巴颜喀拉块体东部活动块体的划分、形变特征及构造意义

Division,deformation and tectonic implication of active blocks in the eastern segment of Bayan Har block

基于活动块体的基本概念，综合对研究区内活动断裂带空间展布、地震活动性等资料的分析将巴颜喀拉块体东部及邻区划分为巴颜喀拉块体（Ⅰ）、华南块体（Ⅱ）、川滇块体（Ⅲ）和西秦岭块体（Ⅳ）等4个一级块体．利用GPS形变场、地球物理场等资料结合F检验法，将巴颜喀拉块体划分为阿坝（Ⅰ1）、马尔康（Ⅰ2）和龙门山（Ⅰ3）3个次级块体，将西秦岭块体划分为岷县（Ⅳ1）和礼县（Ⅳ2）2个次级块体．利用分布在各个块体内部的GPS测站，计算各活动块体及块体边界断裂带的运动变形特征．结果表明：各活动块体的整体运动包括平移和旋转运动；东昆仑断裂带、甘孜一玉树断裂带和鲜水河断裂带的滑动速率明显高于龙门山断裂带的滑动速率；巴颜喀拉块体东部走向北西或北西西的边界断裂表现出左旋拉张的特性；走向北东的边界断裂带，除成县太白断裂带外，均表现出右旋走滑兼挤压的活动特征．巴颜喀拉块体的东向运动存在自西向东的速度衰减，衰减主要被龙日坝断裂带和岷江断裂带分解吸收，其中龙日坝断裂带的水平右旋分解非常明显，约为～4．8±l_6mm／a，岷江断裂带的水平分解较弱．龙门山断裂带被马尔康、龙门山和岷县等次级块体分成南、中、北三段，龙门山断裂带中段上的主压应变率要明显小于龙门山断裂带南段上的应变率，其北西侧变形幅度从远离断裂带较大到靠近断裂带逐渐减小，表明其在震前已经积累了较高的应变能，有利于发生破裂滑动．汶川地震后，地表破裂带和余震分布揭示的断裂带运动性质自南西向北东由以逆冲运动为主，逐渐转为逆冲兼走滑的特征可能与龙门山断裂带中段所受主压应力方向自南西向北东的变化有关．马尔康、龙门山和岷县3个次级块体与华南块体之间较低的相对运动速度以及龙门山断裂带低应变率、强闭锁的特征都决定了汶川地震前龙门山断裂带低滑动速率的运动特征．

Based on the concept of active blocks, we analyze the major active faults and secondaryfaults, historical earthquakes, field geological investigation, and other data, then divide the eastern segment of Bayan Har block and its adjacent regions into four first grade blocks. Bayan Har block（Ⅰ）,South China block（Ⅱ）, Sichuan-Yunnan block（liD, and West Qinling block（Ⅳ）. According to GPS velocities, data of geophysical fields and geomorphological features, we further divide the Bayan Har block into three subblocks, Aba （Ⅰ1）, Maerkang （Ⅰ2）, and Longmenshan （Ⅰ3）,and divide the Xiqinling block into two subblocks, Minxian （Ⅳ1） and Lixian （Ⅳ2）, and then check the relative independence among those blocks by F-test. The modern translation and rotation rates of those blocks were calculated using the GPS data from the Crustal Motion Observation Network of China between 1999 and 2007. The results show that besides the eastward or southeastward translation the subblocks also rotate around the Euler pole; the slip rates for the eastern Kunlun fault, Ganzi-Yushu fault, and the Xianshuihe fault are much higher than that for the Longmenshan fault; the faults trending northwest or northwest west show apparently left-lateral strike-slip and with a tensile characteristic, and those faults trending northeast show obviously right-lateral strike-slip and compression characteristics, except for the Chengxian Taibai fault which shows left lateral and compression characteristics. The eastward motion of Bayan Har block is accommodated mainly by the Longriba fault and Minjiang fault, and the right lateral horizontal component of Longriba fault is obviously ~4.8＋1.6 mm/a, and this may indicate that the eastward motion of Bayan Har block is discontinuous. The Longmenshan fault was divided into three segments by Maerkang subblock, Longmenshan subblock and Minxian subblock. The strain rate on the middle part of the Longmenshan fault is smaller than that on the southern part, and the deformation northwest of the fault shows that near the fault the strain rate becomes smaller, it maybe imply that high strain energy had accumulated before the earthquake, which promoted the surface rupture. The change from thrust motion to thrust and right strike-slip motion revealed by the surface ruptures and focal mechanism of aftershocks of Wenchuan earthquake may be related to the change of the stress on the middle part of the Longmenshan fault from southwest to northeast. The low velocity of Maerkang, Longmenshan, and Minxian subblocks relative to the South China block, the low strain rate and the tightly locked character of Longmenshan fault may determine the low slip rate of the Longmenshan fault.