Late Cenozoic Xianshuihe-Xiaojiang, Red River, and Dali fault systems of southwestern Sichuan and central Yunnan, China (GSA Special Paper 327)

Late Cenozoic Xianshuihe-Xiaojiang, Red River, and Dati Fault Systems of Southwestern Sichuan and Central Yunnan, China Erchie Wang B. C. Burchfiel L. H. Royden Chen Liangzhong Chen Jishen Li Wenxin Chen Zhiliang

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Library of Congress Cataloging-in-Publication Data Late Cenozoic Xianshuihe-Xiaojiang, Red River, and Dati fault systems of southwestern Sichuan and central Yunnan, China I Erchie Wang [et al.]. p. em. -- (Special paper ; 327) Includes bibliographical references. ISBN 0-8137-2327-2 1. Faults (Geology)--China--Szechwan Province. 2. Faults (Geology)--China--Yunnan Province. 3. Geology, Stratigraphic-Cenozoic. I. Wang, Erchie, 1951 - . II. Series: Special papers (Geological Society of America) ; 327. QE606.5.C6L37 1998 55 I .8'72'095138--dc21

98-38073 CIP

Cover: Looking south at left-lateral stream deflections across the Ganzi fault near Ganzi. Such deflections are characteristic for most of the left-lateral fault of the Xianshuihe-Xiaojiang fault system. The Ganzi fault is the northwest continuation of the Xianshuihe fault system.

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Late Cenozoic Xianshuihe-Xiaojiang, Red River, and Dali Fault Systems of Southwestern Sichuan and Central Yunnan, China ABSTRACT The convex northeast, left-lateral Xianshuihe-Xiaojiang, the northwest-trending Red River, and the northwest-, north-, and northeast-trending complex of left-lateral and normal faults of the Dali fault systems are three major active fault systems in the southeastern part of the Tibetan plateau. These fault systems consist of a network of faults that partition displacement during clockwise rotation of crustal rocks around the Eastern Himalayan syntaxis. The current pattern of deformation on these faults can be projected back at least to ~4 Ma. During this time interval, displacement on the Xianshuihe-Xiaojiang and Dali systems has been dominant over displacement on the Red River system. Geological relations along the left-lateral Xianshuihe-Xiaojiang fault system in southwestern Sichuan and central Yunnan show that (1) total displacement along the fault system is ~60 km, (2) displacement began at least by ~4 Ma, and (3) long-term slip rates range from 2–8 mm/yr on faults with small displacement to 9–28 mm/yr on faults with large displacement, but our GPS data indicate current short-term veloci≤2. Total left-lateral displacement on the Xianshuihe-Xiaojiang fault systies to be n≤ tem remains approximately uniform along most of its 1,200 km length, although displacement is locally partitioned onto several faults. Partitioning occurs mainly along left-lateral faults that become more numerous from northwest to southeast. Locally left-lateral displacement is transferred to shortening. In the southern part of the fault system, displacement on discrete faults dies out southward over a distance of 100–200 km; faults of the Xianshuihe-Xiaojiang fault system that reach the Red River fault have displacements of only a few kilometers. This occurs as left slip along the Xianshuihe-Xiaojiang fault system is transferred to extensional structures and, locally, by bending of older structures and shortening across braided fault segments. Extensional and compressional deformation features along the Xianshuihe-Xiaojiang fault system may occur within ~20 km of each other. Total left slip on the XianshuiheXiaojiang fault system remains constant, but southward this slip is accommodated along more fault strands, indicating that left shear is being accommodated by more diffusely distributed shear. The Red River fault, consisting of several northwest-trending strands, is the primary fault of the Red River fault system. We restrict the name Red River fault to the closely spaced faults that can be traced from the Midu basin southeastward into Vietnam, and we consider the Red River fault to be a separate structure from the mylonitic rocks of the older left-lateral Ailao Shan shear zone. Displacement on strands of the Red River fault are dominantly right slip, but they also have thrust and normal components. The main period of thrusting appears to be related to the evolution of the transpressional Ailao Shan shear zone and we do not include it as part of the Red River fault. Right slip began on the Red River fault in pre-Pliocene time, and Wang, E., Burchfiel, B. C., Royden, L. H., Chen Liangzhong, Chen Jishen, Li Wenxin, and Chen Zhiliang, 1998, Late Cenozoic Xianshuihe-Xiaojiang, Red River, and Dali Fault Systems of Southwestern Sichuan and Central Yunnan, China: Boulder, Colorado, Geological Society of America Special Paper 327.

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E. Wang and others displacement of at least 14–48 km occurred prior to deposition of Pliocene strata. During the past ~4 m.y., the total right-lateral displacement on the Red River fault may have been only 5–6 km and its slip rate reduced to 1–3 mm/yr. During this period, the northwestern part of the fault had down-to-the-north normal slip, but an eastern part of the fault had a small component of shortening. Even though the active right-lateral Jianshui and Qujiang faults lie parallel to and ~40 and 70 km north of the Red River fault, our interpretation relates their displacement to left-lateral shear and rotation by the Xianshuihe-Xiaojiang fault system. The Dali fault system consists of a network of young and active northwest- and north- to north-northeast-trending left slip faults. Many of the faults parallel older structures and total offset cannot be easily determined. Sediments deposited in small extensional basins along the Dali fault system suggest that most of the faulting began at ~4 Ma, similar to the beginning of faulting on the Xianshuihe-Xiaojiang fault system and the latest period of movement on the Red River fault system. Faults bound crustal blocks within the Dali fault system that rotate clockwise and are associated with components of normal slip and numerous fault-related basins. Some faults appear to have an older history of pre-Pliocene left slip. The Tongdian fault, like the Red River fault, trends northwest, but both faults appear to have had reduced slip rates once the major component of clockwise rotation began ~4 Ma. During at least the last ~4 m.y., movement of crustal material in the eastern part of the Tibetan plateau and its adjacent foreland is consistent with clockwise rotation, bounded on the east by the Xianshuihe-Xiaojiang fault system, about a pole south of the eastern Himalayan syntaxis. This rotation extends into southern Yunnan and Indochina where it is accommodated by the Dien Bien Phu fault and many other leftslip faults that accommodate clockwise rotation. Within the region of rotating crustal material, the Dali fault system appears to rotate clockwise about a pole of rotation different from that determined by the Xianshuihe-Xiaojiang fault system, and the difference between these regions is accommodated by extension and local right slip. Clockwise rotation of material extends into western Burma, where it appears to be deforming generally north-striking structures, including folds and thrust faults in the accretionary prism of western Burma and the active Sagaing fault, and is absorbed in rollback along the Burma subduction zone. Left-shear between the clockwise rotating crustal material and South China occurs along the Xianshuihe-Xiaojiang fault system. The magnitude of displacement along individual faults dies out toward the Red River fault; however, the fault and adjacent geology is rotated left-laterally and the left-shear passes across the Red River fault onto discrete faults in Indochina. The Red River fault in China now rotates clockwise and has a slow slip rate and small total displacement during the last ~4 m.y. The Tongdian fault may have a similar history. During this period, little crustal material has extruded eastward beyond the Tibetan plateau.

INTRODUCTION During the past two decades, there has been an increased interest in the Tibetan plateau and its surrounding mountain ranges and basins as a natural laboratory in which to study geological processes that range from continental collision tectonics to effects of plateau development on climate. At the same time controversy concerning the origin and evolution of the Tibetan plateau has increased, and many different models for the evolution of the Tibetan plateau have been published. Two-dimensional plateau models present a wide range of interpretations for the origin of the thick Tibetan crust, from underplating of Indian crust beneath Tibet without crustal shortening (Barazangi and Ni,

l982; Beghoul et al., l993) to penetrative shortening of Tibetan crust during indentation of Eurasian lithosphere by a rigid Indian plate (Dewey and Burke, l973). Models that address the threedimensional development of the plateau vary greatly in their interpretations of how India-Eurasia convergence relates to the tectonics of the region east of the indenter. Interpretations include clockwise rotation of lithospheric material around the eastern Himalayan syntaxis according to slip-line theory (Tapponnier and Molnar, l976), spreading of thickened Tibetan crust eastward to the eastern edge of the plateau (England and Housemen, l986; Houseman and England, l986, 1993, 1996), and large-scale eastward extrusion of lithospheric fragments that extend from Tibet to the South China Sea (Tapponnier et al., l982, l986; Briais et

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China al., l993; Leloup et al., l995). These models have been produced at a faster rate than the geological and geophysical information necessary to constrain them. To develop or constrain kinematic and dynamic models for intracontinental deformation, structures used to construct a regional pattern of deformation must be contemporaneous. This can be done best for Holocene and Pleistocene time because it is more difficult to establish contemporaneity for older geological structures, particularly when strains are large and patterns of deformation may change on time scales of 1 m.y. or less (see Zhang et al., l991). Additionally, we must know and remove the effects of younger deformation before we can quantify older parts of the deformational record. Within southern Sichuan and western Yunnan, there are numerous large young and active1 faults that are related to the ongoing convergence between India and Eurasia. This book presents data on three of these fault systems, the left-lateral Xianshuihe-Xiaojiang system, the right-lateral Red River fault system, and the network of left-lateral faults that make up the Dali fault system (Fig. 1). These fault systems are active, lie only a few hundred kilometers east of the eastern Himalayan syntaxis, and have a history that extends back at least several million years. Study of these three fault systems allows us to address problems related to (1) the extrusion of crustal fragments eastward from the Tibetan plateau, (2) how far back into the geological past the present pattern of deformation can be projected, and (3) the relationship between these fault systems and the intracontinental deformation of the India-Eurasia collision zone. REGIONAL TECTONIC SETTING Cenozoic structures in western Yunnan and southern Sichuan are related to the postcollisional intracontinental deformation resulting from the India-Eurasia convergence. Final closure of the Neo-Tethyan ocean that separated the continental crust of India from that of Eurasia probably occurred in Pakistan and the western Himalaya from ~55 Ma (Garzanti et al., l996) to 50 Ma (Searle, l991; see also recent review by Rowley, l996). Farther east in the central and eastern Himalaya the time of final closure and beginning of continent-continent convergence is poorly constrained and remains uncertain. The only data that bear on the closure of the oceanic region near the eastern syntaxis come from north of the Everest region, where lower Eocene (Cuisian-Lutetian >45.8 Ma) marine limestone lies conformably at the top of the Tibetan sedimentary sequence at Tingri (Blondeau et al., l986; Willems et al., l996) and shows no evidence for accelerated loading due to the onset of collision (Rowley, 1998), and where the youngest subduction related igneous rocks in the batholith have yielded U/Pb zircon ages of 41.1 and 41.7 ± 0.4 Ma (Scharer et al., l984). While we use the commonly proposed time for collision at 45 ± 5 Ma, we realize the time of 1 We refer to structures that show evidence for Holocene displacement as active and structures that have Quaternary or uncertain Holocene displacement as young.

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collision is not well constrained. Thus, we regard deformation younger than 40 Ma (late Eocene–Priabonian) or 50 Ma (middle Eocene–lower Lutetian) in the area of western Yunnan region as postcollisional. Since 46.3 Ma India has moved northward at ~68 mm/yr relative to Siberia measured at the eastern Himalayan syntaxis and has rotated counterclockwise by ~30° (Dewey et al., l989; Molnar et al., l993; Rowley, l996). The rate of northward movement of the syntaxis is not well constrained and is dependent on how deformation north of the Himalayan thrust front has been partitioned throughout Tibet and adjacent regions. The region addressed in this report lies ~400–800 km east of the eastern Himalayan syntaxis in western Yunnan and southern Sichuan (Fig. 1) and offers the opportunity to study a variety of problems related to intracontinental deformation. This region contains highly varied topography, including the Tibetan plateau at ~5,000 m elevation, an extensive region of low relief southeast of the plateau at ~2,000–2,500 m elevation, and numerous high peaks and mountain ranges in the northwest and east that culminate in Gongga Shan at 7,556 m. It is one of the most seismically active regions in China (Fig. 2). If there is little significant late Cenozoic shortening between Siberia and South China (South China block), then India must move northward relative to South China at about the overall convergent rate of 68 mm/yr (Rowley, l996). The present-day rate of India relative to Eurasia at the eastern Himalayan syntaxis is 55 ± 5 (R. King personal communication, l996, calculated from NUVEL 1A [DeMets et al., 1994]). Approximately 20 mm/yr of this convergence is absorbed by deformation in the Himalaya (Bilham et al., l997; Molnar and Lyon-Caen, l989; Lyon-Caen and Molnar, l985; Molnar, l987), while the remainder is absorbed by deformation north of the Himalaya over a region that stretches into Mongolia (Molnar and Tapponnier, l975, l977). Thus, at this time the eastern Himalayan syntaxis moves northward relative to South China at about 35 ± 5 mm/yr with much of this motion being accommodated in the area covered by this report. The long-term rates indicate that the eastern Himalayan syntaxis may have moved northward relative to South China by ~2,400 km (compare this to the ~3,400 km of northward motion of India relative to Eurasia over the same time period). These relative displacements must be recorded in the geology of western Yunnan and southern Sichuan, which lie between India and South China. The tectonic interpretation of the region east of the Indian indenter has been highly controversial. Following the early work of Molnar and Tapponnier (l975, l977), Tapponnier et al. (1982, l986) and Peltzer and Tapponnier (l988) have presented analog experiments in plasticine, where India is modeled as a rigid body indenting a softer Eurasia. Comparisons between these models and geology suggest that while shortening in Eurasia north of India and southern Eurasia may accommodate much of the postcollisional convergence, some convergence (perhaps ~30%) may have been accommodated by several hundred kilometers of eastward displacement of two large wedge-shaped fragments of Eurasia. The first of these fragments to be extruded was Indochina, bounded on the west by a right-slip boundary in west-

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Figure 1. Generalized topographic map of the southeastern part of the Tibetan plateau and its foreland covering the region discussed in text. The eastern boundary of the Tibetan plateau is usually taken as an elevation between 3,500 and 4,000 m. The highest peak in the region is Gongga Shan at 7,556 m shown by white triangle. The main fault systems discussed in text are the Xianshuihe-(XS) Xiaojiang (XJ) fault, Red River (RR), and Dali (DS) fault systems. Inset map shows location of Figure 1 relative to the India-Tibetan plateau region. Note that the main fault systems trend southeastward across the margin of the Tibetan plateau into its foreland.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

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Figure 2. Distribution of earthquakes of >M 5.0 between 1900 and 1985 in and around the Tibetan plateau (from Ma, l988). Parts of the Xianshuihe-Xiaojiang (XX), Red River (RR), and Dali (DS) fault systems are the locus of high seismicity and lie within a northeast-trending belt of seismic activity that is one of the most active areas of seismicity in China. Location of Figure 3 is indicated. Shading indicates some active areas of sedimentation along the margin of the plateau.

ern Burma and on the east by a left-slip boundary through western Yunnan (approximately in the position of the currently active Red River fault), and derived from the eastern part of the Zangbo suture in southern Tibet. The second of the fragments to be extruded consisted of eastern Tibet and South China. It was bounded on the south by a right-slip boundary through western Yunnan (along the present Red River fault) and on the north by a complex left-slip fault zone along the Altyn Tagh fault zone and through the Qinling Mountains. Extrusion of this second fragment reversed the displacement sense of strike slip along the Red River region in western Yunnan from left to right slip. In contrast, England and Houseman (l986) and Houseman and England (l986, l993, 1996) have built upon the thin viscous shell formulation of England and McKenzie (1982) and McKenzie (1983) to construct numerical models in which southern Eurasia is treated as a thin viscous sheet indented by a rigid Indian indenter. In this model, the Tibetan plateau grows by shortening and thickening of Tibetan crust. The shortened region spreads eastward around the eastern corner of the India indenter. In their early models no material is extruded eastward beyond the

plateau, although in later models, eastward movement of material beyond the Tibetan plateau occurs not on discrete faults, but as a continuous eastward bulge. The model shows continual clockwise rotational strain east of the eastern Himalayan syntaxis in western Yunnan. Cobbold and Davy (1988), Davy and Cobbold (1988), and Dewey et al. (l989) suggested that the region east of the India indenter undergoes right-lateral shear and large clockwise rotations within a broad north-south-trending zone extending from southern Burma to north of the Tibetan plateau. As in the England and Houseman (l986) and Houseman and England (l986) models, and in marked contrast to the models of Tapponnier et al. (1982, 1986), eastward extrusion of fragments from Tibet into Indochina or eastern China does not occur beyond the zone of right-lateral shear. All these models have different and geologically testable consequences for various time intervals. Recently, some models of the stress and strain patterns east of the syntaxis have tried to incorporate geological observations, and some have been developed directly from earthquake data. Models of the stress, strain, or velocity distribution around the

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syntaxis generally show trajectories that diverge eastward away from the syntaxis (see for example, England and Houseman, l986; Huchon et al., l994; Holt et al., l991, l995). Because the syntaxis moves northward relative to South China at about 35 ± 5 mm/yr, this region must have experienced rapidly changing patterns of deformation in response to the rapidly changing distribution of stress and strain during the past 45 ± 5 Ma of postcollisional convergence. Huchon et al. (1994) have attempted to correlate some geology with predicted stress fields through the period of postcollisional convergence; however, the amount of available geology is limited. Holt et al. (1991, 1995) have attempted to derive the present stress, strain, and velocity fields from earthquake data. The derived fields relate only to present conditions and rely on the assumption that the earthquake data are representative of total strain, a conclusion disputed in this study. These models treat the deformed region east of the syntaxis as a continuum. Our approach is to derive the spatial and temporal development of the strain field from analysis of the spatial and temporal variations in the deformation through geologic mapping. Our work is complementary to those studies mentioned earlier because ultimately it can serve as a test of the models. Here we present the results of mapping and preliminary characterization of the young and active structures in southeast Tibet and the adjacent region, and a discussion of the kinematic and dynamic systems in which this deformation system formed. A second, ongoing part of our work is the mapping of deformational patterns at different times in the past to establish their kinematic and dynamic settings and to assess how these deformation systems relate to the India-Eurasia postcollisional convergence, and how they have changed over time. Because the geology and the deformational history of western Yunnan and southern Sichuan are not well known, our current results must be considered to be preliminary and will no doubt require considerable refinement as tectonic studies in this area proceed. Establishing time slices of contemporaneous deformation in a region as complex and large western Yunnan and southern Sichuan will require an enormous effort. Here we report only on a part of the young and active structures in this region. We consider a number of faults and associated structures to be genetically related and group them into three major fault systems: the left-lateral Xianshuihe-Xiaojiang, right-lateral Red River, and the left-lateral Dali systems (Fig. 1). These three fault systems are usually interpreted to bound a lens-shaped crustal fragment, the Chuan Dian fragment, which has right-slip and left-slip faults on its southwest and northeast sides, respectively (see for example, Kan, et al., l983; Holt et al., l991). The Chuan Dian fragment appears to be moving more rapidly to the southeast than crust adjacent to it and thus may represent the lateral extrusion of a small crustal fragment. The modern Red River fault also forms the southern boundary of the Chuan Dian crustal fragment and thus figures prominently in the controversy about whether largescale east or southeast extrusion of crustal material beyond the Tibetan plateau has occurred.

XIANSHUIHE-XIAOJIANG FAULT SYSTEM The Xianshuihe-Xiaojiang fault system is an arcuate system of left-lateral faults that strikes north in central Yunnan and northwest in western Sichuan and is one of the most active fault systems in China (Figs. 1, 3). It consists of numerous branching and parallel active or young faults that can be traced for more than 1,200 km (Figs. 1, 3). The focus of this section is the latest Cenozoic deformation (~0–4 Ma) along the Xianshuihe-Xiaojiang fault system (Fig. 1). Several previous studies have been published on parts of the system but most of these have focused on earthquakes (Holt et al., l991, l995), historical earthquakes (Hou et al., written communication, l985; Chen and Li, l988) or on active faulting (Allen et al., l991). Timing, total displacement, and history of the fault system have not been studied previously, and it is primarily these topics and their consequences that we address here. Our field study of the Xianshuihe-Xiaojiang fault system was conducted during the summers of l991–1995 and the winter of l993. This section discusses the major faults of the the Xianshuihe-Xiaojiang fault system; however, there are other young or active faults within southern Sichuan and western Yunnan that could be regarded as part of this system. While we consider that these other faults are important to the overall tectonic framework in the eastern part of the Tibetan plateau, their exclusion from this study does not change the conclusions presented here, and they are the subject of our continuing study. We divide the Xianshuihe-Xiaojiang fault system into three segments: the Xianshuihe segment in the northwest, the Anninghe-Zemuhe-Shimian segment in the middle, and the Xiaojiang segment in the south. We include in this fault system several faults in the southern segment, such as the Puduhe, Qujing, Luzhijiang, and Yimen faults (Fig. 3). Geologic setting The Xianshuihe-Xiaojiang fault system cuts through crust with a complex deformational history. It is necessary to give a brief review of the major tectonic and paleogeographic units in southern Sichuan and western Yunnan because they are important in assessing the role of crustal anisotropy and displacement of older tectonic and paleogeographical units. The Xianshuihe-Xiaojiang fault system passes through four major tectonic units, from northwest to southeast: the SongpanGanzi fold belt of the eastern part of the Tibetan plateau, the Longmen Shan thrust belt and its continuation into southern Sichuan, the Yangzi platform, and the South China fold and thrust belts (Fig. 4). Songpan-Ganzi fold belt. The Tibetan plateau in this area lies at an average elevation of ~4,500–5,000 m and is largely underlain by the complexly deformed Triassic rocks of the Songpan-Ganzi fold belt that has been intruded by Triassic and Early Jurassic granitic rocks (Bureau of Geology and Mineral Resources Sichuan Province, l991; Burchfiel et al., l995). Only

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Figure 3. Map showing most of the major faults discussed in text and their location relative to the eastern Himalayan syntaxis. At the eastern Himalayan syntaxis India moves north at about 55 ± 5 mm/yr relative to stable Eurasia. Shading indicates areas of Quaternary deposition. Location of Figures 5, 6, 7, 12, 28, and 29 are shown. The Chuan Dian fragment lies between the Xianshuihe-Xiaojiang, Red River, and Dali fault systems and has been interpreted to be a crustal fragment extruded to the southeast between these three faults of opposite shear sense, an interpretation we reject.

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Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China locally are older rocks exposed. The Triassic rocks are characterized by marine clastic sediments dominated by Middle to Upper Triassic flysch with Upper Triassic molasse and volcanic rocks of the Yidun arc(s) present in the southwest (Fig. 4). Folds in the Triassic rocks generally trend northwest, oblique to the more west-northwest–trending Xianshuihe fault zone. Fold patterns in the Triassic rocks are complex and indicate folding and refolding about several axes. Refolding patterns are well developed where older rocks, commonly metamorphosed to amphibolite grade, are exposed in domes near the southeastern margin of the SongpanGanzi fold belt (Fig. 5). Late Triassic and Early Jurassic plutons intrude the folds, proving many of the structures are Late Triassic to Early Jurassic in age. Post-Upper Triassic sedimentary rocks are rare. Only in the south and west are the deformed Triassic rocks unconformably overlain by coarse clastic rocks of early Cenozoic age (mapped as Eocene and locally Oligocene). Neogene rocks are extremely rare and consist of mainly local deposits of Miocene(?) to Pliocene coal-bearing fine-grained clastics, and diverse Quaternary deposits of fluvial, lacustrine, alluvial, and glacial origin. Coarse clastic rocks are mostly of Quaternary age and were deposited in small basins formed along faults. Some age constraints for sedimentary rocks in the faultcontrolled basins are given in Table 1. Note that no sediments are dated as older than ~2.5 Ma. Eastward along the Xianshuihe fault zone, elevations in the folded Songpan-Ganzi fold belt increase, culminating at Gongga Shan at 7,556 m. Topographic relief here is extreme and the Dadu River east of Gongga Shan is at an elevation of only 1,000 m. Longmen Shan thrust belt and its continuation into southern Sichuan. The Longmen Shan consists of northnortheast–trending, east-vergent thrust faults present near the eastern margin of the Songpan-Ganzi Triassic flysch basin (Burchfiel et al., l995). The thrust sheets contain mainly the Songpan-Ganzi flysch, but along their east side carry Proterozoic metamorphic rocks and Upper Proterozoic and Paleozoic sedi-

Figure 4. Map showing the major tectonic units within the southeastern part of the Tibetan plateau within China. Rocks of the Yangzi platform are unpatterned. They are traversed by a large paleogeographic high, the Kungdian high (KD), shown in shading, which was elevated at various times during Paleozoic and early Mesozoic time. The Kungdian high functioned as an important crustal anisotropy during Mesozoic and Cenozoic deformation. The Danba antiform (DB) is a Cenozoic feature, partly controlled by the location of the Kungdian high, that deforms both hanging-wall and footwall rocks of the Longmen Shan thrust belt. The Yangzi platform has been strongly deformed in the South China fold belts southeast of the Longmen Shan thrust belt. Rocks of Yangzi platform facies can be traced to the northwest between the Three Rivers fold belt (diamond pattern) and the Songpan-Ganzi flysch region (dashed) and Yidun volcanic arc (inverted v’s). The South China fold belt lies along the southeastern margin of the Yangzi platform and is distinguished from the South China fold belts (see text). Shear sense of some of the major strike-slip faults is shown. Coarse stipple around Chengdu is the Chengdu plain, part of the Sichuan basin where Quaternary deposition has taken place.

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mentary rocks from the western margin of the Yangzi platform. These thrust faults can be traced southward from the northern Longmen Shan until they are disrupted by the Danba antiform and are cut by the Xianshuihe fault (Burchfiel et al., l995). South of the Xianshuihe fault the Longmen Shan thrust belt continues as two major southeast-vergent thrust faults that continue into western Yunnan, where they apparently end (Figs. 4 and 5). The geometry of the Longmen Shan thrust belt appears to be controlled by the shape of the Songpan-Ganzi flysch basin, which terminates in western Yunnan against one or more Triassic Yidun volcanic arc(s?) (see Sengor, l984; Mattauer et al., l992; Burchfiel et al., l995). The Longmen Shan thrust belt became active during early Mesozoic time, but the two thrust faults and associated folds that characterize the Longmen Shan belt south of the Xianshuihe fault zone locally involve rocks as young as Upper Eocene–(?)Oligocene, indicating that east-vergent Cenozoic deformation has occurred in this region (Burchfiel et al., 1995). Yangzi platform. The region between the Longmen Shan thrust belt and the South China fold belt is called the Yangzi platform, a region underlain by generally low-grade Proterozoic metamorphic rocks with high-grade rocks known locally (Fig. 4, and following section). Proterozoic rocks are overlain by a thin and incomplete sequence of Paleozoic shallow marine, and locally nonmarine, strata with extensive development of thick Permian basalt in southern Sichuan and most of Yunnan, and by a generally nonmarine Mesozoic to early Cenozoic sequence characterized by terrestrial red beds from the late Middle and Late Triassic to the Eocene. In most of the area, Upper Triassic and Jurassic rocks form foredeep deposits for the Longmen Shan thrust belt. The trend of the Precambrian rocks mimics the trend of an ancient north-south–trending region of relative uplift referred to as the Kungdian high (Fig. 4; Chen and Chen, 1987). Lower and Middle Triassic rocks wedge out onto this paleo-high and are overlapped by Upper Triassic rocks that rest unconformably on Proterozoic metamorphic rocks. Chinese workers have suggested that this paleo-high was positive at various times during the Paleozoic (Zhang et al., 1989). East of the Longmen Shan thrust belt the succession of Mesozoic red beds continues without major unconformities, except locally, into the Upper Cretaceous and in some places into Eocene strata. Nearly all the rocks assigned to the Yangzi platform have been folded, and only local areas have remained undeformed. The Yangzi platform was a depositional platform characterized by a thin and incomplete sedimentary succession of latest Proterozoic to early Mesozoic age, but these strata were deformed during several subsequent deformational events. Thus, we assign part of the region commonly assigned to the Yangzi platform, to the South China fold and thrust belts (see following section). South China fold and thrust belts. The South China fold and thrust belts lie east and southeast of the Longmen Shan thrust belt and contain folds and faults that belong to at least two, and perhaps more, fold belts of different ages. In its southeastern part, folds and thrust faults involve mainly Triassic rocks, dominantly limestone, and Precambrian and Paleozoic rocks near the Yun-

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Figure 5. Regional tectonic map along and adjacent to the northern and central segments of the Xianshuihe-Xiaojiang fault system. For location see Figure 3, and for relations to major tectonic units see Figure 4. Fold axes and thrust faults shown by conventional symbols. AN = Anninghe fault, GZ = Ganzi fault, JQ = Jinhe-Qinhe thrust belt, the southwestern extension of the Longmen Shan thrust belt (LM), LT = Litang fault, SC = Sichuan basin, SM = Shimian fault, XJ = Xiaojiang fault, XS = Xianshuihe fault.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China nan-Vietnam border; it is this part of the region that is commonly referred to as the South China fold belt (Ren, 1990; Fig. 4). In many places these folds are overlain unconformably by unfolded Eocene rocks that rest on a well-developed karst surface of low relief. We will refer to this surface as the sub-Eocene surface, which is well developed in southeastern Yunnan. Folds of the South China fold belt can be dated in Yunnan as post-Late Triassic and pre-Eocene. In a few places in their southern part, the structures are intruded by plutons that have yielded Cretaceous ages (1:1,000,000 Yunnan Geological Map, Bureau of Geology and Mineral Resources of Yunnan Province, 1990), suggesting the structures are early Cretaceous or older. The age (or ages) of deformation of the fold-thrust belt is poorly constrained in Yunnan. From evidence outside Yunnan, deformation is usually assigned a Late Triassic and/or Jurassic age related to one or perhaps two events (Ren, 1990; Suo et al., 1993), but the timing of deformation(s) in Yunnan remains poorly established. North of the South China fold belt, within the region generally regarded as part of the Yangzi platform, is a belt of northeast-trending folds and thrusts that parallel structures in the South China fold belt and involve Proterozoic low-grade metasedimentary and uppermost Precambrian through Jurassic sedimentary rocks. Folds in southern Sichuan that are parallel to, but lie north of the South China fold belt in Yunnan are unconformably overlain by unfolded nonmarine Upper Cretaceous rocks, but in Guizhou Province lithologically similar rocks unconformably overlying the folds are dated as Early Cretaceous (Bureau of Geology and Mineral Resources of Guizhou Province, 1987). Folded rocks below the unconformity are as young as Late Jurassic. Because both the youngest folded rocks and the unconformably overlying rocks are nonmarine, they are not precisely dated, but their assigned ages of Late Jurassic and Late (or Early?) Cretaceous suggest that the folding is probably Early Cretaceous or perhaps as old as Latest Jurassic. This northern belt of folds could be the same age as the South China fold belt, but because the lower limit of folding in the South China fold belt is dated only as post-Late Triassic, contemporaneity remains uncertain. If the rocks in the South China fold belt were folded in Late Triassic or Jurassic time, then the two fold belts have different histories and should be considered to be distinct. A third belt of north- to northeast-trending folds and faults in southern Sichuan and central Yunnan lies northwest of and generally parallels the folds dated as Latest Jurassic–Early Cretaceous. This third belt contains a continuous stratigraphic succession from Paleozoic to Eocene and, rarely, Oligocene rocks. Folding of these rocks occurred in post-Eocene (or locally post-Oligocene) time and is distinctly younger than the folds of the first two belts. Here, folds are generally parallel to the Longmen Shan thrust belt and are part of a widespread Cenozoic deformation system that reactivated the Longmen Shan belt and deformed rocks far to its east (see Burchfiel et al., l995). Rocks of late Oligocene and Miocene age are generally absent from this region, and in many places the folds are unconformably overlain by coal-bearing, fine-grained clastic rocks dated as Pliocene and,

