基于GPS应变、地震应变率与震源应力场对帕米尔高原现今构造变形特征的分析

Present-day crustal deformation of the Pamir constrained by the GPS strain rate,seismic strain rate and earthquake focal stress

研究帕米尔高原的构造变形特征对于理解印度板块向北推挤过程中的应变分配方式以及应力转换模式具有重要的意义.本文利用区域GPS应变场、地震应变场与震源应力场分析帕米尔高原的构造形变特征.主要结论为:(1)该区域变形主要以NNW-SSE或近N-S向的挤压为主,在高原内部伴有明显的近ENE-WSW或E-W向拉张,应力方向在帕米尔高原与塔吉克盆地区域呈现逆时针旋转的趋势,而在塔里木盆地则显示几乎与帕米尔高原的一致的应力状态,这可能与两侧盆地块体的强度差异有关.(2)安德森断层参数A∅显示帕米尔高原北缘与西侧区域为逆断层应力状态,在高原内部为正断层应力状态,这与GPS应变的结果显示的应变主要集中在主帕米尔断裂与阿莱谷地附近而在高原内部应变较低是一致的,另外应力在喀喇昆仑断裂北段的方向基本平行于断层走向,以及断层北端较低的滑动速率,这说明了地壳挤压缩短可能是帕米尔高原主要的的构造变形特征,并不支持由于边界走滑断裂导致的应变分异或者块体挤出的模式.(3)综合考虑地震应变方向与SHmax从帕米尔北部NNW-SSE方向到天山北部的近N-S方向的转换,GPS应变方向在帕米尔高原两侧盆地都存在不同程度的旋转,应力场安德森参数A∅显示的应力状态以及SKS的结果显示的近ENE-WSW方向,我们认为印度板块向北推挤与天山造山带碰撞导致帕米尔高原不对称的径向逆冲是帕米尔高原现今构造变形的主要成因与构造模式.

The aim of this study is to quantify the characteristics of tectonic deformation in the Pamir area based on the GPS strain,seismic strain and focal stress analysis,which has important significance for understanding the strain partitioning and stress transferring in the process of the northward push of the India continent.Results show that(1)the deformation of the Pamir and its surrounding regions is dominated by NNW-SSE or N-S shortening,coupled with nearly ENE-WSW or E-W extension in the interior of the Pamir plateau.In the west of the Pamir plateau and Tajik Basin,the direction of the maximum compressive stress reflects the counterclockwise rotation,while in the Tarim Basin,it is almost consistent with the Pamir plateau,indicating the difference in the strength of both basins as the rigid blocks.(2)The Anderson fault parameters indicate the thrust faulting mechanism in the northern and western margin of the Pamir plateau,and the normal faulting mechanism in the interior of the plateau,which is consistent with the result of the GPS strain.At the same time,the axis of the stress around the Karakorum fault is nearly parallel to the strike of the fault,and the fault slip rate is low at the north end of the Karakorum fault,implying that the deformation in the Pamir plateau is predominated by crustal shortening,incompatible with lateral extrusion or strain partitioning along the large strike-slip fault in the boundary of the Pamir plateau.(3)Integrating that the stress transforming from nearly NNW-SSE in northern Pamir to the N-S direction in the north of the Tien Shan,the strain axis rotates in the both flanks of the Pamir plateau,and the state of the stress field reflected by the Anderson fault parameters,combined with the directions of polarizations of fast shear-waves,we suggest that the deformation of the Pamir plateau can be explained by asymmetric radial thrusting,resulting from the northward push of the Indian plate and collision with the Tien Shan orogenic belt.