青东凹陷北西向断层成因分析

青东凹陷是渤海湾盆地济阳坳陷东侧的新生代盆地,其东界为郯庐断裂。该凹陷古近纪期间为断陷盆地,新近纪至第四纪期间为坳陷盆地。其中发育了近东西向、北西向与北北东向断裂体系。前者为新生的正断层,后两者为复活的基底断层。古近纪期间青东凹陷处于南北向拉张的区域应力状态,北西向基底断层复活中处于斜向拉伸,从而呈现为左行平移正断层。这类基底断层在古近纪早阶段(孔店至沙四下期)活动强,控制了凹陷内北西向展布的沉积格局。古近纪晚阶段(沙四上期以后的古近纪)近东西向新生正断层活动占主导地位,主要形成了近东西向展布的沉积格局。北西向基底断层复活中出现了直接活动、新生雁列断层、直接活动并扩展成东西向正断层3类活动方向。北西西向基底断层多呈现为直接活动,而北西向大型基底断层表现为新生雁列断层,反映了基底断层方位对其复活方式的影响。

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.