thickness of c. 60 km is calculated under the topographically highest part of the collision zone on the basis of calculated seismic interval velocities. If this crust had been constructed from rifted and thinned Australian margin material, the crust may now be more than double its original thickness which would imply over 50% (100-150 km) shortening in the continental portion of the collision zone. The oceanic crust also appears to be anomalously thick beneath the volcanoes on the basis of calculated seismic interval velocities. It appears likely that the oceanic crust has, like the continental crust, been shortened and thickened on predominantly south-dipping thrusts. The fact that these major thrusts are visible beneath and north of the volcanic arc suggests that the collision zone extends north of the non-volcanic arc and is in total about 200 km wide, comprising both continental and oceanic lithosphere, and may have accommodated over 200 km of shortening. The overall, large-scale character of the collision zone is dominated by two sets of divergent struc-
N
57
tures. The southern set is related to the subducting (lower) plate and dips in the same direction as subduction. The northern set, in the upper plate, is antithetic to this. Figure 7 is a cartoon to highlight these primary features.
The deep-seated nature of the orogen In considering the evolution of the collision complex as an orogen, three dates are significant: (1) The Banda allochthon in Timor was apparently uplifted at c. 37 Ma according to K-Ar dating (Sopaheluwakan 1990). (2) Accurate radiometric dating on the Aileu Formation on the north coast of Timor gives an age of metamorphism associated with gradual cooling of 8 Ma (Berry & Grady 1981) and a Mid-Miocene unconformity has been inferred across Timor (Audley-Charles 1968). (3) Pliocene deformation is consistently recorded by sedimentary sequences elsewhere in Timor (Audley-Charles 1968) and the start of the most recent collision has been dated at 2.4 Ma from evidence in the Timor trough (Johnston & Bowin 1981). The extent of crustal material in the central collision complex is defined by seismic velocity and gravity modelling. The crustal material has the form of an inverted triangle about 50 km (37-60) deep and 125 km (135-160) across on the TIMOR profile, which has a cross-sectional area of 3750 km 2. This thickness of crustal material could, conceivably, have been constructed by a number of means. However, it cannot have been constructed entirely from sediments accreted from the Australian plate since 2.4 Ma. Using the plate velocity vectors in DeMets et al. 1990 (Fig. 1),
Australian Continental S Timor Micro-Continent AccretionaryPrism ~li,helf _ Kisaj~ ~, iSubd ,.)., ~lt~ ctionTrench , , ~
Volcanic Arc ~1~ ~
~60 km
Moho ~"
/~5. /
~i~;~ ~ ; ~
0
km
1O0
I
I
I
]
Fig. 7. Cartoon structural cross-section of the Banda orogen based on our interpretation of the BANDA deep seismic profiles. Australian continental shelf sediments are shaded light grey. Highly reflective lower crust is shaded dark grey.
58
A.N. RICHARDSON t~ D. J. BLUNDELL
156 km (65 km/Ma for 2.4 Ma) of convergence normal to the arc could have occurred. This is an upper estimate as it does not take account of decreasing convergence velocity after collision. An upper estimate of the thickness of the sediments between the Australian coast and Timor from the Banda profiles is about 3 km. Assuming that this entire thickness of sediments is added to the growing orogen and that strike-slip movement has not removed material from the normal plane, a cross-sectional area of 468 km 2 would result (3 × 156 km). This is far less than the estimated crustal area of 3750 km 2. A collision event, adding crustal material to the orogen, must therefore have preceded the Pliocene collision event. Nor is it possible that the Banda orogen was constructed entirely from sediments accreted from the Australian Plate since 8 Ma. Using the same argument as above, a sediment volume accumulated over 8 Ma of convergence would amount to only 1560 km 2 in cross-sectional area, which is still substantially less than the estimated 3750 km 2 of crust. There must therefore be an additional piece of crystalline continental crustal material within the collision complex making up a major portion of its bulk. It is thus proposed that a micro-continental fragment, or perhaps an outer margin 'high', lay a few hundred kilometres to the north of the Australian Northwest Shelf and this was incorporated into the collision complex before the arrival of the Australian continental margin. The material separating the micro-continent from the Australian craton was probably oceanic in origin but could have been extremely stretched and attenuated continental lithosphere. This micro-continental fragment collided with the subduction zone at c. 8 Ma in the Miocene. This caused metamorphism
of the Aileu Formation at the leading edge of the micro-continental fragment ( ' m e t a m o r p h o s e d parautochthon' on Timor and Kisar) and folding in the unmetamorphosed rocks. The micro-continent was thrust beneath the forearc region, thus uplifting the Banda Allochthon, and may have induced Late Miocene inversion structures reported from the Timor Sea (MacDaniel 1988; Pattillo & Nicholls 1990). The Australian continental margin arrived at the collision zone 2.4 Ma ago and started downwarping into the subduction zone. The continental crust could not be subducted very far. Subsequently, the Australian continental margin has acted as a bulldozer, shortening and uplifting continental and oceanic parts of the collision zone on deep seated thrusts dipping antithetic to subduction. The oldest thrusts are presumably the more southerly ones. Since then the locus of activity has propagated northwards in a normal fashion until it has reached the backarc region where thrusts are active at the present day. Uplift is currently continuing as evidenced by several hundred metres of elevation of Pliocene coral reef terraces on Alor, Atauro and Wetar (Nishimura & Suparka 1986) and Sumba (Piranzolli et al. 1991). The authors wish to thank Dr D. Snyder for his major efforts in conducting the BANDA survey and supervising the data processing, and to him and their other BIRPS colleagues for ready access to the deep seismic reflection data. BIRPS is supported by the Natural Environment Research Council (NERC) and all BIRPS data are available, at the cost of reproduction, from the Marine Geophysics Programme Manager, British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA. ANR is grateful to NERC for the award of a studentship during the period of this research.
R e f e r e n c e s
AUDLEY-CHARLES, M. G. 1968. The Geology of Portuguese llmor. Geological Society, London, Memoir, 4. 1981. Geometrical problems and implications of large-scale overthrusting in the Banda ArcAustralian Margin collision zone. In: MCCLAY, K. R. & PRICE, N. J. (eds) Thrust and Nappe Tectonics. Geological Society, London, Special Publications, 9, 407-416. - 1986a. Timor-Tanimbar Trough: the foreland basin to the evolving Banda Orogen. In: ALLEN, P. A. 8£ HOMEWOOD, P. (eds) Foreland Basins. International Association of Sedimentology Special Publication, 8, 91-102. - 1986b. Rates of Neogene and Quaternary tectonic movements in the southern Banda Arc based on micropalaeontology. Journal of the Geological Society, London, 143, 161-175. - - & HARRIS,R. A. 1990. Allochthonous terranes of the
South West Pacific and Indonesia. Philosophical Transactions of the Royal Society of London, A331, 571-587. BARBER, A. J. 1979. Structural interpretations of the island of Timor. South East Asian Petroleum Exploration Association Proceedings, 4, 9-12. 1981. Structural interpretations of the island of Timor, eastern Indonesia. In: BARBER, A. J. WIRYOSUJONO,S. (eds) The Geology and Tectonics of Eastern Indonesia. GRDC Special Publication, 2, 183-198. - - - , AUDLEY-CHARLES,M. G. & CARTER, D. J. 1977. Thrust tectonics in Timor. Journal of the Geological Society of Australia, 24, 51-62. BERRY, R. E & GRADY, A. E. 1981. The age of the major orogenesis in Timor. In: BARBER, A. J. & WIRYOSUJONO,S. (eds) The Geology and Tectonics of Eastern Indonesia. GRDC Special Publication, 2, 171-181.
CONTINENTAL COLLISION IN THE BANDA ARC BOWlN, C., PURDY, G. M., JOHNSTON, C., SHOR, G., LAWVER,L., HARTONO,H. M. S. & JEZEK,P. 1980. Arc-continent collision in the Banda Sea region. AAPG Bulletin, 64, 868-918. BRADSHAW, M. T., YEATES, A. N, BEYNON, R. M., BRAKEL,A. T., LANGFORD,R. P., TOTTERDELL,J. M. & YEUNG, M. 1988. Paleogeographic evolution of the North West Shelf region. In: PURCELL,P. G. & PURCELL, R. R. (eds) The North West Shelf Australia. Proceedings of the NW Shelf Symposium 1988, 29-54. BROWN, M. & EARLE, M. M. 1983. Cordierite-bearing schists and gneisses from Timor, Eastern Indonesia: P-T implications of metamorphism and tectonic implications. Journal of Metamorphic Geology, 1, 183-203. CARTER, D. J., AUDLEY-CHARLES,M. G. & BARBER,A. J. 1976. Stratigraphical analysis of island arccontinental margin collision in eastern Indonesia. Journal of the Geological Society, London, 132, 179-198. CHAMALAUN, E H. & GRADY, A. 1978. The tectonic development of Timor: a new model and its implications for petroleum exploration. Australian Petroleum Exploration Association Journal, 18, 102-108. CHARLTON, T. R. 1989. Stratigraphic correlation across an arc--continent collision zone: Timor and the Australian Northwest Shelf. Australian Journal of Earth Science, 36, 264-274. ~, BARBER, A. J. & BARKHAM, S. T. 1991. The structural evolution of the Timor collision complex, eastern Indonesia. Journal of Structural Geology, 13, 489-500. CLOWES, R. M., BRANDON,M. T., GREEN,A. G., YORATH, C. J., SUTHERLANDBROWN, A., KANASEWICH,E. R. & Spencer, C. 1987. LITHOPROBE - Southern Vancouver Island: Cenozoic subduction complex imaged by deep seismic reflections. Canadian Journal of Earth Science, 24, 31-51. DE METS, C., GORDON,R. G., ARGUS,D. E & STEIN, S. 1990. Current plate motions. Geophysical Journal International, 101, 425-478. FITCH, T. J. & HAMILTON,W. 1974. Reply to AudleyCharles & Milsom (1974). Journal of Geophysical Research, 79, 4982. FOWLER, C. M. R. 1990. The Solid Earth. Cambridge University Press. Fucns, K. 1969. On the properties of deep seismic reflectors. Zeitschriftfiir Geophysik, 35, 133-149. HAILE, N. S., BARBER, A. J. & CARTER, D. J. 1979. Mesozoic cherts on crystalline schists in Sulawesi and Timor. Journal of the Geological Society, London, 136, 65-70. HAMILTON,W. 1979. Tectonics of the Indonesian region. US Geological Survey Professional Paper, 1078. HARRIS, R. A. 1989. Processes of aUochthon emplace-
ment, with special reference to the Brooks Range Ophiolite, Alaska and Iimor, Indonesia. PhD thesis, University of London. 1992. Temporal distribution of strain in the active Banda Orogen: a reconciliation of rival hypotheses. Journal of SE Asian Earth Sciences, 6, 373-386. JOmqSTON, C. R. & BOWIN, C. 1981. Crustal reaction
59
resulting from the mid-Pliocene to Recent continent-island arc collision in the Timor region.
Australian Bureau of Mineral Resources Journal of Geology and Geophysics, 6, 223-243. JONGSMA,D., WOODSIDE,J. M., HUSON,W., SUPARKA,S. & KADARISMAN,D. 1989. Geophysics and tentative Late Cenozoic seismic stratigraphy of the Banda Arc-Australian continent collision zone along three transects. Netherlands Journal of Sea Research, 24, 205-229. KARIG, D. E., BARBER, A. J., CHARLTON, T. R., KLEMPERER,S. & HUSSON~,D. M. 1987. Nature and distribution of deformation across the Banda Arc-Australian collision zone at Timor. Geological Society of America Bulletin, 98, 18-32. KLEMPERER, S. L., HOBBS, R. W. & FREEMAN,B. 1990. Dating the source of lower crustal reflectivity using BIRPS deep seismic profiles across the Iapetus Suture. Tectonophysics, 173, 445-454. MACDAmEL, R. P. 1988. The geological evolution and hydrocarbon potential of the western Timor Sea region. Australian Petroleum Exploration Association Journal, 28, 270-284. MASSON, D. G., MILSOM, J., BARBER, A. J., SIKUMBANG, N. & DWlYANTO,B. 1991. Recent tectonics around the island of Timor, eastern Indonesia. Marine & Petroleum Geology, 8, 35-49. MATTHEWS, D. & THE BIRPS GROUP. 1990. Progress in BIRPS deep seismic reflection profiling around the British Isles. Tectonophysics, 173, 387-396. MCCAFFREY, R. 1988. Active tectonics of the eastern Sunda and Banda Arcs. Journal of Geophysical Research, 93, 15 163-15 182, 1989. Seismological constraints and speculations on Banda Arc tectonics. Netherlands Journal of Sea Research, 24, 141-152. MOONEY, W. D. & BROCHER, T. M. 1987. Coincident seismic reflection/refraction studies of the continental lithosphere: a global review.
Geophysical Journal of the Royal Astronomical Society, 89, 1-6. MOORE, J. C., DIEBOLD, J., FISHER, M. A., SAMPLe, J., BROCHER, T. ET AL. 1991. EDGE deep seismic reflection transect of the eastern Aleutian arc-trench layered lower crust reveals underplating and continental growth. Geology, 19, 420-424. NISHIMURA, S. & SUPARKA,S. 1986. Tectonic development of East Indonesia. Journal of SE Asian Earth Science, 1, 45-57. PATT~LLO, J. & NlCnOLLS, P. J. 1990. A tectonostratigraphic framework for the Vulcan graben, Timor Sea region. Australian Petroleum Exploration Association Journal, 30, 27-51. PIRANZOLLI, P. A., RADTKE, U., HANTORO, W. S., JOUANNIC, C., HOANG, C. T., CAUSSE,C. & BOREL BEST, M. 1991. Quaternary raised coral reef terraces on Sumba Island, Indonesia. Science, 252, 18341837. POWELL, D. E. 1982. The North West Australian continental margin. Philosophical Transactions of the Royal Society of London, A305, 45--62. PRICE, N. J. & Atn)LEY-CHARLES, M. G. 1983. Plate rupture by hydraulic fracture resulting in overthrusting. Nature, 306, 572-575.
60
A. N. RICHARDSON •
& -1987. Tectonic collision processes after plate rupture. Tectonophysics, 140, 121-129. RESTON, T. J. 1987. Spatial interference, reflection character and the structure of the lower crust under extension. Results from 2D seismic modelling. Annales Geophysicae, 5 (ser. B), 339-48. RICHARDSON, A. N. 1994. Lithospheric structure and dynamics of the Banda Arc, Eastern Indonesia. PhD thesis, University of London. SERPA, L. 8z VOOGD,B. DE 1987. Deep seismic reflection evidence for the role of extension in the evolution of continental crust. Geophysical Journal of the Royal Astronomical Society, 89, 55-60. SILVER, E. A., REED, D., MCCAFFREY,R. & JOYODIWIRYO, Y. 1983. Back arc thrusting in the eastern Sunda Arc, Indonesia: a consequence of arc-continent collision. Journal of Geophysical Research, 88, 7429-7448. SOPAHELUWAKAN, J. 1990. Ophiolite obduction in the
-
-
Mutis Complex, Timor, eastern Indonesia: an
D. J. BLUNDELL
example of inverted, isobaric, pressure metamorphism. PhD
medium-high
thesis, Free University, Amsterdam. VAN DER WERFF, W., KUSNIDA, D., PRASETYO, H. & WEERING, TJ. C. E. VAN 1994. Origin of the Sumba forearc basin. Marine and Petroleum Geology, 11, 363-374. YON DER BORCH, C. C. 1979. Continent-island arc collision in the Banda Arc. Tectonophysics, 54, 169-193. WANNER,J. 1913. Geologie von West Timor. Geologische Rundschau, 4, 136-150. WARNER, M. 1990. Basalts, water, or shear zones in the lower continental crust? Tectonophysics, 173, 163-174. WOODSIDE, J. M., JONGSMA,D., THOMMERET,M., STRANG VAN HEES, G. & PUNTODEWO. 1989. Gravity and magnetic field measurements in the eastern Banda Sea. Netherlands Journal of Sea Research, 24, 185-203.
Geophysical evidence for local indentor tectonics in the Banda arc east of Timor D A V I D B. S N Y D E R 1, J O H N M I L S O M 2, & H A R D I
PRASETYO 3
1BIRPS, Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK 2 Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK 3 Marine Geological Institute, J1. Dr. Junjunan 236, Bandung 40174, Indonesia Abstract: Two deep seismic reflection profiles and gravity measurements collected across the
Banda arc east of Timor help to characterize crustal and uppermost mantle structures in the region where arc-continent collision is thought to be furthest advanced. Reflectors beneath the Sahul Platform of the Australian shelf indicate geometries consistent with older extensional rift structures overprinted by more recent horizontal shortening. To the north, the negative Bouguer gravity feature associated with the southern parts of the accretionary complex is unusually broad and deep. Post-collisional sediments are thin where this feature is crossed by the seismic lines, implying that older sediments or the crust are anomalously thick. Still further north, the forearc basin is notably narrow near eastern Timor and contains few sediments, most undeformed. The backarc region to the north is remarkable for the presence of a N-S trending line of seamounts culminating in an active volcano, Gunung Api, which is situated 400 km above the WadatiBenioff zone. Reflection profiles across and along the seamount chain indicate underplating at Moho depths and a fault which appears to have acted as a conduit for basalt eruptions at the surface. Collectively, these observations imply anomalously thick and bouyant crust beneath the Banda arc east of Timor, and suggest two possible causes. Either a local promontory in the irregular boundary of the Australian craton was underthrust beneath the volcanic arc and forearc to 50-70 km depths or a Palaeozoic basin similar to the nearby Bonaparte Basin was underthrust and its former crustal structure inverted and thickened to form the buoyant crust. The new seismic reflection data help to locate the anomalously thick crust implied by gravity anomalies by defining the leaky fracture zone containing Gunung Api. It is inferred that the fracture propagates "ahead of the indenting wedge of underthrust, thickened Australian crust.
The Banda arc between Indonesia and Australia possesses some of the best examples of features associated with the collision of an arc and a continental margin (e.g. Hamilton 1979, 1988; McCaffrey 1988), and also displays distinctive segmentation along strike (Silver et al. 1983; Karig et al. 1987; Jongsma et al. 1989; Harris 1991). A pair of recent deep seismic reflection profiles were sited across one of these distinctive segments, immediately to the east of the island of Timor, in order to better characterize structural geometries throughout the lithosphere (Fig. 1). Focal mechanisms of large shallow earthquakes (McCaffrey 1988) and shallow seismic reflection profiles (Silver et al. 1983) and recent GPS measurements (Genrich et al. 1994) indicate that the Wetar thrust, a northward verging thrust on the northern edge of the inactive volcanic arc (Fig. 1), today absorbs most of the 75 mm a-1 (DeMets et al. 1990) relative convergence between Australia and the Banda Sea. In contrast, shallow seismic reflection profiles within the Timor Sea show reflector geometries in
Mesozoic and Cenozoic sediments that are typical of active subduction zone trenches worldwide (Karig et al. 1987). The Timor trough cannot simultaneously absorb plate convergence and transfer the motion of Australia to the volcanic arc. Loci of horizontal shortening have probably migrated northward across the 2 0 0 k m wide convergence zone during the past 2 Ma, with specific faults, active for short periods, acting as mechanisms of strain partitioning (e.g. McCaffrey 1992, 1996). A series of balanced cross-sections of the accretionary complex showed that m a x i m u m horizontal shortening, and hence the most evolved arc-normal convergence, has occurred in eastern Timor (Johnston & Bowin 1981; Harris 1991). Eastern Timor lies midway along the inferred convergence margin of the Australian continent with SE Asia (Fig. 1). Low 3He-4He ratios in volcanic rocks east of Flores were interpreted as due to the subduction and melting of continental crust at c. 150 km depths whereas to the west the
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 61-73.
61
62
D.B. SNYDERETAL
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Fig. 1. Regional location map showing the Archaean blocks on the Australian mainland (shading: ST = Sturt, K = Kimberley), principal landmasses, the 200 m bathymetric contour (short dash) that approximately defines the limit of the continental shelf, the 3 km contour (long dash) that approximates the limits of oceanic crust in the Indian ocean, the new seismic reflection lines (heavy solid lines), older refraction profiles (medium solid lines) with crustal thickness indicated in kilometres, and the 100, 300, and 500 km contours of the Wadati-Benioff zone (Cardwell & Isacks 1978). Large solid dots indicate the transition from subducted continental crust to subducted oceanic crust (to the west) within the subduction zone as determined from He isotope analysis of erupted lavas (Hilton et al. 1993). The vector labelled AUS---~SEA represents the local relative convergence vector between Australia and Eurasia (DeMets et al. 1990). C = Canning Offshore Basin, L = Londonderry High, A = Ashmore Platform, V = Vulcan sub-basin, P = Petrel sub-basin of the Bonaparte Basin, S = Sahul platform, M = Malita Graben, MS = Money Shoal Graben, B = Browse Basin, and WT = Wetar Thrust.
He source was inferred to be subducted oceanic (MORB) rocks (Hilton et al. 1992). The change in isotope ratios coincides with strike-slip faults at the surface (Breen et al. 1989) and crustal-scale fault planes defined by micro-earthquakes (McCaffrey et al. 1985) which are co-linear with the 3 k m bathymetric contour defining the N W margin o f the Australian continental shelf (Fig. 1). Thus defined, the western edge of the subducted continental margin lies c. 300 k m west of the study area. A similar distance along the strike of the volcanic arc
to the east of eastern Timor, the Wadati-Benioff zone (Fig. 1) curves sharply back onto itself to the N W (Cardwell & Isacks 1978). Here the gross Australia-SE Asia convergence vector (DeMets et al. 1990) is almost parallel to the volcanic arc, i m p l y i n g largely strike-slip d i s p l a c e m e n t s on structures parallel to the arc (see also McCaffrey 1996). If eastem Timor represents the most evolved segment of the Banda arc, local anomalously thickened crust or thick crust inherited from a
LOCAL INDENTOR TECTONICS IN THE BANDA ARC former promontory on the Australian shelf margin may be the cause. The current complex tectonic geometries associated with Irian Jaya prevent precise reconstruction of the former continental margin east of present-day Tanimbar, but New Guinea is thought to represent a large indentor wedge of Australian continental crust plowing into and bending the margin of SE Asia (Pigram et al. 1989). The east Timor region may contain a smaller promontory on the western margin of this larger indentor, but one that produced significant localized shortening across the Banda arc. Evidence for a promontory which has already subducted or underthrust the Banda arc must be sought using reflection and refraction profiles, gravity field anomalies and bathymetric observations. This paper describes and discusses this evidence. The variability in crustal structure observed in the unsubducted part of the Australian shelf south of Timor serves as the starting point, and it suggests similar complexity in subducted parts. Inversion of subducted sedimentary basin structures and thickening of sub-basin crust previously thinned during Devonian-Permian rifting provides an additional mechanism that can produce anomalously thick and buoyant crust east of Timor.
Structures of the northern australian shelf The Australian continental shelf south of Timor is covered by a relatively dense network of deep seismic reflection profiles plus gravity, magnetic and bathymetric measurements (e.g. O'Brien et al. 1993; AGSO North West Shelf Study Group 1994). Sedimentary basin structures are well understood, but crustal thickness is poorly constrained. The nearby Australian mainland contains two large Archaean cratons, the Kimberley and Sturt blocks (Fig. 1). Immediately offshore between these two blocks and continuing northwards to the Malita Graben lies the Bonaparte basin with its Petrel and Vulcan sub-basins. It, and the Canning Basin to the southwest, form composite Late Devonian to Early Carboniferous intra-cratonic rift basins with axes perpendicular to the Australian margin and associated with NE-SW extension (AGSO North West Shelf Study Group 1994). Mid-Carboniferous to Early Permian rifting that led to break-up of Gondwanaland and separation of 'Sibumasu' (Sengor 1987) from NW Australia and the opening of 'Neo-Tethys' (Veevers 1988), also initiated a series of generally NE-trending depocentres which constitute the Westralian Superbasin of Yeates et al. (1987). The superbasin's internal platforms and basins are orientated parallel to the continental margin and thus angularly super-
63
imposed on the older radial rift basins. In the study area these later features include the Sahul and Ashmore Platforms, which show little evidence of Mesozoic extension, and the Malita, Vulcan and Browse Basins (Fig. 1). The start of the break-up phase and the subsequent development of the continental margin was recorded as a major Lower-Middle Carboniferous unconformity. MidCarboniferous to Lower Permian basin fill may actually be a sag-phase sequence deposited following break-up and the 100-400% extension thought to have characterized this rift episode (Etheridge & O'Brien 1994). The Sahul Platform occupies c. 30 000 km 2 of the Australian shelf immediately south of eastern Timor. The Malita Graben separates it from offshore parts of the Sturt block of the Australian mainland (Fig. 1). Seismic reflection profiles and drill holes indicate that c. 6000 m of Carboniferous(?) and as much as 10 000 m of later sediments were deposited in the Malita Graben, confirmed by a core of 3300 m of post-Permian sediments on the Sahul Platform (AGSO North West Shelf Study Group 1994). The uniformity of sediment cover on the Sahul Platform suggests thermal subsidence rather than subsidence synchronous with rifting and implies little thinning of this crustal block. To the north, very young structures associated with convergence tectonics overprint older extensional structures (Figs 3 & 4). Although no evidence exists on seismic reflection sections to indicate thicknesses of post-Permian sediments as great as those observed in the Malita Graben, reflectors beneath the Jurassic and Permian levels indicate a possible older basin north of the Sahul Platform and currently underlying the Timor trough (Figs 3 & 4).
Crustal thickness estimates In contrast to the wealth of information available about the upper crustal and basin structure of the Australian continental shelf, its crustal thickness and that of the Banda arc itself are less well documented. Three refraction profiles provide general estimates for the thickness of Australian continental crust in this area, but these show variability and uncertainty commensurate with the basin structure described above (Jacobson et al. 1979). The three profiles were orientated parallel to the shelf margin, and two lie in the Timor trough (Fig. 1). Crustal thickness estimates range from 31-40 km within an area of 500 000 km 2, and profiles 150kin long show crustal thickness variations of 4-8 km (Bowin et al. 1980). A fourth profile within the Banda Sea, east of Wetar (Figs 1 & 2), indicated a typical oceanic crustal thickness of 11 km (Bowin et al. 1980).
64
D. B. SNYDER ET AL.
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By themselves, these thickness estimates are too sparse to characterize the crust, but crustal thicknesses can be extrapolated or inferred over the region by using reflection profiles. Penetrative reflectivity within extended continental margins generally disappears below the Moho (e.g. Matthews 1986). The intersecting reflection and refraction profiles in the study area (Fig. 1) demonstrate that this relationship applies here, at least
locally. By assuming that it applies for the entire northern Australian shelf, a gradual eastward thickening of the crust from about 30-40 km is inferred between eastern Timor and Tanimbar. This pattern of crustal thickening is not locally correlated with the known basin structure unless the Sahul Platform continues to the NE as argued by Charlton et al. (1991) using stratigraphic criteria. Unrecognized basins, similar to the Malita Graben
65
LOCAL INDENTOR TECTONICS IN THE BANDA ARC N
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Fig. 4. Migrated seismic section from the DAMAR profile in the vicinity of the Timor trough. Post-Permian shelf sediments are clearly seen near the sea floor on the inner trough wall, folded beneath the trough floor. Two northward-dipping reflectors indicated (arrows) in the uppermost basement beneath the shelf sediments are similar to the one described in the previous figure. Moho lies at 13-15 s.
66
D. B. SNYDER ET AL.
or Petrel sub-basin, lying north of the Sahul Platform and beneath the Timor trough may explain these relationships, but cannot readily account for the observed regional along-trench crustal thickness variations. The AGSO North West Shelf Study Group (1994) note that one important consequence of the concentration of CarboniferousPermian extension in the lower crust is that extensional structures appear only as subtle reactivation features within the upper crust and basin fill, often displaced or offset from areas of lower crustal thinning. Parts of the Australian shelf overthrust by the Timor accretionary prism may therefore contain additional NE-trending sag basins like the Malita Graben, or more probably, northern parts of the north-trending rifts associated with the Petrel sub-basin.
Gravity field observations and modelling The Australian continental shelf south of Timor is now covered by a relatively dense grid of marine
gravity measurements. To the north, coverage is less complete but recent surveys onshore have mapped the gravity field on nearly all the outer arc islands between Timor and Tanimbar (Richardson 1993). The gravity field of the Banda Sea region is dominated by a steep gradient which forms a continuous arcuate feature running from northern Timor past Tanimbar and the Kai Islands to southern Seram (Bowin et al. 1980). Typically, values of Bouguer gravity increase from the slightly negative, as in the south coast region of Timor, to greater than +150 mGal as at points on the north coast of Timor (Fig. 5). Attempts to model this gradient used profiles crossing the arc at various locations. Of these, the model of McBride & Karig (1987) incorporates measurements from an anomalous region of western Timor where contour trends are locally transverse to, rather than parallel to, the strike of the arc, and the model of Milsom & AudleyCharles (1985) is unsatisfactory because of the lack of seismic control. The model of Jongsma et al. (1989; see also Richardson 1993) (Fig. 6a) was
Fig. 5. Bouguer gravity map of the study area. Reduction density is 2.2 Mg m-3. Contour interval is 10 mGal. Solid lines indicate the (roughly N-S) deep reflection profiles and (roughly E-W) modelled transects. Note the particularly negative values associated with the accretionary complex between the seismic profiles south of the island of Moa and the north-south gravity 'notch' over the shelf immediately to the south. Stippled borders are located near inflection points in east-west transects of the gravity field and define the possible areal extent of an anomalous low-density body in the crust.
LOCAL INDENTOR TECTONICS IN THE BANDA ARC
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67
based on gravity, magnetic and seismic data along a profile which nearly coincides with the TIMOR line (Fig. 2). The model shows thick continental crust in the downgoing plate almost as far north as the north coast of Timor, with low Bouguer anomalies in the south produced by a thick, lowdensity accretionary complex and the high values in the north due to the presence of what was originally sub-oceanic crust and mantle beneath the forearc basin and the volcanic arc. Subduction, rather than limited overthrusting, is hypothesized at the Wetar thrust and a mantle block is therefore shown as virtually isolated by the action of the opposing subduction zones (Fig. 6a). Of the more localized features of the gravity field, one of the most remarkable is the negative free-air and Bouguer anomaly feature centred directly south of the outer arc island of Moa (Figs 2 & 5). This feature was first indicated by measurements on a few widely spaced lines of marine survey, but was later substantiated by free-air gravity anomalies derived from satellite altimetry and measurements on Moa and the adjacent islands. The lowest Bouguer values occur 40 km to the south of the islands and the free-air minimum lies further south in deeper water. Bouguer anomalies, used in preference to free-air anomalies in Fig. 5, are less influenced by bathymetric variations and therefore better at resolving deep structures. On this map the Moa feature appears nearly elliptical, 40 km across from north to south (measured between half amplitude points) and 120 km from east to west, with an amplitude of c. 70 mGal. It is superimposed on a more elongated, lower amplitude feature that just reaches into the southern part of eastern Timor. Directly south of this 'Moa anomaly', the Bouguer anomalies are 30-50 mGal less than on nearby parts of the Australian shelf margin, and NNW of the feature Bouguer anomalies are 30-50 mGal less than in other parts of the forearc basin (Fig. 5). Collectively, these form a NNW trending band of lower gravity. The two deep seismic profiles cross the long axis of the 'Moa anomaly' on either side of its centre, at about the location of the two half-amplitude points. The profiles also lie near the base of local east-west gravity gradients on the Australian shelf (Fig. 5). Other than the discordantly dipping reflections indicated by arrows in Figs 3 & 4, neither seismic section appears to show basins or shelf features significantly different from those imaged on numerous unpublished seismic profile sections further east, where the regional gravity minimum has lower amplitude. These discordant reflectors have geometries consistent with out-of-plane reflections from dipping reflector surfaces (faults?) striking nearly parallel to the seismic profiles and offset by 10-15 km.
68
D . B . SNYDER E T A L
Two gravity profiles orientated along structural strike (Figs 5 & 6b) illustrate the similarity in the along-strike gravity trends along the prism and shelf by eliminating the more dominant northsouth gravity gradients associated with the subduction zone. One simple model indicates that a combination of anomalous, low densities at the base of the crust and at the top of basement can successfully match observed Bouguer values (Fig. 6b). Three sources of low density material are considered here. The first possibility, of a deep post-orogenic slope basin of the type known to occur south of Timor (e.g. Karig et al. 1987), is not supported by the seismic stratigraphy inferred from the seismic profiles (Fig. 3). Similarly, no direct evidence exists for a significantly thicker accretionary complex east of Timor, nor for anomalously lower density material within it, although Woodside et al. (1989) showed it as a 50 km thick tapering wedge. Thickening of the basement of the downgoing crustal layer by thrusts does seem plausible, based on duplex thrusts suggested by lower crustal reflector geometries (Figs 3 & 4) and on the variations in crustal thickness reported from the NW shelf of Australia (Bowin et al. 1980; O'Brien et al. 1993). Because thickening would effectively add low density material at Moho depth, it would have to be very local to produce a gravity field anomaly as localized as that near Moa. If thickening of the downgoing crustal layer is a valid explanation, syn-orogenic thrust duplexing seems more probable than isostatically uncompensated variations in crustal thickness that existed prior to collision. Elsewhere on the Australian shelf such variations are compensated. Thickening due to thrust duplexing could be accomplished as an independent tectonic mechanism or by restoration of crust previously thinned during the development of Palaeozoic sag or rift basins to its normal thickness whilst preserving or thickening the overlying basin strata. The resulting bi-level thickening is illustrated by the density model shown in Fig. 6b. Inherited crustal structure localizing and intensifying the thickening effect of syn-orogenic thrusting seems the most probable mechanism to produce the anomalously low Bouguer anomalies associated with the Australian shelf margin in this region.
Forearc crustal structures The forearc within the study area is unusual in the narrowness of the forearc basin and in the presence of the island of Kisar. Between the two deep seismic profiles the forearc basin is typically only 15-20 km wide and contains only tens of metres of sediment (Fig. 7). The basin appears to not have
acted as a depocentre, suggesting that it has been structurally higher than other parts of the forearc, such as the Savu Sea, enabling along-arc ocean currents to keep the sea floor relatively free of sediments during the past few million years. Recent mapping on the island of Kisar has revealed, from south to north, low-grade metamorphic rocks such as greenstones, amphibolites, and bands of thick mylonitized quartzites interlayered with greenstones and high-grade pelites (Dropkin et al. 1993; Richardson 1993). Some of these rocks bear a strong similarity to the Aileu (Grady & Berry 1977) or Lolotoi/Mutis Complex Formations on Timor and Moa (interpretations of Richardson 1993 and Dropkin et al. 1993 respectively), and are interpreted as dismembered ophiolites thrust onto thinned Australian shelf or oceanic crust or the volcanic arc. Kisar thus is related to the accretionary complex rather than to the volcanic arc, which emphasizes the unusual narrowness of the forearc basin in this segment of the Banda arc. Southward dipping reflections observed on the deep reflection sections (Fig. 7) may represent backthrusts that have translated the forearc (accretionary prism) northward over the forearc basin and thus narrowed it. The entire forearc block from Kisar to Sermata may represent a large rectangular klippe (Fig. 2).
Backarc features: a seamount chain including Gunung Api Anomalous features also occur north of this segment of the volcanic arc. Although the magnetic patterns are very poorly constrained, Lapouille et al. ( 1 9 8 5 ) suggested transform offsets of c. 50 km of anomalies identified as M7-M10 along a NW-SE orientated line passing through Gunung Api. More recent work (e.g. Rehault, pers. comm. 1995) suggests that the anomalies have been incorrectly identified, but the evidence for offset remains valid. The new seismic reflection profiling has revealed a number of previously undocumented sea-floor volcanoes (seamounts) along the TIMOR line and coincident with this inferred transform fault. The northern part of one of these seamounts is shown in Fig. 8. The API cross-line (Fig. 2) showed that between the seamounts lies a low ridge of extruded basalt separating two levels of the Banda Sea ocean floor (Fig. 9). Sediment thicknesses appear to be similar on both sides of the ridge, which therefore coincides with a down-tothe-east offset of 350-400 m in the sea-floor basement. Arcuate reflectors within the ridge suggest either folds and thrusts or primary extrusive structures among the basalt flows. Gunung Api, the seamounts and the inferred ridge basalt flows
LOCAL INDENTOR TECTONICS IN THE BANDA ARC
69
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Fig. 7. Part of the DAMAR seismic reflection section within the forearc basin. This section was migrated but has not been depth converted. Approximately 200 m of relatively undeformed sediment lie within the 10 km wide forearc basin at the centre of the section. Greater sediment accumulations can be observed (arrows) in slope basins both north and south of the forearc basin. Curved arrows indicate possible backthrust.
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Fig. 8. Part of the TIMOR seismic reflection section just south of Gunung Api. This section was migrated and then depth converted using the velocities shown at the right in km s-1. In this depth section the Moho appears as a bright reflection (arrow) at a depth of 10 km where this line crosses the API line. If refraction velocity function M12, of Bowin et al. (1980), shown for comparison at the left, was used instead for the depth conversions, the bright reflection occurs at 14-15 km depths, and no obvious feature is associated with the Moho. Numerous convex-upward reflections (M) are migration noise due to the incomplete removal of the sea-floor multiple before migration and should be ignored.
