旋卷构造应力场与鄂尔多斯地块周缘断裂系成因探讨

当外力的合力不通过活动地块的质量中心时，该地块将产生旋扭运动，运动的结果在其周边形成旋卷构造应力场．在该应力场作用下，与主应力作用面方向一致的断裂将由作用面的力学性质决定其活动方式．与主张应力作用面一致的断裂将产生张破裂，长期作用下将形成断陷盆地；与主压应力作用面一致的断裂将产生逆冲或逆掩活动，或形成褶皱．地质资料表明，长期稳定的华北地块在早第三纪初期解体，其中晋冀陕地块发生过逆时针转动，形成其周缘断陷系；上新世时晋冀陕地块解体，地块东边界西移，出现山西地堑系，形成鄂尔多斯地块．晋冀陕地块周缘的一些主要断裂的构造形迹及力学性质可用旋转构造应力场的主应力作用面解释． 

DISCUSSION ON ROTATIONAL TECTONIC STRESS FIELD AND THE GENESIS OF CIRCUM-ORDOS LANDMASS FAULT SYSTEM

When the resultant of applied forces does not pass through the center of an active landmass,the landmass will rotate,giving rise to a rotational tectonic stress field. The motion of a fault along the principal stress plane is determined by the mechanic features of the plane. Tensile fractures occur on the faults in the direction of the principal extensional stress plane,and fault-depression basins will be formed under a long-term action. Thrusting and overthrusting occur on faults in the direction of the principal compressional stress plane,or folds may be formed as a result. Information on geology shows that the North China landmass,which remained stable and intact for a long time,became disjointed in the Eogene period. In the course of disjunction,anticlockwise rotation took place in the Shanxi-Hebei-Shaanxi (Jin-Ji-Shan) landmass,giving rise to the fault-depression system in its periphery. In the Pliocene epoch the landmass lost stability and its eastern boundary moved westward. As a result,the Shanxi graben system appeared and Ordos landmass was formed. Structural and mechanic features of the main faults around Jin-Ji-Shan landmass can be explained with principal stress plane of a rotational tectonic stress field.