Evolution of an Andean margin: a tectonic and magmatic view from the Andes to the Neuquén Basin (35 degrees-39 degrees S lat)

Contents Dedication: Pablo Groeber (1885–1964) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...

Author: Suzanne Mahlburg Kay | Víctor A. Ramos

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Contents Dedication: Pablo Groeber (1885–1964) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii 1. Overview of the tectonic evolution of the southern Central Andes of Mendoza and Neuquén (35 °–39°S latitude) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 V.A. Ramos and S.M. Kay 2. Upper Cretaceous to Holocene magmatism and evidence for transient Miocene shallowing of the Andean subduction zone under the northern Neuquén Basin . . . . . . . . . . . . . 19 S.M. Kay, W.M. Burns, P. Copeland, and O. Mancilla 3. Deep seismic images of the Southern Andes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 X. Yuan, G. Asch, K. Bataille, G. Bock, M. Bohm, H. Echtler, R. Kind, O. Oncken, and I. Wölbern 4. Neogene tectonic evolution of the Neuquén Andes western flank (37–39°S) . . . . . . . . . . . . . . . . 73 D. Melnick, M. Rosenau, A. Folguera, and H. Echtler 5. Intraplate deformation in the Neuquén Embayment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 A. Mosquera and V.A. Ramos 6. Structural evolution and magmatic characteristics of the Agrio fold-and-thrust belt. . . . . . . . . 125 G. Zamora Valcarce, T. Zapata, D. del Pino, and A. Ansa 7. Synrift geometry of the Neuquén Basin in northeastern Neuquén Province, Argentina . . . . . . 147 E. Cristallini, G. Bottesi, A. Gavarrino, L. Rodríguez, R. Tomezzoli, and R. Comeron 8. The case for extensional tectonics in the Oligocene-Miocene Southern Andes as recorded in the Cura Mallín basin (36 °–38°S) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 W.M. Burns, T.E. Jordan, P. Copeland, and S.A. Kelley 9. Early to middle Miocene backarc magmas of the Neuquén Basin: Geochemical consequences of slab shallowing and the westward drift of South America . . . . . . . . . . . . . . . . 185 S.M. Kay and P. Copeland 10. Evolution of the late Miocene Chachahuén volcanic complex at 37°S over a transient shallow subduction zone under the Neuquén Andes . . . . . . . . . . . . . . . . . . . . . 215 S.M. Kay, O. Mancilla, and P. Copeland 11. Miocene to Quaternary deformation of the Guañacos fold-and-thrust belt in the Neuquén Andes between 37 ° and 37 °30’S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 A. Folguera, V.A. Ramos, E.F. González Díaz, and R. Hermanns iii

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Contents 12. Late Cenozoic extension and the evolution of the Neuquén Andes . . . . . . . . . . . . . . . . . . . . . . . 267 A. Folguera, T. Zapata, and V.A. Ramos 13. Upper Pliocene to Lower Pleistocene volcanic complexes and Upper Neogene deformation in the south-central Andes (36°30’–38 °S). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 F. Miranda, A. Folguera, P.R. Leal, J.A. Naranjo, and A. Pesce 14. The Pliocene to Quaternary narrowing of the Southern Andean volcanic arc between 37° and 41°S latitude. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 L.E. Lara and A. Folguera 15. Geochemistry and isotopic characteristics of the Caviahue-Copahue volcanic complex, Province of Neuquén, Argentina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 J.C. Varekamp, J. Maarten deMoor, M.D. Merrill, A.S. Colvin, A.R. Goss, P.Z. Vroon, and D.R. Hilton Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343

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Geological Society of America Special Papers Dedication Pablo Groeber (1885−1964) Geological Society of America Geological Society of America Special Papers 2006;407;v doi: 10.1130/0-8137-2407-4.v

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Geological Society of America


Dedication Pablo Groeber (1885–1964)

The outstanding characteristics of the Andes of Neuquén and Mendoza were studied for more than 50 years by Dr. Pablo Groeber, who, through his pioneering work, established the first understanding of the geology of this part of the Andes. This German geologist, who received his doctorate degree in geology at the University of München, Germany, in 1897, arrived in Argentina when he was 26 years old. Prior to his arrival in Argentina, he conducted two large expeditions to the central Tien Shan, where he spent more than two years establishing the basis of the modern structural and stratigraphic knowledge of this remote and, at this time, unknown region of central Asia. Soon after being hired by the Geological Survey of Argentina, Groeber began one of the most creative investigations of the Andes of Argentina and Chile. He mapped vast areas of the cordillera, surveying his own topography and producing structural sections and sketches of the most remote parts of the region. He set up much of the present stratigraphic framework of Neuquén and Mendoza, and documented the basic structures and magmatic cycles related to the deformation and uplift of the Cordillera de Los Andes. He also recognized the main depositional sequences, the different uplift pulses, and the migrations in volcanic activity. In those years before isotopic ages were available, he identified and assigned ages to a series of basaltic and andesitic volcanic episodes associated with different Cenozoic diastrophic phases. Groeber wrote fundamental papers that led the way to the present state of knowledge in the region. His publications on the high Cordillera de Los Andes of Mendoza and Neuquén, on the structural evolution of the Neuquén Basin, and his geologic maps along the 70°W meridian form the basis of the modern geology of the region. The reader will find references to Groeber and mention of his seminal proposals in a number of chapters in this volume. During his tenure with the survey, Groeber also was teaching in the Universities of Buenos Aires and La Plata, where he strongly influenced several generations of geologists and supervised several doctoral theses. For a number of decades, his work was the main reference on the geological setting and evolution of the south central Andes. His disciples continued his work, improved his schemes, and refined the correlations of the geologic units, but the main tectonic framework that he proposed in the 1950s is still the core of our understanding of this sector of the Andes. v


Geological Society of America Special Papers Preface Suzanne Mahlburg Kay and Víctor A. Ramos Geological Society of America Special Papers 2006;407;vii-x doi: 10.1130/0-8137-2407-4.vii



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Preface The Andes are the type example of an accretionary orogen that has developed without a continental collision. In an accretionary orogen, magmatism occurs over a subducting slab and is associated with episodes of contractional and extensional deformation that are explained by the relative rollback velocity of the subducting slab, episodes of shallowing and steepening of the subduction zone, and changes in convergence parameters. In this framework, the evolution of the northernmost Patagonian and southernmost Central Andean Cordillera between 35°S and 39°S along with the region to the east (retroarc) is a virtual endmember in understanding the development of an Andean-type accretionary orogeny. In contrast to the general conception of an Andean orogen, the Tertiary to Holocene evolution of this part of the Andes is generally characterized by basaltic to andesitic arc volcanic centers, relatively low relief, both periods of compression and extension, a relatively small amount of crustal shortening, and extensive retroarc mafic volcanism. This evolution stands in marked contrast to a typical Andean model that is strongly influenced by the Neogene history of the central most Andes; a region that constitutes only a fraction of the length of the long-lived orogen and a timeframe that constitutes only a part of its Jurassic to Holocene history. Unlike the southernmost Central Andes, the evolution of the central most Central Andes is marked by andesitic to dacitic stratovolcanoes and gigantic ignimbrites complexes, large amounts of contractional deformation east of the arc that have resulted in extensive crustal shortening, a relatively silicic crust whose mafic part could have been removed by delamination, and the dramatic uplift of one of the world’s largest and highest plateaus— the Puna-Altiplano. A major objective of this volume is to examine the Tertiary to Holocene tectonic and magmatic evolution from the arc to the retroarc in the distinctive and until recently little understood end-member of the Andean accretionary orogen between 35°S and 39°S. The Tertiary to Holocene evolution of the eastern slope and retroarc of the Andes in the Argentine provinces of Neuquén and Mendoza between 35°S and 39°S has been strongly influenced by the Mesozoic development of the Neuquén Basin, which is one of the most productive and historically important hydrocarbon basins in South America. The literature on the region has concentrated heavily on the late Triassic to Paleocene evolution of the basin as the subsequent history has traditionally been of less interest to the petroleum industry. The detailed studies of the region have emphasized the sedimentary aspects of the Triassic to Early Jurassic rifting history of the basin associated with the breakup of Pangea and its subsequent

Mesozoic evolution as an Andean foreland basin. Numerous other papers have dealt with the richness of the Mesozoic fossil assemblages that have yielded some of the world’s most spectacular ammonite, dinosaur and vertebrate fossils. This picture has changed in the past decade due to several initiatives financed by the petroleum industry and government funding agencies. The impetus has come from the role of postMesozoic deformation and magmatism in modifying the Mesozoic configuration of the Neuquén basin and in controlling petroleum migration. Another contributing factor has been that large numbers of oil wells have been drilled through postCretaceous magmatic flows. Questions have been posed concerning the style and effects of Tertiary to Holocene deformation, the age and origin of the extensive retroarc volcanic rocks, and the tectonic controls of deformation and magmatism. A paucity of post-Paleocene sedimentary strata has led to studies that emphasize magmatism and deformation as the keys to understanding the tectonic evolution of this region whose history reflects the development of the long-lived Andean margin. The objective of this volume is to present new data and ideas that have emerged relative to the post-Mesozoic evolution of the Neuquén Basin. These studies have led to major steps in understanding the arc and retroarc evolution of this distinctive segment of the Andes. The papers in this volume constitute a complement to the literature on the young volcanic centers of the Southern Volcanic Zone arc, and to a volume titled The Neuquén Basin: A case study in sequence stratigraphy and basin dynamics (published by the Geological Society of London). The papers in the later volume emphasize the Mesozoic stratigraphy, sedimentary geology, biostratigraphy, paleoecology, paleogeography and deformational history of the Neuquén Basin. Among the results in the chapters of this volume are major revisions in the timing of Mesozoic and Cenozoic magmatic and deformational events in the arc and retroarc, in models for relating the arc and retroarc magmatism and the extensional and contractional deformational history to plate convergence vectors, and in the importance of pre-Tertiary structures in controlling younger deformational styles. Proposals are advanced for transient Miocene shallowing of the Andean subduction zone in association with development of Laramide-style magmatism and uplift far east of the trench, and for an association of Pliocene to Quaternary arc to retroarc extension and retroarc mafic magmatism in the Payenia Large Igneous Province with steepening of the subducting slab over a hydrated mantle. The first chapter, by V.A. Ramos and S.M. Kay, presents an overview of the Mesozoic to Holocene tectonic evolution of vii


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the Andes between 35°S and 39°S with an emphasis on the southern Mendoza and Neuquén provinces of Argentina. The paper presents a synthesis of the current state of knowledge on the structural evolution of the region and draws attention to the importance of the control of older structural and lithospheric scale features, shifting convergence parameters, and changing slab configurations on the spatial and temporal evolution of the region. The second chapter, by S.M. Kay, W.M. Burns, P. Copeland, and O. Mancilla, is complementary to the first chapter in presenting a synthesis of the Upper Cretaceous to Holocene magmatic evolution of the Neuquén basin between 36°S and 38° S and in presenting the evidence for Kay’s original suggestion for a transient Miocene shallow subduction north of the Cortaderas lineament. A survey of the spatial and temporal distribution of the magmatic rocks along with new major and trace element analyses (90 samples), new isotopic data (12 samples), and 12 new 40Ar/ 39Ar ages are used to argue two major points. (1) The configuration of the subduction zone from the trench to the arc has been little modified since the Upper Cretaceous. (2) A transient period of shallow subduction that began at ca. 20 Ma and peaked in the late Miocene was followed by Pliocene steepening that led to eruption of widespread mafic magmas that form the backarc Payenia Large Igneous Province. The southern margin of the shallowly dipping subduction zone is argued to be near the Cortaderas lineament that essentially bounds the limit of Neogene retroarc magmatism. The third chapter, by X. Yuan, G. Asch, K. Bataille, and coworkers at the GeoForschungsZentrum in Potsdam, Germany, reports some of the initial results of the TIC-TAC project whose primary aim is a study from the Chile trench across the forearc to the main Andean cordillera at this latitude. The paper presents some of the first deep seismic images of the southern Andes between latitude 36° and 40°S. The results, which are from receiver function images, show the oceanic Moho of the subducted Nazca plate imaged down to a depth of ~100 km in good correspondence with results from Wadati-Benioff zone seismicity and wide-angle seismic reflections, the continental Moho at a depth of ~40 km beneath the Main Cordillera, and the eastward shallowing of the Moho to ~35 km beneath the western Neuquén basin. An intriguing result shows the continental Moho locally shallowing to an apparent depth of 30 km beneath the Loncopué graben on the eastern slope of the Andes where Pliocene to Holocene extension is concentrated. The fourth chapter, by D. Melnick, M. Rosenau, A. Folguera, and H. Echtler, uses new field observations and the published literature to discuss the Neogene evolution of the western flank of the Andes in Chile between 37°S to 39°S. This evolution is marked by: (1) Oligocene to middle Miocene extension followed by late Miocene shortening coincident with uplift, exhumation, inversion, and a volcanic gap in the Main Cordillera; (2) Pliocene to early Pleistocene extension with reestablishment of the arc and transtension along the intra-arc zone; and (3) late Pleistocene to Holocene narrowing of the arc and localized extension-transtension along the axial intra-arc

zone. The main uplift of the cordillera is placed between 11 and 6 Ma. The cessation of contraction is argued to be linked to an increase in slab angle that also triggered extension along the orogenic front and onset of arc-parallel strike-slip faulting. The crustal-scale dextral strike-slip Liquiñe-Ofqui fault zone, which concentrates deformation south of 38°S and helps to accommodate oblique subduction is reviewed and analyzed. The fifth chapter, by A. Mosquera and V.A. Ramos, presents a synthesis of intraplate deformation in the Neuquén Basin based on previously unpublished two- and three-dimensional seismic images. The discussion emphasizes: (1) the role of Paleozoic basement fabrics in the development of Mesozoic and Cenozoic deformational fabrics, (2) the important role of fabrics inherited from the Late Permian collision of an allochthonous Patagonian terrane whose suture with Gondwana is argued to be under the Neuquén Basin, (3) how the sequence and location of uplifts, inversion of half-graben systems, and strike-slip faults can be used to demonstrate a shifting spatial and temporal pattern of the main stresses across the region, and (4) how the changing deformational pattern can be linked to changes in the convergence parameters between the South America (Gondwana) and oceanic plates to the west. The sixth chapter, by G. Zamora Valcarce, T. Zapata, D. del Pino, and A. Ansa, shows that the Agrio fold-and-thrust belt in the retroarc between 37°S and 38°S was subjected to major contractional deformation in the Late Cretaceous and the Miocene. The paper presents a description of the Agrio belt, crucial new 40Ar/ 39Ar ages that for the first time show the importance of Late Cretaceous contractional deformation in the Agrio belt, and new geochemical data on the Cretaceous to Eocene volcanic rocks from which the 40Ar/ 39Ar ages were obtained. The seventh chapter, by E. Cristallini, G. Bottesi, A Gavarrino, L. Rodríquez, R. Tomezzoli, and R. Comeron, analyzes the syn-rift geometry of the northeastern part of the Neuquén Basin in Neuquén Province using seismic data and a discrete element modeling approach. They present a regional map showing the half-graben faults and transfer zones that developed during the Triassic to lower Jurassic synrift stage. They argue that these faults result from differential subsidence and that the anticlines and synclines associated with them are not due to later tectonic inversion as is the case further west in the Neuquén basin. The eighth and ninth chapters consider aspects of late Oligocene to middle Miocene arc and retroarc deformation and magmatism. The eighth chapter, by W.M. Burns, T.E. Jordan, P. Copeland, and S.A. Kelley, presents a synthesis of the Cura-Mallín basin which is one in a chain of late Oligocene to early Miocene intra-arc sedimentary basins that formed along the Andes between 33° and 43°S. Facies variations, stratal thickness patterns, structural analyses, apatite and zircon fission-track and 40Ar/ 39Ar ages are used to argue that the basin formed by normal faulting, with little or no strike-slip influence. The model is argued to apply to the entire chain of intra-arc basins. The ninth chapter, by S.M. Kay and P. Copeland, presents a synthesis of


Preface early to middle Miocene backarc magmatism between 36°S and 37°S with new major, trace element and isotopic data, and 40Ar/ 39Ar ages. The lack of an arc chemical signature in 24 to 20 Ma alkaline magmas and its appearance in amphibole-bearing mafic lavas after 20 Ma is linked to the initial shallowing of the subducting Nazca plate. This information is put together with the space-time distribution and chemical-isotopic characteristics of Oligocene to middle Miocene retroarc magmas between 33° and 43°S to argue that a regional change from extensional to contractional deformation after 20 Ma is best attributed to an accelerated rate of westward drift of South America over the underlying mantle. The tenth chapter, by S.M. Kay, O. Mancilla, and P. Copeland, examines the evolution of the late Miocene Chachahuén volcanic complex near 37°S that is located some 500 km east of the modern Chile trench. A detailed analyses of mineralogical, geochemical and isotopic data and 40Ar/39Ar ages along with previously unpublished K/Ar ages and mapping shows that the magmatic evolution of this 7.3–4.8 Ma hornblende-bearing basaltic to dacitic nested caldera complex is best explained by formation during a latest Miocene peak in a transient shallowing of a portion of the subducting Nazca plate under the northern Neuquén Basin. The uplift of the Sierra de Chachahuén is also argued to have occurred at this time. Attention is drawn to chemical and structural parallels with similar age volcanic complexes and uplifts far to the east of the trench over the modern Chilean flatslab region between 28°S and 33°S. The cause of transient shallow subduction could be subduction of a small oceanic plateau. Chapters eleven to thirteen principally deal with the Pliocene to Holocene tectonic evolution of the Main Cordillera and the eastern slope of the Neuquén Andes. In chapter eleven, A. Folguera, V.A. Ramos, E.F. González Díaz, and R. Hermanns present five structural transects between 37°S and 37°30´S that demonstrate the existence of late Miocene to Quaternary folds and thrusts that were previously largely unrecognized on the eastern slope of the Andes. They call this region the Guañacos fold and thrust belt and demonstrate that contractional inversion of Oligocene to Miocene extensional structures has affected even Quaternary volcanic rocks in the present orogenic front that is located immediately east of the Southern Volcanic Zone arc. The inversion is mechanically linked with the La Laja strike-slip fault system in the intra-arc in Chile. In chapter twelve, A. Folguera, T. Zapata, and V.A. Ramos describe the contrasting Pliocene to Holocene deformational styles and topographic features north and south of 37.5°S on the eastern slope of the Andes between 36°S and 39°S. They use published data along with new field and seismic evidence to discuss four extensional depocenters that developed contemporaneously with the Guañacos fold-and-thrust belt. They argue that these contrasting deformational styles, which are difficult to reconcile with the nearly constant convergence parameters and slab configuration along the modern margin, can be reconciled with different rates of trench roll back in response to different late

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Miocene to Holocene changes in Wadati-Benioff zone geometry north and south of 37.5°S . The thirteenth chapter, by F. Miranda, A. Folguera, P.R. Leal, J.A. Naranjo, and A. Pesce, deals with the age of volcanism and deformation in the upper Pliocene to lower Quaternary volcanic chain east of the modern Andean front between 36.5° and 38°S. The authors present new K/Ar ages, generally bracket a Miocene deformation event between 9 and 6 Ma, and show that the volcanic centers erupted in two contrasting structural settings. One group is associated with north northwest-trending contractional faults and the other with northeast-trending extensional faults. The difference in structural styles can be explained by strain partitioning in the region. The fourteenth and fifteenth chapters deal with volcanism in the Pliocene to Quaternary Southern Volcanic Zone arc. In chapter fourteen, L.E. Lara and A. Folguera present an overview of the complex arc-backarc magmatic system that developed on the western margin of the Neuquén Basin during the late Cenozoic. They use new 40Ar/39Ar ages, compiled K/Ar ages and geochemical data, and the regional tectonic framework to argue that paired volcanic belts south of 38°S that have been used as evidence for a westwardly migrating volcanic front actually reflect arc broadening followed by narrowing. In their view, the arc front remained stationary. The narrowing of the arc is argued to correlate with a decrease in plate convergence rate and dextral transpression. The last chapter, by J.C. Varekamp, J. Maarten de Moor, M.D. Merrill, A.S. Colvin, A.R. Goss, P.Z. Vroon, and D.R. Hilton, presents a detailed look at the geochemical and isotopic evolution of the Pliocene to Holocene Caviahue-Copahue volcanic complex, which sits at the junction where the eastern Pliocene chain diverges from the main Southern Volcanic Zone arc. The paper presents a large new data set with major and trace elements and Pb, Sr, Nd, and He isotopic data. The authors argue that chemical and isotopic changes between the Pliocene Caviahue complex and the Pleistocene Copahue volcano require a change in magma source components and a change from a fluid fluxing melt regime to a sediment melting regime. They note that the nature of mantle melting could have switched from flux melting to decompressional melting with a change to an extensional regime in the region. The comprehensive picture of the structural, magmatic, sedimentological, and tectonic evolution of the Andes between 35°S and 39°S that emerges through these chapters provides a new framework and perspective on the evolution of the distinctive Andean-type accretionary orogen in this region. The conclusions have major implications for the Cenozoic evolution of the Neuquén foreland basin. ACKNOWLEDGMENTS The editors would like to sincerely thank the reviewers for their constructive reviews and comments on the papers. In alphabetical order, the reviewers were: Adriana Bermúdez (Universidad Nacional del Comahue, Argentina), Richard All-


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mendinger (Cornell University, USA), Susan Beck (University of Arizona, USA), Ben Brooks (University of Hawaii, USA), Mathew Burns (U.S. Geological Survey, USA), Carlos Costa (Universidad Nacional de San Luis, Argentina), Peter Cobbold (University of Rennes, France), Beatriz Coira (Universidad Nacional de Jujuy, Argentina), John Davidson (Durham University, UK), Gloria Eisenstadt (University of Texas at Arlington, USA), Luis Fauque (SEGEMAR, Argentina), Todd Feeley (Montana State University, USA), Estanislao Godoy (SERNAGEOMIN, Chile), Brian Horton (University of California at Los Angeles, USA), Robert Kay (Cornell University, USA), Estanislao Kozlowski (Pan American Oil, Argentina), Eduardo

Llambías (Universidad de La Plata, Argentina), Leopoldo López Escobar (Universidad de Chile, Concepción, Chile), Andrew Meigs (Oregon State University, USA), Constantino Mpodozis (Sipetrol, Chile), Jorge Muñoz (SERNAGEOMIN, Chile), Eric Sandvol (University of Missouri, USA), Charles Stern (University of Colorado, USA), Anthony Tankard (Consulting Geologist, Canada), Stuart Thompson (Yale University, USA), Gustavo Vergani (Repsol-YPF, Argentina), and Jan Witte (Wintershall Energía, SA, Argentina). Suzanne Mahlburg Kay Víctor A. Ramos


Geological Society of America Special Papers Overview of the tectonic evolution of the southern Central Andes of Mendoza and Neuquén (35° −39°S latitude) Victor A. Ramos and Suzanne Mahlburg Kay Geological Society of America Special Papers 2006;407;1-17 doi: 10.1130/2006.2407(01)



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Geological Society of America Special Paper 407 2006

Overview of the tectonic evolution of the southern Central Andes of Mendoza and Neuquén (35°–39°S latitude) Víctor A. Ramos* Laboratorio de Tectónica Andina, Universidad de Buenos Aires, Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos Aires, Argentina Suzanne Mahlburg Kay* Institute for the Study of the Continents and Department of Earth and Atmospheric Sciences, Snee Hall, Cornell University, Ithaca, New York 14853, USA

ABSTRACT The southern Central Andes of Argentina between 35° and 39°S latitude can be divided into two sectors with contrasting geological histories. The boundary between the sectors coincides with the Cortaderas lineament. North of the Cortaderas lineament, the Andes record a foreland expansion of arc magmatism that is associated with contractional deformation in the Malargüe fold-and-thrust belt, and subsidence of the Río Grande foreland basin between 15 and 5 Ma. The peak expansion of deformation into the foreland occurred as late Miocene magmatic arc rocks erupted more than 500 km east of the trench and the San Rafael basement block was uplifted in central Mendoza. This stage was followed by the collapse of the basement uplift by normal faulting, the retreat of the magmatic arc, and the eruption of widespread late Pliocene to early Pleistocene within-plate lava flows in the Payenia region. Extensive Quaternary calderas and rhyolitic domes along the axis of the main Andes reflect crustal melting associated with basaltic underplating. In contrast, the structural evolution of the sector south of the Cortaderas lineament is dominated by the Late Cretaceous development of the Agrio fold-and-thrust belt, which underwent minor reactivations in the Eocene and the late Miocene. The post-Miocene Guañacos fold-and-thrust belt that has since developed along the axis of the main Andes concentrates neotectonic contraction. Arc magmatism in this sector is largely restricted to the axial area of the Andes. Both the sectors north and south of the Cortaderas lineament show evidence of an important episode of extension during the Oligocene to early Miocene, and for renewed extension in the Pliocene and the Pleistocene. The contrasting geological histories north and south of the Cortaderas lineament reflect differences in the geometry of the subducting plate, variations in crustal rheologies inherited from a more restricted distribution of Mesozoic rifts in the northern than the southern segment, and variations in the trench roll-back velocity through time. Keywords: Andes, Neuquén, Malargüe, Agrio, Payenia, flat-subduction, extension. *E-mails: [email protected]; [email protected]. Ramos, V.A., and Kay, S.M., 2006, Overview of the tectonic evolution of the southern Central Andes of Mendoza and Neuquén (35°–39°S latitude), in Kay, S.M., and Ramos, V.A., eds., Evolution of an Andean margin: A tectonic and magmatic view from the Andes to the Neuquén Basin (35°–39°S latitude): Geological Society of America Special Paper 407, p. 1–17, doi: 10.1130/2006.2407(01). For permission to copy, contact [email protected]. ©2006 Geological Society of America. All rights reserved.

