基于sP前驱震相叠加研究南美中部地区岩石圈-软流圈边界形态

The nature of the lithosphere-asthenosphere boundary beneath the central South America area from the stacking of sP precursors

岩石圈-软流圈边界(lithosphere-asthenosphere boundary)是上地幔内具有负速度梯度的地震波速度界面.对俯冲带区域LAB开展地震学探测有助于进一步认识岩石圈和软流圈的相互作用以及与板块俯冲有关的地球动力学过程.本文收集了2006-2012年发生于南美中部地区4个深源地震的垂向宽频带波形资料,利用线性倾斜叠加处理得到了相对走时-慢度域的灰度图,并成功提取了sP在LAB底反射的前驱震相S_(LAB)P.基于改进的一维速度模型IASP91-SA计算得出了6个S_(LAB)P震相反射点的水平分布,并划分为西部(Ⅰ)和东部(Ⅱ)分区:分区工内LAB深度位于60~63 km,平均深度为61 km,起伏为3 km;分区Ⅱ内LAB深度位于78~82 km,平均深度为80 km,起伏为4 km.研究结果显示南美中部地区LAB深度自西向东呈变大的趋势,这可能反映了大陆岩石圈受改造程度的差异.我们推测在靠近海沟的地区,软流圈内部分熔融程度较高且熔体较为富集,对大陆岩石圈的侵蚀作用较强;在远离海沟的地区,软流圈内部分熔融程度降低且熔体分布减少,对大陆岩石圈的侵蚀作用减弱.

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 （sLt^P） are successfully extracted. Based on the one-dimensional modified velocity model （IASP91-SA）, we obtain the horizontal distributions for the six SI.ABP reflected points, which are divided into the western part （ ~ ） 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 kin, 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.