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 the seismic discontinuity with a negative velocity contrast in the upper mantle. Seismic detections on 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 waveform data is processed with the linear slant stack method to get the vespagrams in the relative travel-time to slowness domain, and the sP precursors reflected from the bottom of the LAB (s(LAB)P) are successfully extracted. Based on the one-dimensional modified velocity model (IASP91-SA), we obtain the horizontal distributions for the six SLABP reflected points, which are divided into the western part (I) and the eastern part (II). In part I, the LAB depths range between 60 and 63 km, with the average depth of 61 km and the topography of 3 km; in part II, the LAB depths range between 78 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 to the continental lithosphere. We infer that the continental lithosphere may be subjected to the stronger erosion in the area near the trench, for the higher degree of partial melting and the more fertile melts in the asthenosphere; the continental lithosphere may be subjected to the weaker erosion in the area far from the trench, for the lower degree of partial melting and the less fertile melts in the asthenosphere.