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V.A. Ramos and S.M. Kay

INTRODUCTION The various papers in this volume show that the segment of the Andean Cordillera between 35° and 39°S has distinctive characteristics when compared with other parts of the ArgentineChilean Andes. Most of the Andean chain is under compression, and the present orogenic front is located between a retroarc fold-and-thrust belt and an undeformed foreland region. Magmatic rocks are generally concentrated along an active magmatic arc, except in areas of shallow subduction where volcanism is absent (e.g., Jordan et al., 1983; Kay et al., 1988). In contrast, the segment discussed here (Fig. 1) has some peculiarities, including: (1) a large amount of late Cenozoic magmatic activity in the foreland region (Bermúdez et al., 1993); (2) an active thrust front located west of an inactive Late Cretaceous to Miocene fold-and-thrust belt (Folguera et al., 2004; Ramos and Folguera, 2005); and (3) widespread Pliocene and Pleistocene extension (Polanski, 1963; González Díaz, 1964; Kozlowski et al., 1993; Folguera and Ramos, 2000, 2001, 2002). The objective of this contribution is to present a synthesis of the tectonic evolution of the Neuquén and southern Mendoza segments of the Andes between 35° and 39°S based on information in recent publications and in the chapters in this volume. This synthesis shows that geologic observations in the northern part of the region fit with steepening of the subduction zone fol-

lowing a short period of flat-slab subduction during the Miocene as proposed by Kay (2001a, 2001b, 2002; Kay et al., this volume, chapter 2) and that those in the southern part fit with an oscillatory behavior of the subduction zone. MAIN GEOLOGICAL FEATURES The Andes between 35° and 39°S can be divided into two regions with distinctive evolutionary characteristics. These regions are bounded by a northwest-trending structural feature that was first described by Groeber (1938) and is referred to as the Cortaderas lineament (Ramos, 1981; Ramos and Barbieri, 1989). The Cortaderas lineament is defined by northwest-trending faults that can be traced to the Southern Volcanic Zone arc front where they control the recent craters of the Chillán volcano. The lineament was interpreted as a basement boundary that was reactivated during Andean deformation by Ramos (1981). Other authors consider the Cortaderas lineament to be a reactivated Paleogene north-verging thrust system (Cobbold and Rossello, 2003). Another important observation associated with the Cortaderas lineament is the concentration of Cenozoic magmatic activity to the north and the near absence of Cenozoic volcanism to the south (Ramos and Barbieri, 1989). Kay (2001a, 2001b; Kay et al., this volume, chapter 2) argued that the Cortaderas lineament marks the southern limit of a Miocene shallow subduction zone.

Figure 1. Map from the trench to the backarc showing the main geological provinces and structural features of the south-central Andes between 34°and 40°S. Differences in the regions north and south of the Cortaderas lineament are discussed in the text. Position of the Cortaderas lineament is based on Ramos and Barbieri (1989). FTB—fold-and-thrust belt.


Tectonic evolution of the southern Central Andes, Mendoza and Neuquén The major geologic features of the regions north and south of the Cortaderas lineament are illustrated in the figures and summarized herein. Region North of the Cortaderas Lineament The major geologic features in the region between 37°S and 34°S, to the north of the Cortaderas lineament, are illustrated in Figures 2 and 3. The general characteristics of the area

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just north of the Cortaderas lineament are described in this volume by Kay and Copeland (chapter 9), Kay et al. (chapters 2 and 10), and Folguera et al. (chapters 11 and 12). The discussion herein integrates these features with those farther north in the province of Mendoza. From west to east, the main morphotectonic elements of the region include the main Andean range (Principal Cordillera), the Río Grande foreland basin, the San Rafael basement block, and the widespread Pliocene to Quaternary Payenia backarc volcanic field.

Figure 2. Generalized geological map of parts of the provinces of Mendoza, Neuquén, and La Pampa in Argentina showing the Miocene geologic features of the region north of the Cortaderas lineament discussed in the text. Post-Miocene structures and volcanic rocks in the foreland are shown in Figure 3. Bold dashed lines are principal highways.


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Figure 3. Map of the same region as in Figure 2 showing the principal Pliocene and younger structures and volcanic rocks. Lines show locations of cross sections shown in Figures 4 and 5. Short dashed lines are boundaries between the Argentine provinces of Mendoza, Neuquén, and La Pampa. Longer dashed lines are principal highways (symbol is for Highway 40).


Tectonic evolution of the southern Central Andes, Mendoza and Neuquén Principal Cordillera The Principal Cordillera straddles the Chilean-Argentine border zone (Figs. 1 and 2) and includes the active Andean Southern Volcanic Zone arc front, which at these latitudes (35°–39°S) is located on the Chilean side of the range. Generally, the Southern Volcanic Zone has been divided into three segments based on petrologic and geologic studies (e.g., López Escobar, 1984; Hildreth and Moorbath, 1988; Dungan et al., 2001). These segments are known as the northern Southern Volcanic Zone (Tupungato to Maipo, 33.5°–34.5°S latitude), the transitional Southern Volcanic Zone (Palomo to Tatara–San Pedro, 35°–36°S), and the southern Southern Volcanic Zone (Longaví to Hudson, 36°–46°S). Among other differences, crustal thicknesses are considered to decrease from 65–60 km in the northern Southern Volcanic Zone to ~42–35 km in the southern Southern Volcanic Zone (e.g., Hildreth and Moorbath 1988; Ramos et al., 2004). Recent studies have demonstrated a complex history of forearc subduction erosion and consequent late Cenozoic migration of the magmatic arc toward the foreland in the segment north of 36°S (Kay et al., 2005). North of the Cortaderas lineament, the eastern slope of the Principal Cordillera coincides with the Las Loicas trough (named by Folguera et al., this volume, chapter 12). The Las Loicas trough is a Pliocene to Quaternary volcano-tectonic basin located east of the active Southern Volcanic Zone arc that is controlled by extensional north-northwest–trending faults and filled with thick sequences of Quaternary ignimbrites, lavas, and ashfall deposits derived from large silicic volcanic centers like the Bobadilla, Varvarco, and Domuyo calderas (Fig. 3; see Hildreth et al., 1999; Folguera et al., this volume, chapter 12). East of the Las Loicas trough lies the thick-skinned Malargüe fold-and-thrust belt (Fig. 2), which deforms Mesozoic marine and continental sequences (e.g., Kozlowski et al., 1993).

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The structure of this belt is dominated by large double-plunging anticlines that are cored by the Choiyoi Group (Permian-Triassic) volcanic and plutonic rocks, which constitute the exposed basement of the region. The Mesozoic strata in the belt are separated by an angular unconformity from pre–late Miocene Tertiary volcanics and granitoids, which are in turn unconformably overlain by thick sequences of late Miocene to Pliocene volcanics (Gerth, 1931; Ramos and Nullo, 1993). Tertiary synorogenic conglomerates and sandstones occur in the Principal Cordillera as remnants of the western part of a foreland basin. Cobbold and Rossello (2003) discussed the evolution of the Paleogene deposits. The volume of the Paleogene deposits is relatively minor compared to that of Neogene sequences that are widely preserved in the eastern foothills of the Principal Cordillera from the Río Diamante valley in the north to the Río Grande valley in the south (Fig. 2). The Neogene sequences are interpreted as synorogenic deposits associated with the Miocene uplift and shortening of the main Andean range. According to Kraemer et al. (2000), an older sequence that is constrained between 15.1 Ma at the base and 6.7 Ma at the top by K/Ar ages on interbedded volcanic layers is separated by an angular unconformity from a younger sequence, the age of which, at the base, is constrained to be between 6.7 Ma and 5.4 Ma. Undeformed late Pliocene to Quaternary sequences, which include andesitic and basaltic lava flows, unconformably overlie the Miocene deposits. Neogene normal faults are found along the eastern foothills of the Principal Cordillera. One of these is the Infiernillo fault (Kozlowski et al., 1993; Dajczgewand, 2002) that intersects the Río Salado valley (Figs. 3 and 4). As shown in Figure 4, the Infiernillo fault is a west-dipping normal fault that separates late Miocene deposits from Mesozoic sedimentary strata. Evidence for activity on this fault in Quaternary times comes from distinct pulses of basaltic lavas that have erupted along the fault (Fig. 4).

Figure 4. Cross section across the El Infiernillo normal fault along the Río Salado valley (modified from Dajczgewand, 2002). Location of section is shown in Figure 3.


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Río Grande Foreland Basin East of the Principal Cordillera, synorogenic deposits have accumulated in the large Río Grande foreland basin, which extends from the Río Diamante valley in the north to the southern limit of Mendoza province (Fig. 2). The basin fill includes more than 2000 m of synorogenic deposits that accumulated in two distinct depocenters located north and south of the Río Atuel (Yrigoyen, 1993). The Castillos de Pincheira outcrops, a few kilometers west of the city of Malargüe, expose both Paleogene and Neogene synorogenic sequences. Facies analyses of the Paleogene unit indicate a western provenance. Facies analyses of the Miocene deposits show that they constitute a coarseningupward sequence with both an eastern and western provenance. The thick coarse deposits at the top of the sequence have a dominantly eastern provenance (Kraemer and Zulliger, 1994) and contain basement clasts that record the uplift of the San Rafael block (Fig. 2, see following). These sequences are unconformably covered by thick late Pliocene to Quaternary deposits. Subsidence associated with extension has affected the region of the Río Grande basin in recent times. Active subsidence is presently occurring in the Laguna Llancanelo depression (Fig. 3) on the eastern side of the basin. This depression is bounded on the east by a normal fault (Manceda in Kozlowski et al., 1993). To the north is the Alto Tunuyán depression, which is interpreted as a half-graben (Fig. 3) that is filled by Quaternary clastic sediments. As recognized by Polanski (1963), the eastern side of the Alto Tunuyán depression is bounded by north-south–trending normal faults that affect Pliocene deposits and that can have throws of more than 700 m. One of these is the Cerro Negro de Capiz fault that affects late Pliocene deposits (Yrigoyen, 1993). The Extenso del Campo Bajo valley occurs in the region between the Alto Tunuyán depression and the Laguna Llancanelo.

San Rafael Basement Block Further east is the San Rafael basement block (Fig. 2). This block consists of Middle Proterozoic metamorphic rocks and tightly folded Paleozoic deposits (Moreno Peral and Salvarredi, 1984) that are cut and unconformably overlain by Permian to Triassic granitoids and volcanic rocks of the Choiyoi Group (e.g., Kay et al., 1989). The presence of an old peneplain carved on these basement rocks was first noted by Polanski (1954). The time of uplift and exposure of the erosional surface is constrained by synorogenic deposits of the Río Grande foreland basin that form a bajada of low-energy clastic deposits, which were derived from the Andean foothills. The time of deposition of the sediments is constrained by the presence of mammalian fossils (Soria, 1984) with a middle Miocene Colloncurense age (15–12 Ma; Pascual et al., 2002) and the K/Ar ages of 15.1 Ma and 6.7 Ma in the tuffs in the sequences dated by Kraemer et al. (2000). A rapid period of uplift at 5 Ma is indicated by Pliocene synorogenic deposits with an eastern provenance from the Río Grande basin. This uplifted peneplain was subsequently cut by Pliocene and Quaternary normal faults described by Narciso et al. (2001). Among these normal faults is the west-dipping El Carrizalito fault (Figs. 3 and 5), which indicates activity in the late Pliocene to early Pleistocene recognized by González Díaz (1964). Another is the Llancanelo fault that bounds the western margin of the block further to the south. The eastern margin of the San Rafael block is still actively shortening, as indicated by recent seismic activity as exemplified by the large Malvinas earthquake in the vicinity in 1929 (Bastías et al., 1993). The thrust front, which does not show a surface rupture, is expressed by a frontal monocline developed in Pleistocene basalts (Costa et al., 2004).

Figure 5. Cross section across the San Rafael block showing the Carrizalito fault and the late Miocene peneplain, which is broken by normal faults. One of these faults controls the position of the late Pleistocene basalts that erupted from the Cerro Negro volcano. The late Miocene thrust on the eastern side is inactive. Location of section is shown in Figure 3.


Tectonic evolution of the southern Central Andes, Mendoza and Neuquén Retroarc Volcanic Field The large retroarc volcanic province named Payenia by Polanski (1954) covers vast regions of the foreland between 35° and 37°S (Fig. 3). The maximum volumes of volcanic rocks erupted in the region between the Volcán Nevado and the Payún Matru caldera. These volcanic rocks pinch out to the north where they partially cover the San Rafael block. Regional studies by González Díaz (1972a, 1972b) and Bermúdez et al. (1993) indicate that these late Miocene to Quaternary volcanic rocks erupted in two distinct volcanic episodes. The first episode produced the hornblende-bearing mafic andesites to rhyodacites found in the Cerro Plateado (35°40′S) and Sierra de Chachahuén (37°05′S) volcanic fields, which are located some 500 km east of the modern trench (Fig. 2). The andesitic to rhyolitic volcanic rocks in the Cerro Plateado field are considered to be late Miocene in age (Bermúdez, 1991). Volcanic rocks in the Sierra de Chachahuén field farther south have an arc-like magmatic chemistry and range in age from ca. 7.2 to 4.8 Ma (Kay, 2001a, 2001b; Kay et al., this volume, chapter 9). Younger volcanic rocks from the Volcán Nevado (3810 m) in the region of the Plateado field (Fig. 3) are dominantly composed of trachyandesite and are considered to be Pliocene in age (Bermúdez, 1991). The second episode produced the extensive late Pliocene to Quaternary Payenia volcanic field that is temporally associated with the Cerro Diamante (2354 m), Cerro Payún (3680 m), Payún Matru caldera, and Auca Mahuida volcanic complexes (Figs. 3 and 6; see Bermúdez et al. 1993). Basaltic flows in these fields can reach lengths of over 200 km. The volcanic rocks of this episode generally have an alkaline within-plate chemical signature and are considered to be associated with an extensional regime (Muñoz et al., 1989; Bermúdez et al., 1993; Kay, 2001a, m2001b; Kay et al., this volume, chapter 2). Region South of the Cortaderas Lineament The major geologic features of the region south of the Cortaderas Lineament are illustrated in Figures 1 and 6. In striking contrast with the region north of the Cortaderas lineament, the retroarc east of the Loncopué trough is essentially devoid of Neogene volcanic rocks and synorogenic deposits (Fig. 6). Many of the general characteristics of this region are described in the papers in this volume by Mosquera and Ramos (chapter 5), Zamora Valcarce et al. (chapter 6), and Folguera et al. (chapters 11 and 12). From west to east, the main morphotectonic elements of the region are the Principal Cordillera, the Loncopué trough, the Agrio fold-and-thrust belt, the Chihuidos high, the Añelo basin, and the Neuquén Embayment. Principal Cordillera The Principal Cordillera south of the Cortaderas lineament is located within the southern segment of the Southern Volcanic Zone arc. Quaternary to Holocene arc volcanic complexes in this region include the centers at Chillán, Antuco, Copahue,

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Lonquimay, and Llaima (Fig. 6), which were assigned to the Lonquimay volcanic chain by Burckhardt (1900). These centers overlie Mesozoic marine sediments, Cretaceous plutonic rocks, and Oligocene to Miocene sedimentary and volcanic rocks that are unconformably covered by early Pliocene Cola de Zorro Formation volcanic rocks (e.g., Vergara and Muñoz, 1982; Burns et al., this volume, chapter 8). The Lonquimay chain is flanked to the east by the series of late Pliocene to early Quaternary volcanoes that Burckhardt (1900) assigned to the Pino Hachado chain. Most of the eastern flank of the Principal Cordillera north of the Copahue volcano is affected by the contractional deformation that created the late Miocene to Quaternary Guañacos fold-and-thrust belt (Folguera et al., 2004, this volume, chapter 11). Loncopué Trough The Loncopué trough, which is immediately east of the Principal Cordillera (Fig. 6), is an extensional basin associated with a thick cover of Pleistocene basaltic lavas and late Pleistocene– Holocene monogenic volcanic cones (Ramos, 1978; García Morabito, 2005; Folguera et al., this volume, chapter 12). The volcanic rocks have subdued-arc geochemical signatures (Muñoz and Stern, 1988). The Loncopué trough coincides with an important region of crustal attenuation that is discussed by Yuan et al. (this volume, chapter 3). Agrio Fold-and-Thrust Belt The next major feature to the east is the Agrio fold-andthrust belt (Figs. 1 and 6), which was divided into two sectors by Ramos (1998). The western sector is composed of Jurassic and Early Cretaceous marine deposits that were deformed in a thick-skinned belt, which was produced by the inversion of Early Jurassic half-grabens (Vergani et al., 1995; Folguera et al., 2002) during the Late Cretaceous. These sequences are intruded and overlain by Late Cretaceous to Eocene magmatic rocks (Franchini et al., 2003; Zamora Valcarce et al., this volume, chapter 6). The eastern sector of the Agrio belt consists of Early Cretaceous marine sedimentary rocks that were deformed in a Late Cretaceous thin-skinned belt that detached in Late Jurassic evaporite deposits. Both parts of the Agrio belt were tectonically reactivated and last deformed in the middle to late Miocene (Zapata and Folguera, 2005). Chihuidos High Farther east, the Chihuidos high is a basement arch that is largely covered by Late Cretaceous red bed sequences, which are synorogenic foreland basin deposits associated with the Agrio fold-and-thrust belt. The age of the lower part of the synorogenic sequence is constrained by zircon fission-track ages of 88 ± 3.9 Ma in the Huincul Formation on the southern margin of the basin at Cerro Policía in the province of Río Negro (Corbella et al., 2004). These ages imply that deformation in the Agrio fold-and-thrust belt had started by the Late Cretaceous (before Turonian-Santonian times). According to Mosquera and


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Figure 6. Generalized geological map of the northern part of Neuquén province in Argentina showing the geologic features along and south of the Cortaderas lineament discussed in the text.

Ramos (this volume, chapter 5), the presence of deformed middle Miocene synorogenic deposits on the Chihuidos high shows that the block was uplifted by tectonic inversion of normal faults during the late Miocene. Small post-tectonic basaltic cones with ages of 4.5 ± 0.5 Ma (Ramos and Barbieri, 1989) and an intraplate alkaline chemistry (Kay et al., 2004) occur near the northern boundary of the Chihuidos high (Fig. 6). Añelo Basin The sediments in the Añelo basin (Fig. 6) to the east record the Miocene uplift of the Chihuidos high. This basin represents a foredeep with a fill of a few hundred meters of late Miocene to Pliocene synorogenic deposits (Mosquera and Ramos, this volume, chapter 5), which include the strata exposed at Barranca del Palo and in the Sierra Blanca (Uliana, 1978). Most of the synorogenic sediments that formed in the Miocene

bypassed the Añelo basin and Neuquén Embayment and ended up as far east as the continental margin (Folguera et al., 2005). Neuquén Embayment East of the Agrio fold-and-thrust belt and south of the Añelo basin, Mesozoic sedimentary rocks associated with the Neuquén Embayment are preserved (Fig. 6). These sequences include a large expanse of marine sediments that were deposited and subsequently covered by Late Cretaceous synorogenic deposits. Thin sequences of continental and shallow marine strata of Maastrichtian to Paleogene age represent the first transgression derived from the Atlantic Ocean after the Early Cretaceous opening of the South Atlantic. The more northern part of the embayment is partially covered by Neogene alkaline basaltic lavas associated with volcanic centers north of the Cortaderas lineament.


