滇西地区地壳速度结构及其构造意义

3-D CRUSTAL P-WAVE VELOCITY STRUCTURE IN WESTERN YUNNAN AREA AND ITS TECTONIC IMPLICATIONS

使用2011年8月至2012年8月在南北地震带南段布设的密集流动地震台站记录的波形数据,采用绝对定位方法对滇西地区的1 593个地震进行了初步定位,应用近震走时层析成像方法获得了滇西地区三维地壳P波速度结构。研究结果表明,攀枝花附近的高速异常从地表可一直延伸至中地壳,并在中下地壳深度与滇西北地区的高速异常形成大范围的高速块体。这一高速异常体对青藏高原物质向南逃逸起到一定的阻挡作用,可能是导致川滇活动块体北部次级块体快速抬升的重要因素。位于印度板块与欧亚板块俯冲边界的腾冲、保山地块在下地壳表现为明显的大范围低速异常,推测该低速异常与印度板块东向俯冲引起的地幔热物质上涌有关。金沙江-红河断裂带是川滇块体重要的南部边界,红河断裂带中段的弥渡至红河一带具有高速的地壳结构以及较弱的地震活动性,可能表明该区现阶段处于闭锁状态,是未来需要重点监测的地段之一。

Using the absolute relocation method, we relocate 1593 earthquakes from the seismic waveform data recorded by the dense temporary seismic array from August 2011 to August 2012, which is deployed in the southern segment of the north-south seismic belt. Then seismic traveltime tomography method is applied to obtain the 3-D crustal P-wave velocity structure in the western Yunnan area. The inversion results indicate that high-velocity anomaly extends to the middle crust from the surface in the Panzhihua area, with the high velocity on its west merging into a large-scale high-velocity block. We speculate that the high-velocity block plays a certain impediment to southward escaping of the Tibetan plateau materials, which caused the rapid uplift of the northern sub-block of the Sichuan-Yunnan active block. In the south of the Jinshajiang-Red River Fault, low-velocity anomaly exsists in the lower crust of the Tengchong block and Baoshan block. Since it is located in the subduction boundary between the Indian plate and Eurasian plate, we consider that the low-velocity anomaly may be caused by the high temperature derived from the upper mantle due to the Indian plate＇s eastward subduction. Jinshajiang-Red River Fault is the southern boundary of the Sichuan-Yunnan block. The high-velocity crustal structure and the weak seismic activities in the middle segment of the Red River Fault（ Midu to Red River areas） imply that this region is locked currently. However, as the boundary between the blocks, it should be one of the key monitoring areas in the future.