June 7, 2011

Category: News

A Montgomery Prize was recently awarded to Lepolt Linkimer, a Ph.D. candidate in the Department of Geosciences at the University of Arizona, for the best overall GeoDaze presentation. In addition to receiving a cash prize, Linkimer will be invited to present his talk to our Tucson staff. GeoDaze, a student-run symposium funded by private donations, gives graduates and undergraduates an opportunity to showcase their research.

M&A has supported this event for many years in an effort to promote excellence in research and communication in the geosciences. We also award a Montgomery Prize at another UA symposium — El Dia del Agua — sponsored by the Department of Hydrology & Water Resources.

Linkimer’s abstract follows.

Lithospheric Structure of the Chile-Argentina Flat Slab Region from Double-Difference Tomography

Lepolt Linkimer1, Susan Beck1, George Zandt1, Patricia Alvarado2, Megan Anderson3, Hersh Gilbert4

1 Department of Geosciences, University of Arizona

2 Departamento de Geofísica y Astronomía, Universidad Nacional de San Juan-CONICET, Argentina

3 Department of Geology, Colorado College

4 Department of Earth and Atmospheric Sciences, Purdue University

We obtain earthquake locations and a detailed three-dimensional model of the subduction zone velocity structure in west-central Argentina (lat. 30-32°S and lon. 64-70°W) by applying a regional-scale double-difference tomography algorithm to earthquake data recorded by the recent SIEMBRA (2007-2009) and ESP (2008-2010) broadband seismic networks. In this region, flat subduction of the Nazca Plate including the Juan Fernandez Ridge results in a ~750 km gap in the arc volcanism of the overriding South America Plate and uplift of the thick-skinned basement-cored outcrops of the Sierras Pampeanas.

Our model shows the subducting Nazca Plate as a clear high P-wave velocity (Vp), low S-wave velocity (Vs), and high Vp/Vs ratio anomaly. We observe two distinct low Vp/Vs regions in the South America Plate upper mantle. One is located in the west above the flat slab and consists of low Vp and high Vs suggesting orthopyroxene enrichment. The other one is located above where the slab is dipping (east of lon. -67) and consists of high Vp and high Vs, which may be reflecting melt depletion after partial meting. Our results are consistent with a thicker South America crust (~60–70 km) in the western part of the study area and a much thinner crust in the eastern part (~30-40 km). Our model shows that the middle-to-upper crust in the Cuyania Terrane is mostly high Vp/Vs in contrast to the Pampia Terrane crust, which is mostly imaged as low Vp/Vs. This is consistent with the differences in composition for the Sierras Pampeanas basement: a more mafic composition for Cuyania terrane in the west and a more felsic composition for the Pampia Terrane in the east. The relocated slab earthquakes illuminate the slab geometry suggesting that the subducted Juan Fernandez ridge is more buoyant where the slab seismicity is the shallowest (lon. 68.5-67.5°W).

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