Tectonic evolution of the southern Central Andes, Mendoza and Neuquén MAGMATIC AND DEFORMATIONAL HISTORY Important differences in the regions north and south of the Cortaderas lineament are evident in the sequential episodes of contractional and extensional deformational events that affected this region of the Andes. These differences are discussed in the following within the general context of the tectonic cycles first proposed by Groeber (1953, and references therein). Early Jurassic–Early Cretaceous Extensional Stage Most of the Early Jurassic to Early Cretaceous Andean margin at these latitudes was associated with negative trench roll-back velocity of the subduction zone (see Mpodozis and Ramos, 1989; Ramos, 1999). Normal faults were dominant along the main Andean chain, and volcanic rocks were widespread on both slopes of the Main Andean Cordillera. During this time, deep marine depocenters accumulated thick deposits that interfingered with basaltic and andesitic rocks erupted in an extensional-type magmatic arc. The main difference between the regions north and south of the Cortaderas lineament is that rift systems are more aerially restricted to the north. In detail, widespread grabens and halfgrabens found in the foreland in the south are abruptly replaced by a much narrower zone of rift-related structures in the north. The faulting in these rifts, which started in the Triassic and continued into the Early Jurassic, preceded the inception of subduction along the Pacific margin. The wider zone of rifts in the south coincides with a distinctive basement fabric. An important feature of this basement fabric is the N70°E-trending Huincul fault system that corresponds with the Huincul Ridge (Figs. 1 and 6) and marks a major crustal boundary. Mosquera and Ramos (this volume, chapter 5) interpret this boundary as a Paleozoic suture marking the collision between an allochthonous Patagonian terrane to the south and the Gondwana continent to the north. Evidence that the Huincul Ridge parallels the suture comes from aeromagnetic (Chernicoff and Zappettini, 2004), gravity (Kostadinoff et al., 2005), and seismic (Mosquera and Ramos, this volume, chapter 5) data. Late Cretaceous Contractional Stage Important changes in magmatism and deformation occurred near the end of the Early Cretaceous. One of these changes was the shutdown of the elongated magmatic belt that produced the 93–72 Ma granitoids exposed along the Chilean side of the Principal Cordillera (see Ramos and Folguera, 2005). South of the Cortaderas lineament, the end of this magmatic stage coincided with the eastward migration of the Late Cretaceous magmatic arc and its reestablishment in the western part of the Agrio fold-and-thrust belt. A contemporaneous migration of the frontal arc front did not occur in the northern sector. Another major development at this time was that the deformational

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regime became contractional throughout the region, with the most intense deformation occurring in the southern sector. The more extensive previous rifting in the southern region likely left an extended broken basement that was more susceptible to deformation. Support for major Late Cretaceous contractional deformation south of the Cortaderas lineament comes from several lines of reasoning. First, 40Ar/39Ar ages obtained by Zamora Valcarce et al. (this volume, chapter 6) on pyroclastic breccias and andesitic lavas unconformably overlying deformed Early Cretaceous marine rocks show that deformation had begun by 77 Ma in the western Agrio fold-and-thrust belt. Second, 40Ar/39Ar ages obtained by Zamora Valcarce et al. (this volume, chapter 6) on east-west–trending andesitic dikes suggest that compression along the western margin of the Neuquén Embayment had begun by 100 Ma. Third, zircon fission-track ages in Neuquén Group strata reported by Corbella et al. (2004) show that foreland basin sedimentation had begun by 88 Ma. Finally, fossils in Neuquén Group rocks indicate the development of a deep, eastward-thinning, Late Cretaceous foreland basin related to contractional deformation in the Agrio fold-and-thrust belt. Evidence for Late Cretaceous contractional deformation north of the Cortaderas lineament comes from the observation that andesitic volcanic rocks south of Cerro Domuyo and east of the Cordillera del Viento (Fig. 6) with K/Ar ages of 71.5 ± 5 Ma postdate deformed Cretaceous deposits (Llambías et al., 1979). Other evidence comes from a 69.09 ± 0.13 Ma 40Ar/39Ar biotite cooling age from the Varvarco pluton, which Kay (2001b) and Kay et al. (this volume, chapter 2) interpret as an uplift age for the Cordillera del Viento. Burns (2002) argued from fissiontrack data on zircons that the uplift of the Cordillera del Viento had begun by 70 Ma, and possibly before 80 Ma. Paleogene Compressional Stage There are still uncertainties concerning the distribution of magmatic rocks and the extent of contractional deformation that took place during the Paleogene. Well-preserved Eocene volcanic rocks occur along the Chilean Central Valley in the northern sector, but as noted by López-Escobar and Vergara (1997), similar-age volcanic rocks have not been found in Chile between 36° and 37°30′S. Paleogene volcanic rocks are present at these latitudes in Argentina (Franchini et al., 2003; Ramos and Folguera, 2005). Kay et al. (this volume, chapter 2) argue for a small eastward shift of the volcanic front in the Paleogene based on the age, chemistry, and distribution of volcanic rocks in northern Neuquén. The scarcity of Paleogene synorogenic deposits across the region is linked to the question of the importance of Paleogene contractional deformation. Volumes of these deposits are low north of the Cortaderas lineament and even more limited to the south. A notable exception is in the Pampa del Agua Amarga region in the southern Agrio fold-and-thrust belt, where late Paleocene to early Eocene pyroclastic and tuffaceous deposits


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in the Puesto Burgos Formation unconformably overlie sediments of the Late Cretaceous Neuquén Group (Leanza and Hugo, 2001). The age of these tuffs is constrained by plant fossils. Overall, it is apparent that large parts of the Agrio fold-andthrust belt and the adjacent Chihuidos high were positive areas during the Paleogene and that some synorogenic sediments accumulated in a foreland basin in the north. Cobbold et al. (1999) and Cobbold and Rossello (2003) appealed to transpression to reconcile the relatively small amounts of synorogenic deposits found for this time with the important episode of Eocene contractional deformation that they proposed. Their argument for Eocene deformation was largely based on the orientation of bitumen veins in the Agrio fold-and-thrust belt east of the Cordillera del Viento. Farther south, evidence for Eocene uplift along the Cordillera Principal south of 38°S comes from apatite fission-track ages of 40.6 ± 4.5 Ma (Gräfe et al., 2002). Oligocene to Early Miocene Extensional Stage During the Oligocene to early Miocene, the Neuquén Andes were characterized by generalized extension in the forearc (e.g., Cisternas and Frutos, 1994; Stern et al., 2000), arc (e.g., Vergara et al., 1997; Burns et al., this volume, chapter 8), and retroarc (e.g., Folguera et al., 2003). The eruption of early Miocene alkaline basaltic volcanic rocks at distances up to 500 km from the modern trench has been used as evidence that crustal attenuation in an extensional regime extended far into the foreland (Ramos and Barbieri, 1989; Kay, 2001b; Kay and Copeland, this volume, chapter 9). This period has been associated with a period of negative trench roll-back by Kay and Copeland (this volume). Middle to Late Miocene Contractional Stage The middle to late Miocene was a time of contractional deformation across the region. Important differences occur north and south of the Cortaderas lineament. To the north, magmatism was widespread, the Malargüe fold-and-thrust belt propagated eastward, and synorogenic deposits were widely distributed. To the south, magmatism was confined to the region near the arc axis, and contractional deformation was restricted to inversion of the Cura Mallín basin in the Principal Cordillera and to minor reactivation of the Agrio fold-andthrust belt. The most dramatic changes occurred in the northern segment. Beginning in the region of the Principal Cordillera, the eastward broadening of the magmatic arc at the latitude of central and southern Mendoza was initially recognized by Gerth (1931). This eastward expansion of magmatism occurred at the time of important crustal shortening and uplift in the Malargüe fold-and-thrust belt, and the subsidence that led to the accumulation of the thick Neogene synorogenic deposits in the Río Grande basin. The eastward propagating sequence culminated

in the contractional uplift of the San Rafael block by the end of the Miocene and the eruption of the Cerro Plateado and associated volcanic centers far to the east of the trench (Delpino and Bermúdez, 1985). Bermúdez (1991) pointed out that the late Miocene arc in central Mendoza was over 200 km wide. The eastward expansion of the Miocene volcanic arc across southernmost Mendoza and northern Neuquén that culminated in the eruption and uplift of the Sierra de Chachahuén between 7.8 and 4.8 Ma is discussed by Kay (2001a, 2001b) and Kay et al. (this volume, chapters 2 and 10). The presence of sparse synorogenic deposits in the foreland south of the Cortaderas lineament is interpreted as indicating renewed contraction in this region. Among these deposits are those at Rincón Bayo that unconformably overlie Paleogene sediments in the Chihuidos high (Zapata et al., 2003) and conglomerates and sandstones containing middle Miocene mammal fossils (Repol et al., 2002). Based on the small volume of Miocene compared to Cretaceous synorogenic deposits, Miocene contraction in this region is considered to have been less important than in the Cretaceous (Ramos, 1998; Mosquera and Ramos, 2005, this volume, chapter 5). Pliocene Extensional Stage Differences across the northern and southern sectors are again striking in the Pliocene. The northern sector is characterized by important retroarc basaltic magmatism with a subdued arc to intraplate chemical character (Muñoz et al., 1989; Bermúdez et al., 1993; Kay, 2001b; Kay et al., 2004, and this volume, chapter 2). Large volumes of alkaline magmas erupted from fissures and important volcanic centers like the late Pliocene Payún Matru caldera. Magmatic activity was accompanied by generalized extension in the retroarc and adjacent areas, and the collapse of previously uplifted foreland areas like the San Rafael block (González Díaz, 1964; Bermúdez et al., 1993). Extension propagated from the foreland to the foothills of the Principal Cordillera as retroarc volcanoes, such as those in the Tromen region, erupted between the Principal Cordillera and the eastern foreland (Kay, 2001b; Kay et al., this volume, chapter 2). South of the Cortaderas lineament, extension was limited to the arc and western retroarc (Folguera et al., 2003, and this volume, chapter 11). The magmatic arc expanded toward the retroarc with stratovolcanoes erupting along the Pino Hachado chain (e.g., Muñoz and Stern, 1988; Lara and Folguera, this volume, chapter 14). Basaltic volcanism occurred only as far east as the Loncopué trough (Fig. 6). Pleistocene to Holocene Stage During the Pleistocene, extension propagated to the main axis of the Principal Cordillera. Large volumes of rhyolite erupted from a series of calderas and volcanic domes north of the Cortaderas lineament, including the Planchón, Calabozos, Bobadilla, Varvarco, Domuyito, and Domuyo centers (Fig. 3).


Tectonic evolution of the southern Central Andes, Mendoza and Neuquén The rhyolitic magmas are considered to have been generated by crustal melting associated with basaltic underplating (Hildreth et al., 1999). The volcanic and pyroclastic deposits produced by these eruptions filled the fault-bounded Las Loicas trough (Fig. 3; see Folguera et al., this volume, chapter 12). Volcanic activity also took place in the retroarc in stratovolcanoes like Tromen and in alkaline monogenic basaltic and alkaline complexes like Payún Matrú and Auca Mahuida (see Holmberg, 1964; Llambías, 1966; González Díaz, 1972b; Bermúdez et al., 1993; Kay, 2001b; Kay et al., 2004, and this volume, chapter 2). During the same period, contractional deformation in the Guañacos fold-and-thrust belt straddled the western part of the Cortaderas lineament in the Principal Cordillera (Fig. 6) as the Pliocene-Pleistocene volcanic arc was thrust over Pleistocene bajadas (Folguera et al., this volume, chapter 11). Contractional deformation also occurred along the eastern margin of the San Rafael block, where thrust faults involve Quaternary lavas and the large 1929 Las Malvinas earthquake is interpreted to have had a compressional mechanism (Costa et al., 2004). Farther south, the main Pleistocene to Holocene arc volcanic activity was largely focused along the Lonquimay chain on the western slope of the Andes, where active volcanism continues in the southern sector of the Southern Volcanic Zone today. Mafic volcanism and normal faulting was largely restricted to the Loncopué trough, and evidence for Quaternary shortening is absent (see Folguera et al., this volume, chapter 12) TECTONIC EVOLUTION OF THE ANDES NORTH AND SOUTH OF THE CORTADERAS LINEAMENT The geologic history and features discussed in the previous sections are utilized in the following to propose a model for the contrasting evolution of the regions north and south of the Cortaderas lineament. Important factors in explaining the differences between these sectors are discrete crustal rheologies inherited from Paleozoic and Mesozoic events and differences in underlying subducting plate geometry. Conceptual cartoons in Figure 7, not drawn to scale, emphasize the difference in the various stages of this evolution. A more detailed discussion of the Miocene development of the region just north of the Cortaderas lineament is presented by Kay (2001a, 2001b, 2002) and Kay et al. (this volume, chapters 2 and 10). Early Rifting (Triassic to Early Jurassic) The Mesozoic evolution of the region is shown to begin in Figure 7A with the generalized extension and rifting related to the early breakup of the Pangean supercontinent. This rifting was concentrated north of the Huincul Ridge (Fig. 1), which is interpreted by Mosquera and Ramos (this volume, chapter 5) to parallel the late Paleozoic suture between the allochthonous Patagonia terrane and Gondwana. In accord with this interpretation, Mosquera and Ramos (this volume) suggest that the

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widespread extension under the Neuquén Embayment correlates with the hanging wall of the suture. This structural pattern then controls the later geometry of the Neuquén Embayment (Franzese and Spalletti, 2001). Early Subduction (Jurassic to Early Cretaceous) The initiation of subduction in the lower Jurassic at ca. 180 Ma is based on the appearance of arc magmatic rocks in Chile (see Mpodozis and Ramos, 1989; Parada 1990). The main difference between the northern and southern sectors is that arc volcanic rocks are either more abundant or are better preserved in the north (see Ramos and Folguera, 2005). The rift structures shown in the cartoon in Figure 7B are very similar to those shown in Figure 7A, since it is difficult to separate the faults related to early rifting from those produced by extension in the backarc after subduction began. Tectonic Inversion (Late Cretaceous to Paleocene) The cartoons in Figure 7C show the major differences between the southern and northern regions during the Late Cretaceous to Paleocene. In the southern region, the magmatic front migrated eastward and major contractional deformation occurred in the Agrio fold-and-thrust belt in the foreland. To the north, the arc remained stationary, and contractional deformation in the foreland Malargüe fold-and-thrust belt was less intense. In Figure 7C, the more pronounced eastward migration of the magmatic arc and the greater amount of crustal shortening in the south are tentatively linked to a relative shallowing of the Benioff zone in the south. Furthermore, as contractional deformation in both regions is primarily linked to tectonic inversion of basement faults and only secondarily to thinskinned thrusting, the more intense deformation in the south can be correlated with a greater amount of previous extension. Steepening of Benioff Zone in the Southern Sector (Oligocene to Early Miocene) Major changes occurred in the latest Oligocene to early Miocene as the Nazca plate replaced the Farallón plate, the relative convergence rate increased and became nearly normal (e.g., Somoza, 1998), and negative trench roll-back caused generalized extension across the region (see Kay and Copeland, this volume, chapter 9). As shown in Figure 7D, extension at this time was localized in the Coya Machalí intra-arc basin along the main Andes to the north (e.g., Godoy et al., 1999; Charrier et al., 2002), whereas extension near the Cortaderas lineament to the south was more intense, with effects extending from the Cura Mallín intra-arc basin (e.g., Burns et al., this volume, chapter 8) into the retroarc where alkali olivine basalts were erupting (see Ramos and Barbieri, 1989; Kay, 2001b; Kay et al., 2004; Kay and Copeland, this volume, chapter 9). The cartoon in Figure 7D shows a return to a steeper subduction zone than


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Tectonic evolution of the southern Central Andes, Mendoza and Neuquén

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that which existed in the Late Cretaceous to Paleocene in the southern region, which is in accord with extension in the Cura Mallín basin and into the backarc.

contractional deformation was less intense, the subduction zone for the southern segment in Figure 7E is shown as steeper (~30°) than that to the north.

Shallowing of Benioff Zone in Northern Sector (Middle to Late Miocene)

Steepening of the Benioff Zone (Pliocene to Quaternary)

As recognized by Kay (2001a, 2001b, 2002; Kay et al., this volume, chapter 2), an important transient shallowing of the subduction zone north of the Cortaderas lineament can explain the Miocene advance of deformation into the foreland and the eruption of volcanic rocks with arc-like magmatic signatures up to 500 km east of the trench. This shallowing, which is depicted in the cartoon in Figure 7E, fits well with the wave of deformation in the Malargüe fold-and-thrust belt and the foreland expansion of magmatism discussed already. The age and distribution of magmatic rocks in the Principal Cordillera and the synorogenic deposits in the foreland (Kraemer et al., 2000) are consistent with the eastward advance of the Malargüe thrust front from 15 Ma to 5 Ma as the magmatic arc broadened. During the time of shallow subduction, the San Rafael block and the Sierra de Chachahuén were uplifted in a manner analogous to that of the Sierras Pampeanas over the present Pampean (Chilean) flat-slab segment to the north. Because magmatism did not completely cease, the shallowing of the slab is considered to have been less pronounced than under the Pampean flat slab to the north (Kay, 2001b, 2002; Mancilla, 2001; Kay et al., this volume, chapters 2 and 10). During this same period, the region south of the Cortaderas lineament was subjected to contractional deformation that led to the middle to late Miocene compressional inversion of the Cura Mallín basin, reactivation of the Agrio fold-and-thrust belt, and the uplift of the Chihuidos high. As magmatism in the south was restricted to the region of the Principal Cordillera and

Figure 7. Conceptual cartoons (not to scale) comparing the tectonic evolution of the sectors north and south of the Cortaderas lineament: (A) Triassic–Early Jurassic rifting associated with the breakup of Pangea and inception of early Jurassic subduction. (B) Jurassic–Early Cretaceous subduction with generalized extension associated with negative trench roll-back. (C) Late Cretaceous to Paleogene contraction in the Agrio fold-and-thrust belt (FTB) and less extensive tectonic inversion in the Malargüe fold-and-thrust belt. (D) Extension associated with steepening of the subduction zone related to negative trench roll-back velocity. (E) Differential shallowing of the subduction zone with Miocene contractional deformation and crustal shortening. (F) Steepening of the subduction zone, widespread extension and magmatism in the northern sector; localized extension and magmatism, and initiation of contraction in the Guañacos fold-and-thrust belt. (G) Present setting under contraction. More detailed early Miocene to late Quaternary lithospheric scale cross sections north of the Cortaderas lineament are included in Kay et al. (this volume, chapter 2), and more detailed Pliocene to late Quaternary sections comparing the regions north and south of the Cortaderas lineament are in Folguera et al. (this volume, chapter 12).

The cartoons in Figures 7F and 7G show a return to a steeper subducting slab than in the late Miocene, with the most pronounced steepening in the north. Kay (2001b, 2002; Kay et al., this volume, chapter 2) argued that the end of arclike magmatism far east of the trench followed by widespread Pliocene to Quaternary mafic volcanism with a progressively more intraplate-like chemical signature is best explained by such a steepening of the subducting slab north of the Cortaderas lineament. The widespread within-plate basaltic volcanism of the Payenia volcanic field is thought to be triggered by the reinsertion of hot asthenosphere into the thicker mantle wedge above the steepening slab (Kay et al., 2004). Such a scenario also fits with the Pliocene to Quaternary extensional collapse of foreland in this paper. Crustal melting by basaltic underplating linked to the injection of hot asthenosphere along the cordilleran axis north of the Cortaderas lineament can explain the formation of large Quaternary collapse calderas and the emplacement of rhyolitic domes along the Las Loicas trough (Folguera et al., this volume, chapter 12). Crustal weakening would favor shortening along the PliocenePleistocene arc leading to the development of the Guañacos fold-and-thrust belt along the eastern slope of the Principal Cordillera (see Folguera et al., this volume, chapter 11). The region south of the Cortaderas lineament records a contraction of the Pliocene-Pleistocene arc, which retreated westward to the present position in the southern sector of the Southern Volcanic Zone (Lonquimay) volcanic arc. Pliocene to Quaternary extension in this region is confined to the Loncopué trough. Local negative trench roll-back has been postulated to explain both the extension and minor retreat of the volcanic arc (see Lara and Folguera, this volume, chapter 14). This roll-back could be accompanied by a minor steepening of the subducting slab (Folguera et al., this volume, chapter 12). Roll-back could also play a role in events in the northern sector, but only shallowing followed by steepening of the subducting slab can easily explain expansion followed by disappearance of arc-like magmatic activity and compressional deformation far into the backarc. CONCLUDING REMARKS The tectonic evolution of the southern Mendoza and Neuquén Andes is characterized by changes in the position of the magmatic arc front, periods of expansion of Tertiary to Holocene volcanism into the foreland, and waves of compression or extension that are best linked to changes in the geometry of the Benioff zone. In detail, shifting or expansion of arc magmatism into the foreland is coeval with contraction in the fore-


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land fold-and-thrust belt, whereas retraction of arc magmatism toward the trench is linked with extension and collapse. There is no clear evidence to suggest a correlation of changes in slab geometry with subduction of aseismic ridges, as has been proposed for the Chilean flat slab farther north (e.g., Yáñez et al., 2001; Ramos et al., 2002). The simplest explanation for these changes is that subduction of a ridge produced a transient shallowing in the geometry of the subducted slab (see also Kay et al., this volume, chapter 9). These changes could have been enhanced by the absolute motion of the South American plate as normally encompassed in the overriding velocity of the upper plate (Jarrard, 1986; Sobolev and Babeyko, 2005). The overriding velocity is equivalent to the trench roll-back velocity plus the orogenic shortening rate. An increase in the overriding velocity may result in compression and shortening and positive trench roll-back velocities as proposed by Daly (1989); a decrease in the overriding velocity may produce an extensional regime coeval with negative trench roll-back. Evaluation of the mechanisms that control changes in the tectonic regime in the Neuquén Andes requires taking into account changes that affect a broader area. For example, Miocene compression is well known all along the Andean margin. Silver et al. (1998) argued that the resulting contraction can be related to a relative increase in the overriding velocity of the entire South American plate as Africa slowed down. In another example, the extensional regime that controlled the inception of normal fault-bounded intra-arc basins in the Oligocene is well known to have occurred all along the Andean margin (e.g., Daly, 1989; Mpodozis and Ramos, 1989). This change could be attributed to a general decrease in the overriding velocity affecting the South America plate, which would produce a steepening of the subduction zone and a retreat of magmatic activity toward the trench. On the other hand, local conditions might have enhanced these effects, as seen in the southern sector of the Neuquén Andes, where widespread normal faults related to Mesozoic rifts played a major role in later contractional events. ACKNOWLEDGMENTS The authors acknowledge financial support from the Agencia Nacional de Promoción Científica y Tecnológica (grant ANCPYT 14144/03 to Ramos) and Repsol YPF (to S.M. Kay). Members of the Laboratorio de Tectónica Andina (Universidad de Buenos Aires) and the Cornell Andes Group are thanked for interesting discussions and comments. The paper was improved by careful reviews by Constantino Mpodozis and Robert Kay. REFERENCES CITED Bastías, H., Tello, G.E., Perucca, L.P., and Paredes, J.D., 1993, Peligro sísmico y neotectónica, in Ramos, V.A., ed., Geología y recursos naturales de Mendoza: Buenos Aires, XII Congreso Geológico Argentino y II Congreso de Exploración de Hidrocarburos, Relatorio VI-1, p. 645–658. Bermúdez, A., 1991, Sierra del Nevado. El límite oriental del arco volcánico Neógeno entre los 35°30′ y 36° L.S. Argentina: 6th Congreso Geológico Chileno Actas, v. 1, p. 318–322.

