环鄂尔多斯断陷盆地现今地壳运动与构造环境特征的GPS研究

The Ordos block in itself has been very stable. In contrast, the margins of this stable block have been the site of active Cenozoic tectonics in northeastern China. The block has been surrounded by four rift systems: the Weihe graben to the south, Yinchuan rift basins to the west, the Hetao rift zone to the north, and the Shanxi graben to the east. Circum-Ordos rifts are characterized by extensional shear zones, expecting on the southwestern edge, which has been compressed by Tibet in the NE-NNE direction. Historic records show four M=8 and more than 30 M≥6 earthquakes in these rift zones. Extensive GPS measurements in the past two decades, along with newly constructed campaign sites of CMONOC (Crustal Movement Observation Network of China), provide unprecedented opportunities to studying detailed deformation of faults in this region. In our research, we will try to collect and reprocess more than 1000 GPS sites from 1992 up to now, using the latest geophysical correction models and final satellites orbits to obtain accuracy self-consistent GPS velocities. Then, fault slip rates and locking depths of the rift systems will be estimated using different deformation models, such as screw dislocation model, GPS velocity profiles model and rigid block model. Given that different simulation models may result in different slip rates, the discrepancies between them and which one is better will be addressed. In the next, we will also compare slip rates of GPS results with that of geology, and tries to resolve the discrepancies considering the influence of lithosphere viscosity structure. Finally, we will attempt to interpret the tectonic deformation mechanism and seismicity based on our latest and detailed kinematic model of rift systems, on seismic tomography, paleoseismicity, Cenozoic structure and so on. Systematic and informative research on present-day crustal deformation in circum-Ordos could shed new insights into understanding active tectonic mechanism in this region, and provides important implication for continental dynamics.

内部稳定的鄂尔多斯块体四周被依次形成的渭河断陷盆地、银川-吉兰泰断陷盆地、河套断陷盆地、山西地堑系及断裂所围限，其构造演化历史、形变模式、动力学背景一直是地学界争论的焦点。已有的大地测量与地震地质对该区域断层活动、地壳形变的研究结果之间差异显著。本项目拟对研究区域内1992年以来的大密度GPS监测资料，采用优化的地球物理模型和数据处理技术进行精化处理，获得自洽的高精度速度场；采用多种正反演方法，对控制鄂尔多斯块体周缘大地震的断裂带进行分段、系统研究，精确测定其滑动速率、拉张速率及闭锁状况等运动参量；结合地震地质、地球物理等资料深入探讨断层滑动速率GPS结果与地质结果的差异原因；在此基础上，试图从现今大陆变形动力学的视角探讨鄂尔多斯块体周缘地壳形变的构造背景及形变模式。本项目对新生代华北地区形变机理与大陆动力学研究均具有重要意义。

内部稳定的鄂尔多斯块体四周被依次形成的渭河断陷盆地、银川-吉兰泰断陷盆地、河套断陷盆地、山西地堑系及断裂所围限，其构造演化历史、形变模式、动力学背景一直是地学界争论的焦点。已有的大地测量与地震地质对该区域断层活动、地壳形变的研究结果之间差异显著。鄂尔多斯块体周围内部稳定而断限盆地内地质活跃，历史上曾发生过多次8级地震。本项目鉴于该区域独特的构造背景以及地震活动特征，采用大地测量观测技术系统研究了鄂尔多斯块体以及周缘主要滑动断裂带的现今地壳运动特征及动力学意义。我们采用高精度GPS数据处理软件，采用最新的数据处理模型、策略处理该区域2000年以来各类GPS观测资料，获取了该区域密集的三维形变场。基于速度场结果，通过跨断层GPS速度剖面方法研究了鄂尔多斯块体的顺时针旋转速率以及滑动断裂的滑动速率。研究结果展示了现今运动的基本特征为：鄂尔多斯块体整体呈顺时针旋转，其西南边界与青藏高原东北缘相接，接受印度-欧亚板块俯冲作用的持续影响是推动块体旋转运动的主要动力源；块体的东西两侧的银川盆地和山西地堑构成右旋走滑的力偶系统，走滑运动速率在1-3mm/a；块体南北两侧的河套盆地和汾渭盆地则呈现出走滑和拉张均微弱的运动特征；鄂尔多斯周缘拉张型作用主要集中在山西地堑的北部以及银川盆地的东北角，其中山西地堑北部的拉张由大同盆地和岱海盆地共同作用。基于上述运动学特征，同时结合前人的研究成果，我们认为鄂尔多斯块体内部稳定整体旋转是由于其西南角来自青藏高原北缘的挤压俯冲以及海原断裂东段的走滑运动的结果。此外，GPS资料还证实了东太平板块俯冲中国华北地壳减薄的作用。本项目的价值还体现在对环鄂尔多斯块体周缘断陷盆地的地震复发周期的初步认识，以及构造背景的解释。