Growth and transtensional reactivation of a basin-bounding fault: Chengbei Sag, Bohai Bay Basin, East China

Abstract

Models of extensional fault systems are predominantly based on examples where extension is approximately orthogonal to rift boundaries and deformation can be regarded as plane strain. In contrast, extensional faults in transtensional or strike-slip settings present complex geometric and kinematic features due to rotational deformation, and distinct rift and post-rift phases may be difficult to separate. Well-constrained 3D seismic data from the Bohai Bay Basin, East China reveal the growth history and transtensional reactivation of a basin-bounding fault (Chengbei Fault). During the syn-rift phase, the Chengbei Fault reached its near-final length within ∼24 Ma via the propagation and linkage of six initial fault segments, accompanied by the accumulation of 26% of its total displacement; in the following ∼18.7 Ma of the Paleogene, it accumulated the remaining 74% of displacement without an obvious increase in fault length. Following a phase of relative quiescence in the early Neogene, the Chengbei Fault was reactivated at ∼12 Ma, associated with the enhanced activity of a continental-scale strike-slip fault. At least 17 fault segments developed quickly and linked vertically to the lower fault system of the Chengbei Fault. The plan-view en-echelon and horse-tail configuration, and the flower structure seen in section views of the fault system, indicate that the post-rift activity of Chengbei Fault has had both dip-slip and strike-slip components. A calculation of the rotational deformation using fault heaves and orientations indicates that the tectonic reactivation occurred in a strike-slip-dominated transtensional setting accompanied by an increasing extension component with time. In basins affected by strike slip, kinematic analyses that consider rotational deformation can provide more detailed and convincing information on the tectonic setting.