Bermúdez, A., Delpino, D., Frey, F., and Saal, A., 1993, Los basaltos de retroarco extraandinos, in Ramos, V.A., ed., Geología y recursos naturales de Mendoza: Buenos Aires, XII Congreso Geológico Argentino y II Congreso de Exploración de Hidrocarburos, Relatorio I-13, p. 161–172. Burckhardt, C., 1900, Profils géologiques transversaux de la Cordillère Argentino-Chilienne: Museo de La Plata, Anales, Sección Geología y Mineralógica, v. 2, p. 1–136. Burns, W.M., 2002, Tectonic and depositional evolution of the Tertiary Cura Mallín Basin in the southern Andes (36.5 to 38°S lat.) [Ph.D. Thesis]: Cornell University, Ithaca, New York, 218 p. Burns, W.M., Jordan, T.E., Copeland, P., and Kelley, S.A., this volume, The case for extensional tectonics in the Oligocene-Miocene Southern Andes as recorded in the Cura Mallín basin (36°–38°S), in Kay, S.M., and Ramos, V.A., eds., Evolution of an Andean margin: A tectonic and magmatic view from the Andes to the Neuquén Basin (35°–39°S lat): Geological Society of America Special Paper 407, doi: 10.1130/2006.2407(08). Charrier, R., Baeza, O., Elgueta, S., Flynn, J.J., Gans, P., Kay, S.M., Muñoz, N., Wyss, A.R., and Zurita, E., 2002, Evidence for Cenozoic extensional basin development and tectonic inversion south of the flat-slab segment, southern Central Andes, Chile (33°–36°S.L.): Journal of South American Earth Sciences, v. 15, no. 1, p. 117–140. Chernicoff, J., and Zappettini, E., 2004, Geophysical evidence for terrane boundaries in south-central Argentina: Gondwana Research, v. 7, p. 1105–1117, doi: 10.1016/S1342-937X(05)71087-X. Cisternas, M.E., and Frutos, J., 1994, Evolución tectono-estratigráfica de la Cuenca Terciaria de los Andes del Sur de Chile (37°30′–40°30′Lat.S.): 7th Congreso Geológico Chileno (Concepción) Actas, v. 1, p. 6–12. Cobbold, P.R., and Rossello, E.A., 2003, Aptian to Recent compressional deformation of the Neuquén Basin, Argentina: Marine and Petroleum Geology, v. 20, p. 429–443, doi: 10.1016/S0264-8172(03)00077-1. Cobbold, P.R., Diraison, M., and Rossello, E.A., 1999, Bitumen veins and Eocene transpression, Neuquén Basin, Argentina: Tectonophysics, v. 314, p. 423–442, doi: 10.1016/S0040-1951(99)00222-X. Corbella, H., Novas, F.E., Apesteguía, S., and Leanza, H.A., 2004, First fission track-age for the dinosaur-bearing Neuquén Group (Upper Cretaceous) Neuquén Basin, Argentina: Revista Museo Argentino de Ciencias Naturales (N.S.), v. 6, p. 1–6. Costa, C.H., Cisneros, H., Salvarredi, J., and Gallucci, A., 2004, Nuevos datos y reconsideraciones sobre la neotectónica del margen oriental del bloque de San Rafael: 12 Reunión Sobre Microtectónica y Geología Estructural (Cafayate), Resúmenes, p. 7. Dajczgewand, D.M., 2002, Faja plegada y corrida de Malargüe: Estilo de deformación en la región de Mallín Largo. Trabajo Final de Licenciatura: Buenos Aires, Universidad de Buenos Aires (unpublished), 119 p. Daly, M., 1989, Correlations between Nazca/Farallón plate kinematics and forearc evolution in Ecuador: Tectonics, v. 8, p. 769–790. Delpino, D.H., and Bermúdez, A., 1985, Volcán Plateado. Vulcanismo andesítico de retroarco en el sector extrandino de la Provincia de Mendoza, 35°42′ Lat. Sur. Argentina: Antofagasta, 4th Congreso Geológico Chileno Actas, v. 3, p. 108–119. Dungan, M.A., Wulff, A., and Thompson, R., 2001, Eruptive stratigraphy of the Tatara–San Pedro complex, 36°S, Southern Volcanic Zone, Chilean Andes: Reconstruction method and implications for magma evolution at long-lived arc volcanic centers: Journal of Petrology, v. 42, p. 555–626, doi: 10.1093/petrology/42.3.555. Folguera, A., and Ramos, V.A., 2000, Control estructural del Volcán Copahue: Implicancias tectónicas para el arco volcánico Cuaternario (36°–39°S): Asociación Geológica Argentina Revista, v. 55, p. 229–244. Folguera, A., and Ramos, V.A., 2001, Distribución de la deformación en los Andes Australes (33°–46°S), in Cortés, J.M., Rossello, E., and Dalla Salda, L., eds., Avances en microtectónica: Asociación Geológica Argentina, Serie D, Publicación Especial, v. 5, p. 13–18. Folguera, A., and Ramos, V.A., 2002, Partición de la deformación durante el Neógeno en los Andes Patagónicos Septentrionales (37°–46°S): Revista de la Sociedad Geológica de España, v. 15, p. 81–93.


Tectonic evolution of the southern Central Andes, Mendoza and Neuquén Folguera, A., Ramos, V.A., and Melnick, D., 2002, Partición de la deformación en la zona del arco volcánico de la cordillera Neuquina en los últimos 30 millones de años (36°–39°S): Revista Geológica de Chile, v. 29, p. 227–240. Folguera, A., Ramos, V.A., and Melnick, D., 2003, Recurrencia en el desarrollo de cuencas de intraarco. Colapso de estructuras orogénicas. Cordillera Neuquina (37°30′): Revista de la Asociación Geológica Argentina, v. 58, p. 3–19. Folguera, A., Ramos, V.A., Hermanns, R.L., and Naranjo, J., 2004, Neotectonics in the foothills of the southernmost central Andes (37°–38°S): Evidence of strike-slip displacement along the Antiñir-Copahue fault zone: Tectonics, v. 23, TC5008, doi: 10.1029/2003TC001533. Folguera, A., Folguera, A., Zárate, M., and Ramos, V.A., 2005, La cuenca de antepaís Neógena del Río Negro asociada con el levantamiento de los Andes de Neuquén, in 16th Congreso Geológico Argentino Actas, La Plata: Actas, v. 2, p. 29–36. Folguera, A., Ramos, V.A., González Díaz, E.F., and Hermanns, R., 2006, this volume, Miocene to Quaternary deformation of the Guañacos fold-andthrust belt in the Neuquén Andes between 37°S and 37°30°S, in Kay, S.M., and Ramos, V.A., eds., Evolution of an Andean margin: A tectonic and magmatic view from the Andes to the Neuquén Basin (35°–39°S lat): Geological Society of America Special Paper 407, doi: 10.1130/ 2006.2407(11). Folguera, A., Zapata, T., and Ramos, V.A., 2006, this volume, Late Cenozoic extension and the evolution of the Neuquén Andes, in Kay, S.M., and Ramos, V.A., eds., Evolution of an Andean margin: A tectonic and magmatic view from the Andes to the Neuquén Basin (35°–39°S lat): Geological Society of America Special Paper 407, doi: 10.1130/ 2006.2407(12). Franchini, M.B., López Escobar, L., Shalamuk, I.B.A., and Meinert, L.D., 2003, Paleocene calc-alkaline subvolcanic rocks from Nevazón Hill area (NW Chos Malal fold belt), Neuquén, Argentina, and comparison with granitoids of the Neuquén-Mendoza volcanic province: Journal of South American Earth Sciences, v. 16, p. 399–422, doi: 10.1016/S08959811(03)00103-2. Franzese, J.R., and Spalletti, L.A., 2001, Late Triassic continental extension in southwestern Gondwana: Tectonic segmentation and pre-break-up rifting: Journal of South American Earth Sciences, v. 14, p. 257–270, doi: 10.1016/S0895-9811(01)00029-3. García Morabito, E., 2005, Geología del sector occidental de la depresión de Loncopúe, provincia del Neuquén: Buenos Aires, Trabajo Final de Licenciatura, Universidad de Buenos Aires (inédito), 108 p. Gerth, E., 1931, La estructura geológica de la Cordillera Argentina entre el Río Grande y el Río Diamante en el sud de la provincia de Mendoza: Academia Nacional de Ciencias Actas, v. X, p. 125–172. Godoy, E., Yañez, G., and Vera, E., 1999, Inversion of an Oligocene volcanotectonic basin and uplifting of its superimposed Miocene magmatic arc in the Chilean Central Andes: First seismic and gravity evidences: Tectonophysics, v. 306, p. 217–236, doi: 10.1016/S0040-1951(99) 00046-3. González Díaz, E.F., 1964, Rasgos geológicos y evolución geomorfológica de la Hoja 27d (San Rafael) y zona occidental vecina (Provincia de Mendoza): Asociación Geológica Argentina, Revista, v. 19, p. 151–188. González Díaz, E.F., 1972a, Descripción geológica de la Hoja 27d San Rafael, Provincia de Mendoza: Servicio Nacional Minero Geológico, Boletín, v. 132, p. 1–127. González Díaz, E.F., 1972b, Descripción geológica de la Hoja 30d PayúnMatrú, Provincia de Mendoza: Dirección Nacional de Geología y Minería, Boletín, v. 130, p. 1–88. Gräfe, K., Glodny, J., Seifert, W., Rosenau, M., and Echtler, H., 2002, Apatite fission track thermochronology of granitoids at the south Chilean active continental margin (37º–42ºS): Implications for denudation, tectonics and mass transfer since the Cretaceous, in Proceedings of the 5th International Symposium on Andean Geodynamics, Toulouse, France: IRD Editions, Paris, , p. 275–278.

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from Antarctica to Alaska: Geological Society of America Special Paper 241, p. 51–66. Pascual, R., Carlini, A.A., Bond, M., and Goin, F.J., 2002, Mamíferos Cenozoicos, in Haller, M.J., ed., Geología y recursos naturales de la provincia de Santa Cruz: XV Congreso Geológico Argentino Relatorio II, p. 533–544. Polanski, J., 1954, Rasgos geomorfológicos del territorio de la provincia de Mendoza: Ministerio Economía, Instituto Investigaciones económicas y tecnológicas. Cuadernos de Investigaciones y Estudios, v. 4, p. 4–10. Polanski, J., 1963, Estratigrafía, neotectónica y geomorfología del Pleistoceno pedemontano, entre los Ríos Diamante y Mendoza: Asociación Geológica Argentina, Revista, v. 17, p. 127–349. Ramos, V.A., 1977, Estructura, in Rolleri, E.O., ed., Geología y Recursos Naturales de la Provincia del Neuquén: VII Congreso Geológico Argentino (Neuquén), Relatorio, p. 99–118. Ramos, V.A., 1981, Descripción geológica de la Hoja 33 c Los Chihuidos Norte, provincia del Neuquén: Servicio Geológico Nacional, Boletín, v. 182, p. 1–103. Ramos, V.A., 1998, Estructura del sector occidental de la faja plegada y corrida del Agrio, cuenca Neuquina, Argentina: 10th Congreso Latinoamericano de Geología Actas, v. 2, p. 105–110. Ramos, V.A., 1999, Plate tectonic setting of the Andean Cordillera: Episodes, v. 22, p. 183–190. Ramos, V.A., and Barbieri, M., 1989, El volcanismo Cenozoico de Huantraico: Edad y relaciones isotópicas iniciales, provincia del Neuquén: Asociación Geológica Argentina Revista, v. 43, p. 210–223. Ramos, V.A., and Folguera, A., 2005, Tectonic evolution of the Andes of Neuquén: Constraints derived from the magmatic arc and foreland deformation, in Veiga, G.D., Spalletti, L.A., Howell, J.A., and Schwarz, E., eds., The Neuquén Basin: A case study in sequence stratigraphy and basin dynamics: The Geological Society [London] Special Publication 252, p. 15–35. Ramos, V.A., and Nullo, F.E., 1993, El volcanismo de arco Cenozoico, in Ramos, V.A., ed., Geología y recursos naturales de Mendoza: XII Congreso Geológico Argentino y II Congreso de Exploración de Hidrocarburos (Mendoza), Relatorio I(12), p. 149–160. Ramos, V.A., Cristallini, E., and Pérez, D.J., 2002, The Pampean flat-slab of the Central Andes: Journal of South American Earth Sciences, v. 15, p. 59–78, doi: 10.1016/S0895-9811(02)00006-8. Ramos, V.A., Zapata, T., Cristallini, E., and Introcaso, A., 2004, The Andean thrust system: Latitudinal variations in structural styles and orogenic shortening, in McClay, K., ed., Thrust tectonics and hydrocarbon systems: American Association of Petroleum Geologists Memoir 82, p. 30–50. Repol, D., Leanza, H.A., Sruoga, P., and Hugo, C.A., 2002, Evolución tectónica del Cenozoico de la comarca de Chorriaca, provincia del Neuquén, Argentina: 15th Congreso Geológico Argentino Actas, v. 3, p. 200–205. Silver, P.G., Russo, R.M., Lithgow-Bertelloni, C., 1998, Coupling of South American and African Plate Motion and Plate Deformation: Science, v. 279, p. 60–63. Sobolev, S.V., and Babeyko, A.Y., 2005, What drives orogeny in the Andes?: Geology, v. 33, p. 617–620, doi: 10.1130/G21557.1. Somoza, R., 1998, Updated Nazca (Farallón) –South America relative motions during the last 49 m.y.; implications for mountain building in the Central Andean region: Journal of South American Earth Sciences, v. 11, p. 211–215, doi: 10.1016/S0895-9811(98)00012-1. Soria, M.F., 1984, Vertebrados fósiles y edad de la Formación Aisol, provincia de Mendoza: Asociación Geológica Argentina, Revista, v. 38, p. 299–306. Stern, C., Muñoz, J., Troncoso, R., Duhart, P., Crignola, P., and Farmer, G., 2000, Tectonic setting of the mid-Tertiary coastal magmatic belt in south central Chile: An extensional event related to late Oligocene changes in plate convergence rate and subduction geometry: 9th Congreso Geológico Chileno Actas, v. 2, p. 693–696. Uliana, M., 1978, Estratigrafía del Terciario, in Rolleri, E.O., ed., Geología y recursos naturales del Neuquén: VII Congreso Geológico Argentino, Relatorio, p. 67–84.


Tectonic evolution of the southern Central Andes, Mendoza and Neuquén Vergani, G.D., Tankard, A.J., Belotti, H.J., and Welsink, H.J., 1995, Tectonic evolution and paleogeography of the Neuquén Basin Argentina, in Tankard, A.J., Suárez, R., and Welsink, H.J., eds., Petroleum Basins of South America. American Association of Petroleum Geologists Memoir 62, p. 383–402. Vergara, M., and Muñoz, J., 1982, La Formación Cola de Zorro en la Alta cordillera Andina Chilena (36°–39°S), sus características petrográficas y petrológicas: Una revisión: Revista Geológica de Chile, v. 17, p. 31–46. Vergara, M., Moraga, J., and Zentilli, M., 1997, Evolución termotectónica de la cuenca Terciaria entre Parral y Chillán: Análisis por trazas de fisión en apatitas: 7th Congreso Geológico Chileno (Antofagasta) Actas, v. 2, p. 1574–1578. Yáñez, G., Ranero, G.R., von Huene, R., and Diaz, J., 2001, Magnetic anomaly interpretation across a segment of the southern Central Andes (32–34°S): Implications on the role of the Juan Fernández ridge in the tectonic evolution of the margin during the Upper Tertiary: Journal of Geophysical Research, v. 106, p. 6325–6345, doi: 10.1029/2000JB900337. Yuan, X., Asch, G., Bataille, K., Bock, G., Bohm, M., Echtler, H., Kind, R., Oncken, O., Wölbern, I., 2006, this volume, Deep seismic images of the Southern Andes, in Kay, S.M., and Ramos, V.A., eds., Evolution of an Andean margin: A tectonic and magmatic view from the Andes to the Neuquén Basin (35°–39°S lat): Geological Society of America Special

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MANUSCRIPT ACCEPTED BY THE SOCIETY 22 DECEMBER 2005

Printed in the USA

Geological Society of America Special Paper 407 2006

Upper Cretaceous to Holocene magmatism and evidence for transient Miocene shallowing of the Andean subduction zone under the northern Neuquén Basin Suzanne Mahlburg Kay* W. Matthew Burns*† Institute for the Study of the Continents and Department of Earth and Atmospheric Sciences, Snee Hall, Cornell University, Ithaca, New York 14853, USA Peter Copeland* Department of Geosciences, University of Houston, Houston, Texas 77204, USA Oscar Mancilla* Repsol YPF, Buenos Aires, Argentina

ABSTRACT Evidence for a Miocene period of transient shallow subduction under the Neuquén Basin in the Andean backarc, and an intermittent Upper Cretaceous to Holocene frontal arc with a relatively stable magma source and arc-to-trench geometry comes from new 40Ar/ 39Ar, major- and trace-element, and Sr, Pb, and Nd isotopic data on magmatic rocks from a transect at ~36°–38°S. Older frontal arc magmas include early Paleogene volcanic rocks erupted after a strong Upper Cretaceous contractional deformation and mid-Eocene lavas erupted from arc centers displaced slightly to the east. Following a gap of some 15 m.y., ca. 26–20 Ma mafic to acidic arclike magmas erupted in the extensional Cura Mallín intra-arc basin, and alkali olivine basalts with intraplate signatures erupted across the backarc. A major change followed as ca. 20–15 Ma basaltic andesite–dacitic magmas with weak arc signatures and 11.7 Ma Cerro Negro andesites with stronger arc signatures erupted in the near to middle backarc. They were followed by ca. 7.2–4.8 Ma high-K basaltic to dacitic hornblendebearing magmas with arc-like high field strength element depletion that erupted in the Sierra de Chachahuén, some 500 km east of the trench. The chemistry of these Miocene rocks along with the regional deformational pattern support a transient period of shallow subduction that began at ca. 20 Ma and climaxed near 5 Ma. The subsequent widespread eruption of Pliocene to Pleistocene alkaline magmas with an intraplate chemistry in the Payenia large igneous province signaled a thickening mantle wedge above a steepening subduction zone. A pattern of decreasingly arc-like Pliocene

*E-mails: Kay—[email protected]; Burns—[email protected]; Copeland— [email protected]; Mancilla—[email protected]. †Now at U.S. Geological Survey, Reston, Virginia 20192, USA.

Kay, S.M., Burns, W.M., Copeland, P., and Mancilla, O., 2006, Upper Cretaceous to Holocene magmatism and evidence for transient Miocene shallowing of the Andean subduction zone under the northern Neuquén Basin, in Kay, S.M., and Ramos, V.A., eds., Evolution of an Andean margin: A tectonic and magmatic view from the Andes to the Neuquén Basin (35°–39°S lat): Geological Society of America Special Paper 407, p. 19–60, doi: 10.1130/2006.2407(02). For permission to copy, contact [email protected]. ©2006 Geological Society of America. All rights reserved.

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S.M. Kay et al. to Holocene backarc lavas in the Tromen region culminated with the eruption of a 0.175 ± 0.025 Ma mafic andesite. The northwest-trending Cortaderas lineament, which generally marks the southern limit of Neogene backarc magmatism, is considered to mark the southern boundary of the transient shallow subduction zone. Keywords: Andes, volcanism, tectonics, Neuquén Basin, shallow subduction, geochemistry, Neogene.

INTRODUCTION The history of the continental lithosphere, mantle wedge, and subducting plate of the south Central Andes, between 36.5°S and 38°S, is reflected in the temporal and spatial distribution and chemistry of the Upper Cretaceous to Holocene arc and backarc magmatic rocks of the Neuquén Basin. Together with the structural and geophysical characteristics of the region, the distribution and geochemical features of these magmatic rocks can be used to formulate a model for the magmatic and deformational history and the evolution of the subducting slab in a west to east transect between 36° and 37.5°S through the Neuquén Basin. GEOLOGIC AND TECTONIC SETTING The east-west transect through the Neuquén Basin between 36.5° and 38°S latitude lies just south of where the Holocene volcanic arc front of the Southern Volcanic Zone is displaced to the west (Fig. 1). At this latitude, the currently subducting Nazca plate corresponds to chrons 8–13 (ca. 33–25 Ma; Cande and Kent, 1992). The Holocene centers of the transitional Southern Volcanic Zone segment to the north dominantly erupt andesitic lavas, whereas those of the southern Southern Volcanic Zone segment dominantly erupt high-Al basalts (see review by Stern, 2004). To the east, the backarc can be divided into two regions by the northwest-trending Cortaderas lineament (Fig. 2), which broadly intersects the southern end of the transitional Southern Volcanic Zone segment. North of the Cortaderas lineament, Miocene to Holocene backarc magmatic rocks are widespread in a retroarc region where Mesozoic rifting was less important and Neogene contractional deformation was more important than to the south (see Ramos and Kay, this volume, chapter 1). Particularly notable in the backarc are the Pleistocene to Holocene backarc Tromen and Payún Matrú volcanic centers and the extensive mafic flows that constitute the Payenia and Auca Mahuída volcanic fields (Figs. 1 and 2). To the south of the Cortaderas lineament, Miocene to Holocene backarc magmatic rocks are essentially absent. The ages and locations of the Upper Cretaceous to Holocene magmatic rocks discussed in this paper are summarized in Table 1. They largely overlie or intrude the Mesozoic to early Paleogene sedimentary strata of the Neuquén Basin. The history of the Neuquén Basin can be divided into three general stages (e.g., Vergani et al., 1995): (1) a Triassic to Early Jurassic

Figure 1. Generalized map of Eocene to Holocene magmatic rocks of the Andean Cordillera and Patagonia from 34° to 52°S, modified from the 1:2,500,000 scale, 1997 geologic map of Argentina (Servicio Geológico Minero Argentino, Buenos Aires) and map in Stern et al. (1990). Boxed area labeled “Central Neuquén Basin transect” between ~36.5 and 38°S is region considered in this paper. SVZ—Southern Volcanic Zone.