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rarely, as uppermost Miocene-Pliocene. Thus, the general upper limit for this period of folding is pre-Pliocene, although some folds and thrusts in this belt are locally active. The Pliocene rocks overlie an erosion surface with gentle rolling relief of a few tens or hundreds of meters’ relief. This erosion surface is widespread in Yunnan, but the age of the erosion surface is poorly constrained. It is younger than folded Eocene (and locally Oligocene) rocks but could have developed partly contemporaneously with deposition of the Pliocene strata. We will refer to this erosion surface informally as the sub-Pliocene erosion surface (see next section). This northern belt of northeast-trending folds is overprinted by and partly contemporaneous with an arcuate belt of north- to northwest-trending folds and thrust faults that generally parallel the outcrop of the Precambrian rocks in the Kungdian high (see Figs. 4, 5). In the region where these two deformed belts overlap, there is a complex dome and basin structure that clearly reflects superposition of structures within these two Cenozoic fold belts (Burchfiel et al., 1995). As in the northeast-trending belt, some of the folds and thrust faults in the northwest-trending belt can be approximately dated as pre-Pliocene in age, but locally some of these structures may be still active. All structures within these fold and thrust belts deform rocks of the Yangzi platform, but we refer to all the folded region as the South China fold and thrust belts (Fig. 4). Because rocks that provide upper and lower age limits for folding in the different belts crop out only locally, in many areas the age of specific folds and thrust faults is unclear. Sub-Pliocene erosion surface. This sub-Pliocene erosion surface cuts across all pre-Neogene rocks and is present throughout much of western and central Yunnan. It lies at an elevation of ~1,800–2,200 m and is best developed near the southern part of the Xianshuihe-Xiaojiang fault system, generally south of the Jinsha River. Where Pliocene to Quaternary faulting has occurred, remnants of this surface are preserved within the faulted blocks. Recognition of this surface is important because deformation that affects it can probably be constrained to be Quaternary in age. The sub-Pliocene erosion surface is cut on folded and thrusted rocks as young as Eocene and is characterized by generally low relief. Locally, thin deposits of fine-grained clastic rocks, commonly containing coal, rest unconformably on the surface. Locally, these clastic rocks have been studied in detail because they contain important coal deposits and have yielded numerous Pliocene (5.3–1.6 Ma) and, locally, early Pleistocene fossils (W. Downs, personal communication, l993; He et al., 1985). Because fossils from these rocks are generally not more narrowly dated than Pliocene, the age of any particular deposit is somewhat uncertain. Because only small and rare exposures of Miocene (23.7–5.3 Ma) rocks are present in the region, development of this surface can be constrained only to be older than Pliocene and younger than Oligocene or late Eocene (40–23.7 Ma). We have not studied this surface in the necessary detail to decipher its origin or time of development, and because many of the Pliocene

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rocks are preserved in low areas above the gently undulating surface, they could have been deposited during development of the surface. Therefore, we refer to this surface as the sub-Pliocene surface. In fact, the surface is probably polygenetic, and its time of formation diachronous. Xianshuihe segment The Xianshuihe fault zone generally consists of a narrow left-slip fault zone containing small pull-apart basins where individual fault segments are not aligned (Fig. 5) (Allen et al., l991). The narrow width of the fault zone in this segment differs from segments of the fault system farther south where numerous fault splays are present. This difference in style may be related to the fact that this segment deforms a homogeneous Songpan-Ganzi crust, while the more southerly segments deform a relatively anisotropic crust of the Yangzi platform. From 1725 to 1983, nine earthquakes of M = 7–7.9 and 13 earthquakes of M = 6–6.9 were attributed to motion along this segment of the Xianshuihe-Xiaojiang fault system. Focal depths reach a maximum of ~20 km (Tang et al., l993). Allen et al., (l991) have documented active left slip on this segment of the fault and suggested a slip rate of 15 ± 5 mm/yr along its western portion, and a less well-constrained 5 mm/yr along its eastern portion. More recently, Wen et al. (1996) have proposed a slip rate of 7.2 mm/yr along the Luhuo-Daofu segment of the fault (Fig. 6). Total offset on the Xianshuihe fault zone is ~60 km and can be bracketed by offset geologic features in three areas along the fault. In the Luhuo-Daofu area (Figs. 3 and 6), Upper Triassic and Permian rocks are tightly folded into southwest vergent structures, and along the east side of these folds, a southwest-dipping, east-vergent

thrust fault places them above Upper Triassic volcanic rocks. These structures and formations can be correlated across the Xianshuihe fault, and are offset left-laterally by 60 km across the fault (C–C′, Fig. 6). Twenty kilometers farther west, a large steeply dipping normal fault is offset 50 km across the fault (B–B′, Fig. 6). Several kilometers east of the offset folds and thrust fault, a steeply dipping fault separates the Upper Triassic Xinduqiao and Waduo formations and is offset 65 km across the fault (D–D′, Fig. 6). An east-dipping thrust fault that carries the distinctive flysch of the Yajiang Formation only in its footwall has been offset left-laterally 42 km across the Xianshuihe fault (points A–A′, Fig. 6). South of the Xianshuihe fault these rocks appear to have been affected by significant left-lateral drag. If the apparent drag, suggesting an additional left-shear of 15–20 km, is added, the offset is 57–62 km. Thus, the total displacement on the Xianshuihe fault zone in this area is 42–65 km, and most likely ~60 km. Near Kangding (Fig. 5) the Xianshuihe fault offsets a large pluton that is shown on most maps as Mesozoic in age, but that has recently been dated as late Cenozoic, at least locally (Roger et al., l995). Offsets of irregularly shaped plutons are difficult to measure, but some limits can be suggested. The western side of part of the pluton north of the fault is offset only ~15 km, but the eastern side of the same body apparently is offset 60 km. In the same area, numerous river offsets are also present (Plate 1). East of the fault at Kangding, Precambrian crystalline rocks are overlain on their faulted western margin by a narrow sliver of Triassic rocks, which in turn are overlain along a thrust contact with Sinian and Paleozoic metasedimentary rocks (see Burchfiel et al., l995). Similar, but not identical, relations are present 58–60 km to the south on the south side of the fault (Fig. 5). The faulted contacts between the Paleozoic units probably mark the front of the Longmen Shan thrust belt in this area, but the thrust faults

Figure 6. Geologic map of the Luhou and Daofu areas along the northwestern part of the Xianshuihe fault. Offsets of different geological features used to measure the total offset on this part of the Xianshuihe fault are shown. The 42 km offset at A–A′ is considered a minimum because of left-lateral drag east of point A′. The more reliable offsets appear to be ~60–65 km. Fold axes and thrust faults are shown by conventional symbols. The normal fault (offset B–B′) is shown by solid rectangles on the fault trace. For location see Figure 3.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

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have had a complex history so it is difficult to be sure if these faults have been modified by later thrust faulting and, in places, normal faulting. The Ganzi fault zone, which lies southwest of the western part of the Xianshuihe fault zone, was considered by Allen et al. (l991) to be the left-stepping continuation of the Xianshuihe fault zone (Fig. 5). The Jinsha and Yalong Rivers are offset across the Ganzi fault zone by ~60–80 km and 30 km, respectively. A large Triassic granitic pluton is also offset left-laterally by ~75–80 km (Fig. 5). At this time there is only weak evidence for how the leftstep transfer of displacement from the Ganzi to the Xianshuihe fault zones takes place. Our work in progress suggests that some displacement on the Ganzi fault zone was transferred to the Xianshuihe fault zone, and some was absorbed by shortening on the southeast side of the Ganzi fault zone near its southeastern termination. The partitioning of displacement cannot be quantified at this time. Although we have not studied the Ganzi fault zone in as much detail as other fault zones described in this book, we consider the Ganzi fault zone to place important constraints on the inception of the left-lateral Xianshuihe-Xiaojiang system. Unlike the Xianshuihe segment, which contains only small pull-apart basins, the Ganzi fault zone contains several large sedimentary basins that give a minimum age on the inception of faulting (Fig. 5). The oldest sediments present within these basins are Quaternary (Bureau of Geology and Mineral Resources of Sichuan, 1991), and if they can be assumed to mark the inception of left-slip faulting, it would suggest that faulting started near the beginning of Quaternary time, at ~2.5 Ma. Central segment The complexity of the Xianshuihe-Xiaojiang fault system increases near Shimian where it branches from one to two faults—the Anninghe and Shimian fault zones (Figs. 3 and 7)— and minor unnamed splays. South of Shimian, these faults cut rocks of the South China fold and thrust belts on both sides of the fault. The north-south–striking Anninghe fault consists of several parallel strands and forms the western splay and cuts mainly Precambrian rocks of the Kungdian high. The Shimian fault forms the eastern splay and may consist of several faults. Its curved trace parallels the structural grain of a Cenozoic fold-thrust belt that consists of low-grade Precambrian metamorphic rock, overlying Paleozoic cratonal sedimentary rocks, and Mesozoic to early Cenozoic red beds. Farther south, the southeast-striking Zemuhe fault cuts obliquely through the Cenozoic fold belt and connects to the south end of the Anninghe fault at Xichang. The Zemuhe and Shimian faults join at Qiaojia, and then follow the Jinsha River southward to form the northern end of the Xiaojiang fault segment. Anninghe fault. The active trace of the Xianshuihe fault segment is difficult to recognize for ~40 km south of Shimian, but the juxtaposition of mapped units within Precambrian rocks clearly shows that the fault is present (Fig. 7). From near

Figure 7. Generalized geologic map of the central segment of the Xianshuihe-Xiaojiang fault system where the Xianshuihe fault splays into the Anninghe, Zemuhe, and Shimian faults and rejoins to the south into the Xiaojiang fault. Although a total offset across the Anninghe fault is difficult to determine, the contrast in pre-Cenozoic rocks across the fault indicates a large displacement. Offsets A–A′, and B–B′ are discussed in text. In this segment of the fault system, left slip is partitioned into both strike slip and shortening. Locations of Figures 8, 9 , 10, and 11 are shown. For location see Figure 3.

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Shimian, the fault passes southward into high mountains where it follows a narrow and steep-walled gorge and lies entirely within Precambrian rocks. The active trace of the fault zone occurs as at least three strands present 5 km west and 4 km north of Huajiaba, south of which we refer to the fault zone as the Anninghe fault zone (Fig. 8). From Huajiaba south, the eastern and western strands of the Anninghe fault zone bound the east and west sides of a narrow ridge. Rivers contained in wide valleys flow toward the ridge from both sides and, upon encountering the ridge, are deflected along the ridge or cut through it in narrow gorges. The ridge continues south and ends ~4 km north of Shilong, where active strands of the fault are present along the east side of the wide Anninghe valley and within the mountain slopes to the east as far south as Xichang (Fig. 7). Along both sides of the valley, downto-the-valley normal faults cut Lower to Upper Quaternary rocks (Xu and Pei, 1987). From Huajiaba to Xichang, left-lateral deflection of rivers and streams is common along fault strands of the Anninghe fault zone. For example, Tang et al. (1993) reported 150 m offset of late Pleistocene sediments dated at 25,840 yr by C14 yielding a slip rate of ~6 mm/yr. At Xichang, the active fault zone changes strike to south-southeast into the Zemuhe fault zone, although one active strand can be traced as far south as Dechang (Fig. 7). From 116 B.C. to A.D. 1980, 14 earthquakes of M >5, some as large as M = 7, were reported along the Anninghe fault (Tang et al., l993). Many of these earthquakes occurred in the region where the Anninghe and Zemuhe faults join. Numerous landslides and mass flow deposits are present along the fault, many of which are known to be the result of earthquakes. The curvature of the Xianshuihe-Anninghe fault zone remains approximately constant along the Xianshuihe-Shimian part of the fault zone, but increases from Kangding south (Fig. 5). Because the Xianshuihe-Xiaojiang fault system strikes generally south-southeast, while the Anninghe fault zone strikes northsouth, we interpret displacement across the central segment of the Anninghe fault zone to be left-slip, oblique convergence beginning near Kangding. We interpret features like the topographically high ridge along the northern part of the Anninghe fault zone and parallel folds, thrust faults, and left-slip fault splays east of the Anninghe fault zone to be caused by a component of east-west shortening across the fault zone. There is considerable evidence for oblique shortening across a broad zone both west (Gongga Shan area reaching 7,556 m) and east of the Anninghe fault zone (Burchfiel et al., l995; see also Lithospheric Dynamics Atlas of China, l989), and our GPS studies also show a significant component of oblique shortening that occurs (Shen et al., l995; King et al., 1997). Thus, the active strain in this area is partitioned across a zone broader than the Anninghe and Shimian fault zones, and the slip rate of 6 mm/yr left slip determined by Tang et al. (l993) probably records only part of the strain related to movement on the Xianshuihe-Xiaojiang fault zone in this region. Zemuhe fault. In most places the Zemuhe fault consists of one major strand, but locally several minor anastomosing fault

strands surround fault slivers. Along most of its eastern trace the Zemuhe fault lies within a deep narrow valley in which the fault is marked by a wide zone of fault breccia and gouge that locally reaches 500 m in width. Across the Zemuhe fault, streams are commonly offset left-laterally by hundreds of meters to more than 2–3 km (Plate 2). At Ningnan, a ridge that crosses the fault is offset left-laterally ~4.5 km (Fig. 9 A–B) and several rivers in the same area show left-lateral offsets of 1–3 km (Fig. 9). Near its junction with the Anninghe fault, the Zemuhe fault splits into two strands. The southern strand strikes into the Anninghe valley where ~10 km west of Xichang, it may offset the Anning River left-laterally ~10 km. The northern strand strikes into a broad valley near Xichang that contains the deep Qionghai lake. Near the junction of the Anninghe and Zemuhe faults, there is evidence for local extension (Plate 3), and the Qionghai Lake basin appears to be an expression of that extension. Near Qionghai Lake several active fault traces pass north of Xichang to merge with faults along the east side of the Anninghe valley, forming a connection between the Zemuhe and Anninghe faults. Shimian fault. Near Shimian, the Shimian fault branches southeastward from the Xianshuihe fault zone, crosses the Precambrian rocks of Kungdian high, and curves southward parallel to the trend of folds and thrust faults involving Paleozoic and Mesozoic rocks. The Shimian fault offsets a unique ultramafic body and its contact aureole by 8 km left-laterally (Fig. 10). The Shimian fault also offsets the contacts between Precambrian metamorphic rocks, Sinian volcanic rocks and Sinian sedimentary rocks, and a syncline within Paleozoic and Mesozoic rocks by 8–11 km leftlaterally (Fig. 10). The Shimian fault is considered to be active because it disrupts drainage near Shimian and, in its southern part offsets streams and geological structures left-laterally and forms pull-apart basins filled with Quaternary sedimentary rocks (Fig. 10). The Shimian fault appears to merge with thrust faults of the late Cenozoic northwest-trending fold-thrust belt (Fig. 5; Burchfiel et al., l995), so that it is difficult to determine which of the faults represents the main trace of the active fault. Total offset along the Anninghe-Zemuhe segment is ~50–60 km, similar to the total offset along the Xianshuihe segment. Displacements across the northern part of Anninghe fault cannot be easily measured because the fault cuts through complex Precambrian geology with irregular contacts. Southward, near and southeast of Xichang, Upper Triassic rocks rest unconformably on lower volcanic and upper nonvolcanic units within the Sinian rocks. These rocks are present north and south of the Zemuhe fault (points A and A′, Fig. 7), which indicates an offset of the contact between the upper and lower Sinian units and the Upper Triassic rocks by ~40 km left-laterally. A southern branch of the Zemuhe fault seems to offset the Anning River by ~10 km leftlaterally, suggesting a total left-lateral displacement of ~50 km or more. Along the southern part of the Zemuhe fault, an open syncline in Upper Triassic rocks is present on both sides of the fault. North of the Zemuhe fault, Upper Triassic rocks rest unconformably on Permian basalt on the east limb of the syncline (point B, Fig. 7). On the southern side of the fault, Permian basalt

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Figure 8. Map showing major fault traces along the northern part of the Anninghe fault. Note that the faults terminate wide valleys filled by Quaternary both east and west of the main fault zone. The fault zone is marked by a ridge across which the rivers draining the truncated valleys cut narrow gorges. The faults show clear evidence for active left-lateral strike slip, and the ridge indicates shortening making this part of the Anninghe fault a transpressional fault zone (see inset map). A similar conclusion is reached from the shorter radius of curvature of the Anninghe fault within the Xianshuihe-Xiaojiang fault system. For location see Figure 7.

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Figure 9. Map showing the left-lateral offsets of the rivers and ridges along the southeastern part of the Zemuhe fault near Ningnan. A and B are offset ridge segments mentioned in the text. The ridge labeled B and its continuation to the southeast forms a shutter ridge for several rivers to the northeast. For location see Figure 7.

wedges out toward the west across the synclinal axis (point B′, Fig. 7). The axis of this syncline is offset 57 km left-laterally across the Zemuhe fault. Additionally, the westernmost edge of the Permian basalt is offset by a minimum of 53 km because the basalt is exposed on the north side of the fault. Although none of these offsets is precisely determined, and each has associated uncertainties, the consistency in magnitude of offset across the Zemuhe and Anninghe fault segments indicates a total offset of 50–57 km; thus, almost all of the displacement on the Xianshuihe fault appears to be partitioned southward and southeastward onto the Anninghe and Zemuhe faults. The Shimian fault follows the Jinsha River valley southward to a point ~10 km north of Qiaojia (Figs. 5 and 7) where the Zemuhe and Shimian faults join and continue south into the Xiaojiang fault segment (Figs. 3, 7). In this area the Jinsha River has cut a spectacularly deep gorge that is >2.5 km deep in places (~500 m deeper than the Grand Canyon of the western United States).

The Jinsha River exactly coincides with the Xiaojiang and Shimian faults for a distance of 60 km, which we interpret to be a left-lateral offset of the river by the Shimian and Xiaojiang faults (see following section). Interestingly, displacement across the Zemuhe fault (50–57 km) is smaller than the displacement on the Xianshuihe and Xiaojiang faults (60 km) to the north and south. We infer that the missing displacement occurs by left-lateral displacement mostly along the Anninghe fault and 8–11 km on the Shimian fault, and shortening occurs within the region from west of the Anninghe fault to east of the Shimian fault. Evidence for shortening strain is present within this region, such as the elevated ridge along the Anninghe fault and folds and faults that bound Quaternary basins, but quantitative assessment of how the strain is partitioned in this region cannot be made at this time. But the fact that the total displacements along both the Xianshuihe and the Xiaojiang segments (see following section) are about the same suggests that ~60 km of left-lateral displacement has been complexly partitioned in the central segment of the XianshuiheXiaojiang fault system. The inception of faulting on the Anninghe-Zemuhe segment is difficult to determine, but it appears to be late Pliocene or early Quaternary. Along the Anning River near Xichang, numerous terraces are cut by normal faults that parallel the river. Coarsegrained alluvial and fluvial deposits in these terraces are derived from the surrounding mountains and belong to the well-dated Xigeda Formation. This is the only Cenozoic rock unit in the area and lies unconformably on all pre-Cenozoic units. Fossils from the lower Xigeda Formation are late Pliocene to early Pleistocene in age, while fossils from the upper Xigeda Formation are middle to late Pleistocene in age (Bureau of Geology and Mineral Resources of Sichuan, l991). The coarse-grained character of the lower Xigeda Formation indicates that faulting was active during deposition and began prior to deposition of the Xigeda Formation. Because older Cenozoic deposits are not preserved in this area, the inception of faulting along the Anninghe and Zemuhe faults probably began before late Pliocene–early Quaternary time. The time between the inception of faulting and deposition of the basal beds of the Xigeda Formation is unknown. Similar relations are shown on Chinese geologic maps for the small Butuo basin along the southern part of the Shimian fault (Bureau of Geology and Mineral Resources of Sichuan, 1991; Fig. 11). This basin contains fossiliferous coal-bearing rocks that are correlated with the late Pliocene or early Quaternary lower Xigeda Formation. These rocks are deformed, dip up to 80°, and are unconformably overlain by Quaternary rocks. Although we have not seen these relations in the field, they suggest a similar time for the initiation of basin formation, which we interpret to mark the beginning of movement on the Shimian fault. Xiaojiang segment Of the three segments of the Xianshuihe-Xiaojiang fault system, the Xiaojiang segment is the most complex (Figs. 12, 13).

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

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Figure 10. Geologic map of the northern part of the Shimian fault, showing left-lateral displacement of geological features mentioned in the text. A–A′ is the offset of an ultamafic body. B–B′ is the offset of a syncline in Paleozoic and Mesozoic rocks across the Shimian and a parallel fault in the southeastern part of the map. C–C′ is the offset of early Sinian volcanic rocks. For location see Figure 7.

The Xiaojiang segment shows complex partitioning of displacement on different faults as well as a decrease in the total fault displacement from north to south (Fig. 14). In its southern part, it consists of numerous branching faults and both convergence and extension appear to be present and partitioned onto different groups of faults. At the southern end of the Xiaojiang segment, where it reaches the Red River fault, the total fault offset on the Xianshuihe-Xiaojiang fault system is only a few kilometers. However, it appears that the total left-lateral strain across the system is

largely accommodated by numerous small faults and by broadly distributed shear and may still be ~60 km (see following section). Faults within the Xiaojiang segment traverse obliquely across northeast-trending structures of the South China fold and thrust belts, which consist mainly of Paleozoic, Mesozoic, and rare Cenozoic sedimentary rocks, and in the north, Proterozoic low-grade metamorphic rocks (Fig. 13). These Proterozoic rocks, often referred to as Yangzi platform basement, consist of a thick sequence of mainly low-grade metasedimentary rocks that

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Figure 11. Interpretation of the displacements along the southern part of the Shimian fault. Left-hand diagram shows north-south–trending folds and inferred river courses (dashed lines) that are older than initiation of the Shimian fault. Right-hand diagram shows present structural configuration. Note that some shortening structures were developed during movement on the Shimian fault, and the fault is a left-lateral transpressive structure in which Quaternary rocks, such as in the Butuo basin, are involved. For location see Figure 7.

deform similarly to the overlying unmetamorphosed sedimentary succession. These folds and thrust faults generally trend northeast but, locally, at their southern end near the Red River fault, they curve to trend east-west. In the region around Kunming (Fig. 13), the folds and thrust faults have irregular trends. Note that all the faults of the Xiaojiang segment cut all of the folds and thrust belts and that faults and other structures associated with the Xiaojiang segment also disrupt the sub-Pliocene erosion surface. Thus, structural disruption of the sub-Pliocene erosion surface near the Xiaojiang fault segment is an indication that deformation is young, contemporaneous with, and probably

related to, the Xiaojiang fault system. Because the Xiaojiang fault segment is characterized by numerous Quaternary structural basins of varying size, the age of sediments in these basins provides a crucial time constraint for the Xiaojiang fault system. (The general stratigraphy and age constraints for the sedimentary rocks of the major fault controlled basins are given in Table 1.) The Xiaojiang segment can be divided into a northern, central and southern part. Northern part of the Xiaojiang segment. The northern part of the Xiaojiang segment consists mainly of one fault, the North Xiaojiang fault. It extends from Qiaojia, where the Zemuhe and

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Figure 12. Map of the faults of the Xiaojiang segment of the Xianshuihe-Xiaojiang fault system. For location see Figure 3. South of where the Zemuhe and Shimian faults rejoin at Qiaojia to form the Xiaojiang fault zone, at least six north-trending active faults make up the Xiaojiang segment. Lakes and Quaternary-filled pull-apart structures are the result of extension in the Xiaojiang segment. Locations of detailed figures are shown.

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Figure 13. Generalized geologic map of the Xiaojiang segment of the Xianshuihe-Xiaojiang fault system. This part of the fault system cuts obliquely across the Yangzi platform, the South China fold belts, and the South China fold belt.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Figure 14. Location and magnitude of total offsets along faults in the Xiaojiang segment. See Table 2 for features offset at each locality. For location of figure, compare to Figure 12, which also shows the location of detailed map giving data for some offsets and related structures. Black indicates lakes. AW = Awang basin, DC = Dongchuan basin, HL = Heilongtan basin, HZ = Huize basin, JC = Jiangchuan basin, JS = Jianshui basin, KY = Kaiyuan basin, LL = Luliang basin, ML = Mile basin, QX = Quxi basin, QJU = Qujing basin, QJ = Qiaojia basin, SM = Songming basin, SP = Shiping basin, TH = Tonghai basin, YL = Yiliang basin, YJ = Yuanjiang basin YX = Yuxi basin, YZ = Yangzong Lake, ZY = Zuyuan basin.

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Shimian faults join, south to ~30 km north of Dongchuan where the fault bifurcates (Fig. 12). Several streams and early Quaternary terraces along the Qiaojia basin (Plate 4) on the east and streams all along this segment of the fault are consistently offset left-laterally (Plate 5). It is along this part of the Xiaojiang fault that the Jinsha River is offset left-laterally by 60 km (a–a′, Fig. 14; see also Table 2). West of the fault is an extensive outcrop of Proterozoic metasedimentary rocks in the Tangdian Shan, a mountainous region that rises to an elevation of 4,247 m. East of the fault there are no comparable mountains or exposures of Proterozoic rocks, and topography east of the fault is dominated by the sub-Pliocene erosion surface (Plate 6a, b). Remnants of the sub-Pliocene erosion surface appear to be preserved on the Tangdian Shan, with the typical coal-bearing late Pliocene-early Quaternary sediments overlying the surface at Fawo (Fig. 15). The sub-Pliocene erosion surface is well preserved south of the Tangdian Shan, indicating that the Tangdian Shan was developed after the formation of the sub-Pliocene surface, and probably after deposition of the coal-bearing sediments. These relations suggest that the Tangdian Shan, and their associated structures, developed contemporaneously with faulting along the North Xiaojiang fault. The Jinsha River, at an elevation of 800 m, lies in a deep gorge along the fault and cuts a gorge through the topographically highest region west of the fault (Fig. 15). Thus, the Jinsha River appears to be antecedent to development of the topographically high area west of the fault. South of the Tangdian Shan, displacement on the central Xiaojiang segment is ~10–15 km smaller than displacement on the northern segment (see following section and Fig. 14). This abrupt reduction in total displacement along the Xiaojiang fault is consistent with absorption of 10–15 km of some of the left-slip by 10–15 km of shortening on east-west–trending folds and thrust faults within the Tangdian Shan. The southern boundary of the Tangdian Shan fault is a north-dipping thrust fault along which ~10–15 km of south-vergent shortening can be proven (v–v’, Fig. 14). South of the thrust fault, south-vergent folds may accommodate another 1–2 km of shortening. Northeast-vergent structures in the Lunan Shan to the north may also be related to this shortening deformation. Shortening structures and their resultant topography appear to die out westward (Fig. 15), but this relation cannot be demonstrated at this time. These shortening structures have no counterpart east of the fault (Figs. 13, 15) and most of the shortening structures appear to merge with the North Xiaojiang fault, rather than being truncated by it. The correlation between the Xiaojiang fault and the structures that caused elevation of the Tangdian Shan are illustrated by a simple geometric model shown in Figure 16. Central part of the Xiaojiang segment. The central part of the Xiaojiang segment consists of a western and eastern strand that splays into numerous other strands to the south (Fig. 13). We also include in the central Xiaojiang segment two active left-slip faults that strike parallel to the Xiaojiang fault, the Qujing fault to the east and the Puduhe fault to the west. All four faults cut

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TABLE 1. AGE OF SEDIMENTARY DEPOSITS IN BASINS ALONG THE XIANSHUIHE/XIAOJIANG FAULT SYSTEM Basin

Dianchi basin

Age

Holocene Late Pleistocene Middle Pleistocene Early Pleistocene

Control

Dated by fossils, the paleomagnetic age of the upper part of the profile is 0.45 Ma

Thickness

Youngest Bedrocks

>1,000 m

Mesozoic

Songming basin

Holocene Pleistocene

Dated by fossils

116 632 m

Pliocene

Fuxian Lake

Holocene Pleistocene

Dated by fossils

163 m

Pliocene

Mengzi basin

Holocene Pleistocene

Dated by fossils

30 40 m

Miocene

Dongchuan basin

Holocene Late Pleistocene Middle Pleistocene Early Pleistocene

Dated by fossils and by C14 method

>450 m

Paleozoic

>83 m

Pliocene

14 m

Pliocene

Dated by fossils

5 10 m

Pliocene

Dated by fossils and the paleomagnetic age ranges from 0.73 2.48 Ma

480 m

Proterozoic

163 m

Pliocene

Qujing basin

Holocene Pleistocene

Yiliang basin

Holocene No Pleistocene rocks are exposed on the surface

Lujiang basin

Holocene Pleistocene

Yuanmou basin

Holocene Late Pleistocene Middle Pleistocene Early Pleistocene

Yuxi basin

Holocene No Pleistocene rocks are exposed on the surface

No fossils reported, but they are unconformably underlain and overlain by Pliocene and Holocene rocks

Note: The stratigraphic data are from Luo et al. (1983) and Bureau of Geology and Mineral Resources of Yunnan province (1991), some of which are unpublished.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

TABLE 2. OFFSET FEATURES SHOWN ON FIGURE 14 a a’ b b’ c c’ d d’ e e’ f f’ g g’ h h’

60 km 8 km 11 km 17 km 30 km 30 km 16 km 27 km

i i’ j j’ k k’ l l’ m m’ n n’ o o’ p p’ q q’ r r’ s s’ t t’ u u’ v v’ w w’ x x’ y y’ z z’

2.3 km 7 km 7 km 5 km 23 km 22 km 9 km 10 km 7 km 10 km 4 km 5 km 22 km 15 km 4.5 km 21 km 23 km 0.2 km

Jinsha River offset River offset Permian basalt Pliocene rocks Anticline with Proterozoic rocks in core Syncline with Jurassic rocks in core Late Paleozoic formation offset Anticline and Permian/Carboniferous formations on its north and south flank Permian/Carboniferous faulted against Cambrian Syncline with Upper Triassic core River offset Sinian/Proterozoic boundary Pliocene rocks River offset Thrust shortening Jurassic formations Lower Permian/Devonian formations River offset River offset Ordovician/Devonian formations West boundary of Proterozoic rocks Thrust shortening Syncline with Jurassic in core Jurassic formations River offset River offset

through mainly Paleozoic, Mesozoic, and Cenozoic rocks of the northeast-trending South China fold and thrust belts (Fig. 13), and are characterized by many small and large pull-apart basins filled with Quaternary and locally Pliocene rocks (Figs. 12, 13). Evidence for active left-slip is abundant on various parts of all four faults. East Xiaojiang fault. About 30 km north of Dongchuan, the North Xiaojiang fault bifurcates into the East and West Xiaojiang faults. East of the point of bifurcation, folds of the South China fold and thrust belts curve into parallelism with the East Xiaojiang fault and become more narrow by a component of westnorthwest shortening east of the fault (Fig. 17). From here, the East Xiaojiang fault strikes east-southeast and, 30 km south of Dongchuan, curves to strike north-south (Fig. 13). South of its junction with the West Xiaojiang fault, evidence for active left slip is spectacular (Plate 7), and the East Xiaojiang fault splits into several strands that surround a 10-km–long ridge containing Permian rocks at its northern end (Fig. 18). Rocks within the ridge are clearly out of place with respect to the rocks on either side and appear to correlate with Permian rocks 10–15 km to the north on the east side of the Xiaojiang fault. The main Xiaojiang River flows northward along the west side of this ridge (Fig. 18), but several west-flowing rivers are offset left- laterally across the fault strand on the east side of the ridge. Thus, rivers flow west into the narrow valley east of the ridge, southward along the valley, then westward again until they reach the main north-flowing Xiaojiang River (Fig. 18, Plate 8).

