Abstract:

Hailar basin is a Meso-Cenozoic basin which superimposed on the Inner Mongolia-Greater Hinggan Mountain Paleozoic collision orogenic belt. The basin, trending north-east, is consist of five first-order structural units (three depressions and two uplifts), from west to east are Zhalenuoer depression, Cuogang uplift, Beier Lake depression, Bayanshan uplift and Huhe Lake depression. Beier sag is a second-order structural unit in the south of Beier Lake depression. The control subsidence fault of Beier sag is a listric  normal fault which consists of a convex and two concave fault-bend, and formed extensional duplex in depth. The shear rotation of active axial surface caused by convex fault-bend is the same as the fault plane, the shear rotation of active axial surface caused by concave fault-bend is contrary to the fault plane, and two anticline sandwiching a syncline formed in hanging wall of listric  normal fault. The horizontal extension of the control subsidence fault F₁ is 6 850 m during Late Nantun Period in the Early Cretaceous，determined by the rollover width between active axial surface and inactive axial surface. The geometry and kinematics of extensional fault-bend fold in Beier sag reflect the impact of the fault slip rate, the tectonic subsidence rate, deposition rate on deformation characteristics of sedimentary strata and constrains on the sedimentation and petroleum geological conditions in the depression. The internal angular unconformity of the Upper Member of Nantun Formation in Beier sag developed without erosion or hiatus on deformation due to increases in deposition rate relative to subsidence rate, where half-graben compartments change from sediment-underfilled to overfilled conditions.

Key words: extensional structure, fault-bend folding, growth strata, sedimentation, angular unconformity, Beier sag, Hailar basin