Upper Cretaceous to Holocene magmatism

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Figure 2 (on this and following page). Maps of (A) pre-Pliocene and (B) post-Miocene magmatic units in Argentina erupted over the Neuquén Basin region. Maps are modified from the provincial maps of Neuquén (Delpino and Deza, 1995) and Mendoza (Servicio Geológico Minero Argentino, Buenos Aires). Cortaderas lineament is after Ramos (1978). Circles in early Miocene units in backarc schematically indicate late early Miocene volcanic centers. Labels in the undifferentiated Miocene volcanic fields are for the early Miocene Cura Mallín Formation (cm) and the Miocene Trapa Trapa (tt), Cajón Negro (cn), Quebrada Honda (qh) Formations, and the late Miocene to Pliocene? Pichi Neuquén Complex (pn). PQ—Plio-Quaternary. Holocene volcanic centers of the Southern Volcanic Zone arc are from geologic maps of Chile and Argentina.

prerift and rift stage, (2) an Upper Jurassic to Cretaceous subsidence stage, and (3) a Paleocene to Holocene modification stage. The first stage was largely shaped by the extension and fault-controlled subsidence that preceded and accompanied the initial breakup of the Pangea supercontinent. The widespread Triassic Choiyoi rhyolitic volcanic rocks (e.g., Kay et al., 1989) exposed in the Cordillera del Viento (Fig. 2) and underlying much of the Neuquén Basin erupted at this time. The active rifting of this stage generally terminated as Middle Jurassic Andean tectonism and magmatism began to the west. During the second stage, the discrete rifts and intervening basement blocks of the first stage generally merged into a broad postrift basin that was filled by Middle Jurassic to Paleogene sedimentary strata. In the last stage, the Neuquén Basin strata were modified by Tertiary to Holocene extensional and contractional

deformation and affected by periodic magmatic events across the basin. The magmatic rocks of this third stage are the principal topic of this paper. Distribution, Age, and Chemistry of Neuquén Basin Magmatic Rocks The distribution and ages of the Upper Cretaceous to Holocene Neuquén Basin magmatic rocks described below are shown on maps in Figures 1–4 and summarized in Table 1. Twelve new 40Ar/ 39Ar ages are listed in Table 2. Age spectra are presented in Appendix 1. Analytical techniques are the same as in Jordan et al. (2001). New major- and trace-element data for 90 samples and isotopic data for 12 samples are listed in Tables 3–7 and plotted along with 300 other unpublished and pub-

22

S.M. Kay et al.

Figure 2 (continued).

TABLE 1. ARC AND BACKARC MAGMATIC UNITS FROM THE NEUQUÉN ANDES AND NEUQUÉN BASIN (36.5° TO 38°S LATITUDE) Age Arc and near arc Near to middle backarc Far backarc (Ma) Quaternary/Holocene 200–300 ppm) concentrations, are useful in interpreting mantle and subcrustal processes, whereas characteristics of silicic andesite to rhyolites are useful in interpreting crustal processes. Ratios and concentrations of incompatible elements (concentrated in melts) provide insights into mantle magma sources and tectonic settings. Slab-related processes are reflected in ratios of Ti group elements (Ta, Nb) to rare earth elements (REEs) and alkali (K, Rb, Cs)–alkaline earth (Ba, Sr) elements. Ratios of La/Ta, Ba/La, and Ba/Ta in Neuquén Basin samples are compared with those of Southern Volcanic Zone frontal arc magmas (La/Ta ~ 40–95; Ba/La > 20; and Ba/Ta >

500) and mid-ocean-ridge basalt (MORB) and oceanic-island basalt (OIB) magmas (La/Ta < 12; Ba/La < 15) from Hickey et al. (1986) in Figure 6. Indicators of mantle source conditions include the high field strength elements (HFSEs) plotted in Figure 7. High Ta/Hf ratios reflect an enriched intraplate mantle source, whereas low Ta/Hf ratios indicate depleted MORB or arc mantle sources. High Th/Hf ratios are typical of calc-alkaline arc sources. Relative concentrations of incompatible elements also serve as guides to percentages of partial melting in mantle and crustal source regions. Ratios and concentrations of compatible elements (concentrated in minerals) reflect residual minerals that are either left in the magma source after melting or removed by fractionation processes. The residual mineral assemblage reflects the pressure, temperature, and fluid conditions under which the magma last equilibrated. Trace elements are useful in determining residual mineral assemblages because: (1) olivine, orthopyroxene, and micas have little affinity for REEs, but take Ni and Cr, (2) feldspar takes Eu2+ and Sr, (3) clinopyroxene, and to a greater extent amphibole, take middle and heavy REEs and Sc, (4) garnet takes heavy REEs, and (5) accessory titanite and apatite take middle

24

S.M. Kay et al.

Figure 4. Generalized Holocene magmatic and sedimentary rocks west of the Cordillera del Viento and east of the Southern Volcanic Zone (SVZ). Names and locations (points) are shown for samples with analyses in Tables 3–5. Map is after Burns (2002).

Upper Cretaceous to Holocene magmatism

25

TABLE 2. NEW 40Ar/39Ar GEOCHRONOLOGY FOR NEUQUÉN BASIN MAGMATIC ROCKS (SPECTRA IN APPENDIX 1) 40 Ar/39Ar age Sample Unit and locality Type (±1 sigma) (Ma) BPN11 Varvarcó pluton - hornblende-bearing leucogranodiorite Biotite 69.09 ± 0.13 West side of road, east of river, north of Varvarco (36°49.42 S, 70°40.41 W) TDR21 Cerro Negro, andesite flow Hornblende 11.70 ± 0.20 Puesto on southeast side of Cerro Negro (36°50.55 S, 69°57 W) Chos Malal trough and Tromen Massif Biotite 4.0 ± 0.4 TDR16 Tilhué Formation, Cerro Bayo de Tromen - rhyolite dome (37°41.8 S, 69°34 W) TDR19 Intermediate Group Chapúa basalt with large plagioclase phenocrysts Groundmass 1.44 ± 0.08 Road cut north of Chapúa School ( 37°10.77 S, 70°14.95 W) TDR6 Cerro Waile andesite, El Puente Fm. Groundmass 1.04 ± 0.06 1/3 of way up on road to top of Cerro Waile (37°03.6 S, 70°09.5 W) TDR2 Tromen "escorial" andesitic flow Groundmass 0.175 ± 0.028 Cerro Tromen north of Laguna Tromen (37°05.50 S, 70°05.5 W) Southern Payun Matru Field DR38 Flow from Cerro Tanque, Sierra de Chachahuén Northwest of Cerro Campanario (37°0.54 S, 68°56.64' W) DRC14 Young basal flow from well preserved Cerro Méndez cone Just north of Rio Colorado (37°19.45 S, 68°57.30 W) Auca Mahuida Field RD3 Plateau basalt flow, Auca Mahuida West of Puesto Agua del Macho (37°56'S, 69°06'W) Pampa de Las Yeguas basalt in Ardolino et al. (1996) RD1 Plateau basalt flow, Auca Mahuida East of Total well in Rincón Chico (37°49.5 S, 69°46 W), Cerro Las Liebres basalt in Ardolino et al. (1996) RD8 Plateau basalt flow, eastern Auca Mahuida West of the El Cruce Gomeria (37°41.79 S, 68°27.82 W) Cerro Grande basalt in Ardolino et al. (1996) RD20b Mugearite/benmorite flow from crater rim, Auca Mahuida Quebrada on southeast side of rim (37°46.04 S, 68°54.15 W) Auca Mahuida Group of Ardolino et al. (1996)

REEs and zircon takes heavy REEs, Hf, Th, and U. Increasing pressure can produce a change from pyroxene to amphibole to garnet in the residual mineralogy that can be detected by increasing La/Yb and Sm/Yb ratios. La/Yb ratios provide a guide to the overall steepness of the REE pattern (Fig. 8A), whereas La/Sm and Sm/Yb ratios (Fig. 8B) provide a guide to light and heavy REE behavior. Nd, Sr, and Pb isotopic ratios (Figs. 9 and 10) contain independent source region information because they reflect parent/daughter ratios in closed systems and contaminant addition in open systems. Magmas with higher 87Sr/ 86Sr, 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb and lower 143Nd/ 144Nd ratios are said to be relatively isotopically “enriched.” Upper Cretaceous to Eocene Magmatic Rocks The Upper Cretaceous to Paleogene magmatic history of the Neuquén Basin is not well known. Volcanic and plutonic rocks of this age generally occur along a north-south–trending belt that runs through northwestern Neuquén (Fig. 2A). Unlike younger magmatic rocks, they occur only in the western Neuquén Basin. Radiometric ages support dividing them into: (1) Upper Cretaceous, (2) Paleocene, and (3) latest Paleocene to Eocene groups. All intrude or overlie deformed Mesozoic strata, but are not themselves significantly deformed (Llambías et al., 1978; Llambías and Rapela, 1989; Franchini et al., 2003).

Groundmass

2.07 ± 0.11

Groundmass

1.23 ± 0.17

Groundmass

1.78 ± 0.1

Groundmass

1.55 ± 0.07

Groundmass

1.39 ± 0.14

Groundmass

0.99 ± 0.04

Upper Cretaceous to Paleocene magmatism has been summarized by Franchini et al. (2003). Upper Cretaceous activity is confirmed by K/Ar ages of 74.2 ± 1.4 Ma for a biotite from an andesite dike in the Campana Mahuída region (Sillitoe, 1977), a whole-rock K/Ar age of 71.5 ± 5 Ma for an amphibole-bearing andesite sill cutting the Pelán Unit in the Cerro Nevazón region on the east side of the Cordillera del Viento (Llambías et al., 1978; Linares and González, 1990), whole-rock K/Ar ages from 69 ± 4 to 65 ± 3 Ma for tuffs and veins between Andacollo and Huinganco (Vilas and Valencio, 1978), and a whole-rock age of 67 ± 3.2 Ma for a tonalite stock near El Maitenes in the southern Cordillera del Viento (Domínguez et al., 1984). Franchini et al. (2003) also referred to an unpublished age of 64.7 ± 3.2 Ma for a pluton in the Cordillera del Viento. In addition, Zamora Valcarce et al. (this volume, chapter 6) report 40Ar/ 39Ar ages of 72.83 ± 0.83 Ma and 65.5 ± 0.46 Ma, respectively, for a volcanic bomb and the Cerro Naunauco laccolith in the Collipilli region. A new 40Ar/ 39Ar biotite age of 69.09 ± 0.13 Ma from a granodiorite pluton near Varvarcó in the western Cordillera del Viento in Table 2 is interpreted as a cooling age. Paleocene magmatism is confirmed by a 40Ar/ 39Ar age of 56.64 ± 0.44 Ma for an andesitic sill in the Collipilli region (Zamora Valcarce et al., this volume, chapter 6) and by hornblende K/Ar ages of 59.1 ± 2.9 Ma and 56.5 ± 1.7 Ma from a gabbro and a diorite in the Cerro

5.05 2.10 4.5 1.5 6.6 0.82 31.6 876 27.8 0.34 0.147 1.38 135 4961 3.5 36°49.42 70°40.41 1265 Cret.

13.9 32.1 13.1 3.12 0.796 0.448 2.10 0.295 361 439 3.3 1.3 4.7 3.4 0.50 12.1 5 2 10

La Ce Nd Sm Eu Tb Yb Lu Sr Ba Cs U Th Hf Ta Sc Cr Ni Co

Na2O+K2O FeO/MgO La/Sm Sm/Yb La/Yb Eu/Eu* Ba/La Ba/Ta La/Ta Th/La Ta/Hf Th/Hf Ba/Cs K/Cs Th/U Latitude °S Longitude °W Elevation (m) Age (Ma)

BPN11 67.24 0.46 14.34 4.74 0.13 2.26 5.67 3.11 1.95 0.06 99.95

Fm./Group Locality Sample SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total

1 Varvarcó

4.50 2.05 3.9 1.7 6.7 0.82 22.5 1205 53.5 0.32 0.121 2.06 437 8687 4.1 37.139° 70.699°

15.8 33.0 16.2 4.02 1.17 0.626 2.37 0.332 513 356 0.8 1.2 5.0 2.4 0.30 20.9 9 6 18

Paleocene/Early Eocene

3.90 1.73 3.3 1.6 5.2 0.82 27.2 1091 40.1 0.25 0.119 1.19 489 7354 3.3 37.188° 70.691°

11.3 24.4 13.1 3.43 1.04 0.621 2.19 0.316 437 308 0.6 0.9 2.8 2.4 0.28 21.3 4 4 19 4.35 2.17 2.9 1.5 4.4 0.82 26.1 1211 46.3 0.25 0.096 1.10 662 17913 3.7 37.142° 70.729°

11.1 22.4 15.7 3.87 1.06 0.686 2.55 0.370 365 291 0.4 0.7 2.8 2.5 0.24 30.9 5 4 47 4.57 1.78 3.8 1.6 6.2 0.82 26.2 1122 42.8 0.29 0.110 1.37 20 296 3.5 37.145° 70.667°

14.4 30.7 15.3 3.80 1.05 0.626 2.33 0.330 375 376 19.1 1.2 4.2 3.1 0.34 36.6 21 1 23 4.51 2.04 3.6 1.6 5.7 0.82 20.9 981 47.0 0.28 0.103 1.36 257 7251 3.9 37.135° 70.711°

13.7 30.0 12.9 3.83 1.15 0.633 2.43 0.349 424 287 1.1 1.0 3.9 2.8 0.29 22.4 3 0 20 6.36 2.02 4.1 1.7 7.0 0.82 22.2 952 42.8 0.25 0.114 1.21 385 7362 4.1 37.118° 70.691°

30.5 67.4 35.8 7.41 2.03 1.031 4.38 0.614 673 677 1.8 1.8 7.5 6.2 0.71 13.5 4 4 12 6.08 2.36 3.1 1.9 5.9 0.82 20.1 1275 63.5 0.29 0.070 1.29 1307 20934 3.7 37.279° 70.736°

40.7 99.7 51.0 13.31 3.08 2.012 6.94 1.058 617 819 0.6 3.2 11.8 9.2 0.64 54.6 6 9 29

Eocene

7.95 2.65 3.7 1.9 6.9 0.82 20.7 1046 50.4 0.19 0.086 0.83 280 9561 3.8 37.270° 70.818°

21.7 52.5 26.1 5.84 1.65 0.836 3.15 0.461 525 450 1.6 1.1 4.1 5.0 0.43 13.1 2 5 12 6.41 1.82 2.5 2.9 7.4 0.97 27.6 1902 68.9 0.16 0.075 0.84 26 619 4.5 36°42 69°50.2

13.9 34.6 22.7 5.47 1.55 0.730 1.89 0.269 604 383 14.7 0.5 2.2 2.7 0.20 29.8 94 40 31 7.67 2.46 3.5 3.1 10.9 0.97 25.6 1949 76.2 0.19 0.091 1.33 48 1475 3.7 36°41 69°50.45

21.5 49.4 28.0 6.17 1.55 0.760 1.97 0.283 653 549 11.5 1.1 4.1 3.1 0.28 23.3 10 8 25

7.48 2.32 3.5 3.0 10.6 0.97 35.9 2589 72.2 0.20 0.094 1.33 74 2030 3.5 36°42 69°50.2

19.9 43.8 25.3 5.59 1.56 0.729 1.87 0.305 780 712 9.6 1.1 3.9 2.9 0.28 22.5 13 6 26

8.18 2.74 6.7 4.0 26.6 0.97 33.4 2313 69.3 0.41 0.157 4.49 369 4967 4.4 36°41 69°50.45

55.0 108.7 45.4 8.25 1.88 0.363 2.07 0.285 1402 1835 5.0 5.2 22.8 5.1 0.79 4.8 3 2 7

8.14 2.98 6.1 3.1 18.9 0.97 37.9 2437 64.3 0.45 0.157 4.54 1400 15159 4.2 36°50.55 69°57

43.0 86.8 37.6 7.01 1.53 0.571 2.28 0.348 959 1631 1.2 4.6 19.3 4.3 0.67 10.2 26 8 12

TABLE 3. WHOLE-ROCK CHEMISTRY OF LATE CRETACEOUS TO MIOCENE MAGMATIC ARC ROCKS FROM THE NEUQUÉN ANDES 2 3 4 5 6 7 8 9 10 11 12 13 14 Eocene Cayanta Formation west of the Cordillera del Viento late Paleocene to Eocene in southern Mendoza Between Andacolla and Cayanta Río Guañacos Cerro Bayo de la Esperanza ESA-2 ESA-7 ESA-12 ESA-11 ESA-5 ESA-3 RG-10 RG-7 TDR8a TDR9a TDR9c TDR9b TDR11 50.36 52.98 52.81 53.58 54.21 57.42 55.62 55.80 48.04 49.11 50.35 57.02 54.33 0.66 0.88 1.14 0.88 0.85 0.67 1.39 0.69 1.25 1.22 1.08 0.61 0.73 18.98 18.90 18.27 19.05 17.30 17.95 15.98 19.40 18.06 19.65 19.08 18.12 17.72 7.94 7.83 8.23 7.64 8.76 6.07 8.87 5.80 10.74 9.68 9.06 5.43 6.37 0.21 0.21 0.21 0.17 0.22 0.21 0.29 0.20 0.16 0.13 0.14 0.20 0.22 4.58 3.81 3.79 4.28 4.29 3.00 3.76 2.19 5.89 3.93 3.91 1.98 2.13 8.94 10.58 8.76 9.49 9.56 7.53 7.38 7.69 7.92 7.72 7.41 7.71 10.05 3.34 3.65 3.40 3.89 3.54 4.80 4.50 6.10 5.31 5.62 5.12 5.20 6.01 0.56 0.85 0.95 0.68 0.97 1.56 1.58 1.85 1.10 2.05 2.36 2.98 2.13 0.05 0.22 0.12 0.13 0.21 0.33 0.39 0.28 0.43 0.30 0.29 0.31 0.31 95.61 99.91 97.68 99.80 99.91 99.54 99.77 100.00 98.90 99.41 98.79 99.56 100.01

(continued)

5.80 1.31 4.5 4.7 20.8 0.97 16.0 282 17.6 0.19 0.441 1.47 543 13291 5.0 36°50 69°03

29.1 61.4 30.9 6.51 1.754 0.848 1.40 0.179 955 466 0.9 1.1 5.5 3.7 1.65 18.9 203 74 34

Las Lajas DRC16 49.93 1.96 16.75 8.49 0.16 6.50 8.73 4.43 1.37 0.76 99.08

15

16.0 36.2 21.6 3.72 0.873 0.470 1.70 0.239 577 529 1.8 1.3 3.7 2.5 0.36 6.1 2 2 5

7.32 17.8 11.8 2.82 0.896 0.473 1.78 0.285 651 451 5.6 1.2 3.2 2.1 0.26 10.2 5 3 3

14.2 30.3 12.0 3.16 1.070 0.460 1.77 0.252 647 1436 11.4 1.4 3.3 2.3 0.29 11.4 7 2 8

10.3 23.8 12.4 2.76 0.733 0.362 1.62 0.226 425 957 8.7 1.0 4.1 2.4 0.28 9.0 4 2 5

19.6 43.9 29.3 6.23 1.38 1.06 2.88 0.416 422 477 0.6 1.4 5.1 4.0 0.52 28.6 38 18 26

11.6 25.5 12.7 3.55 1.04 0.583 1.89 0.279 384 235 0.5 0.8 2.9 2.3 0.25 28.5 43 24 30

37.8 87.7 44.7 11.7 2.06 2.24 5.65 0.881 368 633 5.4 3.7 13.4 8.4 0.97 34.8 18 15 28

5.31 8.33 4.70 4.84 Na2O+K2O FeO/MgO 2.64 1.33 1.82 3.17 La/Sm 4.3 2.6 4.5 3.7 3.1 3.3 3.2 Sm/Yb 2.2 1.6 1.8 1.7 2.2 1.9 2.1 La/Yb 9.4 4.1 8.1 6.4 6.8 6.1 6.7 Eu/Eu* 0.79 0.97 1.09 0.51 0.68 0.91 0.51 Ba/La 33.0 61.6 100.9 92.8 24.4 20.3 16.8 Ba/Ta 1486 1755 4989 3439 912 953 649 La/Ta 45.0 28.5 49.5 37.1 37.4 46.9 38.7 Th/La 0.23 0.43 0.23 0.40 0.26 0.25 0.36 Ta/Hf 0.142 0.125 0.128 0.116 0.131 0.108 0.116 Th/Hf 1.46 1.54 1.46 1.71 1.29 1.28 1.60 Ba/Cs 288 81 127 110 802 514 117 K/Cs 4630 1396 19033 1934 Th/U 2.8 2.7 2.4 4.1 3.8 3.8 3.6 Latitude °S 37°19.91 37°19.91 37°19.91 37°25.15 37°25.15 37°25.15 37°27.43 Longitude °W 70°26.62 70°23.69 70°23.69 70°23.69 70°23.18 70°23.18 70°23.18 Elevation (m) 1195 1807 1807 1807 872 872 872 Age (Ma) Note: Analytical techniques for major- and trace-element analyses are described in Kay et al. (this volume, chapter 10).

