Field Trip Guides to the Backbone of the Americas in the Southern and Central Andes: Ridge Collision, Shallow Subduction, and Plateau Uplift

Home , Chile Rise, Chile Triple Junction

Field Guide 13

edited by Suzanne Mahlburg Kay and Víctor A. Ramos

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edited by

Suzanne Mahlburg Kay Cornell University Department of Earth and Atmospheric Sciences Snee Hall Ithaca, New York 14853, USA

Víctor A. Ramos Universidad de Buenos Aires Laboratorio de Tectónica Andina Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) Buenos Aires 1428, Argentina

Field Guide 13 3300 Penrose Place, P.O. Box 9140 Boulder, Colorado 80301-9140 USA 2008

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Library of Congress Cataloging-in-Publication Data

Field trip guides to the backbone of the Americas in the southern and central Andes : ridge collision, shallow subduction, and plateau uplift / edited by Suzanne Mahlburg Kay, Víctor A. Ramos. p. cm. — (Geological Society of America ; ﬁ eld guide 13) Includes bibliographical references. ISBN 978-0-8137-0013-7 (pbk.) 1. Geology, Structural—Andes Region—Guidebooks. 2. Geology—Andes Region— Guidebooks. I. Kay, Suzanne Mahlburg, 1947–. II. Ramos, Víctor A. QE230.F54 2008 558—dc22 2008017652

Cover: Mount Aconcagua, highest peak in the western hemisphere. Late Miocene volcanic sequences form the top of the mountain.

10 9 8 7 6 5 4 3 2 1

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Introduction ...... v

1. Field trip guide: Ridge-trench collision—The southern Patagonian Cordillera east of the Chile Triple Junction ...... 1 Matthew L. Gorring

2. Field trip guide: Andean Cordillera and backarc of the south-central Andes (~38.5°S to 37°S) ...... 23 Tomás Zapata, Gonzalo Zamora Valcarce, Andrés Folguera, and Daniel Yagupsky

3. Field trip guide: Frontal and Main Andean Cordilleras near the southern boundary of the Pampean shallow subduction zone ...... 57 Víctor A. Ramos

4. Field trip guide: Evolution of the Pampean flat slab region over the shallowly subducting Nazca plate ...... 77 Víctor A. Ramos

5. Field trip guide: Neogene evolution of the central Andean Puna plateau and southern Central Volcanic Zone ...... 117 Suzanne Mahlburg Kay, Beatriz Coira, and Constantino Mpodozis

iii

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Introduction

Suzanne Mahlburg Kay* Department of Earth and Atmospheric Sciences, Snee Hall, Cornell University, Ithaca, New York 14853 USA

Víctor A. Ramos* Laboratorio de Tectónica Andina, Universidad de Buenos Aires, Consejo Nacional de Investigaciones Cientíﬁ cas y Técnicas (CONICET), Buenos Aires 1428, Argentina

The fi eld guides in chapters 1–5 of this volume were originally written to accompany the fi eld trips run in conjunction with the Backbone of the Americas Meeting that took place 3–7 April 2006, in Mendoza, Argentina. The conference was sponsored by the Geological Society of America (GSA) and the Asociación Geológica Argentina (AGA) in collaboration with the Chilean Geologic Society. The meeting was organized around three important processes affecting the western margin and cordilleras of the Americas—ridge collision, shallowing and steepening subduction zones, and plateau and orogenic uplift. Some 400 participants from 20 countries along the western margin of the Americas and elsewhere attended the meeting. A report of the meeting can be found in the January 2007 issue of GSA Today (Kay and Ramos, 2007). The fi ve fi eld guides from the pre-meeting, intra-meeting, and post-meeting fi eld trips in this volume accompany the meeting themes. General trip locations are shown in the map and topographic image in Figure 1. The trip in chapter 1 to southern Patagonia highlights the ridge-trench collision theme; the trips in chapters 2–4 examine issues related to shallowing subduction zones in the south-central Andes; and the trip in chapter 5 highlights uplift of the Puna plateau in the context of a steepening subduction zone and lithospheric delamination. Chapter 1, “Ridge-trench collision—The southern Patagonian Cordillera east of the Chilean Triple Junction,” is the guide for premeeting fi eld trip 401 to Patagonia, which was organized to highlight the ridge collision theme. This fi ve-day fi eld guide by Matthew Gorring (Montclair State University) features the southern Patagonian Cordillera, south of the present Chile Triple Junction (46.5°S), where distinctive backarc deformational and magmatic features refl ect the northward-propagation of slab windows related to the ridge-trench collision event that began at ca. 14 Ma. Among the features to be observed are (1) an abrupt northward increase in topography in the Cordillera, (2) young thin- and thick-skinned deformation in the Patagonian fold-thrust belt, (3) mid-Miocene to Pliocene plutons associated with the shutoff of the arc south of the slab window, (4) adakitic volcanic rocks interpreted as slab melts associated with partial melting of the hot trailing edge of the Nazca plate as the slab window opened, and (5) extensive oceanic island basalt–like plateau basalts formed from dynamic asthenospheric fl ow as the slab window opened under the backarc. Chapter 2, “Andean Cordillera and backarc of the south-central Andes (~38.5° to 37°S),” is the guide for pre-meeting fi eld trip 402 to the Neuquén Andes in Argentina, which was organized to highlight a region of the margin where both contractional and extensional deformation have occurred. This fi ve-day fi eld guide by Tomás Zapata of Repsol-YPF, Gonzalo Zamora (Repsol-YPF), Andrés Folguera (University of Buenos

