华北克拉通破坏区最新构造运动起始时间讨论

华北克拉通破坏区是历史破坏性地震频发区,震源机制解和地震地表破裂带等反映出历史地震的发震断层为新生走滑断层,很难用地壳的伸展构造系统来合理解释.首先对1679年三河-平谷M8.0级地震的大厂隐伏凹陷西边界夏垫断裂进行高分辨率地震勘探和上盘钻孔地层进行标定,然后在河套断陷盆地带大青山南麓晚更新世湖相地层中识别出2期角度不整合面(UC1和UC2),并进行了系统测年,综合近年来活动断层比例尺填图和城市活动断层探测成果,明确指出,在华北克拉通破坏区,代表新生代早期地壳伸展运动的铲形正断层的活动性在上新世至第四纪早期逐渐减弱,到晚更新世早期基本停止活动;晚更新世中期以来大青山构造运动为华北克拉通破坏区最新一期构造运动,主要表现为区域剪切应变条件下新生走滑断层形成和扩展,并伴随相关地震活动.最新构造运动的主要动力来源于青藏高原物质东向挤出,以及其对鄂尔多斯块体西南缘强烈东向推挤作用.这些新认识对深化华北克拉通破坏区地震发震机理研究,理解板内最新变形动力学,均具有十分重要的科学价值.

Discussion on Initiation Time of the Latest Tectonic Movement in Break-up Region of the North China Craton

The break-up region of the North China craton is an area where historically destructive earthquakes have frequently occurred. The focal mechanism solutions and earthquake surface rupture zones indicate that these historical earthquakes were dominated by the newly-formed seismogenic strike-slip faults, which are incompatible with the crustal extensional tectonics. This research firstly examines the high-resolution seismic reflection profile and borehole stratigraphy on the hanging wall of the Xiadian fault, which resulted in the 1679 Sanhe-Pinggu M8.0 earthquake ruptures along the western boundary of the Dachang concealed sag. Then, two unconformity contacts (UC1 and UC2) are distinguished by careful checks on structural and stratigraphic features from a cross section of the Late Pleistocene lacustrine at the southern piedmont of the Daqingshan. Combined with the results of active fault mappings with 1:50 000 scale and urban active fault detection in recent years, we conclude that the listric normal faults, which represent crustal extension in the Early Cenozoic, had been weakening in the Pliocene to the Early Quaternary, and had ceased activity by the early Late Pleistocene. The Daqingshan tectonic movement has initiated since the middle of the Late Pleistocene, representing the latest tectonic movement in break-up region of the North China craton, in which new strike-slip faults have formed under a shear-strain crustal condition owing to continuous eastward extrusion of the Qinghai-Tibet Plateau and strong eastward push on the southwestern margin of the Ordos block. These new points of view have important academic value for understanding mechanism of earthquake occurrence and dynamics of the latest crustal deformation in intra-plate.