70
D . B . SNYDER E T AL. TIMOR
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7~--=--""- ' :!:-~'"5::";"".7.."-:~::-.~',,.~-.~::':.--=-::?:.:: -:~=::..:.:i::,c-;~-".:,.:'£7:~%:). ;:",~'~'.,'.~:~:;-:L:-~::~;~-.":.',.-.7~'Z-~i~-t~:'-.::~ •, ,', "--Y'.:"~-":.." : 2 . i ::~:5-.~:S.'%~!!.1 km deep. They proposed that this is due to the wedge thrust front being laterally discontinuous and that in some areas the trough fill may have been accreted into the toe of the wedge. Both the section in Fig. 2 and the adjacent TIMOR line show similar features of overthrusting of the trough sediments by the wedge front. From the reprocessing beneath the frontal part of the accretionary wedge, a pre-existing thickness of sediment in the trough does not appear to have been overridden by the wedge (as seen on the TIMOR line), unless it has been fully accreted to the wedge by its recent advancement. The top of the downgoing Australian plate lies at a depth of
81
3.8 km on the TIMOR line beneath a trough sediment fill of c. 630 m, and at a depth of c. 3.2 km at seabed on the DAMAR line at shotpoint 780. Therefore, the greater seabed depth of the TIMOR line may have produced a preferential site for trough sedimentation in comparison with the lack of sediment fill present on the DAMAR section. It may be possible that the inversion of the normal fault beneath the trough axis on the DAMAR line at shotpoint 760 has also contributed to the reduced bathymetry in the trough, although this is speculative.
The accretionary wedge The accretionary wedge is characterized by a change in the surface topography as the Timor collision zone overrides the Australian margin. The sea floor above the wedge shows a relatively smooth profile, with a lack of penetration of thrusts at the wedge front and only minor penetration further back in the wedge. This suggests that the frontal part of the wedge is inactive, with minor out-of-sequence thicknening further back producing internal thickening. This may be due either to the cessation of subduction at the Timor trough in this region, or that the frontal part of the wedge is inactive as internal thickening attempts to return the wedge to a critical state and allow propagation of the wedge front. The internal structure of the wedge is seismically complex. Picking structural and lithological reflectors is obviously difficult in such a complex zone which is likely to contain a high amount of diffracted energy which may not have been migrated back to a source reflector. The use of migration velocity analysis during this study has, however, led to the conclusion that a number of dipping events within the wedge are real reflectors and not diffraction effects. These reflectors are shallow in dip at the toe of the wedge, increasing in dip southwards and are most likely to be thrust faults developed during forward propagation of the thrust front, developing a back-steepening of the thrusts. These show typical thrust geometries with rollover anticlines in the hanging wall above a lowangled detachment. Correlation of reflectors from the Australian margin is difficult in such a highly deformed zone. A strong seismic reflector is defined by a high acoustic impedance contrast at the contact between two lithological units. The lithological units occupying the intervals between the strong reflectors A-D are considered to be homogeneous throughout as indicated by their relatively transparent internal seismic structure, although some internal layering is apparent. These units are
82
B.D. HUGHES ET AL.
thought to be similar to those observed from the Troubadour 1 well located nearby, and consist of." a Recent-Maastrichtian sequence of calcarenites and marls from seabed to 1.55 km depth; a 550 m thick claystone unit (Turonian-Cenomanian); Jurassic sandstones and claystones (2.1-2.8km depth); and Triassic limestones and claystonesiltstone-sandstone sequence (2.8-3.3 km depth). Therefore, during thrusting, imbrication which structurally juxtaposed two of the main lithologies could be expected to produce a reflector with acoustic impedance and phase similar to a stratigraphic contact. The strong convex-upward nature of many reflectors is thought to correlate to hanging wall geometries within thrust sheets rather than being from faults but it is unclear whether the lithological contacts picked correlate with the original stratigraphic reflectors on the Australian margin or whether these are due to lithological juxtapositions created during faulting.
Imaging of structures beneath the accretionary wedge Previous studies across the Timor trough have not generally imaged the structure of the Australian shelf beneath the accretionary wedge. Improved processing during this study has allowed some structures to be imaged beneath the frontal part of the wedge. The decollement itself is not strongly imaged, although the truncation of stratal reflections within the uniform shelf sediments allows an interpretation of its location. Beneath the wedge in the area of shotpoint 820 there appears to be an upwarping of reflectors C and D. This could be either a processing anomaly or a geological feature. Similar anomalous features are well known during processing and are associated with inaccurate velocity determination of the overlying structure, particularly if this is of a high velocity, resulting in a velocity pull-up. However, the feature at shotpoint 820 appears to be of high amplitude and short wavelength, which is inconsistent with any possible velocity anomaly within the overlying wedge. Therefore, this feature is tentatively interpreted as geological. The upper part appears to be truncated by the wedge decollement with no apparent continuation into the wedge. Thus, this structure must pre-date wedge imbrication of this part of the shelf. The shape of the feature on reflector D is similar to that beneath the zones of structural complexity seen elsewhere on the section (shotpoint 635 and shotpoint 747) and may be associated with similar fault structures, although resolved less clearly due to the overlying wedge.
Conclusions Following the design of a reprocessing sequence, an improvement of SNR and resolution of the DAMAR line has been achieved, with the improvement in data quality allowing an increased confidence in the interpretation. Application of similar, data-specific, processing should significantly improve other datasets from areas of complex geological structure. Recent geodetic measurements reported by McCaffrey (1995) have suggested that subduction at the Timor trough has almost stopped. However, observations by Charlton (1988) that the wedge thrust front in the Timor trough to the south of Timor has not advanced significantly in the last 450 000 years have been interpreted as indicative of episodic propagation of the thrust front during continuing subduction by Masson et al. (1991). From a detailed examination of the frontal part of the accretionary wedge on the DAMAR line, it is difficult to resolve this debate. The lack of recent sediment deposition to the south of the thrust front does not allow any estimation of timing of the most recent advance of the wedge and from Charlton's observations both scenarios are possible. A further indication of possible recent thrust activity within the wedge is the identification of active faults either at the thrust front or within the wedge, affecting the more recent sediments which overlie the wedge. Such faults will produce sediment thickening and sharp topographic variations on the wedge top. The reprocessing of the DAMAR line has allowed an improved interpretation of the internal structure of the accretionary wedge which should allow similar structures to be imaged. However, although some faults appear to outcrop at the seabed with the development of minor breaks in topography (e.g. at shotpoints 785, 810, and 922) and may indicate minor internal thickening, the thrust front and much of the internal part of the wedge appear to be recently inactive with continuous, undeformed sediment drape across the outer slope and the wedge front. The lack of a recent significant sediment thickness in the trough axis is attributed to a lack of deposition, rather than due to recent wedge movement imbricating any pre-existing trough fill. From the improved processing of the DAMAR line it is possible to identify the large-scale internal deformation mechanism within the wedge. This is dominated by high-angle thrusts which have imbricated sediments from the upper part of the subducting Australian passive margin. This indicates that the deformation mechanism within the frontal part of the accretionary wedge consists of the imbrication of coherent blocks rather than by a shearing of the subducting Australian margin sediments into an incoherent melange.
SEISMIC REFLECTION DATA FROM THE TIMOR TROUGH
83
References BAXTER, K. 1993. Quantitative modelling of continent
collision: Application to the Timor region, Eastern Indonesia. PhD thesis, University of Liverpool. BowrN, C., PURDY, G. M., JOHNSTON, C., SHOR, G., LAWVER, L., HARTONO,H. M. S. & JEZEK, P. 1980. Arc-continent collision in the Banda Sea region. AAPG Bulletin, 64, 868-915. CHARLTON, T. R. 1988. Tectonic accretion and erosion in steady-state trenches. Tectonophysics, 149, 233-243. HALE, D. 1984. Dip moveout by Fourier transform. Geophysics, 49, 741-757. HUGHES, B. D. 1994. Reprocessing, modelling, and
interpretation of complex seismic reflection data from accretionary wedges: Application to the Timor Trough, Indonesia. MSc thesis, University of Leeds. KARIG, D. E., BARBER, A. J., CHARLTON, T. R., KLEMPERER, S. & HUSSONG,D. M. 1987. Nature and distribution of deformation across the Banda Arc-
Australian collision zone at Timor. Bulletin of the Geological Society of America, 98, 18-32. MASSON, D. G., MILSOM, J., BARBER, A. J., SIKUMBANG, N. & DWIYAYrO, B. 1991. Recent tectonics around the island of Timor, eastern Indonesia. Marine and Petroleum Geology, 8, 35-49. MCCAFFREY,R. 1996. Slip partitioning at convergent plate boundaries of SE Asia. This volume. PIGRAM, C. J. & PANGGABEAN,H. 1984. Rifting of the northern margin of the Australian continent and the origin of some of the microcontinents in eastern Indonesia. Tectonophysics, 107, 331-353. SNYDER, D., PRASETYO, H., BLUNDELL, D. J., PIGRAM, C. J., BARBER, A. J., RICHARDSON, A. & TJOKOSAPROETRO, S. 1996. Style of crustal deformation across the Banda Arc continent-arc collision zone as observed on deep seismic reflection profiles. Tectonics, in press. YtLMAZ, O. 1987. Seismic data processing. Investigations in Geophysics, No. 2. Society of Exploration Geophysicists.
Extension,
collision and curvature
in the eastern Banda
arc
JOHN MILSOM, STEVE KAYE & SARDJONO
Research School of Geological and Geophysical Sciences, Birkbeck College and University College London, Gower Street, London WC1E 6BT, UK A b s t r a c t : The Kai Islands occupy the region of maximum curvature in the east of the Banda arc, where the Aru trough has been regarded as the surface trace of past subduction and present arccontinent collision. Eocene to Pleistocene sediments on Kai Besar, the easternmost island, have not been deeply buried or imbricated but have experienced large-scale extensional faulting. The associated Bouguer gravity high of more than +200 mGal requires upfaulting of the accretionary complex, the attenuated Australian continental crust on which it rests and the underlying mantle at the western side of the Aru trough. Seismic reflection surveys show the deformation front within the Aru trough SE of the Kai Islands but entirely to its west further north. Instead of continuing NNE to an offset near the coast of New Guinea, the collision trace passes through the narrow and shallow strait between Kai Besar and the other islands, and thus mimics the relatively smooth curve of the Bouguer gravity contours, rather than the discontinuities of the bathymetric troughs. The continuity in deep and shallow structures is strong evidence for the existence of the outer arc as a single geological unit prior to the present phase of arccontinent collision.
The B a n d a arc, in eastern Indonesia, is the site o f the collision b e t w e e n the Australian continental margin, which is m o v i n g north with the IndoAustralian plate, and the B a n d a Sea which lies on the SE Asia plate (Fig. 1). The collision has been
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micro-continental-arc and continental-arc collision, foreland fold and thrust belt formation, obduction and the uplift o f mountain belts m a y be f o u n d currently in progress in the I n d o n e s i a n archipelago. Our present understanding o f the structure o f the N e o g e n e - R e c e n t orogenic belts o f Indonesia is illustrated in a series o f cross-sections in Fig. 9. These present-day e x a m p l e s o f orogenic processes p r o v i d e models for the interpretation o f older orogenic belts such as the A l p i n e - H i m a l a y a n system, the H e r c y n i d e s and the Caledonides.
The authors are indebted to the Director of the Geological Research and Development Centre, Bandung, for permission to publish this paper. H. Faeni and Sudarto of GRDC, and Steve Moss, Simon Baker and Andy McCarthy of the University of London assisted in the preparation of the illustrations. Robert Hall and Derek Blundell invited TOS to attend the meeting on the 'Tectonic Evolution of Southeast Asia' organized at the Geological Society, and arranged sponsorship. Robert Hall, Tim Charlton and Steve Moss made many suggestions for the improvement of the paper.
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Palaeomagnetism of the Sibumasu and Indochina blocks: implications for the extrusion tectonic model BRYAN RICHTER
& MICHAEL
FULLER
Department of Geological Sciences, University of California, Santa Barbara CA 93106, USA. email:
[email protected] Abstract: The Jurassic-Cretaceous Kalaw redbeds of Myanmar yield a prefolding Late
Cretaceous to Early Palaeogene magnetization which records c. 25-30 ° of clockwise (CW) rotation relative to the South China block. This corresponds to 10-15 ° CW relative to the Lower Cretaceous Khorat Plateau VGP. The data also show 5 ° of northward transport relative to the 100 Ma South China VGP or 12° relative to the Khorat Lower Cretaceous VGP. Similar CW rotations are measured in remagnetized Palaeozoic carbonates in Peninsular Thailand and Langkawi Island, Malaysia. These block motions most likely took place between the Late Cretaceous and the Late Oligocene. These and other recently published data have several implications for the extrusion tectonic model: (i) Sundaland has only rotated 25-30 ° CW relative to South China during the Tertiary; (ii) southeastward translation is only 300-500 kin; and (iii) Sundaland is composed of smaller sub-blocks, some of which have moved northward. This is interpreted to indicate that deformation of the Sibumasu block is dominated by the oblique subduction of the Indian Ocean Plate while deformation of the Indochina block is dominated by extrusion, in turn driven by convergence between the Indian Craton and Eurasia.
SE Asia has been located at the intersection of the Eurasian, Indian, Australian, Pacific and Philippine Sea plates throughout much of the Cenozoic (e.g. Hamilton 1979). These plate interactions culminated in the Palaeogene to present collision of India with Tibet and the Neogene to present collision of Australia with Sulawesi, Java and Borneo. The boundary conditions set up by the relative motions between these plates have had a profound impact upon the Cenozoic motions of the terranes and micro-plates which comprise SE Asia. Convergence between the Indo-Australia and Eurasian plates continues today and ultimately the terranes of SE Asia will form a broad orogenic belt between these two plates. It has become clear over the last decade that SE Asia has moved with respect to Eurasia during the Cenozoic. Field data which document these motions have been difficult to obtain and often more difficult to interpret. Thus, much of what is postulated about Cenozoic motions is derived from laboratory experiments (e.g. Tapponnier et al. 1986) suggesting that much of the deformation associated with the India-Eurasia collision has been accommodated by rigid plate-wide clockwise (CW) rotation and southeastward translation of the Indochina and Sibumasu blocks out of the collision zone. The available Mesozoic and Cenozoic palaeomagnetic data from the Indochina and Sibumasu blocks qualitatively support the predictions of these
laboratory models. However, ambiguities regarding the tectonic significance of these palaeomagnetic data remain. Many of the tectonic conclusions are based upon several key sampling localities or well preserved chronostratigraphic successions and very little is known about the intervening regions or time periods. With regard to the actual palaeomagnetic analyses, it has proved difficult to discriminate between primary and secondary magnetizations. A review of the literature on the Khorat Plateau, for example, shows the same basic dataset interpreted first as a primary magnetization, reinterpreted as a secondary magnetization, then finally reinterpreted as a primary magnetization (Achache et al. 1983; Chen & Courtillot 1989; Yang & Besse 1993). It is imperative, therefore, to reevaluate existing datasets as new data become available. Despite these difficulties, palaeomagnetism remains one of the few direct records of ancient plate motions available to us. Towards this end we review the available palaeomagnetic data from Sundaland and we present new palaeomagnetic data collected from the Kalaw Redbeds of the Shan Plateau of Myanmar and from Palaeozoic limestones located in Central and Peninsular Thailand. Moreover, we evaluate the tectonic significance of these palaeomagnetic directions. For example, are they caused by local or plate-wide rotations? Are they controlled by structural deformation? Finally, we conclude by evaluating existing tectonic models and propose modifications which make the models
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 203-224.
203
204
B. RICHTER • M. FULLER
more consistent with the measured rotations and translations.
Regional tectonic framework The data presented in this paper have been collected in Thailand and Myanmar from the Tertiary geological 'plate' known as Sundaland (Fig. 1). Sundaland is composed of several Palaeozoic and Lower Mesozoic terranes (Fig. 2). The origin of these blocks and their suturing ages have, among others, been studied by Metcalfe (1988, 1990). Hutchison (1989) provides an excellent summary of additional studies. The new data presented in this study were collected from the elongate N-S trending Sibumasu block (Shan-Thai). In our discussion we compare these data to published data from the Indochina block (Fig. 2). Sibumasu block
The Sibumasu block consists of portions of Myanmar, Thailand, northwest Malaysia, and Sumatra (Fig. 2). It is bounded to the west by the dextral Sagaing Fault, the Andaman Sea and the dextral Sumatran Fault Zone; to the east by the Bentong Raub line, the extension of the BentongRaub line into the Gulf of Thailand, and finally by the Nan-Uttaradit Ophiolite line in northern
30°N
20°N
10 ° N
Thailand; and to the north by the Red River Fault and the Himalayan Syntaxis (Metcalfe 1988). It may have originally been continuous with the Qiangtang or Lhasa blocks but intense deformation makes it difficult to confirm this. There is some agreement that the long axis of the Sibumasu block was originally orientated approximately E-W and was attached to the northern margin of Gondwanaland during the Early Palaeozoic (e.g. Hutchison 1989). It rifted from Gondwanaland in the Middle or Late Palaeozoic and began travelling northward. Collision timing along the Bentong-Raub line in Peninsular Malaysia is widely thought to be marked by the Late Triassic S-type granites of the Main Range Province (e.g. Cobbing et al. 1986). In contrast, Helmcke (1986), Barr & Macdonald (1991), and Barr et al. (1990) have shown that the proposed suture zones in Northern Thailand (Nan River Belt, Phetchabun Fold belt) have a more complicated collisional history which, although it includes the Late Triassic deformation seen elsewhere, began in the Upper or even Middle Palaeozoic. These contrasting collisional ages can be explained in several ways. (a) The Nan Suture may not correlate directly with the Bentong Raub suture and the Sibumasu block may have been broken into northern and southern halves during the Late Palaeozoic; (b) there may have been post-collision lateral translations along these suture zones which have obscured the original collisional relationships; (c) the northern part of the Sibumasu block may have collided with northern Indochina in the Late Palaeozoic but the collision did not go to completion, forming a partially amalgamated superterrane with a small trapped oceanic basin which finally closed in the Late Triassic; or (d) one or more marginal 'back-arc' basins may have formed and then been consumed prior to the final collisional episode in the Late Triassic. The last two hypotheses appear the most likely. Fortunately, almost all researchers agree that the major blocks which comprise the core of SE Asia had assembled and become sutured to South China (Eurasia) by the Late Triassic-Early Jurassic (Hutchison 1989 and references within). Indochina block
90 *
100 °
110 °
120 °
Fig. 1. Regional geography of Southeast Asia. Light circles show location of sites discussed in this study and heavy circles indicate the sites of Yang & Besse (1993).
The Indochina block is bounded to the north by the Red River Fault (Song-Ma and Song-Da suture zone) and to the east by the South China Sea marginal basin (Fig. 2). The southern boundary is poorly documented and it is not clear if the Indochina block has always been continuous with the Borneo block, which has similar Mesozoic igneous sequences, or if there is an E-W trending Late Palaeozoic suture zone near the Natuna and
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INDOCHINA BLOCK
Diamictite ~ Carbonate~ Marine ~Continental ]~E-] Chert ~ Congl. [-~T] Volcani- ~ Evaporite rTFTT]Stratigraphic (GlacioShale, Redbeds clastics Break Marine?) Mudstone (S/S +Cong.) Fig. 2. Tectonostratigraphicterrane summaryfor SoutheastAsia showingPalaeozoicterrane boundaries and regional stratigraphiccorrelations.Tertiarystrata in peninsular Thailand and the Shan Plateau of Myanmar are only locallypreserved in small Oligo-Mioceneintermontanebasins. Diagrammodified from Metcalfe(1988).
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SIBUMASU BLOCK
O
206
B. RICHTER & M. FULLER
Mekong basins. The western boundary has traditionally been interpreted to be the NanUttaradit suture zone. This suture is thought to extend southward, beneath the Central Thai Basin, into the Gulf of Thailand and ultimately to the Late Triassic Bentong-Raub line in Malaysia (Metcalfe 1988; Hutchison 1989). In contrast, Barr & Macdonald (1991) conclude that the Nan suture actually marks a Permian suture between the Indochina block and the recently identified Sukhothai terrane. They place the Late Triassic suture zone farther west, along the eastern margin of the Northern Thai granite province. Unfortunately, the Late Palaeogene to Neogene Central Thai basin and the Pattani trough almost completely cover the suture zone from 16°N to 7°N and it is not yet possible to correlate definitively sutures along the length of the Indochina and Sibumasu blocks. Precambrian basement is exposed in the Kontum Massif of Vietnam and presumably forms the basement of the entire Indochina block. It may have originated on Gondwanaland, possibly along the eastern margin of Australia, but must have rifted away by the Early Carboniferous (Metcalfe 1988). Moderately to strongly deformed Palaeozoic sediments are widely distributed but the block is dominated by thick, relatively undeformed Mesozoic continental sediments of the postIndosinian Khorat Basin. Most palaeomagnetic studies in the Khorat Plateau are concentrated in this Mesozoic section.
Previous palaeomagnetic studies Bunopas (1982) examined the palaeomagnetism of a wide variety of rock types in Palaeozoic and Mesozoic strata throughout both the Indochina and Sibumasu terranes using orientated hand samples. His reconnaissance study formed the foundation for additional studies on Neogene basalts in Northern and Central Thailand (Barr et al. 1976; Barr & MacDonald 1979; McCabe et al. 1988); upon Upper Palaeogene to Neogene intermontane basins throughout Thailand (Richter et al. 1993); and upon Upper Palaeozoic and Mesozoic continental strata from the Khorat Plateau (Barr et al. 1978; Achache et al. 1983; Achache & Courtillot 1985; Maranate & Vella 1986; Chen & Courtillot 1989; Yang & Besse 1993). Upper Palaeozoic and Mesozoic o f the Khorat Plateau As noted above, the majority of palaeomagnetic data has been collected from Upper Palaeozoic and Mesozoic sediments of the Khorat Plateau. Many
of these early studies failed to recognize the potential significance of secondary magnetizations and their data are only presented in tectonically corrected coordinates. Yang & Besse (1993) further note that many of the demagnetization procedures may not have been sufficient for discriminating between primary and secondary magnetizations. Despite these concerns, the consistency between these studies is remarkable. Mean values of c. 3040 ° of CW deflection with positive inclinations of 300-40 ° are found in almost all of the Khorat studies. Furthermore, structural dips are low and corrected and uncorrected directions commonly overlap within their respective 0~95 error estimates. The primary concern with these earlier data is distinguishing between primary and secondary magnetizations and then assigning an age to that magnetization. Yang & Besse (1993) present new data from Upper Permian limestones, the Upper Triassic Huai Hin Lat formation, the Lower Jurassic Nam-Phong formation, the Upper Jurassic Sao Khua Formation, and the Lower Cretaceous Khok Kruat Formation. These data are of high enough resolution to allow an approximate chronology of magnetizations and rotations to be reconstructed. Their results are reviewed below and summarized in Fig. 3 and Table 2. The Permian Limestone samples yield a high coercivity CW deflected component which fails a fold test at the 95% level (Fig. 3). Synfolding analysis indicates that kappa is at a maximum at 30% unfolding and thus the magnetization is best interpreted as a synfolding remagnetization. At this locality, Upper Triassic continental sediments unconformably overlie steeply dipping, strongly deformed Permian units. Thus, the Late Triassic Indosinian orogeny is a strong candidate for the folding and initial remagnetization. The Upper Triassic Huai Hin Lat formation also gives a CW deflected direction. Based upon the interpretation of a positive fold test in the overlying Lower Jurassic Nam-Phong Fm they conclude that this is a primary magnetization and they present a tectonically corrected Late Triassic VGP at 52.1 °N, 169.8°E (o~95=7.3) (Table 2). This VGP is indistinguishable from the remagnetized Permian VGP. The Lower Jurassic Nam-Phong formation also yields a CW deflected magnetization. This is found in both positive and reversed polarities and passes a fold test at the 95% level (Table 2).The tectonically corrected VGP for this unit is located at 54.4°N, 175.6°E ((x95 =4.9) and is statistically indistinguishable from the Permian and Late Triassic VGPs. However, Yang & Besse (1993) caution that the folding may have occurred anytime between the mid-late Yanshanian orogeny and the Late Tertiary.
PALAEOMAGNETISM OF THE SIBUMASU BLOCK N
N
90 °
Ju and KI Redbeds Khorat Plateau, Thailand
207
The Aptian-Albian Khok Kruat Formation yields medium and high coercivity components which are quite close to each other (I, D = 37.2 °, 26.0 ° and 35.3 ° , 29.8 ° respectively). Bedding corrections are all less than 12° and the fold test is inconclusive. Despite this, Yang & Besse (1993) conclude that this is a primary Lower Cretaceous VGP. As above, we urge caution in assuming at Early Cretaceous age. The small ~95' the inconclusive fold test, and the lack of reversed polarities leave open the possibility of a younger remagnetization event.
Mesozoic of peninsular Thailand and peninsular Malaysia
N
90°
+ In Situ ++ + + +++++1-90 ° Permian Carbonates Khorat Plateau, Thailand
Fig. 3. Lower hemisphere equal area projections of site mean directions measured in the Khorat Plateau by Yang & Besse (1993). Solid symbols denote positive polarities while open symbols denote negative polarities. Ellipses are the 95% confidence interval about the mean (alpha95) but these have been omitted from the Ju + K1 plot for clarity. See Table 1 for exact values and text for discussion.
We do not follow them in assigning an Early Jurassic age to this pole because the age of the folding is so poorly understood. The interpretation of a positive fold test is reasonable, but without good age constraints on the age of folding we can only say that this is a pre-folding VGP with a maximum age of Early Jurassic. The Upper Jurassic Sao Khua Formation yields a high coercivity CW deflected direction (Table 2 and Fig. 3). This unit is poorly exposed and dips are very low. As such, fold tests are inconclusive. Polarity may be a useful argument in that one would expect to see an even distribution between normal and reversed polarities if the magnetization truly formed in the Late Jurassic. The lack of a conclusive fold test and the preponderance of normal polarities leaves open the possibility that this magnetization was acquired during the Cretaceous long normal period.
The southernmost extent of the CW rotations described above is presently not clear and it is useful to review the available palaeomagnetic data from peninsular Malaysia. The principal work from peninsular Malaysia is by McElhinny et al. (1974). They detected two distinct populations of palaeomagnetic data: (1) the Pengerang rhyolites, Singapore gabbro and dykes, Bentong Group, Singa Formation and Sempah Fm. displayed inclinations of 20-40 ° with CW rotated declinations of 20-33°; and (2) the Kuantan-Massai dykes and the Segamat basalts displayed inclinations of 30-40 ° with counter-clockwise (CCW) rotated declinations of 40 °. Haile & Khoo (1980) detected 20-30 ° of CCW rotation in the Maran, Teka and Kluang redbeds of Malaysia while Schmidtke et al. (1990) confirmed the CCW rotated declinations in mafic dikes at Kuantan and mafic flows at Segamat. McElhinny et al. (1974) argued that either the poles or the field directions of three of their four CW rotated sites passed a fold test. They estimated the age of folding to be Late Triassic and thus their CW rotated magnetization was assigned a preLate Triassic age. In the context of new data and statistical analyses, however, it is not clear that the CW rotation is a pre-folding magnetization and even if it is, it could be as young as Late Cretaceous. There are additional unpublished analyses conducted at the UCSB palaeomagnetism laboratory which bear on the review of Malaysian data. Schmidtke (pers. comm. 1991) measured CCW deflections of similar magnitude (30-45 ° ) in samples collected from the poorly dated Singapore and Lanchang dikes, the Jurassic-Cretaceous Raub redbeds, and the Permo-Triassic Kodiang Limestone. The Bukit Kemaman, Bukit Temiang, Lanchang and Tanjung Penyabong dykes display between 6 ° and 23 ° of CCW declination deflection while sites from the Mesozoic Raub and
9
-
-
34.8
18.5
1634.0 59.9
7.9
157.0 105.0
6.2
4.1 7.5
69.3
43.7 80.9 28.5
379.2 21.3 48.5 13,5
21.2 27.3
54.5 42.0 19.4 51.1 48.8 24.8 11.5 60.8 45.1 58.5 115.7 43.3 53.9 5.7
in situ k
-
-3.3
-3.7 -2.9
-5.2
-4.7 -5.0 -5.0
-13.4 -16.7 -11.2 -4.7
11.7 6.7
26.0 13.8 9.4 8.8 0.3 --44.5 20.9 32.3 5.9 13.4 39.3 9.4 -34.3 11.2
in situ A ° Lat.
-
-10.9 36.8 13.0
44.0
67.9 62.7 -35.5
35.4 59.4 19.8 ~).3
-
35.6 24.4 30.6
142.4 31.8 61.1 67.7
66.4 31.1 35.8 42.7
311.6 21.3
44.7
23.4
9.5 45.7
58.8 28.0 32.5 50.6 43.9 42.4 39.6 44.9 45.8 43.1 50.4 51.1 40.4
Corr. D
17.7 9.4 30.7 30.4 24.8 7.1 37.1 19.2 29.1 25.7 24.5 28.4 15.3
Corr. I
21.8
150.2
180.0
-
-
6.1
-
Corr. ct95
8.6
6.0
1.6
-
-
47.1
-
Corr. k
-
11.7 -14.3 -0.4
-32.7 -25.8 37.9 -7.5
-11.6 -29.3 -0.3 7.2
15.9 -6.4
8.5
11.7 16.1 4.3 4.3 7.6 17.1 0 10.9 5.2 7.2 7.9 5.5 12.9
Corr. A ° Lat.
E171.2 ° E174.5 ° E173.8 ° E173.0 °
N49.2 ° E172.4 °
N56.7 ° E181.3 ° N54.1 ° E 1 8 2 . 8 ° N55.4 ° E182.1 °
° ° ° °
E122.9 ° E138.5 ° E12.4 ° E27.4 °
N60.5 ° E164.3 °
N 5 2 . 6 ° E207.3 ° N 6 2 . 4 ° E156.3 ° N59.6 ° E187.3 °
N13.4 ° N52.9 ° N58.8 ° N67.7 °
N46.7 ° E 1 9 8 . 4 °
N53.1 ° E 1 8 0 . 6 °
N20.2 N42.7 N37.1 N33.4
N24.8 ° E172.0 ° N49.9 ° E137 °
N40.4 ° E354.9 ° N69.5 ° E164.3 °
N46.4 ° E190.6 °
Corr. VGP
N46.3 ° E 1 6 7 . 2 ° N41.8 ° E161.9 °
N47.3 ° E 3 5 7 . 4 ° N58.7 ° E 1 9 3 . 8 °
N50.6 ° E197.0 °
in situ VGP
values a n d italics indicate the p r e f e r r e d direction b a s e d u p o n interpretation o f fold tests.
n o r t h w a r d t r a n s l a t i o n s as p o s i t i v e a n d s o u t h w a r d t r a n s l a t i o n s as n e g a t i v e ; in s i t u , g e o g r a p h i c a l c o o r d i n a t e s ; C o r r . , s t r u c t u r a l l y c o r r e c t e d c o o r d i n a t e s . B o l d t y p e i n d i c a t e s m e a n
I, i n c l i n a t i o n (°); D, D e c l i n a t i o n (°); ¢~95, t h e 9 5 % c o n f i d e n c e i n t e r v a l a b o u t t h e m e a n d i r e c t i o n ; k, t h e p r e c i s i o n p a r a m e t e r k a p p a ; A L a t , p r e s e n t l a t i t u d e m i n u s p a l a e o l a t i t u d e , w i t h
Limestone VGP mean
33.5 36.1
19.2 17.8
14.9
60.8
Silurian Setul Limestone, Langkawi Island, Malaysia SM-88-62 N6.2 ° E99.8 ° 9 SM-88-63 N6.2 ° E99.8 ° 5 SM88-62, 63 N6.2 ° E99.8 ° 14
14.1 10.3 11.5
75.2 50.5 56.7
Permo-Triassic Lampang Limestone, Central Thailand T91-8 N18°15.75 ' E99°36.92 ' 4 40.3 T91-9 N18°15.75 ' E99°36.92 ' 4 40.7 T91-10 N18°15.75" E99°36.92 ' 7 40.7 T 9 1 - 8 , 9, 10 N18°15.75 ' E99°36.92 ' 15 41.0
3.1 11.4 8.8 18.9
43.2 48.3 27.1 37.2
Limestone, Peninsular Thailand N7°55 ' E99°55 ' 7 N10.95 ° E99.28 ° 9 N9.9 ° E99.1 ° 7 N7°36.43 ' E100°2.04 ' 6
14.9 24.1
9.2 11.9 17.8 8.5 11.1 12.4 17.1 11.9 11.5 8.0 7.1 10.3 8.3 19.1
in situ tx95
38.0 46.3 37.6 22.6
Permian Ratburi T90-10 T90-27 T90-45 T91-39
317.3 32.1
in situ D
Late Palaeozoic Plateau Limestone, Shan Plateau, Myanmar B 9 1 - 1 2 + 14 N20°48.89" E96°44.039 ' 6 17.6 B91-13 N20°40.89 ' E96°44.039 ' 3 26.5
N in situ (samples) I
58.5 24.1 27.2 42.9 38.3 352.9 40.1 54.3 40.4 41.7 48.0 44.8 341.1 39.3
Location
Juro-Cretaceous Kalaw redbeds of Shan Plateau (Shan-Thai) B91-8 N20°48.37" E96°26.98 ' 7 -10.2 B91-10 N20°47.98" E96°27.14 ' 5 13.8 B91-11 N20°47.98 ' E96°27.14" 5 21.9 B91-17 N20°38" E96°34 ' 8 22.8 B91-18 N20°38.95 ' E96°33.02" 6 36.6 B91-19 N20°42.24 ' E96°38.52 ' 7 77.0 B91-20 N20°45 ' E96°32" 8 -0.2 B92-54 N20°47.06 ' E96°27.83 ' 4 -22.1 B92-55 N20°45.40 ' E96°28.78 ' 5 27.9 B92-57 N20°41.75 ' E96°30.16 ' 7 14.3 B92-58 N20°42.24 ' E96°38.52 ' 5 -34.0 B92-60 N20°38.27 ' E96°33.43 ' 6 21,6 B92-81 N20°42.31 ' E96°30.55 ' 7 70.7 Mean N20°42 ' E96°33 ' 80 18.6
Site
T a b l e 1. S u m m a r y o f the n e w p a l e o m a g n e t i c d a t a p r e s e n t e d in this p a p e r oo
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Fig, 4. Structural cross-section A-A" across northwest Sabah Margin, re-drawn from unmigrated deep seismic line BGR 86-06 in Hinz et aL (1989). Vertical exaggeration is c. 10 x. Horizon 'C', at c. 17 Ma, corresponds to the end of an early Miocene compressional phase in the South China Sea. Abbreviations: A, Early Pliocene unconformity; B, Late Miocene uncf.-Shallow Regional Unconformity of Levell (1987); C, Latest Early Miocene uncf. (Deep Regional Unconformity); D, top NW Sabah Platform (?Oligocene to L. Miocene carbonate); 5, Miocene/Pliocene boundary; 6, Base Upper Miocene.
MIOCENE TECTONICS OF SABAH, MALAYSIA
Onland and regional structure of western Borneo The onland structure in the west of Sabah is dominated by an uplifted belt of folded and faulted turbiditic sediments (comprising Crocker, Sapulut and Trusmadi Formations) believed to have been accreted to Borneo since early Tertiary times, during some form of plate convergence, (Hamilton 1979; Hutchison 1988, i989; Rangin et al 1990b; B6nard et al. 1990). Tongkul (1987) presents a detailed structural and sedimentological model explaining the Crocker Formation of the Kota Kinabalu area as a packet of trench sediments accreted to northern Borneo in Palaeogene to early Miocene times during subduction of an oceanic slab ('Proto-South China Sea'; Fig. 1). It remains unclear how much oceanic lithosphere, if any, was subducted during this accretionary/compressive episode (compare Holloway 1982; Taylor & Hayes 1983; Hutchison 1988; Rangin et al. 1990b; Clennell 1992) but field outcrops in the Mount Kinabalu and Telupid areas suggest that the Crocker sediments are floored by ophiolitic basement. Certainly, the Neogene history of the North Borneo and Palawan area has involved only collision of continental material rifted from south Asia rather than subduction of any oceanic lithosphere. The rifled continental material is divided into a series of blocks by what may be throughgoing structures linking to transforms at the South China Sea spreading centre (Tongkul 1990, 1994). Although the trajectories of the individual blocks were broadly from the north, channelled by the lineaments, they were slightly divergent in rate and direction. The main period of convergence at the Borneo margin occurred during the Early to Middle Miocene, at which time the Palaeogene sediments were uplifted to form the mountains of the Crocker Range, which reach heights of 1-2 km above sea-level, even in their present eroded state. The Deep Regional Unconformity (DRU) is not a time-line of regional extent, but rather marks the period during which collision had the maximum impact on the sedimentary record in a particular place. In some areas, there is a marked angular discordance and a considerable section of missing stratigraphy, while in other places, the DRU is represented by a brief hiatus. The controls on this pattern depend not only upon local rates and degrees of tectonic uplift, but also on tectonic and eustatic events that influenced the source areas and distribution paths of the sediments (Tan & Lamy 1990; Rice-Oxley 1991). Thus the architecture of the unconformity depends on the combined effects
311
of erosion and sediment dispersal patterns. There was a second culmination in convergence in the mid Late Miocene which buckled the unconformity surface in many parts of Zones III and IV (Hazebroek & Tan 1993).
