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Flat-slab subduction and crustal models for the seismically active Sierras Pampeanas region of Argentina

By
Patricia Alvarado
Patricia Alvarado
CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas)–Departamento de Geofísica y Astronomía, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, Meglioli 1160 S (5407) Rivadavia, San Juan, Argentina
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Mario Pardo
Mario Pardo
Departamento de Geofísica, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Blanco Encalada 2002, Santiago, Chile
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Hersh Gilbert
Hersh Gilbert
Department of Earth and Atmospheric Sciences, 550 Stadium Mall Dr., Purdue University, West Lafayette, Indiana 47907, USA
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Silvia Miranda
Silvia Miranda
Departamento de Geofísica y Astronomía, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, Meglioli 1160 S (5407) Rivadavia, San Juan, Argentina
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Megan Anderson
Megan Anderson
Department of Geology, Colorado College, 14 E. Cache La Poudre, Colorado Springs, Colorado 80903, USA
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Mauro Saez
Mauro Saez
Departamento de Geofísica y Astronomía, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan, Meglioli 1160 S (5407) Rivadavia, San Juan, Argentina
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Susan Beck
Susan Beck
Department of Geosciences, University of Arizona Gould-Simpson Building #77, 1040 E. 4th St., Tucson, Arizona 85721, USA
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Published:
June 01, 2009

The Sierras Pampeanas in the west-central part of Argentina are a modern analog for Laramide uplifts in the western United States. In this region, the Nazca plate is subducting beneath South America almost horizontally at about ~100 km depth before descending into the mantle. The flat-slab geometry correlates with the inland prolongation of the subducted oceanic Juan Fernández Ridge. This region of Argentina is characterized by the termination of the volcanic arc and uplift of the active basement-cored Sierras Pampeanas. The upper plate shows marked differences in seismic properties that are interpreted as variations in crustal composition in agreement with the presence of several Neoproterozoic to Paleozoic accreted terranes. In this paper, we combine the results from the CHile-ARgentina Geophysical Experiment (CHARGE) and the CHile-ARgentina Seismology Measurement Experiment (CHARSME) passive broadband arrays to better characterize the flat-slab subduction and the lithospheric structure. Stress tensor orientations indicate that the horizontal slab is in extension, whereas the upper plate backarc crust is under compression.

The Cuyania terrane crust exhibits high P-wave seismic velocities (Vp ~6.4 km/s), high P- to S-wave seismic velocity ratios (Vp/Vs = 1.80–1.85), and 55–60 km crustal thickness. In addition, the Cuyania terrane has a high-density and high-seismic-velocity lower crust. In contrast, the Pampia terrane crust has a lower Vp value of 6.0 km/s, a lower Vp/Vs ratio of 1.73, and a thinner crust of ~35 km thickness. We integrate seismic and gravity studies to evaluate crustal models that can explain the unusually low elevations of the western Sierras Pampeanas.

Flat-slab subduction models based on CHARGE and CHARSME seismic data and gravity observations show a good correlation with the predicted Juan Fernández Ridge path beneath South America, the deep Moho depths in the Andean backarc, and the high-density and high-seismic-velocity lower crust of the Cuyania terrane. The Cuyania terrane is also the region characterized by more frequent and larger-magnitude crustal earthquakes.

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GSA Memoirs

Backbone of the Americas: Shallow Subduction, Plateau Uplift, and Ridge and Terrane Collision

Suzanne Mahlburg Kay
Suzanne Mahlburg Kay
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Víctor A. Ramos
Víctor A. Ramos
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William R. Dickinson
William R. Dickinson
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Geological Society of America
Volume
204
ISBN print:
9780813712048
Publication date:
June 01, 2009

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