La Ce Nd Sm Eu Tb Yb Lu Sr Ba Cs U Th Hf Ta Sc Cr Ni Co 3.4 1.6 5.4 0.87 28.2 1372 48.6 0.31 0.106 1.59 564 3.4 37°25.15 70°23.18 872

15.5 35.2 20.6 4.62 1.30 0.751 2.85 0.380 619 437 0.8 1.4 4.8 3.0 0.32 19.9 5 8 20

5.99 2.81 3.1 1.4 4.4 0.90 33.6 1384 41.2 0.21 0.102 0.88 323 13143 3.8 37°04.69 70°17.91 1606

10.6 25.9 15.9 3.46 1.09 0.575 2.44 0.332 458 358 1.1 0.6 2.2 2.5 0.26 9.2 3 3 11

5.79 2.36 4.0 1.6 6.5 0.90 35.0 1278 36.5 0.27 0.108 1.05 134 3262 3.5 37°05.99 70°21.53 1530 11.7 ± 0.20

15.3 35.6 16.8 3.81 1.00 0.558 2.35 0.334 462 535 4.0 1.2 4.1 3.9 0.42 9.0 4 3 10

TABLE 3. WHOLE-ROCK CHEMISTRY OF LATE CRETACEOUS TO MIOCENE MAGMATIC ARC ROCKS FROM THE NEUQUÉN ANDES (continued) 16 17 18 19 20 21 22 23 24 25 Fm./Group Late Paleocene to Eocene east of the Cordillera del Viento in Neuquén Miocene Locality Cerro Caicayen Cerro Mayal south Sierra del Mayal Cerro Negro Sample TDR22 TDR28a TDR28d TDR28b TDR23a TDR23b TDR23d TDR23c TDR34 TDR21 SiO2 59.93 58.86 52.62 52.94 60.25 60.77 TiO2 0.53 0.53 1.22 2.04 0.65 0.67 22.25 19.24 19.33 17.18 16.98 17.16 Al2O3 FeO 3.80 4.19 4.37 3.06 7.67 8.14 10.29 8.69 6.78 5.87 MnO 0.16 0.23 0.30 0.18 0.20 0.18 MgO 1.44 3.15 4.22 3.24 2.41 2.49 CaO 6.99 5.67 9.17 8.00 6.57 6.56 4.28 7.39 6.93 3.71 3.33 2.61 3.58 3.87 4.24 4.23 Na2O 1.02 0.94 1.36 1.26 1.75 1.57 K2O 0.22 0.03 0.33 0.49 0.20 0.24 P2O5 Total 100.62 100.24 99.55 99.21 100.02 99.73

Upper Cretaceous to Holocene magmatism 27

VL-3 47.91 1.33 18.53 9.98 0.17 6.59 10.78 2.50 0.29 0.21 98.29

15.5 30.8 19.2 4.61 1.279 0.723 2.18 0.331 491 276 0.2 0.5 2.3 2.6 0.37 35.2 196 80 42

2.79 1.51 3.4 2.1 7.1 0.51 17.8 750 42.1 0.15 0.14 0.90 1673 14580 4.3 36.852 71.067 1687 ca. 24

Fm./Group Sample SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total

La Ce Nd Sm Eu Tb Yb Lu Sr Ba Cs U Th Hf Ta Sc Cr Ni Co

Na2O+K2O FeO/MgO La/Sm Sm/Yb La/Yb Eu/Eu* Ba/La Ba/Ta La/Ta Th/La Ta/Hf Th/Hf Ba/Cs K/Cs Th/U Latitude °S Longitude °W Elevation (m) Age (Ma)

1

3.05 1.47 3.3 2.0 6.6 0.51 21.2 757 35.7 0.13 0.15 0.68 404 4898 3.0 37.016 71.013 1480 ca. 24

15.2 30.9 19.4 4.57 1.303 0.740 2.29 0.327 483 322 0.8 0.6 1.9 2.8 0.43 30.4 157 69 36

RB-10 48.65 1.40 18.01 9.09 0.20 6.18 10.81 2.58 0.47 0.30 97.69

2

3.02 1.40 2.9 1.7 5.0 0.51 19.5 1014 52.1 0.11 0.10 0.58 1583 27405 4.7 36.829 71.095 1460 ca. 24

10.1 24.0 15.1 3.47 1.151 0.648 2.02 0.280 461 196 0.1 0.2 1.1 2.0 0.19 31.6 118 61 39

LE-5 48.98 1.08 17.84 8.85 0.16 6.33 10.54 2.61 0.41 0.11 96.91

3

3.90 1.54 4.0 2.2 8.6 0.51 21.2 834 39.4 0.14 0.18 1.00 3543 53801 3.6 36.845 71.066 1875 ca. 24

22.4 46.8 26.4 5.62 1.538 0.717 2.61 0.340 621 474 0.1 0.9 3.2 3.2 0.57 30.2 126 57 35 5.97 2.93 2.5 1.7 4.3 0.51 34.8 1434 41.2 0.14 0.11 0.62 1248 29840 3.1 37.025 71.023 1462 ca. 24

13.5 29.9 22.0 5.39 1.577 0.929 3.12 0.461 694 469 0.4 0.6 1.8 3.0 0.33 23.8 7 8 19 7.91 7.51 5.3 1.8 9.6 0.51 31.3 1284 41.0 0.24 0.18 1.75 180 5277 2.9 37.371 70.974 1443 22.8 ± 0.7

29.0 64.9 25.7 5.51 1.106 0.858 3.03 0.444 249 907 5.0 2.4 7.1 4.0 0.71 5.7 3 0 4 9.89 3.75 4.0 2.5 10.1 0.51 17.1 1202 70.5 0.22 0.09 1.42 388 43046 3.7 37.371 70.974 1443 26.3 ± 1.5

17.0 37.9 20.0 4.23 1.227 0.546 1.69 0.235 654 290 0.7 1.0 3.7 2.6 0.24 21.3 44 35 24 4.39 2.91 2.5 1.8 4.5 0.51 14.7 642 43.8 0.18 0.09 0.71 620 20779 3.0 37.223 70.917 1700 ca. 20

20.3 48.8 35.6 8.20 2.064 1.375 4.49 0.646 406 297 0.5 1.2 3.7 5.1 0.46 31.6 7 12 27

ca. 20

4.95 2.20 2.6 1.8 4.6 0.51 18.6 883 47.4 0.15 0.09 0.65 447 14998 3.0 37.214 70.927

11.8 28.3 17.3 4.55 1.340 0.724 2.60 0.374 467 220 0.5 0.6 1.8 2.8 0.25 22.1 8 7 24 4.37 2.62 3.0 2.0 6.1 0.51 17.7 775 43.8 0.28 0.09 1.12 277 9521 3.2 37.374 70.975 1425 ca. 20

17.1 38.2 22.9 5.66 1.354 0.976 2.78 0.414 515 302 1.1 1.5 4.7 4.2 0.39 27.8 6 5 23 3.56 2.35 2.4 1.6 3.8 0.51 21.7 1115 51.4 0.17 0.08 0.69 213 3137 2.8 37.146 70.993 1991 ca. 20

8.6 20.5 13.6 3.54 1.038 0.624 2.26 0.314 416 188 0.9 0.5 1.5 2.1 0.17 25.3 16 7 23 5.75 2.57 3.1 1.9 6.0 0.51 20.9 856 41.0 0.26 0.11 1.10 188 5725 3.3 36.864 70.965 1456 ca. 12

20.8 49.6 29.3 6.74 1.583 1.027 3.47 0.506 502 433 2.3 1.6 5.3 4.8 0.51 23.6 7 7 22

TABLE 4. WHOLE-ROCK CHEMISTRY: MIOCENE CURA MALLIN AND TRAPA TRAPA FORMATIONS 4 5 6 7 8 9 10 11 12 Cura Mallin Fm. Trapa Trapa Fm. ca. 20 Ma LE-3 RB-11 RR6 AL2 RLL-15 RLL-13 RR-5 AP-3 RR-10 49.08 53.42 71.59 67.31 50.61 52.19 51.68 51.74 54.42 1.35 1.57 0.29 0.41 1.89 1.21 1.39 1.16 1.25 18.46 19.97 14.39 15.35 16.10 18.37 18.73 18.79 18.94 9.03 8.94 2.82 3.20 11.31 9.93 9.01 8.94 8.38 0.18 0.19 0.07 0.13 0.23 0.21 0.14 0.28 0.19 5.88 3.05 0.38 0.85 3.89 4.50 3.44 3.80 3.26 10.00 5.12 2.15 2.48 8.30 8.20 9.56 9.22 7.30 3.03 4.62 4.70 6.01 3.19 4.06 3.12 3.22 4.16 0.87 1.35 3.21 3.88 1.20 0.89 1.25 0.33 1.59 0.26 0.27 0.09 0.10 0.58 0.22 0.05 0.40 98.15 98.51 99.70 99.71 97.30 99.55 98.54 97.54 99.88

4.83 2.40 3.6 2.5 9.1 0.51 27.1 1455 53.7 0.19 0.09 0.92 626 14787 2.8 36.864 70.965 1456 ca. 12

13.7 26.3 17.9 3.81 0.935 0.501 1.50 0.225 634 371 0.6 1.0 2.7 2.9 0.25 15.7 3 8 21

4.53 2.51 3.1 1.7 5.2 0.51 23.8 913 38.4 0.34 0.09 1.16 294 9792 3.8 37.223 70.917 1700

15.7 37.4 19.1 4.98 1.228 0.829 3.01 0.432 451 373 1.3 1.4 5.3 4.5 0.41 17.5 3 5 15

6.78 2.18 4.6 1.7 7.9 0.51 24.6 1015 41.3 0.23 0.11 1.07 455 10013 3.5 37.368 71.019 1545 ca. 15

23.3 51.6 23.7 5.03 1.402 0.691 2.94 0.461 508 573 1.3 1.5 5.3 4.9 0.56 11.7 7 3 11

13 14 15 Trapa Trapa Fm. 28; Ta/Hf < 0.15) and fluid mobile element enrichment (Ba/La > 20). In detail, there are differences among them. The Cayanta Formation samples west of the Cordillera del Viento are generally similar to Holocene Southern Volcanic Zone arc samples. In both cases, basaltic to mafic andesitic samples have high Al and low Ti contents, arc-like La/Ta (40–64), Ba/La (21–27), and Ta/Hf (0.07–0.12) ratios, and relatively flat REE patterns (La/Yb = 4–7, La/Sm = 3.3–4.1, Sm/Yb = 1.5–1.9). The initial 87Sr/ 86Sr and 143Nd/ 144Nd ratios of a Cayanta basaltic andesite (Table 7) are near those of the Southern Volcanic Zone lavas (Fig. 9). The Cayanta Formation flows differ from the Southern Volcanic Zone lavas in that they typically have amphibole phenocrysts. In contrast, Cerro Bayo de La Esperanza and Cerro Caicayén region samples are more like early Paleocene samples in that they have higher alkali contents, higher La/Ta, Ba/Ta, La/Yb, La/Sm, and Sm/Yb ratios, and lower Ta/Hf ratios than Cayanta samples (Figs. 5–8). Mafic Bayo de la Esperanza region samples (48%–57% SiO2) are particularly notable for their high Na2O (4.4%–6.0%), Sr (604–1402 ppm), and Ba (to 1835 ppm) contents and high La/Ta (64–72), La/Sm (up to 6.7), and Sm/Yb (2.9–4.5) ratios. Cerro Caicayén quartz diorites (59%–62% SiO2) also have high La/Ta* (55–100, where Ta* = Nb/16), Ba/Ta* (most >1400), La/Yb (>9) and La/Sm (up to 14) ratios, but differ in having lower Sm/Yb ratios ( 3.5; C 87 88 to +4.2; Sr/ Sr = 0.7037–0.7040) virtually devoid of arc-like components (La/Ta < 14; Ba/La 0.45). The least arc-like signatures are found in the Sierra de Chachahuén and Matancilla region magmas, which erupted farthest from the arc (La/Ta 1.8, Fig. 7), variable La/Yb, La/Sm, and Sm/Yb ratios (Fig. 8), low 87Sr/ 86Sr ratios at a given εNd (Fig. 9), and high 206Pb/ 204Pb ratios (Fig. 10). Kay et al. (this volume, chapter 10) attribute the intraplate-like character of the older Vizcachas lavas (La/Ta 18), La/Sm (~6–11), and Sm/Yb (>3) of the silicic Vizcachas lavas are attributed to removal of accessory REEbearing phases (e.g., titanite), the importance of which was diminished in the Chachahuén lavas (La/Yb = 7–15; La/Sm ~ 2.5–7; Sm/Yb ~ 1.8–3.2). The Chachahuén volcanic complex is considered to be contemporaneous with the petrologically similar Plateado-Nevado center to the north (Fig. 2B; Bermúdez, 1991; Bermúdez et al., 1993). Pliocene to Holocene Magmatic Rocks West of the Cordillera del Viento Pliocene to Pleistocene volcanic rocks are widespread between the Holocene Southern Volcanic Zone arc and the western side of the Cordillera del Viento (Figs. 2B and 3). Early Pliocene volcanic rocks that form the base and surrounding region of the Pleistocene to Holocene arc are assigned to the Cola de Zola Formation, which has an age of ca. 5.6–3 Ma (Vergara and Muñoz, 1982). Pliocene basaltic to dacitic rocks to the east in Argentina between 37°S and 37.5°S are assigned to the Centinela Formation, K/Ar ages of which range from 3.2 ± 0.2 to 2.6 ± 0.1 Ma (Rovere, 1998). Farther north, a similar age range seems reasonable for the younger part of the Pichi Neuquén Complex. The Pliocene Centinela Formation flows near the western side of the Cordillera del Viento are followed by the Pleistocene olivine basalt flows of the Guañacos Formation, which has K/Ar ages of 1.4 ± 0.2 to 1.2 ± 0.1 Ma (Rovere, 1998). The Cerro Colorado olivine basalt flow mapped on the west side of the Cordillera del Viento to the north by Pesce (1981) is likely to be of this age. The characteristics of other similar late Miocene to Quaternary volcanic rocks northwest,

west, and southwest of the Cordillera del Viento are summarized by Miranda et al. (this volume, chapter 13), Lara and Folguera (this volume, chapter 14), and Folguera et al. (this volume, chapter 12). Holocene Southern Volcanic Zone centers are discussed by Hildreth and Moorbath (1988), Tormey et al. (1991), and Dungan et al. (2001) and references therein. Southern Volcanic Zone rocks from the Copahue volcano near 38°S are addressed by Varekamp et al. (this volume, chapter 15). The chemistry of representative samples from the Pichi Neuquén, Centinela, and Guañacos Formations is presented in Table 5 and plotted in Figures 5–8. These analyses along with those in Rovere (1998) show that these Pliocene to Pleistocene centers have arc-like characteristics similar to those in the Eocene Cayanta Formation, Miocene Cura Mallín and Trapa Trapa Formations, and younger Southern Volcanic Zone centers (Planchon, Cerro Azul, San Pedro, Antuco, Llaima, Villarrica and Puyehue; see Tormey et al., 1991). A new analysis of an Antuco basalt (51% SiO2) in Table 5 with La/Ta = 47, Th/Hf = 0.679, Ta/Hf = 0.09, Ba/La = 23, La/Yb = 5.0, and Sm/Yb = 2.7 is representative of the Southern Volcanic Zone samples. Pliocene to Holocene Magmatic Rocks East of the Cordillera del Viento Pliocene to Holocene arc magmas are contemporaneous with voluminous backarc magmas erupted east of the Cordillera del Viento (Table 1; Fig. 2B). Between 36°S and 38°S, these eruptions produced the small mafic Pliocene flows west of the Auca Mahuída and Payún Matrú fields, the Pliocene to Holocene mafic to silicic volcanic rocks of the Tromen region (Fig. 4), and the voluminous latest Pliocene to Pleistocene mafic alkaline flows in the Auca Mahuída, Payún Matrú, and Llancanelo fields. Early Pliocene Backarc Alkaline Flows. The oldest postMiocene alkaline volcanic rocks in the transect (Fig. 2B) include the flows west of the Payún Matrú field (González Díaz, 1979) and the Parva Negra and Horqueta Norte flows west of the Auca Mahuída field (Ramos and Barbieri, 1988). González Díaz (1979) argued for a Pliocene age for the flows west of the Payún Matrú field on the basis of geomorphology and two whole-rock K/Ar ages of 8 ± 4 Ma and 4 ± 1 Ma. Ramos and Barbieri (1988) reported a K/Ar whole-rock age of 4.5 ± 0.5 Ma for the Parva Negra flow. Chemical analyses for the Parva Negra and Horqueta Norte flows are presented in Table 6 and plotted in Figures 5–8. These flows are olivine alkaline basalts (46%–48% SiO2) with intraplate-like low La/Ta (14–16) and Ta/Hf (0.36– 0.43) ratios and Ba/Ta (192–570) and Ba/La ratios (14–37) that show a variable intraplate signature. The Near to Middle Backarc—Tromen Region. The distribution of post-Miocene volcanic units in the Tromen region is shown on the maps in Figures 2B and 4. The more detailed map in Figure 4 is based on the Chos Malal and Buta Ranquil geologic maps of Zollner and Amos (1973) and Holmberg (1976) and uses the volcanic stratigraphy of Groeber (1946). New 40Ar/ 39Ar ages in Table 2 and locations of samples with analy-

Upper Cretaceous to Holocene magmatism ses in Table 6 are shown on Figure 4. The magmatic sequences on Figure 4 are divided into three groups in the following discussion: (1) an older andesitic and rhyolitic group that includes the Coyocho (Basalto II) and Tilhué (Andesite III) Formations, (2) an intermediate-age basaltic to mafic andesitic group that includes the Chapúa (Basalto III), Maipo (Basalto IV), and El Puente (Basalto V) Formations and is here designated the Chos Malal group, and (3) a younger, mostly andesitic Tromen Formation group that includes Basaltos VI and VIII. The oldest group incorporates the eroded Coyocho Formation mafic flows west of Cerro Waile and Cerro Tilhué, east of Cerro Michico and in the Cerro Bayo region. They are mapped as being older than the Tilhué Formation, which includes the hydrothermally altered rhyolite complex in the Cerro Bayo region, rhyolites on the south side of Cerro Tromen, the wellpreserved biotite-bearing rhyolitic (75%–76% SiO2) ignimbritedome complex and pyroclastic flows at Cerro Tilhué, and the tuffs west of Cerro Tromen (Tilhué Formation on map). The Cerro Bayo, Cerro Tromen, and Cerro Tilhué complexes could be aligned along a northeast- to southwest-trending fault zone. A biotite in a Cerro Bayo rhyolite yielded the new 40Ar/ 39Ar age of 4.04 ± 0.4 Ma, shown in Table 2. Samples analyzed from the Coyocho mafic flows have basaltic andesitic to andesitic compositions and are characterized by arc-like Ba/La ratios (up to 35), La/Ta (24–34), and Ta/Hf (0.17–0.22) ratios, relatively flat REE patterns (La/Yb = 10–16), and moderate Eu negative anomalies (0.62–0.90). Samples from Cerro Tilhué and Cerro Bayo and a pumice clast from a tuff northeast of the Chapúa School are rhyolites with 75%–76% SiO2, 4.3%–4.9% Na2O, and 4.0%–4.7% K2O. Their trace elements show relatively flat REE patterns (La/Yb = 11–16), large negative Eu anomalies (Eu/Eu* is 0.2 in Cerro Tilhué to 0.6 in Cerro Bayo), arc-like Ba/La (18–29) ratios, and intraplate La/Ta (12–17) and Ta/Hf (0.37–0.44) ratios. The pumice is compositionally most like the Cerro Tilhué rhyolite. – Nd (+2.1 to +2.7), and The 87Sr/ 86Sr ratios (0.7041–0.7042), C Pb isotopic ratios in the Cerro Tilhué and Cerro Bayo rhyolites overlap those of younger Tromen region and Southern Volcanic Zone magmas (Figs. 9 and 10). The intermediate-age Chos Malal flows are concentrated in the depression between Cerro Tromen and the Cordillera del Viento that Zapata et al. (1999) designated the Chos Malal trough. Published K/Ar ages for these flows include duplicate ages of 2.3 ± 0.5 and 2.1 ± 0.5 Ma for a flow north of the Chapúa School and an age of 1 ± 0.2 Ma for a flow near Tricao Malal (Valencio et al., 1970). New 40Ar/ 39Ar ages in Table 2 are 1.44 ± 0.08 Ma for the groundmass of a flow near the Chapúa School and 1.04 ± 0.06 Ma for the groundmass of a silicic andesite from Cerro Waile (Fig. 4). As shown in Table 6, most Chos Malal flows have basaltic to basaltic andesite compositions (50%–55% SiO2). Some are characterized by large plagioclase phenocrysts. A sample from Cerro Waile is a silicic andesite. Overall, they are characterized by relatively flat REE patterns (La/Yb = 7–16) with minimal

43

heavy REE depletion (Sm/Yb < 3), moderate to small negative Eu anomalies (0.7–0.9), and arc-like La/Ta (27–34), Ba/La (17–24), and Ta/Hf (0.15–0.24) ratios. As shown in Figure 9 and Table 7, Chos Malal mafic flows are slightly less isotopi– Nd near +3.2) than cally enriched (87Sr/ 86Sr = 0.7038–0.7040; C 87 86 – Nd = +2). the Cerro Waile andesite ( Sr/ Sr = 0.7044; C The last group includes the youngest flows from Cerro Tromen and the surrounding region. New analyses in shown in Table 6 and plotted in Figures 5–8 along with those in Llambías et al. (1982) and Stern et al. (1990) show that these flows are primarily andesitic in composition. Duplicate 40Ar/ 39Ar groundmass determinations presented in Table 2 yield an average age of 0.175 ± 0.025 Ma for the young “escorial” flow. Chemically, Cerro Tromen andesites in Table 6 have ~60% SiO2, 2.5%–3% K2O, 1% TiO2, and FeO/MgO ratios ~ 2. Compared to the Cerro Waile Chos Malal trough andesite, they have higher K2O contents, generally less arc-like La/Ta (17–23), Ba/Ta (327–550), and Ta/Hf ratios (0.19–0.2), and higher La/Sm and Sm/Yb ratios. Andesitic blocks in the Agua Carmonina Formation deposits east of Cerro Tromen (Fig. 3) are chemically similar. A basalt (7% MgO, 206 ppm Cr) from south of Cerro Michico (Fig. 3) has distinctly intraplate-like La/Ta (20), Ba/La (19), and Ta/Hf (0.20) ratios and a flat REE pattern – Nd (La/Yb = 6.0, Sm/Yb = 2). The 87Sr/ 86Sr ratio (0.7039), C (+3.4), and Pb isotopic ratios of the “escorial flow” (Table 7; Figs. 9 and 10) overlap those of Southern Volcanic Zone flows to the west. Auca Mahuída and Southern Payún Matrú Fields. Further east are the extensive late Pliocene to Pleistocene Auca Mahuída, Payún Matrú, and Llancanelo alkaline volcanic fields (Fig. 2B). These fields are composed of basaltic to hawaiitic shield volcanoes, monogenetic to polygenetic cones, and differentiated mugearite to trachyandesite flows, domes, and pyroclastic rocks that are concentrated in the high-standing regions. The general characteristics of these fields have been summarized by Bermúdez et al. (1993) and Muñoz Bravo et al. (1989). Only the southern Payún Matrú and Auca Mahuída fields are discussed here. The general distribution of centers in the southern Payún Matrú region is shown on maps by Holmberg (1964), González Díaz (1972, 1979), and Pérez and Condat (1996). The region includes extensive flows in and north of the Sierra de Chachahuén region, as well as the cones and flows near the Río Colorado (Fig. 4). Dated flows have largely yielded Pleistocene ages. Among these are whole-rock K/Ar ages of 1.1 ± 0.5 Ma and 1.0 ± 0.4 Ma for Sierra de Chachahuén flows west of Cerro Ureta (37°2′S, 68°56′W) and north of Cerro Ratón (37°2.11′S, 68°51′W) given in Pérez and Condat (1996). Pérez and Condat (1996) reported a K/Ar age of 2.26 ± 0.07 Ma for a flow from Cerro Tanque in the northern Sierra de Chachahuén. A new 40Ar/ 39Ar groundmass age of 1.48 ± 0.14 Ma for a flow from Cerro Méndez north of the Río Colorado is shown in Table 2. The distribution of centers in the Auca Mahuída field is shown on maps by Holmberg (1964), Ardolino et al. (1996),