The Central Sabah-Western Sulu Sea basin system According to Hutchison (1992) the Central Sabah Basin can be regarded as an onland extension of the Southeast Sulu Sea Basin (Fig. 5). Results of scientific drilling (Nichols et al. 1990; Rangin & Silver 1990a, b) suggest that the Southeast Sulu Sea Basin was initiated in latest Oligocene or earliest Miocene time, by rifting of a pre-existing island arc and ophiolitic terrain (Fig. 2) whose onland extensions are now exposed in the Upper Segama area of Sabah, and in the Philippine island of Panay (Hinz et al. 1991). Subsequently, a limited amount of oceanic spreading occurred along a NE-SW axis, opening up a marginal basin bounded to the north by a rifted and subsided arc (Cagayan Ridge) and ophiolite terrain (the Northwest Sulu Sea Basin and Palawan Island), and to the south by the volcanic arc of the Sulu islands. The close proximity of the pole of rotation to the marginal basin meant that eastern Sabah lay at the hinge zone of the spreading (Hutchison 1992). Lithospheric stretching near this pole was presumably not sufficient to generate new oceanic crust (White & McKenzie 1989), but there is scattered evidence of basaltic magmatism along the axis of the Central Sabah basin in the relevant time interval, i.e. from the late Oligocene to the middle of the Miocene (Kirk 1968; Tamesis 1990; Clennell 1992). It is evident that the eastern Sabah to southeastern Sulu Sea region experienced mainly NW-SE extension from Early to Middle Miocene time. At some time in the late Miocene, opening of the Southeast Sulu Sea Basin was extinguished by the subduction of the spreading ridge southwards and eastwards beneath the Sulu Archipelago, and the larger Philippine islands of Negros and Panay (Hinz & Block 1990; Hinz et al. 1991). At this time the tectonic development of eastern Sabah became disconnected from that of the Sulu Sea, probably by the growth of a transform fault system running from the Sulu archipelago towards the Balabac Strait (Hinz et al. 1991). The geometry and kinematic linkage of this system remains obscure, but wrench faulting has apparently affected a series of upper Neogene basins running from the Balabac Basin (Beddoes 1976) through to the Sandakan Basin (Fig. 5).
312
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Fig. 8. Histogram of available plutonic ages from Sumatra, incorporating this study and previously published data.
(Cobbing et al. 1992) suggesting that most of the plutonism was post-orogenic. In addition, a partly coeval plutonic arc of subduction related I-type granitoids can now be identified in western Sumatra (219 _+4 to 183 _+ 13 Ma). Plutons of this subduction-related arc intrude Palaeozoic metasediments of the West Malaya (Sibumasu) terrane deformed by the collision, supporting an Early Triassic suturing age. Thus it is tentatively suggested that the Early Mesozoic plutonism may in fact be made up of two separate magmatic events: an Early Triassic event dominated by collision-related crustal S- and I-type granites in peninsular Malaysia, Thailand and the Indonesian tin-islands, and a Late Triassic to Early Jurassic (220-180 Ma) post-collisional event. The latter is represented by an I-type plutonic arc in western Sumatra (Episode B) and coeval S-type and crustal I-type granitic magmatism in the Main Range Province, Indonesian tin-islands and eastern Sumatra (Episode B1). This magmatism was probably related to tectonic release and adiabatic decompression, with resulting anatexis, in the back-arc region, with granites channelled along deep-seated faults. A postulated change in the convergence angle of the oceanic plate resulted in a more oblique subduction regime in the Early Jurassic that brought the Episode (B-B 1) plutonism to an end and resulted in transpressional strike-slip along the Sundaland continental margin which was taken up along older fault structures.
Episode C ( 1 6 9 - 1 2 9 Ma)
Middle Jurassic to Early Cretaceous plutonism in Sumatra is represented by an extensive I-type, subduction-related belt (Bungo batholith, Sulit Air suite) focused along the western edge of the Mergui microplate broadly coincident with, but laterally more extensive than, the Episode B plutonic belt. The plutonism appears to have been channelled along the junction between the Permian volcanic arc and the Palaeozoic continental margin metasediments, interpreted as an Early Triassic suture. This period of plutonism is correlated with northwest-directed subduction beneath the Sundaland continental margin, in line with postulated northwestward spreading based on identified sea floor magnetic anomalies in the eastern Indian Ocean (Patriat & Achache 1984). This plutonic arc may also extend north into the Shan scarp region of Burma on the basis of limited geochronological evidence (Cobbing et al. 1992). Episode C plutonism terminated in the latest Early Cretaceous (or early Middle Cretaceous) following the collision, accretion and local obduction of the allochthonous Woyla terranes of southern Sumatra, some 125 Ma ago (McCourt et al. 1993). These ophiolitic rocks can be correlated with similar rocks in western Burma (Mawgyi andesites), Tibet (Donqiao ophiolite) and possibly SE Kalimantan (Alino Formation and Meratus ophiolite), and it is likely that they represent fragments of an oceanic arc system that
MESOZOIC---CENOZOIC PLUTONISM IN SUMATRA collided with, and was thrust over, the continental margin of Sundaland at this time (cf. Mitchell 1993).
Episode D (120-75 Ma) Subsequent to the accretion event, possible northdirected subduction was reestablished and I-type granitoid magmas were emplaced into the now cratonized Woyla microplate, with the majority of the plutons focused along the main suture line and related faults (Sikuleh granite, Manunggal batholith, Ulai intrusion, Garba pluton, Sulan pluton). Based on Mitchell (1993), it is probable that this subduction-related arc extends north into Burma west of the Sagaing Fault, where the oldest dated plutons are of mid-Cretaceous age. This western Burma Arc (?equivalent to the Central Valley Province of Cobbing et al. (1992)) is made up of I-type granodioritic to tonalitic plutons, with K-Ar ages of 106+7, 103+4, 9 8 + 4 , 9 4 + 4 and 91 __.8 Ma, and like its Sumatran equivalent intrudes a sequence of deformed oceanic rocks, basaltic andesites and basalt pillow lavas, the Mawgyi andesites (Mitchell 1993). Broadly contemporaneous plutonism is also recorded from the Western Province of Thailand and Burma (Cobbing et al. 1992) and corresponds to a mixed population of I- and S-type granites with high initial ratios indicative of a significant crustal component in most cases. Clarke & Beddoe-Stephens (1987) proposed that this belt of Upper Cretaceous S- & I-type crustal granites also extends into eastern Sumatra, as indicated by the 80 Ma Hatapang granite. It is suggested that this plutonism was related to anatexis, the result of crustal thickening accompanying thrusting that was contemporaneous with subduction and VAG, I-type magmatism in the Central Valley Province and western Sumatra. Middle to Late Cretaceous magmatism continued northwards through the Mogok Belt into Assam (Mitchell 1993) and reported ages of 113-82 Ma on the Gandise batholith in Tibet (Debon et al. 1986) may indicate a further extension of this plutonism. The general absence of plutonic rocks with ages in the range 75-60 Ma, coincides with the well documented latest Cretaceous deformation throughout this region, including Sumatra (de Coster 1974; Hamilton 1979; Cameron et al. 1980; Pulunggono & Cameron 1984). Exactly why plutonism ceased is not clear. The model proposed here involves a suggested change from high angle to oblique subduction along the continental margin, related to a change in oceanic spreading patterns and plate configurations, that resulted in the accretion of a continental sliver, the West Sumatra terrane. This terrane is now present as a series of fragments such as Sikuleh, Natal and possibly
331
Bengkulu (Fig. 2) that correspond to the southerly extension of the West Burma terrane of Metcalfe (1994). Much of the evidence for this event, however, has since been destroyed during tectonism and disruption related to the Early Tertiary collision of India and Eurasia.
Episode E (60-?50 Ma) Following the Late Cretaceous deformation event, a new subduction regime was established along the continental margin of Sumatra as evidenced by a short-lived but extensive plutonic episode from 60-50 Ma (Episode E). This I-type, VAG plutonic arc (Lassi pluton, Nagan granodiorite etc.) was superimposed on the earlier Cretaceous and Jurassic arcs via deep-seated older fault structures in the continental margin which acted as magma conduits. Limited regional evidence suggests that this mainly Early Eocene (57-52 Ma) plutonism extends into Burma and Thailand where it is of combined I- & S-type character (Cobbing et al. 1992; Mitchell 1993). It is suggested that this plutonic episode was brought to an end by the Middle Eocene collision of India and Eurasia at about 50 Ma, approximating to the timing of proposed ophiolite emplacement in the IndoBurman Ranges of western Burma (Mitchell 1993). A further important consequence of the collision of India and Eurasia was the indentation and related deformation of the Lower Tertiary margin of Asia and the probable extrusion and clockwise rotation of much of SE Asia, including Sumatra. The shape of this margin prior to collision was, as suggested by Tapponnier et al. (1986), a simple slightly convex line extending from Sumatra to the western Makran. As a preliminary model it is proposed that this margin was characterized by a series of subparallel, outwardly younging plutonic belts representing prolonged convergence and subductionrelated plutonism, along the margin, from the Early Mesozoic to the Early Tertiary.
Episode F (30-0 Ma) Subsequent to the India-Eurasia collision, and a related major reorganization of plate motions and spreading patterns in the Indian Ocean, NNE directed subduction was established along the Sundaland margin. Available plutonic ages from Burma, 38 + 1 Ma (Mitchell 1993) suggest that subduction-related activity was taking place along the margin by the Early Oligocene, although plutonism in Sumatra was apparently not established until the Early Miocene (Episode F). Wajzer et al. (1991) reported Late Oligocene ages (3028 Ma) from the Air Bangis granite of central Sumatra, but concluded that these plutons, and
332
W.J. McCOURT
ET AL.
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Fig. 9. The main granite provinces of SE Asia, modified from Cobbing et al. (1992) for Sumatra. Inset shows the principal tectonostratigraphic terranes of SE Asia based on Metcalfe (1988, 1990).
MESOZOIC---CENOZOIC PLUTONISM IN SUMATRA contemporaneous volcanics of the Langsat volcanic arc, had formed elsewhere along the Sundaland margin and were tectonically juxtaposed against the Woyla Group of Sumatra sometime prior to the Middle Miocene. Rock et al. (1983) proposed that the Langsat Volcanics were of Palaeogene age, a conclusion confirmed by Wajzer et al. (1991), who assigned them a Late Eocene to Early Oligocene age on the basis of whole rock K-Ar dates (4038 _+ 1 Ma). Thus the proposed age of the Air Bangis plutonism is almost identical to that noted above from Burma. The younger dates (30-28 Ma) from the Air Bangis granites could therefore reflect their collision with and accretion to the Sumatran margin and this could relate to the proposed midOligocene collisional event responsible for the recorded inversion in the forearc basins of Sumatra and Java as proposed by Daly et al. (1991). Following this mid-Oligocene event, widespread andesitic volcanism was established in Sumatra and the main Neogene magmatism of the Barisan arc was initiated. Subduction-related VAG plutonism was widespread by the end of the Early Miocene and in the Middle Miocene the entire Barisan arc became volcanic. Middle Miocene to Early Pliocene, I-type granitoid plutons are essentially
(EUR)ASIA
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Fig. 10. Cartoon, not to scale, illustrating a possible plate-tectonic setting along part of the Asian margin prior to the Eocene collision of India (modified from Metcalfe (1990). SE Asian terranes as follows: SC, South China; C, Qiangtang; I, Indochina/East Malaya; L, Lhasa; S, Sibumasu; WB, West Burma; WY, Woyla.
334
W.J. McCOURT ET AL,
Sumatra can be recognized, albeit in disrupted form, throughout m u c h of SE Asia (Fig. 9). Thus the Triassic to Early Jurassic plutonism correlates with the Eastern and Main Range Granite Provinces of Thailand and Malaysia, whereas the Middle Jurassic and Cretaceous plutonism can be correlated with a combination of the Western and Central Valley Provinces of Thailand and Burma. It is suggested as a preliminary model that prior to the Eocene collision of India the Sundaland margin was orientated approximately W N W and made up of a series of outwardly younging subductionrelated plutonic arcs (Fig. 10), some of which probably extended along the southern margin of the Asian plate. The present distribution and geometry of these arcs in SE Asia is the result of the effects of the collision of India and Eurasia, i.e. indentation, extrusion and strike-slip faulting, as predicted by the model of Tapponnier et al. (1982, 1986). The more recent major dextral strike-slip m o v e m e n t s along the SFS and related master faults outside of Sumatra, the onset of which coincided with the opening of the A n d a m a n Sea c. 11 Ma ago
(Curray et al. 1979), has further complicated this scenario. We also propose that breaks in plutonic activity correspond to periods of oblique approach, that, in some instances, relate to the collision and accretion of allochthonous terranes. We further suggest that one of the underlying factors that controls the development and siting of the various plutonic arcs is the availability of deep-seated faults along the continental margin, that probably extend down to the site of m a g m a generation at or close to the subduction zone. This paper is published with the permission of the Directors of the Geological Research and Development Centre, Bandung, and the British Geological Survey, Nottingham. The work in Sumatra was carried out as part of a bilateral technical cooperation project between the governments of Indonesia and the United Kingdom and funded jointly by the Indonesian Directorate General of Geology and Mineral Resources (DGGMR) and the Overseas Development Administration (ODA) of the British Foreign Office. We thank A. H. G. Mitchell and S. J. Moss for suggestions which improved the text.
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the Andaman Sea and Burma. In: WATKINS, J., MONTADERT, L. & DICKENSON, P. W. (eds) Geological and Geophysical Investigations of Continental Margins. AAPG Memoir, 29, 189-198. DALY, M. C., COOPER,M. A., WILSON, I., SMITH,D. G. t~ HOOPER, B. G. D. 1991. Cenozoic plate tectonics and basin evolution in Indonesia. Marine and Petroleum Geology, 8, 1-21. DEBON, E., LE FORT, P., SHEPPARD,S. M. E & SONET, J. 1986. The four plutonic belts of the TranshimalayaHimalaya: a chemical, mineralogical, isotopic and chronological synthesis along a Tibet-Nepal section. Journal of Petrology, 27, 219-250. DE COSTER, G. L. 1974. The geology of the Central and Southern Sumatra Basins. In: Proceedings Indonesian Petroleum Association 3rd Annual Convention. 77-110. EUBANK, R. T. & MAKK1,A. Ch. 1981. Structural geology of the Central Sumatra Back-Arc Basin. In: Proceedings Indonesian Petroleum Association, lOth Annual Convention. 153-196. FOLEY, S. E • WHELLER, G. E. 1990. Parallels in the origin of the geochemical signatures of island arc volcanics and continentaI potassic igneous rocks: the role of residual titanates. Chemical Geology, 85, 1 18. FONTAINE,H. & GAFOER,S. 1989. The pre- Tertiary fossils of Sumatra and their environments. CCOP Technical Secretariat Publication, Bangkok, Thailand. GAFOER, S. 1990. Tinjauan Kembali Tataantratigrafi Pratersier Sumatra Bagian Selatan. [A review of the pre-Tertiary sequences of Southern Sumatra. Abstract in English.] In: Prosiding Persidangan Sains Bumi dan Masyarakat Universiti Kebangsaan Malaysia, 9-10 July 1990.
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The Mentawai fault zone and deformation of the Sumatran Forearc in the Nias area M . A. S A M U E L
& N. A. H A R B U R Y
SE Asia Research Group, Research School of Geological and Geophysical Sciences, Birkbeck College and University College London, Gower Street, London W C I E 6BT, UK. Abstract: The Sumatran Forearc is the site of oblique plate convergence between the IndoAustralian and SE Asian-Eurasian Plates. It is generally accepted that the forearc sliver is not behaving as a rigid plate and that the rate of slip increases along the fight-lateral Sumatran Fault System from southeast to northwest. Two contrasting hypotheses have been invoked to explain this pattern of decoupling: by using slip vectors to suggest arc-parallel stretching; and a second major right-lateral strike slip zone, parallel to the Sumatran Fault System, to accommodate the oblique subduction. This zone, termed the Mentawai fault zone, lies just to the east of the outer-arc ridge and is indicated as intersecting with other faults on Nias Island. In this paper data are presented from Nias where a thick forearc succession is excellently exposed in three subbasins with half-graben geometries and broadly comparable Palaeogene-Recent histories. Mud diapirism has generated melanges which cut the Oligocene to Recent strata. Outcrop data, LANDSAT, Synthetic Aperture Radar and aerial photographs were used to determine fault offset patterns, and marked extension of the island along north-striking right lateral faults and ESE striking left-lateral faults is revealed. Extension was further accommodated along a set of ENE striking normal faults. In the region where the proposed Mentawai fault zone comes onto Nias there is an important, long-lived, basin-bounding fault. This structure had in excess of 5 km normal throw during the Oligocene-Miocene whilst minor contractional reactivation of the fault occurred during the Pliocene phase of uplift and deformation which affected all of Nias. Data from onshore and offshore the Nias region suggest that all the features visible from seismic sections collected over the Mentawai fault zone to the south of Nias can be explained in terms of inversion of originally extensional structures and mud diapirism. It is suggested that strike-slip motion is of limited importance along the 600 km long Mentawai fault zone. Rather this highly structured zone represents a deformation front whose origin can be explained by Pliocene to Recent subduction-driven inversion at the outer margin of the forearc.
The Sumat~an Forearc has been recognized as a type example of an obliquely convergent margin for over twenty years (e.g. Fitch 1972). In recent years two contrasting models of deformation of the Sumatran Forearc have been proposed (McCaffrey 1991; Diament et al. 1992). This paper outlines these m o d e l s and presents new data on the Sumatran Forearc, from the area of Nias, which allow a critical assessment of these two models. Furthermore the results lead to an improved understanding of the d e f o r m a t i o n processes acting within, and at the margin of, the forearc.
Deformation of the Sumatran Forearc The Sumatran Forearc forms part of the Sunda subduction system which extends from Sumba in the east to B u r m a in the north (Figs 1 and 2; Moore et al. 1980b; Curray 1989). Plate tectonic models predict convergence rates varying from 7.8 c m a -1 near S u m b a w a to 6 c m a -1 near the A n d a m a n Islands (Minster & Jordan 1978; DeMets et al. 1990) with the direction of convergence close
to n o r t h - s o u t h ( N e w c o m b & M c C a n n 1987; McCaffrey 1991). The Sumatran arc has a classic morphology of trench, accretionary prism, outer-arc ridge, forearc and volcanic chain with active andesitic volcanism (Karig et al. 1979) and seismicity displays a distinct Benioff zone (Page et al. 1979). Recent work has shown that the outer-arc ridge along Sumatra comprises the outer part of the forearc rather than forming part of the accretionary prism which is located further to the southwest (Samuel 1994; Samuel et al. 1995). The outer-arc ridge is wholly submerged off Java whereas the ridge gains sub-aerial expression in the Sumatran Forearc. A further important difference between the Javan and Sumatran forearcs is the subduction direction which varies from nearly orthogonal off Java to oblique west of Sumatra where the trench strikes N140°E. Fitch (1972) proposed that oblique convergence in the Sumatran area could be a c c o m m o d a t e d by the right-lateral Sumatran fault zone. This fault zone connects to the A n d a m a n Sea in the northwest
From Hall, R. & Blundell, D. (eds), 1996, Tectonic Evolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 337-351.
337
338
M.A. SAMUEL • N. A. HARBURY
where extension and sea floor spreading is an extreme effect of oblique convergence (Fig. 2; Curray et al. 1979). To the southeast the zone extends to the Sunda Strait where further extension is occurring (Huchon & Le Pichon 1984; Harjono et al. 1991) and where it probably crosses the forearc (Curray 1989). A sliver plate (the Burma Sliver Plate; Curray et al. 1979) is thus decoupled from the Eurasian and Indo-Australian plates and moves northwest with respect to the Eurasian Plate. This plate has been called the Sumatran Sliver Plate or forearc sliver in the Sumatran area (Jarrard 1986; Diament et al. 1992). McCaffrey (1991) presented slip vector data that suggested the oblique convergence in the Sumatran area was not completely decoupled. The results indicated increasing decoupling northwestwards (Fig. l a). Modelling of the pattern of slip vectors by McCaffrey (1991) suggests that the forearc is not behaving as a rigid plate but is subject to arc-parallel stretching at a uniform strain rate of 3--4 x 10-8 per year. He showed that the northwestward motion of the forearc relative to the SE Asian plate should increase from near zero at the Sunda Strait to 4 5 - 6 0 m m a -1 in northwest Sumatra. McCaffrey (1991) acknowledged that the deformation processes acting to stretch the forearc were poorly understood. Shallow strike-slip earthquakes in the forearc have nodal planes that strike north and east across the forearc rather than parallel to it (McCaffrey 1991) and it was suggested the deformation of the forearc was most likely to occur on strike-slip faults crossing the margin than on arc-parallel strike-slip faults or arc-perpendicular normal faults. There are a number of geological indications that support the concept that deformation must be occurring in the Sumatran Forearc. For example there does not appear to have been sufficient movement along the Sumatran Fault Zone to account for the 460 km of opening of the Andaman Sea in the last 13 Ma (Curray et al. 1979). Indeed Bellier et al. (1991) suggest that the rate of slip along the southern end of the Sumatran Fault zone is only 6 mm a-1 which is considerably less than the 40 mm a-1 rate of opening of the Andaman Sea (Curray et al. 1979). Diament et al. (1992) proposed an alternative solution to explain the decrease in rate of slip to the southeast along the Sumatran Fault system. A single channel seismic reflection survey led to the identification of a complex set of structures which they correlated for 600 km along the outer margin of the Sumatran forearc and named the Mentawai fault zone (Fig. l b, c). These features were apparent on all seismic sections from southern Sumatra to Siberut although the detailed structure of the zone was highly variable; from north to
south, faulted anticlines, faulted blocks, horst and graben systems and flexures were all described. Diament et al. (1992) interpreted the structures as comprising a right-lateral strike slip zone (Fig. lc). They based their arguments on three points: (1) it is possible to interpret some features, identified on seismic lines, as positive flower structures; (2) the fault zone is straight and appears to run continuously for hundreds of kilometres; (3) a pronounced difference is commonly found in the depth to acoustic basement on either side of the fault zone. Diament et al. (1992) further suggested that displacement on the Mentawai fault zone may be relayed to the Batee fault which they tentatively map as linking with the Mentawai fault zone on Nias (Fig. lc). Nias is therefore of prime interest as it potentially lies at the intersection of two major fault zones. Furthermore it is the only island along the outer-arc ridge which is cut by the Mentawai fault zone (Fig. lc). Deformation in the Nias area Previous work by Moore & Karig (1980) suggested that Nias comprised part of an uplifted accretionary complex. Two main stratigraphic units were defined. The lower unit, the 'Oyo Complex' was interpreted as a tectonic melange that formed prior to the Early Miocene, when deep marine sediments were unconformably deposited above it in trench slope basins (Moore et al. 1980a). The Miocene to Pliocene sedimentary succession was reported as a shallowing-up sequence. Uplift was continuous, with compressional deformation occurring largely along arcward-dipping thrust and reverse faults. More recently Pubellier et al. (1992) have suggested that Nias comprises part of an Eocene Tethyan suture zone. Their interpretation is based largely on the apparent recognition of in situ Eocene rocks forming part of the non-ophiolitic sedimentary succession on the island. The research of Samuel (1994) and many previous workers (e.g. Douville 1912; Moore et al. 1980a) shows however that the rocks in question are of Upper Oligocene and Lower Miocene; they contain both a reworked Eocene fauna and younger microfossils. As part of an integrated study of the Sumatran Forearc by the University of London SE Asia Research Group the structural and stratigraphic evolution of the whole of Nias has recently been studied in detail (Samuel 1994; Samuel et al. 1995). The studies have led to a substantial reinterpretation of the sedimentological and structural setting of the island and the work has shown that Nias consists of three main sub-basins and a basement high (Fig. 2). These detailed sedimentological, biostratigraphic and palaeobathymetric studies of the successions on Nias have led to the construction
SUMATRAN FOREARC & MENTAWAI FAULT ZONE
339
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340
M . A . SAMUEL 8Z N. A. HARBURY
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341
SUMATRAN FOREARC 8,~ MENTAWAI FAULT ZONE
One of the fundamental differences in the new stratigraphy for the forearc compared, for instance, with the scheme of Moore e t al. (1980a), is the recognition that the Oligocene and Lower Miocene sediments on Nias form part of a single, stratigraphically continuous, sedimentary succession (Fig. 3). Workers such as Moore & Karig (1980) believed that the Oligocene deep marine sedimentary rocks only occurred in the melanges whereas this study, particularly in the central part of
of a new stratigraphy for Nias (Fig. 3). When these studies are combined with structural data collected over the entire island a new picture of the geological evolution of Nias emerges (Fig. 3). There is strong evidence that the Oligocene to Early Miocene history of the Sumatran Forearc was extensional/transtensional and below we outline some of the key points of this history before concentrating on specific aspects of the deformation of Nias.
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359
BANTIMALA COMPLEX EVOLUTION, SULAWESI Table 1. K-At ages of muscovites from metamorphic rocks of the Bantimala Complex. Sample no.
Rock type
Mineral
Age (Ma)
BT-11b Mg-47 L-01B BT-08e BT-17
eclogite eclogite eclogite pelitic schist pelitic schist
muscovite muscovite muscovite muscovite muscovite
132 + 7 124 _ 6 113 _ 6 114 _+6 115 _ 6
amphibolite facies. In a tectonic slice, the metamorphic grade increases toward the northeast (tectonically towards the top). Glaucophane schist and eclogite associated with serpentinite are locally present as tectonic blocks and slices and occur along the boundary fault of one tectonic slice consisting of metamorphic rocks. Most of the metamorphic rocks of greenschist to amphibolite facies were originally eclogite and have suffered retrogressive metamorphism. Wakita et al. (1994b) reported K-Ar ages of micas from schists ranging from 113-132 Ma (Table 1). The ages of eclogite vary widely, although the ages of greenschists are concentrated c. 114-115 Ma.
Schist breccia 'Schist breccia' is one of the main lithologies of the Bantimala Complex. Most are sedimentary breccias consisting mainly of schist fragments, although it is sometimes difficult to distinguish tectonically brecciated schist from sedimentary breccia. The metamorphic grades of the schist fragments are the same as that of schists in the tectonic slices of the Bantimala Complex. The relationship between chert and 'schist breccia' of the Bantimala Complex has been described as an 'unusual unconformity' in the Paring River (Haile et al. 1979; Wakita et al. 1994b). The 'brecciated schist' grades into sandstone, which is overlain by radiolarian chert. As the chert is intercalated with thick beds of 'schist breccia' at several horizons, as well as with sandstone layers of various thicknesses, the three rock types, i.e. chert, sandstone and 'schist breccia' are considered to be contemporaneous deposits of an unstable sedimentary basin.
Chert and siliceous shale Chert layers range from 1-20 cm thick and are interbedded with thinner shale layers less than 1 cm thick. The bedded chert is mostly red or reddish brown, and sometimes pale green or grey in colour. It is composed mainly of skeletons and fragments of radiolarians, and a small amount of shale. The chert sometimes includes well preserved middle
4°mr 3.21 2.94 3.76 2.5 2.37
%40Ar
%K
93.9 90.4 96.6 92.2 94.5
5.99 5.94 8.34 5.48 4.98
Cretaceous radiolarians (upper Albian to lower Cenomanian) including Holocryptocanium barbui,
Thanarla conica, Archaeodictypomitra vulgaris and Rhopalosyringium majuroensis. The chert is underlain by 'schist breccia' and coarse grained sandstone. It is intercalated with sandstone beds and laminae in the lower part of the succession. The chert is intercalated with rhyolite tuff layers along the Pateteyang River (Fig. 6e). The rhyolite is pale green in colour, is usually fine but sometimes coarse grained. The chert locally grades into siliceous shale towards the stratigraphic top in some localities. The shale is grey or reddish-brown in colour, and composed of radiolarian skeletons, terrigenous fragments and other detrital materials. One of the authors (Munasri) extracted radiolarians including Holocryptocanium barbui, Pseudodictyomitra pseudomacrocephala and Thanarla veneta from a siliceous shale collected in the Pangkajene River.
Jurassic Paremba Sandstone Jurassic shallow marine sedimentary rocks, called the Paremba Sandstone (Sukamto & Westermann 1992), are incorporated as tectonic slices in the Bantimala Complex. The lower part of the Paremba Sandstone along the Bontolio River is composed of thin bedded sandstone and shale, intercalated with thin limestone layers. Some shallow marine sedimentary structures such as ripple and convolute laminations are recognized (Fig. 6c). The upper part of the formation is rich in conglomerate (Fig. 6d) which includes pebbles mainly of basalt and schist. Ammonites (e.g. middle Liassic Fuciniceras), gastropods and brachiopods of the Lower and Middle Jurassic are reported from the Paremba Sandstone (Sukamto & Westermann 1992).
Cretaceous Balangbaru Formation Cretaceous flysch sequences assigned to the Balangbaru Formation are widely distributed in the northeast and north in the Bantimala area. Detailed descriptions were given by Hasan (1990, 1991) and the following is based on Hasan (1990).
360
K. WAKITA ET AL.
(a)
~)
(c)-
(a)
(e)
(13
Fig. 6. Outcrop photographs of the Bantimala Complex. (a) Cretaceous limestone blocks embedded within a sheared shale matrix along the Cempaga River. (b) Highly sheared shale matrix including various kinds of rocks as clasts, along the Pateteyang River. Plio-Pleistocene faults cut the shale matrix of the Cretaceous melange. (c) Jurassic Paremba Sandstone, showing ripple marks, interbedded with shale, along the Bontorio River. (d) Jurassic Paremba Sandstone with intercalations of conglomerate, along the Bontorio River. (e) Rhyolite layers intercalated in radiolarian chert, along the Pateteyang River, near the mouth of the Sanggi River. (f) Turbidite of the Balangbaru Formation, in the upstream part of the Balangbaru River.
The formation is subdivided into three members, in ascending order: the Allup, Panggalungan and Bua Members. The Allup Member is composed of pebbly sandstone, conglomeratic breccia and interbedded sandstone with shale. The Panggalungan Member consists of interbedded sandstone
and shale and chaotic breccia deposits. The Bua Member is composed of interbedded sandstone, shale (Fig. 6f) and conglomerate. Hasan (1990) interpreted the facies associations of the Allup, Panggalungan and Bua members as inner fan, outer fan to basin plain and middle fan respectively. The
BANTIMALA COMPLEX EVOLUTION, SULAWESI mineral composition of the sandstones varies from the bottom to top of the formation. Garnet, spinel, glaucophane and chloritoid are characteristic of the lower parts of the Balangbaru Formation, while zircon, apatite and tourmaline increase in proportion towards the stratigraphic top. The mineral assemblages suggest that the lower sediments were derived principally from metamorphic rocks of the Bantimala Complex, and the upper sediments have a continental or magmatic arc provenance. Hasan (1990) reported planktonic foraminifera, such as Globotruncana helvetica, G. arca, G. foricata and Heterohelix globulosa, from the fine grained sandstone and siltstone of the Panggalungan and Bua members of the Balangbaru Formation indicating a lower Turonian to upper Maastrichtian range. One of the authors (Wakita) extracted radiolarians from shale and siliceous shale of all members of the Balangbaru Formation. Archaeodictyomitra spp., Pseudodictyomitra spp., Rhopalosyringium majuroensis, Thanarla spp., Praeconocaryomma sp., Stichomitra sp. Archaeodic~omitra and Pseudodictyomitra are most dominant genera among them. The assemblage remains very similar from the stratigraphic bottom to top of the formation, and is also very similar to that from chert of the Bantimala Complex reported by Wakita et al. (1994b). Rhopalosyringium majuroensis ranges from upper Albian to upper Turonian (Schaaf 1984) and the common occurrence of Pseudodictyomitra spp. indicates that assemblage ranges from Lower to middle Cretaceous. No species indicating an age younger than Coniacian has been found in samples of the Balangbaru Formation. The radiolaria indicate the age of the Balangbaru Formation is between upper Albian and Turonian. Sukamto (1975, 1978, 1982, 1986) and Hasan (1990) excluded the Balangbaru Formation from the Bantimala Complex. However, the new radiolarian age data suggest that the age of the Balangbaru Formation is not different from the age of chert in the B antimala Complex. The Balangbaru Formation also occurs as tectonic slices stacked with the other members of the Bantimala Complex. The authors therefore propose, on the basis of new radiolarian data, that the Balangbaru Formation is part of the Bantimala Complex. The formation is almost contemporaneous with sandstones deposited on chert of the Bantimala Complex, and on the ultramafic rocks.
Tectonic evolution The following lithologies and structures of the Bantimala Complex are critical elements in the tectonic evolution of the Bantimala area: high pressure metamorphic rocks, Jurassic shallow
361
marine sedimentary rocks, schist breccia overlain by radiolarian chert, melanges including blocks of various kinds of rocks, and tectonically stacked slices of various rocks unconformably overlain by Palaeogene volcanic rocks. To explain the l i t h o l o g i e s and structures of the Bantimala Complex, the authors propose the following hypothetical tectonic evolution: Cretaceous subduction, collision and accretion of a microcontinent, and Neogene tectonic stacking of slices caused by westward collision of another microcontinent. Figure 7 shows a possible tectonic evolution of the Bantimala area based on this hypothesis. K-Ar ages of high pressure metamorphic rocks range from 132-113 Ma, and indicate that there was subduction from Jurassic to early Cretaceous towards the 'West Kalimantan Continent' as indicated by the distribution of Jurassic to Cretaceous granites. Jurassic shallow marine sedimentary rocks are the most important piece of evidence for a microcontinent that subducted, collided and accreted in the early Cretaceous 'Bantimala trench'. Shallow marine clastic formations contemporaneous with the Paremba Sandstone of the Bantimala area are found further to the east in central and SE Sulawesi, and in the Banggai-Sula area (e.g. the Buya Formation; Surono & Sukarna 1993). On the other hand, there is no indication of unmetamorphosed Jurassic rocks to the west of the Bantimala area. Various sizes of continental fragments drifted northward and accreted along the Asian continental margin since the break-up of the Gondwanaland (Nur & Ben-Avraham 1983; Maruyama et al. 1989). The microcontinent on which the Jurassic Paremba Sandstone was deposited is thought to be one of them. Subduction of the oceanic plate caused the formation of high pressure metamorphic rocks at least from Late Jurassic to Early Cretaceous times, and brought the microcontinent with its overlying Jurassic shallow marine sedimentary rocks into the 'Bantimala trench'. After the arrival at the trench, the microcontinent was subducted, collided and accreted within the accretionary wedge. After the collision and accretion of the microcontinental block, subduction ceased at the 'Bantimala trench'. Underthrusting of the light and buoyant continental fragment caused the rapid uplift and exhumation of high pressure metamorphic rocks. Their 113-132Ma K-Ar ages indicate the time of cooling during exhumation. After metamorphic rocks of the Bantimala Complex appeared at the surface, they were eroded and provided 'schist breccia' and sandstone to a sedimentary basin in which radiolarian remains were deposited at a relatively high rate during the late Albian to early Cenomanian (Wakita et al. 1994b).
K. WAKITA ET AL.
362
late Albian
Jurassic West Kalimant an Cont inent
Shallow marine sediment at ion
schist breccia
radiolarian chert
Exhumat ion Collision & accret ion
High P/T met amorphic rocks
C e n o m a n i a n - early T u r o n i a n olist ost rome
O l i g o c e n e - Pliocene
BalanclbaruFormation sla
chert
Fig. 7. Tectonicevolution of the Bantimala area, South Sulawesi.
The radiolarian biostratigraphy and lithostratigraphic relationships reveal that the ages of 'schist breccia', chert, and the Balangbaru Formation are similar. The occurrences of slump deposits and 'schist breccia' layers within chert suggested that they were deposited in an unstable sedimentary basin. Successive exhumation of metamorphic rocks caused the uplift of the basement of the basin. The basement provided fragments of the other members of the Bantimala Complex into melanges which were originally formed as olistostromal deposits. Ultramafic rocks of the Bantimala Complex were exhumed at almost the same time as the high pressure schists. The Balangbaru Formation was deposited unconformably on the ultramafic rocks. After deposition, the tectonic slices of the Balangbaru Formation and its ultramafic basement were tectonically juxtaposed with other slices of the Bantimala Complex. The most distinct feature of the Bantimala Complex is tectonic stacking of slices. The structure is very similar to that of accretionary prisms and accretionary complexes elsewhere in the world. The stacking of structures dipping to the east are, however, opposite to that expected from a westward-dipping, oceanic plate subduction toward the Sundaland continent during Cretaceous time. Sukamto (1982) suggested that tectonic stacking and mixing in the Bantimala Complex were formed prior to Late Cretaceous, because the Balangbaru
Formation was considered to overlie the Bantimala Complex unconformably. However, as the Balangbaru Formation is found as tectonic slices in the Bantimala Complex, the stacking must have happened after its deposition. Although the geological map of Sukamto (1986) indicates that Palaeogene propylitized volcanic rocks unconformably cover the tectonic slices and their boundaries (Fig. 3), we doubt that the stacking of slices occurred in before the Palaeogene. We need to check the nature of boundaries and relationship between members of the Bantimala Complex and Palaeogene propylitized volcanic rocks. Thrust faults locally cut the Miocene Camba Formation. Some of the faults obviously moved later than Miocene time. Coffield et al. (1993) and Bergman et al. (1996) argued that stacking of the Lamasi Complex of South Sulawesi was caused by westward obduction of the ophiolite in Oligocene time and Miocene to Pliocene collision of microcontinent. A similar scenario is acceptable for the tectonic stacking of the Bantimala Complex. Obduction of an oceanic plate and subsequent collision of a microcontinent caused westward-thrusting during Oligocene to Pliocene time (Parkinson 1991; Coffield et al. 1993; Bergman et al. 1996). Tectonic stacking of the Bantimala Complex is similarly interpreted to result from ophiolite obduction followed by microcontinent collision in Tertiary time.