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Large alluvial fans developed along the west-flowing portions of the rivers have forced the south-flowing river segments to the west side of the narrow valley, where Quaternary sedimentary rocks are strongly deformed, with local dips of 70°. It appears that the ridge once may have extended farther south, perhaps as far as 2 km south of Dongchuan, but has been breached in several places, probably by headward erosion of tributaries from the north-flowing Xiaojiang River, thus allowing the south-flowing river segments to cut across the ridge and enter the main Xiaojiang River. One of the rivers that enters the East Xiaojiang fault system from the east (stream B in Fig. 18) flows ~8 km southward along the ridge before it flows west through the ridge. Another river located just to the north (stream A in Fig. 18) now flows directly into the main north-flowing Xiaojiang River, but it also appears to have flowed southward around the ridge until recently, as evidenced by the presence of a large abandoned river channel that accommodated its southward flow along the ridge (Fig. 18). South of Dongchuan evidence for active left-slip is common and includes offset rivers, shutter ridges, disrupted drainage, and small pull-apart basins (Plate 9). These features occur as far south as Yiliang, where the Niulan River is offset left-laterally 23 km along an eastern splay of the East Xiaojiang fault, and the Yiliang basin forms a narrow left-stepping pull-apart basin (Figs. 13, 21). Five kilometers north of where the East Xiaojiang fault curves southward to strike north-south, a small north-south–striking fault branches off the main East Xiaojiang fault (Fig. 19). Where these two faults join there is a small, narrow pull-apart basin containing steep east-sloping alluvial fans and scree cones. The branch fault can be traced for 30 km south where it appears to end abruptly with no apparent transfer of its displacement. Total offsets on the East Xiaojiang fault can be firmly established by offsets of numerous geological features along the fault. The most reliable of these are shown in Figures 14 and 20. Structures of the northeast-trending fold-thrust belt are consistently offset 27–30 km, with the best constrained offsets being ~30 km (Fig. 14, e–e′, f–f′, h–h′). Along the central and southern part of the fault two rivers are offset by 23 and 22 km (Fig. 14, n–n′, y–y′). The river offsets are considered minimum offsets, but because the East Xiaojiang fault loses its displacement to the south (see following section), these river offsets may reflect the total displacement along the south and central part of the East Xiaojiang fault system. West Xiaojiang fault. From its junction with the North Xiaojiang fault, the West Xiaojiang fault strikes southward through Songming and farther south it begins to splay (Fig. 13). In the north it is marked by a single fault trace with several small, leftstepping pull-apart basins, and it passes through the large Songming basin (Fig. 20). In its northern part the fault lies in a deep gorge cut into the sub-Pliocene surface, but ~10 km north of the Songming basin it crosses a knickpoint in the drainage system and to the south it crosses the sub-Pliocene erosion surface with little associated relief until it reaches Fuxian Lake. The West Xiaojiang fault is seismically active. Since the 16th

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Figure 15. Simplified geologic map of the Tangdian Shan area. The structure and reduced offset on the North Xiaojiang fault indicate ~15 km of shortening is absorbed in the Tangdian Shan. A north-south profile shows a dominance of shortening in the Tangdian Shan, elevation of the sub-Pliocene surface to nearly 4 km at Fawo, and the antecedent character of the Jinsha River. See Figure 12 for location.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

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Figure 16. Generalized three-dimensional diagram illustrating the relation between strike slip on the North Xiaojiang fault and shortening in the Tangdian Shan. Shortening appears to die out westward where left-lateral faults begin and continue south.

century, there have been 15 earthquakes on the fault: one M = 8, four M >6, and the other ten M = 5–6. The M = 8 earthquake occurred in 1833 and caused a surface break 80 km long with a maximum offset of 5.8 m (Chen and Li, l988). Stream offsets at many places also indicate that displacement along the West Xiaojiang fault is active (Chen and Li, l988). Three geological features of the northeast-striking South China fold-thrust belts intersect the West Xiaojiang fault and are offset 11–16 km across the fault (Fig. 14, c–c′, d–d′, g–g′). Throughout the central part of the West Xiaojiang fault segment, offsets on the fault are fairly uniform, but southward there is a reduction in displacement (see following section). Where the West Xiaojiang fault passes through the Songming basin it offsets coal-bearing fine-grained clastic rocks of Pliocene age ~17 km (d–d′, Fig. 14; Fig. 20) with a possible range of 11–17 km. This offset is equal to the total offset on the fault and indicates that the inception of this branch of the fault was in latest Pliocene or earliest Quaternary time. Puduhe-Yuxi fault zone. The Puduhe fault trends northsouth ~35 km west of the West Xiaojiang fault zone (Fig. 13). It can be traced for 200 km from the Jinsha River in the north, south to the Yuxi fault (Figs. 13, 21). Along its southern part there are several basins filled with Quaternary sedimentary rocks, including the large Dian Chi basin, which is mostly under Dian Chi Lake, near Kunming. North of Dian Chi Lake, we have no evidence that the fault is active, but the topography, disrupted drainage, valleys aligned along the fault, and ridges of displaced geology suggest young displacement on the fault. Along the west side of Dian Chi Lake there is evidence for young, probably active faulting, and it is clear that Quaternary faulting is responsible for the formation of the lake and its underlying sedimentary basin. East of the lake, the sub-Pliocene surface is well developed and is warped down toward the lake. The Dian Chi basin has been drilled in several places, and the thick-

ness of Quaternary rocks suggests that several subbasins beneath the lake are separated by faults (Luo et al., l983). The thickest deposits (~1,000+ m) of Quaternary sedimentary rocks in the Dian Chi basin are along its west side (Fig. 21). The lake, at 1,800 m elevation, is bounded on the west by an isolated mountain range with a steep to near vertical slope along its east side and with a maximum elevation of 2,800 m at Xi Shan (Plate 10). Numerous small faults and abundant fault breccia are present in many places along this steep eastern slope. The west side of the range slopes gently for 20 km to merge with the sub-Pliocene surface, which is overlain by horizontal Pliocene deposits at an elevation of ~2,000 m. From the south end of Dian Chi Lake, the Yuxi fault strikes south-southwest into the Yuxi basin (Fig. 21). The Yuxi fault shows abundant evidence for active left slip, and we consider it to form the southern part of the Puduhe fault zone, although it is clearly not a direct continuation of the Puduhe fault because it lies farther east and has a more easterly strike. Along the northern part of the Yuxi fault, rivers are consistently offset left-laterally by several hundred meters and shutter ridges are well developed (Plate 11). The fault passes southward through low mountains and along the base of the steep slope on the west side of the Yuxi basin. Here left-lateral stream offsets are common and the fault cuts finegrained coal-bearing sediments with abundant fossils of late Miocene and Pliocene age (Will Downes, personal communication, l993; Fig. 21). The northeastern side of the basin is marked by a steep slope that is probably caused by normal faults, although we found only weak evidence that the faults were active. Along the southeastern side of the basin, the sub-Pliocene surface is gently warped down into the basin. The geometry of these faults (Fig. 21) suggests that the Yuxi basin is a pull-apart basin associated with local downwarping along its southeastern side. Within the southern part of the Yuxi basin the evidence for active faulting becomes less clear. There is no evidence for the

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E. Wang and others

Figure 17. Geologic map of the Huize area north of Dongchuan showing the bending of older structures and shortening related to movement on the Xiaojiang fault. The dashed line outlines the possible orientation of the large anticline cored by Proterozoic rocks before left-lateral shear deformation. Small arrows show enhanced shortening of older structures adjacent to the fault. See text for discussion and Figure 12 for location.

presence of the Yuxi fault farther south, and the east-west–trending Qujiang fault is continuous across the projected trend of the Yuxi fault (Figs. 13, 21). Thus, the Puduhe-Yuxi left-lateral fault zone appears to end north of the Qujiang fault. Structural features are difficult to correlate across the Puduhe fault because pre-Neogene structural trends are complex

and, unlike those in most of Yunnan, irregular. North of Dian Chi Lake structures trend east-west on the west side of the fault and north-south on the east side of the fault. However, ~40 km north of Dian Chi Lake structures trend northeast on both sides of the fault, and in this area offsets of several geological features can be determined. From north to south three geological features can be

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Figure 18. Active faults of the East and West Xiaojiang fault along the Dongchuan basin (patterned Holocene). The largest fault blocks along the East Xiaojiang fault are underlain by rocks of Sinian to Paleozoic age and are parts of fault slivers that were displaced southward, forming the Quaternary-filled Dongchuan basin and diverting rivers flowing westward into the basin. The displaced slivers have been breached by small tributary streams from the Xiaojiang River, forming isolated hills along the fault slivers. Pleistocene strata form a terrace along the west margin of the basin and are locally deformed. See text for discussion and Figure 12 for location.

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E. Wang and others

Figure 20. Geologic map along the East Xiaojiang fault in the Xundian and Songming areas where the fault offsets the Niulan River, a syncline cored by Triassic and Jurassic strata in the South China fold belts, and Pliocene rocks of the Songming basin. These data yield the total magnitude of offset for the East Xiaojiang and West Xiaojiang faults in this area. See Figure 12 for location.

Figure 19. Map showing the left-lateral offsets of streams and rivers along the northern segment of the East Xiaojiang fault in the Gongshan area where the Xiaojiang fault changes strike from north-northwest to north-south. The Awang basin, a small pull-apart basin, is located where the fault steps left before joining the main branch of the East Xiaojiang fault. See Figure 12 for location.

matched across the fault: the unconformity between Cambrian and Sinian rocks, a steeply dipping contact between Devonian and Ordovician rocks, and a syncline cored by Jurassic red beds (Fig. 14, z–z′, aa–a′a′, p–p′). These three features yield left-lateral offsets of 7, 7.5, and 10 km, respectively. Along the Yuxi fault, rocks on both sides are mostly Precambrian metamorphic rocks and offsets are difficult to determine. However, ~8 km south of Dian Chi Lake a steep contact between Precambrian metamorphic rocks and unmetamorphosed Sinian sedimentary rocks is offset left-laterally by 5 km (Fig. 14, l–l′). Because the Yuxi fault ends 30 km farther south, the

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Figure 21. Generalized tectonic map and interpretive diagrams for the Kunming area that is bounded by the East Xiaojiang and Puduhe-Yuxi faults on the east and west, respectively. Rocks shown on some maps as Pliocene east of the Dian Chi Lake are considered lower Pleistocene, as shown here, by Lou et al. (l983). Northeast-trending right-slip faults that lie between and end against the north-trending faults of the Xiaojiang fault system are interpreted to be the result of counterclockwise rotation of fault blocks, and basins influenced by the corners of rotating blocks are also shown (see inset a and b). The pull-apart origin for Fuxian Lake and the Jiangchuan basin is also shown. Dark gray denotes lake. See Figure 12 for location.

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E. Wang and others

Puduhe-Yuxi fault must lose 10 km of displacement between a locality north of Dian Chi Lake and the southern end of the Yuxi fault. The Puduhe fault must also lose displacement to the north because it ends near the Jinsha River. Measured offsets indicate that, beginning ~40 km north of Dian Chi Lake, the Puduhe fault may be losing displacement southward, partly by transferring displacement to north-south shortening on thrust faults west of the Puduhe fault. These northdipping thrust faults, called the Jiaozi Shan thrust faults, are associated with an abrupt change in the topography at the north end of the Dian Chi Lake and carry Precambrian and lower Paleozoic rocks southward above Permian rocks that are in turn thrust southward above Mesozoic red beds (Figs. 13, 21; Plate 12). The thrust faults curve north into the Puduhe fault and similar thrust faults are absent east of the Puduhe fault. It is possible that the thrust faults are pre-Neogene, but because the sub-Pliocene surface is well developed south of the thrust faults, and because a similar surface is present north of the thrust faults at 400 m higher elevation, the thrust faults appear to displace, and thus postdate, this surface. However, there are no Pliocene deposits north of the thrust faults, so that it is not certain that it is the same surface. On Chinese geologic maps, these thrust faults are shown having irregular outcrop patterns to the west. Some of this irregularity is due to landsliding of Permian rocks from north to south on the present topography, also suggesting that the topography is young and may be related to Quaternary movement on the thrust faults. A similar relationship on the Tianba thrust fault may be present farther south, north of the Yuxi basin (Fig. 21), but we have no evidence that the north-dipping thrusts here have young displacement. We also have no evidence to show how displacement is lost on the Puduhe fault to the north of Kunming. Southern part of the Xiaojiang segment. The southern part of the Xiaojiang fault segment is probably the most complex part of the entire Xianshuihe-Xiaojiang fault system. The East and West Xiaojiang faults splay southward and lose displacement in complex ways; only minor displacement remains on the East Xiaojiang fault zone where it reaches the Red River fault (Fig. 13). East Xiaojiang fault. The north-south–trending Yiliang basin is the largest Quaternary basin along the southern part of the East Xiaojiang fault. Steep slopes formed by faults bound the basin on both sides, and the left-stepping character of the East Xiaojiang fault through the basin suggests that the basin is a narrow pullapart feature (Fig. 21). Near the southern tip of the basin several streams and ridges are offset left-laterally along the fault (Plate 13) and hot springs are present along the fault. South of the Yiliang basin the East Xiaojiang fault loses its linear character and splays into several faults (Fig. 22). Along this part of the East Xiaojiang fault older folds and thrust faults are nearly parallel to the East Xiaojiang fault, and it is difficult to determine which faults are strands of the East Xiaojiang fault, which faults are reactivated older faults, and which faults are older faults that are not part of the East Xiaojiang fault zone. Geomorphological evidence indicates that some shortening has occurred across strands of the East Xiaojiang fault. Where the

East Xiaojiang fault consists of several branches, the topography along the fault zone consists of north-south–trending ridges that rise sharply 300–500 m above the more gentle topography of the sub-Pliocene surface. Strands of the East Xiaojiang fault dip steeply east and west, and consistently show hanging wall–up stratigraphic offsets (Fig. 22, A–C cross section). We interpret the change in topographic character along the fault to result from a component of shortening across the East Xiaojiang fault zone. On its easternmost strand, Devonian limestone is thrust eastward onto Eocene-Oligocene beds of the Lunan basin (Fig. 22). Folds and thrusts in the Paleozoic rocks in this area are truncated by the

Figure 22. Geologic map along the southern part of the East Xiaojiang fault in the Yiliang area. The East Xiaojiang fault splays into several arcuate strands and older northeast-trending structures of the South China fold belts are bent into parallelism with the fault, such as the syncline north of Yiliang. Unlike most of the East Xiaojiang fault, in this area associated structures are compressional as shown by the cross section. See Figure 12 for location.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China sub-Eocene karst surface; thus this thrust fault is post-Eocene and is not part of the older Mesozoic South China fold and thrust belts. North of the cross section in Figure 22, near Yiliang, the karst surface is overlain by subhorizontal, early Quaternary, unconsolidated, lateritic soil suggesting this karst surface is a multiple surface formed not only by pre-Eocene erosion, but also by pre-Quaternary erosion (sub-Pliocene erosion surface?). In addition, the western side of the basin is marked by steep slopes suggesting young displacement on the basin margin thrust fault. These data suggest that thrust faulting along the west side of the Lunan basin is Quaternary in age and related to movement on the East Xiaojiang fault. Even with the complications of numerous fault strands, interactions between old and young faults, and a component of thrust movement on some fault strands, there is evidence for leftlateral displacement of ~5–10 km on streams crossing the East Xiaojiang fault (Fig. 23, Plate 14). Displacement along the East Xiaojiang fault appears to be young. For example, the small Huaning basin, lying along the western strand of the East Xiaojiang fault, contains Pliocene and Quaternary sedimentary rocks that dip southward at ~5–15° and rest unconformably on older rocks in the north. Because Quaternary rocks are present only at the south end of the basin, it is likely that tilting of the Pliocene rocks postdates the youngest rocks in the sequence and is thus Quaternary in age. The Neogene and Quaternary rocks are faulted along both the eastern and western sides of the Huaning basin but did not show definitive evidence for active displacement, although some streams that cross the east side of the basin show left-lateral bends suggestive of active faulting. Disrupted drainage is common south of the basin, suggesting that young and possibly active displacement on these strands of the East Xiaojiang fault continues south of the Huaning basin (Fig. 23). Except for the Xinzhai fault, the western strands of the East Xiaojiang fault do not offset the east-west–trending Jianshui fault, indicating that most of the active displacement on these fault strands ends at or north of the Jianshui fault (Fig. 13). North of the Jianshui basin these arcuate strands of the East Xiaojiang fault outline a rhombic-shaped mountain underlain by low-grade Proterozoic rocks. Uplift of this mountain may be related to a component of shortening across the East Xiaojiang fault, as well as by shortening caused by active right-slip on the eastwest–trending Qujiang and Jianshui faults (see following section). Only the easternmost fault, the Xinzhai fault, continues to the Red River Valley (Fig. 14). The eastern strands of the East Xiaojiang fault pass through the Panxi basin with a left-stepping geometry that suggests the basin is a left-lateral pull-apart structure (Fig. 22). Farther south, several of these strands abut, but do not offset, an east-trending ridge of Carboniferous limestone. Only the easternmost strand of the East Xiaojiang fault can be traced as far south as the easttrending Jianshui fault. Because this fault does not exist south of the Jianshui fault, it may transfer its left-slip displacement eastward onto the Xinzhai fault (Fig. 14). Disrupted drainage in the Jianshui basin and some left-lat-

31

eral stream offsets indicate that active left-slip continues southward on the Xinzhai fault (Plate 15) but dies out southward before reaching the Red River. The Xinzhai fault also appears to offset the east-west–striking Jianshui fault left-laterally, and several narrow Quaternary basins, interpreted as pull-apart basins, lie along the fault. These observations indicate that the East Xiaojiang fault loses ~20 km of displacement between the Yiliang basin and the Jianshui fault, a distance of ~120 km (Fig. 14). For example, the Niulan River is offset left-laterally by ~22 km (Fig. 14, n–n′); ~100 km to the south, one strand of the East Xiaojiang fault leftlaterally offsets a syncline of Triassic rocks by ~7 km (Fig. 14, j–j′); 30 km farther south, the Jianshui fault appears to be offset left-laterally across the Xinzhai fault by ~1–3 km, but this offset is difficult to measure. Most strands of the East Xiaojiang fault end north of Jianshui basin (Fig. 14). This loss of displacement

Figure 23. Right side of figure shows the left-lateral stream offsets along the East Xiaojiang fault in the same area shown in Figure 22. Arrows show some of the obvious offsets. Left side of diagram shows a reconstruction of drainage after fault displacements are removed. In this area the fault is left-lateral transpressional. Stipple pattern shows location of Quaternary basins along the fault. For location see Figure 12.

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appears to be by shortening and bending within the fault zone, but it is not clear exactly how this is accomplished. West Xiaojiang fault. South of the Songming basin, the West Xiaojiang fault steps left through Yangzhong Lake where it forms a narrow pull-apart structure (Fig. 21). Southward, the fault splits into two branches that bound two large basins of different character. The Fuxian basin contains one of the deepest lakes in China (average depth ~80 m, maximum depth ~180 m, Fig. 21), This basin is bounded on both sides by normal faults that are exposed at the northern and southern ends of the lake, creating steep slopes that disrupt the gently rolling sub-Pliocene surface. The apparent youthfulness of the topography suggests young to active faulting; evidence for active faulting is clear both north and south of the Fuxian basin, but we found no direct evidence for active faulting along the lake. A narrow horst separates the Fuxian basin from the smaller Jiangchuan basin to the southwest. In contrast to the Fuxian basin, the Jiangchuan basin has a normal fault only on its eastern side, and its western side is formed by gentle downwarping of the sub-Pliocene erosion surface (Fig. 21). The lake lies on the northeastern side of the basin, indicating that tilting of the subPliocene surface is young and controls the position of the lake. A small, narrow fault-bounded Quaternary basin extends northward from the north end of the Jiangchuan basin, but relief along the basin margins is only 100–200 m, suggesting that the bounding faults have small displacement. Faults bounding the narrow horst between the Fuxian and Jiangchuan basins continue south of the Jiangchuan basin. On the west side of the horst, the Baipo fault curves west, and left-lateral stream offsets indicate that this fault is active (Plate 16). Southward, the Baipo fault continues to curve west and forms part of the northern margin of the Tonghai basin (Fig. 21). On the eastern side of the horst, the Xiongguan fault also curves west, but north of the Tonghai basin, it merges with or ends against an eastwest–trending north-dipping normal fault (Fig. 21). Streams that cross the Xiongguan fault near Shangying show active left-slip displacements. Near its southern end, the Xiongguan fault bounds a small extensional basin, but none of these faults reaches as far south as the Tonghai basin. Total offsets on the southern part of the West Xiaojiang fault indicate that it is losing displacement to the south and ends near the northern margin of the Tonghai basin. On both sides of the Fuxian basin, folds and faults truncated by the sub-Pliocene erosion surface are complex and in places record superposed folding and faulting events. A tentative match of older folded and faulted structures across the Fuxian basin indicates ~2–5 km of northeast (pull-apart) extension across this basin. We have no measurable offsets along the Baipo fault, but contacts within Permian and Paleozoic units are offset left-laterally by 1–2 km across the Xiongguan fault. The West Xiaojiang fault loses ~15 km displacement between the Songming and Tonghai basins, a distance of 120 km (Fig. 21). This displacement appears to be absorbed by extensional structures along the southern part of the West Xiaojiang fault. This is in contrast to the East Xiaojiang fault zone,

which appears to lose displacement mainly by shortening and bending of rocks within the fault zone. Qujing fault zone. The Qujing fault zone lies 20–50 km east of the Xiaojiang fault (Fig. 13). The fault cuts through the subEocene and sub-Pliocene karst plateau that is developed across strongly deformed Paleozoic and Triassic rocks (e.g., Plate 17a) and lies at an elevation of ~2,000 m. Earthquakes and river offsets indicate the fault is an active left-slip fault zone parallel to the Xianshuihe-Xiaojiang fault system; thus, we consider it to be part of that fault system. The Qujing fault zone is marked by numerous Quaternary basins, some of which contain hot springs, as are the other active faults of the Xianshuihe-Xiaojiang fault system. These basins and the fault zone disrupt the generally gentle relief on the sub-Eocene and sub-Pliocene karst surfaces (Plate 17b). This fault zone is not shown on previous geologic maps but has been recognized as an important fault zone during our studies. The Qujing basin is the largest and most complicated basin along the Qujing fault zone (Fig. 24). Its northwestern part is bounded by steep slopes caused by normal faulting. The left-stepping geometry of the fault zone in this part of the basin suggests it is a pull-apart basin. A high ridge rising to >400 m above the basin floor and to >200 m above the general elevation of the karst plateau separates the Qujing basin into two parts (Fig. 24). The western side of the ridge appears to be a north-striking normal fault, but the southern and eastern sides of the ridge are formed by a curved, steeply west- and north-dipping thrust fault that carries Paleozoic rocks above Neogene rocks (Plate 18). Faults on both sides of the ridge are young and probably contemporaneous, and the pattern of the faults indicates coeval northeastward extension and local northwestward shortening. The close association of shortening and extension is reminiscent of the small-scale extensional pull-aparts and compressional mole tracks commonly associated with active strike-slip faulting. The southeastern part of the Qujing basin contains mainly Pliocene rocks (Ciying Formation) and is marked by a normal fault on its western side. The northern part of the Qujing fault is young, and the topographic expression of the fault and the presence of earthquakes in this area indicate that the fault is active. The Luliang basin lies south of the Qujing basin and contains both Pliocene and Quaternary deposits, and the subPliocene erosion surface forms the basin floor and slopes gently north (Fig. 24). The same fault zone that forms the western side of the southern part of the Qujing basin forms the eastern side of the Luliang basin (Fig. 24, Plate 19) and is marked by a steep slope suggesting normal faulting. The reversal of normal fault displacement between the two basins is a characteristic feature of strike-slip–related basins. At Mile, the Qujing fault zone bifurcates and the two faults merge again north of Kaiyuan (Fig. 13). Several small basins filled with Quaternary deposits lie along the faults. Their geometry is suggestive of pull-apart basins at different types of releasing bends or fault intersections. At Kaiyuan, the fault again bifurcates to the south (Fig. 25), the eastern branch striking into the northern

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

33

Figure 24. Geologic map of the Qujing and Luliang basins along the northern part of the Qujing fault (center). Interpretive diagram for local structures discussed in text is shown at right. Possible predisplacement reconstruction of the Neogene basin is shown at left. The interpretation shown suggests that the two basins were originally one basin displaced by the Qujing fault. See Figure 12 for location.

part of the complex Mengzi basin and the western branch continuing through a small basin at Gejiu (Fig. 25). The Mengzi basin has the geometry of a left-stepping pullapart (Fig. 25), indicating left-slip continues from the Qujing fault south through the Mengzi basin. Earthquakes in the Mengzi basin are common, suggesting active deformation. In the northwestern part of the Mengzi basin, the eastern branch of the Qujing fault appears to merge with a northwest-dipping thrust fault north of Jijie that places Middle Triassic limestone on Paleogene conglomerate and Pliocene coal-bearing sediments (Fig. 25). Both the eastern and western sides of the basin are marked by normal faults. The fault on the eastern side of the basin cuts the sub-Eocene karst surface that is well preserved ~700–1000 m above the basin floor (Plate 20). The western side of the basin is marked by a high, steep scarp, and the sub-Pliocene surface lies 1,000-1,400 m above the basin. Faults along the west side of the basin bound a series of fault blocks downthrown toward the basin. At one place basin sediments abut the fault with only small alluvial fans present, although there is >1,500 m of relief on the

basin margin, suggesting that normal fault displacement is very young if not active (Plate 21). This boundary of the basin also shows left-lateral displacement, as indicated by the left-lateral offset of 30 m and 50 m of two Quaternary alluvial fans in the area of Panzhihua (Li Desheng, unpublished data, 1983). The Nanxihe fault trends southeast from the southeastern corner of the Mengzi basin (Fig. 25). Although this fault is not seismically active, offsets of drainage along the fault suggest active left-lateral displacement. Pliocene rocks are also offset along the fault. This fault may connect with the eastern boundary fault of the Mengzi basin. As shown in Figure 25, the formation of the Mengzi basin is partially attributable to left-lateral movement along the Nanxihe fault. The Gejiu fault, the western branch of the Qujing fault, passes through a small basin at Gejiu (Fig. 25, Plate 22) The basin floor lies ~1,100 m below the karst plateau that is well developed above both sides of the basin. Brecciated rock is widespread along the margins of the basin, and the geometry of the basin-bounding faults suggests it is a narrow pull-apart basin.

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Figure 25. Map of the Mengzi basin (see location in Fig. 12). A karst plateau, parts of both the subEocene and sub-Pliocene karst surfaces, surrounds the Mengzi basin and is characterized by depressions shown by the numerous closed contours. A generalized model for formation of the Mengzi basin as a modified pull-apart structure is shown in the inset.

The Gejiu basin has internal drainage and lake sediments fill its southern end. We interpret the basin to be partly a large karst sink. The basin appears to be drained from underground by a major south-flowing river that exits at a large cave ~10 km south of the basin. At least two similar but smaller basins are present farther south. Along these internally drained basins aligned along the Gejiu fault, rivers disappear at the surface within each basin and reappear from a cave at lower elevation south of each basin. We interpret this line of basins to be primarily karst basins developed along the fault. The town of Gejiu thus appears to have sev-

eral significant societal dangers: earthquakes, potential fault displacements, unstable building foundations, landslides, and the potential for collapse of the karst sink. The modification of the Gejiu structural basin by karst processes is common to many of the other Quaternary basins developed in the limestone terranes of Yunnan, particularly in the Triassic limestone province of the South China fold belt, such as the Mengzi and Kaiyuan basins. The time of inception of the Qujing fault zone is similar to the other faults of the Xianshuihe-Xiaojiang fault system. It cuts through folds of early Cretaceous age(?), disrupts the sub-

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China Eocene, and in places the sub-Pliocene erosion surface, and displaces fine-grained Pliocene rocks whose distribution and rock types record little evidence of having been deposited during active faulting. All the rocks deposited in basins associated with the Qujing fault are shown on geologic maps as Quaternary. Only in the Qujing, Luliang, and Mengzi basins do fossils confirm the Quaternary age of the sediments (Li Shengde, unpublished data, 1984). The limited evidence suggests that these fault-controlled basins began to form during latest Pliocene to Quaternary time, which is probably close to the time of inception of the Qujing fault zone. Total displacement on the Qujing fault zone is poorly constrained. The footwalls of the normal faults that bound both parts of the Qujing basin have no Neogene rocks, and it is possible that the Neogene rocks in the basin were deposited in one basin that has since been offset by left slip (Figs. 14, 24). If this interpretation is correct, the maximum left-slip displacement would be ~17 km, but without detailed study this remains poorly constrained. About 60 km north of Qujing the fault is difficult to trace and appears to die out. The only other place along the Qujing fault zone where a possible offset occurs is near Kaiyuan where the Nanpan River is offset left-laterally at least 3 km and perhaps as much as 8 km (Fig. 26). At its southern end, the fault passes through Gejiu, but does not reach the Red River (Fig. 13). The Nanxihe fault, southeast of the Mengzi basin, continues into Vietnam and its fate is unknown. Thus, like other faults of the Xianshuihe-Xiaojiang system, the Qujing fault zone appears to die out before reaching the Red River. Like the Puduhe fault zone, it also dies out northward. Other left-lateral faults. Three left-slip faults, the Yimen, Lufeng, and Luzhijiang faults, lie west of the Puduhe fault (Fig. 12). All three faults strike north-south parallel to the other faults of the Xianshuihe-Xiaojiang fault system. All three faults zones are locally active, but how far along strike the faults are active is unknown. The Yimen and Luzhijiang faults appear to have had an older history of respective west- and east-vergent thrust movement. Yimen fault. The north-striking Yimen fault lies within the easternmost part of the Mesozoic Chuxiong basin (Fig. 13). The north-south elongated Luoci basin lies along the fault and contains a thin section of mostly Pliocene rocks; Pleistocene rocks are limited to the north end of the basin. The Yimen fault forms a steep scarp along the eastern side of the Luoci basin, suggesting a component of young or active normal displacement. River offsets along the base of the steep scarp indicate the fault has active left slip. Lufeng fault. The Lufeng fault lies west of the Yimen fault and forms the western boundary of the Lufeng basin where the Lufeng fault makes a left step (Fig. 13). Along the western side of the Lufeng basin, the fault forms a steep scarp and shows clear left-slip stream offsets of 100–200 m. Even though the topography along the west side of the basin indicates a down-to-the-east normal component, Precambrian rocks east of the fault are juxtaposed against Jurassic and Cretaceous rocks west of the fault. At the southern end of the Lufeng basin a west-vergent thrust fault is mapped juxtaposing the same units. The thrust is probably early

35

Tertiary in age and has no topographic expression. The geological relations suggest the thrust was locally reactivated as a normal fault. North from the Lufeng basin, geologic maps show the thrust fault continues for 20 km, but how much of this length has active left slip is unknown. At the south end of the Lufeng basin the active fault steps east, and the east-west–trending fault connecting the left step has a north-side-down normal component forming the abrupt south end to the basin. The left-step segment of the Lufeng fault continues ~10 km farther south before it appears to end. Along this segment of the fault there are indications of left slip, from stream offsets, and down-to-the-east displacement, but the evidence is weak and inconclusive.

