Anisotropic Fast Axes at the Central Andean Trench: From Parallel to Perpendicular Orientation
Abstract
In subduction zones anisotropy is often found to be dominated by fast axes oriented sub-parallel to the trench. Interpreting the fast-polarization directions in terms of mantle deformation has led to the hypothesis of trench-parallel mantle flow due to pressure gradients induced by slab geometry or trench migration. However, the relation between seismic anisotropy and the mantle-flow field in subduction zones remains poorly understood. Here, we compare splitting measurements from real and synthetic waveform data to constrain the seismic anisotropy at the Central Andean trench. We have studied shear-wave splitting along two EW-oriented profiles in the Central Andes at 21°S and 25.5°S in order to infer variations of fast-axes directions and delay times from the Pacific coast to the western margin of the Interandean Zone. Using both, recordings from teleseismic SKS and local S phases, we aim to discriminate between the effects from the lithosphere of the overriding South-American plate and anisotropy induced by mantle flow beneath the subducting slab. The analysis of SKS-splitting along the profiles from West to East reveals fast polarizations that vary abruptly from EW (closer to the trench) to NW-SE (at a distance of about 300 km from the trench). The corresponding delay times are relatively small and vary between 0.4 and 1.2 seconds. The splitting analysis from local events (within the slab) reveals similar delay times. However, the corresponding fast axes deviate from the SKS results and are predominantly oriented NE-SW beneath the Altiplano plateau. We compare our observations with splitting measurements on synthetic waveforms generated by anisotropic finite-difference models of the subduction zone. We test for influences of changes in mantle-flow direction and crystal alignment of olivine due to its dependence on water content, respectively. Our results contribute to the understanding of mantle dynamics in subduction zones.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.S53E2539W
- Keywords:
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- 7208 SEISMOLOGY / Mantle;
- 7240 SEISMOLOGY / Subduction zones;
- 8170 TECTONOPHYSICS / Subduction zone processes