青藏高原东北缘构造应力场及动力学特征

Tectonic stress field and dynamic characteristics in the northeastern margin of the Tibetan Plateau

青藏高原东北缘是高原物质挤出的重要通道,也是深入认识青藏高原构造隆升和横向扩展机制的重要区域.本文利用中国地震台网在青藏高原东北缘布设的宽频带地震仪记录的波形资料,利用CAP全波形反演方法及基于P波初动极性和S/P振幅比的HASH方法,获得了研究区内M_(L)≥3.0地震震源机制解781组,同时收集了M_(W)≥4.6中强地震震源机制解96组,利用STASI阻尼区域应力场反演算法对该区的构造应力场分布特征进行了分析.结果显示,研究区震源机制P轴、T轴以及最大主应力轴σ_(1)和最小主应力轴σ_(3)总体上倾伏角较小,揭示了近水平的挤压或剪切应力环境.σ_(1)以NE向为主,且由巴颜喀拉块体西南部向外呈扇形辐射,至祁连造山带逆时针偏转为NNE向,至秦岭造山带顺时针偏转为ENE向—ESE向,分布特征与区内走滑型边界断裂活动性质相对应.水平最大主应力S_(Hmax)方向与σ_(1)分布特征基本一致,应力结构类型以走滑型为主,且R值具有明显的空间差异分布特征.柴达木—陇西块体和巴颜喀拉块体东南部与秦岭造山带和四川盆地的过渡区域,深浅地壳S_(Hmax)方向具有差异性,深部地壳变形与浅部可能发生解耦,其余区域不同地壳深度的S_(Hmax)方向一致性较好.联合基于原位地应力实测获得的地壳浅表层地应力状态、基于地震各向异性获得的地壳和上地幔变形特征以及基于GPS观测获得的地壳浅表层形变特征,讨论了青藏高原东北缘岩石圈垂向变形特征及动力学机制.结果表明青藏高原东北缘应力场与应变场特征一致,地表形变主要受控于区域构造应力场.壳幔变形机制和深部动力学特征极为复杂,很难用单一模式解释,青藏高原东北缘可能以岩石圈连续增厚变形与下地壳通道流的共同作用模式为主.

The northeastern margin of the Tibetan Plateau is an important channel for material extrusion and lateral growth of the Tibetan Plateau,and it plays an important role for in-depth understanding the mechanism of the tectonic uplift and lateral growth in and around the Tibetan Plateau.Using the broadband records from 103 permanent seismic stations from the Chinese National Seismic Network(CNSN)deployed in the NE margin of the Tibetan Plateau,we determined the focal mechanisms of 781 earthquakes(M_(L)≥3.0)by the cut-and-paste(CAP)method and the HASH method.In addition,focal mechanisms of 96 moderate earthquakes(M_(W)≥4.6)in the study area are collected as a supplement.The distribution characteristics of the tectonic stress field in this area are analyzed using a damped inversion method,STASI.The results show that the plunge angles of the P axes,T axes,the maximum principal stress axes,σ_(1),and the minimum principal stress axes,σ_(3),are generally small,revealing a nearly horizontal compression or shear stress environment.σ_(1)axes are mainly NE,and is fan-shaped radiation from the southwestern part of the Bayan Har block.To the Qilian thrust belt,σ_(1)deflects counterclockwise to the NNE direction,and to the Qinling orogen,σ_(1)deflects clockwise to the ENE-ESE direction.Directions of S_(Hmax)are basically consistent with the distribution characteristics ofσ_(1).Stress regime in the study area is mainly strike-slip,and the R value presents obvious spatial differentiation characteristics.In the transition area between the Qiadam-Longxi block and the southeastern part of the Bayan Har block,the Qinling orogenic belt and the Sichuan Basin,the S_(Hmax)direction of the deep and shallow crust are different,indicating decoupled upper and lower crustal deformations.And the S_(Hmax)directions of different crustal depths in the other regions have good consistency.Finally,combined with the shallow crustal stress state obtained by the in-situ stress measurements,the deformation characteristics of the crust and upper mantle based on seismic anisotropy,and the deformation characteristics of the shallow crustal based on GPS observations,as well as the analysis results in this study,we discuss vertical deformation and dynamic mechanism of the lithosphere in the NE margin of the Tibetan Plateau.The results show that the crustal stress field and strain rate field are basically coupled,and the crustal deformation is mainly controlled by the background tectonic stress field.Besides,the deformation of the crust and mantle and deep dynamic characteristics are extremely complex,and it is difficult to explain it with a single model.The NE margin of the Tibetan Plateau may be dominated by the joint modes of tectonic extrusion and lateral channel flow in the lower crust.