Figure 26. The Kaiyuan basin was formed as a pull apart structure by a left step along the Qujing fault. Left-lateral offset along the Qujing fault in this area is between 3 km (A–B) and 8 km (A–C) depending upon how the deflection of the Nanpan River is interpreted. For location see Figure 12.

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Luzhijiang fault zone. The Luzhijiang fault zone consists of two north-south–striking segments that are often shown as a continuous fault, the Luzhijiang fault zone (Fig. 13). The connection between the two segments is not obvious on satellite images, and it remains unresolved whether there is a continuous active fault zone. The northern segment, called the Yuanmou fault, extends ~300 km from southwestern Sichuan into western Yunnan and is clearly expressed on satellite images. The southern segment, called the Yangwu fault, extends ~75 km and ends at the Red River fault (Fig. 12). The region between these two faults contains mapped normal and thrust faults along the eastern margin of the Mesozoic Chuxiong basin that could form faults connecting the Yuanmou and Yangwu faults. However, the traces of possible connecting faults are almost entirely within Precambrian rocks and lie within the Luzhi River valley, obscuring evidence for recent displacements. The Yuanmou fault is seismically active (Lithospheric Dynamics Atlas of China, 1989) and bounds the eastern side of the Yuanmou basin, separating the basin from a high plateau to the east, underlain by generally horizontal Jurassic and Cretaceous sedimentary rocks (Fig. 13). The Yuanmou basin contains early Pleistocene lacustrine strata that were deposited in a lake formed by east-side-up movement on the Yuanmou fault. Presence of these lake strata provides evidence that the earliest movement on the fault was at least as old as early Pleistocene in age. Younger west-vergent thrusting on the Yuanmou fault placed Jurassic redbeds on the early Pleistocene strata that were extensively folded and faulted near the fault. Earthquake focal mechanisms indicate active left-slip (Liu et al., l986) on the Yuanmou fault. In contrast, field evidence for strike-slip movement is poorly developed, and we found only one reliable left-lateral stream offset of ~100–200 m. The Yuanmou basin is elongated parallel to the Yuanmou fault, similar to many of the other Quaternary basins related to faults along the Xianshuihe-Xiaojiang system (Fig. 13). There are two smaller basins that lie farther west and may have been connected to the Yuanmou basin at one time. The oldest sediments within these two basins are dated as early Pleistocene, although in two small adjacent basins, late Pliocene strata (Shagou Formation) are present (W. Downs, personal communication, l993). Sedimentary rocks in the Yuanmou basin are characterized by fine-grained lacustrine and fluvial deposits, and they unconformably overlie Proterozoic metamorphic rocks (Bureau of Geology and Mineral resources of Yunnan Province, 1990). The sedimentary record in the Yuanmou basin indicates that lacustrine conditions ceased in middle Pleistocene time (Bureau of Geology and Mineral resources of Yunnan Province, 1990). The sediments in the basin are being actively eroded by a northflowing tributary of the Jinsha River, and it is clear that the Jinsha River drainage captured and drained the Yuanmou lake basin in middle Pleistocene time. The Jinsha River has eroded through the lake sediments and into the Proterozoic substrate, forming a gorge several hundred meters deep. The Yangwu fault is expressed as a clear linear feature in the

field and on satellite images. The fault generally separates Proterozoic metasedimentary rocks from Triassic rocks along much of its trace. Large and small fault-bounded lenses of Precambrian and Triassic rocks are interleaved along the fault, and an elongate ridge formed by Triassic coal-bearing strata extends for several kilometers along the fault. Stream offsets indicate active left-slip displacement on the Yangwu fault (Fig. 27, Plate 23). The Yangwu fault ends within the Red River Valley to the south. To the north its trace becomes obscured where it intersects the northwest-trending Jianshui and Qujiang faults (Fig. 13). Right-slip faults associated with the Xiaojiang segment. Within the southern segment of the Xianshuihe-Xiaojiang fault system are numerous faults that lie between the main fault strands of the system but do not cut them. Most of these faults strike to the northeast and occur in the central part of the segment between Tonghai Lake and Dongchuan (Figs. 13, 21). They often follow old northeast-trending structural features, but in several places they offset rivers right-laterally, indicating that at least some of these faults are active and related to the Xianshuihe-Xiaojiang system (Plate 24). About 40 km north of the Dian Chi Lake, a northeast-trending fault offsets a river 4 km right-laterally (Fig. 14, s–s′). Ten kilometers farther north, a curving fault through the Liangwang Shan bounds Neogene and Quaternary deposits and

Figure 27. Left-lateral disruption of drainage along the Yangwu fault, the southern continuation of the Luzhijiang fault. A long ridge that parallels the fault on its east side for ~7–8 km is either a shutter ridge or a displaced sliver on the fault, but geological relations in the field are not clear. For location see Figure 12.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China offsets a contact between Devonian and Ordovician rocks 5 km right-laterally (Fig. 14, t–t′). Numerous other northeast-striking faults appear to bound parts of the Quaternary basins along the main north-striking left-lateral faults. Wherever we can find geological offsets of older rocks, they are consistently right-lateral, but we can not date the movement on most of these faults. We interpret these faults to be part of the Xianshuihe-Xiaojiang system and their right slip to be the result of counterclockwise rotation of crustal fragments between the main left-lateral faults (Fig. 21). These northeast-trending faults have small displacements and thus slip rates are probably so slow that it would be difficult to recognize them as active faults. If this interpretation is correct, it suggests that some Quaternary basins in the region owe part of their development to counterclockwise rotation of these faults. The Dian Chi and Songming basins have shapes that are not compatible with a simple pull-apart geometry on northsouth–striking faults. The sub-Pliocene erosion surface is locally warped down into these two basins. Parts of the basins are bounded by components of normal displacement on the northsouth left-slip faults. Their positions at the extensional corners of counterclockwise rotating crustal fragments suggest subsidence at the extensional corners of the rotating blocks, as a cause of the irregular geometry of the basins (Fig. 21). Other right-slip faults possibly related to the Red River fault system. The northwest- to west-striking Qujiang and Jianshui faults interact with the southernmost part of the XianshuiheXiaojiang fault system south of Tonghai Lake (Fig. 13). These two active faults lie 50–75 km north of and are parallel to the Red River fault. Their relations to the Xianshuihe-Xiaojiang and Red River fault systems are not clear. They could be regarded as the most northerly expression of right slip on the Red River system. However, both faults contain abundant evidence for an older, but Cenozoic, left-lateral displacement of a few tens of kilometers, and their young right slip could be considered to result from counterclockwise rotation on preexisting east-trending faults between faults of the Xiaojiang fault system. A discussion of this topic is considered in the section on the Red River fault. Discussion of the Xianshuihe-Xiaojiang fault system Data presented earlier show that the total displacement on the Xianshuihe-Xiaojiang fault system is partitioned among different faults within the system, and that displacement on discrete faults within the system dies out to the south before reaching the Red River. The Xianshuihe segment has ~60 km of total left-lateral displacement from Ganzi to Shimian. The total horizontal displacement appears to be reasonably constant on the Xianshuihe segment even though Allen et al. (1991) suggested the slip rate on the Xianshuihe fault changed from 15 ± 5 mm/yr in the northwest to ~5 mm/yr in the southeast. If their slip rates are accurate, it suggests that short-term slip rates may change temporally even though the long-term total horizontal displacement may be constant. From near Shimian southward, the Xianshuihe fault bifur-

37

cates into the Anninghe and Shimian faults and the faults rejoin father south at Qiaojia. From the displacement of geological features, it appears that horizontal displacement is divided with ~47–53 km on the Anninghe-Zemuhe faults (Fig. 7) and ~8–11 km on the Shimian fault (Fig. 10). Shortening is associated with this part of the fault zone and is expressed by the elevated ridge along the northern part of the Anninghe fault, the thrust faults and folds across the region from Gonga Shan to east of the Shimian fault, and Pliocene-Quaternary uplift of Gongga Shan (Chen et al., l996). The smaller curvature of the Anninghe part of the fault system relative to the Xianshuihe-Shimian part of the fault system is consistent with transpression along this part of the fault system between Shimian and Qiaojia. South of Qiaojia, the northern part of the Xiaojiang fault zone has 60 km of displacement where it crosses and offsets the Jinsha River, but to the south the displacement becomes partitioned on several faults (Fig. 14). South of Dongchuan, the total displacement on the West and East Xiaojiang faults is significantly ~60,000 yr [radiocarbon data]; we use long term to refer to periods of millions of years. In the discussion presented here long term refers to ca. 4–2 Ma.) Long-term slip rates range from 15–30 mm/yr on the Xianshuihe and northern end of the Xiaojiang faults to 12–28 mm/yr on the Anninghe-Zemuhe faults, 2–5 mm/yr on the Shimian fault, 9–23 mm/yr on the East Xiaojiang fault, and 3–8 mm/yr on the West Xiaojiang faults. Slip rates become slower southward as displacement on these faults dies out. These long-term slip rates are higher than slip rates determined from our GPS studies by a factor of about two; this discrepancy will be discussed in following paragraphs. The current pattern of tectonic activity along the Xianshuihe-Xiaojiang fault system in western Yunnan differs from effects in the area of the eastern syntaxis predicted by the thin viscous sheet models of England and McKenzie (1982), England and Houseman (l986, l988), and Housemen and England (l986). The clockwise curvature of principal stress and strain axes around the eastern syntaxis in their models is in the same sense as

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China similar axes deduced from simple interpretations for stress and strain axes for the Xianshuihe-Xiaojiang fault system. However, they differ significantly in the amount of curvature. In the models principal axes of deviatric compressive stress strike east-west in the region east of the syntaxis, and only much farther to the southeast do they strike northwest. Axes deduced from the faults would strike at least N45°W directly east of the syntaxis. Molnar and Tapponnier (1977) applied slip-line theory related to indentation to interpret the pattern of active faults around the eastern syntaxis. While this interpretation has some similarities to the pattern of active faults, it is difficult to compare their study with our results because their study uses instantaneous solutions and is at the initial stage of indentation, whereas our interpretation applies to the late stage of indentation after large-scale rotation of material has already occurred around the syntaxis. Patterns of principal stress, strain and velocity deduced from earthquake data by Holt et al. (l991, 1995) are similar to what we interpret from the faults (see following paragraphs), but our interpretations of the origin of some right-lateral faults, such as the Qujiang and Red River faults and faults in the region south of Yunnan, are different. A discussion of these differences follow the data and discussions on the Red River fault system given in later paragraphs. In general, the displacements on the Xianshuihe-Xiaojiang fault system suggest clockwise rotation of about 1.2°–2.4°/m.y. for material west of the fault system relative to South China. Rotations on a smaller scale also occur within the Xianshuihe-Xiaojiang fault system. Within the fault system small crustal fragments are rotating counterclockwise bounded by right-slip northeast-striking faults that are themselves rotating counterclockwise between the major faults (Fig. 21). Movement on these northeast-striking faults may have controlled the shapes of several Quaternary basins, such as the Dian Chi and Songming basins. There are numerous northeast-trending faults that end against north-trending faults of the Xianshuihe-Xiaojiang system for which we have no evidence of age. It is unclear how any of them may be active and related to counterclockwise rotation within the main fault system because their slip-rates and total offsets are probably small. Along most of its length the Xianshuihe-Xiaojiang fault system cuts obliquely through older folds and thrusts, and the anisotropy of the crust played little if any role in localizing the faults. However, in the central part of the Xianshuihe-Xiaojiang fault system, in particular along the Shimian segment, left-slip faults appear to have developed from an earlier or contemporaneous system of arcuate folds and thrusts produced by northeastsouthwest shortening (Burchfiel et al., 1995). This fold and thrust belt formed sometime after Eocene(?)-Oligocene time and overprints an older, still Cenozoic, northeast-trending fold-thrust belt that is parallel to the Longmen Shan farther north in Sichuan (Fig. 5). The region of intersection of the two fold and thrust belts forms a characteristic dome and basin pattern of superposed structures. The northwest-trending and younger fold-thrust belt is a region of high topography with one peak, Gongga Shan, reaching 7,556 m, several peaks above >6,000 m, and many peaks >4,000 m. Youth-

39

fulness of the high topography suggests that the northwest-trending belt is a late Cenozoic (800 km southeast into Vietnam (Figs. 1, 29). They lie within the broad zone of deformation that reflects the India-Eurasia convergence, and they are related to the complex stress and strain pattern that exists in the region around the eastern Himalaya syntaxis. In combination with the Xianshuihe-Xiaojiang fault system to the east, these three fault systems outline a lens-shaped region, the Chuan Dian crustal fragment, interpreted to be bounded by right-slip and left-slip faults on its southwest and northeast sides, respectively (Fig. 3). The Chuan Dian crustal fragment has been interpreted to be moving more rapidly to the southeast than crust adjacent to it (Kan et al., 1983; Holt et al., l991, 1995). Thus, the Red River and Dali fault systems not only figure prominently in the controversy over whether large-scale extrusion of crustal material occurred, but also whether they form the boundaries of a small crustal fragment that has been interpreted to be actively extruding from the region of the southern Tibetan plateau. Study of this smaller crustal fragment yields important information on the structure and mechanisms of crustal extrusion. The crust in western Yunnan is composed of a collage of crustal fragments that were assembled by late Mesozoic time, prior to the collision between India and Eurasia (e.g., see Sengor, l984; Sengor et al., l988; Metcalfe, l993, l996). This collage was, and continues to be, deformed and disrupted during the postcollisional convergence between India and Eurasia (Wang and Burchfiel, l997). Postcollisional deformation in western Yunnan has been so strong and complex that identification of and relations between crustal fragments that form the collage have been difficult to establish (Wang and Burchfiel, l997). We divide western Yunnan into seven tectonic units that we consider appropriate for examining the young and active tectonics of the region (Fig. 29). These include the faults and folds that make up the Three Rivers fold belt, sometimes called the Henduan Shan orogenic belt, that lies mostly in the westernmost part of Yunnan, but continues southward beyond the Chinese border. This structural belt can be subdivided into several parts on the basis of age and structural style (Wang and Burchfiel, l997), but here we have separated only the Cenozoic Lanping-Simao fold belt in this region (Fig. 29). To the east of the Lanping-Simao fold belt lies a complex of folds and faults that have northwest, north, and northeast trends that we have divided into six tectonic units: the Yunling collage, Dali highland, Chuxiong basin, Tibetan plateau, Yangzi platform, and the South China fold and thrust belts (Fig. 29). Structures within these units are older than the Red River and Dali fault systems. Some of the tectonic units are separated from one another by major late Cenozoic and active faults, whereas the boundaries of other units are older and may have influenced or controlled many of the late Cenozoic and Holocene structures, or may place limits on the magnitude of displacements on the young structures. The Red River fault, senso stricto, separates the Lanping-Simao fold belt from the tectonic units to the northeast and, in our interpretation, cannot be traced west of Dali.

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Lanping-Simao fold belt. The Lanping-Simao fold belt (Fig. 29) consists mainly of Mesozoic and early Cenozoic red beds with the exception of some Lower and Middle Jurassic marine rocks that are intercalated in the west. Local unconformities are present within the red bed sequence, but, in general, Mesozoic and early Cenozoic rocks form a conformable succession. On the western side of the fold belt, the Mesozoic rocks have complex relations to older rocks. They contain Triassic volcanic rocks near their base and rest on a variety of Paleozoic and (?) older rocks. In many places Middle Jurassic rocks rest unconformably on the older rocks. Along the eastern side of the fold belt, the Mesozoic rocks also have complex relations with a narrow and discontinuous zone of Paleozoic rocks, metamorphic rocks with uncertain protolith ages, and plutonic rocks. The longest part of this zone consists of high-grade metamorphic rocks and forms the Ailao Shan that can be traced southeastward from the Midu basin in a progressively widening belt into Vietnam (Fig. 29, shown as the eastern part of unit VIII). West of the Ailao Shan is a zone of low-grade metamorphic rocks that contains numerous small bodies of ultramafic rocks and serpentinite that in many places form part of a melange. Everywhere faults separate the Paleozoic, Mesozoic, and early Cenozoic rocks from the higher grade rocks of the Ailao Shan. Upper Triassic rocks west of these faults contain thick conglomerate with clasts of volcanic and sedimentary rocks, but they contain no clasts from the crystalline rocks of the adjacent Ailao Shan. Rocks similar to those in the Ailao Shan are exposed in the shorter Diancang Shan, northwest of Dali, and Xuelong Shan farther northwest (Fig. 29). Rocks in both the Diancang Shan and Ailao Shan are locally mylonitic and have yielded Cenozoic cooling ages (Harrison et al., l992a; Leloup et al., l993, l995), but the mylonitic rocks in the Xuelong Shan have not been dated. The Mesozoic rocks of the LanpingSimao belt narrow and pinch out northward against Triassic volcanic rocks along the Tongdian fault. Mesozoic rocks of the Lanping-Simao belt were folded in Eocene, Oligocene, and postOligocene time. Folds trend north to northwest and form arcuate patterns, whereas the belt of metamorphic rocks in the Ailao Shan, Diancang Shan, and Xuelong Shan form a linear belt. Folds of the Lanping-Siamo fold belt generally strike obliquely, with a more northerly strike, into the metamorphic rocks. Chuxiong basin. The Chuxiong basin lies northeast of the Lanping-Simao fold belt and is separated from it by faults along the east side of the Ailao Shan metamorphic rocks (Fig. 29). It consists of a thick sequence of Mesozoic to early Cenozoic red beds that rests mainly on Precambrian metamorphic and granitic rocks and locally on Paleozoic rocks similar to those of the Yangzi platform to the east and northeast. Based on the sequence of Precambrian and Paleozoic rocks, it can be argued that the basement of the Chuxiong basin is the continuation of the Yangzi platform. There is evidence of local Cretaceous deformation near Yuanmou in the northeastern part of the Chuxiong basin, but in general the sedimentary sequence is continuous from Triassic to Eocene and even Oligocene time. In the northern half of the basin, the Mesozoic rocks are folded along generally north-north-

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Figure 29 (this and facing page). Tectonic map of the Red River and Dali fault systems in western Yunnan. For location see Figure 3. a) The major faults of the Red River and Dali fault systems and the location of other figures. Inset in upper right shows the main tectonic units discussed in the text. Some faults of the Xianshuihe-Xiaojiang fault system are shown where they adjoin the Red River fault system. b) More detailed maps of the Red River fault and its relation to the rock units of the northeastern part of the Lanping-Simao unit in the Ailao Shan (lower left) and faults of the southeastern part of the Red River fault and adjacent Cenozoic rocks. Inset map in upper right shows location to regional structures (XX = Xianshuihe fault; RR = Red River fault) .

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

west– to north-trending axes and cut by thrust faults with similar trend. The active Chenghai fault forms the northwestern boundary of the Chuxiong basin (Fig. 29). The northern boundary of the basin is arbitrarily taken as the northern limit of Jurassic rocks. These Jurassic rocks rest on Triassic, Paleozoic, and Precambrian rocks and are not present farther to the north. Tibetan plateau. The Tibetan plateau tectonic unit lies north of the Chuxiong basin and topographically forms a high plateau whose underlying rocks belong to several different paleogeographic units. In the area of Figure 29, it consists of the southeasternmost part of the strongly folded Triassic flysch in the Songpan-Ganzi basin, and the southernmost part of the Triassic Yidun volcanic arc (Fig. 4). The Songpan-Ganzi flysch basin ends southward at a northeast-striking belt of faults, at least one of which is a thrust fault. South of these faults the Triassic units are marine and dominated by limestone with some nonflysch clastic units, and they rest on Paleozoic rocks of Yangzi platform type. The Triassic volcanic rocks of the Yidun arc grade southward into nonvolcanic marine units that also lie above Paleozoic rocks of Yangzi type, and both the Triassic and Paleozoic rocks continue into the Dali highland. The rocks of the Tibetan plateau unit are strongly folded, refolded,

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and intruded by Triassic and Early Jurassic igneous rocks. There are no Jurassic or Cretaceous rocks present in this part of the Tibetan plateau and Paleogene and Neogene rocks rest unconformably on the folded, intruded, and locally metamorphosed Triassic and older rocks. Deformation in the Tibetan plateau unit is complex and occurred in both Mesozoic and Cenozoic time. Dali highland. The Dali highland is surrounded by late Cenozoic (5.5 km) cored by metamorphic rocks of probable Devonian protolith age that yield Cenozoic metamorphic ages (Lacassin et al., l996). Unlike the Chuxiong basin to its east, the Dali highland does not contain Jurassic, Cretaceous, or early Paleogene rocks. The folded and thrusted Paleozoic and Triassic

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rocks are unconformably overlain by Upper Eocene and Oligocene coarse clastic rocks that are themselves involved in Cenozoic thrusting and folding. Yunling collage. The Yunling collage is also surrounded by late Cenozoic (800 km northwest to the Midu basin in western Yunnan. In this book, however, we discuss only that part of the fault that is in China. Regionally, the Red River fault consists of several fault strands that form the southwestern boundary of the Chuxiong basin, Yangzi platform, and South China fold belt or lie within the southwesternmost part of these tectonic units (Fig. 29). The northeastern side of the narrow belt of metamorphic rocks that are variably mylonitized to form the Ailao Shan shear zone and that crop out in the Ailao Shan, which we assign to the easternmost part of the LanpingSimao fold belt (see Wang and Burchfiel, l997), is marked by a fault. This fault has a complex history, the younger part of which we assign to the Red River fault and an older part that may be related to the Ailao Shan shear zone or an early period of movement on the Red River fault. The Red River fault consists of several fault strands that

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China show evidence for active right-lateral displacement along most of its length. The fault strikes N45°W to the northwest of Yuanjiang; but to the east of Yuanjiang, its strike changes to N60°W, then returns to a N45°W strike and continues into Vietnam (Fig. 29). The character of the fault also changes at Yuanjiang, dividing it into two segments: a northwestern segment with a component of normal displacement, and a southeastern segment with a component of thrust displacement. Cenozoic sedimentary rocks occur northwest of Majie (Fig. 29a) and in a narrow belt along the Red River from near Mesha east to Huangcaoba (Fig. 29b), and they provide evidence for the sense of displacement and temporal history of the fault. The ages of Cenozoic strata east of Mesha range from Eocene to Quaternary, but both their stratigraphy and age are poorly determined. Older maps show early Cenozoic strata to be present in several places, but recent geologic maps (Bureau of Geology and Mineral Resources of Yunnan, 1990) have changed the age of most of these rocks to Triassic. Only one small area of early Cenozoic rocks is still shown on the most recent geologic map of Yunnan (1:1,000,000 Geological Map of Yunnan, 1990) along the eastern part of the Red River. Our studies on these Cenozoic strata are in progress, and we have shown the distribution of these rocks on Fig. 29b as they appear on existing Chinese maps (Bureau of Geology and Mineral Resources of Yunnan Province, 1990). The oldest strata east of Mesha consist of ~100–200 m of maroon, red, and tan mudstone, sandstone, limestone, intercalated with gypsum, coal, and some conglomerate, which form a narrow belt of nearly continuous outcrop east of Nansha and discontinuous outcrops west of Nansha. In the eastern part of the belt they are mapped as resting unconformably on Triassic limestone, now marble; however, the contact is not clearly exposed. Clasts in the conglomerate are all well-rounded, pebble-size resistant rock types, such as well-cemented sandstone and limestone. These strata are overlain by red, maroon, purple, tan, and gray sandstone and mudstone, often with disrupted bedding and interbedded massive diamictite, probably debris flows, which contain angular cobbles and boulders of sandstone, limestone, and in some units metamorphic and igneous rocks probably derived from the Ailao Shan to the south. Some coloration of the strata is primary, but much of it is clearly secondary and cuts across bedding indicating important fluid flux within these rocks during deformation. These strata are assigned a late Eocene–Oligocene age based on their correlation with lithologically similar rocks in other parts of Yunnan (Bureau of Geology and Mineral Resources of Yunnan, l990; Fig. 29b). Several hundred meters of yellow, red, and green sandstone and mudstone with variable amounts of diamictite continue the section upward. Some beds contain abundant rounded and angular clasts as much as 60 cm in diameter of metamorphic and igneous rocks, rarely mylonitic, derived from the Ailao Shan. These strata are overlain by yellow and tan conglomerate and sandstone dominated by well-rounded, pebble- to boulder-size clasts of metamorphic and igneous rocks, but also containing clasts of sandstone and limestone derived from Mesozoic and

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Paleozoic rocks north of the Red River. Some dark gray to black limestone clasts appear to be derived from limestone within the Cenozoic strata. North of Nansha several hundred meters of dark gray to black massive Cenozoic limestone rests unconformably on Paleozoic and Mesozoic rocks of the South China fold belt. The limestone consists of microbreccia, at least partly developed from karstification, limestone pebble and cobble conglomerate, and bedded limestone. The relations between the dark gray limestone and the yellow and tan pebble and boulder conglomerate remain unresolved because of rapid and complex facies changes that existed in this area. These strata contain plant material (spores and pollen) that are similar to those found in well-dated Oligocene–lower Miocene strata in the Lanping-Simao fold belt, and these strata are assigned a similar age (Bureau of Geology and Mineral Resources of Yunnan Province, l990). We designate these rocks as Miocene on Figure 29b. Pliocene rocks (Sanying Formation), dated by fossils (Bureau of Geology and Mineral Resources of Yunnan, 1990), are present between Yuanjiang and Mesha (Fig. 29b). They consist of 350–800 m of sandstone, mudstone, conglomerate, and lacustrine deposits locally containing gypsum. Most of the rounded, pebble- to cobble-size clasts in the conglomerate consist of limestone and sandstone from Mesozoic rocks in the Chuxiong basin to the north and from the high-grade metamorphic rocks of the Ailao Shan to the south. Locally debris-flow diamictite is intercalated, but it does not appear to be as common as in Miocene strata east of Yuanjiang. In some places the strata are purple, red, and maroon, but the color cuts across sedimentary layering and is clearly secondary. Near Majie are important outcrops of fossiliferous, coalbearing Pliocene conglomerate and sandstone (Fig. 30). Clasts in the conglomerate consist of well-rounded pebbles and cobbles of limestone and sandstone derived mainly from Mesozoic rocks from the Chuxiong basin to the north, with rare material derived from the Ailao Shan metamorphic rocks or the red beds of the Lanping-Simao tectonic unit to the west. Even though there is more than 1,000 m of relief along the east side of the Ailao Shan, significant thicknesses of Quaternary deposits are uncommon, except in the Midu basin and east and west of Yuanjiang. Where present, Quaternary strata consist of alluvial fan deposits along the northeast side of the Ailao Shan, and fluvial and terrace deposits along the Red River. West and east of Yuanjiang, older Quaternary strata rest with angular unconformity above deformed Pliocene and Miocene rocks, respectively, and are deeply eroded by the most recent period of stream incision. These Quaternary sediments contain abundant conglomerate, dominated by clasts of high-grade metamorphic and mylonitic rocks derived from the Ailao Shan. Clasts in these strata are commonly 1–2 meters in diameter and well rounded. Quaternary fluvial and alluvial fan deposits in the Midu basin are derived from Paleozoic and Mesozoic rocks from the LanpingSimao, Chuxiong, and Dali highland tectonic units. The northwestern segment of the Red River fault, northwest of Yuanjiang, consists of a steeply east-dipping linear fault that

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Figure 30. Geologic map of the northern part of the Red River fault in the Majie area where a minimum offset can be determined. Geological units and streams are consistently offset right-laterally along the fault. The maximum and minimum offsets of Permian units are 40 km (A–F) and 20 km (C–F) respectively, and the offsets of the Triassic granite are 54 km (B–G) and 25 (B–D), respectively. The amount of the offset of the Red River is 6 km (K–L). Cross section X–Y shows the Pliocene sedimentary rocks resting unconformably on the Ailao Shan metamorphic rocks. The Pliocene rocks also rest on the sub-Pliocene erosion surface that forms the gentle relief at the crest of the Ailao Shan. See Figure 29 for location.

