Structural features and determination of deformation stages of Hongqi sag in Hailar basin during the early cretaceous

Hailar basin is a meso-cenozoic basin which superimposed on the Inner Mongolia-Greater Hinggan Mountain Paleozoic collision orogenic belt. The north-east trending basin consists of five first-order structural units (three depressions and two uplifts) including Zhalenuoer depression, Cuogang uplift, Beier Lake depression, Bayanshan uplift and Huhe Lake depression from west to east, respectively, The Hongqi sag is a second-order structural unit in the north of Beier Lake depression. Research indicates that the Hongqi sag underwent four deformation stages during the Early Cretaceous: Stage of NW-SE extension during the deposition of Xing'anling Group and the Nantun Formation, NE trending half-garben controlled by large scale dustpan-like normal fault as depression-controlling fault developed; Stage of NW-SE compression during the deposition of Damoguaihe Formation and the first member of Yimin Formation, thrust fault formed due to the intensive inversion of early normal fault, the thrust of the hanging wall lead to the uplift of the NW boundary of the sag near the depression-controlling fault, and the block of thrust sliding of the deep part of the depression-controlling fault in the shallow area lead to the rotary uplift of the SE slope zone during the inversion, hence, the size of the sag dwindled obviously, and the large scale NE trending fault-bend fold formed in the center of the sag which consists of the central fault-uplift zone of Hongqi sag; Stage of EW trending extension during the deposition of lower part of the second member of Yimin Formation, the early depression-controlling fault showed features of normal fault, and the newly formed depression-controlling fault had features of sinistral discrete slip; Stage of EW compression during the deposition of upper part of the second member of Yimin Formation, the early depression-controlling fault showed features of thrust of hanging wall again, inverted anticline developed in the hanging wall near the fault, forming fault-bend fold in the sag. During each deformation stage, there were some differences of the structural features of Hongqi sag in space.