Characteristics and mechanism of Late Mesozoic extensional faults in West Shandong Uplift

Abstract：Field survey performed in West Shandong Uplift indicates that extensional faults are well developed and characterized by complicated structural styles in West Shandong Uplift. The main structure style is the compound extensional fault system composed of high-angle faults and low-angle décollement faults. The high-angle faults, which dominate the sedimentation in the southern depression, constitute the main part of the extensional fault system in West Shandong Uplift. In the fault zone, there are dynamic breccia, fault clay, small folds and scraping trace making up signs of faulting activity. Regional décollement faults composed of shallow level faults and deep level faults were found in West Shandong Uplift. The most outstanding décollement faults of the shallow level are between Lower Cambrian and Archean and between Ordovician and Carboniferous. There are different structural deformational styles developed near the décollement surface: above it (in the lowermost part of the Cambrian) there occurred intense structure deformation resulting in folding and shattering, whereas the structures in the underlying Archean are relatively simple, only with extrusion and rupture. The décollement of the deep level controlling the formation and distribution of the shallow level décollement is mainly developed in the low-velocity layer of the crust. The result of the tectonic stress field numerical simulation shows that the extensional fault system has been formed by two stages of large scale extensional movement since Late Mesozoic. Mechanically, the extensional fault system is closely related to the Late Mesozoic-Cenozoic strike-slip activity of Tan-Lu fault zone, the subduction of the Pacific Plate and the magma upwelling.