Three dimensional P-wave velocity structure around Xiaojiang fault system and its tectonic implications
Three dimensional P-wave velocity structure around Xiaojiang fault system and its tectonic implications
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发表时间：
2013
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通讯作者：
Wu Jian
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作者：
Wu Jian
小江断裂系是青藏高原东南缘的重要断裂带，在青藏高原物质的侧向逃逸中起着重要作用。本文利用固定地震台和时间密集地震台阵的走时资料，对小江断裂系及其周边地区的三维P波速度结构进行了成像。结果表明，小江断裂系在中上地壳主要表现为低速，在中下地壳，小江断裂系中部为低速异常，北方和南方均为高速异常，攀枝花附近的北方高速异常从下地壳延伸到地表，认为小江断裂系中部地区的低速异常是上地幔派生的高温造成的，其高速度与上地幔的温度有关北方地区的速度异常很可能与晚古生代地幔柱活动导致大量基性和超基性地幔物质侵入地壳有关，这些高速异常具有较大的机械强度，对青藏高原物质向南逃逸起到一定的阻碍作用，并导致川滇活动块体北方子块体的快速隆升，南部地区北倾的高速异常是川滇活动块体向南挤压的进一步障碍，导致上覆上中地壳低速区的强烈变形和高地震活动率，同时这种高速块体有助于上地幔的稳定，使其南部地区的地幔物质受青藏高原物质向南逃逸的影响较小。
As the southeastern margin of Tibetan Plateau,Xiaojiang fault system plays an important role in the lateral escape of Tibetan Plateau materials.In this paper,3-D P-wave velocity structure in Xiaojiang fault system and its surrounding areas was imaged by using travel time data from permanent seismic stations and a dense temporal seismic array.The results show that in the upper and middle crust,Xiaojiang fault system mainly represents low velocity,while high velocity anomalies exist to its east.In the middle and lower crust,a low velocity anomaly is shown in the central area of Xiaojiang fault system,and high velocity anomalies exist both in the northern and southern areas.The northern high velocity anomaly near Panzhihua extends from lower crust to the surface,and the southern one in the lower crust extends to the upper mantle.We consider that the low velocity anomaly in the central area of Xiaojiang fault system is caused by high temperature derived from upper mantle.The high velocity anomalies in the northern area are most likely related to the late Paleozoic mantle plume activity which caused a large amount of basic and ultrabasic mantle material intruded into the crust.These high velocity anomalies,with greater mechanical strength,play a certain impediment to southward escaping of Tibetan Plateau materials,and lead to the rapid uplift of the northern sub-block of the Sichuan-Yunnan active block.The north-dipping high velocity anomaly in the southern area is a further barrier for southward extrusion of Sichuan-Yunnan active block,leading to strong deformation and high rate of seismicity within the low velocity area in the overlying upper and middle crust.Meantime,this high velocity block contributes to stabilization in the upper mantle,which causes mantle materials of its southern region less affected by the southward escape of Tibetan Plateau materials.