44

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and Delpino and Bermúdez, 2005, personal commun.). The 40Ar/ 39Ar ages in Rossello et al. (2002), which range from 1.7 ± 0.2 Ma to 0.88 ± 0.03 Ma, and new 40Ar/ 39Ar ages in Table 2 show that these rocks erupted in the last 2 m.y., which is in accord with the general Pleistocene age assignments in Holmberg (1964) and Uliana (1978). The groundmass 40Ar/ 39Ar ages in Table 2, correlated with the mapping units of Ardolino et al. (1996), are: 1.78 ± 0.1 Ma for a Pampa de las Yeguas basalt, 1.55 ± 0.07 Ma for a Cerro Las Liebres basalt, 1.39 ± 0.14 Ma for a Cerro Grande basalt, and 0.99 ± 0.04 Ma for a Cerro Auca Mahuída mugearite. The 40Ar/ 39Ar ages are in accord with the relative volcanic sequence proposed by Ardolino et al. (1996) but not the late Miocene to Pleistocene age assignments. The chemistry of southern Payún Matrú and Auca Mahuída field samples analyzed by Kay (2001b) are plotted in Figures 5–9. As in other analyses from this region (e.g., Delpino and Bermúdez, 1985; Bermúdez and Delpino, 1989; Muñoz and Stern, 1988, 1989; Muñoz Bravo et al., 1989; Stern et al., 1990; Bermúdez et al., 1993; Saal et al., 1993, 1995; Saal, 1994), the southern Payún Matrú and Auca Mahuída volcanic rocks are olivine-bearing alkali basalts, hawaiites, benmorites, trachyan-

desites, and trachytes (Fig. 5) characterized by intraplate La/Ta (11–16) and Ta/Hf (0.38–0.52) ratios and transitional arc-like Ba/La ratios (most 16–25). La/Yb ratios range from 8 to 13 in the basalts and from 12 to 18 in more differentiated silicic samples. The higher ratios reflect light REE enrichment in the more silicic flows; all Sm/Yb ratios are 35° in Somoza, 1998) from 49 to 26 Ma. The convergence data permit a more normal convergence regime in the Upper Cretaceous before 69 Ma. The distribution of magmatic rocks is consistent with the arc front having been west of or on the western margin of the Cordillera del Viento from ca. 70 to 50 Ma and shifting to the eastern side at the time of major change in convergence obliquity at ca. 49 Ma. The magmatic record and regional

stratigraphy indicate that the principal period of pre-Miocene contractional deformation in the Neuquén Basin was in the Upper Cretaceous. Evidence for a major Eocene contractional deformation suggested by Cobbold et al. (1999) is less clear. The timing of major periods of deformation is discussed from a magmatic and tectonic viewpoint in the following. Latest Upper Cretaceous (Campanian-Maastrichtian) Support for Upper Cretaceous deformation of Lower Cretaceous and older strata in the western Neuquén Basin and Upper Cretaceous uplift of the Cordillera del Viento comes from magmatic and stratigraphic evidence. Stratigraphic evidence is based on Upper Cretaceous to Paleogene Neuquén and Malargüe Group strata being deposited in a foreland basin east of the Cordillera del Viento (Kozlowski et al., 1987; Barrio, 1990). Magmatic evidence comes from deformed Neuquén Basin strata being cut or overlain by Upper Cretaceous to Eocene magmatic rocks as old as ca. 74 Ma (see Franchini et al., 2003) and by dikes possibly as old as 102 Ma (Zamora Valcarce et al., this volume, chapter 6). A lack of discordance between Upper Cretaceous Neuquén Group and latest Upper Cretaceous– earliest Paleocene Malargüe Group strata to the east shows that deformation was restricted to the general region of the arc in the western Neuquén Basin (Kozlowski et al., 1987). Support for deformation in the latest Upper Cretaceous comes from the biotite 40Ar/ 39Ar cooling age of 69.09 ± 0.13 Ma from the Varvarcó leucogranodiorite in the western Cordillera del Viento (Table 2). Since unrealistically high geothermal gradients are needed to keep biotite from cooling below 300 °C at depths of less than ~3 km (see Kurtz et al., 1997), the present outcrop must have been at 3 km depth or less at 69 Ma. The relatively coarse grain size (up to 0.5 mm) and the amphibole-bearing mineral assemblage of the Varvarcó pluton are consistent with an initial emplacement depth of at least 5–6 km. The case for an Upper Cretaceous emplacement age for the pluton comes from ages of similar magmatic rocks in the Neuquén Basin (see Franchini et al., 2003, and preceding) and from the Lo Valle volcanic rocks to the north in Chile (40Ar/ 39Ar ages of 70–69 Ma; Gana and Wall, 1997). Given an Upper Cretaceous emplacement age for the pluton, 2–3 km of Upper Cretaceous uplift is needed in the Cordillera del Viento region. Contractional deformational at this time fits with a more normal plate convergence regime and evidence for synchronous deformation along much of the Andean margin (see Cobbold and Rossello, 2003). Evidence that the Varvarcó pluton was near the surface by 56.5 ± 0.6 Ma age comes from ages of the Cayanta volcanic rocks (Jordan et al., 2001) that lie unconformably on the western margin of the Cordillera del Viento. A 5–6 km uplift of the pluton between the Upper Cretaceous and Paleocene is consistent with the ~7000 m of pre-Eocene uplift called upon by Kozlowski et al. (1996) to explain the discordance between the

Upper Cretaceous to Holocene magmatism Carboniferous Andacollo Group and the Eocene lavas. Further evidence for Upper Cretaceous deformation comes from fissiontrack ages presented by Burns (2002). Paleocene to Eocene Ages of magmatic rocks in the Neuquén Basin do not provide evidence for the major Eocene contractional deformational event postulated by Cobbold et al. (1999). This event was postulated based on: (1) model ages and orientations of bitumen veins that cut deformed Cretaceous strata on the east side of the Cordillera del Viento, and (2) ages of magmatic rocks in the Sierra de Huantraico (Cobbold and Rossello, 2003). The age for bitumen vein emplacement comes from thermal models that project that organically rich Mesozoic sedimentary source rocks reached thermal maturation in the Eocene. Contemporaneous deformation is predicated on high fluid pressures related to hydrocarbon generation triggering motion on regional detachments. A northwest-southeast orientation for both the veins and major regional contractional structures is considered to reflect tensile failure perpendicular to the least compressive regional stress. This orientation is argued to be the one expected in the right-lateral Eocene transpressional regime predicted from plate convergence vectors (Pardo-Casas and Molnar, 1987). The lack of contemporaneous foreland basin strata is explained by a transpressional regime. Problems for this model are that deformed Neuquén Basin strata are cut by Upper Cretaceous magmatic rocks and that latest Paleocene–earliest Eocene Cayanta lavas unconformably overlie uplifted Cordillera del Viento rocks. The argument for Eocene deformation in the Sierra de Huantraico is complicated by the possibility that the Eocene magmatic rocks used to delimit the deformation age are actually early Miocene in age (Kay and Copeland, this volume, chapter 9). While support for major deformation in the Eocene is unclear, evidence for changes in the magmatic and tectonic regime in the Neuquén Basin as the convergence obliquity changed at ca. 49 Ma comes from the eastward displacement of the arc front. This offset is marked by the cessation of Cayanta volcanism west of the Cordillera del Viento and the initiation of Caicayén group magmatism to the east. The combination of high La/Ta and Sm/Yb ratios in the ca. 50–45 Ma magmatic rocks from Caicayén and Bayo de Esperanza (Figs. 6 and 8) is reminiscent of regionally high ratios in magmatic rocks associated with eastward offsets of the Miocene arc front west of the northern Southern Volcanic Zone (Kay et al., 2005). These Miocene offsets are associated with periods of compressional deformation, and it is reasonable to expect that some Eocene deformation accompanied the eastward shift of the Eocene arc in the Neuquén Basin. Kay et al. (2005) argued that high La/Ta and Sm/Yb ratios at the time of arc migration reflect adjustments in the slab geometry and peaks in forearc subduction erosion (see Fig. 11).

47

A Model for Shallowing and Steepening of the Nazca Plate to Explain the Miocene to Holocene Magmatic and Deformational Characteristics of the Neuquén Basin Many aspects of the Miocene to Holocene magmatic and structural evolution of the Neuquén Basin north of the Cortaderas lineament can be explained by shallowing followed by steepening of the subducting Nazca plate as argued by Kay (2001a, 2001b, 2002). Lithospheric-scale cross sections illustrating the model in a transect near 37°S are shown in Figure 11B and are discussed below. They draw heavily from the model for shallowing of the subduction zone below the Chilean (Pampean) flat slab between 28°S and 33°S (e.g., Kay et al., 1991, 1999; Kay and Abbruzzi, 1996).

Early Miocene (Extensional Regime over a Steep Subduction Zone) The lithospheric cross section at ca. 24–20 Ma shows an active volcanic arc and widespread backarc eruptions in an extensional tectonic regime over a relatively steeply subducting slab. The period began as the oceanic Farallón plate broke up and the near-normal subduction regime between South America and the Nazca plate that persists today emerged (see Pardo Casas and Molnar, 1987; Somoza, 1998). Unlike some Oligocene volcanic rocks studied by Muñoz et al. (2000) in the arc region to the south, all of the magmas that erupted in the intra-arc Cura Mallín basin show a frontal arc character as indicated by high La/Ta, Ba/Ta, and Ba/La ratios (Fig. 6). Higher Ta/Hf ratios relative to older and younger arc magmas in the region (Fig. 7) indicate that the mantle wedge had a more intraplate-like character than before or after (Burns, 2002). The largely bimodal basaltic–basaltic andesite and rhyolitic compositional range of the Cura Mallín magmas (Fig. 5) and their relatively flat REE patterns (low La/Yb and Sm/Yb ratios; Fig. 8) are in accord with their eruption through a thin crust in an extensional setting. Further discussion of the setting of the Cura Mallín intra-arc basin can be found in Jordan et al. (2001) and Burns et al. (this volume, chapter 8). In the backarc, a steep subduction zone is consistent with the lack of a subduction-related geochemical signature in the alkali olivine basalts that erupted as far as 550 km east of the modern trench. Their intraplate, OIB-like chemical signatures (see Kay and Copeland, this volume, chapter 9) are shown by low La/Ta, Ba/Ta, and Ba/La (Fig. 6) ratios and high Ta/Hf ratios that reach an extreme in the lavas erupted the furthest east of the arc (Fig. 7). A noncontractional or extensional backarc regime fits with the widespread eruption of alkali olivine basalts. The lack of reported structural evidence for major extension in the backarc appears to suggest that the stress regime was only mildly extensional.

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Late Early Miocene (Change to a Contractional Regime and Initial Shallowing) The lithospheric section at 19–16 Ma shows volcanism from the arc into the middle backarc over a shallower subduction zone than that before 20 Ma. Evidence for a change to a nonextensional stress regime in the arc comes from the end of sedimentation in the Cura Mallín intra-arc basin (Burns et al., this volume, chapter 8). A change from a bimodal volcanic assemblage in the Cura Mallín Formation to a more restricted mafic andesitic composition range in the Trapa Trapa Formation (Figs. 5 and 6) is in accord with a longer residence time for magmas in the crust. A frontal arc setting for the Trapa Trapa lavas is consistent with high La/Ta, Ba/Ta, and Ba/La ratios (Fig. 6). Evidence for a change in the mantle source region comes from a return to typical arc Ta/Hf ratios (Fig. 7). Similarities in La/Yb, La/Sm, and Sm/Yb ratios between Cura Mallín and Trapa Trapa mafic magmas show that no major pressure change occurred in residual mineral assemblages in equilibrium with the erupted magmas. Evidence for a shallower subduction zone under the backarc comes from changes in the character of backarc magmatism. This change is marked by the cessation of widespread alkali olivine basalt eruptions, and their replacement by mafic andesitic to trachydacitic magmas erupting from volcano and caldera complexes in the mid-backarc. Kay and Copeland (this volume, chapter 9) point to the higher La/Ta, Ba/Ta, and Ba/La (Fig. 6) ratios and lower Ta/Hf (Fig. 7) ratios in these midbackarc magmas to argue for introduction of a subducted component into the mantle source below the Sierra de Huantraico by 19 Ma. Evidence that the magmas were more hydrous by 19 Ma comes from the presence of large clinopyroxene phenocrysts in the basalts and hornblende in the mafic andesites. The presence of water is in accord with subduction-related fluids entering the mantle source. The easiest way to explain a more hydrated mantle richer in arc-like subducted components is for a shallower slab to have extended under the Sierra de Huantraico by 20 Ma. The presence of clinopyroxene and amphibole phenocrysts requires a period of magma evolution in the middle to lower crust as would be expected if magma ascent is slowed in a contractional stress regime. Other evidence for a contractional stress regime has already been discussed above. Middle to Late Miocene (Continued Contraction and Shallowing) The lithospheric section at 14–10 Ma shows a shallower subduction zone with volcanic centers in the arc and near backarc and contractional deformation in the backarc. The most important changes are in the backarc. Evidence for little magmatic change in the arc comes from middle to late Miocene (Trapa Trapa, Cajón Negro, and Quebrada Honda) magmas having the same general petrologic character, arc-related La/Ta, Ba/Ta, Ba/La, and Ta/Th ratios (Figs. 6 and 7) and REE ratios (Fig. 8), as early Miocene Trapa Trapa magmas.

Three general stages can be recognized in the middle to late Miocene magmatic history of the backarc. The first is characterized by a virtual magmatic lull from 16 to 14 Ma, the second by andesitic eruptions from ca. 14 to 10 Ma, and the third by another lull from ca. 10.7 to 7 Ma. The backarc magmas of this period have chemical signatures that are consistent with subducted components influencing a mantle wedge above a subducting slab. Among the lavas erupted are the ca. 12 Ma Cerro Negro hornblende andesites, which have high La/Ta (> 30) and Ba/Ta (>1000) ratios and low Ta/Hf ratios that are markedly more arc-like than those of the early Miocene backarc lavas (Figs. 6 and 7). Similarly, the Miocene Huincán magmas in southern Mendoza (Fig. 2A) show clear trace-element evidence for a subducted component (Baldauf, 1997). Support for backarc contractional deformation in the late Miocene comes from chemical signatures in magmatic rocks that are interpreted to reflect relative crustal thickening. The argument is based on REE patterns that have shapes influenced by pressure-sensitive mafic mineral assemblages that equilibrate with mantle-derived magmas in the crust (Hildreth and Moorbath, 1988; Kay et al., 1987, 1991). The best depth indicators are Sm/Yb and La/Yb ratios as they can reflect amphibole that is stable at intermediate pressure and garnet that is stable at higher pressure. Using this reasoning, similar Sm/Yb and La/Yb ratios in ca. 12 Ma Cerro Negro andesites and Eocene Cayanta andesites are consistent with little crustal thickening under the western Neuquén Basin between 56 and 12 Ma. The picture changes in the late Miocene, as shown by La/Yb (Fig. 7A) and Sm/Yb (Fig. 7B) ratios in Miocene to Pliocene Neuquén Basin magmatic rocks. Comparisons show that these ratios are: (1) lower in ca. 14 Ma Huincán I andesites than in ca. 10–6 Ma Huincán II andesites, (2) lower in ca. 12 Ma Cerro Negro andesites than in younger than 4 Ma Tromen andesites, and (3) lower in Cerro Negro andesites than in Huincán andesites. Based on this logic, Baldauf (1997) and Nullo et al. (2002) argued that higher ratios in Huincán II than Huincán I andesites reflected a crustal thickness increase in Mendoza in response to crustal shortening between ca. 14 and 10 Ma. In the same way, lower ratios in ca. 12 Ma Cerro Negro andesites than in Tromen andesites younger than 4 Ma can be interpreted as reflecting crustal thickening in response to crustal shortening between 12 Ma and ca. 4 Ma in Neuquén. Generally higher ratios in Mendoza than Neuquén andesites are consistent with greater crustal thicknesses in Mendoza, where crustal shortening estimates are higher (Zapata et al., 1999). Overall, the data support an episode of backarc crustal thickening between 12 and 10 Ma under the Neuquén Basin. Latest Miocene (Maximum Development of Shallow Subduction Zone) The lithospheric section at ca. 8–5 Ma is drawn as the shallow subduction zone reached its maximum development under the Neuquén Basin. A key feature in this interpretation is the

Upper Cretaceous to Holocene magmatism Chachahuén volcanic complex that erupted in the block-faulted Sierra de Chachahuén, some 500 km east of the modern trench. To the west, the status of ca. 8–5 Ma magmatic activity is poorly known. Candidates for arc magmas at this time are volcanic rocks in the Pichi Neuquén Complex on the west side of the Cordillera del Viento. Analyses of Pichi Neuquén basaltic to basaltic andesites (Table 5; late Miocene in Figs. 5–8) show that they have generally higher La/Ta (>56) and Ba/Ta (>1300) and lower Ta/Hf (500), Ba/La (>16), and Ta/Hf (800 km. Subduction and continental deformation has created two significant plateaus, the Altiplano and Puna at 3.8 and 4.5 km elevation, respectively. The maximum elevation is more than 6000 m. The Wadati-Benioff zone reaches a depth of more than 650 km (Cahill and Isacks, 1992). In the Southern volcanic zone, the mountain belt is much narrower and lower. The mean elevation is 0.45, εNd = +3.6–+4.2; La/Ta < 14; Ba/La < 16; 87Sr/ 86Sr = 0.7037–0.7040) indicate a backarc mantle devoid of arc-like components. These basalts erupted at a time of extension all along the margin during a period of rapid, near-normal Nazca–South America plate convergence when spreading ridges between the Pacific, Nazca, and Antarctic plates were becoming more parallel to the Chile Trench. Ridge rotation along with slab roll-back in response to slow relative motion between South America and the underlying mantle can explain why isotopically enriched magmas erupted far to the east of the trench in a generally extensional regime. Subsequently, 19–15 Ma basaltic to trachyandesitic backarc lavas with weak arc-like La/Ta (15–26), Ba/La (15–32), and Ta/Hf (0.2–4.5) ratios and a more depleted isotopic signature (εNd = +3.9–+4.7; 87Sr/ 86Sr = 0.7033–0.7037) erupted in a contractional regime. Their chemical features fit with incipient shallowing of the Nazca plate under the northern Neuquén Basin. A contractional regime that extended all along the margin can be explained by westward acceleration of South America over the underlying mantle as Nazca–South America plate convergence slowed. Keywords: Andes, plateau magmatism, Miocene, Neuquén Basin, trace elements, isotopes, plate motions, shallow subduction. *E-mails: [email protected]; [email protected].

Kay, S.M., and Copeland, P., 2006, Early to middle Miocene backarc magmas of the Neuquén Basin: Geochemical consequences of slab shallowing and the westward drift of South America, in Kay, S.M., and Ramos, V.A., eds., Evolution of an Andean margin: A tectonic and magmatic view from the Andes to the Neuquén Basin (35°–39°S lat): Geological Society of America Special Paper 407, p. 185–213, doi: 10.1130/2006.2407(09). For permission to copy, contact [email protected]. ©2006 Geological Society of America. All rights reserved.

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INTRODUCTION Magmatism has been a common occurrence in the southern and south-central Andes since the Mesozoic rifting that led to the breakup of South America and Africa. Among these magmas are the early to mid-Miocene volcanic rocks that erupted in the Neuquén Basin (Figs. 1 and 2) after a change from slower, more oblique South America–Farallon plate convergence to more rapid, near-normal South America–Nazca plate convergence at ca. 25–24 Ma (e.g., Pardo Casas and Molnar, 1987; Somoza, 1998). This change, which is generally taken to mark the initiation of the modern Andean cycle, generally coincides with the first magmatic eruptions reaching far into the backarc of the central and eastern Neuquén Basin since the Triassic. At the same time, arc volcanism reinitiated in the Main Andean Cordillera to the west after a gap of >15 m.y. (e.g., Jordan et al., 2001). In a larger picture, the origin and evolution of these magmas are part of a late Oligocene–early Miocene story that includes the accumulation of volcanic/sedimentary sequences in forearc, intra-arc, and backarc basins before, at, and after the breakup of the Farallon plate (see Jordan et al., 2001). A number of workers have related the evolution of these sequences to the breakup of the Farallon plate and subsequent changes in the convergence regime between South America and the subducting oceanic plate (Kay et al., 1993; Muñoz et al., 2000; Jordan et al., 2001). The late Oligocene–earliest Miocene sequences have been related to an extensional setting (e.g., Muñoz et al., 2000; Jordan et al., 2001) and the younger ones to a contractional setting. The problem with a simple correlation with South American–Nazca convergence parameters is that the extensional regime has been argued to persist until ca. 20 Ma (Jordan et al., 2001) or even 13–8 Ma (Radic et al., 2002), whereas the last major change in convergence vectors is placed ca. 25 Ma (Somoza, 1998). The purpose of this paper is to put ca. 24–15 Ma backarc magmatism in the Neuquén Basin into the context of southcentral Andean evolution and to explore how a transition from a ca. 24–20 Ma extensional regime to a contractional one after 20 Ma fits with the broader tectonic picture. To do this, the spatial distribution, ages, and chemical characteristics of Neuquén Basin backarc volcanic rocks are evaluated in light of new 40Ar/ 39Ar ages, whole-rock major- and trace-element analyses, and Sr, Nd, and Pb isotopic analyses. The data are used to show that: (1) Neuquén Basin backarc volcanic rocks formerly assigned Eocene to late Miocene ages erupted from 24 to 15 Ma, (2) ca. 24–20 Ma volcanic rocks are alkali basalts with oceanicisland basalt (OIB)-like chemical signatures that erupted from monogenetic and simple polygenetic centers in an extensional setting, and (3) ca. 19–15 Ma volcanic rocks are basalts to trachyandesites-dacites with weak arc-like signatures that generally erupted from volcanic complexes in a more contractional setting. These interpretations are then used to propose that a subducted component was introduced into the backarc mantle

Figure 1. Generalized map showing distribution of latest Oligocene to early Miocene sedimentary and volcanic rocks in Chile and western Argentina between 33°S and 44°S. Early Miocene volcanic rocks in the Somuncura plateau province are concentrated west of the solid line in the outcrop region. Generalized outcrop patterns are from 1:1,000,000-scale geologic map of Chile (1980, Servicio Nacional de Geología y Minería, Santiago) and 2,500,000-scale geologic map of Argentina (1997, Servicio Geológico Minero Argentino, Buenos Aires). Extent of subsurface exposure is as in Jordan et al. (2001). Dashed box labeled Neuquén Basin shows generalized region of early Miocene volcanic rocks that are the main focus of this paper.

beneath the Neuquén Basin at ca. 20 Ma in association with the initiation of transient Miocene shallowing of the subducting Nazca plate (Kay, 2001; Kay et al., this volume, chapter 2). Finally, a regional tectonic evaluation shows that a ca. 24–20 Ma extensional regime associated with the eruption of the Neuquén Basin alkaline basalts and contemporaneous formation of forearc, intra-arc, and backarc basins at a time of rapid South America–Nazca convergence (Somoza, 1998) can be explained


Slab shallowing and the westward drift of South America

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Figure 2. (A) Generalized geologic map of the Neuquén Basin region showing the distribution of Tertiary to Holocene magmatism and the Cortaderas lineament. Map is simplified from 1:2,500,000scale geologic map of Argentina (1997) and geologic maps of provinces of Mendoza (1993) and Neuquén (Delpino and Deza, 1995). Modifications in ages are in accord with radiometric dates in this volume. Names of Southern Volcanic Zone arc centers are given for reference.

by: (1) readjustments in South America–Nazca plate interactions in response to ridge rotations and ridge jumps among plates in the Pacific Ocean to the west (Tebbens and Cande, 1997; Tebbens et al., 1997), and (2) trench roll-back related to a slow margin-normal velocity for South America over the underlying mantle in the hotspot reference frame. The subsequent contractional regime after 20 Ma is shown to coincide with: (1) a time of near-parallel alignment of the Chile Trench with oceanic ridges to the west, and (2) trench advance as the South American plate began moving westward over the underlying mantle in the hotspot reference frame. The general features of the isotopic data from the early to middle Miocene Neuquén Basin volcanic rocks along with those from Oligocene to middle Miocene volcanic rocks between 33°S and 45°S latitude can be explained in the context of this model.