BANTIMALA COMPLEX EVOLUTION, SULAWESI
Conclusions The Bantimala C o m p l e x consists of tectonic slices of sandstone, shale, conglomerate, chert, siliceous shale, basalt, ultramafic rocks, schist, and 'schist breccia' The rock components of the complex range in age from Jurassic to middle Cretaceous. The c o m p l e x was u n c o n f o r m a b l y overlain by Cenozoic sedimentary and volcanic formations and was intruded by Tertiary igneous rocks. Schists in the Bantimala C o m p l e x range mainly from greenschist to amphibolite facies and locally include glaucophane schist and eclogite. The K-Ar ages of the schists range from 113-132 Ma. Schist was unconformably overlain by radiolarian chert and clastic rocks during the late Albian to early Cenomanian. Radiolarian biostratigraphy reveals that the Balangbaru Formation ranges from upper Albian to Turonian and belongs to the Bantimala Complex. Jurassic shallow marine sedimentary rocks are the most important pieces of evidence for a lost m i c r o c o n t i n e n t that subducted, collided and accreted in the early Cretaceous ' B a n t i m a l a trench'.
363
Underthrusting o f a light and buoyant continental f r a g m e n t c a u s e d the rapid uplift and exhumation of high pressure metamorphic rocks including eclogite. Tectonic stacking of slices in the Bantimala Complex was mainly caused by Oligocene ophiolite obduction and M i o c e n e - P l i o c e n e collision of a microcontinent. This paper is one of the results of the joint project between the Research and Development Centre for Geotechnology (RDCG) and the Geological Survey of Japan (GSJ) under the ITIT programme 'Research on Mineral Resources Assessment of Oceanic Plate Fragments'. The authors thank Drs A. J. Barber and C. D. Parkinson of Royal Holloway, University of London for their critical review, effective suggestions and discussion of the geology of this area. We also express thanks to Dr Ir. S. Suparka, director of RDCG for his helpful support during our geological survey. We are grateful to Dr R. Sukamto of the Geological Research and Development Centre for his kind offer of unpublished data and information on the field area. Thanks are also extended to Drs K. Mori, T. Nakamori and Y. Iryu of Tohoku University and Dr Y. Sato of the Geological Survey of Japan for the identification of fossils in limestone clasts of the melange of the Bantimala Complex.
References ASlKIN, S. 1974. The geological evolution of central Java and vicinity in the light of the new global tectonics. PhD Thesis, Bandung Institute of Technology [in Indonesian with English abstract]. BARBER, A. J., TJOKROSAPOETRO,S. & CHARLTON,T. R. 1986 Mud volcanoes, shale diapirs, wrench faults, and melanges in accretionary complexes, eastern Indonesia. AAPG Bulletin, 70, 1729-1741. BERGMAN, S. C., COFFIELD, D. Q., TALBOT, J. P. & GARRARD, R. J. 1995. Tertiary tectonic and magnetic evolution of western Sulawesi and the Makassar Strait, Indonesia: evidence for a Miocene continent-continent collision. This volume. BERRY,R. F. & GRADY,A. E. 1987. Mesoscopic structures produced by Plio-Pleistocene wrench faulting in South Sulawesi, Indonesia. Journal of Structural Geology, 9, 563-571. CLENNELL,B. 1991. The origin and tectonic significance of melanges in Eastern Sabah, Malaysia. Journal of Southeast Asian Earth Sciences, 6, 407-429. COFFIELD, D. Q., BERGMAN, S. C., GARRARD, R. A., GURITNO, N., ROBINSON, N. M. & TALBOT, J. P. 1993. Tectonic and stratigraphic evolution of the Kalosi PSC area and associated development of a Tertiary petroleum system, south Sulawesi, Indonesia. In: Proceedings Indonesian Petroleum Association 22nd Annual Convention. 678-706. HAILE, N. S., BARBER, A. J. & CARTER, D. J. 1979. Mesozoic cherts on crystalline schist in Sulawesi and Timor. Journal of the Geological Society, London, 136, 65-70. HAMILTON,W. 1979. Tectonics of the Indonesian region. US Geological Survey, Professional Paper, 1078.
HASAN, K. 1990. The Upper Cretaceous Flysch Succession of the Balangbaru Formation, Southwest Sulawesi, Indonesia. PhD Thesis, University of London. -1991. The Upper Cretaceous Flysch succession of the Balangbaru Formation, Southwest Sulawesi. In: Proceedings Indonesian Petroleum Association 20th Annual Convention. 183-208. HEHUWAT, E H. A. 1986. An overview of some Indonesian melange complexes - a contribution to the geology of melange. Memoir of Geological Society of China, 7, 283-300. HUTCHISON, C. S. 1989. Geological Evolution of SouthEast Asia. Oxford University Press, Oxford Monographs on Geology and Geophysics, 13. LASH, G. G. 1987. Diverse melanges of an ancient subduction complex. Geology, 15, 652-655. MARUYAMA,S., LIU, J. G. & SENO, T. 1989 Mesozoic and Cenozoic Evolution of Asia. In: BEN-AVRAHAM,Z. (ed.) The Evolution of the Pacific Ocean Margins. Clarendon Press, Oxford, Oxford Monographs on Geology and Geophysics, 75-99. NUR, A. & BEN-AVRAHAM,Z. 1983. Break-up and accretion tectonics. In: HASHIMOTO,M. & UYEDA, S. (eds) Accretion Tectonics in the Circum-Pacific Regions. Terrapub, Tokyo, 3-18. PARKINSON, C. D. 1991. The Petrology, Structure and Geologic History of the Metamorphic Rocks of Central Sulawesi, Indonesia. PhD Thesis, University of London. SCHAAF, A. 1984. Les radiolaires du Cr~tacd Infdrieur et Moyen: Biologie et Systematique. Sciences G6ologiques Memoire, 75.
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K. WAKITA ET AL.
SIMANDJUNTAK,T. O. 1990. Sedimentology and Tectonics of the Collision Complex in the East Arm of Sulawesi, Indonesia. Geology of Indonesia, 13, 1-35.
SUKAMTO, R. 1975. Geologic Map of Indonesia, Ujung Pandang Sheet. 1:1,000,000. Geological Survey of Indonesia. - 1978. The structure of Sulawesi in the light of plate tectonics. In: Proceedings of Regional Conference on Geology and Mineral Resources of SE Asia. 121-141. -1982. The geology of the Pangkajene and Western part of Watampone, South Sulawesi, scale 1:250,000. Geological Research and Development Centre, Bandung. 1986. Tectonik Sulawesi Selatan dengan acuan khusus ciri-ciri himpunan batuan daerah Bantimala. Dissertation, ITB, Bandung.
8Z WESTERMANN, G. E. G. 1992. Indonesia and Papua New Guinea. In: WESTERMANN,G. E. G. (ed.) The Jurassic of the Circum-Pacific. Cambridge University Press, USA, 181-193. SURONO & SUKARNA,D. 1993. Geology of the Sanana Sheet, Maluku, 1:250,000. Geological Research and Development Centre, Bandung. WAKITA, K., MUNASRI ,~ BAMBANG, W. 1994a. Cretaceous radiolarians from the Luk-Ulo Melange Complex in the Karangsambung area, central Java, Indonesia. Journal of SE Asian Earth Sciences, 9, 29-43. --, MUNASRI,SOPAHELUWAKAN,J., ZULKAm~AaN,I. & MIYAZAKI, K. 1994b. Early Cretaceous tectonic events implied in the time-lag between the age of radiolarian chert and its metamorphic basement in Bantimala area, South Sulawesi, Indonesia. Island Arc, 3, 90-102.
The Tertiary evolution of South Sulawesi: a record in redeposited carbonates of the Tonasa Limestone Formation M O Y R A E. J. W I L S O N & D A N W. J. B O S E N C E
SE Asia Research Group, Department of Geology, Royal Holloway University of London, Egham TW20 OEX, UK Abstract: South Sulawesi, situated at the junction of three major plates and with an almost complete Tertiary sequence, is an ideal location in which to study syntectonic sedimentation. Redeposited carbonate facies of the lower/middle Eocene to middle Miocene Tonasa Limestone Formation in the Barru area prove to be reliable indicators of tectonic activity. South of the Barru area contemporaneouscarbonate sediments formed on a relatively stable shallow-waterplatform, known as the Tonasa Carbonate Platform. Redeposited carbonate facies and interbedded marls from the Barru area are described and interpreted in this study. The immaturity and provenance of clasts indicate that the redeposited facies were derived from the faulted northern margin of the Tonasa Carbonate Platform. A relay ramp between at least two major NW-SE trending faults is the inferred configuration of this margin. Three main phases of faulting are indicated by the redeposited facies: late Eocene to early Oligocene, middle Oligocene and early to middle Miocene. This is consistent with other outcrop and seismic data from the region and with the inferred plate tectonic situation during the Tertiary.
Sulawesi is located in an exceedingly complex tectonic region, where three major plates have been interacting since the Mesozoic. With reference to the hotspot frame the Pacific-Philippine plate is moving WNW, the Indo-Australian plate NNE and both are colliding with the relatively stable Eurasian plate (Hamilton 1979; Daly et al. 1987, 1991). The convergence zone of this triple junction is a composite domain of micro-continental fragments, accretionary complexes, m61ange terrains, island arcs and ophiolites. Successive accretion from the east of oceanic and microcontinental material, and the associated development of island arcs, have all controlled the stratigraphic development of Sulawesi. South Sulawesi (Fig. 1), located on the eastern margin of Eurasia, has an almost complete stratigraphic sequence representing the period between the late Cretaceous and the present day (Fig. 2; Sukamto 1975; Hamilton 1979; Van Leeuwen 1981). South Sulawesi is therefore an ideal location in which to study the effects of local or regional tectonics preserved within a sedimentary sequence. Carbonate deposits of the middle Eocene to middle Miocene Tonasa Limestone Formation comprise a major part of the Tertiary succession in the western part of South Sulawesi (Fig. 2). The aim of this paper is to document platform, slope and deep water carbonate lithologies in the vicinity of Barru (Fig. 1), which preserve evidence of local, contemporaneous tectonic activity. Sulawesi is formed of distinct north-south
trending tectonic provinces (Sukamto 1975). In the west, the north and south arms of Sulawesi are composed of thick Tertiary sedimentary and volcanic sequences overlying pre-Tertiary basement complexes (Sukamto 1975; Van Leeuwen 1981). The Barru area lies within this western arc or province (Sukamto 1975; Hamilton 1979). Central Sulawesi is composed of sheared metamorphic lithologies and in the east a highly tectonized melange complex is present (Sukamto 1975; Hamilton 1979; Parkinson 1991). The eastern periphery of this melange has been overthrust by a dismembered and imbricated ophiolite sequence (Sukamto 1975; Silver et al. 1978; Simandjuntak 1990; Parkinson 1991). Emplacement of this ophiolite and resulting formation of the melange occurred during the middle Oligocene (Parkinson 1991). The microcontinental fragments of the Buton-Tukang Besi Block and Banggai-Sula are thought to have collided with the eastern part of Sulawesi during the early-middle Miocene (Fortuin et al. 1990; Davidson 1991; Smith & Silver 1991) and late Miocene-early Pliocene respectively (Garrard et al. 1988; Smith & Silver 1991). The Tertiary stratigraphy of western Sulawesi is comparable with many of the Tertiary basins in neighbouring east Kalimantan. West Sulawesi, the East Java Sea and east Kalimantan are thought to have comprised a widespread basinal area, the formation of which commenced during the early-middle Eocene (Van de Weerd & Armin 1992).
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 365-389.
365
366
M.E.J. WILSON 8~ D. W. J. BOSENCE
Geology and stratigraphy of South Sulawesi South Sulawesi is structurally separated from the rest of the western arc of Sulawesi by a NW-SE trending depression which passes through the Sengkang Basin (Fig. 1; Van Leeuwen 1981). Geologically and geomorphologically South Sulawesi is divided by a present day N-S trending depression known as the Walanae Depression (Fig. 1). The Walanae Depression has been described as a major left-lateral strike-slip zone (Sukamto 1975; Van Leeuwen 1981). Seismic (Grainge & Davies 1983) and present-day outcrop constraints suggest significant normal displacement on basin-bounding faults occurred during the Tertiary. The Barru area of this study includes the northernmost outcrops of the Tonasa Limestone Formation and is located on the southern margin of the depression passing through the Sengkang Basin, to the west of the Walanae Depression (Fig. 1).
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Two inliers of the pre-upper Cretaceous basement complex of Sulawesi are exposed west of the Walanae Depression (Fig. 1). They comprise tectonic slices of metamorphic, ultrabasic and sedimentary lithologies (Hamilton 1979; Sukamto 1982). Deep marine clastics of the upper Cretaceous Balangbaru and laterally equivalent Marada Formation overlie the basement complexes unconformably (Fig. 2; Van Leeuwen 1981; Sukamto 1982; Hasan 1991). Palaeocene-Eocene volcanics of the Langi Formation and marginal marine siliciclastics, shales and coals of the Eocene Malawa Formation overlie with an angular unconformity the Balangbaru Formation in the eastern and western parts of west South Sulawesi respectively (Sukamto 1982). The upper part of the Malawa Formation interdigitates with shallow marine carbonates of the middle Eocene to middle Miocene Tonasa Limestone Formation to form a transgressive sequence. Deposition of at least 400 m of shallow marine carbonates occurred in the area between Maros and Tonasa II (Fig. 1; Garrard et al. 1989; Crotty & Engelhardt 1993). These sediments of the Tonasa Limestone Formation are thought to have formed on a relatively stable, gently subsiding, large-scale platform c. 80 km across (personal observation; Garrard et al. 1989), named here as the Tonasa Carbonate Platform. An intra-formational midOligocene unconformity occurs within shallowwater carbonates of the Tonasa Limestone Formation in the eastern Biru area (Figs 1 and 2, Van Leeuwen 1981). From the early Miocene onwards in the Biru area there was a deepening of the environment, and carbonate sedimentation was strongly influenced by local tectonism (Van Leeuwen 1981). This study describes and interprets deep marine late Eocene to mid-Miocene carbonates of the Tonasa Limestone Formation, from the Barru area to the north of the Tonasa Carbonate Platform. During the middle to late Miocene carbonate production was terminated by the influx of volcaniclastic deposits of the Camba Formation. These volcaniclastics were derived from a N-S trending volcanic arc which developed in South Sulawesi (Sukamto 1982; Yuwono et al. 1985). East of the Walanae Depression, lithologies are quite distinct from those to the west and the oldest lithologies are of Eocene age (Figs 1 and 2; Sukamto 1975). The lithologies are dominated by volcanics and volcaniclastics of the Salo Kalupang, Kalamiseng and Camba Formations (Sukamto 1982; Yuwono et al. 1985). Eocene shallow-marine carbonates of the Tonasa Limestone Formation outcrop only as a fault-bounded sliver at the eastern margin of the Walanae Depression (Fig. 1; Sukamto 1982).
TERTIARY EVOLUTION OF S SULAWESI
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Structure and stratigraphy of the Tonasa Limestone Formation in the Barru area The Tonasa Limestone Formation in the Barru area is bounded to the south by pre-Tertiary metamorphic and ultrabasic basement lithologies of the Bantimala Block (Fig. 3; Berry & Grady 1987). A smaller inlier (8 km across) of metamorphic basement lithologies, known as the Barru Block (Berry & Grady 1987), is located to the southeast of Barru (Fig. 1). The overall structure of these basement complexes is of relatively rigid blocks tilted to the east and bounded on the remaining sides by eastward dipping thrusts or sinistral wrench faults (Sukamto 1982; Berry & Grady 1989). A number of generally NW-SE trending faults also cut the Tertiary sequence in the Barru area (Fig. 3). The eastern and southeastern flanks of both basement blocks are unconformably overlain by an almost continuous stratigraphic sequence from the Balangbaru Formation through to the Camba Formation. Tertiary angular unconformities occur at the base of the Malawa Formation and in some
367
localities between the Tonasa Limestone and Camba Formations. The Tertiary lithologies dip eastwards at 10-25 ° . North of the Bantimala Block the Tertiary lithologies are folded into a WSW-verging, regional-scale N N W - S S E trending anticline, named here the Rala anticline (Fig. 3). On the northeast limb of the Rala anticline there is a complete stratigraphic sequence from the Malawa Formation through to the Camba Formation, with no apparent unconformities. On the southwestern limb of this anticline Tertiary lithologies dip 20--40 ° to the WSW and the carbonate sequence is considerably condensed or absent. An angular unconformity separates the Camba Formation from the older underlying lithologies on this western limb. Igneous bodies composed of diorite-granodiorite and trachyte (Sukamto 1982) intrude the Tonasa Limestone Formation and older lithologies in a number of localities (Fig. 3). Although the age of the intrusives in the Barru area is not known, similar lithologies from other areas have been dated using K-Ar techniques as middle to upper Miocene (Sukamto 1982). The Tonasa Limestone Formation in the Barru area reaches a maximum thickness of 1100 m in the Rala section (Fig. 4). The basal few metres of the carbonate sequence are interbedded with siliciclastics of the Malawa Formation. The earliest carbonate sediments are composed of wackestones, packstones and floatstones (Fig. 4). Some beds contain a rather limited fauna of miliolids, gastropods and occasional pseudomorphs after gypsum or anhydrite. Other facies contain large alveolinids or broken branching corals as well as miliolids. These lower to middle Eocene facies (T a, T. Wonders 1993 pers. comm.) indicate a normal marine back reef-barrier environment with minor restriction. Usually the initial carbonate sediments pass upwards into thick successions of metre-scale bedded packstones and rudstones composed of abundant large Nummulites and coralline algae. The coralline algae often encrust the large benthic foraminifera to form rhodoliths, suggesting shallow marine, relatively turbulent conditions prevailed (Fig. 5a). In the Rala section only, a subsequent deepening of the environment is indicated by marls and wackestones containing abundant, monospecific, large, flat Discocyclina typical of the lower limits of the photic zone (T. Wonders 1993 pers. comm.). Most areas contain decimetre-scale planar-bedded packages of bioclastic packstones, which include planktonic foraminifera and show occasional evidence of current or wave reworking (Fig. 4). This facies shows an overall fining-upward trend and is interpreted as outer shelf or slope deposits
368
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TERTIARY EVOLUTION OF S SULAWESI Lithology
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with a upper Eocene age (N15, T. Wonders 1993 pers. comm.). In the northern and eastern parts of the Barru area (Fig. 3) these packstones are overlain by a thick sequence of upper Eocene to middle Miocene marls interbedded with redeposited carbonate facies (Fig. 5b), which are the main focus of this paper.
Redeposited carbonate and marl facies of the Barru area Marls
This easily weathered facies is interbedded with coarser, cemented carbonate facies, and is especially abundant in northern and eastern parts of the Barru area. The percentage thickness of the marls in the Tonasa Limestone Formation decreases from the Bangabangae section (E-F; 69% thickness) moving southwards towards the Bantimala Block (Figs 3 & 6). Marls are best exposed in stream cuttings, especially in the Bangabangae section, and although poorly exposed
369
in intervening areas this facies is considered to be laterally continuous between sections. The marls are pale green-grey in colour, poorly cemented and often fissile. Planar lamination on a millimetre scale is a common feature of the marls. In some localities the marls appear more homogeneous, and water-worn surfaces reveal mottling caused by the presence of randomly orientated burrows on a millimetre to centimetre scale. A pelagic biota, including planktonic foraminifera and nannofossils is well preserved and occurs in abundance in this facies. The fine grained laminated nature of the marls indicates that this facies was deposited in a low energy environment largely from suspension. The presence of abundant, well preserved tests of pelagic organisms indicates deposition in a deep marine basinal setting above the carbonate compensation depth. Burrowing in some localities suggests an aerobic environment.
Bioclastic packstone facies
This facies is seen in the upper parts of most sections in the Barru area interbedded with marls and other coarser carbonate facies. The bioclastic packstone facies occurs most abundantly in the Rala section (13.8% thickness of total deeper water facies) and is best exposed in stream sections at this locality (Figs 3 & 6). Although only well exposed in stream cuttings, beds of this facies are thought to be laterally extensive on a scale of tens of metres. In this facies beds are planar-bedded with bed thicknesses varying from 10-90 cm. Bed contacts are usually planar and sharp. The grain size varies from fine to coarse sand grade between adjacent beds. Sedimentary structures were not observed and burrows with millimetre to centimetre diameters are common in this facies. Although delicate tests of planktonic foraminifera are frequently well preserved, more robust fossils such as coralline algae, large and small benthic foraminifera and echinoids are common as fragments. Based on 'bundling' of beds and grain composition, this facies can be subdivided into two. (a) Packages of medium bedded packstone which generally lack intervening marly units were found in only one locality in the Rala section (see below, Fig. 12). Faunal elements contained in a single bed are consistently of one age. This subfacies is very similar to the bedded packstones described above, which underlie the marls (Figs 3 & 4). (b) Single packstone beds interbedded with marls commonly contain both grains which are contemporaneous with and older than the intervening marls. This subfacies often includes
370
M. E. J. WILSON & D. W. J. BOSENCE
Fig. 5. Carbonate facies of the Tonasa Limestone Formation from the (a) lower and (b) upper parts of the Rala Section. (a) Shallow-marine Nummulites and coralline algal rudstone. The coralline algae encrusts the large benthic foraminifera forming rhodoliths. Scale is in centimetres. (b) Typical outcrop of marls interbedded with redeposited carbonate facies. The lower resistant, bioclastic packstone bed is separated from a clast-supported breccia unit by rather easily weathered green-grey marls. Vertical field of view is 1.5 m.
sand grade, angular clasts from all the formations underlying the Tonasa Limestone Formation. A redeposited origin is inferred for this facies because it is interbedded with basinal marls and contains fragmented shallow-water bioclasts. Sedimentary structures which might indicate the mode of reworking are absent. The packages of bioclastic packstones may perhaps have a similar origin to the outer shelf-slope packstones underlying the marls (Fig. 4). In terms of clast content the single beds of bioclastic packstones are comparable with the graded bioclastic packstone facies described below. Non-graded packstone beds are often documented in sequences containing abundant graded packstones and have been interpreted as grain flow or modified grain flow deposits (Lowe 1976; Cook & Mullins 1983; Gawthorpe 1986; Eberli 1987). However, criteria indicating grain flow deposits such as dish structures, inverse grading, diffuse lamination and outsized clasts (Middleton & Hampton 1976; Cook & Mullins 1983) have not been identified in this facies.
Graded bioclastic pack-grainstone facies This facies is well exposed and common throughout the upper part of most sections in the Barru area. Graded, bioclastic pack-grainstone facies
are interbedded with marls. Coarser breccia facies are almost invariably overlain by this facies (see below). The percentage thickness of this facies decreases northwards away from the northern margins of the basement blocks (Fig. 6). Beds of this facies are often laterally continuous on a scale of tens of metres, although a 10% decrease in bed thickness over 5 m has been observed. Bed thicknesses of this facies vary from 5 cm up to 110 cm (Fig. 7). The basal bed contacts are sharp and often planar. Less commonly this facies has an erosive base and exhibits rare groove or flute casts (see below for palaeocurrent data). Beds are graded both in terms of grain size and composition; fining upwards from pebble-coarse sand grade to fine sand-silt grade (Fig. 8a). Heavier gravel or sand grade schist, ultramafic or sandstone clasts tend to be concentrated in the lower parts of beds. Rare water escape structures and imbrication of planar clasts are present. Planar lamination on a millimetre scale is a common sedimentary structure in the upper part of beds (Fig. 8a). The upper bed contact may be planar or slightly undulose and sharp or gradational into marls. This facies incorporates a large variety of often fragmented bioclasts, including large and small benthic foraminifera, coralline algae, echinoderm and very rare coral debris. It contains up to 25% well preserved planktonic foraminifera, especially in the upper finer part of beds. Lithic clasts include
TERTIARY EVOLUTIONOF S SULAWESI
N
,~ Bangabangae section
F m i"~"~"~"~"~'~ 775 ~'1
10km
~
Upper coarse volcaniclastic member of the Camba Fm.
371
5km Rala section
Dm! ~,,~,,~,-~
Lower member of the Camba Fro. - Deep madne shales interbeclded with volcaniclastics
1200
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~ ~
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Total thickness of made & redeposited facies Total number of redeposited beds Clast-supported breccia Graded bioclastic packstones topping the above Marl supported breccia Graded bioclastic packstone Bioclastic packstone Planktonic foraminifera wacke/packstone
Mad
C I I~ s f' m c G P C:
I"'::::::::.Marl '-
: : : : : ~
Rala section 635 rn 173 39.64 11.85 0.40 12.63 13.77 1.10 20.22 0.40
8.82 6.17 7.82 69.02 6.76
% % of limestone conglomerates, packstones and marls 100 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
........
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.........
Ban~labangae section 408 m 76 0.93 0.47
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207,
....
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0 % % Amalgamation of redeposited carbonate beds 100 5O 0 DISTAL ~ '
PROXIMAL
BASlNWARD Fig. 6. Three composite stratigraphic sections through the redeposited facies interbedded with marls showing northwards proximal to distal and basinward trends. See Fig. 3 for location of the stratigraphic sections (A-B, C-D, E-F).
metamorphic (see below) and quartzose sandstone lithologies, a wide range of limestone facies and marl clasts. Both packstone and grainstone textures occur, although in the majority of beds the matrix is marly or composed of finely fragmented bioclasts. Trace fossils on a millimetre to centimetre scale, including Taphrhelminthopsis are common at bed contacts and may be parallel or perpendicular to bedding. Silica replacement is frequent within this facies and occurs in three main forms: as chalcedony within individual bioclasts or litho-
clasts, as nodules and as irregular but apparently continuous 'beds'. Silica nodules are parallel to bedding, up to 15 cm thick and 5 0 c m across and sometimes follow centimetre diameter burrows. Because this facies is interbedded with basinal marls and contains fragmented lithoclasts and shallow marine bioclasts, it is considered to be redeposited. Sedimentary structures such as an erosive base, normal grading and parallel lamination are typical of calciturbidites (Crevello &
372
M.E.J.
W I L S O N ( ~ D, W, J, B O S E N C E
Graded bioclastic pacidgrainstone facies
Planktonic foraminifera wacke/packstone facies
6) U p p e r bed contact is planar or slightly
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T 8
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overlyingUndul°se iand n sharp t O or m gradational a r l s
'i~i~i~i~i~i~i~ '..~.~.~,.~.~.~.~.~.~.~.~. i~i~i~i~i~i~i~'i~i~ ..~ x.~.~ 900
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~.~ NP23 latest early/early late Oligocene
700
/
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470
:, ;, I
i D28 .k-=_;=_;=_;=_;=_;,j,'_-.:-._, .-...-=! D25 I . . . . ~ ; ~ " ,." ~ 11
~ - - ' ~ / ' ~ E n v e l o p e of bed ~' thickness of the _~.~_[~ _carbonate ---redepositedfacies
_i
=lcwI
~= BP - Bioclastic packstone ~ ~ MSB- Marl supported breccia• CSB - Clast supported breccm I GBP - Graded b oc ast c packstone i~ PFW - Planktonic foraminifera wackestone -_A_rk_os_ic packst_one_ J
" ! _
600
~
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~ ") D 0 nn~E] ~ o ~ Carbonate facies
1 2 3 4 5 10152025 Thickness of redeposited carbonate units (m)
I mini Pre late Cretaceous i [r~ t ~ ?Palaeocene/Eocene - - sandstone = [ ] Eocene carbonate I [ ] O h"goce n e carbo n ate i
[ ] Miocene carbonate rI.--I - I Undated carbonate ma Contemporaneous m basinal sediments I [ ] V olcanic casts
:_.:':.,"_'U-.-.,i
',',-&e-,; ~ . . . . . .
,, F~ Lithic clas '~,~ ii ~ .UlOClaSIs ... iI ~ . fraomented H r ~ _. ° . . . . I 1151oclasIs WhOle == jr.
Fig. 12. Point-counting data plotted against the bed thicknesses for the redeposited facies in the Rala measured section. Note the increased percentage of clasts derived from earlier formations or reworked from older shallow marine carbonates of the Tonasa Limestone Formation when the redeposited beds are thickest.
382
M.E.J.
WILSON & D. W. J. BOSENCE
plate reorganization is thought to have resulted in Eocene basin formation, in detail the mechanics of this basin formation are as yet unclear. Initiation and formation of different parts of the basin have variously been related to extensional back-arc formation (Hamilton 1979; Daly et al. 1987, 1991) and a foreland-fore-arc flexural origin (Williams et al. 1988; Hutchison 1989). Two major fault trends are apparent from seismic data in the Makassar Strait offshore Sulawesi (Fig. 13). Approximately NE-SW trending faults have been related to back-arc extension due to roll-back of a plate subducting eastwards under eastern Sulawesi (Daly et al. 1987, 1991; Letouzey et al. 1990). Although Eocene-Oligocene volcanics in eastern South Sulawesi are thought to delimit the plate margin, detailed analysis has not been undertaken and it is not clear if these volcanics are related to subduction (Letouzey et al. 1990; Van de Weerd & Armin 1992). In the Kalosi area upper Miocene-Pliocene igneous rocks have been related to subduction of continental crust (Bergman et al. 1996). Another possibility is that the Makassar Strait extension may represent a failed armextremity of sea floor spreading in the Celebes Sea to the northeast (Hutchison 1988; Moss 1994). The second group of faults trends NW-SE and includes the Adang and Sangkulurang faults (Fig. 13).
NW
TT-2
TT-1
1~ -~:--~
SE
I
~~ z , ~
"-~r-__-S,-_z' % t
km
~
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m TT-2
62 821
4
early to mid- i
.~
"
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1 5 ~
~ ,\\ ~ "~, \ ~\ \\.
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KEY Deepmarine shales+ marls
Redeposited :: ,,mestonetaces
Shallowmarine
Oligocene
~ 2182 to early ~i~~ Miocene ~]
', ,\ ~
i [] ,
~
elastics(Marginal). marine
~ ~ ~
clastics
Non marine Basement
...................
~ ~. ~ ~~ ~J,~i
2892. . . . . . . . . . 3055 Eocene 3238 no recovery 3560 . . . . . . . . . . ]~P'--7] Cretaceous-inferred
Fig. 14. Seismicsection and borehole data from the inverted Taku Talu fault, Makassar Straits (after Situmorang 1987). See Fig. 13 for location of boreholes.
Offshore analogue o f the northern faulted margin The late Eocene to middle Miocene redeposited carbonate facies in the Barru area are inferred to have been derived from the faulted northern margin of the Tonasa Carbonate Platform. An analogous example of syn-tectonic carbonate sedimentation is revealed from seismic and borehole data in the Makassar Straits, from the vicinity of the NE-SW trending Taku Talu fault (Fig. 14). Eocene to middle Miocene carbonate breccias containing clasts from the underlying basement were deposited as a thickened hanging wall sequence, whilst shallow-water carbonate sedimentation continued on the footwall block of the Taku Talu fault (Fig. 14). Thus the Taku Talu fault was a major active Eocene to early Miocene normal fault which underwent later inversion during the middle Miocene (Situmorang 1987). ~
Outcropping Eocene to Holocene.TT-2 Boreholelocation(seeFig. 14) [ sedimentary basin fill pre-Cretaceous continentalcrust ..-,.-j Subduct-iotick nFaUl markSzone t downthrowside
Fig. 13. Map to show the location of Tertiary basins in Kalimantan and western Sulawesi. Modified after Van de Weerd & Armin (1992).
A d a n g fault The Adang fault is a major NW-SE trending structure separating the North and South Makassar Basins (Fig. 13). This lineament is thought to extend westwards onshore forming the southern
TERTIARY EVOLUTION OF S SULAWESI boundary of the Kuta Basin and has been linked across Borneo with the Lupar Line forming a 'trans-Borneo shear' (Woods 1985; Kusuma & Darin 1989; Wain & Berod 1989; Bransden & Matthews 1992; Van de Weerd & Armin 1992). Both strike-slip (Woods 1985; Hutchison 1989; Biantoro et al. 1992) and normal displacements (Biantoro et al. 1992; Rangin et al. 1990) on the Adang fault have been inferred. It has been suggested that the Adang fault was an important Eocene to early Miocene normal fault downthrowing to the north, which subsequently underwent strike-slip reactivation during the middle-late Miocene (Kusuma & Darin 1989; Rangin et al. 1990; Biantoro et al. 1992). During the late Eocene to Miocene the Adang fault had a strong influence on sedimentation patterns. The Paternoster Platform and Barito Basin were the sites of shallow-water carbonate development, whilst deeper sedimentation occurred in the Kuta Basin to the north (Van de Weerd et al. 1987; Kusuma & Darin 1989; Wain & Berod 1989). Seismic data across the Makassar Straits (D. Coffield pers. comm. 1995) suggest there was no fault linkage between the NW-SE trending Adang Fault and the main north-bounding fault to the Tonasa Carbonate Platform. It is suggested that NW-SE trending structures influenced sedimentation patterns in Sulawesi and Kalimantan. NNESSE trending faults such as the Walanae Fault also had an effect on Tertiary sedimentation patterns in western Sulawesi (Van Leeuwen 1981; D. Coffield pers. comm. 1995). Seismic data N-S across the Sengkang Basin reveal normal block-faulting, with blocks being downthrown to the centre of the basin (A. Ngakan pers. comm. 1994). Indeed, the northbounding faults to the Tonasa Carbonate Platform and faults with a similar trend to the east may mark the southern boundary of the NW-SE trending depression, which runs through the Sengkang Basin and appears structurally to separate South Sulawesi from the rest of the western arc of Sulawesi (Fig. 1). Timing
Redeposited carbonate facies in the Barru area suggest three phases of tectonic activity in the area during the late Eocene-early Oligocene, earlylate Oligocene and early-middle Miocene. Other evidence for possible tectonic activity in the region is documented below and possible causes are discussed (Fig. 15). E a r l y - m i d Eocene. A basal angular unconformity
to many initially transgressive Tertiary sequences marks widespread basin initiation (Cater 1981; Pieters et al. 1987; Van de Weerd et al. 1987;
383
Kusuma & Darin 1989; Wain & Berod 1989; Letouzey et al. 1990; Bransden & Matthews 1992; Hutchison 1992; Van de Weerd & Armin 1992). The angular unconformity between the Malawa Formation and underlying Balangbaru Formation is one such contact. Basin initiation possibly occurred as early as the Palaeocene in some localities (Bishop 1980; Cater 1981; Wain & Berod 1989). Carbonate production, including the deposition of the Tonasa Limestone Formation, had begun in many areas by the middle to late Eocene. Active faulting and graben formation has been inferred from seismic (van de Weerd et al. 1987; Bransden & Matthews 1992) and outcrop data (Tyrrel et al. 1986; Kusuma & Darin 1989). Basin formation is thought to be a response to widespread Eocene plate reorganisation. Late E o c e n e - e a r l y Oligocene. This period corre-
sponds with the first phase of coarse redeposited facies in the Barru area, indicating the initiation (or reactivation) of faulting on the margin of the Tonasa Carbonate Platform. The geometry and timing on major faults from regional seismic data suggest that localized extension was underway by the early Eocene (P9/P10), and rifting was widespread by the late Eocene (Van de Weerd et al. 1987; Letouzey et al. 1990; Bransden & Matthews 1992). The major variation in preserved thicknesses across faults, and hence syn-tectonic deposition is observed during the late Eocene and early Oligocene (Bishop 1980; Bransden & Matthews 1992). During the late Eocene a significant tectonic event caused widespread high angle faulting and subsequent erosional truncation affecting a number of areas (Bransden & Matthews 1992). This period is thought to represent the main extensional phase in the region (Letouzey et al. 1990; Bransden & Matthews 1992). The middlelate Eocene is also the age of the oldest oceanic crust in the Celebes Sea (Weissel 1980). Middle Oligocene. A mid-Oligocene unconformity
has been reported from many offshore (Bransden & Matthews 1992) and onshore areas (Van Leeuwen 1981; Kusuma & Darin 1989; Van de Weerd & Armin 1992). This unconformity, the major shift in facies belts, localized channelling and thickness variations have been related both to the mid Oligocene eustatic lowstand (Bransden & Matthews 1992) and tectonic activity (Cater 1981; Van de Weerd et al. 1987; Wain & Berod 1989; Sailer et al. 1992; Van de Weerd & Armin 1992). Sailer et al. (1992) reported exposure and subsequent deepening prior to the middle Oligocene (29.5-30 Ma) sea-level fall of Haq et al. (1987). In Sulawesi the second phase of coarse redeposited facies in the Barru area, an unconformity in the
384
M.E.J. WILSON (~ D. W. J. BOSENCE W.Kalimantan Basins
Age
Upper Kutai Basin
SE.Kalimantan Basins
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Redeposited carbonate facies
[~
~ (Marginal) Shallow marine limestones [,_'1 clastms
marine
in
~ Non marine I',','i clastics
Fig. 15. Tertiary stratigraphic correlation chart, showing similarities between the sequences in Kalimantan, East Java Sea and Sulawesi. Data taken from Bishop (1980), Cater (1981), Van de Weerd et al. (1987), Kusuma & Darin (1989), Wain & Berod (1989), Bransden & Matthews (1992) and Van de Weerd & Armin (1992).
