利用虚震源反射成像法研究江南造山带东段上地壳结构

Crustal structure across the Jiangnan orogen from teleseismic virtual-source reflection method

江南造山带是华南地区扬子地块与华夏地块碰撞的产物,其地壳构造记录了两地块的碰撞过程,研究江南造山带的地壳构造有助于重建扬子地块与华夏地块的碰撞过程。本研究在江南造山带上布设了两条流动地震台阵,利用虚震源反射法提取其所记录的远震事件初至P波在地表的反射波(PPdp)波形,重构了研究区内两条测线下方的上地壳结构。结果显示:江绍断裂两侧上地壳沉积层的厚度变化明显,推断该断裂是扬子地块与华夏地块的东边界;相较华夏地块,江南造山带与扬子地块的层位连续性更强,符合江南造山带先与扬子地块合为整体后再与华夏地块碰撞的多期构造过程及其对应产生的亲扬子地块属性;江绍断裂西北侧的地层不整合以及赣东北断裂区域的断陷构造,可为了解古华夏洋向扬子陆块俯冲及碰撞和随后的构造运动过程提供参考依据。

The Jiangnan orogen belt(JOB) was formed by the collision between the Yangtze block(YB) and the Cathaysia block(CB). The crustal structure of the JOB recorded the collisional process between the two blocks. Hence,studying crust of the JOB can illuminate the process between the YB and the CB,and thus better understand the tectonic evolution of the South China Plate(SCP). In this study,we applied the teleseismic virtual reflection method to derive the phase of the reflection waves(PPdp) of large teleseismic earthquakes recorded by two temporary seismic arrays that were deployed around the JOB and obtained the upper crust structures. The results suggest that the thickness of the upper crust sediments on two sides of the Jiangshao fault changes obviously. It is also inferred that the Jiangshao fault is the eastern boundary between the Yangtze block and the Cathaysia block. The JOB and the YB have more apparent stratigraphic continuity than that of the CB. This result agrees with a multi-stage tectonic model which suggests that the JOB first bounded the YB and then collided with the CB.Hence the characteristics of the JOB are closer to that of the YB. The stratigraphic unconformities in the northwest of the Jiangshao fault and the fault structure in the northeast Jiangxi region provides a basic profile for understanding the subducting and colliding processes between the ancient Cathaysian ocean and the YB as well.