Growth and linkage of normal faults experiencing multiple non-coaxial extension: A case from the Qikou Sag, Bohai Bay Basin, East China

Oblique-slip faults, which are essentially reactivated pre-existing fault caused by non-coaxial stress, are common in rift basins. Large faults with lengths exceeding thousands of kilometres are mostly rooted faults and would have experienced numerous reactivations. Using 3D seismic and drilling data, we investigated the geometry, kinematic features, growth and linkage, and three-dimensional evolution of the Nandagang fault system (NDGFS) on the Qinan Slope in the Qikou Sag, Bohai Bay Basin. A hybrid fault model incorporating a lengthening stage and a displacement accrual stage was used to analyse the growth and linkage of NDGFS. By using the throw/displacement backstripping approach, ratios of maximum displacement to fault length (D-max/L) of the evolving NDGFS were also obtained to delineate faulting stages of the hybrid fault model. From the pre-Cenozoic basement to the Quaternary strata, NDGFS comprises three fault systems separated vertically by two unconformities, namely the lower, the middle and the upper systems. On planar view, NDGFS encompasses three structural trending zones with variable vertical growth styles. The E-W and NEE-SWW trending zones are mainly dominated by growth faults, while the NE-SW zones are characterised by dip-linkage faults. For NDGFS, both the middle and deeper fault segments experienced a two-stage hybrid faulting and reached their final lengths in ca. 20%-30% less time than the usual faulting lifetime proposed by previous researchers. In contrast, the upper faults, especially the segments within the Neogene Guantao (Ng) and Minghuazhen (Nm) formations, only experienced a lengthening stage. The slip rates and the pre-existing faults have strong influences on the duration that faults accomplished their lengthening stages. A D-max/L ratio of 0.015 obtained from the evolving NDGFS is determined to be effective in delineating between the lengthening stage and the displacement accrual stage, which may have a broad application for normal faults in similar geological settings.