THE NATURE OF THE LITHOSPHERE-ASTHENOSPHERE BOUNDARY BENEATH THE CENTRAL SOUTH AMERICA AREA FROM THE STACKING OF sP PRECURSORS

The lithosphere-asthenosphere boundary (LAB) is a seismic discontinuity with negative velocity contrast in the upper mantle. Seismic detections of the LAB in subduction zone region are helpful to understand the interaction between the lithosphere and asthenosphere and the geodynamic process associated with the slab subduction. In this paper, the vertical broadband waveforms are collected from four deep earthquakes occurred from 2006 to 2012 beneath the central South America area. The seismic waveforms are processed with the linear slant stack method to get the vespagrams in the relative travel-time vs. slowness domain, and the sP precursors reflected at the bottom of the LAB (s_LABP) are successfully extracted. Based on the one-dimensional modified velocity model (IASP91-SA), the horizontal distribution for the six s_LABP reflecting points is obtained and divided into the western part (I) and the eastern part (II). In the part I, the LAB depth ranges between 60 km and 63 km, with the average depth of 61 km and the topography of 3 km; in the part II, the LAB depth ranges between 78 km and 82 km, with the average depth of 80 km and the topography of 4 km. Our results reveal the increasing LAB depths from west to east in the central South America area, and the trend may possibly represent the reformation differences of the continental lithosphere. We infer that near the trench, the continental lithosphere may be subjected to the stronger erosion for the higher degree of partial melting and more fertile melts in the asthenosphere; while far from the trench, the continental lithosphere may be subjected to the weaker erosion for the lower degree of partial melting and less fertile melts in the asthenosphere.