小江断裂带周边地区三维P波速度结构及其构造意义

doi:10.6038/cjg20130713

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摘要

作为青藏高原的东南边界,小江断裂带在高原物质的侧向逃逸中发挥着重要的作用.本文利用流动地震台阵及固定台站的走时观测资料,对小江断裂带及周边区域的壳幔三维P波速度结构进行了研究.结果表明,在中上地壳,小江断裂带内部主要为低速异常,其东侧主要为高速异常.在中下地壳,小江断裂带中部为低速异常,北部和南部主要为高速异常,其中北部的高速异常可延伸到地表附近,南部的高速异常可一直延伸到上地幔.我们推测,小江断裂带中部的低速异常与深部热作用有关;北部的高速异常可能是晚古生代地幔柱活动导致大量基性和超基性幔源物质侵入地壳引起的,它的存在对青藏高原物质向南逃逸起到了一定的阻挡作用,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;南部顶界面向北倾斜的高速异常体对川滇活动块体向南滑移起到了进一步的阻挡作用,导致其上覆的中上地壳低速异常区发生较强的变形和强烈的地震活动,同时在上地幔深度范围起到了稳定的作用,使其南部区域的介质受青藏高原物质向南挤出的影响明显减小.

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关键词 ：地震层析成像, P波速度结构, 小江断裂带, 青藏高原东南缘

Abstract：

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.

Key words： Seismic tomography P-wave velocity Xiaojiang fault system Southeastern Tibetan Plateau

收稿日期: 2013-01-15

PACS:

P315

基金资助:

国家自然科学基金(41074068);中国地震局地球物理研究所基本科研业务专项(DQJB11B12);地震行业科研专项(200708035)资助.

作者简介: 吴建平,1964年生,研究员,博士生导师,主要从事地震学与地球内部物理学研究.E-mail:wjpwu@cea-igp.ac.cn

链接本文:

http://www.geophy.cn/CN/10.6038/cjg20130713 或 http://www.geophy.cn/CN/Y2013/V56/I7/2257

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