CHARACTERISTICS OF MESOZOIC COMPRESSIVE-TORSIONAL STRIKE-SLIP FAULTS IN THE ZHANHUA-CHENGDAO AREA OF BOHAI BAY BASIN

The study area is located in the Bohai Bay Fault Basin, and unlike other fault basins consisting of normal faults, the study area is also characterised by reverse faults formed by extrusion processes. In the course of the study, it was found that the study area was subjected to shear stresses at the same time, forming three compressional-torsional strike-slip faults.

Using the tectonic map, 3D seismic body profile, geotectonic background of northern China and the equilibrium profile back stripping method, in order to analyse the geometry, kinematics and genesis of three compressional-torsional strike-slip faults: the Chengbei 30 fault, the Chengdong fault and the Zhuanggu 29 fault.

The result：1、On the flat surface, The Chengbei 30 fault and the Chengdong fault are in the NE-SW direction, and the Zhuanggu 29 fault is in the NNW-SSE direction, which is close to conecting the first two faults. On the profile, three faults cut through to the Archean strata. The tendency of the Chengbei 30 fault is NW and the fault surface is steep. Palaeozoic strata of the faulted upper plate is thinner than the lower plate. In the present section, the fault is influenced by the Cenozoic tensional stress field and exhibits positive fault characteristics, but the fault drop decreases from NE to SW. The tendency of the Zhuanggu 29 fault and the Chengdong fault is SE and the fault surface is shovel-shaped with a steep top and a gental bottom. Along the strike from N to S, the fault changes from a reverse fault to a positive fault, creating a dolphin effect.

2、According to the results of equilibrium profile back stripping, three faults were active from the end of the Indosinian movement(T₃) .They were influenced by the left-slip movement of the Tan-lu fault zone and its derived extrusion component. And the Zhuanggu 29 fault was also influenced by NE-SW compressional pressure from subduction of the North China Plate. At the end of the Yanshan movement (K₂), the Tanlu fault zone changed from left-slip to right-slip. For Zhuanggu 29 fault, the Palaeozoic and Mesozoic strata of the faulted upper plate are substantially uplifted, forming folds.