长江中下游及邻区地震各向异性对区域构造演化的启示

Seismic anisotropy in and around the region of the Middle and Lower Reaches of the Yangtze River and its insight into the regional tectonics

大别山东缘和长江中下游地区是我国东部构造格局最复杂的地区之一.为深入理解和认识该区的构造演化过程和壳幔变形特征,本文利用NW-SE向跨越长江中下游地区的密集宽频带流动地震台站数据,开展了远震接收函数Pms各向异性研究,并结合已有研究进一步分析了该区地壳与上地幔各向异性特征及其产生机制.结果表明:长江中下游及邻区地壳各向异性呈明显的横向分区性,华北克拉通东南部以近NWW向为主,而长江中下游和华南东北部以NE向为主.在垂向上,该区地壳与上地幔各向异性方向也存在明显差异,Pms各向异性快波方向与XKS(PKS,SKS,SKKS)快波偏振方向整体上存在30°~60°左右差异,形成了该区壳幔各向异性的“立交桥”式分层结构.本研究认为印支期和燕山期的区域构造演化差异以及华北与华南块体的流变学强度不同是造成该区壳幔各向异性“立交桥”式分层结构的重要因素.华北地区Pms各向异性特征反映了华南与华北块体在印支期陆陆碰撞所形成的中下地壳变形痕迹,华南东北部地区的Pms各向异性则主要受控于燕山期古太平洋板块俯冲造成的陆内挤压作用.燕山期古太平洋板块的北西向俯冲作用不仅导致了软流圈物质沿NW或近W向流动和橄榄岩矿物晶体的定向排列,也在地壳和岩石圈尺度产生了陆内挤压作用并最终导致了目前观测到的中下地壳变形和各向异性特征.

The region lying on the eastern margin of the Dabie Mountains and the Middle and Lower Reaches of the Yangtze River Belt (MLYB) constitute one of the most structurally complex areas in eastern China. To profoundly understand the tectonic evolution and deformation characteristics of the crust and upper mantle of this region, we calculate the seismic anisotropy by fitting the Pms delay time in teleseismic P-wave receiver functions recorded by a dense portable seismic array across the MLYB. We further elucidate the distinct characteristics and origin of the crustal and upper mantle seismic anisotropy by integrating previous similar studies in and around MLYB. Our research reveals obvious lateral zoning in crustal anisotropy in MLYB and its surrounding North China Block (NCB) and South China Block (SCB). The southeastern part of the NCB predominantly displays crustal anisotropy in nearly NWW directions, while the MLYB and the northeastern part of SCB show a predominance in the NE direction. Vertically, there is a conspicuous disparity in anisotropic directions in the crust and upper mantle of this region. The fast wave directions of Pms anisotropy exhibit a general difference of about 30° to 60° from the XKS (PKS, SKS, SKKS) fast wave polarization directions, delineating a "flyover" style structure of crust-mantle anisotropy in this region. This study suggests that the differences in the rheological strength between NCB and SCB and in regional tectonic evolution during the Indosinian and Yanshanian periods play a crucial role in creating this "flyover" style anisotropy. The Pms anisotropy characteristics in NCB reflect the deformation traces of the middle and lower crust formed during the Indosinian continental collision between NCB and SCB, while that in the northeastern part of SCB are dominantly controlled by the intraplate compression formed during the northwestward subduction of the Paleo-Pacific plate. The subduction of the Paleo-Pacific Plate in the Yanshanian period caused not only the asthenospheric flow along NW or W direction and the lattice-preferred orientation of mineral alignment, but also intraplate compression in the crust and lithosphere and culminated the observed deformation and seismic anisotropy features of the middle and lower crust in and around the MLYB.