Influences of the Pre-Existing Faults on Later Faults: Insights from Planar Sandbox Experiments

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Abstract The Nanpu Sag, located in the northern Huanghua Depresssion of the Bohai Bay basin, is a Meso-Cenozoic petroliferous extensional sag. Based on the interpretation of 3D seismic data and the results of balanced cross-section restoration, the structural evolution of the fault arrays during different Cenozoic geological periods was studied systematically and quantitatively, and the results suggest that the distribution of the fault arrays seems complex but rather regular, it was concluded that the fault arrays undergone two important evolutionary periods during the Cenozoic, the pre-existing faults before the second and third members of Shahejie formation(Es2+3)impacted profoundly on the forming and developing of later faults. Two series of comparative sandbox models were conducted depending on the presence of the pre-existing faults or not and reproduced the evolutionary process of the fault arrays in the Nanpu Sag. The study discovered that the faults in the Nanpu Sag have classical syndepositional characteristics during different geological periods in the Cenozoic, Sedimentary period of Es2+3 implied the key tectonic change; the pre-existing faults formed before Es2+3 reacted in later extension as weak structures and impacted on the formation and development of the later faults, especially the variations of fault strike, the strikes and the sizes of the pre-existing faults controlled the distributions of the later faults. So far, the evolution of the fault arrays of sedimentary basin has not been reported, the study gave important evidence and planar sandbox models provided reliable proofs for the research on the faults. The fault is basic and key element in basin especially fault-block or rift basin research. The formation, evolution and relations of faults are the research core and also the key problem on oil and gas exploration in basin. Generally, the faults distribution and arrays are very complex during long-term geological evolution, geologists tried to explain them by Anderson's model---the classical theory of structural geology, but confined to the strict conditions--- brittle, infinite and homogeneous deformation, Anderson's model is not applied to any geological conditions. The evolution of extensional basin has obvious sequences, with the development of faults, pre-existing faults surely exerted the profound effect on the later faults even in a single extension direction. Structural physical modeling is an effective method for fault research. Sandbox modeling, as an effective method for study of basin deformation mechanisms, has been giving many insights into the mechanisms of rift or pull-apart basins. It has been demonstrated by many experiments that the boundary geometries and extension directions are the two most key factors controlling the basin structural styles (McClay and White, 1995; Mart and Dauteuil, 2000; Dooley and McClay, 1997). If a rift basin has zigzag boundaries, its extension direction is not always perpendicular to its boundaries, so faults developed within the basin will have different orientations from the basin boundaries (Zhou and Qi, 1999; Tong, 2003; Waldron, 2005). Based on the results of planar Sandbox modeling for the middle-south part of the Huanghua Depression and the similarities among the depressions and between the depressions and the whole Bohai Basin (Nabelek, et al., 1987; Xu, et al., 1996), Zhou and Zhou.(2006) presented that the formation of the Huanghua Depression and the Bohai basin share the same dynamic genesis: S-N extension in Cenozoic period, and the "slab window" effect formed from the subducted nearly E-W trending spreading ridge between the Pacific and Kula Plates provided the driving force.