marks the eastern margin of the Ailao Shan. Between Jinbaoshan and Yuanjiang, the fault is characterized by right-lateral stream offsets and triangular facets developed on the northeast side of the Ailao Shan that are suggestive of normal faulting (Allen et al., l984). However, how much normal displacement may have occurred is unclear. Foliation in the rocks of the Ailao Shan dips steeply east, and it is unclear how much of the development of the triangular facets is related to normal faulting and how much is related to erosion along the foliation surfaces. This segment of the Red River fault is generally parallel to both mylonitic and metamorphic foliation in the Ailao Shan. Regionally, however, the fault is discordant to the Ailao Shan high-grade rocks because they end near Daqiao (Fig. 29b) and the fault continues northwest, separating unmetamorphosed Mesozoic and early Cenozoic rocks of the Lanping-Simao fold belt on the southwest from Triassic and, rarely, Paleozoic rocks of the Chuxiong basin on the northeast. At Midu, the fault enters the Midu basin, an extensional basin of Quaternary sediments formed at the intersection of the northwest-trending, right-lateral Red River fault and the

northeast-trending, left-lateral Chenghai fault (Fig. 29). The southwestern and northwestern sides of the basin are bounded by steep mountain fronts, suggesting that the two faults have an important component of normal displacement; however, evidence for active right-slip along the southwest side of the basin is not common. The Red River fault continues northwest from the Midu basin at least to the larger Erhai basin north of Dali (Fig. 29a). This part of the fault separates Mesozoic rocks on the southwest from lowgrade metamorphic Paleozoic rocks of the Dali highlands on the northeast and is the most northwesterly fault we call the Red River fault. Right-lateral stream offsets are well developed and indicate that this segment of the Red River fault is active. A second active fault lies to the east of the Ailao Shan mountain front from Chunyuan to Yaojie (Fig. 29a). This fault is a splay of the Red River fault and is marked by right-lateral stream offsets and local scarps. It cuts through both Triassic rocks of the Chuxiong basin and, at its eastern end, Cenozoic rocks that lie unconformably on rocks of the Chuxiong basin.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China Two areas are important to understanding the history of the northwest segment of the Red River fault. At the north end of the Ailao Shan, in the area northwest of Majie (Fig. 30), the Red River is displaced right-laterally 5.3 km across the Red River fault (Allen et al., l984). Along the offset part of the river, the fault has consistently deflected tributary streams right-laterally indicating ongoing active displacement, as do river deflections noted by Allen et al. (1984) along the fault to the northwest. There is also an older history to the Red River fault in this area. Pliocene conglomerate (see previous section) rests unconformably on three different tectonic units. To the north they rest on Triassic rocks of the Chuxiong basin. On the south they rest on the metamorphic rocks of the Ailao Shan, which are dominantly nonmylonitic marble. Locally between these two tectonic units, the Pliocene strata rest on fault-bounded slivers of Permian limestone and Mesozoic granitic rocks derived from south of the Ailao Shan metamorphic rocks, and the strata are not offset by the faults that bound these slivers. These fault slivers have been displaced 20–54 km and indicate pre-Pliocene right-lateral displacement on the Red River fault (see following section). The clasts in the Pliocene conglomerate are dominated by Mesozoic rocks from the Chuxiong basin, indicating that the present course of the Red River, which flows from the Lanping-Simao red bed terrain, could not have supplied this material. The Pliocene rocks are folded, with dips up to at least 45°, along northwest-trending axes, and an erosion surface with gentle relief truncates the folded Pliocene and older rocks. The erosion surface and the Pliocene rocks lie at least 200–300 m above the present Red River, indicating that the present gorge through which the Red River flows has been incised since the formation of the undated, but probably Pleistocene, erosion surface. Structure and stratigraphy of the Pliocene and Quaternary rocks from Mesha to east of Yuanjiang (Fig. 29b) suggest the most active period of normal faulting on this segment of the Red River fault was mainly of late Pliocene or early Quaternary age. Pliocene rocks dip uniformly southwest toward the Ailao Shan, and conglomerates in these strata contain rounded pebbles from both the Mesozoic rocks to the north and the metamorphic rocks to the south. Although the outcrop is poor, beds at the base and top of the Pliocene section appear to dip at about same angle, suggesting that their present 20°–30° southwest dip occurred after deposition. The well-developed triangular facets adjacent to the Pliocene rocks (Plate 26) and the uniform southwest dip of the Pliocene strata are suggestive that the rotation of these beds was caused by a northeast-dipping normal fault, although we were never able to find an exposure of the fault. Following rotation of the Pliocene strata, they were eroded to low relief and covered by early Quaternary boulder conglomerate derived exclusively from the Ailao Shan metamorphic and igneous rocks. These conglomerates extend from Mesha to southeast of Yuanjiang and are cut by right-lateral faults along strands of the Red River fault that lie northeast of the Ailao Shan mountain front, but they are not tilted like the Pliocene strata or strongly folded like the Miocene strata east of Yuanjiang. Deep erosion of the

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early Quaternary conglomerate has left these deposits as remnants on interfluves above present drainage. Active sedimentation occurs in the basin around Yuanjiang, which has the characteristics of a sag basin. Thus the area between Mesha and east of Yuanjiang indicates an important period of normal faulting sometime in late Pliocene to early Quaternary time on a southern strand of the Red River fault, but the present activity on the northwestern segment of the fault is right-lateral strike-slip, locally on faults that lie northeast of the Ailao Shan. The eastern segment of the Red River fault, east of Yuanjiang, is more complex than the northwestern segment and contains fault movement that may be more properly assigned to the older Ailao Shan shear zone. It consists of at least three major faults (Fig. 29b) that cut the narrow belt of Cenozoic sedimentary rocks east of Yuanjiang. The southern fault forms the northern boundary of the high-grade metamorphic rocks of the Ailao Shan. It dips 20°–60° south but is rarely exposed, and places the high-grade rocks above strongly folded Eocene-Oligocene and Miocene sedimentary rocks. The thrust fault is marked by tens of meters of gouge and abundant slickensided surfaces that contain down-dip striae. Footwall strata are variably deformed but consistently become steeper to near vertical and locally overturned near the fault. Folds within the Cenozoic rocks parallel the thrust fault, are locally overturned to the north, and are cut by small south- and north-vergent thrust faults. Commonly the strata near the fault show alteration that cuts across bedding. Mylonitic rocks in the hanging wall contain the older left-shear fabric of the Ailao Shan shear zone, and their foliation dips moderately to steeply north into the thrust fault. In some places, the mylonitic foliation is folded to dip south in harmony with the thrust displacement, and locally it is parallel to the fault. Along most of its length, this southern fault shows no evidence for active displacement, except from Honghe to ~10 km east of Nansha, and for a short distance east of Xinjie. In both these places the most active trace of the Red River fault merges with the thrust fault and streams are right-laterally offset (Fig. 29b). Before entering Vietnam, the thrust fault dips steeply south and there is no evidence that it is active. In most places, the trace of the Red River fault does not follow a major topographic break, in contrast to the northwestern segment of the fault, but it is commonly marked by saddles and shoulders along ridges leading from the Red River valley into the Ailao Shan. The most active trace of the Red River fault is parallel to the eastern boundary of the Ailao Shan high-grade rocks but lies to the north within the narrow belt of Cenozoic rocks or the Mesozoic rocks that underlie them, with the two exceptions mentioned earlier. The fault dips from near vertical to about 60° north near Honghe, and offset streams and scarps attest to its active rightslip movement (Plate 27). Locally where it dips about 60° north, the fault shows a thrust component and places Triassic limestone above Miocene strata. Southeast of Yuanjiang, the fault is vertical and juxtaposes strongly folded Miocene rocks against unfolded early Pleistocene strata, indicating that folding of the Miocene rocks is mainly pre-Quaternary. In the area of Xinjie, this fault

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merges with the thrust fault to the south; the combined fault has a steep dip to the south and offset streams indicate active right slip. Farther to the southeast there is no evidence for active right slip on this fault. North of the two faults described earlier, there are numerous short faults within the Cenozoic strata that cut folds and have thrust components both to the north and south. There is no evidence these faults are active. North of the continuous belt of Cenozoic rocks there are faults that cut Permian and Triassic rocks and one fault that can be traced from the Vietnam border west to at least Honghe (Fig. 29b). This fault dips 20°–60° north, displaces Permian and Triassic rocks, and in two places, east of Honghe and west of Huangcaoba, it places Permian and Triassic rocks, respectively, above Miocene strata, and it is a south-vergent thrust fault. The same fault shows right-lateral stream offsets for at least 20 km east of Huangcaoba where it dips steeply to the northeast. East of Yuanjiang, all three major faults show at least local evidence for active right-lateral displacement, except near the Vietnam border, and the central fault appears to be the most active. The central fault has scarps that indicate a component of active thrusting showing this fault is both a right-slip and thrustslip fault. The other two faults appear to be mainly older thrust faults, locally reactivated by right shear. Evidence also suggests that the thrust movement on these two faults is Miocene and younger, but whether thrusting occurred before right-slip began on the Red River fault is uncertain. Constraints on the timing, nature, and magnitude of displacement along the Red River fault. From the geological relationships along the Red River fault, it is difficult to determine timing of events. Everywhere Quaternary rocks are exposed they are generally horizontal, and where they are in contact with Neogene rocks they rest unconformably on them. Where the Quaternary rocks are cut by the Red River fault, displacement is mainly right-lateral; along the northwestern segment, there is locally a normal component. Along its northwestern segment folding of Pliocene rocks preceded the latest episode of right slip. Along the eastern segment, the fault shows a component of active thrustslip, but the Quaternary rocks are unfolded, indicating the folding, and perhaps most of the thrusting, of the Neogene rocks is pre-Quaternary. The latest Quaternary episode of right slip is consistent with the interpretation of Leloup et al. (l993, l995), based on isotopic data, that right-slip faulting began ~4 Ma. Our interpretation of how the dated rocks in the Diancang Shan relate to the Red River fault, however, is different from theirs. The Pliocene rocks near Mesha have probably been rotated by normal faulting along the Red River fault, but they do not appear to be folded. In contrast, all along the eastern segment of the Red River fault the Cenozoic strata are strongly shortened, but because the Cenozoic strata are poorly dated, exact correlation of deformational events cannot be made. At the north end of the Ailao Shan, in the area of Majie and Jinbao Shan, Pliocene coal-bearing deposits (Sanying Formation) rest unconformably on high-grade metamorphic rocks (Fig. 30).

These Pliocene strata are folded northeast of the Red River fault, but locally south of the fault they rest on a broad subhorizontal surface that extends along the range even where the Pliocene rocks are absent (Plate 28), such as in the Yuanyang and Jinping areas (Fig. 29b). This surface is similar to the surface that underlies Pliocene rocks in many places in Yunnan that we refer to the sub-Pliocene surface; however, the folded Pliocene rocks near Majie also are truncated by a younger erosion surface of low relief, which makes the correlation of erosion surfaces uncertain. The incision by the present river system into these erosion surfaces and the deposition of Quaternary boulder conglomerate dominated by material from the Ailao Shan in the Mesha-Yuanjiang area indicate much of the present relief along the Red River fault is Quaternary in age. Although river and stream offsets indicate active right-slip faulting (also see Allen et al., l984), normalslip displacement along the northwest segment of the Red River fault may have been important in the earliest part of this episode of strike-slip faulting, at least locally near Mesha and in the Midu basin, but there is only weak evidence for active normal faulting. Total right-lateral offset on the Red River fault remains uncertain, but limited evidence suggests it may be between 6 and 20–54 km. At the north end of the Red River fault near Majie (Fig. 30), the Red River is right-laterally offset ~6 km. Allen et al. (l984) reported right-lateral stream offsets of 5.3 km along the Red River fault, and they suggested, that since the Red River shows this offset, most of this displacement occurred after the entrenchment of the river. They further suggested that the offset occurred in the last 2–3 m.y. based on the depth of entrenchment of the Red River gorge. The highland surface into which the Red River is entrenched can be shown in several places to be cut on Pliocene rocks, and evidence from provenance of the Pliocene strata indicates they were not derived from the current course of the Red River. Thus, the total offset during the last few million years would be 5–6 km, with several hundred meters of associated normal faulting. Along the eastern segment of the Red River fault there is little evidence for the magnitude of young displacement. Cenozoic rocks are exposed at low elevations along the Red River and bounded by highlands >1,000 m higher, suggesting the present site of the Red River was topographically low before Quaternary time. Remnants of erosional terraces are cut on the folded Cenozoic rocks, suggesting that the Red River already occupied a valley along its present course, possibly during Pliocene time. Thus, some displacement on the faults along this segment could date from early Pliocene or older time, and the total offset on the Red River near Majie is not the total offset on the Red River fault. Larger pre-Pliocene offset on the Red River fault is suggested by geological relations in the Majie area (Figs. 29a, 30). Uniquely mylonitized granitic rocks, shown as late Mesozoic on geologic maps (1:1,000,000 Geological Map of Yunnan, Bureau of Geology and Mineral Resources of Yunnan Province, 1990), bounded by strands of the Red River fault and derived from the south side of the fault, suggest a right-lateral displacement of between 25–54 km (Fig. 30 A–D and B–G). Permian rocks in

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China this area consist of two distinct types; northeast of the Red River fault they are limestone and Emei Shan basalt, whereas on the southwest side they are limestone and melange, often well foliated. A sliver of Permian rocks, clearly derived from the southwest side of the fault, is right-laterally offset ~20–40 km (Fig. 30 A–F and C–F). The magnitudes of all offsets of Triassic and Permian rocks must be considered minimum offsets because the rocks are fault-bounded slivers both north and south of the Red River fault (Fig. 30). These data suggest the Red River fault has at least 5–6 km and probably at least 20–54 km of right-lateral displacement. Northwest of Majie (Fig. 30), Pliocene conglomerate unconformably overlies two of these fault slivers, the faults that bound the slivers, and the two major tectonic units—Ailao Shan metamorphic rocks of the Lanping-Simao unit and Chuxiong basin— juxtaposed by the fault (see also, Bureau of Geology and Mineral Resources of Yunnan Province, l990). This would suggest that the Red River fault has had 5–6 km of displacement during late Pliocene/-Quaternary time, and the larger displacement is older, probably post 17–20 Ma (see following section). Such an interpretation figures importantly in the relations between the Red River fault and the Xianshuihe-Xiaojiang fault system (see following section). Relations of the Red River fault to the Ailao Shan shear zone. The Red River fault closely follows the mylonitic rocks of the Ailao Shan shear zone of late Oligocene–middle Miocene age, suggesting the faulting was controlled by the older crustal anisotropy of the shear zone. However, the detailed geological relations between the two structures make this conclusion uncertain. Structural and thermochronological studies have been interpreted to show that the metamorphic rocks in the Ailao Shan and Diancang Shan underwent important left-lateral shear in early Miocene time. Pressure-temperature studies indicate that left-lateral shear was at amphibolite grade and isotopic studies on leucogranites indicate shearing occurred at 22.4 and 26.3 Ma in the Ailao Shan and 22.4 and 24.2 Ma in the Diancang Shan (Scharer et al., l990; Liu et al., l992; Leloup et al., l995). Cooling ended ductile shearing in the Ailao Shan at ~20 Ma (Harrison et al., 1992a) and ~17 Ma in the Diancang Shan (Leloup et al., 1993). The Diancang Shan shows a second period of cooling at ~4.7 Ma, interpreted to be related to the onset of right-lateral strike-slip and normal faulting along the Red River fault (Leloup et al., 1993). Large magnitude displacements on the Ailao Shan shear zone figure importantly in tectonic interpretations of the IndiaEurasia collision zone by Tapponnier et al. (1982, 1986), Harrison et al. (l992b), Leloup et al. (1993, 1996), and Briais et al. (1993). In the most recent work on the Ailao Shan shear zone by Leloup et al. (1995), the authors suggest a finite left-lateral offset across the shear zone of 700 ± 200 km during the period 35–17 Ma, as Indochina was extruded to the southeast away from the India-Eurasia collision zone. The same authors also suggest right-lateral strike-slip and normal faulting began on the Red River fault ~5 Ma (Leloup et al., l995; see also Leloup et al.,

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l993). Early estimates of the amount of right slip were several hundred kilometers, but these estimates varied greatly. The most recent study by Leloup et al. (l995) has reduced right-slip displacement to a few tens of kilometers, similar to the displacement we suggested earlier. However, data presented previously suggest that most of the 20–54 km right-slip displacement on the Red River fault may be of pre-Pliocene age and older than ~5 Ma. Although there are several possible tectonic causes for cooling in early and middle Miocene time, relative uplift of the Ailao Shan and associated erosion and deposition of Miocene-Pliocene sedimentary rocks northeast of the Ailao Shan could be related events (e.g., Harrison et al., 1992a). At the north end of the Ailao Shan, the Pliocene rocks that rest unconformably on the highgrade metamorphic rocks form the oldest overlap relationship, and prove the high-grade rocks were at the surface by Pliocene time. The clasts in the poorly dated upper Eocene–Oligocene and Miocene conglomerate east of Yuanjiang indicate that the mylonitic rocks were exposed at least during Miocene time and possibly during Eocene-Oligocene time. Unfortunately, the age of these sedimentary rocks is poorly constrained and currently cannot be accurately related to the cooling ages reported by Harrison et al. (l992a). Relations of the Ailao Shan metamorphic and mylonitic rocks to the Lanping-Simao fold and thrust belt to the southwest have suggested that uplift and cooling of the Ailao Shan rocks may have occurred within a transpressional setting during Oligocene-Miocene time (Wang and Burchfiel, l997). The thrust fault along the north side of the Ailao Shan high-grade rocks, east of Yuanjiang, might be related to the same tectonic setting. The Eocene-Oligocene and Miocene strata are in thrustfault contact with the metamorphic rocks of the Ailao Shan, but the depositional setting of the Cenozoic strata remains unclear. If thrusting and exhumation of the Ailao Shan mylonitic rocks are related, it would suggest a transpressional tectonic setting similar to that suggested by structural relations along the southwest side of the Ailao Shan (Wang and Burchfiel, l997). In our opinion, to assign both the older left slip and younger right slip to an Ailao Shan–Red River shear zone confuses the possible position of major fault displacements in this area. The structural relationship between the mylonitic rocks in the Ailao Shan shear zone and the position of the active Red River fault supports only an indirect relationship between them. Harrison et al. (1992a) show mylonitic rocks across the entire metamorphic core of the Ailao Shan and adjacent to the active Red River fault. We have observed in the cross section exposed at YuanjiangMoujiang the rocks adjacent to the active Red River fault are nonmylonitic amphibolite, and mylonitization irregularly increases in intensity southwest away from the Red River fault. The most highly mylonitized rocks occur in a 2-km-wide zone near the western limit of the Ailao Shan metamorphic rocks. East of Yuanjiang, the most active central strand of the Red River fault lies mostly within Mesozoic and Cenozoic rocks and does not bound the high-grade rocks in the Ailao Shan. In three other cross sections, the Ailao Shan metamorphic rocks show only limited or no mylonitization. North of Jinping near the Vietnam border,

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there are no mylonitic rocks across most of the Ailao Shan, and the only strongly mylonitized rocks form a zone ~200–500 m wide north of Jinping (Fig. 29b), 17 km southwest of the Red River fault. A cross section at Manfei, northwest of Yuanjiang, contains only rare mylonitic rocks; steeply dipping mylonites at the western margin of the Ailao Shan metamorphic rocks are present but with a down-dip lineation. Where the Ailao Shan metamorphic rocks end northwest of Majie, the rocks are mostly nonmylonitic marble. Further complicating the interpretation between mylonitic rocks and the Red River fault is the presence of mylonitic rocks similar to those in the Ailao Shan, with subhorizontal lineations, left-lateral kinematic indicators, and mylonitic leucogranite north of the Red River fault near Xiajie (Fig. 29; Plate 29a and b). These rocks (Yaoshan Group) form a fault sliver north of the Red River and, farther north, form the metamorphic core of an anticline flanked by Triassic rocks. Chinese maps (Bureau of Geology and Mineral Resources of Yunnan Province, l990) show a depositional contact between the Triassic metasedimentary rocks and the metamorphic rocks. Thus, the strands of the Red River fault do not follow in detail the mylonitic rocks of the Ailao Shan shear zone, and until the relationship becomes more clear, the two structural features should remain separated. There is equivocal evidence for large magnitude left slip along the Red River. A sliver of Jurassic strata within the Red River fault zone north of Yuanyang (Fig. 29b) is similar to Jurassic red beds in the Chuxiong basin ~100 km to the northwest. Only pre-Jurassic rocks of the Yangzi platform now lie north of

the sliver. Because no Jurassic rocks are exposed for 50 km north of the Red River fault, it remains unknown if Jurassic red beds similar to those in the Chuxiong basin could have extended into the area where only the older rocks are now exposed north of the fault. If Jurassic rocks were present north of the fault, the sliver could have been derived by local dip slip and not have required large strike-slip displacement. Additionally, long fault-bounded slivers of Triassic rocks are present between strands of the Red River fault in southern Yunnan whose facies are more similar to red beds of the Chuxiong basin than to the limestone in the South China fold belt directly to the north. Because both the Jurassic and Triassic rocks are unconformably overlain by Miocene strata, their current distribution is probably related to deformation along the older Ailao Shan shear zone, rather than the Red River fault. These rocks may provide evidence for ~200 km of left-lateral displacement, but until their facies relations are better understood, the tectonic position of these Jurassic and Triassic strata remains unresolved. It would also imply that active strands of the Red River fault could have an older history related to movement on the Ailao Shan shear zone, but not directly related to the mylonitic and metamorphic rocks in the shear zone. The anisotropy control of the older shear zone on the younger faults may be important, but complex, and requires additional study. Jianshui fault. The Jianshui fault lies ~50 km north of and parallel to the Red River fault. It has a curvilinear trace extending ~130 km from Huanian in the west to near Jijie in the east (Fig. 31). Fourteen destructive earthquakes have been recorded on the Jianshui fault since the historical record began in the 15th cen-

Figure 31. Map of major structural features along the right-lateral Jianshui fault (location on Fig. 29). Location of detailed figures are also shown.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China tury (Han et al., l991). The Jianshui fault is left-laterally offset along the east side of the Jianshui basin by the Xinzhai fault, a branch of the left-lateral Xiaojiang fault, that divides it into a western and eastern segment. Both segments of the Jianshui fault show evidence of active right slip. The western segment cuts through mainly Precambrian rocks, whereas the eastern segment cuts through Paleozoic, Mesozoic, and Cenozoic rocks. Right-lateral stream offsets indicate that the Jianshui fault is active along its entire length. Along the western segment of the Jianshui fault, the Daqiao River, the main river in the area, crosses the fault and has been displaced right-laterally, and the consistent right-lateral offset on its tributaries to the west is striking. Maximum displacement on the rivers and streams in this area is 3 km (Fig. 32). Smaller offsets of streams and terraces of ~100–200 m are common (Plates 30, 31). The fault strikes westward into the Huanian basin, an elongate basin filled with Quaternary sedimentary rocks (Fig. 32). The shape of the Huanian basin suggests that it formed as an extensional basin, but the origin of the basin is complex because the active left-lateral Yangwu fault strikes into the basin from the south (Fig. 32). The basin is entirely surrounded by steep mountains, and the slopes along the southwestern sides of the basin have spurs with triangular facets suggesting that they are controlled by normal faults. Within the mountains northeast of the Huanian basin, Mesozoic rocks north of the Jianshui fault are

51

repeated by north-striking normal faults that terminate against the Jianshui fault (Fig. 32). South of the western end of the Jianshui fault are two short segments of west-vergent thrust faults and associated folds. They crop out near the base of a west-facing slope. The thrust faults could be active, but positive field evidence is lacking. Because the Jianshui fault appears to die out quickly northwest of the Huanian basin, we interpret that the Jianshui fault probably ends by losing displacement on extensional faults in and near the Huanian basin and by shortening on the thrust faults along the southeastern part of the basin (Plate 32). Along the central part of the Jianshui fault, the shallow Yilong Lake abuts the fault on its south side (Fig. 31). The maximum depth of the lake is 5 m, and all the sediments in the lake basin are Quaternary in age (Bureau of Geology and Mineral Resources of Yunnan, unpublished data, 1977). The sub-Pliocene erosion surface is warped down to the north into the lake and into the Quaternary filled lowland to its west, both of which make up the Shiping basin. Geological relations suggest that the Shiping basin is a large sag pond along the Jianshui fault. North of the lake, several lens-shaped ridges underlain by Devonian limestone form an en echelon array along the Jianshui fault (Plate 33). The lenses are surrounded by gouge and block the south-flowing streams into the lake. Only Precambrian rocks crop out north of the faulted lenses of Devonian rocks. The

Figure 32. Map at the western end of the Jianshui fault (for location see Fig. 31). Right-lateral movement along the Jianshui fault appears to be absorbed by north-trending normal faults and crustal shortening along arcuate thrust faults and folds at the north and south ends of the fault respectively. Rivers crossing the fault in this area are consistently offset right-laterally.

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source for the faulted lenses of Devonian rocks is uncertain because Devonian rocks abut the fault both to the east and west of the faulted lenses and evidence indicates the fault had an older left-slip displacement (see following section). East of Yilong Lake, near Sijia, the Jianshui fault lies in a narrow gorge along which several lenses of bedrock forming shutter ridges are present within the fault zone and streams are offset right-laterally ~500–2000 m (Fig. 33). The fault dips steeply north and appears to have a small reverse component. At the eastern end of the western segment, the Jianshui fault strikes into an area of Neogene and Quaternary sedimentary rocks (Fig. 34). We interpret the sedimentary rocks to consist of two successions. An older succession consists of multicolored, fine-grained, lacustrine coal-bearing sediments. Similar coalbearing, fine-grained clastic rocks in Yunnan generally yield Pliocene ages and are known to range from latest Miocene to early Quaternary. The younger succession consists of Quaternary rocks that were deposited in a depression or basin, the Jianshui basin, whose geometry is similar to the present distribution of Quaternary rocks. The Pliocene rocks lie on the sub-Pliocene erosion surface, and we interpret these rocks to have had a more regional distribution than their present outcrop extent. The present internally drained Jianshui basin is probably related to active faulting on both the Jianshui and Xinzhai faults and formed during Quaternary time. The Neogene and Quaternary sedimentary rocks are strongly folded both south of the Jianshui fault and north and south of its eastward projection (Fig. 34). The surface trace of the Jianshui fault appears to end within the Neogene rocks, but the right-lateral displacement probably continues eastward (see following paragraph). Drill holes have reached the base of the folded Neogene section at ~600 m (Chen Jiangzhong et al., unpublished data, 1992). Structurally the Jianshui basin contains two parts: a region of folded Pliocene-Quaternary strata flanked to the north and

south by unfolded strata of the same age that dip gently into the basin. Axial traces of folds in the Pliocene-Quaternary rocks trend northeast and their sigmoidal en echelon geometry clearly indicates they were formed in a zone of right-shear (Fig. 34). The Pliocene sedimentary rocks contain folds that are overturned both east and west with numerous internal thrusts and are truncated by an erosion surface that is highly undulating (Plate 34). In many places the erosion surface appears to be folded and actively eroded. On one of the easternmost folds, the erosion surface is folded and forms an anticline that plunges north and south (Plate 35). Evidence strongly suggests that the erosion surface is currently being folded and that folding has been progressive during Quaternary to Holocene time. In the low topography of the easternmost part of the basin, a low hill is formed by a south-plunging anticline that folds the erosion surface and deflects a river flowing southeast into the enclosed Jianshui basin (Fig. 34). The evidence from the folding of the erosion surface and deflection of the river indicates that the folding is young and locally active. North and south of the folded rocks, Pliocene and Quaternary strata rest unconformably on deformed Precambrian and Paleozoic rocks. The erosion surface at the base of the Neogene rocks is correlated with the sub-Pliocene erosion surface, and the overlying sediments dip gently into the modern enclosed Jianshui basin (Fig. 31). The eastern side of the Jianshui basin is marked by the north-striking Xinzhai fault, a strand of the Xiaojiang fault zone (see previous section), which appears to have left-slip, strike-slip, and west-side-down normal-slip displacement. The origin of the Jianshui basin must involve the complex interaction of active left- and right-slip faults. The main part of the basin appears to have formed as an enclosed sag basin during Quaternary time. Where the basin sediments were deposited above the Jianshui fault they were right-laterally sheared and folded, but the fault did not form a continuous surface break. Along the easternmost part of the western segment of the Jianshui fault, right-lat-

Figure 33. Map showing right-laterally displaced ridges and streams along the Jianshui fault in the area of Sijia between the Jianshui and Yilong basins. For location see Figure 31.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

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Figure 34. Structural map of the eastern end of the western segment of the Jianshui fault. Fold axes in the Neogene rocks at the east end of the fault indicate shortening by right shear. The Jianshui fault does not break through the Neogene rocks, but probably continues at depth beneath the basin. The Miandian fault is considered to be the eastern continuation of the Jianshui fault, but is offset by the Xinzhai fault, a strand of the Xiaojiang fault system. A simplified model for the structural relations is shown in the inset, upper left. Note the deflection of the river (dashed line with arrows) in the eastern part of the Jianshui basin by an active south-plunging anticline. See Figure 31 for location.

eral stream offsets are common along the fault (Fig. 34). Separation of the Pliocene rocks in the basin along the eastern end of the western segment of the Jianshui fault is ~5–6 km, some of which could be related to south-side-down normal displacement. Beyond the area of folded rocks the sag component of the basin has continued to develop to the present day. The Miandian fault forms the western part of the eastern segment of the Jianshui fault. It lies east of the Jianshui basin and has characteristics similar to the western segment of the Jianshui fault (Fig. 31). The Miandian fault offsets streams right-laterally

from a few tens of meters (Plate 36) to ~1–1.5 km, indicating that it is an active fault. The two segments of the Jianshui fault are not aligned, and it appears that they are offset 1–3 km left-laterally by the Xinzhai fault. The eastern segment of the Jianshui fault forms the southern boundary of a structural basin filled with folded Neogene sediments (Fig. 31). Basin strata consist of coal-bearing Pliocene and Quaternary deposits locally unconformably underlain by late Paleogene conglomerate. The northern boundary of the elongate Neogene basin is marked by a steep northeast-dip-

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ping thrust fault that carries Triassic limestone southward over Neogene rocks (Plate 37). Some folds in Neogene rocks near the thrust fault trend parallel to the thrust fault, suggesting a causal relation between the thrust fault and the folds. Farther south Neogene rocks are folded along northeast-trending axes, and we relate these folds to right-lateral shear along the Jianshui fault. Eastward the Jianshui fault curves to the southeast (Fig. 31). Along both the east-west– and southeast-trending parts of the eastern segment of the Jianshui fault, there is high topography to the south indicating the fault has a down-to-the-north normal-slip component, consistent with a releasing bend geometry. The eastern termination of the Jianshui fault is complex. It strikes into Neogene and Quaternary rocks that fill small pull-apart basins along the southern continuation of the left-slip Qujing fault (Fig. 31; see previous section). The relations between the two active fault systems are complex, but we interpret that extension within the small pull-apart basins results mainly from movement on the Qujing fault. Beyond this point there is no fault that could be considered the eastward continuation of the Jianshui fault, and we interpret right slip along the fault to end in extension within the small pull-apart basins. The thrust fault along the northern margin of the Neogene basin lies in the shortening quadrant of the two conjugate faults and is another example of the close spatial rela-

tion between contemporaneous shortening and extension present throughout the major fault systems in western Yunnan. Like the other right-slip faults that lie parallel to and north of the Red River fault, the Jianshui fault appears to follow an older left-slip fault, as indicated by 10–20 km offsets of rock units and folds of the South China fold belt. The older left-slip displacement is much larger than the right-slip displacement. The small right-slip displacement of only a few kilometers that we measured may result from a slow slip rate and/or the fact that right slip began only during Quaternary time. Qujiang fault. The right-lateral Qujiang fault lies ~75 km north of the Red River fault and is the northernmost fault possibly related to the right-slip Red River system (Fig. 29a). It extends from the Chahe area in the west ~100 km to the Quxi area in the east (Fig. 35). Like the Jianshui fault, the western part of the Qujiang fault is convex south, but since it does not extend as far east, it does not have the sigmoidal shape of the Jianshui fault. The Qujiang fault is divided into two segments at Gaoda. The western segment is characterized by a linear fault that lies in steep-sided valleys, except in its central part where it passes through the elongate E’shan basin. The fault cuts through mainly Proterozoic metasedimentary and Sinian sedimentary rocks. Eastward, along the eastern segment, the fault curves gently north and bounds the small Quxi basin on its north side forming

Figure 35. Map showing the major structural features along the Qujiang fault and in the region of the Tonghai basin. Large arrows point to areas where right shear is partially absorbed by extension in the west and by shortening in the east. Location on Figure 29. Location of detailed figures are indicated.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China a steep south-facing escarpment. At Wujie a short fault splays east from the Qujiang fault west of the small Quaternary basin at Gaoda (Fig. 35). Active right-lateral displacement is demonstrated by both earthquakes and deformational features. The 1970 Tonghai earthquake, M = 7.7 (Zhang and Liu, 1977), is one of a series of earthquakes recorded on the fault. Its epicenter was at Wujie, and the surface break extended 60 km eastward from the E’shan basin. Measured right-slip on the 1970 surface break was 2.2 m at Gaoda (Liu et al., l988, The Geodetic Survey Brigade for Earthquake Research, 1975) and 2.7 m in the Meizishu area (Lithospheric Dynamics Atlas of China, l989). Active right-lateral displacement is expressed by numerous stream offsets all along the fault. Along the western segment, in the Chahe area, streams are offset as much as 1,500 m (Fig. 35). West of the E’shan basin, the Qujiang River and its tributaries have been disrupted by right slip, and a ridge and the river have been offset 1,500 m (Fig. 36). Nearby a large alluvial fan has been displaced ~500 m from its source. At the west end of the E’shan basin, a stream is offset right-laterally ~200 m (Plate 38). East of the E’shan basin there are abundant drainage offsets; 500 m on two alluvial fans dated as Pleistocene (Han Mukang et al., unpublished data, 1980), near the village of Nuibaidian, and stream offsets of up to 1,200 m in the Xiaozhi area (Fig. 37). The E’shan basin, an east-west elongate basin, is bounded by the Qujiang fault on its south side and another parallel fault on its north side (Figs. 35, 38). Sinian and Proterozoic rocks are exposed in low hills within the basin. The fault on the north side does not

55

extend east of the basin, and it can be interpreted to curve south into normal faults. We interpret this basin to be the result of extension at the east end of a fault sliver that moved west relative to the area north (Fig. 38 inset). Westward, the northern fault passes into an area underlain by low-grade Proterozoic metamorphic rocks. The mapped extension of this fault curves gently southwest and has a south-directed thrust component. The geometry of the fault suggests that right slip is partially transferred into shortening west of the basin and extension within the eastern part of the basin (Fig. 38). Unfortunately, we found no definitive evidence the fault is active because only rarely does it cut young rocks. The western termination of the surface break for the 1970 Tonghai earthquake ended in the E’shan basin. It is likely that the strike-slip movement was partially absorbed by the extension within this basin. The eastern segment of the Qujiang fault lies at the foot of a south-facing escarpment that forms the northern boundary of the Quxi basin (Fig. 35). Evidence for active right slip is abundant along this curved segment of the fault. In the Gaoda area, a series of streams are consistently offset right-laterally (Fig. 39). Along the eastern segment of the fault river displacements reach 2 km. Even where a component of normal displacement is prominent, right-lateral offset on streams is clearly evident (Plate 39). This part of the fault is characterized by a zone, as much as 100 m wide, of cataclasites and gouge derived from purple and white Sinian country rock. Erosion of the gouge has produced spectacular hoodoo morphology. The Quxi basin is underlain by multicolored fine-grained

Figure 36. Right-lateral disruption of drainages along the Qujiang fault 5 km west of the Eíshan basin. Hills A and B are interpreted to be offset pieces of a formerly continuous ridge. If this interpretation is correct, the offset is 1.5 km as indicated. See Figure 35 for location.

