Discovery of Compressional Structure in Wuerxun–Beier Sag in Hailar Basin of Northeastern China and Its Geological Significance

The Hailar Basin is a Mesozoic-Cenozoic basin, which is superimposed on the Paleozoic collision orogenic belt of Inner Mongolia–Greater Hinggan Mountain. The basin, trending north-east, consists of five first-order structural units (three depressions and two uplifts), they are, from west to east, the Zhalenuoer Depression, the Cuogang Uplift, the Beier Lake Depression, the Bayanshan Uplift and the Huhe Lake Depression, respectively. The Wuerxun Sag and the Beier Sag are two second-order structural units in the south Beier Lake Depression. Research indicates that in the Early Cretaceous Epoch, the Wuerxun-Beier sag underwent two long-term compressions: NW-SE trending during the Damoguaihe Period to the early Yimin Period, and near EW trending during the end of the Yimin Period, respectively. The compressions led to the inversion of some depression-controlling normal faults in the extensional faulted depression, and formed new fault-propagation fold, fault-bend fold, imbricate structure, duplex structure, pop-up structure, and triangular zone etc. Because of the differences in geometric and kinematic features and the variance in the ratio of the uplift rate of compressional structures to the deposition rate of isostructural growing strata in space, syn-compression strata show different depositional features in different structural positions. These structural features are very similar to those of the orogenic belt. The discovery of two phases of compressional structures in the Wuerxun–Beier Sag proved that the crust of Northeast China was not always in the state of extension and thinning in the Early Cretaceous, but showed the features of superimposition and thickening during the two compression periods, and thus formed the structural framework of the alternative distribution of the compression-depression basins and the intra continental (intra plate) orogenic belt in space.