Quechua II contraction in the Ayacucho intermontane basin: Evidence for rapid and episodic Neogene deformation in the Andes of central Perú |
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| Institution: | 1. Laboratório de Geocronologia, Instituto de Geociências, Universidade de Brasília, Brasília, DF 70910-000, Brazil;2. Géosciences-Environnement Toulouse, Université de Toulouse, UPS (SVT-OMP), CNRS, IRD, 14 Avenue Édouard Belin, F-31400 Toulouse, France;3. Institut des Sciences de l''Évolution de Montpellier (ISE-M, UMR 5554, CNRS/UM/IRD/EPHE), c.c. 64, Université de Montpellier, Place Eugène Bataillon, F-34095 Montpellier Cedex 05, France;1. Department of Earth Sciences and the Environment, University of Geneva, Rue des Maraichers 13, Geneva 1205, Switzerland;2. HRH Geology, 19 Silverburn Place, Aberdeen AB23 8GE, United Kingdom;3. Institut für Geologie und Paläontologie, Corrensstrasse 24, D-48149 Münster, Germany;4. Deptartment of Earth, Ocean and Atmospheric Sciences, The University of British Columbia, Vancouver V6T 1Z4, Canada;5. Institute of Mineralogy and Geochemistry, University of Lausanne, Switzerland;6. Institute of Geosciences, Mineralogy, J. W. Goethe University, Frankfurt 60438, Germany;1. Departamento de Procesos y Energía, Facultad de Minas, Universidad Nacional de Colombia, Medellín, Colombia;2. Grupo de Investigación en Geología y Geofísica (EGEO), Universidad Nacional de Colombia, Medellín, Colombia;3. Smithsonian Tropical Research Institute, Balboa, Ancon, Panama;4. Departamento de Materiales y Minerales, Facultad de Minas, Universidad de Colombia, Medellín, Colombia;5. Departamento de Física y Geociencias, Universidad del Norte, Barranquilla, Colombia;6. School of the Environment, Washington State University, Pullman, WA, USA;7. Departamento de Ciencias de la Tierra, Universidad EAFIT, Medellín, Colombia;8. Instituto de Geociências, Universidade de São Paulo, São Paulo, Brazil;1. Centre for Tectonics, Resources, and Exploration (TRaX), Department of Earth Sciences, The University of Adelaide, 5005, Australia;2. Institute of the Earth Sciences, St. Petersburg State University, 7/9 University Embankment, St. Petersburg 199034, Russia |
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| Abstract: | In the Ayacucho basin of central Perú the regional Quechua II contractional deformation is bracketed by 40Ar/39Ar isotopic age determinations to a maximum duration of about 300,000 years, and probably less than 150,000 years, centered on 8.7 Ma. The strongly deformed Huanta Formation beneath the Quechua II angular unconformity was deposited during a period of extension that began before 9.05 ± 0.05 Ma. Deposition of a thick succession of alluvial fan deposits interbedded with flows of basaltic andesite in the Tingrayoc Member continued up to about 8.76 ± 0.05 Ma with the later part of the sedimentary record reflected by lacustrine deposits of the Mayocc Member. The upper limit on contractional deformation is constrained by an age of 8.64 ± 0.05 Ma on a unit of tuff near the base of the Puchcas volcanics, which in places was deposited upon near-vertical beds of the Huanta Formation. The Ayacucho Formation was deposited, locally unconformably, upon the Puchcas volcanics beginning slightly before 7.65 ± 0.10 Ma.Extended periods of neutral to tensional stress interrupted by rapid well-developed pulses of contractional deformation demonstrate the episodic behavior of Andean orogeny in Perú. The very short duration for the Quechua II event implies that driving forces for episodic deformation may be related to coupling along the orogen boundaries and strain accumulation and release mechanisms in the continental crust instead of much longer-term variations in the configuration of converging plates. |
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