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Figure 37. Right-lateral disruption of drainages and development of shutter ridges along the Qujiang fault in the Xiaozhi area. For location see Figure 35.

clastic rocks, dated as Pliocene in age (Han Mukang et al., unpublished data, 1980; 1:1,000,000 Geological Map of Yunnan, Bureau of Geology and Mineral Resources of Yunnan, 1990), but unpublished data assign these rocks to the early Pleistocene (Geological Institute of Yunnan province, personal communication, 1993). The basin strata unconformably overlie strongly folded Sinian sandstone and conglomerate. The Pliocene rocks and their basal unconformity are warped down to the north into the Quxi basin and into the Qujiang fault that bounds the basin on its north side. The surface marking the unconformity is continuous with the topographic surface of the gently rolling upland south of the basin, and we correlate it with the sub-Pliocene erosion surface. Like the Jianshui basin, the Pliocene rocks probably had greater regional extent but were preserved in the Quxi basin when it formed in Quaternary time. The eastern boundary of the Quxi basin is formed by a strand of the north-south–trending leftlateral East Xiaojiang fault (Fig. 35) and the Qujiang fault ends at this through-going fault. Before the Qujiang fault reaches the East Xiaojiang fault, it curves north and becomes a west-dipping thrust fault. Streams are offset immediately west of the curved thrust fault, indicating active right slip was transferred rapidly into the thrust because the active East Xiaojiang fault is not deformed by the Qujiang fault and there is no continuation of the fault farther east. Another longer arcuate thrust fault lies north of the eastern part of the Qujiang fault (Fig. 35), but there are no young rocks along this fault, and it is unknown if this fault is an active thrust fault developed within the shortening quadrant of the two conjugate strike-slip faults. The Quxi basin lies within

the extensional quadrant of the two conjugate strike-slip faults. It appears to have been formed as a sag basin caused by northeastsouthwest extension and bounded on the east by the Xiaojiang fault (Fig. 40). Relations of the Qujiang fault to the Xiaojiang segment. The Qujiang fault interacts in a complex way with generally northstriking faults that are part of the Xianshuihe-Xiaojiang fault system. North of the E’shan basin, north-south–trending faults of the active left-slip Yuxi fault must end because they do not pass through the basin (Fig. 35). Strands of the Yuxi fault appear to curve west at their southern ends and have a southeast-vergent thrust component. There are no young sediments along the thrust faults so they cannot be shown to be active, but they do not offset the fault on the north side of the E’shan basin and the westernmost thrust fault appears to merge with it. The relations suggest that a combination of shortening and extension on opposite sides of the Yuxi fault absorbs its strike-slip displacement (also see previous section). The Wujie fault, a steeply southeast-dipping fault with reverse displacement, branches northward from the Qujiang fault at Wujie (Fig. 35). The northwest-vergent Wujie fault places Sinian quartzite over Paleozoic rocks. A zone of gouge >100 m wide, eroded into spectacular hoodoo morphology, marks the fault. Right-lateral stream offsets near the Qujiang fault suggests the Wujie fault has an active component of right slip (Fig. 35). Displacement occurred on this fault during the 1970 Tonghai earthquake (Zhang and Liu, 1978), suggesting that some of the right-slip on the Qujiang fault was absorbed by oblique shortening on the Wujie fault. Thus, the

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Figure 38. Generalized structural map of the Eíshan basin. In set map at lower left shows a simplified model for the formation of the basin where slip E–F on northern branch of the Qujiang fault is transferred and added to slip A–B on the southern branch of the fault to yield total slip C–D east of the Eíshan basin. The transfer from the northern fault is accomplished through extension and opening of the eastern part of the Eíshan basin. Location is shown on Figure 35.

last vestiges of displacement on at least two faults of the Xiaojiang fault system die out at the Qujiang fault. The Qujiang fault appears to end both to the east and west at intersections with two north-south left-slip faults of the Xianshuihe-Xiaojiang system. At its eastern end, some, and perhaps all, of the displacement on the Qujiang fault is transferred into shortening on one and possibly two thrust faults (see previous section). Only faults of the Xiaojiang fault east of the Quxi basin continue south of the Qujiang fault, and the strand of the East Xiaojiang fault that is continuous across the east end of the Qujiang fault does not appear to be deformed by movement on the Qujiang fault. At its western end, the Qujiang fault enters a basin filled with a very thin section of Quaternary rocks at Dianzhong (the Quaternary rocks are so thin they are not shown

on any geologic map; Fig. 35). The north-south–trending left-lateral Yangwu fault strikes into the western side of the basin from the south (Fig. 29). Along its eastern and northern sides, the Dianzhong basin is marked by normal faults that are clearly expressed in the field and on satellite images, suggesting that they are young or active. There is no known westward extension of the Qujiang fault, and we interpret the fault to end within the extensional Dianzhong basin. Not all the extension in the Dianzhong basin need be related to absorption of right slip on the Qujiang fault because the left slip on the Yangwu fault would also cause extension in the basin by forming a left step with the Luzhijiang fault (see previous section). Age of the Jianshui and Qujiang faults. Time of inception of the Jianshui and Qujiang faults is not clear. The Quxi and Jian-

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Figure 39. Map showing right-lateral drainage offsets along the Qujiang fault in the Gaoda area. See Figure 35 for location.

shui basins both contain Pliocene rocks, but there is little evidence that the basins formed at that time. The Pliocene rocks rest on the widespread sub-Pliocene erosion surface that extends far beyond the basin margins. There have been no studies on the sedimentology of these basins to know how they relate to the faults that currently bound them. Both the Jianshui and Qujiang faults have an older left-lateral history whose age and relations to these two basins are also unclear. The evidence from other basins directly related to the right-lateral history on these two faults suggests their inception was some time in the Quaternary. The folds

Figure 40. Diagram showing the model for the origin of the Quxi basin that developed south of the Qujiang fault (Q. J. Ft.) at its intersection with the southern part of the East Xiaojiang fault (X. J. Ft.). Location of basin is on Figure 35 at the southwest corner of the intersection of the two faults with the town of Quxi in its center.

in the Jianshui basin are clearly related to right slip on the Jianshui fault and affect the Pliocene section, and their inception must follow deposition of the Pliocene strata and perhaps part of the Quaternary strata. Pull-apart basins, such as the E’shan, Shiping, and Dianzhong basins, contain only Quaternary strata. These strata, as well as the Quaternary rocks in the Jianshui basin, are poorly studied, and their ages are not well established. Accepting the ages shown on Chinese maps (Bureau of Geology and Mineral Resources of Yunnan Province, l990) would suggest rightlateral movement on the Jianshui fault did not begin until some time in the Quaternary. Regional relations would suggest the same Quaternary initiation for most of the faults in this area, but the question clearly requires more detailed study. Perched and abandoned stream channels along the western segment of the Qujiang fault suggests its movement began after early or middle Pleistocene time. On the drainage divide ~10 km west of the E’shan basin and on the crest of a ridge above the eastern side of the E’shan basin are abandoned stream channels that have yielded middle Pleistocene fossils (Han Mukang et al., unpublished paper, 1980). The channels lie at the same level as the surrounding sub-Pliocene surface. It can be interpreted that these channels were formed by streams flowing on the subPliocene erosion surface before faulting began, and that the disruption of the surface did not occur until the Qujiang fault and the related E’shan basin formed. The evidence, which is not conclusive, affects the estimate of the long-term slip rate on this fault (see following section).

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China Total displacements and slip rates on the Jianshui and Qujiang faults. Total right-lateral displacement on the Jianshui and Qujiang faults cannot be accurately determined because older left-lateral displacement caused offsets of a few tens of kilometers on all pre-Pliocene rocks. The largest right-lateral displacements of rivers that cross both faults is ~3 km. At the western end of the Jianshui basin, Pliocene rocks are separated by ~5–6 km, but the actual displacement may be smaller (see previous section), suggesting that 5–6 km may be considered a maximum offset. Because the time of inception of these faults can not be accurately determined, other than Quaternary, a long-term slip rate of ~1.2–2.4 mm/yr must be regarded as only a minimum. If the evidence provided by the middle Pleistocene abandoned stream channels is correct, it suggests that faulting did not begin until the middle Pleistocene or later. This would suggest a more rapid slip rate, perhaps 5–6 mm/yr or faster. Until the field relations and ages of these deposits are better established, the longterm slip rate on these faults remains unclear. Origin of some basins along the Jianshui and Qujiang faults. Origin of the Tonghai, Quxi, and southern Jianshui basins along the Jianshui and Qujiang faults remains somewhat uncertain. These three basins are different from most of the other basins in the region because they lie in the region where the two major strike-slip fault systems intersect (Figs. 31, 35). All three basins are bounded on one or more sides by steep slopes related to faults that disrupt a region dominated by a gently rolling well-preserved upland surface 100–300 m above the basins that we correlate with the sub-Pliocene erosion surface. Quxi and southern Jianshui basins. The Jianshui basin consists of two parts: a northern part that is characterized by active folding, discussed earlier, and a southern part that contains unfolded Pliocene and Quaternary rocks (Fig. 34). Both the Quxi and southern Jianshui basins are similar and bounded by eastwest–trending right-lateral and north-south–trending left-lateral faults on their northern and eastern sides, respectively (Figs. 34, 35). If these fault systems are considered a conjugate set, these two basins generally lie within a quadrant of northeast-southwest extension, and the strike-slip faults along their northern and eastern sides would have a component of normal slip. In both basins the sub-Pliocene erosion surface and its overlying Pliocene beds dip gently northward to northeastward toward the faulted basin margins. The geometry of the basins suggests they are extensional and form as sag basins along their boundary faults (Fig. 40). Tonghai basin. The origin of the Tonghai basin remains somewhat unclear. It also lies in the region where major strikeslip fault systems intersect (Figs. 29, 35). The Tonghai basin forms a rhombic-shaped, down-faulted basin within the regional sub-Pliocene erosion surface. The shallow Jili Lake is present in its northeastern part. Evidence for faulting is abundant on all sides of the basin except along its eastern side. The southern side of the basin is bounded by a straight, steep slope underlain by Paleozoic rocks (Fig. 35). Along the western end of this slope, valleys with narrow flat floors are elevated a few tens of meters above alluvial fans that spread northward into the basin. It is not

59

clear whether the elevation of the valley floors is the result of faulting or a former high stand of the lake. The straightness of the slope, possible triangular facets, and the possible vertical offsets of the valleys provide morphological evidence for a fault at the base of the slope (Plate 40). Alluvial fans on the basin side of the slope all head at the mouths of the present streams, indicating that no lateral movement has occurred; we interpret this slope to be formed by normal faulting along the Tonghai fault. The western and eastern parts of the northern side of the basin are bounded by two steep, straight, east-west–trending slopes connected by a curved northwest-trending slope (Fig. 35). The active left-slip Baipo fault, described above as part of the West Xiaojiang fault zone, appears to merge with or slightly offset one of the northern boundary faults of the basin. The morphology of the two east-striking slopes suggests they are formed by south-dipping normal faults. Within the western part of the Tonghai basin are low hills underlain by Triassic rocks. The morphology of the hills suggests that they are north-tilted fault blocks bounded by normal faults on their south sides. Headward erosion by streams into an upland capped by the sub-Pliocene erosion surface on the northern and southern sides of the basin has not advanced very far, suggesting that the faulting is young and probably active. The southwestern side of the Tonghai basin is also marked by a straight, but more indented, slope than the slopes on the northern and southern sides of the basin (Fig. 35). Because of extensive human modification of the land surface by dams, fields, and irrigation canals, evidence for active faulting is difficult to interpret. In only a few places ridges and streams appear to be left-laterally offset. Flat valley floors are raised across the trace of the fault at the base of the slope, indicating that the fault has a component of normal slip. At the northwestern end of a valley that narrows to the northwest along this fault, an abundant gouge marks the fault and a ridge is offset left-laterally. The evidence indicates that a young or active fault, the Guanying fault, with combined left slip and normal slip, is present along the southwestern side of the basin. The evidence, however, is not conclusive. The eastern side of the basin shows no evidence for young or active faulting. The boundary between basin and highlands is steep but irregular and deeply embayed, and the relation of this boundary to any known structures and to the formation of the Tonghai basin remains unknown. The Tonghai basin is the most difficult Quaternary basin in the region to explain. It lies within what appears to be a shortening quadrant between the two major conjugate fault systems (Fig. 35). The position of the Tonghai basin, as well as the southern Jianshui and Quxi basins, is in the region where the faults of the Xianshuihe-Xiaojiang fault system lose displacement or end. The fact that the right-slip Jianshui and Qujiang faults are deformed by left-lateral shear suggests that the rate of left-lateral shear dominates over right-lateral shear north of the Red River fault. We tentatively suggest that the east-trending basin-margin faults of the Tonghai basin formed as right-slip faults in a counterclockwise rotating crustal fragment bounded by the East Xiao-

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jiang and Yuxi faults (Fig. 41), causing some east-west extension. The evidence from faults on the northern and southern sides of the basin indicates a component of north-south extension in the basin as well. This is more difficult to explain because this quadrant in a conjugate fault system should be in north-south compression. We tentatively suggest that this extension is related to north-south movement of crustal rocks as displacement becomes transferred from more uniform left-shear to the south of the basin into localized left-slip on discrete faults of the Puduhe-Yuxi and Xiaojiang fault zones to the north of the basin. South of the Tonghai, Jianshui, and Quxi basins left-lateral shear is present, but it appears to be accommodated more uniformly across the width of the fault system by more distributed deformation and rarely localized on discrete left-lateral faults (see following section). Chuxiong fault. The Chuxiong fault lies ~75 km north of the Red River fault and trends northwest across the central part of the Chuxiong basin (Figs. 29, 42). The Chuxiong basin consists

Figure 41. Diagram showing the interpretation of the Tonghai basin (for location see Fig. 35) as a pull-apart basin between two right-slip faults developed by left slip and counterclockwise rotation along two branches of the Xiaojiang fault zone. See more detailed discussion in the text.

of a thick sequence of Triassic through Eocene and locally Oligocene sedimentary rocks. These rocks are folded and faulted along generally north-south trends. Pliocene rocks form only local and scattered outcrops in the central part of the basin and are characterized by coal-bearing lacustrine deposits. As elsewhere southeast of the Tibetan plateau, the Pliocene rocks rest on a relatively gentle erosion surface that we correlate with the subPliocene erosion surface, which forms a plateau of gentle relief at ~2,500 m elevation across the Chuxiong basin. The Chuxiong fault is poorly developed, even though it appears to be marked by a northwest-trending belt of small and intermediate earthquakes whose focal mechanisms are reported to be right-lateral (Lithospheric Dynamics Atlas of China, 1989). The only place where a fault can be located extends from Chuxiong ~40 km to the southeast (Fig. 42). It is marked by a steep escarpment and two small basins, one with Quaternary rocks and the other with Pliocene rocks, that are bounded on the south side by the fault. The Pliocene rocks are folded into a syncline whose axial trace parallels the fault. The ridge along the south side of the fault forms the major drainage divide between the Yangzi and Red Rivers. Topographic features indicate that the fault has had north-side-down normal displacement and the surface of the Quaternary basin slopes south into the fault, suggesting that the fault is young or active. Although focal mechanism studies have yielded evidence for active right-lateral faulting, we could find no surface evidence for active right-lateral displacement. Even though other small Quaternary basins are aligned along the projection of the fault, such as at Nanhua (Fig. 42), there is no evidence for a fault to the northwest or southeast of the 40-km-long fault trace mentioned earlier. Thus, either the fault is very young or has such a slow slip rate that it has not developed clear surface features or developed into a longer through-going structure. Currently, it is unclear if this is a regionally important active fault. Similar to the Jianshui and Qujiang faults, the Chuxiong fault had older left-slip displacement. Jurassic and Cretaceous red beds are strongly folded along north-south axes in a belt about 50 km wide (Fig. 42). As the fold axes are traced toward the fault they bend southeastward or northwestward on the north and south sides of the fault, respectively, indicating left-lateral shear strain (Fig. 42). Left slip on the Chuxiong fault has been related to a broad zone of left-lateral shear associated with middle Cenozoic ductile shear on the Ailao Shan shear zone by Tapponnier et al. (l986). The constraints that can be placed on the time of left slip depends on how the Chuxiong fault is related to the folds. The folds involve Middle Eocene (and perhaps Oligocene rocks) and are unconformably overlain by Pliocene rocks. If the fault is a tear fault it formed at the time of folding, but if it is unrelated to the folds it would be younger. Rocks of the appropriate age are not present to distinguish between these two relationships. The folds in the Pliocene rocks are gentle and open, suggesting that they are not directly related to the period of folding that produced tight folds within the Mesozoic and early Cenozoic rocks, but are related to initiation of right-lateral displacement of late Pliocene to Quaternary age.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Figure 42. Generalized geologic map of the Chuxiong basin. See Figure 29 for location. Units of Early, Middle, and Late Jurassic, Early and Late Cretaceous, Paleocene, and Eocene age are also shown.

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Dali fault system Within west-central Yunnan is a network of young and active faults that we have grouped into the Dali fault system (Figs. 29. 43). This fault system contains north-, northeast-, and northwest-trending left-slip faults with variable components of normal displacement. Two northwest- to north-trending faults along the Lancang River are probably active, but we have no evidence for their slip sense. The northwestern continuation of the Red River fault and the older Ailao Shan shear zone and their relation to the Dali fault system are not clear (Fig. 43). Evidence indicates that if there were a continuous through-going Red River fault and/or Ailao Shan shear zone, they have been disrupted northwest of Dali by the younger Dali fault system that probably was established during Pliocene-Quaternary time. The network of faults of the Dali fault system bound tectonic units that contain older structure, and the young faults often follow older structural features, suggesting that crustal anisotropy has played an important role in their localization (c.f., Figs. 43, 44). One of the most striking deformational features of the Dali highland area is the wide distribution of young basins that total more than 100. Some of these basins are bounded by young or active structures, and nearly all the basins are Quaternary in age (Table 3). Because we have designated the Jianchuan fault to be the western boundary of the Dali highland, the western one-third of the highland contains numerous active faults and associated basins (Fig. 43). The selection of boundaries for tectonic elements in this highly faulted area is somewhat arbitrary, and a separate small structural unit could have been defined for the western part of the Dali highland. We recognize the arbitrary nature of such divisions, but it will not affect the general interpretations that are proposed in the following section. The Dali and Red River fault systems merge at the western boundary of the Midu basin where the northwest-trending, rightslip Red River fault intersects the north-trending, left-slip Chenghai fault zone of the Dali fault system (Fig. 43). Stream deflections along the north-trending Chenghai fault at the western side of the basin are clear evidence for active left slip. The geometry of this basin is consistent with an origin as a modified pullapart basin formed in the extensional quadrant between two conjugate strike-slip faults. The fault present between the Midu and Erhai basins is a vertical fault that separates Mesozoic rocks on its southern side from lower Paleozoic rocks on its northern side. It is associated with right-slip stream offsets (Plate 41) and is the probable continuation of the Red River fault; however, it is not aligned with the southern margin of the Midu basin whose bounding fault has evidence of north-side–down displacement, but only weak evidence for right-lateral displacement. The active trace of the Red River fault projects from the southeast into the middle of the Midu basin and does not have an obvious trace across the basin floor to connect with the active right-lateral fault extending west from the Midu basin. Near Dali, faults that lie along the projected continuation of

Figure 43. Map of major young and active faults of the Dali fault system in the Dali highland and adjacent areas (for location see Figure 29). BB = Baihanchang basin, DB = Daju basin, HB = Heqing basin, JB = Jianchuan basin, JIB = Jinyuan basin, LB = Lashi basin, LJB = Lijiang basin, SB = Shaxi basin, SYB = Sanying basin, YB = Yongsheng basin. Line A–B is location of cross section in Figure 46.

the Red River fault form a complex pattern, and we consider them to be part of the Dali fault system (Fig. 43). The Erhai basin is bounded on its western side by east-dipping normal faults with little or no evidence of strike-slip movement that bound the more than 4-km-high adjacent Diancang Shan (Plate 42). Some of the

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TABLE 3. AGE OF SEDIMENTARY DEPOSITS IN BASINS ALONG THE DALI FAULT SYSTEM Basin

Age

Dali basin

Holocene Late Pleistocene Middle Pleistocene Early Pleistocene

Yongsheng basin

Holocene Late Pleistocene Middle Pleistocene

Binchuan basin

Holocene Late Pleistocene Middle Pleistocene Early Pleistocene

Control

Thickness

Contains fossils

2,000 m

0.015 0.04 Ma (C14 age) 0.2 0.25 Ma

150 200 m

0.001 Ma (C14 age) 0.1 Ma (C14 age) 500 m 1.73 Ma (calculated from present sedimentation rate)

Heqing basin

Holocene Pleistocene

Contains fossils

500 m

Lijiang basin

Holocene early middle Pleistocene

0.73 2.48 Ma

1,000 m

Jianchuan basin

Holocene Late Pleistocene Middle Pleistocene Early Pleistocene

0 9880 (C14 age) 35830 yr (C14 age) Dated by fossils and paleomagnetic data

12 28 m 8 25 m 53 200 m 40 248 m

Eryuan basin

Holocene Late Pleistocene

Contains fossils

0 50 m 100 250 m

Chenghai Lake

Holocene Pleistocene

Midu basin

Holocene Pleistocene

0 4000 yr (C14 age) 0.52 Ma (calculated from present sedimentation rate)

1,200 m

Dated by fossils

600 700 m

Note: The stratigraphic data are from He et al. (1985), Peng and Jiao (1986), Ma (1988), Le and Jin, unpublished data (1990), Wu (1992), Zhao (1965). Some data from Bureau of Geology and Mineral Resources of Yunnan province are unpublished.

alluvial fans north of Dali are traversed by fault scarps indicating active normal faulting. These faults are parallel to, but are not aligned with, the Red River fault. The northeastern boundary of the Erhai basin is a topographically prominent, but irregular, escarpment that suggests it may be bounded by a normal fault; but if so, the fault is not exposed and is probably hidden beneath Erhai Lake. Metamorphic and mylonitic rocks of the Diancang Shan are bounded by the Erhai normal fault on the eastern side and arcuate gentle south- and west-dipping faults on its southern and western sides (Plate 43), respectively, which carry unmetamorphosed Mesozoic rocks in their hanging walls (Figs. 43, 44). At the southeastern end of this arcuate fault zone, footwall rocks contain mylonitic metamorphic rocks with top-to-the-southsoutheast shear sense and subhorizontal to gently south-south-

east–plunging lineations. This fault zone appears to have been at least partly responsible for the tectonic unroofing of the Diancang metamorphic core (Leloup et al., l993). Faults along the west side (Plate 43) cut the foliation, sometimes mylonitic, and thus record a second period of unroofing of the Diancang Shan metamorphic rocks. We find it difficult to accept the interpretation of Leloup et al. (l993) that the faults on the western side of the Diancang Shan are the offset continuation of the Red River fault. Their characteristics are more like those of a detachment fault related to formation of an extensional metamorphic core complex, rather than a right-lateral strike-slip fault. Our preliminary interpretation of faults bounding the Diancang Shan is that they record at least two periods of unroofing of the metamorphic rocks at ~20 Ma and ~4 Ma to present. Other faults, such as the Yangbi fault (Fig. 43),

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that parallel the Red River fault, lie even farther west of the Diancang Shan; we find it even less likely that these faults are a direct continuation of the Red River fault. Thus, it is difficult to find a direct northwest continuation of the Red River fault beyond the Midu basin, or perhaps beyond Dali, and we suggest that the name Red River fault not be carried farther northwest. Chenghai fault. The Chenghai fault trends north to northnortheastward along the eastern boundary of the Dali highland (Figs. 43, 44). It is a clear and continuous fault except where it passes through the mountains between the Midu and Binchuan basins. All along its trace it shows evidence for active left slip. Rivers and streams that cross the fault consistently show left-lateral offsets (Plate 44). The Chenghai fault displaces the Jinsha River left laterally ~3 km (Fig. 45, Plate 45) and a small Quaternary basin is present on the down-thrown western side of the fault (Fig. 45). The Jinsha River has recently captured the drainage into this basin by westward headward erosion through the elevated footwall of the Chenghai fault. Most of the Quaternary basins along the Chenghai fault have internal drainage and will be subject to capture in the near future. There is also clear evidence for a component of west-sidedown normal displacement on the Chenghai fault in both the Binchuan and Chenghai basins (Plates 44, 46) and east-sidedown normal displacement in the Midu basin. The Binchuan basin has a component of normal displacement on its eastern side, and the Chenghai basin at Chenghai Lake has faults with normal displacement on both sides that dip into the basin forming a narrow graben. Young and only partially modified fault scarps and triangular facets indicate active normal displacement. At its northern end the Chenghai fault branches into three faults that curve west. Along the curved part of the faults are the Jinyuan and Yongsheng basins that are modified pull-apart basins similar to the Midu basin at the southern end of the Chenghai fault (Fig. 46 and Plates 46, 47). Sediments in all these basins, except the Midu basin, dip east except for the youngest alluvial fan deposits that dip west from the elevated footwall of the Chenghai fault. With local exceptions, the rivers that parallel the basins lie near or along their eastern sides, indicating active eastward tilting of the basin floor. The basins appear to have formed at releasing bends along the curved Chenghai strike-slip fault. Along most of its length, the Chenghai fault has formed along an older thrust fault reactivating it as a left-slip normal fault (Figs. 44, 46). The older gently west-dipping thrust fault carried Paleozoic and Mesozoic rocks eastward above Mesozoic and Cenozoic rocks of the Chuxiong basin. The west-dipping Chenghai normal fault carries Paleozoic rocks in its hanging wall and Mesozoic rocks in its footwall along most of its length except where remnants of the older thrust fault are preserved east of the active left-slip normal fault. Because the Chenghai fault shows both normal and left slip and is parallel in both strike and possibly dip with an older listric thrust fault, it is interpreted to be a listric left-slip normal fault that reactivated an older thrust fault. Jianchuan fault. The Jianchuan fault bounds the western side of the Dali highland (Figs. 43, 44) and forms a sharp linear

fault zone on the ground and on Landsat images. All along the fault evidence for active left-slip abounds but becomes less obvious to the south between the Shaxi and Qiaohou basins (Fig. 43). North of the Baihanchang basin, streams and ridges along the fault are consistently offset ~1 km (Plate 48). Near the southern end of the fault a young stream and alluvial fan have a left-lateral offset of ~50 m. Where the Jianchuan fault terminates to the south against the Yangbi fault, the small Qiaohou basin is bounded by normal faults on its northwestern and southwestern sides (Fig. 43). This basin is geometrically similar to the Midu basin and is interpreted as a modified pull-apart basin (see following section). At three locations along the Jianchuan fault are small faultparallel basins formed at releasing bends or along splays off the main fault and occupied, from south to north, by the Shaxi, Jianchuan, and Baihanchang basins. These basins have normal faults on their eastern and western sides. Within the Baihanchang basin, small grabens are present in Quaternary rocks along the faults. Along the flanks of these grabens are sets of terraces that are formed by down-faulted fragments of the sub-Pliocene erosion surface, which is well developed in this area and cut on deformed Triassic rocks. All the basins are reported to contain only Quaternary rocks (1:1,000,000 Yunnan Geological Map, Bureau of Geology and Mineral Resources of Yunnan, 1990; Table 3). The Jianchuan basin also contains one of the most complete and well studied sections of Cenozoic rocks in Yunnan. The sequence consists of Eocene, Oligocene, Miocene and Pliocene rocks with local interbedded volcanic rocks. These rocks are not confined to the Jianchuan basin, but are more widely distributed to the west and have no direct relation to the development of the Jianchuan fault (Fig. 44). Only the Quaternary strata are related to development of the Jianchuan basin. At its north end, the Jianchuan fault enters the Jinsha River valley and is probably partly responsible for the sharp bend in the Jinsha River in this area. Even though the fault lies beneath the Jinsha River for ~30 km, evidence for active left slip is present by left bends of its tributaries along the west side of the river in the south and across alluvial fans along the east side of the river in the north. The fault continues north and curves westward where it continues as the northwest-trending Benzilan-Zhongdian fault (Fig. 43). Lijiang fault. The northeast-trending Lijiang fault consists of northeast and southwest segments separated by the central part of the Lijiang basin where it makes a left step. Many studies (Liu et al., 1986; Pan et al., 1987; Holt et al., 1991) show the two segments of the Lijiang fault to be a continuous, active left-lateral fault zone, but we have not found evidence at the surface for a fault connecting the two segments. The fault bounds and connects parts of the Jianchuan, Lashi, and Lijiang basins. These basins are filled with Quaternary sediments and show evidence for active subsidence suggesting that the Lijiang fault is of the same age and related to the left slip and extension common to the Dali highland. Unlike the Jianchuan and Chenghai faults, we could find no offsets of geological units or features across either

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Figure 45. Map showing the offset of ~3 km of the Jinsha River by leftlateral movement on the Chenghai fault. See Figure 29 for location.

