EVOLUTION OF NORMAL FAULTS WITH MULTIPLE NON-COAXIAL EXTENSION: AN EXAMPLE FROM THE QIKOU SAG, BOHAI BAY BASIN, EAST CHINA

Understanding how normal faults grow and link through time is fundamental in structural geology. 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 kilometers are mostly rooted faults and have experienced numerous reactivations. Using 3D seismic and drilling data, we investigated the geometry, kinematic characteristics, growth and linkage, and three-dimensional evolution of the Nandagang fault system (NDGFS) on the Qinan Slope in the Qikou Sag, Bohai Bay Basin. From the preCenozoic basement to the Quaternary strata, NDGFS comprises three fault systems separated vertically by two unconformities, namely the lower, the middle and the upper fault systems. On planar view, NDGFS contains three structural trending zones with diverse vertical growth styles. The E–W and NEE–SWW trending zones are mainly dominated by growth faults, while the NE–SW zones are characterized by dip-linkage faults. The angle between late stress and the strike of pre-existing fault and shale thickness both controlled regularly vertical growth. Geometrical and kinematic evidence implied NDGFS was consistent with a hybrid fault model incorporating a lengthening stage and a displacement accrual stage. By using the original throw/displacement backstripping approach, ratios of maximum displacement to fault length (D_max/L) of the evolving NDGFS were obtained to delineate faulting stages of the hybrid fault model. For NDGFS, both the middle and deeper fault segments experienced a two-stage hybrid faulting and reached their final lengths in ca. 18%–25% less time than the usual faulting lifetime (ca. 30%) 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.