*E-mails: [email protected]; [email protected]

Kay, S.M., and Ramos, V.A., 2008, Introduction, in Kay, S.M., and Ramos, V.A., eds., Field trip guides to the Backbone of the Ameri- cas in the southern and central Andes: Ridge collision, shallow subduction, and plateau uplift: Geological Society of America Field Guide 13, p. v–vii, doi: 10.1130/2008.0013(00). For permission to copy, contact [email protected]. ©2008 The Geological Society of America. All rights reserved.

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80° W 70° W 60° W 50° W Peruvian 10° N flat-slab

CVZ Northern Altiplano Volcanic 0° Puna Zone Nazca plateau South Peruvian American flat-slab Plate 5 10° S NAZCA 4 Argentina-Chile Central PLATE 3 flat-slab Volcanic Nazca Ridge Zone Juan Fernández 2 SVZ plateau 20° S Chile Rise 5

Nazca rench Plate slab Chilean flat-slab 30° S 1 Chile T window AVZ Juan Fernández 4 Ridge 3 ANTARCTIC 2 PLATE Chile Rise Southern 40° S Volcanic Zone Chile Trench 1 Antarctic Austral Volcanic Zone 50° S Figure 1. Map and topographic relief images of South Plate 1000 km America show the locations of the fi ve fi eld trips. The numbers 1–5 refer to the chapters in the fi eld guide.

Aires), and Daniel Yagupsky (University of Buenos Aires) features the Cretaceous to Recent deformational history of the Andes adjacent to the Neuquén Mesozoic sedimentary basin. The region has experienced multiple deformational events in response to changes in the confi guration of the convergent plate boundary. Among the features to be observed are (1) the Mesozoic statigraphic sequence of the western Neuquén basin, (2) the uppermost Cretaceous Agrio fold-and-thrust belt whose origin is linked to moderate shallowing of the subduction zone by the authors, (3) the late Oligocene–early Miocene Cura Mallín extensional basin related by the authors to steepening of the subducting plate, (4) contractional structures related to the inversion of the Cura Mallín basin and the formation of the Guañacos fold and thrust belt, (5) mafi c volcanic rocks and extensional structures in the Loncopué trough related to recent steepening of the subduction zone, and (6) the spectacular young backarc Cerro Troman volcano. Chapter 3, “Frontal and Main Andean Cordilleras near the southern boundary of the Pampean shallow subduction zone,” is the guide for fi eld trip 403, which was the intra-meeting fi eld trip to the high central Andes for all participants. This one-day fi eld guide by Víctor Ramos (University of Buenos Aires) is for a transect above the currently amagmatic southern hinge of the Pampean fl at slab (commonly called the Chi- lean fl at slab). The trip provides a view of the Andean deformation front in the Precordillera and the main features of the Frontal and Main Cordilleras in Argentina. Among features to be observed are (1) late Paleo- zoic sedimentary and magmatic rocks in the Frontal Cordillera, (2) Triassic volcanic and plutonic sequences, (3) the sedimentary sequences and inverted normal structures of the Triassic Cuyo rift, (4) Mesozoic to Mio- cene magmatic arc and sedimentary basin rocks in the Main Cordillera, (5) Miocene foreland basin deposits, (6) the thin-skinned Aconcagua fold and thrust belt that deforms Jurassic to Miocene sequences, and (7) the late Miocene Cerro Aconcagua center (6967 m)—the highest peak in the Western and Southern hemispheres (the top of the Backbone). Chapter 4, “Evolution of the Pampean fl at-slab region over the shallowly subducting Nazca plate,” is the post-meeting guide for fi eld trip 404, which was organized to highlight the shallowing subduction zone

