Analysis of formation mechanism of NW-striking faults in Qingdong Sag

The Qingdong Sag is a Cenozoic basin in the east of the Jiyang Basin,a part of the Bohai Bay Basin,and is bounded by the Tan-Lu fault to the east.It is a faulted basin during the Paleogene and a depression basin during the Neogene to the Quaternary.Nearly EW,NW and NNE-striking fault systems occur in the sag.The former is newly-formed normal fault while the later two are reactivated basement faults.The sag was under NS extension during the Paleogene.The NW-striking faults suffered oblique extension and showed sinistral normal faulting.This kind of basement faults had intense activity during the early Paleogene,i.e.Kongdian to lower Sha-4 period and therefore controlled a NW-trending sedimentary pattern.The new nearly EW-striking normal faults predominated during the late Paleogene since the upper Sha-4 period,leading to the formation of a nearly EW-trending sedimentary pattern.The NW-striking basement faults show three methods of reactivity such as direct activity,formation of new en echelon faults and direct activity plus propagating into EW-striking normal faults.The NWW-striking basement faults are dominated by the direct activity whereas the large NW-striking basement faults exhibit the formation of new en echelon faults,suggesting that the reactivity methods are affected by the orientation of the basement faults.