青藏高原南部右旋走滑边界带的厘定

It is a generally acknowledged fact that lateral extrusion or escape is one of the important deformation way in Tibetan plateau. However, for overall eastward extrusion of the plateau,there are still some questions about the southern boundary. The main debate is whether there is dextral strike-slip boundary in the southern Tibetan plateau,and whether JiaLi fault is the southern boundary zone.If yes, how about its space distribution, geometric shape and quaternary movement characteristics? If no,where is the southern boundary zone? So far, there is not enough evidence to solve this debate. Furthermore, there are a series of faults echeloned southwest area beside JiaLi fault, discontinuously distributed in Naqu, Jiali, Linzhi and Motuo county. Their extending length are generally smaller, most of them are dozens of kilometers. They are obviously active during late Quaternary and cut north-east and nearly east-west trending faults with dextral strike-slip movement. Larges Earthquakes in the region are associated with them. It is one newborn active fault zone like the Zhangjiakou-Bohai fault zone. It may be the dextral strike-slip boundary zone of the eastward extrusion of the Tibetan Plateau during Quaternary period. We shall comprehensively study the latest movement characteristics of JiaLi fault and NW faults in the southwest area of JiaLi fault by combination of land measurement,geological and geomorphic survey, and determine the dextral slip boundary in the southern Tibetan plateau. The main research contents include: (1) Study the present motion characteristics of faults and blocks by three GPS profiles observation across the fault and exsiting GPS data; (2) Study the space distribution,geometric characteristics, movement way of JiaLi fault and NW faults to the southwest area of JiaLi fault through the remote sensing image data interpretation, field geological survey at four key regions; (3) Determining the latest active time and sliding rate of these faults by analysing the deformation and displacement of geomorphic faces at all levels and precise dating times of geomorphic faces; (4)Determining the latest movement characteristics of faults by the comprehensive analysis of GPS profile observation and geology survey results. Determining dextral strike-slip boundary zone in the southern Tibetan plateau by analysis of different types of model through finite element numerical simulation, and establishing deformation mechanism of dynamics model.

侧向挤出或逃逸作为青藏高原变形的重要方式之一已成为不争的事实。但对于高原整体向东挤出的南界尚存疑较多，主要集中在嘉黎断裂带是否是青藏高原南部右旋走滑边界带？如果是，其空间展布、几何形态与运动特征如何？如不是，南部右旋走滑边界带可能位置何在？本项目采用大地测量与地质调查相结合，综合研究嘉黎断裂带及西南侧北西向断裂带的最新运动特征，用以厘定青藏高原南部右旋走滑边界带。主要包括：①通过不同构造部位3条跨断层GPS剖面观测结合已有GPS数据，获取断裂和块体的现今运动特征；②通过遥感资料解译、4个重点区域地质调查，研究断裂的空间展布和几何特征与运动方式；③通过对断裂二侧各级地貌面变形和位移的对比分析和精确测年，确定其最新活动年代和滑动速率；④综合分析GPS剖面观测和地质调查成果，确定断裂最新运动特征，通过对不同类型模型进行有限元数值模拟分析，厘定青藏高原南部右旋走滑边界带，建立变形机制的动力学模型。

侧向挤出或逃逸作为青藏高原变形的重要方式之一已成为不争的事实。但对于高原整体向东挤出的南界尚存疑较多，主要集中在嘉黎断裂带是否是青藏高原南部右旋走滑边界带。本项目采用大地测量与地质调查相结合，综合研究嘉黎断裂带及西南侧北西向断裂带的最新运动特征，厘定青藏高原南部右旋走滑边界带。GPS观测研究结果揭示构造结西侧右旋走滑的最大区域位于嘉黎断裂带北侧的边坝县城与金岭乡之间，走滑速率达8-10mm/a，而嘉黎断裂带、那曲-墨脱断裂带和雅鲁藏布江断裂带的右旋走滑速率均小于5mm/a；东构造结顶部附近不存在明显的右旋走滑带；东构造结东侧，嘉黎断裂带南支的右旋走滑速率和挤压速率明显增大，分别为5-6mm/a和7-8mm/a，而北支断裂发生了反转表现为弱左旋走滑和拉张运动，右旋走滑的最大区域位于嘉黎断裂带与阿帕龙断裂带之间，存在9-12mm/a右旋走滑速率差异，阿帕龙断裂带的右旋走滑速率为2-3mm/a，挤压速率为1-2mm/a。表明尽管东构造结形成于中生代，但现今仍控制着周边地壳运动的顺时针转动。应变场资料显示东构造结西侧北西向右旋走滑不明显，而东侧在墨脱县东南和波密县南部存在明显的北西向右旋走滑带，其往南与实皆断裂相连通，共同构成青藏高原右旋剪切带的南部边界带。地质地貌资料表明，嘉黎断裂带和那曲—墨脱断裂带晚第四纪以来均存在右旋走滑特征，但滑动速率较小，一般小于4mm/a，与前人提到的青藏高原右旋走滑边界带10-20mm/a走滑速率差异较大，应该不是青藏高原右旋剪切带的南部边界带。布设在东构造结周边的地震台阵观测资料揭示在东构造结东南侧地震活动形成北西向的团块状密集条带，其与GPS得到的面膨胀率和剪切应变场中的北西向低值与高值异常区域位置基本重合，揭示可能存在一条北西向的新生断裂带。综合分析认为青藏高原右旋剪切带的南部边界为边坝至墨脱达木一线的北西向新生断裂带。