Biru area (Van Leeuwen 1981) and minor localized evidence for exposure on the Tonasa Carbonate Platform (personal observation) during the mid Oligocene, all suggest a tectonic cause. During the Oligocene sea floor spreading began in the South China Sea (Holloway 1982; Ru & Pigott 1986; Daly et al. 1991) and the east Sulawesi ophiolite was emplaced (Parkinson 1991). Early to middle Miocene. In the Barru area west-
verging folding affects both the Tonasa Limestone Formation and the overlying middle-late Miocene volcaniclastics. The latest Oligocene to middle Miocene phase of the Bantimala Redeposited Facies therefore records a period of faulting and uplift prior to folding (and possible inversion). Based on seismic evidence from the east Java Sea and Makassar Strait, contemporaneous faulting and syn-tectonic graben fills have been inferred until the early Miocene (Bishop 1980; Cater 1981, Letouzey et al. 1990). Inversion of many of the earlier faults with normal displacement occurred slightly later during the early to middle Miocene (Fig. 14; Letouzey et al. 1990; Bransden & Matthews 1992). This compressive regime, active
up to the present-day, is widely attributed to the collision of microcontinental fragments onto eastern Sulawesi during the middle to late Miocene (Daly et al. 1987, 1991; Van de Weerd & Armin 1992). The middle to late Miocene is also the time when a volcanic arc developed in western Sulawesi (Sukamto 1975; Yuwono et al. 1985; Coffield et al. 1993; Bergman et al. 1996). Similar to the major bounding faults of the north margin of the Tonasa Carbonate Platform, many of the faults mentioned above had major normal displacements. Inferring an additional strike-slip motion would depend on along-strike linkage of faults, a factor which is difficult to ascertain from the available seismic and outcrop information. A transtensional Palaeogene history for the region has been suggested on the basis of the length of faults, apparent reversal and observed variability of fault trends (Bishop 1980; Bransden & Matthews 1992).
C o n c l u s i o n s
Late Eocene to middle Miocene redeposited facies of the Tonasa Limestone Formation in the Barn]
TERTIARY EVOLUTION OF S SULAWESI area provide a unique example of the use of carbonate sedimentology in comparing local and regional tectonic events. In the shallow-water carbonates of the Tonasa Carbonate Platform to the south, tectonic effects are impossible to distinguish from eustatic effects. However, clast composition and facies types in the redeposited carbonates reveal that tectonic and not eustatic changes were the main controls on sedimentation. In fact, redeposited carbonates in the Barru area prove to be a remarkable natural seismograph, recording regional tectonic changes and a number of local and regional conclusions can be inferred from them. (1) The spatial distribution, b a s e m e n t clast content and p r o x i m a l - d i s t a l trends within redeposited facies suggest derivation from two main source areas. These areas were from the northern margins of the Bantimala and Barru Blocks. (2) The textural immaturity and provenance of clasts indicate that the redeposited facies were derived from the faulted margins of a carbonate platform. Redeposited facies were derived from at least two faulted line-sources and were deposited as sheet flows forming carbonate aprons at the base of the slope. (3) An east-dipping relay ramp between two main N W - S E trending faults is the preferred configuration of the northern margin of the Tonasa Carbonate Platform. (4) The redeposited facies indicate three phases of tectonic activity. These occurred during the late
385
Eocene to early Oligocene, the middle Oligocene and the early to middle Miocene. This is consistent with other similar sequences in Kalimantan and the East Java Sea and with inferred regional plate tectonic changes. (5) The northern faulted margin of the Tonasa Carbonate Platform has a similar trend to N W - S E orientated structures in the Makassar Straits and Kalimantan. These structures are inferred to have i n f l u e n c e d s e d i m e n t a t i o n patterns during the Eocene to middle Miocene. The senior author gratefully acknowledges BP Exploration, UK for their generous financial support during the course of her PhD study, of which this work forms a part. The SE Asia Research Group, Royal Holloway, University of London, especially Dr Tony Barber and Diane Cameron, are thanked for their administrative and technical support. In Indonesia, Alexander Limbong, the senior author's counterpart from GRDC, Bandung, during three gruelling 'non-stop' field seasons deserves special thanks. GRDC, Bandung, Kanwil, South Sulawesi, particularly Darwis Falah and family, BP offices in Jakarta and Ujung Pandang and LIPI all provided technical and practical support. Dr Ted Finch and Prof. Fred Banner at University College London, and Dr Toine Wonders, Consultant, UK, are thanked for their excellent biostratigraphic work. The constructive comments from referees Dr Dana Coffield and Dr Neil Harbury and those by Dr Tony Barber, Rob Bond, Nigel Deeks and Dr Steve Moss towards improving this paper were much appreciated. Keith Denyer is thanked for producing the photographic plates.
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Tertiary Tectonic and magmatic evolution of western Sulawesi and the Makassar Strait, Indonesia: evidence for a Miocene continent-continent collision STEVEN
C. B E R G M A N
1, D A N A
& RICHARD
Q. C O F F I E L D 2, J A M E S A. G A R R A R D
E T A L B O T l, 3
4
1ARCO Exploration & Production Technology, Exploration Research & Technical Services, 2300 W. Plano Pkwy., Plano TX 75075-8499, USA. e-mail
[email protected] 2 Atlantic Richfield Indonesia, The Landmark Center, Tower B, Jl. Jend Sudirman, Kav. 70A, Jakarta, Indonesia 12910. e-mail
[email protected] 3Now at: KT Geoservices, 661 N. Plano Rd, Suite 317, Richardson TX 75081, USA 4 ARCO International Oil & Gas Co., Asia-Pacific New Ventures Exploration, 2300 Plano Pkwy., Plano TX 75075-8499, USA. Now at: ARCO Alaska, 700 G Street, Anchorage AK 99701, USA Abstract: New field and laboratory data from western Sulawesi, Indonesia, integrated with available data establish its Late Cenozoic igneous framework and a new model for its tectonic evolution. Western Sulawesi contains three major Neogene N-S-trending tectonic domains (from W to E): (1) an active foldbelt, in which Pliocene and Miocene volcanogenic rocks are involved in W-vergent thrusting which extends into the Makassar Strait; (2) a central belt comprised of a deformed submarine Miocene volcanoplutonic arc built on an Oligocene-Eocene clastic and carbonate platform with Latimojong Mesozoic basement metamorphic and sedimentary rocks thrust over its eastern margin on W-vergent faults; and (3) an accreted Cretaceous-Palaeogene(?) ophiolite (Lamasi Complex) between the Latimojong basement block and Bone Bay. The Lamasi Complex ophiolite includes dioritic plutons, basaltic sheeted dykes, pillow lavas, greenstones, tufts and volcanic agglomerates with depleted (MORB-like) Sr & Nd isotope and REE characteristics of probable normal oceanic crust with possible subduction-related or back-arc affinity. New K-Ar, 4°Ar-39Ar, Rb-Sr, and Nd-Sm isotope data suggest Cretaceous to Eocene crystallization and Oligocene to Miocene obduction. Late Miocene to Pliocene extrusive and intrusive rocks form a cogenetic volcanoplutonic complex of calc-alkalic to mildly alkalic, potassic, and shoshonitic felsic and mafic magmatic rocks of bimodal composition which were erupted and intruded during a short episode of Middle Miocene to Pliocene (3-18 Ma) lithospheric melting. Based on new Rb-Sr, Nd-Sm, and U-Pb isotope, and major and trace element geochemical data, parental source rocks of the Miocene melts were Late Proterozoic to Early Palaeozoic crustal and mantle lithospheric assemblages which became heated and melted owing to a continent-continent collision in which west-vergent continental lithosphere derived from the Australian-New Guinea plate was subducted beneath eastern-most Sundaland. The timing of this magmatism and subsequent cooling and denudation history are constrained by 113 new K-Ar, 4°Ar-39Ar, and fission track ages. The new tectonic model differs significantly from previous models: 'the Makassar Strait is now interpreted as a foreland basin bound on both sides by converging Neogene thrust belts, in contrast to previous models suggesting Late Tertiary oceanic spreading or continental rifting. West-vergent obduction of a pre-Eocene oceanic, primitive arc, or back-arc crust onto western Sulawesi occurred during late Oligocene to Miocene times. The Late Miocene western Sulawesi magmatic arc is envisioned as a continent-continent collision product, in contrast to previous models involving a normal ocean--continent or ocean-ocean subduction-related magmatic arc (west or east vergent) or post-subduction rifting. The east Sulawesi ophiolite extends into western Sulawesi, suggesting that Bone Bay resulted from collapse of the overthickened Miocene orogen. The new tectonic model illustrates the central role western Sulawesi plays in unlocking the complex evolution of Indonesia as well as the temporal and magmatic details of a continent-continent collision zone.
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolution of SoutheastAsia, Geological Society Special Publication No. t06, pp. 391-429.
391
392
s.C. BERGMAN ET AL.
The objective of this paper is to integrate new and published geological, petrological, isotopic, stratigraphic, structural and geochemical data for western Sulawesi in order to test available models and formulate a new tectonic model for the region. Sulawesi is located in a complex tectonic position, at the intersection of three major lithospheric plates: the westward-moving Pacific Plate, the northward-moving Australian Plate and the relatively stationary Eurasian Plate (Fig. 1). The complex plate tectonic position of Sulawesi is manifested in a varied Tertiary structural and stratigraphic record (Figs 1-3). The south arm of Sulawesi is dominated by Miocene and younger volcanic and plutonic rocks forming a magmatic belt that many workers have regarded as a subduction-related volcanic arc involving a westwarddipping (Sukamto 1978; Hamilton 1979), or eastdipping (Katili 1978) oceanic plate. Some recent workers interpret the magmatic arc as a postcollisional rift-related magmatic belt (Yuwono et al. 1988b; Leterrier et al. 1990; Kalavieris et al. 1992; Priadi et al. 1994). The present study presents new field and laboratory evidence based on three seasons of geological research (19901993) associated with the exploration for hydrocarbons in the Kalosi Production Sharing Contract (PSC) (see Coffield et al. 1993 for a summary of the petroleum systems framework). The results demonstrate that previous 'conventional' tectonic models for the Late Tertiary evolution of western Sulawesi and the adjacent Makassar Strait require significant modification. The authors propose that the Miocene to Recent tectonic framework of western Sulawesi and Makassar Strait was dominated by compressional continent-continent collision processes.
Geological and tectonic framework Sulawesi is located at the southeast limit of the Sunda Platform crustal domain and north and west of the Australian Continental Platform, its probably derivative fragments forming Irian Jaya, Sula, Buru, Seram and the Tukang Besi Platform. Sulawesi formed along the Oligocene-Miocene collision zone between the Eurasian Plate and micro-continental fragments derived from the Indian-Australian Plate (Hamilton 1979; Hutchison 1989a; Rangin et al. 1990; Daly et al. 1991). The four arms of Sulawesi and adjacent islands form four distinct megatectonic provinces. The northern arm is composed of late Palaeogene to Neogene subduction related volcanic arc rocks resulting from the west-dipping subduction of the Molucca Sea Plate (Jezek et al. 1981; Bellon & Rangin 1991). The east and southeast arms are
composed of Mesozoic and younger allochthonous metamorphic and ophiolitic rocks which were obducted during the Oligocene epoch (Parkinson 1991). The south arm is dominated by Miocene and younger volcanic and plutonic rocks which form a magmatic belt superimposed on the Mesozoic basement of the southeastern margin of Sundaland (Katili 1978; Silver et al. 1983a, b). The fourth megatectonic province contains Late Palaeozoic and Mesozoic Australian-derived microcontinents which have been accreted to the eastern margin of Sulawesi, comprising Banggai, Sula, Buton, Kabaena and Tukang Besi, among other islands. The present work concerns parts of the south and north arms of Sulawesi near their boundary in an area approximately bound by Mamuju, Palopo, Parepare and Barulatong (Fig: 2), which will be referred to as 'western Sulawesi' for simplicity. The present-day tectonic framework of western Sulawesi is dominated by active uplift, and compressional and strike-slip faulting. The region is seismically active and nearly all historic earthquake hypocentres are < 50 km below the surface (cf. USGS National Earthquake Information Center, Boulder, Colorado, PDE database which shows > 40 shallow events between November 1929 and July 1994 with Richter magnitudes 3-5 in the Mamasa-Parepare areas). The study area is 500-800 km north of the Sunda-Banda volcanic arc/subduction zone in which the Australian Plate is being subducted beneath Sumbawa, Flores and the Flores Sea. Miocene and younger collision of Australian continental crust has resulted in the accretion of Timor to the upper plate of Eurasia and a south-dipping thrust zone has developed north of Flores in response to the continued compression. Several major NNW trending strike-slip faults cut through the south arm; these faults have traditionally been interpreted as sinistral from field-based structural observations (Berry & Grady 1987) and LandSat imagery interpretations (M. Crawford pers. comm. 1991), although there is evidence for right-lateral offset on some faults. Our recent work has shown that some of these strike-slip faults are transpressional and possess recent compressional offset. Late Tertiary compressional structures include oblique-trending folds and shallow-angle thrust faults. The presence of several Quaternary basins within and adjacent to the south arm of Sulawesi indicate the presence o f Late Tertiary subsidence, possibly representing piggy-back basins adjacent to thrust sheets, or due to transtensional processes. Geological data relevant to the study area include published 1:1 000000 and 1:250000 scale geological maps (Djuri & Sudjatmiko 1974; Ratman 1976; Sukamto 1975, 1982; Ratman & Atmawinata 1988) and various literature information (e.g. van
Fig. 1. Present-day tectonic elements of Indonesia showing the distribution of active trenches, Quaternary subduction-related volcanic arcs, and major structures. L~
394
S.C. BERGMAN ET AL.
Bemmelen 1949; Audley-Charles 1977; Hamilton 1979; Sasajima et al. 1980; Van Leeuwen 1981; Garrard 1989; Hasan et al. 1991; Coffield et al. 1993), which are summarized in a geological sketch map in Fig. 2 and in a schematic stratigraphic section in Fig. 3. Geological studies in western Sulawesi include early Dutch work initiated during the first part of this century (Abendanon 1915; t'Hoen & Ziegler 1917; Reyzer 1920; Sax 1931a, b; Sung 1948). More recent pubfished information on geochronology and petrology of volcanic and plutonic rocks of western Sulawesi include Sasajima et al. (1980), Van Leeuwen ( 1981 ), Sukamto & Simandjuntak (1983), Leterrier et al. (1990), Kalavieris et al. (1992), Yuwono 1987, Yuwono et al. (1988), Van Leeuwen et al. (1994) and Priadi et al. (1993, 1994). The stratigraphy of western Sulawesi is dominated by Neogene rocks but also includes formations as old as Jurassic (Fig. 3). Five main lithostratigraphic sequences are recognized (Garrard et al. 1992; Coffield et al. 1993): (1) Mesozoic (mainly Late Cretaceous) metasedimentary rocks of the Latimojong, Balangbaru and Marada Formations, which include flysch deposits believed to have formed in a forearc basin setting, and ophiolites of the Lamasi Complex CretaceousPalaeogene?); (2) a Palaeogene (Middle to Late Eocene) syn-rift sequence composed of siliciclastic, coal, volcanic and carbonate sedimentary deposits of the Toraja and Malawa Formations; (3) an Eocene to Middle Miocene carbonate post-rift sequence including the Makale and Tonasa Formations; (4) a Middle Miocene to Pliocene basaltic to dacitic volcano-plutonic and epiclastic sedimentary sequence including the Enrekang and Camba Volcanic series and the Buakayu Formation, and consanguineous granitic to gabbroic intrusives; and (5) Pliocene and younger synorogenic, nonmarine to upper bathyal sedimentary deposits including the Walanae Formation, formed during widespread thrusting, regional uplift and erosion. These five sequences occur in a variety of structural positions. The older sequences are typically structurally highest in the sections and in the hinterland to the east; the younger sequences occur in structurally lower positions than in the sequences towards the west or foreland. Importantly, the western promontory of the south arm of Sulawesi (near 3°S latitude) is a relatively recent geomorphological product, resulting from uplift due to Neogene thrusting which extends for 200-300 km from the Majene foldbelt in the west to the Kalosi foldbelt in the east. Makassar Strait
The Makassar Strait is characterized by a central
deep-water trough with depths as great as 15002500 m. In contrast to most of the Makassar Strait margins which contain a broad shelf, the western Sulawesi margin north of Majene possesses a very narrow shelf (< 10-20 km) and steep slope. The tectonic evolution of the Makassar Strait has been the subject of considerable debate since Alfred Russell Wallace delineated his Australian-Asian faunal boundary along its axial trend in 1858 (for a Wallace Line review, see George 1981). Most workers have incorporated phases of Palaeogene, Miocene or Quaternary rifting which resulted from NW-SE extension along the Makassar Strait (Katili 1971, 1975, 1978; Carey 1976; Audley-Charles 1977; Hamilton 1978, 1979; Burollet & Salle 1981; Situmorang 1982a, b, 1984; Faugeres et al. 1989; Shaw & Packham 1992; Effendi 1993, among many others)i Rifting is thought to have involved oceanic (Hamilton 1979) or continental crust (Burollet & Salle 1981). Malecek et al. (1993) proposed that the Strait is underlain by trapped Cretaceous oceanic crust. Alternative models involving a foredeep to the western Sulawesi orogen have been advanced by van Bemmelen (1949), although this interpretation has largely been neglected by recent workers, until now. Most recent interpretations tend to draw long continuous normal faults down the axis of the Strait, or attempt to close up the basin by matching the Sulawesi western coastline with the eastern coastline of Borneo (Katili 1978). Whereas this procedure may work well for passive margins such as West Africa and east South America, the recently active (15 Ma to present) fold and thrust belt of western Sulawesi has produced a rapidly evolving Neogene coastline which does not lend itself to coastline matching. Seismic sections orientated NW-SE show the salient tectonostratigraphic sequences and structural styles. Whereas Eocene extensional faulting is recorded in certain deeper parts of the sections, Oligocene passive platform conditions are present, and Late Miocene to Pliocene fold and thrust belt development is apparent (Fig. 4). The leading edge of the thrust front is evident on the seismic section, as is a series of stacked thrust sheets with a series of overlying piggy-back basins. These piggy-back basins contain distinct stratigraphic packages which record the movement of the underlying thrust sheets. Contemporaneous sedimentary packages in front of the leading thrust are entirely parallel bedded and undeformed whereas those in the piggy-back basins contain diverging reflection packages which onlap the adjacent growing ramp anticlines. The Samarinda anticlinorium, a fold and thrust belt on the opposite (western) side of the northern Makassar Strait mirrors the Majene foldbelt. The Samarinda anti-
MIOCENE W SULAWESI CONTINENT
395
COLLISION
120" 00'
KALOSI
PSC.
03*
03 ° 30'
0
10
20
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1
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1
VOLC/LNICS& CLAt~I'ICS L EOCENE - M. MIOCENE
INTRUSIVES
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INDEX MAP llllllll,i
i PRE TERTIARYBASEIkrcNT LATIMOJOI~
I
i
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LAMASl COMPLEX OPHIOLITE
AGE ?
..........
I
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I
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Fig. 2. Generalized geological map of western Sulawesi.
clinorium is a Neogene east-vergent compressional trend which has developed in the Kutei Basin along strike to the north of the west-vergent Meratus Mountains orogen. The origin of these structures
is controversial and calls upon inversion of preexisting rift structures (Biantoro et al. 1992) or gravity sliding from the hinterland of the Kutei Basin (Ott 1987; van de Weerd & Armin 1992).
S.C. BERGMAN ET AL.
396
STRATIGRAPHY FORMATION
LITHOLOGY
TECTONICS
Ma
0 ,==.
40
:)CENE
~- sroc. p, ~C~.Et •
WALANAE
FM.
NEW K-Ar ages 10 ,
,
.
,
,
,
.
.
.
,
.
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F
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OPHIOLITE OBDUCTION
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,
,
,
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POST-RIFT
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55
60
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m
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~,r~
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VVV
+
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Fig. 3. Stratigraphic column of western Sulawesi.
Fig. 4. Interpreted E-W seismic section 201 through the Makassar Strait (after Coffield et al. ]993).
,
,
~T
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I
~- /
°F-
~ ,.,.~
°ILl I @
° Z _1
MIOCENE W SULAWESICONTINENT COLLISION Herein, the Makassar Strait is interpreted as a present-day foreland basin bound on both sides by converging Neogene fold and thrust belts.
Nature and distribution of CretaceousPalaeogene ? and Neogene igneous rocks Two groups of igneous rocks are distinguished on the basis of field relations, age, composition, petrology and isotope and trace element geochemistry: Cretaceous-Palaeogene(?) ophiolitic rocks of the Lamasi Complex and MiocenePliocene igneous rocks of the Camba-EnrekangMamasa Complex. Lamasi Complex ophiolitic rocks are intensely deformed, typically metamorphosed and mainly bounded by thrust faults. Associated sedimentary deposits include red cherts and mudstones. The Lamasi Complex ophiolites include a variety of intrusive and extrusive lithologies. Intrusives such as sheared diorite plutons (> 20 km diameter), amphibolite gneisses (metagabbros) and sheeted basalt dyke complexes have been recognized. Extrusives include basaltic pillow lavas, basalt to andesite lava flows and agglomerates, and andesite, dacite and rhyolitic pyroclastic deposits comprising block breccias, tuff breccias and tuffs, Neogene igneous rocks of the CambaEnrekang-Mamasa Complex and derivative volcaniclastic deposits cover more~han 75% of the surface of western Sulawesi (Fig. 2). Volcanic outcrops consist of massive lavas and pyroclastic sections with > 5 km thicknesses; plutons and laccoliths of batholithic dimensions (> 50-100 km diameter) are intimately associated with this volcanic assemblage. Volcanic and intrusive sequences are variably deformed; many volcanic sequences are steeply dipping to vertical and batholithic-scale laccoliths are detached and bounded by thrust faults. The volcanic rocks range from basalt to rhyolite in composition, but are dominated by basaltic to dacitic stratovolcano complexes, lava domes and flows, and pyroclastic sequences. Plutonic rocks range from gabbroic and dioritic stocks, sills and dykes to granitic-quartz monzonitic to monzodioritic stocks, plutons and large laccoliths. Lamprophyre dykes are common. The age of the Camba-Enrekang-Mamasa igneous sequences is mainly Middle to Late Miocene (averaging 8 _+2 Ma), although the total age range observed is 3-18 Ma (see below).
Analytical results and interpretation Petrology A summary of lithological and location data for intrusive and extrusive rocks included in the
397
present study is provided in Table 1. (For more details see table 1 of Supplementary Publication No. SUP 18099 (11 pp) available from the Society Library or the British Library Document Supply Centre, Boston Spa, Wetherby, W. Yorks LS23 7BQ, UK.) Analytical methods and details are provided in the Appendix. The locations of various types of analytical data are summarized in Figs 5-7. Modal analyses of eight selected phaneritic Miocene intrusives of the Camba-EnrekangMamasa Complex (Table 2) range from quartz monzonite and monzonite to diorite in the IUGC classification scheme. The Miocene Camba-Enrekang-Mamasa Complex comprises basaltic and gabbroic lithologies rich in augite, through andesites/diorites rich in augite and amphibole with rare enstatite, to granitic-monzonitic lithologies relatively poor in quartz and rich in K-feldspar, albite and biotite. The most primitive basaltic lithologies are phlogopitebearing lamprophyres (olivine minettes) and alkali gabbros. Volcanic rocks are dominantly subalkaline to shoshonitic, with leucite or analcime observed in many trachytes or trachyandesites. Palaeogene ophiolites of the Lamasi CoMplex include a variety of intrusive and extrusive lithologies. Intrusive rocks include basalt, diorite and gabbro of varying degrees of metamorphism (greenschist-amphibolite grade). Extrusives are dominantly basalt, andesite and dacite.
Major and trace element geochemistry Major and trace element compositions of 59 samples define specific compositional groups and trends for the Lamasi and Miocene complexes, illustrated in Harker variation diagrams and compositional plots (Figs 8-14) and REE and spider diagrams (Figs. 11-12) (selected averages are listed in Table 3; detailed analytical data are in table 2 of Supplementary Publication SUP 18099 (see above)). The Miocene igneous rocks range from basalt to rhyolite in the TAS classification scheme (Fig. 8), and are dominated by high K calc-alkalic and shoshonitic series (see Bergman 1987 for an overview of K-rich alkaline igneous rocks). On an AFM diagram, the Miocene igneous suite defines a calc-alkaline trend (Fig. 10). The presence of basaltic trachyandesites, trachyandesite, and borderline trachyte-dacite compositions indicates an alkaline affinity of the intrusive and extrusive units. Compositions are dominated by relatively alkaline basaltic and dacitic rocks, with a conspicuous lack of andesites. Miocene intrusive and extrusive lithologies are geochemically similar and define the same trends on major element cross plots (Fig. 9). Compositions of Miocene intrusive and extrusive rocks from a
lithology
phlogopite olivine basalt dyke arkosic sandstone alkali gabbro sill syenogabbro sill basalt crystal lithic lapilli tuff basalt crystal lithic lapilli tuff graded basalt crystal lithic lapilli tuff trachyandesite crystal lithic lapilli tuff monzodiorite pluton quartz monzodiorite pluton andalusite siltstone homfels basalt block in flow breccia quartz monzodiorite pluton monzodiorite stock dacite porphyry lava dacite porphyry lava coarse crystal lithic dacite tuff crystal lithic dacite lapilli tuff welded crystal lithic dacite lapilli tuff dacite porphyry lava quartz monzodiorite pluton quartz monzodiorite pluton
sample
90SUL1 90SUL2 90SUL3 90SUIAa 90SUL5 90SUL6a 90SUL6b 90SUL7 90SUL8 90SUL9 90SUL10a 90SUL 11 90SUL12 90SULI3 90SUL14 90SUL15 90SUL16a 90SUL 18a 90SUL18b 90SUL20 90SUL21 a 90SUL21 b
Table 1. Sample location summary
Awang R. Awang R. Sipe Hill Tou R. Subing R. Sadang R. Sadang R. Sadang R. Palopo pluton Palopo pluton Palopo pluton aureole Bambulu R. Palopo pluton Rantepao stock Mt. Tambakkuku Mt Tambakkuku Mt Tambakkuku Batu R. Batu R. Loca R. Mamasa Mamasa
locality Miocene Miocene Miocene Miocene Miocene Miocene Miocene Miocene Miocene Miocene Cretaceous Palaeog./Cret Miocene Miocene Miocene Miocene Miocene Miocene Miocene Miocene Miocene Miocene
age 290 295 290 290 730 830 830 830 1070 1050 250 10 475 835 400 365 400 100 100 120 1100 1100
(m)
elev
3 3 3 3 3 3 3 3 2 2 2 2 2 2 3 3 3 3 3 3 2 2
deg 11.9 11.8 11.7 11.3 9.9 9.9 9.9 7.6 57.4 56.8 57.1 50.6 57.5 57.2 30.7 28.9 28.9 33.2 33.2 34.0 56.4 56.4
min
lat S
119 119 119 119 119 119 119 119 120 120 120 120 120 119 119 119 119 119 119 119 119 119
deg
43.9 43.8 43.6 43.7 44.7 45.8 45.8 46.9 4.2 4.9 7.5 7.2 5.7 58.9 47.0 47.1 47.6 43.5 43.5 42.3 22.7 22.7
min
long E
bp b b
bh h b bs bp
b bh bh
b b
K
A
1 1 1 1 1 1 1 l 1
i i i i i i i i
i
G
Analyses
z a
sn
P
az
snp
a
Z
sn
sn
az z
sn
a
sn snp
sn
z a
azs
sn
z sn
z
sn
a sn
sn sn sn
sn
I
~D
90SUL22a 90SUL22b 90SUL23 90SUL24 90SUL25 90SUL26a 90SUL26b 90SUL26c 90SUL26d 90SUL27 90SUL28 RAG/90/73 RAG/90/74 92SUL1 92SUL2 92SUL3 92SUIA 92SUL5 92SUL6 92SUL7 92SUL8 ND92/29 RAG92-004 RAG92-006 RAG92-008
monzonite pluton fluvial alluvium draining pluton dacite porphyry lava vitric rhyolite lava quartz monzodiorite pluton andesite crystal lithic lapilli tuff andesite crystal lithic lapilli tuff andesite block in tuff breccia andesite block in tuff breccia trachyte porphyry dome quartz monzonite gneiss clast in Walanae fine grained volcaniclastic sandstone siliciclastic sandstone vertical dacite lava diorite/gabbro xenolith in dacite lava vertical dacite lava thrust sheet horizontal dacite lava horizontal dacite lava horizontal dacite tuff dacite pyroclastic flow lapilli tuff vertical Eocene sandstones dacite lava dome tuffaceous sandstone trachyte/shoshonite tuff breccia leucite shoshonite lapilli tuff
Balakalua Balakalua Buntubuntu Sumarorong Polewali pluton Pare Pare stratavolcano Pare Pare stratavolcano Pare Pare stratavolcano Pare Pare stratavolcano Allakuang quarry Enrekang Sadang River Sadang River Biloka River Biloka River Biloka River Biloka River gorge Benteng dam Waru Balu Lambou/Kalosi Tirasa Mtn/Benteng end of New Rangas Rd end of New Rangas Rd end of New Rangas Rd
Miocene Recent Miocene Miocene Miocene Miocene Miocene Miocene Miocene Miocene Pliocene Miocene Miocene Miocene Miocene Miocene Miocene Miocene Miocene Miocene Eocene Miocene Miocene Miocene Miocene
1035 1035 975 850 70 60 60 60 60 100 200 300 300 189 190 185 160 30 20 55 270 425 400 5 5
59.1 59.1 0.9 10.1 24.2 55.2 55.2 55.2 55.2 59.4 33.1 12.7 12.6 34.6 33.9 34.4 38.2 41.3 34.3 42.1 19.8 37.4 37.6 37.6 37.6
119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 118 118 118 20.4 20.4 18.9 19.8 20.8 42.5 42.5 42.5 42.5 47.8 45.0 43.7 43.8 40.6 41.9 40.5 41.7 4O.0 34.8 37.3 49.4 37.9 49.2 49.2 49.2 z a a
sn sn
snp
s
s
b
b b
b
b
a
snp
az
a
a
Z
Z
b b b b
aZS
snp
sn
sn
bh b
b b b
400
s . c . BERGMAN ET AL.
Table 2. Modal compositions of selected Miocene intrusive rocks sample
quartz (vol%)
plagioclase
K-feldspar
4 7 11 2 11 4 5 22
61 50 55 63 49 45 38 41
17 24 14 10 26 33 27 32
90SUL8 90SUL9 90SUL12 90SUL13 90SUL21 a 90SUL22a 90SUL25 90SUL28
biotite
hornblende
augite
6 3 8 9 19
Normalized volume percentages based on c. 500 points using a 1 mm grid.
exhibit flat to LREE depleted patterns similar to N-type MORB and possess spider diagram patterns similar to MORB and supra-subduction zone rocks (SSZ; arc or back-arc) defined by slight negative Nb anomalies.
200-300 km wide area define continuous trends on these variation diagrams, suggesting a cogenetic relationship. Lamas• Complex ophiolites are compositionally distinct from Miocene intrusives and extrusives in terms of major and trace element contents. Lamas• ophiolitic rocks range from basalt to rhyolite in major element composition (Fig. 9), are marginally thole••tic on an AFM plot (Fig. 10), and are depleted in most trace elements compared with the Miocene magmatic units. Lamas• ophiolites
118 `>30' 2 ° 30'
Conventional K - A r and 4°Ar-39Ar geochronology Fifty-three conventional K-Ar ages of biotite, hornblende, plagioclase, san•dine, phlogopite, white
119 °
120 °
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I
120 ° 30' 2 ° 30'
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3°Makassar Strait
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Pare Pare/ 9
90.2%b,0,, .
90-27
,
119 °
Fig. 5. Sample location map for all samples included in the present study.
4°
I 120 °
120
° 30'
119 °
118 ° 30'
120 °
I /
2 ° 30' b 7.3.+0.2 b 2.4.+0.1 sn 5 . 3 _ + 0 . ~
, /
120 ° 30'
I
2 ° 30'
/
Mamuju
/I
L ] ~
Makassar Strait ~
p120.0-.+1.0
~1
b 6.2_+0.2 h 10.7.+1.1 / Hal•DO I ,, ~+,, " ~ = b 5.4_+0.2, h 9.6_+0.9' I h I~'R°+fl~u'~I~ n4-n P 4 b N A p172.4tf,18+2.1plat" am56.9, . . . . . . . . ' ~- I' -~ ' ~ ........ ;~--' '~'-]' A m" • NA ~am 203t~one ~1_ 3 ° wr9 7_904 -/ "-p123.6+1.6, Nl~20.4tl~.2+0.2ptat " -" p1120-25 NA p1137.3tf # a y l + b 10.0-~-0.4 ' "~b 7.6-+0.20 wr 4~7_3.1 I b 11_+0.4 b 12.5_+0.5 & I • b9.6_+0.3 p1201_+9(0.10%K) [ b 12.8_+0.3 NA p1312tf; am 123.2tf / . phi 17.5_+0.4~ Kalosi• "+^ • • I • b 7.7-+0.2 . . . . ~ b 9.8_+0.3 wm ~4t_z A/A p 126.4tf; am 371tf |
" t/ ') - ' (~"
b77+03 b 72±0.3 • ' - ' b 7.0±0.3 i • " a a m asa b 7 3+0 3 • " -" • b 9.8_+0.3
, --'i
• . , , - ' - - ' - " - ' ~ h ~ l m q / ~ n+n-~ b 7.2_0.3, sn 6.7_+0.3 pl 7.8_+0.8, b 7.3_+0.3 ~A~T^~ u o. r_-lJ.,~,r~.o~-u.~ b 8.8_+0.3, pl 7.7_+0.8• b 7.1-+0.3 "~'~ } b7.6.+0.2 _-"I•.b8.1_+0.3 ii b 8.05-_0.31 b 7.4~_0.'~2b~8.1_+0.3 p162_+13 (0.02%K) sn 0
25
I
I
0
50 km i
~
/
I
25 mi
b 6.8+0.3 Pare P ~
4° 118 ° 30'
Enrekang
b 8.5-+0.3
~
7.8_+0.39b4.8_+0.2
I 119 °
I 4° 120 ° 30'
120 °
Fig. 6. K-Ar and 4°Ar-39Ar (A/A) age results location map. Age _+ 10 (Ma) follows phase dated: wm = white mica, pl = plagioclase, see Table 4 for other abbreviations.
118 ° 30'
119 °
3 ° --
120 °
.,~ ~j
2 o30'
/ . . . . J Makassar (,•
Strait
+
1 2 0 ° 30'
I
a 7.3+1.2, z 13.0.+0.6 a 7.2.+1.0 e ....... D ivlu[ll~u 7RR+t3a _mea46.+0.6 .¥ ........ z 8.8_0.5
2 o30'
a 2 1+0 5 z 6 4+0 4 s4 4+0 5 ~ _ . ...... t ...... ; / " D ~ i,-,,-,,-, z 10.8+~6 -~-ea 8.4_+4.3 . . . . ~,v + zS~0.~ ••a8.7-+8.8 -B o n_e T a 6.2_+1.1, z 6.3_+0.4 - z8.6_+0.6 | Bay
~
I I , _
•
~
,
a 20.4_+5.2 . z 4.3_+0.6 a 4.0+_2.9 a 3.8_+1.6 a 2~.6+_6.5
/ a 9.2+_1.4, z 9.8+_0.5
- 1 k % t " / /
t • a 12.9.+3.8, z 92-+10 n 7 (~+1 9 7 . . . . . a 5.8.+0.9 Kalosi • • \ ,t~ u_~,iL, LZ.U_TV.'~ • a4.1-+1.0 a 8.2+9.5 z 93-+9 \ " f-l,,Ja 2.4_+1.8, z 6.8_+0.5 • '..a 5.5_+1.6, z 67.+8 l a4.2_+06 z 5 0 + 0 3 s4.5_+0.9 ~ _~11+~~ Wjl~a6.3+~.3, z128_+12 / \ -A. . . . . . . . . . . . . . . . . . . . ~ a, ,z, ,~ a 4.5+0 9 • / "z 123_+9 \,"v f~ z67_-',~'4• a9.9.+1.6 z69-+5
?
118 ° 30' Fig. 7. Fission abbreviations.
a• 25'r.i
\ z##:,
/
119 ° track age results location
120 ° map. Central
FT age + ]~ (Ma)
fo|]ows
1200 30'
p h a s e d a t e d : see T a b l e 4 f o r
402
S . C . B E R G M A N ET AL.
