利用P波接收函数研究青藏高原东南缘地幔转换带结构

摘要

The deep structure of the southeastern Tibetan plateau is of great importance to a better understanding of the India-Eurasia collision as well as the evolution of the Tibetan plateau.In this study,we collected waveform data of 566 permanent and temporary seismic stations in southeastern Tibetan plateau,then extracted a total of 77853 high quality P-wave receiver functions using the maximum entropy deconvolution method.By the Common Conversion Point (CCP) stacking technique,we mapped the topography of the 410-and 660-km discontinuities ('410' and '660' hereinafter),and further investigated the lateral variation of the mantle transition zone (MTZ) beneath this region.The background velocity model deduced from H-κ stacking results and a previous body-wave tomographic research were applied to the correction of the crustal and upper mantle heterogeneities beneath southeastern Tibetan plateau for CCP stacking.Our results reveal two significantly thickened MTZ anomalies aligned nearly in the southnorth direction.The magnitudes of both anomalies are ～ 30 km above the global average of 250 km.The southern anomaly located beneath the central-Yunnan sub-block and the Indo-China block is characterized by a slightly deeper '410' and a greater-than-normal '660',while the northern anomaly beneath western Sichuan has an uplifted '410' and a depressed '660'.Combining with previous studies in the adjacent regions,we suggest that after slab break-off between the Indian plate and Neo-Tethys oceanic plate,the remnant of the latter stagnating at the bottom of the MTZ may have caused the southern anomaly in our receiver function images.The anomaly in the north,on the other hand,might be the consequence of the delamination of the overlying lithosphere sinking into the MTZ.The origin of the Tengchong volcano is probably connected to the upwelling of the hot and wet asthenospheric material caused by the slab break-off process.%研究青藏高原东南缘的深部结构对于理解印度—欧亚板块的碰撞机理和青藏高原的形成演化具有重要的科学意义.本研究对布设在研究区域内566个固定和流动地震台站的波形资料进行了处理,获得77853条高质量P波接收函数,应用接收函数共转换点(CCP)叠加技术获得了研究区域下方精细的地幔转换带间断面起伏形态及转换带厚度变化图像.结果表明:研究区域南北方向上具有两个明显的转换带增厚异常区,南侧异常区位于滇中次级块体与印支块体下方,可能是新特提斯洋板片与上部印度板块间断离并部分滞留在转换带底部的结果;北侧川西地区异常增厚可能与上方岩石圈拆沉并降至转换带有关;腾冲火山起源可能是板块俯冲过程中发生断离造成软流圈物质部分熔融,湿热物质上涌所致.