Seismic attenuation above the Pampean flat slab in South America

Abstract:

The Pampean flat slab extends nearly horizontally beneath the South American Plate at ~100 km depth. Consequently, this region lacks arc volcanism as the slab immediately contacts the overriding lithospheric mantle. Previous studies indicate slab dehydration and partial eclogitization of the lower crust of the overriding plate. In this study, we use data from 52 seismic stations in the region to investigate the seismic attenuation structure above the Pampean flat slab. We invert for t^* and estimate path-average attenuation structure for both P and S waves in the study area. The results show that there is a high-attenuation zone beneath the back-arc, with path-average 1/Q_p > 0.010 and 1/Q_s > 0.006. The low Q_p/Q_s values (< 1.0) may suggest strong scattering attenuation in the overriding lithosphere. We suspect that the high attenuation anomalies in the back-arc are related to the slab dehydration. We also detect high attenuation anomalies beneath the Holocene volcanoes near the southern edge of the Pampean flat slab, which may indicate partial melting in the regular mantle wedge.