Table 3. Summary of igneous, metamorphic and sedimentary rock major and trace element geochemical data averages
Lithology
number of samples
Miocene volcanic rocks
36
Miocene intrusive rocks
19
Cretaceous metamorphic rock Lamasi Ophiolite
1 13
Palaeogene clastic rocks
5
Miocene volcanic rocks
36
Miocene intrusive rocks
19
Cretaceous metamorphic rock Lamasi Ophiolite
1 13
Palaeogene clastic rocks
5
Miocene volcanic rocks
36
Miocene intrusive rocks
19
Cretaceous metamorphic rock Lamasi Ophiolite
1 13
Palaeogene clastic rocks
5
TiO 2
A1203
(wt%) Fe203
MgO
CaO
57.05 9.70 59.35 8.51 38.60 53.14 5.36 63.88 18.94
0.70 0.20 0.65 0.27 0.34 1.11 0.49 0.48 0.19
13.83 2.50 14.97 2.00 7.49 15.52 1.86 11.91 4.21
3.42 1.67 1.87 1.45 3.12 3.60 1.11 2.69 1.70
4.02 2.60 3.94 2.26 1.15 5.05 1.68 1.43 0.97
6.43 4.72 5.48 3.22 25.70 8.30 3.72 6.54 10.50
2.15 1.41 3.24 1.42 0.80 6.05 1.80 2.40 1.45
5.23 2.06 4.92 2.39 3.61 9.29 2.24 4.36 1.58
(ppm) Li 19 10 29 12 20 6 5 30 20
Be 6 3 5 1 3 4 1 4 1
Rb 169 104 179 99 21 10 13 35 22
Cs 16 19 7 4 2 2 1 6 4
Sr 610 372 553 266 581 191 182 277 261
Ba 1225 824 996 740 132 104 139 221 134
Y 24 9 19 7 12 28 15 14 11
La 56 35 41 17 13 5 4 12 5
(ppm) Hf 6.4 3.5 4.8 1.5 1.5 3.2 1.9 3.7 1.2
Nb 16.9 9.0 13.5 5.0 7.0 5.4 1.4 7.4 1.8
Ta 1.4 0.8 1.0 0.2 < 0.5 < 0.6
W 5.5 4.6 2.3 1.2 < 1 3.3 0.6 2.0 1.0
Sc 20 11 18 13 10 32 10 11 6
V 129 69 125 87 92 264 127 92 52
Cr 142 107 124 99 34 135 164 181 214
Co 18 11 20 13 9 28 13 14 9
SiO 2 mean std. dev. mean std. dev. mean std. dev. mean std. dev.
mean std. dev. mean std. dev. mean std. dev. mean std. dev.
mean std. dev. mean std. dev. mean std. dev. mean std. dev.
< 0.7
FeO
FeO*
For complete data see table 2 of Supplementary Publication SUP 18099 (see text).
mica or whole rock separates define the cooling histories of the major igneous complexes (summarized in Table 4 and illustrated on a map in Fig. 6; detailed data provided in table 3 of Supplementary Publication SUP 18099 (see above)). High temperature cooling ages (> 300°C) of 38 Miocene and Pliocene Camba-EnrekangMamasa Complex intrusive and extrusive rocks overlap and range from 2.4-17.5 _+0.2-0.4Ma (average 8 +_2.5 Ma). Where multiple phases were dated, concordant ages are generally observed with the exception of two samples which contain hornblende ages significantly older than their biotite ages. This discordance may reflect the higher blocking temperature of hornblende (500°C) than biotite (300°C), suggesting protracted cooling in the interval 500-300°C (see below). Six conventional K-At plagioclase or whole rock ages of seven samples of the Lamasi Complex range from 20-201 Ma. The rocks possessing older ages (162-201 Ma) contain 0.02-0.10 wt% K in mineral separates or whole-rock splits and are interpreted as anomalously high due to excess
radiogenic 4°Ar. In contrast, some Lamasi Complex samples with higher K contents (0.2-0.5 wt%) give younger K-Ar ages in the range 20-24 Ma and are interpreted as reset cooling ages thus dating the ophiolite emplacement. Two plagioclase separates with moderate K contents (0.10-0.14wt%) give Palaeocene or early Cretaceous ages (46_ 3 and 120 _+5 Ma) which are interpreted as a partially reset higher temperature cooling events. Ten 4°Ar-39Ar ages of plagioclase or amphibole separates from five rocks of the Lamasi Complex are summarized in Table 4 (Bergman, unpublished data). Unfortunately, the plagioclase and amphiboles possess very low K contents, yielding a variety of ambiguous ages with a bewildering and poor interpretation potential. Nevertheless, three groups of ages are distinguished: excessively old ages due to excess 4°At, high T (> 400°C) or partially reset cooling ages, and low T (< 300°C) cooling ages. The separates with among the lowest K contents (0.001-0.06 wt%) possess the oldest ages in the range 203-371 Ma. These are regarded as erroneously old due to excess 4°At contamin-
403
MIOCENE W SULAWESI CONTINENT COLLISION
MnO
Na20
K20
P205
CO2
SO 4
S
F
C1
0.13 0.20 0.10 0.05 0.16 0.18 0.04 0.18 0.14
2.78 1.37 2.98 1.04 1.40 3.55 1.30 0.73 0.97
3.18 1.56 3.81 1.66 0.90 0.35 0.56 0.92 0.62
0.40 0.32 0.29 0.21 0.07 0.17 0.13 0.07 0.02
1.04 3.92 0.94 1.89 19.00 0.05 0.04 4.45 8.15
0.44 0.49 0.05 0.05 ,6" c.)
i
I
5 10 15 20 Apparent Fission Track Age (Ma) SPHENE 90SUL25 Polewali Pluton
t-"
U- 2 .
0
>6
I
i
I
I
5 10 15 20 Apparent Fission Track Age (Ma) APATITE 90SUL25 m Polewali Pluton
~4 O"
0
5 10 15 20 Apparent Fission Track Age (Ma)
Fig. 15. Fission track age histograms for apatite, sphene and zircon from the Polewali pluton.
Hollister et al. 1987; Rutter et aL 1989). The presence of andalusite porphyroblasts within the Palopo pluton contact metamorphic aureole (sample 90SUL10, elevation 250m) limits the intrusion pressure to < 3.8 kbar, the aluminosilicate triple point. This indicates that the elevated pressures inferred for samples 90SUL13 and 8 probably reflect earlier phases of crystallization in the cooling history of the pluton, possibly during magma ascent through the lower and middle crust (15-23 km). The top of the pluton (sample 90SUL9 from 1050 m elevation) crystallized at pressures of 1.0 __.0.5 kbar (3-4 km subsurface depths), whereas the lower-most sample (90SUL12, 0.5 km
415
elevation) crystallized at pressures of 1.7 _ 0.5 kbar (5-6 km subsurface depths). Combining these inferences with the fission track and K-Ar geochronology data for the Palopo pluton and its contact aureole, its cooling and uplift history can be constrained. The Palopo parental granitic magma was formed in the lower crust during a Miocene melting event, ascended toward the surface, and intruded the Cretaceous deposits at depths of 3-5 km during 6-10 Ma and rapidly cooled through 500-300°C by 6-8 Ma. The pluton finally cooled through 60-120°C and ascended through 2-3 km depths by 2-3 Ma, presumably during denudationrelated uplift and erosion associated with thinskinned thrusting. Tectonic framework
The Miocene plate tectonic framework of Sulawesi has been the subject of ongoing vigorous debate (Katili 1978; Sukamto 1978; Hamilton 1979, 1989; Van Leeuwen 1981; Hutchison 1982, 1989a, b; Silver et al. 1983a, b; Wood 1985; Nishimura 1986; Leterrier et al. 1990; Letouzey et al. 1990; Rangin et al. 1990; Audley-Charles 1991; Audley-Charles & Harris 1991; Daly et al. 1991; Rangin & Silver 1991). Most workers envision a Miocene westward-dipping subduction zone to produce the widespread Miocene volcanic and plutonic rocks in western Sulawesi. The present study, however, indicates Palaeozoic to Proterozoic lithospheric parental rocks unlike any known in Sundaland (see Hutchison 1989a for details of Sundaland) were melted to produce the Miocene igneous rocks. Such Palaeozoic to Proterozoic lithospheric assemblages characterize the northern Australian Plate, such as in western Irian Jaya (Pieters et al. 1983; Pigram & Panggabean 1984) and derivative continental blocks including Sula, Buru, Seram and the Tukang Besi Platform. Silurian or Permian granitic rocks form part of an Early Palaeozoic continental assemblage and extensive Miocene collision of arc terranes and associated crustal melting are known along the northern margin of NE Australia-New Guinea (Audley-Charles 1991). This similarity to the inferred crustal parental material of the Miocene magmatic rocks of western Sulawesi suggests that a crustal fragment derived from the northern Australian plate was accreted onto SE Sundaland during the Miocene due to westward vergent plate motion of the Pacific plate or was transported along and juxtaposed against SE Sundaland by a transform margin. It is also possible that the accreted crustal block was one of the allochthonous Pacific microplates which characterize eastern Indonesia (Hutchison 1989a, b; Audley-Charles 1991; Audley-Charles & Harris 1991). Early Miocene emplacement of the East
416
S . C . BERGMAN E T A L
25 20
AGE (Ma) 15 10
20
5
E
°4e°e° ~
rein100
25, 20
...
AGE (MliI 15. 10.
20
5. ~
100
~',,.,0e 2km sediment ~
Australian basement
i
I
Fig. 9. Reconstruction of the Bird's Head-Sula-Banggai-East Sulawesi region prior to development of the Sorong fault. The Bird's Heads block is drawn in its present-day orientation with respect to north. In order to keep SW Obi as an identifiable body, the inferred wrench faults bounding this terrane have not beeen restored.
Tomori basinal depocentres. However, as the Salawati Basin has a double depocentre, there is some ambiguity as to which depocentres should be aligned• Both alignments give a reasonable correlation of the basin isopachs, but placing the Tomori Basin north of the eastern (Sele Straits) depocentre of the Salawati Basin (Fig. 9) gives a more plausible basemap for palaeogeographic reconstructions (such as that for the Miocene described below - see Fig. 10). The net offset on the Soron~ fault zone based on this reconstruction is about 900 km. The Salawati and Tomori Basins show strong stratigraphic similarities up until fairly recent times, and this probably indicates that offset of the two half-basins occurred relatively recently. The restored composite basin in Fig. 9 is roughly elliptical in shape, with an oblique orientation suggestive of a strain ellipse resulting from leftlateral shear parallel to the Sorong fault zone. This probably suggests that the basin was formed largely as a result of transtension on the Sorong system. As most of the Tertiary stratigraphic thickness that gives rise to the elliptical shape is accounted for by the Plio-Quaternary clastic sequences, and additionally as the Salawati Basin
in particular shows little evidence of fault-related deformation prior to the Early Pliocene, it is likely that movement on the Sorong fault zone occurred primarily during the Pliocene and Quaternary periods. The precise age of the onset of clastic sedimentation in the Salawati Basin has not been stated in the published literature, but a stratigraphic column illustrated by Livingstone (1992) indicates the earliest deposition of the clastic Klasafet Formation in the Upper Miocene. Livingstone (1992) also showed an idealized burial plot for the Waipili-1 well in the northern part of Salawati island which indicates that the top of the Kais platform sequence began to subside rapidly between 6-7 Ma ago. This might indicate the time at which the Sorong fault system began to develop. A further constraint on the age of the Sorong fault zone comes from regional plate tectonics. It was suggested earlier that the orientation of the fault system parallel to the relative convergence vector between the Pacific and Indo-Australian plates indicates that the present-day Sorong system acts as a transcurrent boundary between these two plates (Fig. 1). The motion of Australia relative to the Pacific in this region is at a rate of 128 km Ma -1
478
x.R. CHARLTON
on a bearing of 068 ° (using the Euler poles of Minster & Jordan 1978). At this rate the 900 km displacement would be achieved in a period of 7 Ma. Assuming that plate motion has been constant through this period, this provides a minimum age for the Sorong system. This estimate is very similar to the commencement of subsidence in the Salawati Basin indicated by Livingstone (1992). The absence of significant pre-Pliocene faulting, together with the largely non-clastic nature of the Miocene basin fill, suggests that the Miocene carbonate basin pre-dated (and is therefore genetically unrelated to) the development of the Sorong fault system. Figure 10 shows a possible interpretation of the Miocene basinal palaeogeography based on extrapolating the well established palaeoenvironments of the Salawati Basin into the Tomori Basin, and taking into account the limited geological data from Banggai-Sula and east Sulawesi. It would seem reasonable from Fig. 10 that the combined Salawati-Tomori Basin originated at least in part as the foreland basin to the east Sulawesi orogenic belt. If this interpretation is correct, then the clear evidence from the Salawati Basin for strong basinal subsidence in the Lower Miocene suggests that orogenesis was underway in east Sulawesi by this time. This is somewhat
Fig. 10. Miocene palaeogeography of the eastern Salawati Basin (after Gibson-Robinson et al. 1990) extrapolated into the repositioned Tomori Basin.
earlier than the usually accepted Middle Miocene age (e.g. Davies 1990). There is, however, a discrepancy between the age of the east Sulawesi orogen as inferred from onshore geology (which suggests important orogenesis during the Middle Miocene or earlier) and the offshore seismic and well data which indicates fold-and-thrust belt development in the Tolo Gulf during the Pliocene (Davies 1990). As already described, the strike of the Tolo Gulf foldand-thrust belt is markedly oblique to that of the orogenic belt onshore in east Sulawesi. It appears that there were two distinct phases of development in the east Sulawesi orogen: an earlier pre-Middle Miocene phase which gave rise to a foreland basin on its eastern flank (the Miocene Salawati-Tomori Basin), and a later phase of fold-and-thrust belt development during the Pliocene. The earlier phase of deformation arose from arc-continent collision (Davies 1990), whilst the Pliocene deformation is probably related to the westward translation of eastern Sulawesi in the Sorong fault system. The Southeast Arm of Sulawesi shows clear structural and stratigraphic similarities with the East Arm, and probably formed a direct continuation of the the east Sulawesi orogenic belt prior to the Pliocene. The Tolo Gulf foldbelt probably developed contemporaneously with oroclinal bending of eastern Sulawesi, with the Southeast Arm rotating anticlockwise with respect to the East Arm. In addition to the Bird's Head and eastern Sulawesi, three displaced terranes are provisionally repositioned in Fig. 9. These terranes are relocated to give the tightest geographic fit consistent with the regional geology. Banggai and most of the Sula islands are treated as a single terrane which is fixed relative to eastem Sulawesi and the Tomori Basin. The positioning of Banggai-Sula is thus largely defined by the re-positioning of the Tomori Basin north of the Salawati Basin. However, some freedom remains as to body rotation, and in Fig. 9 Banggai-Sula (and eastern Sulawesi) have been rotated about 15 ° anticlockwise relative to the Bird's Head in order to achieve the tightest geographic fit. If, as is likely, the Bird's HeadMisool block has itself undergone body rotations relative to Australia during the Neogene, the inferred 15 ° rotation of Banggai-Sula would be additional to that in the Bird's Head. The second displaced terrane in Fig. 9 comprises Sulabesi island, eastern Mangole and the southwestern part of Obi island. The boundaries of this terrane may be a simplification: Mangole island is cut obliquely by a number of ENE-WSW trending steep faults which may be left-lateral faults (as suggested in the restoration) or could be normal faults. In Fig. 9 Sulabesi is restored by reversing
SALAWATI • TOMORI BASIN CORRELATION an assumed anticlockwise rotation of about 90 ° relative to Banggai-Sula and also relative to eastern Mangole. Sulabesi and eastern Mangole are then translated about 175 km eastward relative to Banggai-Sula (cf. 180 km displacement without rotation estimated by Garrard et al. 1988, based on the similarity of structural style and the nature of Jurassic sediments in Sulabesi and the Taliabu Shelf immediately west of the Sula islands). The third displaced terrane in Fig. 9 is the Tamrau Terrane of the northern Bird's Head. Most authors (e.g. Pigram & Davies 1987) have interpreted very large displacements between the Tamrau Terrane and the Bird's Head, in part based on the apparent necessity of accommodating the large strike-slip offsets of the Sorong fault zone to the west on the single fault strand of the Sorong fault. However, the geology of the Tamrau Terrane is not greatly different from that of the Bird's Head (cf. Visser & Hermes 1962), and it is suggested that the Tamrau Terrane can be repositioned by a relatively small eastward translation of about 60 km. This would place the Netoni Igneous Complex (Pieters et al. 1981) north of the Anggi Granites of the eastern Bird's Head, and re-aligns the eastem end of the Tamrau Terrane with the eastern edge of the continental Bird's Head block. (It has been suggested by several authors that the Netoni Block is itself a distinct 'mini terrane' within the Sorong fault; however, the overlap of the Cretaceous Amiri Sandstone across the supposed Tamrau-Netoni terrane boundary (Hartono et al. 1989) does not support this distinction). A consequence of this relatively limited displacement on the Sorong fault through the Bird's Head is that most of the movement on the Sorong system must be taken up north of the Tamrau Terrane. In Fig. 9 most of the inferred left-lateral movement is taken up along the northern edge of the Tamrau Terrane such that Sulabesi island is re-positioned immediately northwest of the Tamrau Terrane. This aligns N-S trending normal faults in the Sula islands with similar faulting on the eastern flank of the Salawati Basin, and also juxtaposes the Jurassic-Cretaceous shale sequences of the Sula islands with comparable sequences in the Tamrau Terrane.
479
Implications for hydrocarbon exploration The reconstructions in Figs 9 & 10 may also have some implications for future oil exploration in this region. In the Salawati Basin, nearly all oil production is from pinnacle reefs which developed near the shelf-slope break around a semi-enclosed Miocene basin. If this palaeogeography can be extrapolated into the repositioned Tomori Basin as suggested in Fig. 10, then the belt of pinnacle reefs should follow the eastern margin of the basin to the south and west of the Banggai island group. In addition, it now seems widely agreed that the primary source rocks for the Salawati oils are Miocene deep marine and poorly oxygenated calcareous mudstones and/or marly limestones which were deposited either contemporaneously with or immediately above the Kais pinnacle reefs. It is probably more than coincidence that these restricted basinal sediments were deposited in the semi-enclosed Miocene Salawati embayment (Fig. 10). The palaeogeographic map also indicates that a similar embayment would be expected at the northern end of the Tomori Basin, and such a restricted basin might well be the source for oil and gas discoveries along the western flank of the Tomori Basin. However, the predominant structure of the northern Tomori Basin is that of a foreland basin, and any oil generated in this region would be more likely to migrate updip towards the foreland; that is towards the postulated pinnacle reef trend on the eastern flank of the Tomori Basin. Unfortunately, most of the extrapolated pinnacle reef trend lies under water depths of 10002000 m, and is therefore unlikely to be a commercial prospect with present-day economics. This work was initiated as part of the London University study of the Sorong fault zone under the leadership of Prof. Robert Hall. Fieldwork in this area was sponsored by the Royal Society, NERC grant GR3/7149 and the London University Consortium for Geological Research in Southeast Asia. Thanks to Chris Gibson-Robinson and Peter Lunt (Petromer Trend Corp.) and Andy Livsey (Simon Petroleum Technology, Jakarta) for useful discussions, and to Tony Barber (Royal Holloway University of London) and Dick Garrard (ARCO) for useful comments in review.
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L., VACHARD, D., E~r AL. 1994. Une plate-forme carbonatEe d'~ge rhEtien au centre-est de Sulawesi (rEgion de Kolonodale, CEl~bes, IndonEsie).Comtes Rendus Acad~mie des Sciences de Paris, 318(II), 809-814. DAVIES,I. C. 1990. Geological and exploration review of the Tomori PSC, eastern Indonesia. Proceedings of the Indonesian Petroleum Association, 19, 41-67.
480
T.R. CHARLTON
Dow, D. B. & SUKAMTO,R. 1984. Western Irian Jaya: the end-product of oblique plate convergence in the late Tertiary. Tectonophysics, 106, 109-139. FROIDEVAUX,C. M. 1974. Geology of Misool island (Man Jaya). Proceedings of the Indonesian Petroleum Association, 3, 189-196. • 1977. Tertiary tectonic history of the Salawati area, lrian Jaya, Indonesia. Proceedings of the Indonesian Petroleum Association, 6, 199-220. GARRARD, R. A., SUPANDJONO, J. B. & SURONO. 1988. The geology of the Banggai-Sula microcontinent, eastern Indonesia. Proceedings of the Indonesian Petroleum Association, 17, 23-52. GIBSON-ROBINSON, C., HENRY, N. M., THOMPSON, S. J. & HARYONO TRI RAHARIO 1990. Kasim and Walio fields - Indonesia. In: BEAUMONT,E. A. (compiler) Stratigraphic Traps, I, AAPG Treatise of Petroleum Geology, 257-295. HALL, R., NICHOLS, G. J., BALLANTYNE,P., CHARLTON,T. & ALl, J. 1991. The character and significance of basement rocks of the southern Molucca Sea region. Journal of SE Asian Earth Sciences, 6, 249 258. HAMILTON, W. 1979. Tectonics of the Indonesian Region. US Geological Survey Professional Papers, 1078. HANDIWmlA, Y. E. 1990. The stratigraphy and hydrocarbon occurrences of the Salodik Group, Tomori PSC area, East Arm of Sulawesi. Paper presented at the 19th Annual Convention of the Indonesian Association of Geologists. HARTONO, U., AMRI, C. & PIETERS, P. E. 1989. Geological
map of the Mar Sheet, Irian Jaya. 1:250,000. Geological Research & Development Centre, Indonesia. HERMES, J. J. 1968. The Papuan geosyncline and the concept of geosynclines. Geologie en Mijnbouw, 4 7 , 81-97. KATILI, J. A. 1978. Past and present geotectonic position of Sulawesi, Indonesia. Tectonophysics, 45, 289-322. KERTAPATI, E. K., SOEHAIMI, A. & DJUHANDA, A. 1992. Seismotectonic map of Indonesia. Geological Research & Development Centre, Indonesia. LINTHOUT, K, HELMERS,H & ANDRIESSEN,P. A. M. 1991. Dextral strike-slip in Central Seram and 3-4.5 Ma Rb/Sr ages in pre-Triassic metamorphics related to Early Pliocene clockwise rotation of the BuruSeram microplate (E. Indonesia). Journal of SE Asian Earth Sciences, 6, 335-342. LIVINGSTONE, H. J. 1992. Hydrocarbon source and migration, Salawati Basin, Irian Jaya. Eastern Indonesia Exploration Symposium, Simon Petroleum Technology/Pertamina. LUNT, P. & DJAAFAR,R. 1991. Aspects of the stratigraphy of western Irian Jaya and implications for the development of sandy facies. Proceedings of the Indonesian Petroleum Association, 20, 107-124. MILSOM, J., MASSON, D. 8,: NICHOLS, G. 1992. Three trench endings in eastern Indonesia. Marine Geology, 104, 227-241. MINSTER, J. B. & JORDAN, T. 1978. Present-day plate motions. Journal of Geophysical Research, 83, 5331-5354. PIETERS, P. E., HARTONO, U. & AMRI, C. 1981.
Preliminary geologic map of the Mar Quadrangle,
lrian Jaya. 1:250,000. Geological Research & --,
Development Centre, Indonesia. PIGRAM, C. J., TRAIL, D. S., DOW, D. B., RATMAN, N. & SUKAMTO, R. 1983. The stratigraphy of western Irian Jaya. Bulletin of the Geological
Research & Development Centre, Indonesia, 8, 14--48. PIGOTT, J. D., TRUMBLY, N. I. & O'NEAL, M. V. 1982. Northern New Guinea wrench fault system: a manifestation of late Cenozoic interactions between Australian and Pacific plates. In: Watson, B. (ed.)
Transactions of the Third Circum-Pacific Energy and Mineral Resources Conference, 613--620. PIGRAM, C. J. & DAVIES, H. L. 1987. Terranes and accretion history of the New Guinea orogen. BMR Journal of Australian Geology & Geophysics, 10, 193-211. - & PANGGABEAN,H. 1984. Rifting of the northern margin of the Australian continent and the origin of some microcontinents in eastern Indonesia. Tectonophysics, 107, 331-353. -& SUKANTA,U. 1982. Geological data record of the Taminabuan 1:250,000 sheet area, Irian Jaya. Open file report, Geological Research & Development Centre, Indonesia. -& -1989. Geology of the Taminabuan Sheet area, lrian Jaya. 1:250,000. Geological Research & Development Centre, Indonesia. , CHALLINOR,A. B., HASIBUAN, E, RUSMANA, E. & HARTONO, U. 1982. Lithostratigraphy of the Misool Archipelago, Irian Jaya, Indonesia. Geologie en Mijnbouw, 61,245-279. , SURONO & SUPANDJONO, J. B. 1985. Origin of the Sula Platform, eastern Indonesia. Geology, 13, 246-248. REDMOND,J. L. & KOESOEMADINATA,R. P. 1976. Walio oil field and the Miocene carbonates of Salawati Basin, lrian Jaya. Proceedings of the Indonesian Petroleum Association, 5, 41-57. RI~HAULT,J. P., MALOD, J. A., LARUE,M., BURHANUDDINN, S. ~ SARMILI,L. 1991. A new sketch of the central North Banda Sea, Eastern Indonesia. Journal of SE Asian Earth Sciences, 6, 329-334. ROBINSON, K. M. 1987. An overview of source rocks and oils in Indonesia. Proceedings of the Indonesian Petroleum Association, 16, 97-122. RUSMANA,E., KOSWARA,A. & SIMANDJUNTAK,T. O. 1984.
[Report on the geology of the Luwuk Quadrangle, 1:250,000]. Open file report, Geological Research --,
& Development Centre, Indonesia• [In Indonesian]. SUKIDO, SUKARNA, D., HARYONO, E. & SIMANDJUNTAK, T. O. 1986. Preliminary geological
map of the Lasusua-Kendari Quadrangle, Sulawesi. 1:250,000. Geological Research & Development Centre, Indonesia. SATO, T., WESTERMANN, G. E. G., SKWARKO, S. K. & HASIBUAN, E 1978. Jurassic biostratigraphy of the Sula islands, Indonesia. Bulletin of the Geological
Research & Development Centre, Indonesia, 4, 1-28. SIMANDJUNTAK,T. O. 1990. Sedimentology and tectonics of the collision complex in the East Arm of Sulawesi, Indonesia. Geologi Indonesia, 13(1), 1-35.
SALAWATI & TOMORI BASIN CORRELATION SIMBOLON, B., MARTODJOJO, S. & GUNAWAN,R. 1984. Geology and hydrocarbon prospects of the preTertiary system of Misool. Proceedings of the Indonesian Petroleum Association, 13, 317-340. SMITH, R. B. & SILVER,E. A. 1991. Geology of a Miocene collision complex, Buton, eastern Indonesia. Bulletin of the Geological Society of America, 103, 660-678. SPT (SIMONPETROLEUMTECHNOLOGY)/PERTAMINA.1992. Eastern Indonesia: Biostratigraphy, Geochemistry and Petroleum Geology. Unpublished non-exclusive consultancy study. SUKAMTO,R. 1975. Geologic map of Indonesia, sheet 8: Ujung Pandang. 1:1,000,000. Geological Survey of Indonesia. SUPANDJONO,J. B. & HARYONO,E. 1993. Geology of the
Banggai Sheet, Sulawesi-Maluku.
1:250,000.
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SURONO & SUKARNA, D. 1993. Geology of the Sanana Sheet, Maluku. 1:250,000. Geological Research & Development Centre, Indonesia. --, SIMANDJUNTAK, T. O., SITUMORANG, R. L. & HADIWIJOYO, S. 1987. Geologic map of the Batui Quadrangle, Sulawesi. 1:250,000. Geological Research & Development Centre, Indonesia. TJIA, H. D. 1973. Irian fault zone and Sorong melange. Sains Malaysiana, 2(1), 13-30. VINCELETTE,R. R. & SOEPARJADI,R. A. 1976. Oil-bearing reefs in Salawati Basin of Irian Jaya, Indonesia. AAPG Bulletin, 60, 1448-1462. VISSER, W. A. & HERMES,J. J. 1962. Geological results of the exploration for oil in Netherlands New Guinea.
Koninklijk Nederlands Geologie en. Mijnbouw Genootschaap Verhandlingen, Geologische Serie, 20.
The geology and tectonic evolution of the Bacan region, east Indonesia JEFFREY
E A. M A L A I H O L L O
& ROBERT
HALL
SE Asia Research Group, Department of Geological Sciences, University College London, Gower Street, London WCIE 6BT, UK
Abstract: Bacan is located in the zone of convergence between the Eurasian, Philippine Sea and Australian plates. The oldest rocks in Bacan belong to the Sibela Continental Suite and are probably of Precambrian age. The complex includes continental phyllites, schists and gneisses of upper amphibolite facies. Isotopic dating yielded extremely young ages due to interaction with hydrothermal fluids. Juxtaposed against the continental rocks is the mostly unmetamorphosed, arc-related Sibela ophiolite, probably derived from the Philippine Sea plate. Isotopic dating yielded a Cretaceous age with an Oligocene-Miocene overprint. In north Bacan, the oldest formation is the Upper Eocene Bacan Formation which comprises interbedded arc volcanic and turbiditic volcaniclastic rocks, metamorphosed under conditions between the prehnitepumpellyite and greenschist facies. A similar Lower Miocene sequence, assigned to the same formation, is exposed in south Bacan. The Oligocene Tawali Formation on Kasiruta, NW of Bacan, consists of arc basalts and volcaniclastic turbidites, metamorphosed to zeolite facies. The Bacan and Tawali Formations represent different parts of an arc, active from Late Eocene until Early Miocene, resulting from northward subduction of the Australian plate under the Philippine Sea plate. There is a major Lower Miocene unconformity, representing collision of the Australian continent with the Philippine Sea plate, above which shallow marine limestones of the LowerMiddle Miocene Ruta Formation were deposited. This deposition was interrupted by sudden influxes of volcaniclastic sands, forming the Amasing Formation. The Upper MiocenePleistocene Kaputusan Formation, rests locally unconformably on older rocks, and includes three members. The Goro-goro Member consists of arc andesites, originating from four eruption centres, which erupted from Late Miocene to Pleistocene, with the oldest in south and the youngest in north Bacan. The Pacitak Member consists of shallow marine pyroclastic rocks. The Mandioli Member formed fringing coastal reef limestones. The volcanic rocks of this formation were produced by eastward subduction of the Molucca Sea plate. Quaternary basalts are related to movement along the Sorong fault. It is concluded that most of the Bacan region has been part of the Philippine Sea plate since the Cretaceous; volcanic rocks of different ages all have an arc character and chemistry, and lithological variations reflect different positions within the volcanic arc; and there is evidence for continental crust of Australian origin in the Bacan area by the Early Miocene.
Three major plates, the Eurasian, Philippine Sea and Australian plates, converge in the Bacan region (Fig. 1). The Eurasian plate has a complex eastern margin which includes continental and volcanic arc crusts accreted during collision with the Philippine Sea plate (e.g. Rangin 1991). The Philippine Sea plate has a long arc history (Hall et al. 1995a), extending back at least to the Cretaceous, and is currently converging with Eurasia in the Philippines, and overriding the Molucca Sea plate in the south. The Australian plate has been moving northwards since the Cretaceous. During much of the Palaeogene, oceanic crust of the Australian plate was subducted under the southern edge of the Philippine Sea plate (Hall et al. 1995b). A strike-slip fault system, the Sorong fault, subsequently developed at this plate boundary. Bacan contains rocks of continental, ophiolitic and arc affinities (Fig. 2), and therefore an under-
standing of the tectonic evolution of Bacan should greatly enhance our knowledge of the development of this area of complex plate boundaries. This paper briefly describes and summarizes the geology of each of the formations in the Bacan region, discusses them in the light of regional tectonic events, and proposes a synthesis of the tectonic evolution of this region. A detailed description of the lithostratigraphy of Bacan (Fig. 3), with chemical and palaeoenvironmental +analyses can be found in Malaihollo (1993).
Geology and tectonic evolution of Bacan Sibela continental suite The oldest rocks in the Bacan region form part of the Sibela Continental Suite. The complex includes continental phyllites, schists and gneisses of upper
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 483-497.
483
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amphibolite facies (c. 600°C, c. 5 kbar), with strong penetrative fabrics indicative of a polyphase deformation and recrystallization history, typical of Barrovian type dynamo-thermal metamorphism (Brouwer 1923; Hall et al. 1988). There is also evidence for retrograde metamorphism associated with post peak-metamorphism deformation. The protoliths were pelites with minor amounts of sandy and carbonate/marly horizons. Whole rock chemistry of the pelites suggests that they were derived from a cratonic area and were deposited on an active margin underlain by continental crust. Based on regional stratigraphical arguments (Hamilton 1979) and the Pb isotopic signatures of Quatemary volcanic rocks (Vroon 1992), these metamorphic rocks are postulated to be of Precambrian or Palaeozoic age. This suggestion has not been confirmed isotopically, as K-Ar and Ar-Ar step heating results yielded extremely young ages (< 0.21 Ma; Malaihollo 1993), interpreted to be a result of interaction with a hydrothermal fluid carrying fractionated argon which were subsequently concentrated between the sheet silicate layers, causing an apparent young age. This mechanism could account for other unusually young ages from high-grade metamorphic rocks in the region (cf. Linthout et al. 1991). There are highly foliated metasedimentary rocks
in the south Saleh Islands and the SE comer of north Bacan. These rocks have a similar character to the Sibela Continental Suite, particularly in having brown metamorphic biotite which throughout the whole region is found only in the Sibela Continental Suite, but are of lower metamorphic grade. This paper interprets the Sibela Continental Suite to be derived from the Australian plate, where these rocks formed either part of the Mesozoic-Tertiary passive margin of north Australia or fragments rifted from Australia during Gondwana break-up. Many authors have suggested or implied that the continental rocks in Bacan were introduced into the region via strike-slip motion of the Sorong fault system from the Bird's Head region of Irian Jaya (e.g. Hamilton 1979; Silver et al. 1985) or central Papua New Guinea (Pigram & Panggabean 1984). 87Sr/86Sr isotopic compositions of volcanic rocks (E. Forde, pers.comm. 1993) suggest that the Upper Miocene Kaputusan Formation on south Bacan was erupted through continental crust, indicating the presence of continental material under the Bacan region by the Late Miocene. The presence of a Lower Miocene post-collisional intrusion (the Nusa Babi Monzodiorite) in Bacan suggests the presence of continental crust by the Early Miocene (as discussed below).
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Possible Tectonic Events Quaternary volcanism related to movements on Sarong Fault.Upliftof Sibela Block. Thrustingin Halmahera, movement on splaysof Sarong Fault.
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Arc volcanic and turbidites, Repeated, high concentration, proximal volcaniclasticturbidites, i Metamorphosed to PrehniteInner and middle fan facies, Pumpellylte to Lower Greenschist facies. I Volcanism related to the northward movement of Pillowlavas with volcanic arc chemistry. Australiaunder the Zeolitefaciesmetamorphism. PhilippineSea plate, I
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Fig. 3. Cenozoic stratigraphy of the Bacan region with related local tectonic events. Timescale after Harland et al. (1990).
Sibela ophiolite
Juxtaposed against the continental suite is the Sibela ophiolite (Hall et al. 1988). There are two types of material recognized in this suite: ophiolitic rocks, most of which are of lower crustal plutonic and mantle origin (microgabbros, gabbros and serpentinised harzburgites) with few volcanic rocks, and spatially-related rocks including unusual amphibole cumulates and many rocks with magmatic-tectonic fabrics. This dismembered, incomplete ophiolitic complex is mostly unmetamorphosed, although locally there are mylonitic rocks which have been affected by upper amphibolite-lower granulite facies metamorphism (c. 1000oc, c. 5 kbar) related to ductile deformation of hot rocks at or near their place of formation, and local recrystallization in shear zones (Malaihollo 1993). Chrome spinel compositions suggest that the peridotites may have formed in an arc setting. This is supported by geochemical evidence from cogenetic metagabbros. Petrographic study reveals that most of the cumulate rocks consist mainly of cumulus amphibole with minor pyroxene and intercumulate plagioclase, indicating crystallization
under hydrous conditions, implying an arc-related setting. The cumulate rocks in the Sibela ophiolite are very different from those of the Halmahera ophiolite, which consist of olivine, orthopyroxene and clinopyroxene (Ballantyne 1992). The Halmahera cumulates are genetically related to the ophiolite which is interpreted to have formed in a supra-subduction zone, normally associated with the initiation of a subduction zone, from the intraPacific subduction (Ballantyne & Hall 1990; Ballantyne 1991). Preliminary Nd-Sm ages on the Halmahera cumulates are Jurassic (M. E Thirlwall, pers. comm. 1992). Ar-Ar and K-Ar dating of the Sibela cumulates and metagabbro (Malaihollo 1993) yielded Cretaceous (97-94 Ma) and Oligocene-Miocene (37-25 Ma) ages, although amphiboles analysed contain excess argon and therefore plateaux ages will be slightly older than closure ages (Lanphere & Dalrymple 1976). The Cretaceous age may be linked to volcanic activity recorded in the east Halmahera ophiolite (9480Ma; Ballantyne 1992), suggesting a link between the Sibela ophiolite and the east Halmahera ophiolite. The Oligocene-Miocene ages are related to the Oligocene volcanism (discussed below).
