华北地块中部活动构造特征及汾渭地堑成因探讨

Active tectonics in the Central North China Block and the cause of the formation of the Fenwei Graben

研究区位处华北克拉通中部,受太平洋构造域和特提斯构造域的影响,新生代开始裂解,先后形成了渤海湾盆地和汾渭地堑。第四纪以来,华北地块中部活动日趋活跃,不同走向的断裂和盆地也表现出不同的活动性。NW向和近EW向断裂主要表现为左旋走滑性质,而NE向断裂则主要表现为正断性质。NNE向断裂虽也表现出正断性质,但右旋走滑性质正逐渐占主导地位。汾渭地堑中,NE向盆地与NNE向的盆地也因此表现出一定的活动差异。盆地形成初期,两种走向的盆地沉降速率差别不大,但第四纪以来,NE向盆地的沉降速率已大大超过NNE盆地。对于第四纪以来这种构造运动的表现,同时结合GPS观测数据及青藏高原的大规模隆升与华北地块中部山地相对隆升和盆地沉降事件在时间上的高度一致性,推测华北地块中部中新世以来的构造活动主要受印度与欧亚板块碰撞的挤出作用影响,在NWW和NE向剪切应力的作用下,华北地块中部自南北两端受NWW SEE向拉张的合力作用下开始裂解。随着青藏高原隆升速度的加快,两种剪切应力也随之增强,其合力的影响范围逐渐向中心向东不断扩展,形成了一系列的NNE和NEE向盆地。同时,NE向剪切应力增幅明显强于NWW向剪切,导致其合力由NW向拉张逐渐转变为NNW向拉张,引起不同走向断层活动性的差异与不同走向盆地沉降速率的相应改变。

The study area is located in the center of the North China Craton which was affected by two global domains： the Paleo-Pacific and the Tethys tectonic domains.In the Cenozoic,the North China Craton was under destruction,and the Bohai Bay Basin and the Fenwei Graben developed.Until now,the activity of the North China Craton is still increasing and different trending faults and basins show different activities.The NW-trending and EW-trending strike-slipping faults are sinistral.The NE-trending faults are predominantly normal.Though the NNE-trending faults display normal,the slip components are gradually obvious.Therefore the NE-trending basins show activity different from that of the NNE-trending basins in the Fenwei Graben.In the Quaternary,the sedimentation rates of the NNE-trending grabens are larger than those of the NE-trending grabens,though there was no difference of the sedimentation rates when the basins were formed.The Qinghai-Tibet Plateau and the uplifting mountains in the North China Block happened in the same period.This paper proposes that the active tectonics in the center of the North China Block since the Miocene was mainly impacted by a far-field extrusion resulting from the collision of the Indian Plate to the Eurasian Plate.The extrusion caused the NE-trending dextral shearing and the NWW-trending sinistral shearing that played a role in the activities of the center of the North China Block.The join effect of those shearing resulted in the destruction in the north and south parts of the North China Block in the Miocene.The acceleration of the Qinghai-Tibet Plateau uplifting caused two shearing events having different amplitudes,the resultant force also switched from the NW-trending extension to the NNW-trending extension,and transferred its impact to the east and north parts of the study area,resulting in the development of grabens,the variation of activity of faults with different strikes,and the migration of the subsidence centers and depocenter since the Pliocene.