New evidences for the presence of the NS-trending extensional structures in northwestern China: An example from the Early Cretaceous half-graben fault depressions in Jiuquan, Gansu

The Early Cretaceous half-graben fault depressions in Jiuquan are aligned along the N-S or N-E directions, and classified into three types:Domino, opposite dip and trailing dip types. The filling material is obviously constrained by the NS-or NE-trending normal faults. The volcanic rocks are composed, without exception, of alkaline basalts that may be of mantle-melted origin. The half graben fault depressions in Jiuquan are interpreted to be built up of nearly NS-trending extensional structures triggered by the intracontinental mantle thermal upwelling during the Early Cretaceous. Till the Cenozoic, the compressional processes of the Indian plate and the northward thrusting of the North Qilian Mountains may give rise to the transition of the southern parts of these Early Cretaceous half-graben fault depressions into mountain ranges. The present-day southern boundaries of the half-graben fault depressions have been overlain by the North Qilian Mountains. Furthermore, the compression of the Indian plate caused the NS-or NE-trending normal faults to be overturned to form the thrusts.