GEOLOGY • TECTONICS OF BACAN, E INDONESIA There are two possible interpretations for the association of ophiolitic and cumulate rocks. The first possibility is that the ophiolitic rocks represent older crust of possibly Jurassic age (cf. east Halmahera-Gag ophiolite) intruded by Cretaceous arc cumulates related to intra-oceanic subduction. This interpretation is supported by chemical similarities between the cumulates and zoned ultramafic complexes, normally associated with arc magmatism. Alternatively, both the ophiolitic and arc cumulate rocks may be related to the same intra-oceanic subduction during the Cretaceous. The age and nature of the original crust upon which the arc was built is unknown.
Juxtaposition of continental and ophiolitic rocks The continental and ophiolitic rocks are juxtaposed in the Sibela Mountains. Differences in metamorphic character indicate that metamorphism occurred before amalgamation of continental and ophiolitic rocks. The continental suite is interpreted to be part of the Australian continental basement, whereas the ophiolitic rocks represent the Philippine Sea plate. Regional mapping shows that since the Early Miocene, rocks from these two plates have the same geological history, indicating an Early Miocene collision (Hall et al. 1995a). There are three possibilities for the mode of juxtaposition of the continental and ophiolitic rocks: (1) the Sibela Mountains represent a suture zone resulting from collision between the Australian and Philippine Sea plates; (2) the continental rocks collided with the Philippine Sea plate and were subsequently translated by strike-slip faulting as part of a composite fragment to their present position; (3) the continental rocks remained part of the north Australian Margin until the late Neogene and were then translated by strike-slip faulting on the Strong fault into the region. These possibilities are discussed below.
487
erupted in an arc setting. Turbiditic rocks were deposited by dilute, low concentration, possibly distal or low energy currents. The Bacan Formation was affected by a thermal event at c. 15 Ma. In south Bacan the oldest rocks dated are a very similar sequence of Early Miocene age. These interbedded volcanic and volcaniclastic turbidite rocks were metamorphosed to prehnitepumpellyite facies (c. 240-330°C, c. 2 kbar) due to burial metamorphism, with local hydrothermal alteration. Whole rock and mineral chemistry indicates an arc origin for the volcanic rocks. Graded volcaniclastic arenites indicate deposition by turbidity currents which dissipated their energy as they travelled down slope. Isotopic dating shows that this formation was affected by a thermal event atc. 8Ma. Undated metabasites in the Saleh Islands were metamorphosed at conditions between upper prehnite-pumpellyite and lower greenschist facies (c. 250-360°C, c. 4 kbar). The metamorphic character of these rocks suggests static, regional metamorphism. Geochemical and lithological evidence from the metabasites indicates that they represent an arc-related calc-alkaline sequence, possibly formed in a backarc, and they have similar chemical characteristics to the Upper Eocene and Lower Miocene sequences. The Upper Eocene, Lower Miocene and the Saleh Island sequences are similar in all aspects, except age. More dating is needed, although this may prove difficult, since despite examination of > 200 thin sections, only four samples were found to be suitable for biostratigraphic or isotopic dating. This is attributed to lack of fossils in the original sediments, metamorphism which destroyed fossils, and alteration by thermal overprints of the volcanic rocks. The rocks assigned to the Bacan Formation could represent either different parts of a single, continuously active arc, or separate arcs active at different times between the Late Eocene and Early Miocene.
The Tawali Formation The Bacan Formation In north Bacan the oldest rocks exposed belong to the Upper Eocene Bacan Formation. This comprises interbedded basic-intermediate volcanic and volcaniclastic turbiditic rocks. The Bacan Formation was folded and subsequently metamorphosed under conditions transitional between the prehnite-pumpellyite and lower greenschist facies (250-330°C, c. 2 kbar upwards), with characteristics of burial metamorphism. Hydrothermal alteration may be related to the Neogene Kaputusan volcanism. Mineral and whole rock geochemistry indicates that the volcanic rocks were
The oldest rocks on Kasiruta belong to the newly defined Tawali Formation, named after the island of Kasiruta (also known as Tawali Besar). Rocks forming part of this formation were originally assigned to the Bacan Formation (Yasin 1980), but here they are assigned to a new formation because of differences in lithology and the character of metamorphism and deformation. The lower part of the Tawali Formation consists of basaltic pillow lavas and interbedded sediments (Jojok Member), and it is overlain by fossiliferous volcaniclastic turbidites (Marikapal Member). This formation is affected by burial metamorphism transitional
488
J.F.A. MALAIHOLLO & R. HALL could be a function of different structural positions during collision with the Australian continent or different thermal environments due to different positions in the pre-collision arc. Thus, the simplest explanation for all these Upper Eocene, Oligocene and Lower Miocene volcanic arc formations is that they represent arc volcanism at the edge of the Philippine Sea plate due to the subduction of the Indo-Australian plate under the Philippine Sea plate (Fig.4). This is supported by the fact that all of these formations record similar palaeomagnetic declinations and inclinations which are characteristic of the Philippine Sea plate (Hall et al. 1995a; Ali & Hall 1995).
between low and high temperature zeolite facies (c. 180°C, < 2 kbar). Whole rock chemistry indicates that the Lower Oligocene basalts are highly differentiated arc lavas, erupted in a deep, open marine environment above the CCD. The Upper Oligocene volcaniclastic rocks are products of repeated, high concentration, proximal turbidity currents with associated slumped deposits. The Tawali Formation is interpreted as an equivalent of the Bacan Formation although there are several differences between them. There is an apparent lack of Oligocene rocks on Bacan, although as noted above, only a few samples of the Bacan Formation have proved dateable. There are also differences in lithology and metamorphic character (particularly the lower grade of metamorphism suffered by the Tawali Formation relative to the Bacan Formation), and trace element differences between the Tawali and the Bacan Formations. Rocks of similar lithology and age to the Tawali Formation have now been identified throughout the Halmahera-Waigeo region (the Tawali Formation in Halmahera, the Anggai River Formation in Obi and the Rumai Formation in Waigeo; Hall et al. 1991). The Oha Formation of Halmahera is probably not of Upper CretaceousEocene age as previously suggested (Hakim & Hall 1991), but is interpreted as part of the same arc as the Bacan Formation. Despite differences, all these formations range in age from Upper Eocene to Upper Miocene and they all pre-date the c. 22 Ma unconformity (discussed below) interpreted to be related to the arrival of the continental crust in the region. Differences in lithology could be explained if the different formations represent different parts of the arc (e.g. arc slopes, backarc basin, forearc slopes). Differences in the grades of metamorphism
~
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Early Miocene unconformity Above the Bacan and Tawali Formations, there is a major regional unconformity. This is characterized by a major change in lithological, metamorphic and structural character. Rocks below the unconformity are folded arc volcanic rocks and turbidites, metamorphosed up to greenschist facies, whereas the rocks immediately above it are mostly unfolded, unmetamorphosed carbonates. The age of the unconformity is estimated to be c. 22 Ma (Early Miocene) throughout the Halmahera region (Hall et al. 1995a). Nusa Babi intrusive rocks The Nusa Babi Monzodiorite (NBM) includes quartz-monzodiorite dykes and plugs with associated aplite dykes of monzogranite composition. These intrude the Bacan Formation and have been reported to intrude the Sibela Complex (Yasin
BacanFm.arc BacanFm.&AnggaiRiverFm, (forearcturbidites
(back-arcpillowbasalts)
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GEOLOGY & TECTONICS OF BACAN, E INDONESIA
1980), but do not intrude the Ruta and Amasing Formations. The NBM is locally altered, possibly due to auto-metasmorphism. Magma evolution was primarily controlled by fractionation and the addition of water. Whole rock geochemistry indicates that the NBM is of plutonic arc or collisional magmatism origin. K-Ar dating yields an Early Miocene age (Baker & Malaihollo 1996). The geochemistry, isotopic age and the fact that the NBM intrudes the Bacan Formation indicate a postcollisional origin, probably related to the arrival of the Sibela Continental Suite in the region.
489
The Saleh Diorite
The Saleh Diorite intrudes the Sibela Continental Suite in the Saleh Islands and is juxtaposed against it in central Bacan. This intrusion consists of amphibole-bearing diorite and micro-diorite. Locally these rocks have suffered low temperature alteration, probably due to auto-metamorphism, and subsequent hydrothermal alteration. Whole rock geochemistry indicates that the Saleh Diorite is different from the NBM and is of arc or postcollisional origin. K-Ar dates suggest the c. 15 Ma age is related to the initiation of the Halmahera arc.
The Ruta and A m a s i n g Formations
The Lower-Middle Miocene Ruta Limestone lies unconformably above the Bacan and Tawali Formations. There are four microfacies recognized in the Ruta Limestone: skeletal wackestonespackstones, algal boundstones, foraminiferal packstones and bioclastic-lithoclastic packstones. These represent deposition on an open platform, a platform margin build-up (patch reef), a tidal bar downslope from a patch reef (open platform) and foreslope talus respectively, all forming part of a shallow marine carbonate platform. The deposition of the Ruta Formation began in the Early Miocene, predominantly as open platform, platform margin build-up and foreslope talus facies• Between the Early~ Miocene and Middle Miocene, deposition was locally interrupted by sudden and high influxes of volcaniclastic material forming sandstones of the Amasing Formation. Three facies in the Amasing Formation have been recognized: shallow marine with storm horizons, shoal or estuarine, and beach deposits. These facies represent shallowing upwards from an open carbonate platform to a beach environment. Deposition of the Ruta Formation continued until the Middle Miocene and its upper part is dominated by the tidal bar facies. There was widespread development of carbonate platforms at this time throughout the Halmahera-Waigeo region (Hall et al. 1995a), in north Irian Jaya (Pigram & Davies 1987; Pigram et al. 1990) and in the Philippines (Mitchell et al. 1986). Widespread carbonate platform development suggests that most of the Bacan-Halmahera-Waigeo region was in a quiet tectonic setting, in an equatorial position and was part of an extensive shallow marine area. In north Irian Jaya, the Middle Miocene Moon Volcanics (Pieters et al. 1989) may be linked with the Maramuni arc of Papua New Guinea which Dow (1977) suggested represented post-collision subduction reversal. However, the character and distribution of this volcanism is still poorly known and if an arc existed, it may have been limited to the eastern part of New Guinea (Ali & Hall 1995).
The Kaputusan Formation
The Kaputusan Formation was deposited above the Ruta and Amasing Formations. It consists of three members: the Goro-goro Volcanic, the Pacitak Volcaniclastic, and the Mandioli Limestone Members• Field mapping and aerial photographic interpretation indicates that the Kaputusan Formation rests directly upon the Bacan Formation in some areas implying an unconformable contact at its base. The Goro-goro Member consists of twopyroxene andesites (TPAN), hornblende-pyroxene andesites (HPAN), hornblende andesites (HBAN), and hornblende-biotite andesites (HBIAN). Associated with the volcanic rocks are subaqueous pyroclastic flows with related base surge deposits. The petrography, mineral chemistry, whole rock major and trace element chemistry of these rocks are typical of island-arc volcanic rocks. They are mostly fresh, akhough locally they are metamorphosed to zeolite facies, interpreted as due to hydrothermal activity. Magma diversification of the Goro-goro Member was achieved mainly by fractionation of plagioclase, pyroxene, Fe-Ti oxide, amphibole and biotite, with evidence for replenishment, immiscibility, resorption and assimilation. Magmatic conditions were < 8 kbar and 2-10 wt.% H20 with PH2'-' < Ptotal" The Goro-goro Member U includes at least four eruption centres (as defined by bulk trace element ratios): in south Bacan (mostly HBIAN), the Goro-goro area (mostly TPAN), and the north Mandioli and the Kaputusan areas (largely HPAN and HBAN). The Mandioli group are shoshonitic rocks, which may indicate eruption in an extensional setting within a dominantly convergent zone, such as a strike-slip zone where similar rocks commonly occur (e.g. Ellam et al. 1988). K-Ar ages indicate that these centres ~ were erupting from the Late Miocene (c. 7.5 Ma) to the Pliocene (c. 2.2 Ma). Each of these centres appears to have been active for c. 2 Ma, with a history of multiple eruptions. The Upper MioceneUpper Pliocene Pacitak Member consists of •
490
J.F.A. MALAIHOLLO ,~ R. HALL
Discussion
reworked pyroclastic and volcaniclastic material from the Goro-goro Member, deposited in an oxygen-rich, nearshore, shallow marine environment. The Upper Miocene-Lower Pliocene Mandioli Member consists of wackestones and packstones which formed fringing coastal reefs. The Kaputusan Formation is interpreted to be the product of the eastward subduction of the Molucca Sea plate under Halmahera, and can be correlated with the Weda Group of Halmahera and the Woi Formation of Obi (Hall et al. 1995a; A l i & Hall 1995).
The history and motion of the Philippine Sea plate has been complex, although recent palaeomagnetic results are contributing to a clearer picture. The work of Hall et al. (1995a, b) suggests that between c. 50-40 Ma the plate experienced a c. 50 ° clockwise rotation with a southward translation; no rotation between 40 to 25 Ma, and from 25 to 0 Ma the plate rotated clockwise 35 ° with northward translation. The first rotation may be related to the change of motion in the Pacific plate (Clague & Jarrard 1973) at about 42 Ma. Australia was moving north throughout the Tertiary and we interpret the regional unconformity at c. 22 Ma as the result of collision of Australia with a volcanic arc at the southern edge of the Philippine Sea plate (discussed below). This may have caused or contributed to renewed clockwise rotation of the Philippine Sea plate.
Q u a t e r n a r y deposits Although currently there are no active volcanoes on Bacan, Quaternary volcanic rocks are present. These are fresh olivine-, pyroxene- and plagioclase-phyric basalts. Mineral and whole rock geochemistry indicate that these are arc basalts which erupted through a quartz-rich basement (Sibela Continental Suite). There is evidence of a withinplate basalt component from whole rock and mineral chemistry, possibly indicating magmatism related to movements along the sinistral Sorong fault. This is supported by the linear distribution of the volcanic centres, and in accordance with the view of Silitonga et al. (1981). Detritus from the Sibela Continental Suite can be found solely in the Quaternary deposits, indicating that it has become available for erosion only recently, suggesting extremely fast rates of uplift (c.
Pre-Tertiary and early Tertiary evolution of Bacan There are two types of pre-Tertiary rocks in Bacan: continental crust and ophiolitic/arc material. The continental crust is presumably of Australian margin or micro-continental origin; north of this margin was oceanic crust. Ophiolite/arc material in Bacan records Cretaceous and Oligo-Miocene ages, both of which can be correlated with ages recorded in Halmahera which now forms part of the
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GEOLOGY ~ TECTONICS OF BACAN, E INDONESIA
Philippine Sea plate. This implies that the ophiolitic rocks of Bacan were part of the Philippine Sea plate since at least the Cretaceous. Plate tectonic reconstructions show that almost all the oceanic floor of the Mesozoic west Pacific has long since been subducted. The ophiolites of Halmahera and Waigeo, which represent the oldest parts of the Philippine Sea plate, were formed in an intraoceanic arc setting (Ballantyne 1991, 1992) and the Cretaceous Halmahera volcanic arc was situated at sub-equatorial latitudes (Hall et al. 1995a). There is no lithostratigraphic evidence on Bacan for the history of the region between the Cretaceous and Late Eocene. 45-22 Ma
Between the Late Eocene and Early Miocene there was extensive arc activity throughout the BacanHalmahera region. Sukamto et al. (1981) suggested that Oligocene volcanism in the Bacan-Halmahera region was the result of west-dipping subduction from the Pacific side, implying that the region lay on the Eurasian plate. Recent palaeomagnetic evidence (Hall et al. 1995a) from the Tawali Formation shows that the region was part of the Philippine Sea plate. Oceanic crust of the Australian plate was subducted northwards, forming an arc at the southern margin of the Philippine Sea plate and fragments of this arc extend from northern New Guinea, through the Bacan-Halmahera-Waigeo region, into the east Philippines (Rangin et al. 1990; Hall 1995; Hall et al. 1995a). The Oligocene Tawali Formation is interpreted as part of this volcanic arc. The Upper Eocene to Lower Miocene Bacan Formation is genetically related to the Tawali Formation and represents temporal and spatial variations of the same arc (Fig. 5). There are two possible correlatable volcanic arcs in the east Indonesia-west Pacific area: the OligoceneMiocene volcanic rocks of Irian Jaya and Papua New Guinea (e.g. Pigram & Davies 1987) or the Eocene-Oligocene Palau-Kyushu remnant arc (Karig 1975; Sutter & Snee 1980). The PalauKyushu and the West Mariana ridges are associated with opening of the Parece Vela Basin (at 3017 Ma) and these remnant arcs are attributed to subduction of Pacific ocean (e.g. Uyeda & BenAvraham 1972; Seno & Maruyama 1984) and are therefore different from the arc represented by the Tawali Formation. Correlation of volcanic rocks in Bacan and the Bird's Head was suggested by Van Bemmelen (1949) and Verstappen (1960). The arc volcanic rocks of north Irian Jaya, for example the Arfak (Ratman & Robinson 1981; Pieters et al. 1982), Batanta (Sanyoto et al. 1985), Yapen (Atmawinata et al. 1989) and Mandi Formations
491
(Pieters et al. 1989) and Papua New Guinea, for example the Bismarck Volcanic Province (Dow 1977), are interpreted to be related to subduction between the Australian and Pacific plates (Dow 1977; Pigram & Davies 1987) and may be a continuation of the Bacan-Tawali Formation volcanic arc. 22 Ma unconformity
A collision between the Australian margin and an island-arc has been implied or suggested by many authors (e.g. Dow 1977; Jaques & Robinson 1977; Pieters et al. 1983; Pigram & Davies 1987), although suggestions of the age of this collision vary. Pigram & Symonds (1991) review estimates of its age ranging from Eocene to Late Miocene; for east New Guinea they argue for a collision unconformity at c. 30Ma. Charlton et al. (1991) tentatively suggested a mid-Oligocene age for collision on Waigeo, based on a poorly defined Upper Oligocene age of the Mayalibit Formation. However, new isotopic dating of volcanic rocks from this formation indicates a narrow age range, of uppermost Oligocene-lowermost Miocene (our unpublished results). At the south end of Mayalibit Bay on Waigeo these volcanic rocks dip at up to 40 ° beneath Miocene limestones, suggesting they are below the unconformity. The presence of the Australian continental rocks on Bacan could be due to: (1) Early Miocene collision; (2) Early Miocene collision followed by Neogene strike-slip translation; or (3) Pliocene or younger pure strike-slip translation. Although there is no indisputable evidence, the proximity of the Sibela Continental Suite to the arc, interpreted to be the result of subduction of the Indo-Australian plate under the Philippine Sea plate, and the juxtaposition of the continental rocks with the ophiolite interpreted to represent Philippine Sea plate material favours a collisional interpretation (1 or 2). The character of the Nusa Babi Monzodiorite, suggesting post-collisional melting of a continental source, and the evidence of a continental crustal contribution to Late Miocene volcanic rocks also suggest the presence of a continental basement beneath central and south Bacan by the early Neogene. On Bacan there is a regional unconformity of Early Miocene age which is interpreted as resulting from the collision of Australian and Philippine Sea plates (Fig. 6). There is a change from folded, metamorphosed arc rocks to unmetamorphosed carbonates and a possible stitching intrusion of Philippine Sea plate rocks (the Bacan and Tawali Formations) and Australian plate rocks (Sibela Continental Suite) by the Nusa Babi Monzodiorite which is of Early Miocene age. This interpretation
492
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Fig. 6. Simplified tectonic setting of the Bacan region during the Early Miocene. Continuation of subduction culminated in collision and thrusting of the continental crust, under the arc-forearc region of the Philippine Sea plate. Following collision, the Philippine Sea plate rotated westward.
is supported by a similar lithostratigraphy throughout the Bacan-Halmahera region (Hall et al. 1988 1991), where most of the Lower to Middle Miocene is represented by carbonate rocks deposited above Upper Eocene-Lower Miocene arc rocks of Philippine Sea plate affinity and rocks of Australian continental affinity. It is therefore concluded that the collision in east Indonesia occurred in the Early Miocene and the authors agree with Ali & Hall (1995) who suggest that much of the evidence from east Indonesia and New Guinea can be understood in terms of pre-Miocene intra-oceanic arc tectonics, an arc-continent collision at about 22 Ma, and Neogene strike-slip tectonics. 19-15 Ma
After the Early Miocene collision, the region was uplifted and deposition of shallow marine carbonates followed. A tectonically quiescent period is envisaged at this time. Localized uplift contributed influxes of volcaniclastic material onto the carbonate platform (Fig. 7), probably derived from the pre-Miocene formations. Initiation of Molucca Sea subduction and s u b s e q u e n t arc v o l c a n i s m
Northward movement of Australia during the Neogene occurred without subduction at the boundary of the Australian and Philippine Sea plates, which was the left-lateral strike-slip Sorong fault system. However, arc volcanism did result from the eastward subduction of the Molucca Sea
plate under Halmahera (the Philippine Sea plate) at the Halmahera trench (Fig. 8). On Bacan there is a local unconformity of Late Miocene age, and in places the Upper Miocene Kaputusan Formation rests directly on the Bacan Formation. The Kaputusan Formation is the equivalent of other Upper Miocene arc sequences on Halmahera and Obi. Although the oldest isotopic age obtained from the Kaputusan Formation is c. 7.5 Ma, there was a thermal event at c. 15 Ma recorded by widespread resetting of isotopic ages in the Bacan Formation and rocks on Obi. The Saleh Diorite, interpreted as a precursor to the Kaputusan volcanics, was intruded at c. 15 Ma. The oldest isotopic ages obtained from volcanic sequences on Obi are c. 12 Ma (Baker & Malaihollo 1996). Thus, initiation of subduction possibly started at c. 15 Ma and the first volcanic products were erupted at c. 12Ma. Late Neogene-Recent
At c. 3 Ma there was more than 60 km shortening between east and west Halmahera, attributed to movement along the Sorong fault (Nichols & Hall 1991). Splays of the Sorong fault running through the Bacan region may have contributed to the ending of Kaputusan volcanism. Quaternary volcanism was later reactivated along the fault splays (Fig. 9). These faults are also responsible for the shaping of Bacan coastlines and the creation of deep basins surrounding Bacan. The culmination of collision processes in the Molucca Sea will eventually result in the transfer of the Bacan-
GEOLOGY & TECTONICS OF BACAN, E INDONESIA
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H a l m a h e r a region f r o m the edge of the Philippine Sea plate to the Eurasian margin (Hall & Nichols 1990).
Implications for regional tectonics This study has p r o v i d e d n e w geological data and elucidated the tectonic d e v e l o p m e n t of the Bacan
region. In a regional context, it has contributed to an u n d e r s t a n d i n g o f the tectonic history o f the convergent zone b e t w e e n the Australian, Philippine Sea and Eurasian plates in east Indonesia. S o m e o f the m o s t important implications of this w o r k are: (1) M o s t of Bacan is interpreted as part o f the Philippine Sea plate. Cretaceous links are indicated by the presence of the Sibela ophiolite,
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Fig. 8. Simplified tectonic setting of the Bacan region during the Late Miocene. Westward subduction of the Molucca Sea plate under the Philippine Sea plate started at c. 15 Ma, probably from the south and moved northward. Volcanicity started in Bacan at c. 7.5 Ma. Shallow marine sequences and fringing reefs formed around volcanic centres.
494
J.F.A. MALAIHOLLO ~; R. HALL
Fig. 9. Simplified tectonic setting of the Bacan region during the Late Pliocene-Quaternary. The Sorong fault system transform boundary cut through the Bacan region, contributing to the cessation of Miocene-Pliocene volcanism. Quaternary volcanism erupted through continental crust along strands of the Sorong fault. Faults controlled coast lines and basins developed between strands of the fault.
(2)
(3)
(4)
(5)
and younger connections are indicated by the similarity of Eocene-Oligocene arc sequences which also record a similar palaeomagnetic history (Hall et al. 1995a). The Bacan and Tawali Formations are most likely to be arc-related sequences produced as a result of subduction of oceanic crust of the Australian plate, probably directly linked to the Indian Ocean, under the Philippine Sea plate. The Sibela Continental Suite arrived in the Bacan region after collision of Australia or a rifted fragment of Australia with the Philippine Sea plate. The collision of Australia with the Philippine Sea plate can be dated at c. 22 Ma in the Bacan region. Younger Neogene-Recent movements on strands of the Sorong fault have modified the geology of the region and permitted emplacement of younger volcanic rocks.
Several different models for the tectonic development of NE Indonesia have been proposed, e.g. the indentor model of Charlton (1986); the terrane model of Silver et al. (1985); and the marginal basin model of Ben-Avraham (1978), and all were based upon limited geological data. The BenAvraham model considers Bacan-Halmahera as part of the Australian continent, which is wrong as almost all of the region has been part of the Philippine Sea plate since Cretaceous. Both the Charlton and Silver et al. models imply that the Sibela continental block was translated by postMiocene strike-slip motion to the Bacan region,
which has been disputed by results of this study. Ali & Hall (1995) and Hall (1995) offer alternative interpretations of the region which incorporate the results of this and other recent geological and geophysical studies. In a wider context, if the terrane definition of Howell et al. (1985) is followed, the Sibela Continental Suite, the Sibela ophiolite, the Bacan, Tawali and Kaputusan Formations could be considered as five different tectono-stratigraphic terranes. This may lead to interpretations such as those of Struckmeyer et al. 1993, who apparently assign separate histories to each of these 'terranes'. Karig et al. (1986) argued that rocks in the north Philippines, which have a similar character to the Sibela Continental Suite, Sibela ophiolite, Bacan and Kaputusan Formations, were separate allochthonous terranes juxtaposed by strike-slip faulting. Whilst not arguing against their interpretation, this study has shown that the juxtaposition of rocks of different character does not necessitate the notion of allochthonous terranes. In the Bacan region, the only fragment that may genuinely be an allochthonous terrane is represented by the Sibela Continental Suite. For the most part, the region has always been at the edge of the Philippine Sea plate, and different 'tectono-stratigraphic terranes' reflect different tectonic regimes at the edge of the plate. The use of stratigraphical similarities is often used in the literature to correlate tectono-stratigraphic terranes and ultimately tectonic history (e.g. Hamilton 1979; Pigram & Panggabean 1984). Using this technique, the Bacan region could be
GEOLOGY • TECTONICS OF BACAN, E INDONESIA correlated with parts of Papua N e w G u i n e a (Oligocene pillow lavas against continental basement; Dow 1977), the Zamboanga Peninsula of Mindanao (high grade continental rocks juxtaposed against meta-ophiolite unconformably overlain by Middle Miocene volcanic rocks; Rangin 1991) and north Philippines (see above; Karig et al. 1986). This type of correlation may lead to erroneous interpretations since each of these regions has different tectonic affinities (Bacan was and is still part of the Philippine Sea plate; North Papua New Guinea was part of the Philippine Sea plate, but now is part of the Australian plate; Z a m b o a n g a and north Philippines were and are still part of the Eurasian plate). Similarities in the stratigraphy are consequences of similar tectonic histories (areas recording the collision of continental with oceanic plate), and one should be cautious in correlation and interpretation of m o v e m e n t of terranes along strike-slip fault. All too often tectonic reconstruc-
495
tion of a region is based on meagre geological data, such as in the Bacan region, resulting in a diversity of interpretations. Although this study has provided the most comprehensive geological dataset from the Bacan region, the history of the region in the Early M i o c e n e and before the E o c e n e is still unclear. Bearing this in mind, one should be cautious in interpreting the tectonic evolution of other, similarly complex, older regions. This work was supported by NERC award GR3/7149, grants from the Royal Society and the University of London SE Asia Geological Research Group, and financial assistance from Amoco Production Company. We thank S. J. Baker, P. D. Ballantyne, E T. Banner, T. R. Charlton, E. M. Finch, G. J. Nichols and S. J. Roberts for their contributions and discussion, and C. C. Rundle and D. C. Rex for guidance and help with isotopic dating. Logistical assistance was provided by GRDC, Bandung and the Director, R. Sukamto with field support by D. A. Agustiyanto, A. Haryono, and S. Pandjaitan.
References
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margin of the Australian Continent and the origin of some microcontinents in Eastern Indonesia. Tectonophysics, 107, 331-353. & SYMONDS, P. A. 1991. A review of the timing of the major tectonic events in the New Guinea Orogen. Journal of SE Asian Earth Sciences, 6, 307-318. --, DAVIES, P. J., FREARY, D. A., SYMONDS, P. A. & CHAPRONIERE, G. C. U. 1990. Controls on Tertiary Carbonate Platform evolution in the Papuan Basin: new play concepts. In: CARMAN, G. J. & Z. (eds). Petroleum Exploration in Papua New Guinea. Proceedings of the First PNG Petroleum Convention, 185-195. RANGIN, C. 1991. The Philippine Mobile Belt: a complex plate boundary. Journal of SE Asian Earth Sciences, 6, 209-220. --, JOLIVET,L. & PUBELLIER,M. 1990. A simple model for the tectonic evolution of southeast Asia and Indonesia region for the past 43 m.y. Bulletin de la Socidtd ggologique de France, 8 VI, 889-905. RATMAN, N. & ROBINSON,G. P. 1981. Geological Map of the Manokwari Quadrangle, lrian Jaya. Geological Research and Development Centre, Bandung, Indonesia. SANYOTO, P., PIETERS, P. E., AMRI, CH., SIMANDJUNTAK, W. & SUPRIATNA,S. 1985. Preliminary Geological Map of Parts of the Sorong, Kasim, west Waigeo and Misool Quadrangles, lrian Jaya. Geological Research and Development Centre, Bandung, Indonesia. SENO, T. & MARUYAMA, S. 1984. Paleogeographic reconstruction and origin of the Philippine Sea. Tectonophysics, 102, 53-84. SILITONGA, P. H., PUDJOWALUJO,H. & MOLLAT, H. 1981. Geological reconnaissance and mineral prospecting on Bacan island (Moluccas, Indonesia). In: BARBER, A. J. & WmYOSUYONO, S. (eds) The Geology and Tectonics of Eastern Indonesia. Geological Research and Development Centre, Bandung, Indonesia, Special Publication, 2, 373-81. SILVER, E. A., GILL, J. B., SCHWARTZ,H., PRASETYO,H. & DUNCAN, R. A. 1985. Evidence for a submerged and displaced continental borderland, north Banda Sea, Indonesia. Geology, 13, 687-691. STRUCKMEYER,H. I. M, YEUNG, M. & PIGRAM,C. J. 1993. Mesozoic to Cainozoic plate tectonic and palaeogeographic evolution of the New Guinea Region. in: CARMAN, G. J. & CARMAN, Z. (eds) Petroleum Exploration and Development in Papua New Guinea. Proceedings of the Second PNG Petroleum Convention, 261-290. SUKAMTO, R., APANDI, T., SUPR1ATNA, S. & YASIN, A. 1981. The geology and tectonics of Halmahera Island and surrounding areas. In: BARBER, A. J. & WIRYOSUYONO,S. (eds.) The Geology and Tectonics of Eastern Indonesia. Geological Research and Development Centre, Bandung, Indonesia, Special Publication, 2, 349-62. SUTTER, J. F. & SNEE, L. W. 1980. K/Ar and 4°Ar/39Ar Dating of Basaltic Rocks from Deep Sea Drilling Project Leg 59. Initial Reports of the Deep Sea Drilling Project, 59, 729-734.
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GEOLOGY •
TECTONICS OF BACAN, E INDONESIA
UYEDA, S. & BEN-AVRAHAM, Z. 1972. Origin and development of the Philippine Sea. Nature Physical Sciences, 240, 176-178. VAN BEMMELEN, R. W. 1949. The Geology of Indonesia. Volume Ia, General Geology. Government Printing Office, The Hague. VERSTAPPEN, H. TH. 1960. Geomorphological observa-
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tions on the north Moluccan - northern Vogelkop Island Arcs. Nova Guinea Geology, 3, 11-38. VROON, P. Z. 1992. Subduction of Continental Material in the Banda Arc, Eastern Indonesia. PhD thesis, University of Utrecht. YASIN, A. 1980. Geologic Map of the Bacan Quadrangle, North Maluku. Geological Research and Development Centre, Bandung, Indonesia.
Dating of Neogene igneous rocks in the Halmahera region: arc initiation and development SIMON
BAKER
& JEFFREY
MALAIHOLLO
SE Asia Research Group, Department of Geological Sciences, University College, London WC1E 6BT, UK Abstract: Potassium-argon ages of Neogene to Recent igneous rocks from the Halmahera region
record a history of intra-oceanic arc development since the late Middle Miocene following an earlier phase of collisional plutonism. Arc formation from the Middle Miocene onwards was due to the east-directed subduction of the Molucca Sea plate beneath the Philippine Sea plate as it arrived at the Eurasian margin. The distribution of ages within the Neogene arc indicates a northward migration of volcanic activity during the Late Miocene to Pliocene. Results of the dating work show that after collision with the Australian margin at c. 22 Ma there was a period of volcanic quiescence and limestone deposition before a new arc formed. This arc began erupting at around 11 Ma on Obi as a result of subduction of the Molucca Sea plate. Initiation of subduction is thought to have occurred around 15-17 Ma and may have been responsible for disturbing potassium-argon ages of pre-Neogene rocks. Dates from fresh rocks show that the volcanic front migrated northwards through Bacan and Halmahera throughout the Late Miocene to Early Pliocene. Limestone deposition was curtailed as arc activity migrated north while volcanism died out from the south. No Neogene volcanism younger than 8 Ma is observed in the Obi area while on Bacan subduction-related volcanism ceased at c. 2 Ma. Late Pliocene crustal deformation caused a 30-40 km westward shift of the volcanic front. Quaternary volcanic rocks exposed in Bacan and the extreme south of Halmahera are not direct products of subduction but, rather, display geochemical characteristics of both subduction and fault-related magmatism. These volcanic rocks are distributed along splays of the Sorong fault system. The formation and propagation of the Halmahera arc is a consequence of the clockwise rotation of the Philippine Sea plate as the southern edge moved across the northern Australian margin and impinged on the east Eurasian margin. The ages of initiation of volcanism and subduction track the developing plate boundary as subduction propagated northwards.
The principal islands of the Halmahera group (Halmahera, Bacan and Obi) lie in northeastern Indonesia in the province of Maluku, straddling the equator between 127°E and 129°E. These islands lie at the junction of three major plates (the Philippine Sea plate, the Australian plate and the Eurasian plate) where the A l p i n e - H i m a l a y a n and the C i r c u m - P a c i f i c orogenic belts meet. The present-day tectonics are therefore complex (Fig. 1). These belts meet in the region of the Sorong fault system which is responsible for transferring crustal fragments of Philippine Sea and Australian origin into the complex of island arcs and small ocean basins that forms the Eurasian margin. Present-day tectonics are the result of the northward m o v e m e n t of continental Australia (Australia and New Guinea) into the Pacific region throughout the Tertiary. Clockwise rotation of the Philippine Sea plate since c. 25 Ma led to the development of the Sorong Fault Zone as it collided with the northern Australian margin (Hall et al. 1995a, b). Current motion between these two plates is taken up by sinistral m o v e m e n t on the Sorong fault system in northern New Guinea.
The currently active H a l m a h e r a arc, at the eastern edge of the Molucca Sea (Hamilton 1979; Moore & Silver 1983), formed as a consequence of eastward subduction of the Molucca Sea plate beneath the Philippine Sea plate as the latter rotated clockwise. Geophysical evidence from earthquake data indicates a seismic zone dipping at c. 45 ° to the east to depths of about 200 k m (Cardwell et al. 1980). To the south the subduction zone appears to be terminated by a strand of the Sorong fault system just north of the eastern tip of Mangole (Sula Platform). Opposing the Halmahera arc is the Sangihe-north Sulawesi arc approximately 250 k m to the west which is the product of westward subduction o f the Molucca Sea plate; beneath the Sangihe arc the slab dips at 55-65 ° and reaches a depth of 600 k m (Cardwell et al. 1980). The oldest rocks known from the Sangihe arc are of early Middle Miocene age and particularly voluminous arc activity occurred between 5 and 14 Ma (Hamilton 1979). The present Halmahera arc lies north of the equator and is built upon a Neogene arc that extended from Obi northwards through Bacan to
From Hall, R. & Blundell, D. (eds), 1996, Tectonic Evolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 499-509.
499
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EARLY MESOZOIC OROGENY IN FUJIAN, SE CHINA extremely thick (maximum thickness of 3400 m in Da'tian) molasse sequence was rapidly deposited and the width of the basin reached a maximum.
