秦岭造山带上地幔各向异性及相关的壳幔耦合型式  

Crust-mantle coupling mechanism beneath the Qinling Orogen Belt revealed by SKS-wave splitting

作　　者：吴逸影 邓斯壮 钮凤林[2] 贺巍 吴汉宁[1] WU YiYing;DENG SiZhuang;NIU FengLin;HE Wei;WU HanNing(State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Northern Taibai Str.229,Xi′an 710069,China;Department of Earth,Environmental and Planetary Sciences,Rice University,Houston,Texas,TX 77005,USA;Shaanxi earthquake Agency,Xi′an 710068,China)

机构地区：[1]西北大学地质学系,大陆动力学国家重点实验室,西安710069 [2]莱斯大学地球科学学院,美国休斯顿770053 [3]陕西省地震局,西安710068

出　　处：《地球物理学报》2021年第5期1608-1619,共12页Chinese Journal of Geophysics

基　　金：国家自然科学基金(91855211,41674070,41702233,41774030)资助.

摘　　要：秦岭是具有复杂地壳结构、经历长期构造演化的复合型大陆造山带.本文通过地震资料精细反演上地幔各向异性,探索秦岭造山带构造演化及成因动力.采用最小切向能量法、最小特征值法和“叠加”分析法求得覆盖秦岭造山带及周边地区41个地震台站的SKS横波分裂结果:快波偏振方向(φ)和快、慢波的时间延迟(δt),据此绘制了秦岭造山带上地幔各向异性图.将已发表的地表GPS观测结果与上地幔各向异性相结合作上地幔变形因素分析,发现秦岭造山带自西向东的上地幔变形因素不是单一垂直连贯变形或地幔流动,而是共存的.同时,其上地幔变形的主控因素有区域性变化.研究表明秦岭造山带西、中部上地幔变形以壳幔垂直连贯变形为主,属壳幔强耦合,东部壳、幔耦合变弱,上地幔变形以简单地幔流动为主控因素.同时,SKS快波偏振方向(φ)于秦岭造山带显示出南缘略向北凸、北缘略向南凸的弧形展布,反映了造山带两侧刚性较强的扬子地块与鄂尔多斯地块旋转与秦岭造山带南北缘弧形流动有关.The Qinling Orogenic Belt(QOB)is an east-west extended Proterozoic mountain range located between the Ordos block in the north and the Yangtze block in the south.It is featured by complicated crustal deformation structures due to its long history of tectonic evolution.Measuring the subsurface deformation field beneath the QOB thus becomes essential for deciphering major tectonic events that shaped the belt.Seismic anisotropy is generally believed to be caused by fabric structures that are close related to the subsurface strain field.Seismic anisotropy is often measured with two parameters,the fast polarization directionφand delay timeδt between the fast and slow directions,from splitting or birefringence of shear waves.In this study,we measured(φ,δt)from SKS recorded by 41 broadband stations that cover the QOB and it surrounding areas.We employed a 2-D grid searching method which searches for an optimum(φ,δt)that minimizes the transverse energy or the second eigenvalue of the covariance matrix.Stacking of SKS data from multiple earthquakes provides robust measurements of the two splitting parameters.The measured seismic anisotropy shows an overall E-W fast polarization direction across the QOB with slight variations at its western and eastern ends.The fast polarization direction is consistent with a eastward mantle flow that escapes from the NE margin of the Tibetan plateau and flows between the Ordos and Yangtze blocks.We also compared SKS splitting with GPS measurements and crustal anisotropy,and found that the NE marging of the Tibetan plateau and western part of QOB exhibit a vertically coherent deformation(VCD)across the entire lithosphere.In the eastern part of the QOB,there is a significant difference(>15°)in the fast polarization direction measured from SKS and Pms,a P-to-S converted wave at the Moho,indicating some decoupling of deformation between the crust and upper mantle.

关 键 词：秦岭造山带 地震各向异性 地幔变形 壳幔耦合 

分 类 号：P315[天文地球—地震学]