Figure 44. Generalized geologic map of the Dali fault system and its relation to the rock units in the Dali highland, Yunling collage, and Lanping-Simao fold belt tectonic units. See Figure 29 for location.

segment of the fault; thus displacement on the fault is probably not very large. The northeastern segment of the Lijiang fault lies along the base of a steep, northeast-trending slope that bounds a left-stepping jog in the Lijiang basin (Fig. 43), a basin reported to contain at least 1,000 m of Quaternary strata (Ma, 1988). Parallel to this slope there is abundant evidence for several faults (Plate 49) with south-side-down normal displacement as well as active left-slip recorded by numerous stream deflections. Displacement along these faults disrupts the sub-Pliocene erosion surface that is well developed both north and south of the Lijiang fault and east of the Lijiang basin. To the northeast, the fault curves northward and shows a west-side-down component before it intersects the northwest-trending Daju fault (Fig. 43). On Landsat imagery there appears to be evidence for a fault continuing northward beyond the Daju fault. The southwestern segment of the Lijiang fault consists of at

least three faults. Two of these faults are parallel, topographically well expressed, and connect the Jianchuan, Lashi, and Lijiang basins (Fig. 43). The rhomb-shaped Lashi basin is bounded by faults marked by steep slopes on all four sides and the morphology of the slopes suggests young (or active?) normal faulting. Above the basin on the west and south is a well-developed, undulating erosion surface, which we correlate with the sub-Pliocene surface, cut on folded Triassic limestone. On its east side, the Lashi basin lies ~200 m above the Lijiang basin and is separated from it by a sill of Permian basalt and lower Triassic clastics. The normal fault along the southeastern side of the basin is a branch of one of the faults forming the southwestern segment of the Lijiang fault. The normal fault along the northeastern side of the basin continues north beyond the basin. The geometry of the basin suggests a pull-apart origin, modified by karst processes, at the intersection of branching left-lateral faults. A third fault, shown on Chinese maps (Bureau of Geology and Mineral Resources of Yunnan Province, l990), trends northeastward from the Jianchuan basin to the Lijiang basin and aligns with the northeastern segment of the Lijiang fault. Most of the boundaries of the Lijiang basin are not marked by continuous faults, but rather by en echelon faults that locally bound segments of the basin and strike into the mountains away from the basin. Along the northeastern and southern boundaries of the Lijiang

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Figure 46. Cross section of the northern segment of the Chenghai fault. The Chenghai fault probably reactivates an older east-vergent thrust fault at depth. Remnants of the older thrust fault are present locally east of the Chenghai fault at several localities. See line A–B in Figure 43 for location.

basin, the faults appear to bound a series of rotated blocks that have more subsidence toward the basin and are responsible for forming of the basin. Southwest of Lijiang, a lake is present on the downthrown side of one of these fault blocks, suggesting active subsidence. These normal faults terminate against both the northeastern and southwestern segments of the Lijiang fault. Because of the similarity of the geometry of the basin-forming faults to that of the two segments of the Lijiang fault, we suggest that the southwestern segment of the Lijiang fault is probably also young and active, although we have no direct evidence for the displacement history along the fault itself. The Lijiang fault is intimately related to the formation of the Lijiang basin but in a complex way. The two segments of the fault are not simple left-slip faults connecting two pull-apart structures to form the jog in the Lijiang basin. They are faults that are part of the overall left slip and extensional region in the Dali highlands bounding a series of largely north- to northwest-trending normal faults with differential subsidence. Local evidence indicates active left slip and normal faulting, and their relation to active basin formation suggests that both segments are active. Heqing fault. The northeast-trending Heqing fault (Fig. 43) shows abundant stream offsets that indicate active left slip. Deng et al. (l993) have studied the Heqing fault and determined a slip rate of 2.8 mm/yr. It terminates in Quaternary basins at both ends, the Heqing basin to the northeast and the Sanying basin to the southwest. Both basins lie in an extensional position at the end of the left-slip fault. The Heqing basin is formed by a major westdipping normal fault along its eastern side (Plate 50) and contains 300 m of Pleistocene strata. Along the west side of the Heqing basin, faults form local basin-margin faults; and like the Lijiang basin, the faults are en echelon and strike west from the basin

margin into the adjacent mountains. At the southwestern end of the Heqing fault, the Sanying basin is marked by east-dipping normal faults along its western side. The Sanying basin consists of two small northwest-trending basins separated by an intervening horst, and all three structures end against the Heqing fault. The southern and western part of the Dali highland is traversed by a series of narrow extensional basins that trend northwest from the Erhai basin in the south to the Sanying basin in the north where they are offset to the northeast and continue northward as the Heqing and Lijiang basins. The offset and change is at the Heqing fault, which we interpret to be transfer structure between the two sets of extensional basins. Daju fault system. The northeast boundary of the Dali highland is poorly defined. It consists of at least four northwest-trending faults that extend from the Jinyuan basin through the Daju basin (Fig. 43). These faults do not significantly offset older geological units or structures and apparently do not form a continuous fault zone (Figs. 43, 44). We selected this poorly defined group of faults as the northern boundary for the Dali highland because it terminates the region of abundant Quaternary basins and illustrates relations between the faults of the Dali fault system. Young and active faults extend farther northeastward, and with further study, a more clearly defined northern boundary to the Dali fault system may be determined. In our opinion this does not greatly affect the interpretation presented in the following section. Three of the faults forming the Daju fault system bound parts of Quaternary basins and have morphological expression, suggesting that they are young or active faults. The fault along the northeastern side of the Jinyuan basin makes a linear contact at the base of a steep slope and merges with one of the curved fault splays at the north end of the Chenghai fault (Fig.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China 43). The morphological features along this basin-bounding fault indicate normal faulting, and we found no evidence for strike-slip on the fault. Two faults form part of the boundaries of the Daju basin farther northwest. Relations between the faults at the Daju and Jinyuan basins suggest that the basins are pull-apart structures and that the faults have a component of right-slip displacement; we have no direct evidence for the sense of slip, however, our interpretation suggests that the faults may have a component of left slip. Both faults form sharp boundaries at the margins of the basin and have morphological features suggestive of normal faulting, a conclusion reached in a recent study by Leloup et al. (1995). Yulong Snow Mountains. The Yulong Snow Mountains rise to 5,396 m, forming the highest part of the Dali highland and the highest mountain in this part of the Tibetan plateau (Fig. 43). To the south of the mountains, the Pliocene erosion surface is warped and developed at an elevation of ~2,700–3,200 m, and the basin floors of the Daju and Lijiang basins are at elevations of ~1,700 m and 2,400–2,700 m, respectively, adjacent to this high range. Metamorphic rocks are exposed in the northwest part of the Yulong Snow Mountains where they form the core of a major anticlinal structure (Lacassin et al., l996). Faults present around the margins of the range indicate that an important component of unroofing the rocks in the core of the range occurred mainly by normal faulting, a conclusion not reached by Lacassin et al. (1996). Most of the normal faulting was probably Quaternary, and normal faults on the eastern and northeastern sides of Yulong Snow Mountains are active and continue relative uplift of the range. Along the northwestern side of the Lijiang basin, fresh normal fault scarps cut Holocene alluvial fans at the base of the Snow Mountains (Plate 51). Numerous landslide blocks and more coherent fault blocks formed by Paleozoic limestone have been displaced downward toward the Lijiang basin along a series of east-dipping listric normal faults and attest to the large scale and youthfulness of the faulting. The Jinsha River cuts a deep gorge, Tiger Leaping gorge, through the structurally highest part of the Yulong Snow Mountains, indicating the river is antecedent and the uplift of these mountains is very young (see also Lacassin et al., l996). The Jinsha River is at an elevation of ~1,700 m, where it passes through the mountains and the gorge contains more than 3,500 m of relief. Benzilan-Zhongdian fault. In contrast to the Daju fault zone, the northwest-trending Benzilan-Zhongdian fault forms a distinct northeast boundary to the Yunling collage (Figs. 43, 44), and it has a clear linear trace both on satellite images and on the ground. The main fault can be traced from west of its intersection with the Lancang fault at Deqin southeast through the Zhongdian and Xiao Zhongdian basins to its merger with the Jianchuan fault (Figs. 43, 44). The northwestern part of the fault cuts at a high-angle across older north-trending rock units and structure, but the southeastern part of the fault parallels older structural features and its location may have been influenced by crustal anisotropy. Several shorter faults branch from or are parallel to the main through-going fault and may be part of a system

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of faults that make up the Benzilan-Zhongdian fault, but we have not studied these shorter faults. The sense of active slip on this fault is clear and is shown by numerous left-lateral stream deflections between the Jinsha River and Zhongdian. Curiously, three earthquakes of magnitude greater than 6 have occurred on the fault (June 7, 1933, M = 6.5; June 27, 1961, M = 6; and September 28, 1966, M = 6.4). A study of focal mechanisms has indicated that they were caused by right-slip (Kan et al., 1983), in contrast to the consistent left-lateral deflections shown by the streams, a conflict of evidence that remains unresolved. The Benzilan-Zhongdian fault passes through the Zhongdian basin and north of the three subbasins that make up the Xiao Zhongdian basin, but the geometry of the basins does not show an obvious structural origin. To the contrary, the basins are suggestive of significant development by karst and lacustrine processes. They lie entirely within Triassic limestone, and the Zhongdian basin and parts of the subbasins of the Xiao Zhongdian basin have internal drainage, irregular basin outlines, and almost no evidence of having been filled by alluvial fan sediments. Locally the basins contain lakes and enclosed depressions. The Zhongdian basin has a horizontal basin floor that extends to the irregular mountain front and its present shape was probably formed by lacustrine sedimentation. If structure controlled the formation of these basins, it has been so strongly modified by karst and lacustrine processes that it is impossible to recognize. These basins lie at the morphological edge of the Tibetan plateau. To the east, rivers that flow into the Jinsha River have cut deep canyons and will shortly capture and destroy these basins. There is clear evidence for ~30 km of Cenozoic left slip on the Benzilan-Zhongdian fault. Northwest of Zhongdian, northstriking Triassic and Paleozoic rock units are consistently offset left-laterally ~30 km (Fig. 44). The Jinsha River is also left-laterally offset ~35 km, indicating that the left-lateral displacement is Cenozoic in age. Over what time period this has occurred remains unclear. Some of the shorter faults also offset geological units left-laterally, and the total displacement along all the faults probably exceeds ~35 km. The Benzilan-Zhongdian fault is parallel and on strike with the Jianshui, Qujiang, and Chuxiong faults to the southeast that have a history of older left-lateral displacement that changed to younger right-lateral displacement. We suggest that much of the left-lateral displacement on the Benzilan-Zhongdian fault is of the same age as on the three faults to the southeast, and left slip continues to the present. Tongdian fault. The Tongdian fault is marked by a steepsided valley that extends ~200 km from the Qiaohou basin in the south to Baijixun in the north where it merges into the north-trending Lancang fault that lies within the Lancang River valley (Figs. 29, 47). North of the intersection with the Tongdian fault, the Lancang fault forms the western boundary of the Yunling collage. We have not examined the northern part of the Lancang fault, but Chinese maps (e.g., Bureau of Geology and Mineral Resources of Yunnan Province, l991) show offset Jurassic and Cretaceous rocks along the fault that suggest right-lateral displacement.

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The Tongdian fault forms a major geological boundary dividing Triassic metasedimentary rocks containing arc volcanic rocks and melange in the western part of the Yunling collage to the east from Mesozoic red beds of the Lanping-Simao fold belt to the west (Fig. 44). Along its northwestern part, the Tongdian fault splits into two branches that bound a rhombic-shaped high mountain range called Xuelong Shan that is underlain by a highgrade metamorphic complex and characterized by gneiss, marble, and schist (Fig. 44). The Madeng basin (Fig. 47) forms a narrow elongate basin along the fault. Mesozoic rocks from the Lanping-Simao tectonic unit extend across nearly the entire basin as narrow peninsulas and isolated hills, suggesting that the sediments in the basin are thin. Pliocene coal-bearing strata, the oldest rocks in the basin, occur in the northwest part of the basin and in a narrow belt along the southeast side. At the southeast end of the basin the Pliocene strata are situated as much as several hundred meters above the present basin floor and are bounded by a fault from Triassic rocks of the Yunling collage. Older Quaternary alluvial deposits appear to have filled the basin mainly from its southern and western sides, and the basin may have been enclosed during their deposition. The Madeng basin is connected to the topographically lower Qiaohou by a river flowing in a narrow gorge that probably captured the Madeng basin drainages during later Quaternary time. The older Quaternary strata in the southern part of the basin are eroded into long, flat-topped isolated mesas and peninsulas rising >100 m above the younger Quaternary deposits along the present river system that drains the basin to the southeast. Most of the erosion of the older Quaternary strata is in the southern part of the basin, suggesting that headward erosion has not proceeded northwestward along the entire basin. We interpret the Madeng basin as a sag basin formed by asymmetric northeastward tilting into the Tongdian fault, but the evidence for active faulting is lacking. Triangular facets are present along the southeastern part of the basin above the river that follows the Tongdian fault (Plate 52a). If the tops of the mesas underlain by older Quaternary stata are projected to this side of the basin, they reach the top of these facets. We suggest that these facets are formed mainly by erosion along the contact between the Pliocene and older Quaternary strata that may be a fault. A few, but not a majority of the streams crossing the northeastern basin-bounding fault have apparent left-lateral deflections (Fig. 47, Plate 53). However, young fans and terraces on most of the tributaries crossing the fault have no evidence for active scarps or young displacement, and the fault appears to be inactive. The present asymmetry of the basin (Plate 52b) is caused by erosion along the river that lies along the northeast side of the basin. How much of the present morphology of the basin is inherited from its earlier structural history and how much is caused by younger erosion remains unclear, but there is little evidence for active faulting along the northeastern basin margin. Although there is little evidence for active faulting, older Pliocene coal-bearing beds in the northwest part of the basin are strongly folded, with northeast-trending fold axes (Fig. 47) con-

Figure 47. Map of the Madeng basin along the Tongdian fault. At least three, but not all, of the rivers appear to be left-laterally offset. Those rivers with left-lateral offsets are shown with arrows. Inset shows interpretation of fold pattern in the Pliocene rocks at the north end of the Madeng basin. For location see Figure 29.

sistent with formation within a left-lateral shear zone. Timing of the folding is unclear because on the most recent map of Yunnan (1:1,000,000 geologic map, Bureau of Geology and Mineral Resources of Yunnan Province, l990) the folded rocks are shown as early Quaternary, whereas on older maps they are shown as Pliocene. We have not been able to discover the reason for the change in age assigned to these rocks. The folded rocks are overlain unconformably by subhorizontal upper(?) Quaternary allu-

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China vial fans (Plate 54). Evidence for Pliocene-Quaternary and young left-lateral displacement is present, but not demanding. The Tongdian fault is generally considered to be the northern extension of the Red River fault zone (e.g., Leloup et al., l995) because these two fault zones have many similarities. They both have a similar strike, have a clear linear fault trace on Landsat imagery and on the ground, and form the northeastern boundary of the Lanping-Simao fold belt. In addition, the metamorphic rocks in the Xuelong Shan are mylonitic with S-c fabrics, domino structure, and rotation of plagioclase crystals indicating that they were formed by left-lateral shear (Zhang et al., l991; Leloup et al., l995) similar to the rocks in the Ailao Shan shear zone. Their position along the Tongdian fault is also similar to that of the rocks in the Ailao Shan. We suggest that the major displacement across the Tongdian fault that juxtaposed the contrasting tectonic units may be middle Cenozoic or older, but it is not a direct continuation of the younger Red River fault. We include the Tongdian fault in the Dali fault system, but we do not interpret it to be the direct continuation of the Red River fault. In contrast to the Red River fault, the Tongdian fault may have late Neogene or early Quaternary left-slip and southwest-side-down normal displacement. Additionally, the two faults are not aligned and do not have mapped continuity (Fig. 29). Like that of the Red River fault, the young inactivity of the Tongdian fault may be related to the onset clockwise rotation during formation of the Dali fault system, with the Tongdian fault at a high-angle to the velocity vector within clockwise rotating crust. It may, however, be the continuation of older major fault displacement that juxtaposed the Lanping-Simao tectonic units against the tectonic units to the northeast, but whether this is the trace or modified trace of the Ailao Shan shear zone is under study and has not been determined at this time. Discussion of the Red River and Dali fault systems The relative displacements on the young and active faults that form the Red River and Dali fault systems in Yunnan are summarized in Figures 29 and 43. We interpret the displacements on this network of faults to be contemporaneous and thus to have occurred within the same displacement field. The pattern of faults is right slip on northwest-striking faults east of the Dali highland, left slip and normal faulting on north- or north-northeast– striking faults, and left slip on some of the northwest-trending faults within and west of the Dali highland. This network of faults is difficult to explain in isolation from the entire displacement field in the region (discussed in more detail in following section). Leftlateral displacements on the concave-west Xianshuihe-Xiaojiang fault system in concert with the northward movement of India relative to South China indicate crustal material is sheared rightlaterally and rotates clockwise around the eastern Himalayan syntaxis. Within and west of the Dali highland, clockwise rotation is at two scales. At a large scale it extends from at least the western boundary of the Yunling collage eastward to at least the Xianshuihe-Xiaojiang fault system, and at a small scale crustal

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fragments within the Yunling collage and Dali highland rotate clockwise bounded by left-lateral faults (Figs. 48 and 49). Part of the left slip on the northwest-trending faults, such as the Benzilan-Zhongdian fault and possibly the Tongdian fault, is probably caused by the smaller scale clockwise rotation. Accepting a 5–6 km total displacement on the Red River fault from Midu southeast into Vietnam during Pliocene-Quaternary time yields a slow, long-term slip rate of ~1–3 mm/yr. This would explain lack of seismic activity on the fault (see Allen et al., l984) by a long recurrence interval. It would suggest that the prominent surface expression of the Red River fault is due to erosion along an older Cenozoic fault at the contact between the resistant rocks of the Ailao Shan to the southwest and the more erodable sedimentary rocks to the northeast. The Red River fault lies within the large-scale clockwise rotating crustal fragment bounded on the east by the Xianshuihe-Xiaojiang fault system and left-slip Dien Bien Phu fault south of the Ailao Shan (Figs. 49 and 50). The northwest-trending Red River fault should have left slip within such a rotational system, but the extension within the Dali highland suggests crust east of the highland has a small relative eastward component of motion producing the right slip on the Red River fault. That eastward component might come from a different pole of rotation for material in the Dali highland and along the Xianshuihe-Xiaojiang fault system. The geometry of the faults and extensional areas suggests an eastward component of extension within the material rotating clockwise between the Dali highland and the Xianshuihe-Xiaojiang fault system (Fig. 51). The fact that active right slip on the Red River fault appears to end before entering Vietnam would also support this interpretation. We interpret the youngest cooling ages in the Diancang Shan (Leloup et al., l995) to be the result of extensional unroofing related to inhomogeneous clockwise rotation and not directly to right slip on the Red River fault. The morphology and internal structure of the Diancang Shan appear to be rotated clockwise ~15° from their possible continuation in the Ailao Shan to the southeast. Part of this rotation could have been accommodated along the complex curved faults that dip away from the Diancang Shan along its southern and southwestern sides as the metamorphic rocks in the Diancang Shan were tectonically exhumed from below the nonmetamorphic red beds that lie adjacent to the range. The juxtaposition of the Lanping-Simao fold belt (including the Ailao Shan) and all other tectonic units to the northeast indicates a through-going fault of large magnitude. Much of the displacement may be related to ductile left-shear from 35–17 Ma on the Ailao Shan shear zone (see Leloup et al., 1993, l995). Harrison et al. (1992A) interpreted exhumation and cooling of the midcrustal mylonites to have been by normal faulting along the northern boundary of the Ailao Shan during late-stage shearing along the Ailao Shan shear zone at ~20 Ma. In contrast, our studies suggest that the Ailao Shan shear zone was part of a left-lateral transpressive tectonic regime (Wang and Burchfiel, l997). The south-dipping thrust fault forming the boundary of the Ailao Shan east of Yuanjiang would be part of that transpressive regime

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Figure 48. Diagram showing interpretation of the young and active faults in the Dali fault system. Left-lateral displacement is a result of small blocks rotating clockwise relative to crust to their east. Evidence for active left slip along the Zhongdian (ZD) and Daju (DJ) faults is poorly developed, but clear for the north- to north-northeast–striking faults. Quaternary basins within the Dali highlands form as pull-apart and rotational basins at the edges of rotating blocks. See text for discussion. This covers the same area Figure 43. CH = Chenghai fault, DC = Diancang Shan, HQ = Heqing fault, JC = Jianchuan fault, TD = Tongdian fault.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Figure 49. Interpretation of evolution of the Red River fault. PrePliocene right-lateral displacement is a minimum of 14–48 km. Following formation of the Xianshuihe-Xiaojiang fault system, the left slip may be only 6 km from Pliocene to Holocene time. During this time period, the Red River fault has been rotated clockwise within a region west of the Xianshuihe-Xiaojiang fault system and rotated counterclockwise within the left-shear of the Xiaojiang fault zone. Rotation of crustal fragments in the western part of the Red River fault system is about a different pole than the regional rotation about the Xianshuihe-Xiaojiang fault system, creating extension in the Dali Highland area and along the northwestern part of the Red River fault. Within the Xiaojiang fault zone, the Red River fault has components of both right-slip and thrusting, whereas to the north the region within the fault zone is dominated by northeastsouthwest extensional basins.

and exhumation would be related to shortening. The presence of mylonite and metamorphic clasts in the Miocene, and possibly Eocene-Oligocene rocks north of the Ailao Shan and east of Yuanjiang, suggests that unroofing of the Ailao Shan metamorphic rocks did occur rapidly in middle or early Cenozoic time. There is little direct evidence for the relation between this exhumation and the inception of the right-lateral displacement on the Red River fault. The inception of the Dali fault system was probably in Pliocene or early Quaternary time. Structural basins related to faults along this part of the Dali fault system indicate that deformation was active in Quaternary time and followed the formation of the sub-Pliocene erosion surface and deposition of the finegrained, coal-bearing Pliocene rocks that overlie this surface. The basin sediments have been dated in only a few places even though all the basins are shown on geologic maps to contain only

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Quaternary sediments (see Table 3). Leloup et al. (1993, 1995) reported rapid cooling of metamorphic rocks in the Diancang Shan at ~4.7 Ma that may be related to the beginning of exhumation in Pliocene time. The limited data available indicate inception of the fault system between ~5 and 2.5 Ma. Right-slip faulting and associated normal faulting along the Red River fault probably began in pre-Pliocene time, but after termination of left slip on the Ailao Shan shear zone at ~17–20 Ma (Leloup et al., l995). Evidence for total right-lateral displacement on the Red River fault suggests at least 20–54 km, which is similar to the magnitude given by Leloup et al. (1995). However, most of that displacement appears to be older than the deposition of the Pliocene strata in the Majie area. The northwestward continuation of this pre-Pliocene Red River fault remains unknown. During the formation and development of the Red River fault system in Pliocene and Quaternary time, right-lateral displacement on the Red River fault continued, but the total displacement in this time interval was probably only 5–6 km as suggested by Allen et al. (1984, Figure 49). Quaternary rocks and rivers are displaced by right slip, but almost everywhere Quaternary strata are unfolded and lie unconformably on the older Neogene rocks. Vertical relative uplift rates during the past ~5 Ma along normal faults within the Dali fault system are probably less than 1 mm/yr except on a few of the major faults. According to the studies by Ma (1988) and Gou et al. (1986), cores from the Erhai basin contain lower Quaternary rocks at the base of the basin section. The top of Diancang Shan is relatively flat, suggesting that it may be capped by a remnant of the sub-Pliocene erosion surface and that relative uplift of the range followed development of this surface. The difference in elevation between the Diancang Shan and the base of Quaternary sedimentary rocks in the Erhai basin is ~2,500 m and yields a maximum relative vertical uplift rate of only ~l mm/yr along one of the largest normal faults in the Dali system. The cause of right-slip on the Jianshui, Qujiang, and possibly the Chuxiong faults is not immediately clear. All three faults had an older history of left-lateral displacement before right-slip occurred; some of the left-lateral displacement on the BenzilanZhongdian may also date from this time. The three faults are parallel to the Red River fault and could be regarded as faults that express the most northerly extent of right-lateral shear related to the Red River fault. A second interpretation is possible because the Jianshui and Qujiang faults are bounded by north-south–striking faults that belong to the Xianshuihe-Xiaojiang fault system. The right slip could be related to counterclockwise rotation of the crustal material between the left-lateral faults of the Xiaojiang fault zone, causing right slip on preexisting east-west–trending faults. We favor this latter interpretation because (1) right-lateral shear along the Red River fault during the past ~4 m.y. has been at a slow rate, (2) right slip on the Jianshui and Qujiang faults had its inception during the period of reduced activity on the Red River fault, (3) both faults end at active left slip, northsouth–striking faults that are not deformed by right-lateral move-

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Figure 50. Generalized tectonic model relating displacement along the Xianshuihe-Xiaojiang fault system to rotation of crustal material around a poorly defined point south of the eastern Himalayan syntaxis. The crustal material does not rotate as a rigid body but is strongly internally deformed. The rotation affects the active Sagaing fault and the Indo-Burman ranges.

ment, and (4) both faults are possibly bent by left-lateral shear. The Chuxiong fault is so poorly developed that we make no attempt to suggest a cause for its right slip. If the inception of the Dali fault system is Pliocene–early Quaternary, this is about the time that can be inferred for the beginning of the Xianshuihe-Xiaojiang fault system. A tentative conclusion is that the current system of faults in central Yunnan and southwestern Sichuan began about the same time and that they may be no older than Pliocene. Since that time they have dominated over the Red River fault system. The youthfulness of the present pattern of faults is perhaps surprising and suggests that short-term data from slip on active faults, from active deformation derived from earthquakes, and from geodetic studies cannot be projected back in geologic time more than a few million years. Not only are these fault systems young, but they may

evolve rapidly as shown by the relations between the XianshuiheXiaojiang and Red River fault system and the Red River and Dali fault systems. SYNTHESIS AND CONCLUSIONS The Xianshuihe-Xiaojiang, Red River, and Dali fault systems are three of the major active fault systems east of the eastern Himalayan syntaxis in the southeastern part of the Tibetan plateau and its foreland. These fault systems consist of a network of interrelated faults that partition the displacement of rocks during clockwise rotation relative to the syntaxis (or left-lateral displacement relative to South China). Evidence from these fault systems indicates that their faults are young and that the present pattern of deformation can be projected into the past only to ~4–5

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Figure 51. Diagram showing cumulative displacement from east to west across the Xianshuihe-Xiaojiang fault system at different places, and the deflection of the Red River fault at the intersection of the two fault systems. Total displacement on the Xianshuihe-Xiaojiang fault system appears to be approximately constant no matter how many splays the system contains. Displacement on individual faults appears to die out toward the Red River fault, but bending of the Red River fault by ~60 km suggests that the left-shear continues across the fault to the south. Compare to Figure 4 for location.