553
Permian
SouthChina SeaBlock
SouthChinaBlock
Foreland fold-and-thrust belt Widely developed folds and low-angle thrusts in west Fujian have been recognized by regional geological surveys and drill cores that penetrated into both the sedimentary cover and the basement. The foreland fold and thrust belt of west Fujian is characterized by multistage and multilevel thrusting of the cover and basement decoupling. The most prominent folds were developed in the Lower Permian coal-bearing sediments, and the folding in these rocks is intense and complex. This coal-bearing Lower Permian sequence also acted as the detachment zone which enabled the development of imbricate thrusts (Fig. 4) and folds in the overlying sedimentary cover. Other detachment zones in the basin were in the Lower and Upper Carboniferous strata. Low angle thrusts in the basin usually occur in groups withvery similar dips (15-20 °) and strikes (ENE-WSW). These occur not only as cover thrusts related to different layers of detachment, but also as steeper thrusts in the basement of the basin. Kinematic studies of the thrusts reveal two main stages of thrusting. The earlier thrusting episode has been related to Early Mesozoic north-south convergence between two continental blocks (Fig. 5) and was overprinted by Early Cretaceous thrusting from west to east (e.g. Tao 1987). The northwest limbs of the anticlines are shorter and steeper, and at some localities these anticlines are even overturned. Microfabric studies show that the direction of the earlier stage of thrusting was from
I
II
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Early Triassic
Late Jurassic
~
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Oceanic Crust
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Fig. 5. Schematic cross-section to show the evolution of west Fujian foreland basin. Symbols are the same as those in Fig. 3.
SSE to NNW, with the root of the thrust belt, therefore, in the south of the basin.
Permian-Triassic granites Permian to Triassic S-type granites in the west Fujian foreland basin generally cut across the fractures and the anticlinal axes. They are alkaline granites with high 87sr]S6Sr ratios and low Fe3+/ (FEZ++ Fe 3+) ratios. In addition, they are also LREE enriched (Wang & Liu 1986). A discriminant diagram of Rb against Y + Nb (Fig. 6) suggests a syncollisional and volcanic arc origin. Most of these granites postdate the closure of the previous ocean basin and may be related to the partial melting of the partly subducted oceanic crust or to voluminous sediments accumulated between the two continents before the final closure of the intervening ocean basin.
Early Mesozoic continental collision in SE China Fig. 4. Structural profiles to show the thrusting activity in west Fujian. See Fig. 1 (I and II ) for the locations of the profiles.
Studies of the geology of SE Asia and of south China in the last decade or so have culminated in the recognition of several continental blocks
554
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~
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Fig. 6. Discriminant diagram of Rb against Y+Nb for Permo-Triassic granites from west Fujian. The determination of tectonic settings is based on Pearce et al. (1984). ORG = ocean ridge granites; Syn-COLG = syn-collisional granites; VAG = volcanic arc granites; WPG = within plate granites. The original data are from Fujian Bureau of Geology and Mineral Resources (1985). that were assembled mainly during the Late Palaeozoic-Early Mesozoic (e.g. Seng0r 1990). Most of the blocks originated from Gondwana. The Middle-Late Triassic collision between the South China block and the Indochina block along the Black River (or Song Da) suture and the Red River (or Song Ma) suture is well documented, which is in marked contrast to the little attention that has been paid to the possible suture zones further east in the northern shelf of the South China Sea and the south China margin. According to Zhu (1987), there existed a pre-Cretaceous active margin along the northern shelf of the South China Sea, which passes through the southern part of Hainan Island and may be connected to the Red River suture. The similarity in Cambrian sedimentary successions, ore deposits, trilobites and brachiopods between Hainan Island and Australia, together with the discovery of Upper Palaeozoic glacio-marine
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deposits of similar origin to those in Tibet, west Yunnan and SE Asia (Yu 1989) suggest that Hainan Island is a fragment that rifted from Gondwana at or after the end of the Palaeozoic. The location of the suture zone in Hainan Island and its vicinity remains a problem to be solved. Wang (1986) proposed that the suture zone extends along the Reiqiong Strait that separates Hainan Island from mainland China. Others (Zhen 1989) have argued that it is located in northern Hainan (Fig. 7). Further east, Hsti et al. (1990) postulated the existence of a Mesozoic (Triassic or Jurassic) collision between the Huanan and the Donnanya Blocks. However, it is worth mentioning here that the suture zones to the south and to the east of Fujian in SE China are of different origins and ages (e.g., Zhou 1992b). The deep-water flysch deposits of the Lower Triassic Xikou Formation, the intense deformation of these deposits, the formation of S-type granites and the first episode of thrusting in west Fujian fold-and-thrust belt have all been interpreted above to be genetically related to a continent-continent collision which took place to the south of the west Fujian foreland basin in the Early Triassic. This collision marked the beginning of the Early Mesozoic orogeny in the region. However, deformation did not stop as collision ceased. Continuing post-collisional convergence made the orogeny a long process and led to the uplift of colliding continental blocks, resulting in the deposition of thick molasse in the adjacent west Fujian foreland basin (Figs 3 & 5). The difficulty in identifying the suture zone has been due mainly to the massive magmatic and deformation activity related to the subduction process to the east of Fujian in the Taiwan Strait in the Cretaceous (Zhou e t al. 1992b), and to submergence of the area beneath the South China Sea. However, geophysical data from the region record some evidence of the existence of an ENE trending tectonic zone. The Bouguer anomaly trend in east Fujian swings from NE to ENE at
//Pearl River /" Basin /' _--
. . . . . .
........
//
,oo
Fig. 7. Postulation of the positions of Early Mesozoic suture zones on south China margin. Lines 1, 2, 3 are suture zones proposed by HsiJ et al. (1990), Zhen (1989) and Wang (1986), respectively.The area marked 4 is the possible location of the Early Triassic suture between the South China and South China Sea blocks suggested in this paper.
EARLY MESOZOIC OROGENY IN FUJIAN, SE CHINA the southern border of the west Fujian foreland basin. Folding and thrusting have resulted in the thickening of the crust in the basin (Fig. 2). Along the coast from Nan'ao to Hong Kong is an ENE tending tectonic zone characterized by a relatively low aeromagnetic anomaly and gravity gradient, reflecting proposed ultrabasic igneous rocks at a depth of 8-12 km (unpublished report of No. 909 Aeromagnetic Survey Brigade, Ministry of Geology and Mineral Resources of China). In the Pearl River Mouth Basin, three ENE trending aeromagnetic lows are also attributed to deep basic intrusive rocks (Guong et al. 1989). In order to unravel the basement structures beneath the Tertiary basins of the northern South China Sea shelf, an integrated geological-geophysical approach has been used to interpret several N-S trending profiles for which there are gravitational, magnetic, seismic and borehole data supplemented by onshore data. Forward and inversion iterative modelling was applied to the profiles. These data enabled the postulation of Palaeozoic metamorphic rocks and Upper Mesozoic sedimentary and metamorphic rocks in the basement. Fault mapping revealed an ENE fault system cross-cut by younger NW trending faults. All these phenomena are consistent with a collisional event. A schematic evolution for west Fujian is proposed in Fig. 5, though at this juncture we can only conjecture as to the position of the suture zone (Fig. 7).
Discussion According to Seng/3r (1985), the Cimmerian Continent rifted from the northern margin of Gondwana during the latest Palaeozoic to the earliest Mesozoic. This continent disintegrated as it moved through the Tethyan domain. Palaeomagnetic and palaeontological data suggest a possible Gondwana origin for the Yangtze, south China and Hainan Island blocks. The evidence of Early Mesozoic continental collision in SE China suggests that, before the Cimmerian Continent united with Eurasia during the Late TriassicMiddle Jurassic (Sengtir 1985), some of the continental blocks (e.g. South China block and South China Sea block in this paper) had already collided with each other during their northward drifting. The collisions of continental blocks that constitute the Cimmerides therefore took place diachronously.
555
The so-called 'Indosinian' movement in Indochina and south China is the regional geological phenomenon that resulted from this diachronous collision process. As mentioned above, the Early Mesozoic orogeny in SE China was not a short event, because N-S post-collisional convergence continued until the end of the Jurassic. The two regional unconformities within the syn-orogenic molasse and separating the molasse and the underlying flysch (Fig. 3), reflect two main stages of tectonic relaxation (or tectonic quiescent periods of Blair & Bilodeau 1988) in the collision zone. The coarse sediment above the unconformities actually represents flexural rebound of the thrust belt as the uplifted area was eroded, and the finer sediment above the two coarse units might represent renewed uplifts that were related to the continuing N-S compression. The traditional practice of relating these two unconformities to, respectively, 'Indosinian' and 'Yanshanian' orogenies should therefore be reassessed. The sedimentary sequence and evolution of the west Fujian foreland basin are comparable with those of typical foreland basins (e.g. the Appalachian foreland basin, Tankard 1986). Thrusting and drcollement took place along ramps at different levels, and the detached pieces could not displace freely on a perfect 'sole' of the southern Appalachian type (Zhu 1989). Finally, the Lower Triassic deep marine fine turbidites in the foredeep of the west Fujian foreland basin conformably overlies the Upper Permian deposits (Fig. 3), which shows that the transition from passive continental margin to active continental margin was gradual. The fact that neither mud nor shale was deposited over the fine turbidites also reveals that the compression was a slow and continuous process. In addition, the lack of evidence of metamorphism shows that the continental collision was a mild collision. This paper was prepared at the University of Wales, Aberystwyth in early 1994, while the senior author was a recipient of a Royal Society Visiting Fellowship. R. J. Whittington, W. R. Fitches, T. Horscroft, J. Charvet, M. Allen and B. C. Burchfiel are thanked for their constructive comments and contributions to the manuscript. Robert Hall is thanked for improving the figures and manuscript. This research was supported by the National Science Foundation of China (project NSFC49202034).
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Index
References in italics are to Tables or Figures accretionary prisms and complexes Banda arc 53, 57 Borneo 252, 253 Songpan Ganzi 99 Sumatra 23-4 Timor trough 81-2 Visayas 515-16 Aceh sliver microplate 22-3 Adang fault 382-3 Aibi-Xingxing suture 99 Aifam Group 468 Ailaoshan belt 540 Ailaoshan suture 99, 106 Aileu Formation 57 Air Bangis granite 331,333 Aitape Basin 527 Ala Shan terrane 99 Altyn Tagh fault 124, 125 Amasing Formation 489 Ambon 445 crustal isotopic signature 447-8 amphibolite, Darvel Bay 265 Andaman Sea 160 Anggi granite 468 apatite fission track analysis Papua New Guinea 528-31 Thailand 244-5 western Sulawesi 404-6, 424 4°mr/39Arages, Kaibobo complex 457-9 arc-continent collision 525 Arip Volcanics 253 Aru trough 86-7 deformation front 92-4 gravity data 89-92 seismic reflection data 88-9 Asem-Asem Basin 252 Ashmore platform 62, 63 asphalt 431 ATLAS model 154 Aurora volcanic ridge 518 Australian plate collision with Philippine Sea plate 137 continental shelf characters crustal thickness 63--6 gravity survey 66-8 marginal slope features 69-81 reflection profile 49-51 structure 63 isotope characteristics 446 plate motion 32 Ayu trough 168
Bacan 445 crustal isotopic signature 448-9 K/Ar dating 502, 503, 505 plate setting 483 Sibela continental suite 483-4 Sibela ophiolite 486-7 tectonic evolution 490-3 Tertiary stratigraphy 487-90, 501 Bacan Formation 487, 491 Bakit Mersing line 248, 252 Balangbaru Formation 359-61,366, 367 Ban Ang Formation 236 Ban Thalat Formation 236 Banda arc 11, 47, 64, 451 backarc region 56, 68-70 forearc structure 68 indentor model 70-2 orogen evolution 57-8 reflection profiles methods 48-9 results 49-56 Banda arc (east) see Aru trough Banda orogen 194-5, 197, 451 Banda Ridges 445 crustal isotopic signature 449 Banda Sea Miocene history 462 Miocene setting 144-5 tectonic blocks 140, 141 tectonic setting 175-7 Bangdu Formation 541 Banggai granite 474 Banggai Island stratigraphy 474 Banggai-Sula continental fragment 160, 466 Banggai-Sula Spur 445 Banggong suture 99, 107 Bantimala melange complex 354-5 lithology 355 outcrop distribution 356-61 structure 355-6 tectonic evolution 361-2 Banyak thrust 25 Baoshan block 101,540 diamictite 541-4 terrane motion 544-6 Barail Formation 134 Barisan Mts 321-2 Barisan orogen 189-91,197 Barito Basin 252 Batu fault 21 Bawang Dacite 253 557
558 Belaga Formation 248, 250, 251,253 Bentong-Raub line 204 Bia Formation 475 Biak Formation 475 biostratigraphy, Pak Lay sediments 229-30 Bird's Head 139, 142-3, 146 Pb isotope ratios 446 stratigraphy 470, 471 tectonic reconstruction 477 tectonic setting 165, 174 Bisa, K/Ar dating 503 Bislig Bay 517 Black River suture see Song Da Bliri volcanics 527 Bobang Formation 474 Bogal Limestone Formation 468 Bonaparte basin 62, 63 Borneo (Kalimantan) 252 Schwaner Mts 252-3 tectonic setting 157-9, 168-70 see also under Kalimantan Bouguer gravity measurements Aru trough 89-92 Australian continental shelf 66 Boyan Melange 252 Boyan suture 99, 110-11 Browse basin 62, 63 Buan Formation 251 Bunga basalt 253 Bunta Formation 474 Burma see Myanmar Buru, tectonic setting 168 Buton 160, 167, 431-3,445 displaced terrane 466 palaeomagnetism study methods 434-5 results 435-4 1 results discussed 441-2 Buya Formation 474
Cagayan ridge 164 Camba Formation 354-5, 367 Camba-Enrikang-Mamasa Complex 397 Cambro-Ordovician terrane distribution 111 Canning basin 62, 63 Carboniferous studies Fujian province 551 rifting 104 Salawati stratigraphy 468 terrane distribution 112 Tomori stratigraphy 474 Carcar Formation 520 Caroline plate 5-6, 141, 145-7, 466 tectonic setting 168, 177 Cascades fault zone 519 Cathaysia block 100 Cathaysialand 113
INDEX Celebes Sea 140, 147, 164, 171 Cenozoic tectonic setting for SE Asia 124-5 Champa Formation 236 Changning-Menglian Belt 540, 546 Changning-Menglian suture 99, 107-8 China, South 539, 549 collision tectonics 525 Asia-India 129, 133-5 Australia-Philippine Sea plate 137 Fujian evidence 553-5 Indochina 225, 230 Philippines 512-16 Sulawesi 171-4 continental fragments 445 Cotabato fault 518 Cotabato trench 39-40 Cretaceous studies Khorat Plateau 237-8 palaeomagnetism 207 Salawati stratigraphy 468 Sumatra plutonism 324, 325, 330 terrane distribution 114 Tomori stratigraphy 475 Crocker Formation 256-8 crustal provenance analysis Pb-Nd isotope analysis methods 446 results 447-50 results discussed 450-1
Daguma fault 518-19 DAMAR line 48, 49-56, 64, 76, 78-82 Damuchang Formation 541 Dangerous Grounds Block 308 Daram Sandstone Formation 470 Darvel Bay Complex 263-4 metamorphic geology 264-73 origins 273-7 ophiolite emplacement 277 declination 435,436, 437, 439 Devonian studies Fujian province 550-1 rifting 104 Salawati stratigraphy 468 terrane distribution 111 diamict, Yunnan 541-3 Dingjiazhai Formation 540, 541,542 displaced terranes 466 Dongnanya block 100 Dungun Graben 295-6
East Java Sea 384 East Malaya terrane 99, 102 East Natuna Basin 299 East New Guinea composite terrane 140, 141 East Papua composite terrane 140
~NDEX East Talaud bank 40-2 Elat Formation 87 Embaluh Group 251,253, 254 Eocene studies Asia-India collision 129 plate settings 380-2 Sulawesi tectonics 383
Facet Limestone Group 468 Fafanlap Formation 470 Faumai Formation 470 Finnisterre terrane 140, 141, 145, 148 Flores thrust 11 forearc environments, examples of see Sumatra margin 'Fu'an-Nan'jin fault 550 Fujian province 549 foreland basin 549-50 collision tectonics 553-5 fold and thrust belt 553 granites 553 stratigraphy 550-3
Gabaldon basin 519 Gamta Limestone Formation 468 Gaoligongshan metamorphics 540 Garba pluton 331 garnet amphibolite, Darvel Bay 272-3 geochemistry Darvel Bay 267-8, 269 Papua New Guinea 532-3 Sumatra granitoids 325-8 western Sulawesi 397-400, 425 geochronology s e e 4°mr/39Ar;K]mr; Rb/Sr; Sm/Nd; U/Pb glacigenic sediments, Yunnan 541-3 Gondwana terrane rifting 103-5, 539, 544-5 gravity see Bouguer Gujuti Formation 504 Gunung Api 68
Hainan Island 99, 103, 554 Halmahera 29, 32, 499-500 K/Ar dating methods 500-2 results 502-6 results discussed 506-8 stratigraphy 500 tectonic setting 165,491 Hatapang granite 331 Himalaya trench 8 Himalayan front 9-11 hornblende gneiss, Darvel Bay 265 Huai Hin Lat Formation 206, 209, 236 Huanan Alps orogen 549
559
Huon-Finisterre arc terrane 13 hydrocarbon potential Lao PDR 246 Malay Basin 281,285 Penyu Basin 281 Salawati Basin 479 Thailand 244 Tomori Basin 479
inclination 435,436, 437, 439 indentor model, Banda arc 70-2 India-Asia collision kinematic model 125-8 forward modelling 135-7 model constraints 133-5 rotation evidence 131-3 Tertiary development 129-31 Indian Ocean, seafloor spreading 127 Indoburman flysch 134, 135 Indochina block/terrane 99, 102 tectonic history 204-6 tectonic setting 157 Triassic collisional tectonics 225, 230 Indonesia orogenies 188-96 tectonic development 186-8 tectonic history 204-6 tectonic setting 186 Indosinian orogeny 230, 549, 555 Indus Yarlung Zangbo suture 99 Irian Jaya see Bird's Head; Salawati Basin isothermal remanent magnetism measurement methods 435 results 435, 436, 437 Izu trench 8 Izu-Mariana trench 16
Jass Formation 468 Java, tectonic setting 160 Java trench 7, 8 Java-Sumatra margin, collision evidence 133-4 Javan Forearc 337 Jinshajiang suture 99, 106 Jurassic studies Fujian province 552 palaeomagnetism 206-7 rifting 104 Salawati stratigraphy 468 Sumatra plutonism 324, 325, 330 terrane distribution 114 Tomori stratigraphy 475
K/Ar dating
Bacan 502, 503, 505
560 K/Ar dating (contd) Bisa 503 Halmahera arc 506-8 Khorat granites 244 Obi 503, 504 Papua New Guinea 531 Pak Lay 229 Rajang Group 253 Sumatra granitoids 328-9, 333 western Sulawesi 400-3 Kai Islands 87-8 Kaibobo granite 456 Kaibobo ultramafic complex 4°Ar/39Ardating 457-9 closure temperature 459 implications for Banda Sea 462 implications for obduction 461-2 P-T-t modelling 459-61 petrography 457 Kais Formation 471 Kalaw red beds 208, 210-11,212-13, 216 Kalimantan (Borneo) stratigraphy 384 Rajang Group 251-2, 253, 254-6 see also under Borneo Kalomba Formation 475 Kaputusan Formation 489-90, 505 Kasiruta stratigraphy 487-8 Kayasa Formation 505-6 Kayoa 505 Kazakhstan terrane 99 Kelabit Formation 256 Kelalan Formation 250, 256 Kembelangan Group 468 Kemum Formation 468 Kemum terrane 143 Kerabai volcanics 253 Keskain Formation 468 Ketapang batholith 252 Ketungau basin 254 Khlong Marui fault 159 Khok Kruat Formation 207, 209 Khorat Group 225, 230, 231,233 Khorat Plateau 233-5 hydrocarbon potential 244, 246 inversion history 237-45 palaeomagnetic studies 206-7 stratigraphy 235-7 tectonic development 245 Kimberley block 62, 63 Kintom Formation 475 Kisar Island 68 Klamogun Formation 471 Klasafet Formation 472 Klasaman Formation 472 Klondyke Formation 515 Kluet Formation 324 Kongshuhi Formation 541,542 Kontum massif 206
INDEX Kuantan Formation 324 Kulapis Formation 313, 316 Kun Lun fault 125 Kunlun suture 99 Kunlun terrane 99 Kurogegawa terrane 99, 103 Kutei Basin 384
Labang Formation 312, 316 Lamasi Complex ophiolite 397, 400 Lampang Group 208, 212, 214 Lancangjiang suture 99, 107 Langi Formation 366 Lanping-Simao fold system 539, 540 Lao PDR 539 Khorat Plateau 233-5 hydrocarbon potential 244, 246 inversion history 237-45 palaeomagnetic studies 206-7 stratigraphy 235-7 tectonic development 245 Pak Lay Fold Belt history of research 225-8 recent studies 229-31 summary history 231 Lassi pluton 331 Laurasia-Gondwana marginal terranes 539, 544-5 leaky transform fault 70 Legaspi Lineament 517 Lelinta Shale Formation 468 Lenggurur thrust belt 146 Lhasa terrane 99 Lianga fault 517 Ligu Formation 468 Long Bawang Formation 256 Longmen Thrust 124, 125 Lubok Antu Melange 248 Luogengdi Formation 541 Luok Formation 475 Lupar fault 254 Lupar Formation 248, 250, 251,253 Lupar Line 248, 252 Lupar Line ophiolite 248 Lurah Formation 256 Luwuk Formation 475 Luzon, Miocene tectonics 515
Macolod Corridor 511,520-1 Makassar Strait 394-7 tectonic setting 162 Malacca microplate 323, 324 Malawa Formation 354, 366 Malay Basin 281,292-3 basement 283-4
INDEX heat flow 282 hydrocarbons 281,285 stratigraphy 283, 293 structure 284-7, 293-7 Malay blocks, tectonic setting 159-60 Malay Peninsula palaeomagnetic studies 207-10 tectonic setting 539 Maleta basin 62, 63 Manday suture 99, 110 Manunggal batholith 331 Mariana trench 8, 15-16 Marinduque Basin 521 Matano Formation 475 Matindok Formation 475 Mekong basin 282, 299-300 Melanesian orogeny 195-6, 197 melanges Java 353 Nias Island 342 Sulawesi 356 Melawi Basin 254 Menanga Formation 474 Mengliong Group 540 Mentarang Formation 256 Mentawai fault 21, 24 Mentawai fault zone 338, 345-6, 346-50 Mentawai sliver microplate 21-2 Meratus Mts 252 Meratus suture 99, 110 Mergui microplate 323, 324 metachert, Darvel Bay 271-2 metamorphic studies Darvel Bay 264-73 Gaoligongshan 540 Sulawesi 140-2 metatuff, Darvel Bay 270-1 Miangas ridge 35 Mindanao collision zone 511, 512-15 Mindoro-Panay collision zone 511 mineralogy (heavy), Yunnan glacigenics 543 Miocene studies Asia-India collision 130-1 Banda Sea 462 Malay and Penyu Basins 283 Papua New Guinea 533,534-6 Sulawesi tectonics 384 Sulu Sea Basin 311-16, 317 Misool Island, stratigraphy 468, 470, 472, 571 Molucca Sea 29, 147 bathymetric transect features 32-43 geodynamic framework 32 lithospheric boundaries 43-5 subduction 492 tectonic setting 174-5 Molucca-Sorong fault 467 Moon volcanics 489 Mukus assemblage 323, 324
561
Muna 434 Mussau trench 6 Myanmar (Burma) collision evidence 134-5 palaeomagnetic studies methods 212 results 212-13 stratigraphy 210-11 tectonic setting 539
Nagan granodiorite 331 Nalang Formation 236 Nam Con Son Basin 300 Nam Set Formation 236 Nam Thom Formation 236 Nam Xoi Formation 236 Nam-Phong Formation 206, 209 Nambo Formation 474 Nan-Uttaradit ophiolite zone 225 Nan-Uttaradit suture 99, 107 Nanaka Formation 474 Nasa Formation 236 natural thermal remanent magnetism 435,436, 437 143Nd/144Nd and provenance methods of analysis 406-7, 425, 446 results 447-50 results discussed 450-1 Neogene studies Halmahera arc stratigraphy 500 orogenies 188-96 Philippine tectonics 511-12 docking 512-16 extension 520-1 post docking 516-20 Sundaland rotations 217-18 see also Miocene; Pliocene New Guinea convergence evidence 145-6 indentor 43 Pb isotope ratios 446 plate motion 12-13 see also Papua New Guinea New Guinea Limestone Group 470 New Guinea Mobile Belt 525, 526 New Guinea orogen 147 New Hebrides trench 8, 13 Nias basin 21 Nias Island 338-41 deformation studies 341-6 fault pattems 346 significance of structures 346-50 Nofanini Formation 474 North China terrane 99 North Sula-Sorong fault 467 Northeast China terrane 99 Nusa Babi monzodiorite 488-9, 503
562 NW Borneo-Palawan Trough 308 NW Sabah Platform 308 Nyaan Volcanics 253 Nyalau Formation 251
Obi displaced terrane 466 K/Ar dating 503, 504 Tertiary stratigraphy 501 oblique subduction/convergence 3, 19 Oligocene studies Asia-India collision 129-20 Malay and Penyu Basins 283 New Guinea orogen 147 Papua New Guinea 534 South China Sea extension 248 Sulawesi tectonics 383-4 Ombilin granite 324, 329 ophiolites Darvel Bay 263-4, 277 Lamasi Complex 397, 400 Lupar Line 248 Nan-Uttaradit 225 Sibela 486-7 Opol Formation 513 Ordovician studies 111 orogenic events Banda 57-8, 194-5, 197, 451 Baresan 189-91,197 Huanan Alps 549 Indonesia 188-96, 230, 549, 555 Melanesian 195-6, 197 New Guinea 147 Pacific 225 Sabah 258 Sarawak 256 South China Caledonian 549 Sulawesi 191-4, 197 Sunda 188-9, 197 Talaud 191,192, 197 Yanshanian 549, 555
Pacific orogeny 225 Pacific Plate, Mesozoic subduction 549 Pak Lay Fold Belt history of research 225-8 recent studies 229-31 summary history 231 palaeogeography, terrane distribution 111-14 palaeomagnetic studies methods 212, 434-5 regional results Asia rotation evidence 131-3 Banda Sea 142 Buton 435-42
INDEX Khorat Plateau 206-7 Malay Peninsula 207-10 Myanmar 212-13 relation to plate setting 154-6, 217-19 Pangaea 539, 544-5 Papua New Guinea apatite fission track analysis methods 528-30 results 530 results discussed 530-1 geochemistry 532-3 plate setting 525 stratigraphy and structure 525-7 tectonic summary 533-6 thermochronology 531-2 Pasir Basin 252 Pb isotope ratios methods of analysis 446 results 447-50 results discussed 450-1 Pedawan Formation 250 Penyu Basin 281,297-8 basement 283-4 stratigraphy 283 structure 287-8 Permian studies Fujian province 551,553 Khorat Plateau 235 palaeomagnetism 206 rifting 104 Salawati stratigraphy 468 Sumatra magmatism 323-4, 329 terrane distribution 112 Tomori stratigraphy 475 petroleum see hydrocarbon potential Phalat Formation 236 Phetchbun Fold belt 233, 234 Philippine arc 511 Philippine archipelago Neogene tectonics 137-9, 164-5, 511-12 docking 512-16 extension 520-1 post docking 516-20 Philippine fault 15, 511 Philippine Sea plate 5, 32, 466, 491 boundary evidence 147 coupling with Australia 137, 145-6 rotation estimates 138-9 tectonic setting 162 Philippine trench 15, 32, 44 bathymetry 33-5 Phon Hong Group 235, 236, 244 Phu Phanang Formation 236 Phulekphey Formation 236 Piring granodiorite 253 Piyabung Volcanics 253 plate motions 4, 5-6 boundary convergence 3
INDEX boundary zone deformation 6-7 Plateau Limestone 208, 212 Pliocene studies 536 Poh Formation 475 Prince Alexander Mts 527 provenance analysis see under crustal Pujada ridge 35-9 Pumenqian Formation 540
Qaidam terrane 99 Qamdo-Simao terrane 99 Qiangtang terrane 99 Qinling-Dabie suture 99, 109-10 Quaternary studies Bacan stratigraphy 490 Halmahera arc 506
radiometric dating s e e 4°Ar/39Ar, K/Ar; Rb/Sr; Sm/Nd; U/Pb Rajang Group 253 Kalimantan 251-2 Sarawak 248-51 Ratburi Limestone 208, 212, 214 Raub-Bentong suture 99, 108-9 Rb/Sr dating Sumatra granitoids 324, 328-9 western Sulawesi 406-7, 425 Red River fault 124, 125, 157, 204 Red River suture see Song Ma rifting events 103-5 Ruta Formation 489 Ryukyu trench 8, 15
Sabah 307-8 Central basin 311-16 Darvel Bay Complex 263-4 metamorphic geology 264-73 origins 273-7 ophiolite emplacement 277 NW region deep regional unconformity (DRU) 308-10 structure 311 Rajang Group 254-6 Sabah orogeny 258 Sagaing suture 99, 110 Sahul platform 62, 63 Sainabouli Province see Pak Lay Fold Belt Salawati Basin correlation to Tomori Basin 475-6 hydrocarbon potential 479 stratigraphy 467-72 structure 467 tectonic significance 476-9 Saleh diorite 489, 503
563
Saleh Island stratigraphy 487, 489 Salodik Formation 474, 475 Sampolakosa Formation 435,438, 439, 440 Sandakan Formation 314, 316 Sangihe arc 29, 32, 39 Sangihe basin/trough 39, 40 Sat Khua Formation 207, 209 Sapulut Formation 256 Sarangani-Davao depression 39 Sarawak Ketungau Basin stratigraphy 254 Rajang Group stratigraphy 248-51 Sarawak orogeny 256 Saysomboun Formation 236 Schwaner Mts 252-3 SE Asia, tectonic setting in Cenozoic 155, 158, 161,163, 166, 169, 170, 172, 173, 176, 178 SE Asia (SEA) plate 5 Sebuku Formation 256 seismic sections Sandakan basin 314, 315 Vientiane Plain 239-243 Selangkai Formation 251,253 Semitau ridge 254 Semitau terrane 99, 103 Sepauk tonalite 252, 253 Sepik Basin 526 Seram 445, 455-7 crustal isotopic signature 448 tectonic setting 167-8 ultramafic complex study 4°Ar/39Ar values 457-9 closure temperature 459 implications for Banda Sea 462 implications for obduction 461-2 P-T-t modelling 459-61 petrography 457 Seram trough 86 Serantak Volcanics 253 Setul Limestone 208, 212, 214 Shan boundary 99, 110 Shan Plateau palaeomagnetic studies methods 212 results 212-13 stratigraphy 210-11 Shan-Thai block 126, 132 see also Sibumasu block Shazipo Formation 541,544 shear faults 3 Sibela continental suite 483-4 Sibela ophiolite 486-7 Sibolga granite 324, 329 Sibumasu block/terrane 99, 100-2 tectonic history 204 tectonostratigraphy 205 Triassic collision studies 225, 230
564 Sibuyan sea fault 519-20 Siguangping Formation 541 Sikuleh granite 331 Silantek Formation 248, 254 Silurian studies 468 Simao block/terrane 99, 101 Sintang intrusives 254 Sirga Formation 470 slip partitioning 3, 6-16, 19 slip vectors 19 sliver microplates 19, 21-3 Sm/Nd isochron, western Sulawesi 407, 425 Snellius ridge 42-3 Solomon trench 13 Song Da (Black River) suture 105-6, 204, 554 Song Ma (Red River) suture 99, 105-6, 204, 554 Songpan Ganzi accretionary complex 99 Sorol trough 6 Sorong fault 12-13,467 Sorong fault system 465-6 Sorong-Sulabesi fault 467 South China block/terrane 98-100 Cenozoic motion 124, 125 South China Caledonian orogen 549 South China Sea models of tectonic evolution 247-8 spreading mechanism 133 tectonic setting 157 see also Malay Basin; Penyu Basin South Sula fault 467 South West Borneo terrane 99, 103 Sturt block 62, 63 Sukadana granite 252, 253 Sula Islands 474 Sula platform 167 Sulabesi Island 467 Sulan pluton 331 Sulawesi Bantimala melange complex 354-5 lithology 355 outcrop distribution 356-61 structure 355-6 tectonic evolution 361-2 collisional tectonics 171-4 displaced terrane 466 metamorphic belt 140-2 tectonic setting 160-2 Tertiary magmatic evolution 392-4 distribution of igneous rocks 397 geochemistry 397-400, 425 geochronology 400-11,424-5 petrogenesis 411-13 petrology 397 thermal history 141-15 Tertiary stratigraphy 365-9, 394 depositional environments 375-7 facies description 369-75 Tertiary tectonics 383-4, 415-23
INDEX Sulawesi Group 475 Sulawesi orogeny 191-4, 197 Sulu Sea 164 basin tectonics 311-16, 317 Sumatra plutonism 323-5 geochemistry 325-8 geochronology 328-9 tectonic setting 160, 322-3 Sumatra fault 21, 24, 160 Sumatra fault system (SFS) 322 Sumatra margin accretionary prism transfers 23-4 plate setting 20 sliver plates 21-3 subduction rates 20 tectonic model 24-7 Sumatra trench 8 Sumatran Forearc 7, 337-8 see also Nias Island Sumba 445 crustal isotopic signature 449-50 metamorphic belt 140-2 Sunda orogeny 188-9, 197 Sundaland 250, 281 collision evidence 133-5 deftned 291 marginal features 353,354 northern shelf basin studies see Malay; Penyu; West Natuna; East Natuna; Mekong; Nam Con Son palaeomagnetic studies history of study 206-10 methods 212 results 212-17 results discussed 217-19 setting for sites 210-12 significance of results 219-22 tectonic setting 171,204 tectonic summary 300-5 see also Sumatra suturing events 105-11
Tagoloan fault 514 Taiwan collision zone 511 Talaud orogeny 191,192, 197 Talaud ridge 40 Tamangil Formation 87 Tanamu Formation 474 Tang Ting fault 125 Tanimbar trough 86 Tanjong Formation 313 Tarim block/terrane 99, 124, 125 Tatau formation 250, 251 Tawali Besar, stratigraphy 487-8 Tawali Formation 487-8, 491
INDEX tectonic overview of SE Asia 155 Temburong Formation 256 Tengchong Block 540 diamictite 541-4 terrane motion 544-6 terrane evolution origins 98-103 palaeogeography 111-14 Tertiary studies Bacan stratigraphy 487 Bacan tectonics 490-3 Salawati stratigraphy 471 Sumatra plutonism 331-3 Sunda Shelf kinematics 301 Tomori stratigraphy 475 see also Eocene; Miocene; Oligocene; Neogene; Pliocene Tetambahu Formation 474 Tethys Ocean, phases of 103-5,546 Thai-Malay Peninsula 159-60 Thailand hydrocarbon potential 244 palaeomagnetic studies 207-10 rotation 217-18 tectonic setting 539 Thangon Formation 236 thermochronology, Papua New Guinea 531-2 Three Pagodas fault 159 thrust faults 3 Tibet block, Cenozoic motion 124, t25 Timor role in Banda orogen 57 structural model 47 TIMOR line 48, 49-56, 64, 76, 78-82 Timor trough 11, 51-3, 86 evolution 75-6 seismic reflection profile 81 Tipuma Formation 468 Tokala Formation 474 Tomori Basin 472-3 correlation to Salawati Basin 475-6 hydrocarbon potential 479 stratigraphy 470, 471,474-5 structure 473-4 tectonic significance 476-9 Tomori Formation 475 Tonasa Limestone Formation 354, 365, 366 depositional environment 377-80 facies 368-75 origins 375-7 stratigraphy 367-9 tectonic environment 383-4 Tondo Formation 435, 438, 439, 440 Tonga trench 8, 13-15 Torricelli Intrusive Complex 527 Torricelli terrane 147 Triassic studies collisional tectonics 225
Fujian province 551-2, 553 palaeomagnetism 206 rifting 104 Salawati stratigraphy 468 Sumatra plutonism 324, 329-30 terrane distribution 112 Tomori stratigraphy 475 Trusmadi Formation 256 Tukang Besi platform 160, 167, 434
U/Pb dating, western Sulawesi 410-11,425 Ujung Kulon fracture zone 21 Ulai intrusion 331
Verde Passage fault 519-20 Visayas, Neogene accretion 515-16 volcanic arc, role in Banda orogen 55-6, 57 Vulcan basin 62, 63
Waaf Formation 470 Waigeo, tectonic setting 491 Walanae Depression 366 Wallace Line 394 Wang Chao fault 159 Weber basin 87 Weda Group 503, 505 Weduar Formation 87 Weryahan Formation 87 West Andaman fault 22, 23 West Burma Block 99, 102 West Kutei basin 253 West Malaysia see Malay Peninsula West Natuna Basin 282, 298-9 stratigraphy 283 West New Guinea composite terrane 140, 141 West Philippine basin 32 Wetar thrust 11, 61 Wharton basin 19 Woi Formation 504 Woniusi Formation 541,543-4, 544 Woyla suture 99, 110 Woyla terranes 99, 103
Xianggui block 100 Xikou Formation 551-2, 554 Xilao-He suture 99
Yangtze block 100 Yangtze para-platform 549
565
566 Yangtze Platform 539, 540 Yanshanian orogenies 549, 555 Yap trench 6 Yefbie Shale 468 Yongde Formation 541, 544 Yunnan (SW) 539-41
INDEX glacigenic sediments 541-3 origins 544-6
'Zhen'he-Da'pu fracture 549-50 Zishi Formation 541
