Structural inversion, imbricate wedging, and out-of-sequence thrusting in the southern Junggar fold-and-thrust belt, northern Tian Shan, China

Accurate definition of structural style in subsurface interpretation is critically important for understanding the deformation history of fold-and-thrust belts, as well as assessing the petroleum prospectivity of structural traps. Using two- and three-dimensional seismic reflection surveys, well data, field mapping, forward models, and balanced cross sections, we describe the structural styles across the actively deforming southern Junggar fold-and thrust belt in northwestern China, a basin undergoing petroleum exploration and development operations. Subsurface interpretations indicate several folds in the basin overlie Jurassic normal faults that were tectonically inverted in the Late Jurassic to Early Cretaceous. Following inversion, multiple detachment levels propagated northward from the Tian Shan and formed a series of imbricated fault-related folds. The most prominent fold trend in southern Junggar consists of the Tugulu, Manas, and Huoerguosi anticlines, which trap hydrocarbons in elastic Eocene reservoirs. These structures exhibit complex internal geometries, with coeval forethrusts and backthrusts forming imbricated structural wedges. In the latest stages of deformation, and continuing at present, the uppermost thrust sheet, the Southern Junggar Thrust (SJT), truncated the backlimbs of these structural traps, implying the SJT is a tectonically active, out-of-sequence thrust. From these interpretations, we present a model for how the southern Junggar fold-and thrust belt developed from Jurassic to present. Moreover, we detail how fold growth, fault activity, and structural style affected charge histories, trap formation, and reservoir compartmentalization. Our results have direct implications for assessment of the southern Junggar petroleum system as well as other complex fold-and-thrust belts.