HR: 09:15h
AN: S41A-06    [PDF]
TI: Anisotropy in the Chile-Argentina Flat Slab Subduction Zone, South America
AU: * Anderson, M L
EM: anderson@geo.arizona.edu
AF: Department of Geosciences, Gould-Simpson Building University of Arizona, Tucson, AZ 85721 United States
AU: Zandt, G
EM: zandt@geo.arizona.edu
AF: Department of Geosciences, Gould-Simpson Building University of Arizona, Tucson, AZ 85721 United States
AU: Fouch, M J
EM: fouch@asu.edu
AF: Department of Geological Sciences, Arizona State University, Tempe, AZ 85287 United States
AU: Triep, E
EM: triep@andes.unsj.edu.ar
AF: Department of Geophysics and Astronomy, National University of San Juan, San Juan, SJ 5400 Argentina
AB: The South American subduction zone exhibits a dramatic change in dip of the subducting Nazca plate from latitudes $30\deg$ S to $36\deg$ S beneath Chile and Argentina. At $30\deg$ S the slab flattens at a depth of approximately 100 km under Argentina and extends at that depth almost 300 km eastward before continuing its descent into the mantle. South of $33\deg$ S, the slab has a uniform dip of approximately $30\deg$. We are analyzing teleseismic and local earthquakes for seismic anisotropy to better understand the deformation and mantle flow associated with this change in subduction geometry. Earthquakes were recorded by 22 portable broadband seismic stations as a part of the CHile ARgentina Geophysical Experiment (CHARGE). In this study, we analyzed teleseismic SKS and SKKS arrivals from earthquakes with epicentral distances $85\deg$-$140\deg$ from the network as well as S arrivals from local earthquakes in the subducting slab. Preliminary shear wave splitting analyses of local S waves exhibit splitting times of 0.1-0.4 s with a heterogeneous azimuthal distribution of fast axes at some stations in the north, directly above the flat slab. The highest quality results for shear wave splitting from teleseismic events give similar results in this region. We observe larger splitting times ($\sim$0.8 s) with fast directions oriented E-W at stations in the northern part of our network, eastward of the flat slab region. In the southern part of our network, we observe generally N-S fast directions and a broad range (0.2-0.6 s) of splitting times. Preliminary results for the southern portion of our study region are consistent with a model in which N-S mantle flow resulting from retrograde motion of the subducting Nazca plate controls the direction and magnitude of the mantle anisotropy as suggested by Russo and Silver (1994). In the flat slab zone, however, the fast polarization axis is perpendicular (i.e. E-W) to that predicted by the retrograde slab model. We therefore suggest that the local bending geometry of the subducting slab could produce mantle flow in an E-W direction.
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
DE: 8102 Continental contractional orogenic belts
DE: 8123 Dynamics, seismotectonics
DE: 8162 Rheology--mantle
SC: Seismology [S]
MN: 2003 Fall Meeting