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theme. This fi ve-day fi eld guide by Víctor Ramos (University of Buenos Aires) provides a view of the Paleo- zoic to Recent history of the region over the Pampean fl at-slab region (also known as the Chilean fl at slab). The trip features the Andes between 31° and 32°S latitude in a currently amagmatic segment characterized by fl at-slab subduction. Among features highlighted are (1) neotectonic activity at the boundary between the basement uplifts of the westernmost Sierras Pampeanas and the Precordillera belt, (2) Precordillera belt Paleozoic stratigraphy and evidence for Ordovician accretion of the Cuyania (greater Precordillera) terrane and Devonian accretion of the Chilenia terrane, (3) Neogene structure and sedimentary and volcanic rocks of the Precordillera fold and thrust belt, (4) western Precordillera late Paleozoic Gondwana sequences and Triassic rift sediments and structures, and (5) a transect from the Frontal Cordilleras to the Coastal Cordillera in Chile that complements the guide in Chapter 3. Chapter 5, “Neogene evolution of the central Andean Puna plateau and the southern Central Volcanic Zone,” is the guide for post-meeting fi eld trip 405, which was organized to highlight the plateau uplift theme as well as a steepening subduction zone and crustal and lithospheric delamination. This seven-day fi eld guide by Suzanne Kay (Cornell University), Beatriz Coira (Universidad Nacional de Jujuy, Argentina; Days 1–5) and Constantino Mpodozis (Antofagasta Minerals, Chile; Days 6–7) provides a view of plateau evolution in the context of the southern central Andean Puna–Altiplano plateau in Argentina between 23° and 27.5°S latitude and the southernmost Central Andean Volcanic Zone arc in Chile between 26.5° and 27.5°S latitude. Differences in the evolution and uplift of the northern and southern Puna are highlighted on the fi rst fi ve days. Featured are (1) the giant Miocene northern Puna Coranzulí ignimbrite and the Pliocene to Pleistocene southern Puna Cerro Galan and Cerro Blanco ignimbrites, (2) the central Puna Pleistocene shoshonitic lavas and the southern Puna latest Miocene to Pleistocene intraplate and calc-alkaline lavas, (3) latest Miocene to Recent southern Puna normal and strike-slip faults, (4) Tertiary sedimentary sequences and post-middle Miocene internally drained salar basins, (5) Ordovician to Cretaceous basement and deformational styles, and (6) the structure of the southeastern margin of the plateau. The last two days highlight the late Miocene to Pliocene displacement of the arc front from the Maricunga Belt to the southern Central Volcanic Zone. Featured are (1) the Miocene Maricunga Belt arc front centers on the western margin of the plateau, (2) early Miocene mafi c rocks and Miocene-Pliocene volcanic rocks between the Maricunga Belt and Central Volcanic Zone, (3) evidence for early to middle Miocene backarc contractional deformation, and (4) the dramatic Central Volcanic Zone volcanic centers near the international border including Ojos del Salado and Tres Cruses.

ACKNOWLEDGMENTS

We would like to thank the reviewers and trip participants who made signiﬁ cant contributions to improv- ing the presentation and content of these guides. Special thanks go to Dwight Bradley (U.S. Geological Survey), Eric Erslev (Colorado State University), Alan Glazner (University of North Carolina), Teresa Jordan (Cornell University), Robert Kay (Cornell University), and Constantino Mpodozis (Antofagasta Minerals, Chile). Repsol-YPF is acknowledged for offsetting some of the ﬁ eld trip costs.

REFERENCE CITED

Kay, S.M., and Ramos, V.A., 2007, Backbone of the Americas: From Patagonia to Alaska—A super rock star event: GSA Today, v. 17, no. 1, p. 30–31.

MANUSCRIPT ACCEPTED BY THE SOCIETY 10 JANUARY 2008

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