REGIONAL SETTING OF THE NEUQUÉN BASIN The early to middle Miocene lavas that are the focus of this study occur from 68.8°W to 70°W and from 36°S to 37.5°S, where they overlie the Mesozoic–early Tertiary sedimentary sequences that form the infill of the northern and central parts of the Neuquén Basin (Figs. 1 and 2). These lavas erupted in a backarc position relative to the ca. 24–20 Ma Cura Mallín Formation volcanic/sedimentary sequences, which formed in an intra-arc basin, and the middle Miocene arc sequences that followed (e.g., Suárez and Emparan, 1995; Jordan et al., 2001; Burns, 2002; Burns et al., this volume, chapter 8). The Cura Mallín sequences have been related to an extensional regime by Jordan et al. (2001), Radic et al. (2002), and Burns et al. (this volume, chapter 8). Within the context of the Neuquén Basin,


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Figure 2. (continued) (B) Thematic Mapper (TM) satellite image showing distribution of K-Ar and 40Ar/ 39Ar ages in early Miocene backarc volcanic rocks in the region discussed in this paper. Ages in the Sierra de Chachahuén and La Matancilla region are from González Diaz (1979) and Pérez and Condat (1996). Ages in the Sierra de Huantraico–Sierra Negra region are from Ramos and Barbieri (1988), Cobbold and Rossello (2003) and Table 1. Ages in the Cajón de Molle– El Manzano region are from Nullo et al. (2002). Dots with DR labels are localities of samples with analyses in Table 2B.

the backarc lavas erupted north of or along the Cortaderas lineament (Fig. 2; Ramos, 1978), which generally marks the southern extent of post-Oligocene backarc magmatic activity in the Neuquén Basin (Fig. 2). The significance of the Cortaderas boundary, which lies along a persistent NE-trending regional structural grain and projects into an offset in the modern Southern Volcanic Zone arc (Fig. 2), has been unclear. Kay et al. (this

volume, chapter 2) argue that this lineament marks the southern boundary of a transient Miocene shallow subduction zone. Regionally, the 24–15 Ma backarc volcanic rocks in the Neuquén Basin are part of the widespread array of Andean magmatic and sedimentary rocks from 33° to 45°S that formed in late Oligocene to early Miocene forearc, intra-arc, and backarc basins, and the later Miocene arc and backarc complexes that fol-


Slab shallowing and the westward drift of South America lowed (see Fig. 1). To the west and south of the Neuquén Basin are ca. 29–20 Ma arc and forearc sequences, the surface and subsurface expressions of which extend from the eastern edge of the Central Valley near 36°S to the coast at 43° to 44°S (see LópezEscobar and Vergara, 1997; Muñoz et al., 2000). The presence of thick sedimentary sequences and variable intraplate to arc-like chemical affinities in associated mafic to silicic lavas has been interpreted to reflect extensive lithospheric thinning in forearc and intra-arc basins (Muñoz et al., 2000). To the east and south are the volcanic and sedimentary sequences in the Lonquimay Basin in the main Cordillera near 38°S, and the ca. 32–28 Ma arc volcanic sequences in the Ventana Formation (Rapela et al., 1988), which preceded the sedimentary sequences in the Nirihuau Basin (e.g., Cazau et al., 1987) at ~41° to 43°S. East of the Nirihuau Basin are the late Oligocene pre-plateau and voluminous plateau lavas and the early Miocene post-plateau alkaline flows, differentiates, and small-volume ultra-Na and ultra-K lavas of the Somuncura magmatic province (e.g., Corbella, 1984; Ardolino and Franchi, 1993; Kay et al., 1993, 2004). Volcanic sequences of similar age, which are not shown on Figure 1, occur to the south in the Meseta Canquel region at ~44° to 46°S (e.g., Baker et al., 1981; Marshall et al., 1986). To the north of the Cura Mallín Basin are the extensive early Miocene arc and backarc magmatic-sedimentary sequences in the Coya Machalí, which have been associated with a thin crust in an extensional to neutral tectonic regime (e.g., Charrier et al., 1996, 2002; Kurtz et al., 1997; Kay et al., 2005). Further north in the Chilean flat-slab region (not shown on Fig. 1) are the early Miocene Doña Ana arc sequences and small ca. 22 Ma Máquinas backarc olivine alkali basalt flows, which formed in a mildly extensional to neutral tectonic regime (see Kay et al., 1991; Kay and Mpodozis, 2002). DISTRIBUTION, AGES, AND CHEMISTRY OF EARLY MIOCENE BACKARC VOLCANIC ROCKS IN THE NEUQUÉN BASIN In this section, the distribution, ages, and chemistry of the 24–15 Ma volcanic sequences in the Neuquén Basin (Fig. 1) are examined. As shown on the map on Figure 2A and the Thematic Mapper satellite (TM) image in Figure 2B, these volcanic rocks occur in three regions. The first is in the general area of the Cajón del Molle and Puntilla de Huincán region in southern Mendoza, the second is in the La Matancilla–Sierra de Chachahuén region, and the third is in the Sierras de Huantraico and Sierra Negra regions (Fig. 3). Radiometric ages are listed in Table 1. Whole-rock chemical and isotopic analyses are listed in Table 2 and plotted in Figures 4–10; sample localities and descriptions are in Appendix 1. Molle Sequence in Southernmost Mendoza The early Neogene mafic volcanic sequences in the Cajón del Molle and Puntilla de Huincán region in southern Mendoza (Fig. 2) are historically important because this is the area where

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Groeber (1946 and earlier) defined the Basalto 0, Basalto Molle, and Basalto Palaoco units. These names have been widely used for the oldest mafic flows covering the Mesozoic sequences of the Neuquén Basin. More recently, Nullo et al. (2002) reexamined the volcanic stratigraphy of this region and regrouped these volcanic rocks into the Molle, Palaoco, and Puntilla de Huincán sequences of the Molle eruptive cycle. Referring to Figure 2B, the Molle sequence includes the flows in the Cajón del Molle, El Alambrado, and El Manzano regions; the Puntilla de Huincán sequence includes the flows in the Puntilla de Huincán region to the east; and the Palaoco sequence includes the flows in the Sierras de Palaoco and Barda Blanca regions to the north. Nullo et al. (2002) further correlated the Molle and Puntillo de Huincán sequences and assigned them early Miocene ages based on K-Ar dates of 17 ± 2 Ma in the Cajón del Molle and 19.4 Ma at El Manzano. The Palaoco sequence was assigned an age of less than 19.4 Ma, but greater than 13 ± 1 Ma. In the rest of this paper, only the Molle and Puntillo de Huincán sequences are addressed. They are simply referred to as the Molle sequence. Chemical analyses for five Molle and Puntilla de Huincán samples from Baldauf (1997) and Nullo et al. (2002) are reproduced in Table 2 and plotted in Figures 4–8. They include two alkali basalts (47–48% SiO2; Fig. 4) with 0.4–1.5% TiO2 and 1.3–1.4% K2O (one is fairly primitive with FeO/MgO ~ 1.1, 277 ppm Cr, and 133 ppm Ni) and two trachyandesites (54–55% SiO2; Fig. 4) with ~1% TiO2 and ~2% K2O. Their trace-element patterns in Figure 5C are consistent with a close genetic relationship between them, because: (1) the rare earth element (REE) patterns of the basalts and andesites are respectively characterized by ratios of La/Yb = 8–9 and 11–12, La/Sm = 2.3–2.6 and 5, and Sm/Yb = 2.3–2.6 and 2.4–2.6 (Fig. 6), (2) Ba/La ratios are all 19–25 (Fig. 7), (3) La/Ta ratios are all 20–24 (Fig. 7), and (4) Ta/Hf ratios all are 0.21–0.27 (Fig. 8). Along with a dacite (~64% SiO2) with 2.5% K2O, La/Yb = 24, La/Sm = 9, Sm/Yb = 2.6, Ba/La = 32, and La/Ta = 17, these samples have a number of chemical similarities to 19–15 Ma sequences in the Sierras Huantraico and Negra region discussed in the following sections (Figs. 4–8). In contrast, basaltic andesites from the Sierra de Palaoco analyzed by Nullo et al. (2002) have markedly higher La/Ta (>50) and Th/Hf (~1.5–2.5) ratios and lower Ta/Hf (~0. 7–1.0) ratios. Given that these ratios are like those of younger than 15 Ma volcanic rocks farther to the north and that their ages are considered to be younger than 19 Ma, but older than 13 Ma (see Baldauf, 1997; Nullo et al., 2002), the Palaoco sequence in this region is not considered further here. La Matancilla and Sierra de Chachahuén Regions Early Miocene alkali olivine basalt flows are widespread in the second area, which includes the La Matancilla region and the Sierra de Chachahuén of southern Mendoza (Fig. 2). The best known are the flows in the La Matancilla region that González Díaz (1979) assigned to the Palaoco Formation. The K-Ar ages


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Figure 3. Map of the Sierra de Huantraico region modified from Ramos and Barbieri (1988) showing distribution of volcanic units, radiometric ages, and principal faults and fold axis. Dots show sample localities for 40Ar/ 39Ar ages in Table 1 (light dots) and K-Ar ages (black dots) in Ramos and Barbieri (1988). General localities of 40Ar/ 39Ar ages in Cobbold and Rossello (2003) are in areas with boxed ages. Formation names are discussed in text.


Slab shallowing and the westward drift of South America

Latitude (°S) 40 39 New Ar/ Ar ages HDR18 37°15.76

TABLE 1. GEOCHRONOLOGY FOR EARLY TO MIDDLE MIOCENE VOLCANIC ROCKS Longitude Sierra de Huantraico Region Type (°W) 69°39.13

HDR25

37°35.09

69°28.57

HRD20

37°24.96

69°31.97

HDR14b

37°35.80

69°42.13

HDR4

37°33.80

69°50.00

Latitude (°S) New Ar/ Ar ages DRC21 36°36.8

Longitude (°W)

40

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Filo Morado olivine basalt. Road cut just east of Filo Morado station. Huantraico: Desfiladero Negro basaltic dike. Desfiladero Negro southeast of Sierra de Huantraico. Southern Huantraico olivine-clinopyroxene basalt flow. South of road just west of top of Bajada Ternero. Southern Huantraico hornblende-bearing andesite flow. Puesto to west of Puesto Gonzalez near Huantraico syncline axis. Cerro Villegas olivine basalt flow. Puesto Cerro de los Liebres west of Cerro Tormenta.

Age (Ma)

Groundmass

23.4 ± 0.4

Hornblende

25 ± 4

Groundmass

19.1 ± 0.8

Hornblende

19.8 ± 0.7

Groundmass

14.8 ± 1.2

Sierra de Chachahuén Region

Type

Age (Ma)

Matancilla region basalt flow South of Puesto Jumial, Hoja Matancilla (36°36.8 , 68°35.5 )

Groundmass

23.76 ± 0.08

Sierra de Chachahuén basalt flow Near Puesto Zuñiga.

Whole rock

20.4 ± 1.0

Basalt flow in southern Sierra de Huantraico. Cerro Tormenta. Cerro Sur de los Overos. Cerro Bayo de Huantraico andesite.

Whole rock Whole rock Whole rock Whole rock

36 ± 2 21 ± 2 22 ± 2 18 ± 2

39

68°35.5

K-Ar ages from Perez and Condat (1996) Ch-14 37°04.81 68° 53.99 K-Ar ages from Ramos and Barbieri (1988)

40

Ar/39Ar ages from Cobbold and Rossello (2003) Flow near Cerro Bayo de Sierra Negra. Filo Morado flow. Cerro Bayo de Sierra Negra dike. Cerro Bayo de Sierra Negra dike. Cerro Bayo de Sierra Negra dike. Cerro Bayo de Sierra Negra dike. Cerro Bayo de Sierra Negra dike.

that he reported are plotted on the TM scene in Figure 2B. They are: (1) 26 ± 2 Ma for a flow between Loma Alta and Loma Negra, (2) 24 ± 1 Ma for a flow between Mendieta and Puesto Loma Negra, (3) 23 ± 2 Ma for a flow in the Loma de las Ramadas south of Cerro Las Lajas, (4) 22 ± 5 Ma for a flow at the Loma de los Ojos de Agua north of Cerro Las Lajas, (5) 21 ± 2 Ma for a flow on the western border of Cerro Amarillo west of Puesto Peligroso, (6) 21 ± 5 Ma for a flow at Puesto Ranquil north of Cerro Las Lajas, (7) 19 ± 1, 18 ± 3, and 18 ± 6 Ma for flows in the Punta de la Barda region, and (9) 16 ± 5 Ma for a flow at Cerro El Ramblón. A new whole-rock 40Ar/ 39Ar age of 23.76 ± 0.08 Ma (DRC21) in Table 1 for the Loma Negra flow south of Puesto Jumial is in agreement with these K-Ar ages. In the Sierra de Chachahuén to the south (Figs. 2A and 2B), early Miocene alkali olivine basalt flows associated with marine-fossil–bearing sandstones and sandy conglomerates have been mapped as Chachahuén Unit 1 by Pérez and Condat (1996). They reported a whole-rock K-Ar age of 20.4 ± 1.0 Ma (Fig. 2B) for a basalt flow in this variable-thickness sequence, which reaches a maximum of 70 m near the center of the Sierra de Chachahuén. The distribution of these basalts supports an association with a NE-SW–trending fault zone, and outcropscale faults support normal fault motion (D. Ragona, 1999, personal commun.; Kay et al., this volume, chapter 10). Chemical analyses for five La Matancilla region and three Sierra de Chachahuén lava flows are presented in Table 2B. The Chachahuén samples are alkali basalts (~48% SiO2) with

22.1 ± 0.5 22.2 ± 0.5 16.1 ± 0.2 15.3 ± 0.4 18.9 ± 0.4 15.8 ± 0.1 15.2 ± 0.1

~2.2% TiO2, 1.3–1.6% K2O, FeO/MgO = 1.1–1.3, 206–330 ppm Cr, and 155–256 ppm Ni, and the La Matancilla samples are alkali basalts and hawaiites (45.6–49.5% SiO2) with ~2–3.2% TiO2, 1.4–2.1% K2O, FeO/MgO = 1.3–3.5, 190–330 ppm Cr, and 116–184 ppm Ni (see Fig. 4 and Table 2B). Among common trace-element features are overlapping REE patterns (La/Yb = 14–20, one at 29; La/Sm = 3.9–6.4; Sm/Yb = 3.7–6.4, Fig. 6B) and intraplate-like high field strength element (HFSE) signatures (La/Ta = 9–11, Fig. 7; Ta/Hf = 0.45–0.67, Fig. 8) and Ba/La (10–16, Fig. 7B) ratios. Their 87Sr/ 86Sr ratios and εNd values (0.7037–0.70740 and +4.3–+4.7, respectively; Table 3) plot in the ocean-island basalt field (Fig. 9). Sierra de Huantraico and Sierra Negra The third area where early Miocene backarc volcanic rocks occur is in the Sierra del Huantraico–Sierra Negra region (Figs. 2A and 2B). A modified version of a map of the region compiled from the 1:200,000-scale Buta Ranquil (Holmberg, 1976) and Los Chihuidos Norte (Ramos, 1981) geologic sheets by Ramos and Barbieri (1988) is shown in Figure 3. All of the Miocene and older volcanic units have been affected by the contractional deformation that produced the Huantraico syncline and related folds shown on the map. Ramos and Barbieri (1988) assigned the pre-Pliocene volcanic rocks on the map in Figure 3 Eocene to late Miocene ages. The siliceous tuffs mapped in the Carrere Formation were


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assigned an Eocene age based on a K-Ar age of 36 ± 2 Ma from an overlying lava flow near Puesto González. This flow is part of a group of widespread basaltic to mafic andesitic flows, sills, and agglomerates across the region. The oldest rocks in this group were mapped as the Lower Palaoco Formation and assigned to the Eocene based on the 36 ± 2 Ma K-Ar age mentioned already. The younger rocks in this group were designated as the Upper Palaoco Formation and assigned to the early Miocene based on K-Ar dates for flows in the La Matancilla region (González Díaz, 1979). Small subsidiary basaltic centers mapped as the Cerro Cabras Basalt were also assigned to the early Miocene based on K-Ar ages of 21 ± 2 and 22 ± 2 Ma at Cerro Tormento and Cerro Sur de los Overos. Andesitic to dacitic volcanic rocks in the Sierra de Huantraico and the Cerro Bayo de Sierra Negra were mapped as the Pichi Tril Andesite and were designated as Miocene based on a K-Ar age of 18 ± 2 Ma for a Cerro Bayo de Huantraico andesite. Finally, dikes cutting Mesozoic sedimentary sequences around the Sierra de Huantraico that were mapped as the Desfiladero Negro dikes were assigned to the late Miocene based on a 9 ± 1 Ma K-Ar age for a dike in the subsurface in the Aguada San Roque region, south of Auca Mahuída (Fig. 2; see Ardolino et al., 1996). The ages listed in Table 1 and plotted on Figure 3 show that these age assignments need to be revisited, because new 40Ar/ 39Ar ages, like four of the five K-Ar ages reported by Ramos and Barbieri (1988), indicate early to middle Miocene ages. The new 40Ar/ 39Ar dates include groundmass ages of 25 ± 4 Ma for the dike forming the Desfiladero Negro ridge, 23.4 ± 0.4 Ma for a flow in the Filo Morado ridge, 19.1 ± 0.8 Ma for a basaltic flow on the south side of the road at the Bajada Ternero, and 14.8 ± 1.2 Ma for a basalt flow near Cerro Villegas along the Cortaderas lineament. A hornblende from a mafic andesite flow near the southern axis of the Huantraico syncline yielded an age of 19.8 ± 0.7 Ma. The plateau spectra for these ages are shown in Appendix 2. Additional 40Ar/ 39Ar ages cited by Cobbold and Rossello (2003) are 22.2 ± 0.5 Ma for a basalt flow from the Filo Morado ridge, 22.1 ± 0.5 Ma for a flow and 18.9 ± 0.4 Ma for a dike just west of Cerro Bayo de Sierra Negra, and five ages ranging from 16.1 ± 0.2 to 15.2 ± 0.1 Ma for dikes surrounding the Cerro Bayo de Sierra Negra. Based on these new ages, the volcanic rocks of the Palaoco Formation, Cerro Cabras Basalt, and Desfiladero Negro dike unit of Ramos and Barbieri (1988) are reassigned to: (1) an early Miocene Filo Morado basaltic sequence that includes the Palaoco Formation flows from the Sierra Negra and the northern Sierra de Huantraico and most of the Cerro Cabras Basalt, (2) a younger early Miocene Huantraico basaltic to mafic andesitic sequence that includes the Palaoco Formation flows and Desfiladero Negro dikes from the central and southern Sierra de Huantraico, and (3) an early middle Miocene Cerro Villegas basalt. The volcanic rocks in these units along with those in the Pichi Tril Andesite and the Carrere tuffs are discussed in the following sections.

Sample SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Volatiles Total La Ce Nd Sm Eu Tb Yb Lu Sr Ba Cs U Th Hf Ta Sc Cr Ni Co

TABLE 2A. WHOLE ROCK CHEMISTRY: NEUQUÉN BASIN VOLCANIC ROCKS Early Miocene Molle Eruptive Cycle Cajon de Molle Puntilla Huincán Bajada Pajarito I-99 6-99 7-99 9-99 8-99 47.09 48.09 54.07 55.15 63.63 1.54 1.35 0.99 1.01 0.29 18.38 16.41 16.08 16.62 16.08 9.48 9.63 6.89 6.71 2.50 0.17 0.17 0.15 0.15 0.09 5.73 8.48 4.41 4.07 1.29 10.94 9.63 7.82 7.29 3.82 2.82 2.88 3.35 3.58 4.04 1.27 1.38 2.10 2.01 2.51 0.46 0.37 0.30 0.31 0.14 1.68 1.03 2.00 0.96 3.81 99.55 99.42 98.16 97.86 98.20 16.0 34.0 20.0 4.80 1.73 0.7 2.10 0.310 713 303 2.2 0.70 2.3 3.0 0.80 30.0 30 24 30

17.0 35.0 20.0 4.90 1.69 0.7 1.90 0.280 637 325 2.7 0.90 3.0 3.2 0.70 27.0 277 131 39

22.0 43.0 21.0 4.50 1.45 0.6 2.00 0.290 582 550 1.7 1.40 5.3 4.2 1.00 20.0 209 83 25

22.0 45.0 21.0 4.50 1.41 0.6 1.90 0.300 614 553 0.9 1.30 5.4 4.2 0.90 19.0 97 39 21

26.0 46.0 17.0 2.90 0.85 0.3 1.10 0.170 714 834 3.3 2.10 7.1 3.6 1.50 3.0

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