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Ma (see also Burchfiel and Royden, l991). The history of the three fault systems indicates that, in general, such fault systems may originate and evolve rapidly. In this particular case, the change in geometry and rates of deformation relates to the northward migration of the eastern Himalayan syntaxis during the past 4 m.y. A similar evolution can be shown for the older structures that now lie southeast of the syntaxis (Wang and Burchfiel, l997) and may be typical for other major fault systems in similar collisional settings. Because of the rapid evolution in this region and the problem of relating short-term data, such as active slip rates and earthquake data, to long-term data, we have tried the approach presented here for the Xianshuihe-Xiaojiang fault system to analyze its long-term (~ 5 m.y.) history and then compare it with our GPS studies of active crustal movements. The geology of southern Sichuan and western Yunnan permits determination of (1) total displacement on faults within the fault system, (2) time of fault initiation, (3) long-term slip rates, and (4) the interaction between faults within the system and with other geological structures. Such an approach is complementary and adds to short term data analyses. We conclude that by looking at the long-term (>60,000 yr) history of the fault system, short-term transient effects can be eliminated and alternate interpretations of short-term data may be resolved. This can be a problem, for example, in using earthquake data, where short-term data may be mistakenly interpreted to apply to long-term phenomena. Total left-lateral displacement on the Xianshuihe-Xiaojiang fault system is ~60 km throughout most of its 1,200-km length. Displacement, however, becomes partitioned on different faults, even though the total displacement remains approximately constant (Fig. 51). Mostly, partitioning occurs on different left-lateral fault strands that become more numerous from northwest to southeast along the fault system. Locally left-lateral displacement is transferred to shortening, such as to the thrust faults in the Tangdian Shan or Jiaozi Shan in the central part of the fault system. In the southern part of the fault system, displacement on discrete faults gradually dies out southward, within a distance of 100–200 km, until only small displacements of a few kilometers remain on fault strands that reach the Red River. Loss of fault displacement appears to take place by absorbing left slip in extensional structures along most of the fault strands; however, on the East Xiaojiang fault some displacement appears to be lost by bending of older structures and shortening across braided faults. Surprisingly, these two different types of deformation occur within ~20 km of each other. Even on single faults within the system contemporaneous left-slip, normal-slip, and thrust-slip motion occurs in close proximity, such as along the Qujing and Puduhe faults (Figs. 21, 24). Evidence from numerous extensional basins along the Xianshuihe-Xiaojiang fault system suggests that the fault system may be no older than ~2–4 Ma. With the exception of two basins, the sedimentary deposits in the basins are shown on geologic maps as Quaternary in age; however, we have been able to find only published paleontological data to support these ages for a few

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basins. The Quaternary rocks consist of coarse-grained strata, with abundant conglomerate, in contrast with the Pliocene rocks in the region that are characteristically fine-grained, coal-bearing strata. It can be argued that the changes in rock types are the result of climatic changes at the beginning of Quaternary time and are not related to the onset of extensional tectonism associated with left-slip faulting; however, the data suggest that while this may be partly true, the onset of extensional tectonism and related left-slip faulting occurred in late Pliocene to early Quaternary time. Pliocene rocks generally have a distribution beyond the extensional basins, and they lie above a regionally extensive sub-Pliocene erosion surface that was developed prior to left slip and associated extension and shortening. Exposure of Pliocene rocks within the few basins in which they occur is related to the preservation of more widely distributed rocks in structurally down-dropped basins, in contrast with Quaternary rocks that appear to have facies that can be related to basin-margin faults. In the Songming basin, Pliocene rocks are offset by the West Xiaojiang fault by an amount equal to the total magnitude on the fault, indicating that left-slip faulting began in late Pliocene or Quaternary time. Evidence also suggests that shortening related to leftslip faulting displaced the sub-Pliocene surface and its overlying Pliocene strata, for example, in the Tangdian Shan and possibly west of the Puduhe fault. All these relations indicate that left-slip faulting was under way in early Quaternary time and could have commenced somewhat earlier. It, however, followed deposition of Pliocene strata that are generally not dated well enough to determine what part of the Pliocene they represent. Thus we suggest an age of ~4–2 Ma for the inception of left-slip faulting. Accepting an age of 2–4 Ma for the beginning of left-slip faulting on the Xianshuihe-Xiaojiang fault system, long-term (2–4 m.y.) slip rates range from 2–8 mm/yr to 9–28 mm/yr on faults of small and large displacement respectively. Allen et al. (l991) found a short-term slip rate of 15 ± 5 mm/yr on the Xianshuihe fault that is consistent with our long-term results; our time range for the inception of faulting would yield 15–30 mm/yr for the long term slip rate on the Xianshuihe fault. The slower of the long-term slip rates (using 4 Ma as the time of initiation) is larger by a factor of ~2 compared with shortterm slip rates we have determined from our GPS studies in this region (King et al., l997), and with a slip-rate of 7 ± 2mm/yr determined by Wen et al. (1996) for the Xianshuihe fault in the area studied by Allen et al. (1991). There are at least three possible explanations for this difference: (1) the slip rates on the faults have changed and were higher during the early history of the faults, (2) the time of initiation of the faults is twice as old (~8 Ma) as that inferred from the geological data, or (3) the dating of the sediments of Pliocene and Quaternary age is incorrect. We have no evidence to support or reject the first explanation. If the second explanation is correct, it would indicate that the basins along the faults did not begin to form until the last half of the evolution of the faults. Why this should be the case is unknown, but the evidence from some faults shows that the offset of Pliocene rocks is the maximum offset on the fault. Examination of the

third explanation is in progress, but no resolution is possible at this time. Thus, the reason for this difference between long-term and short-term GPS slip rates remains unknown and is under study at this time. The Red River and Dali fault systems are dominated by rightslip and left-slip faults, respectively. In contrast to the XianshuiheXiaojiang fault system, most of the faults are parallel to older structures and total offsets cannot be uniquely determined. Extensional basins of several types are present along most of the faults. Sedimentary rocks related to basin development are Quaternary; in the rare basins where older Cenozoic rocks are present, the older sediments have a wider distribution and are unrelated to the basin. The data suggest that faulting on both systems began about the same time as on the Xianshuihe-Xiaojiang system. The Red River fault is the most prominent fault in the Red River fault system and has had a complex late Cenozoic history. The name Red River fault should be restricted to the young and active faults that can be traced from Dali southeast into Vietnam. Although its location may have been controlled by the crustal anisotropy of the strongly foliated rocks in the Ailao Shan, it should be considered as a separate structure from the mylonitic rocks of the older left-lateral Ailao Shan shear zone, at least until the relations between the two structures can be established. The oldest movement on faults that parallel the Red River is north to northeast thrusting of high-grade rocks of the Ailao Shan above poorly dated Eocene-Oligocene and Miocene conglomerate containing clasts of mylonitic rocks from the Ailao Shan shear zone. Thrust movement can be documented only eastward from Yuanjiang and is mainly pre-Pliocene in age. This deformation may be partly responsible for unroofing midcrustal rocks in the Ailao Shan shear zone, and it may be more logically assigned to a late stage of the development of the transpressional Ailao Shan shear zone. The oldest right-lateral movement that we regard as belonging to the Red River fault is along its northwestern part where displaced Triassic granite and Permian limestone are found in faultbounded slivers displaced 20–54 km. Most of this displacement is probably pre-Pliocene in age. How this displacement is related to the thrust faults southeast of Yuanjiang is currently unknown. Right-lateral faulting on the Red River fault continued into Pliocene and Quaternary time, but the total displacement during this period may be only 5–6 km and is shown by the offset of the Red River and its tributaries (Allen et al., l984). North-side-down normal faulting accompanied right slip along the northwestern part of the Red River fault. Along the southeastern part of the fault, local shortening accompanied right slip. This difference in fault displacement may be related to the regional tectonic position of the Red River fault. The young and active right-slip Jianshui, Qujiang, and Chuxiong faults lie ~50–75 km northeast of and parallel to the Red River fault. We do not regard the Jianshui and Qujiang faults as part of the Red River fault system. Rather they are interpreted to be right-slip faults formed by counterclockwise rotation of crustal material between left-lateral faults of the Xianshuihe-

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China Xiaojiang fault system. The Chuxiong fault is so poorly developed that the origin of its right-slip remains unclear. Faults of the Dali fault system all appear to be left-lateral. The northwest- and north- to northeast-striking faults show both young and active left-lateral displacement with variable components of normal-slip displacement and contain numerous associated sedimentary basins suggesting that the faults were initiated in late Pliocene to Quaternary time. Northwest-striking faults that branch, intersect, or curve into the north- to northeast-striking faults, with the exception of the Tongdian and Daju faults, have well-documented evidence for active left slip. Only on the northwest-striking Benzilan-Zhongdian fault can a total offset of ~30 km be determined. But even on this fault, how much of this displacement occurred in late Pliocene-Quaternary time, and how much may be older, remains unknown. All the left-slip faults within the Dali fault system are interpreted to result from clockwise rotation of small crustal fragments within a region of large-scale clockwise rotation bounded on the east by the Xianshuihe-Xiaojiang fault system and on the west by rightlateral shear along structures that lie to the west of the area covered here. These right-slip faults are directly related to the northward movement of India relative to Eurasia and South China. The Red River fault and faults of the Xianshuihe-Xiaojiang fault system have opposite senses of slip, and they bound a crustal fragment, the Chuan Dian fragment, which has been interpreted to have been extruded southeastward from north of the eastern Himalayan syntaxis. In contrast, our data indicate that during at least the past ~4 Ma the Chuan Dian fragment has been part of a larger crustal region that is rotating clockwise around the eastern Himalayan syntaxis and that the Red River fault is also being rotated clockwise. At the southeast end of the wedgeshaped Chuan Dian fragment, extension is the dominant style of deformation, not compression as would be expected for an extruded fragment. More important, the Xianshuihe-Xiaojiang fault system deforms the Red River fault by counterclockwise rotation. The rotation is equal to the total magnitude of left-shear on the fault system (Fig. 49), indicating that the XianshuiheXiaojiang fault system has been the dominant tectonic feature since its initiation. Most of the right-lateral displacement on the Red River fault preceded the major displacement on the Xianshuihe-Xiaojiang fault system in the area where they intersect. Relations in the area of intersection of the two fault systems also indicate that, although left-slip displacement on the faults of the Xianshuihe-Xiaojiang fault system becomes less southward, the total left-shear across the system remains constant. This indicates that the left shear is being accommodated by more or less continuous deformation rather than discontinuous deformation on faults. The fact that the Ailao Shan and part of the LanpingSimao fold belt also appear to be affected by the same left-shear lends support to the interpretation that left-shear passes southward across the Red River fault into southern Yunnan. The part of the Red River fault subjected to left-lateral shear rotates counterclockwise, and we interpret the active right slip and shortening within this segment of the fault to be caused by the rotation.

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On a more regional scale, we interpret movement of crustal material as a clockwise rotation about a generalized pole south of the eastern Himalayan syntaxis and bounded on the east by leftlateral shear on the Xianshuihe-Xiaojiang fault system. The leftlateral shear passes through the Red River fault into southern Yunnan and Indochina where the left-lateral Dien Bien Phu fault would be the counterpart of the Xianshuihe-Xiaojiang fault system (Fig. 50). West of these eastern-bounding faults there are other numerous left-slip faults in the region south of the Red River fault that contribute to the clockwise rotation (Fig. 51). Within this region of clockwise rotating crust, rotation appears reasonably uniform; however, there are clearly differences within this highly faulted region that are important. The clockwise rotation in the Dali fault system is on shorter faults that appear to rotate about a pole different from that of the Xianshuihe-Xiaojiang system. This difference may have caused the abundant extensional basins in the region of the Dali highland and along the southern part of the Xianshuihe-Xiaojiang fault zone and may also cause a small component of right-slip on the Red River fault. Rotation in this western part of the region south of the Red River fault system is expressed by numerous northeast-trending left-lateral faults, and movement of crustal material is absorbed at the subduction zone in Burma. In western Burma the clockwise rotation appears to be deforming generally north-striking structures, such as the Sagaing fault that records at least some of the present right-shear of India relative to South China. A study by Wu (l991) showed that the southern continuation of the active right-slip Nujiang fault in China becomes offset by northeaststriking left-slip faults in Burma, suggesting that the Nujiang fault in Burma may have taken up some of the India-Eurasia right-shear; it has recently become inactive, however, when the left-slip faults began to evolve. Some right-slip faults are present south of the Red River and Dali fault systems in southwestern Yunnan. They are interpreted to be the result of secondary counterclockwise rotation between the left-slip faults, as suggested for some of the faults in central Yunnan (see previous section). Assuming that the movement on left-slip faults yields the direction of motion of the adjacent crustal blocks, the western and eastern parts of the region south of the Red River and Dali fault systems have different directions of motion. In the west the motion is southwest, whereas in the eastern part of the Xianshuihe-Xiaojiang system it is almost directly south. This would imply an important component of east-west extension that is expressed by numerous small basins throughout the region. Part of the difference in motion may be due to the westward movement in Burma, which we ascribe to trench rollback at the Burma subduction zone, the effects of which die out to the southeast. The difference in motion may also result in right slip on some active northwest-trending faults. The interpretation we presented earlier indicates that during the past ~4 m.y. rotation of crustal material is accommodated by faults to the north and south of the Red River and Dali fault systems and by continuous (at some scale) deformation between the two faulted regions. It further suggests that in this area there has

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been little if any eastward extrusion of crustal material beyond the eastern limits of the Xianshuihe-Xiaojiang fault system and its southern continuation. The geological studies presented here were completed before we obtained results from our GPS network in southwestern Sichuan and western Yunnan that was established and first measured in 1993. We have data over a four-year time period for this network (measured in l993, l995, and 1997). GPS velocities relative to Chengdu (our base station), which is tentatively regarded as on stable South China, can be compared with the long-term rates for the Xianshuihe-Xiaojiang, Red River, and Dali fault systems; however, there are only a few localities where the two data sets can be compared directly (Fig. 52; King et al., 1997, and unpublished data). The GPS data show the following similarities with the geological data: (1)

clockwise rotation of crustal material around the eastern Himalayan syntaxis, (2) oblique convergence in the AnningheShimian segment of the fault system, (3) a southward decrease in velocity along the eastern part of the fault system in its central and southern segments, (4) a decrease in velocity along the northern part of the Xiaojiang fault where convergence in the Tangdian Shan was interpreted to absorb part of the southward movement along this part of the fault, (5) an increase in the velocity from east to west across the southern part of the Xianshuihe-Xiaojiang fault system, and (6) uniform velocity across the Red River fault. Although the velocities and directions of movement determined from the two data sets were comparable after the first two campaigns, our most recent unpublished GPS data covering a four-yr time period suggest short-term velocities that are slower

Figure 52. Comparison of short-term velocities determined by GPS measurements between l993 and l995 (upper numbers) and geologically determined long-term velocities (lower numbers, assuming either initiation of faults at 4 or 2 Ma, respectively) at the few locations where data can be directly compared. GPS velocities are relative to Chengdu. Arrows show direction (essentially the same for the geological and GPS determinations) and magnitude; scale is shown at lower left. Northernmost-opposing arrows along the Xiaojiang fault are for shortening in the Tangdian Shan. The 55 ± 5 mm/yr shown at the syntaxis is the velocity of India relative to Siberia. For details see King et al. (1997).

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China by a factor of ~2 than that predicted from long-term geological data. This difference is currently under study. The driving forces for the clockwise rotation around the eastern syntaxis are complex. We suggest the following interpretation: (1) Clockwise movement of material north and northeast of the syntaxis is driven by crustal thickening and gravitational potential energy; material moves from the region of high topography and thickened crust in Tibet toward the region of lower topography and thinner crust in southern Yunnan. Continued shortening within the Himalaya and regions to the north play a role in maintaining high topography and thick crust. (2) Rollback of the suducting slab beneath Burma, caused by a rate of subduction that exceeds the rate of convergence (for an analogous case, see Royden, l993), creates space into which material in southern Yunnan and northwestern Indochina can move. Western Burma has moved west since the corner of India moved past Burma because the Arakan fold belt bulges too far west to accommodate the passage of northeastern India. Thus the rotation east of the syntaxis and trench rollback beneath western Burma are complementary processes but probably decoupled within the shallow lithosphere along the Sagaing fault. This interpretation was derived from the geological results presented here. Recent geodynamic model studies by Royden (l996) and Royden et al. (1997) have suggested a pattern of deformation, similar to the one described here, that is a natural result of continental convergence where the lower crust is weak so that upper crustal motions are decoupled from the behavior of the lower crust and mantle. In addition, Royden (1996) and Royden et al. (1997) predict that the area east and southeast of the syntaxis should undergo significant clockwise rotation and associated extensional tectonism. We emphasize that our studies focused on long-term (2 km deep. Plate 6a looks north into the upper part of the gorge where small tributaries of the Jinsha River have cut into the plateau surface that is just visible at the right-hand side of the view. Also visible are remnants of the sub-Pliocene surface in the middle of the view that are on the opposite side of the Jinsha River gorge. Plate 6b is a view immediately to the right (east) of the view in Plate 6a and shows the subPliocene surface that forms a plateau at more than 2.5 km elevation. Quaternary sedimentary rocks underlie the village of Huize in the right center of the view. Hills on the plateau are underlain by strongly folded (with near vertical dips), resistant Upper Proterozoic and Paleozoic rocks that form part of the South China fold belts.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Plate 7. View looking east across the Xiaojiang River south of Dongchuan. The East Xiaojiang fault cuts across the ridge in the foreground (dotted line) and offsets the stream in the lower part of the view left-laterally. This is typical of offsets along the Xiaojiang fault in the Dongchuan area.

Plate 8. Looking west into the Xiaojiang River valley and across the Dongchuan basin. A major strand of the East Xiaojiang fault (dotted line) bounds the basin on the west side. Rivers that flow into the basin from the east are deflected to the south (to the left) and flow down the basin axis. The Xiaojiang River flows north (to right) and at a lower elevation than the basin floor. Its tributaries have breached the ridge along the fault and captured the south-flowing rivers in the basin.

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Plate 9. Looking east across the Xiaojiang River near Dongchuan. This view is typical of left-lateral displacement of streams along the East Xiaojiang fault (dotted line).

Plate 10. Looking north along the Puduhe fault (dotted line), which, south of Kunming (marked by smoke plume in upper center), bounds the west side of the Dian Chi basin. The sub-Pliocene erosion surface has been faulted up >1 km relative to its position below the Dian Chi basin. It is currently being eroded and forms the karst exposed in the left foreground.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Plate 11. Looking east across the Yuxi fault south of Dian Chi Lake. Active left-slip along the Yuxi fault has consistently deflected streams left-laterally from 100 to l,000 m, forming well-developed shutter ridges.

Plate 12. Looking east along the Jiaozi Shan thrust belt. The Carboniferous limestone on the top of the mountain at the upper left has been thrust south (right) above Jurassic red beds that form the base of the slope. Trace of the thrust is shown by the barbed line. The sub-Pliocene erosion surface forms the lowrelief surface south of the thrust belt. The Jiaozi thrust belt merges to the east with the Puduhe fault. The Puduhe fault lies on the east side (far side) of the distant ridge. Dian Chi Lake is located behind this ridge and is the ridge from which Plate 10 was taken.

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Plate 13. Looking west across the East Xiaojiang fault at the southern end of the Yiliang basin. Streams show consistent left-lateral deflections and shutter ridges are well developed. Dotted line shows position of the fault.

Plate 14. Exposure of the East Xiaojiang fault south of the Huaning basin showing well-developed subhorizontal slickenlines that are typical of the faults in the Xianshuihe-Xiaojiang fault system. Notebook in center of photograph for scale.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Plate 15. View to the east across the active left-slip Xinzhai fault (dotted line) in the Xiaozhi area. Streams are deflected left-laterally and shutter ridges are well developed along the fault.

Plate 16. View to the northwest across the Baipo fault near its southern end where streams are deflected left-laterally across the fault (dotted line).

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Plate 17. The sub-Pliocene and sub-Eocene erosion surfaces have different surface expressions in different places in the region around the southern part of the Xiaojiang fault system. a, The sub-Eocene surface shows a characteristic karst morphology where it is developed on Triassic limestone of the South China fold belt in the Pingyuanjie area (upper part of plate). The Triassic limestone (Trls) underlying the surface is strongly deformed (lower part of plate). b, View to north showing the sub-Pliocene surface on top of the mountains above the Xiaolongtan basin near Kaiyuan. The surface has gentle relief and merges with the basin from all sides, a characteristic of similar karst basins in the region not controlled by faults. The basin contains coal-bearing Neogene, probably Pliocene, sedimentary rocks. Two strands of the Xiaojiang fault enter from the two sides of the basin (dotted lines) in such a way as to indicate the basin is a pull-apart basin.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Plate 18. View to the north along the northern part of the Qujing fault in the Ciying area. The steep cliff at the base of the mountain marks a steeply north-dipping reverse fault (see text). Neogene rocks form the low foreground. Top of the mountain is formed by the sub-Pliocene erosion surface.

Plate 19. Looking east at the Qujing fault in the Luliang basin. The eastern margin of the basin is characterized by many down-dropped fault blocks, such as the low ridge above the buildings. The subPliocene erosion surface forms the top of the mountain. Dotted line shows the fault trace, but the dotted line is broken in the middle because the fault is behind the ridge and not visible.

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Plate 20. Looking east at the steep eastern side of the Mengzi basin that is formed by a normal fault. There is no evidence that this fault is active and its trace is not evident; thus the fault trace is not shown. The top of the mountain is formed by a subhorizontal erosion surface cut on Triassic limestone of the South China fold belt. In this region this surface is overlain by Eocene rocks, and we refer to it as the sub-Eocene surface. The relationship of this surface to the sub-Pliocene surface farther west is not known in this area.

Plate 21. Looking west at the western margin of the Mengzi basin characterized by several down-to-thebasin fault blocks that apparently preserve parts of the sub-Pliocene erosion surface (surface coincident with the top of the telephone pole in the foreground). The fault at the base of the mountains is hidden in the trees, thus its trace is not shown. The sub-Pliocene erosion surface forms the tops of the mountains in the clouds that lie >1,500 m above the basin floor.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Plate 22. Looking south along the Gejiu fault. The town of Gejiu lies in a narrow pull-apart basin below the sub-Pliocene erosion surface that caps the mountains on either side of the town. The basin has also been modified by karst activity and the southern part of the town has been built on a karst lake. Water entering this enclosed basin goes below ground, flows south, and reappears from a cavern at a lower elevation beyond the ridge at the south end of the town (see text for discussion).

Plate 23. Looking east across the northern part of the Yangwu fault. Streams in this area are offset leftlaterally along the fault (dotted line).

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Plate 24. View to the north across the West Xiaojiang fault that lies in the distance and passes through Yangzhonghai Lake (faintly visible in the upper center of the photograph). The prominent valley lies along the trace of a northeast-striking fault (dotted line) that is one of a family of faults that strikes northeast and terminates against north-south–trending left-lateral faults. Some faults of this strike have right-lateral displacements, and this family of faults is interpreted to be right-slip faults developed by counterclockwise rotation between north-striking left-slip faults (see text for discussion). The subPliocene erosion surface caps the mountains on both sides of the fault.

Plate 25. Looking north at left-lateral stream deflections and development of shutter ridges near the north end of the Nan Tinghe fault, 6 km northeast of Yuanxian county (see Fig. 28 for location). Dotted line shows location of fault.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Plate 26. Looking southwest at triangular facets along the Red River fault in the Yuanjiang area. The northern part of the Red River fault shows a component of young normal displacement as well as active right slip. The facets are cut on the metamorphic rocks of the Ailao Shan that form the mountains above the Red River valley.

Plate 27. Looking north across a northern strand of the Red River fault south of the Gejiu area. The Red River fault consists of at least two strands in this area. The northern strand (dotted line) appears to be the most active, typically with right-lateral stream deflections as shown here.

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Plate 28. Looking northwest across the Red River along the Ailao Shan range in the Jinbaoshan area. The gentle relief at the crest of the range (arrows) is formed by the sub-Pliocene erosion surface, which here is overlain by scattered outcrops of coal-bearing Pliocene sedimentary rocks.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Plate 29. Gneissic rocks with a well-developed mylonitic foliation exposed north of the Red River fault near Xinjie. Foliation and lineation are similar in orientation to the mylonitic rocks in the Ailao Shan south of the Red River fault. a, Looking vertically at the mylonitic foliation that in outcrop and thin section, show well-developed s/C fabrics that indicate left-lateral shear. b, The mylonitic rocks contain a penetrative sub-horizontal lineation.

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Plate 30. Looking north across the Jianshui fault (dotted line) in the Daqiao area. Streams are consistently offset right-laterally ~500 m with well-developed shutter ridges.

Plate 31. Right-lateral stream offsets and development of shutter ridges along the Jianshui fault in the area west of the Shiping basin. Fault is along the top of the terrace in the center of the photograph (dotted line). Streams are typically offset as much as as much as ~500 m.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Plate 32. Looking south into the Huanian basin at the west end of the Jianshui fault. The Jianshui fault trends west and follows the deep valley at the left center of the photograph with two vertical arrows. It intersects the Yangwu fault that trends north and follows the valley with the single vertical arrow. The two faults intersect near the village at the right side of the Huanian basin. Westward movement on the south side of the Jianshui fault is absorbed by folds that form ridges marked by oblique arrows (see Fig. 32 and discussion in the text).

Plate 33. Looking south into the Shiping basin partially filled by the shallow Yilong Lake. The Jianshui fault trends from lower right to upper left (dotted lines) at the base of the slope. Several elongate slivers of Devonian rocks lie along the fault zone and form low hills on which the silos are built. In the far distance the sub-Pliocene surface dips gently north into the basin. See Figure 31 for location and text for discussion.

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Plate 34. Strongly folded Neogene sedimentary rocks in the Jianshui basin at the eastern end of the Jianshui fault. Beds are cut by thrust faults and are locally overturned.

Plate 35. Looking southeast at the folded Neogene sedimentary rocks in the eastern part of Jianshui basin. The elongated hill in the middle ground is an actively forming anticline that folds a young erosion surface. Neogene rocks below the folded erosion surface are more strongly folded than the erosion surface, demonstrating the progressive growth of the folds. The en echelon pattern of the active folds shows continued growth due to right-lateral movement along the Jianshui fault.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Plate 36. Looking south across the Miandian fault that offsets a stream right-laterally in two places (see dotted lines). The largest stream offset is along the line of trees in the center of the photograph and a second smaller offset is present in the center foreground.

Plate 37. Looking west at the north-dipping thrust fault (dotted line) in the Malishu area where Triassic limestone (Trls) has been thrust south onto light-colored Neogene (Pliocene—N2)) sedimentary rocks (see Fig. 31 for location). The thrust fault merges eastward with the Qujing fault and thrust movement is likely associated with shortening at the intersection of the right-lateral Miandian and left-lateral Qujing faults.

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Plate 38. Looking north across the Qujiang fault at the west end of the Eíshan basin at a typical right-lateral stream deflection and shutter ridge development across the fault.

Plate 39. Looking north across the northern margin of the Quxi basin and the steep escarpment formed by the normal-slip component across the right-lateral Qujiang fault. Trace of two strands of the Qujiang fault are shown by the dotted lines. The stream that forms the prominent valley in the center of the photograph (arrow) is offset right-laterally across the fault and enters the Quxi basin at the location shown by the arrow on the left side of the photograph. Another smaller stream is offset at the right side of the photo. The ridge behind which the fault passes in the center of the photograph is formed by Pliocene lacustrine deposits.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Plate 40. Looking south at the steep southern margin of the Tonghai basin that is marked by a north-dipping normal fault. The youthfulness of the fault is indicated by the steep-sided canyons that have just begun to erode headward into the footwall. The gentle surface at the top of the escarpment is the subPliocene erosion surface that surrounds most of the Tonghai basin. The escarpment on the north side of the basin has a similar appearance.

Plate 41. Looking north across the northernmost part of the Red River fault (dotted line) in the Dingxiling area, 10 km northwest of the Midu basin. Right-lateral stream offsets along this part of the fault indicate that the fault is active.

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Plate 42. Looking south at the triangular facets along the normal fault that forms the northeast side of the Diancang Shan. Alluvial fans along the base of the steep slope are very small relative to the rivers that feed them, suggesting continued downward movement of the hanging wall of the fault. There is no evidence for lateral displacement along the fault at the base of the Diancang Shan adjacent to Erhai Lake.

Plate 43. Looking southeast along the southwest slope of the Diancang Shan. Two moderately southwest-dipping normal faults mark this side of the Diancang Shan (dotted lines). The gentle slope to the left side is the almost unmodified footwall of one of the normal faults. The surface marked by the long arrow on the left is at the top of the Diancang and may be the sub-Pliocene surface. The surface marked by the long arrow on the right between the two faults is similar to the surface on the top of the Diancang Shan and may be the sub-Pliocene erosion surface(?) down dropped by the normal faults.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Plate 44. View to the east at the eastern boundary of the Binchuan basin. A river cutting through Jurassic red beds is offset left-laterally along the Chenghai fault. The Chenghai fault also dips west (toward the viewer) and has a component of normal displacement.

Plate 45. Looking east across the Chenghai fault in the Jinjiang area where the Jinsha River is clearly offset left-laterally ~3 km.

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Plate 46. Looking west into down-dropped normal fault blocks at the northern end of the Chenghai fault. Two basins, located by the two arrows, containing Quaternary rocks are present at different elevations across two faults. The higher and closer Yongsheng basin is located by the arrow to the left, and the lower and more distant Jinyuan basin is located by the arrow to the right. The difference in elevation between the two basins is ~600 m. The Yongsheng basin has been down-dropped from a higher elevation from which the photograph was taken. The upland surface in the far distance is the sub-Pliocene surface.

Plate 47. Looking north in the same region as Plate 46 along the strand of the Chenghai fault that forms the eastern boundary of the lower Jinyuan basin. The fault consists of several west-dipping active splays that drop remnants of older subhorizontal surfaces (arrows) down to the west. The faults are marked by thick fault breccia and waterfalls. The footwall of these faults forms the west side of the tilted fault block that separates the Yongchen basin (above and to the right) from the Jinyuan basin.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Plate 48. Looking west at a typical left-lateral stream offset along the southern part of the Jianchuan fault (dotted line).

Plate 49. Looking north at the west end of the eastern segment of the Lijiang fault (dotted line). Normal displacement is demonstrated by the numerous steep fault scarps, whereas evidence for strike-slip displacement is poorly developed in the streams that cross the fault trace here.

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Plate 50. Looking east at the west-dipping normal fault that bounds the east side of the Heqing basin. Triangular facets are well developed and headward erosion on the scarp is in an early stage. The small alluvial fans at the base of the slope indicate rapid downward movement of the basin relative to the mountains.

Plate 51. Looking west at the western margin of the northern Lijiang basin. The Yulong Snow Mountains, the highest mountains in the region (>5,500 m), lie in the clouds and are underlain by metamorphosed Paleozoic rocks. The arrow points to a normal fault scarp within Holocene alluvial fans along the east side of the range.

Xianshuihe-Xiaojiang, Red River, and Dali fault systems, China

Plate 52. a, Looking east-southeast along the Tongdian fault at the east side of the Madeng basin. We interpret the steep escarpment along the east side of the basin to be mainly the result of the erosion along the inactive basin-margin fault following capture of the basin drainage in late Quaternary time. b, View to the southwest, just to the right of Plate 52a, showing the gentle northeastward slope into the basin. Hills in the center of the photograph are underlain by Mesozoic rocks from the Lanping-Simao tectonic unit present in the center of the basin and suggest that the basin fill is thin. The asymmetry of the basin suggests it is a sag basin tilted northeast into the Tongdian fault.

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Plate 53. Looking northwest across the Tongdian fault in the Madeng basin. A few rivers show left-lateral deflections such as shown here. Dotted line shows one trace of the fault that shows possible left-lateral movement.

Plate 54. View to the west of Pliocene coal-bearing rocks (N2) at the northwest end of the Madeng basin. These rocks have been folded along northeast-trending axes oblique to the northwest-trending Tongdian fault, a relationship that suggests left-lateral displacement along the fault. Holocene or late Pleistocene sedimentary rocks (Q) rest unconformably on the folded Pliocene rocks dating their deformation as late Pliocene or Quaternary.

ACKNOWLEDGMENTS This study was a cooperative effort between scientists at the Institute of Geology, Kunming, the Chengdu Institute of Geology and Mineral Resources, Chengdu, and the Massachusetts Institute of Technology, with support from the Institute of Geology, Kunming, and the Chengdu Institute of Geology and Mineral Resources, Chengdu. In particular, the directors of these institutions, Dr. Liu Baojun in Chengdu, and Wang Zuguan in Kunming, have given continual support and encouragement. The following scientists are also gratefully acknowledged: Pan Guitang and Liu Yiping in Chengdu. Over the years we have received exceptional support for this and several other projects from Li Tingdong, Xu

Baowen, and Xiao Xuchang at the Ministry of Geology, Beijing. Support from the American side was provided by National Science Foundation grants EAR8904096, EAR9614970, and INT 9005305, and National Aeronautics and Space Administration grants NAGW-2155 and SENH-0046, awarded to B. C. Burchfiel and L. H. Royden. The manuscript was carefully reviewed by K. Burke and C. Sengor who provided important suggestions for improvement; their help is gratefully acknowledged. REFERENCES CITED Allen, C. R., Gillespie, A. R., Han, Y., Sieh, K. E., Zhang, B., and Zhu, C., 1984, Red River and associated Yunnan province, China: Quaternary geology, slip rates, and seismic hazard: Geological Society of America Bulletin, v. 95, p. 686–700.

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Late Cenozoic Xianshuihe-Xiaojiang, Red River, and Dali Fault Systems of Southwestern Sichuan and Central Yunnan, China E. Wang, B. C. Burchfiel, L. H. Royden, Chen Liangzhong, Chen Jishen, Li Wenxin, Chen Zhiliang Contents Abstract . . . . . ... . .... . . . . . ....... . .. . . .. ... .. .. ... . . .. . .. . ...... . ....... . . .. .. 1 Introduction . .... . .... . ... ... . . . . . . .. .. ... .. . ..... . . .... . .. ... . . .... . ......... . 2 Regional tectonic setting . .. . ... . ............. . .... . .... . . . .. . ........... .. .. .... . 3 Xianshuihe-Xiaojiang fault system .... .. . .. .... . ..... . . . ................. . ... . ..... . 6 Geologic setting .......... . ....... . .. . .. . ... .. . . . . ............. ... ..... . .... 6 Xianshuihe segment ...... . ....... . .. .. .... . . . ... . . . ........... . .... . ... ... 12 Central segment ...................... . .... . ......... . .................... 13 Xiaojiang segment ................................................ . ........ 16 Discussion of the Xianshuihe-Xiaojiang fault system . . . . . .. . ............ . ........... 37 Red River and Dali fault systems .... . ................... . ......................... 39 Geologic setting .... . . : ................ . ............... . ................... 40 Red River fault system ...................................................... 44 Dali fault system .. . ......................... . .... . ........................ 62 Discussion of the Red River and Dali fault systems ............................... . 69 Synthesis and conclusions .......... .. ..... . ..................... . ............... 72 Acknowledgments ....... . .................. . .... . ... .. ................. . ..... 106 References cited ...... . ..... . .............. . . .... . .. . .. ...... . ........... .. .. 106 ISBN 0-8137-2327-2