Effects of fault slip distribution on the geometry and kinematics of the southern Junggar fold-and-thrust belt, northern Tian Shan

Abstract

At the foreland edge of fold-and-thrust belts (FTBs), tectonic shortening could either be accommodated by thick-skinned faulting and/or structural wedges developed near range-fronts, or be transferred along detachment layers further into basins, forming thin-skinned structures. Although the geometry and kinematics of each structure have been well-investigated, studies that characterize slip distribution along a specific fault as well as the effects of variations in magnitude of fault displacement on the development of fault-related folds are still lacking. In this study, we present a case study of the southern Junggar Thrust (SJT), a seismically-active fault developed in the southern Junggar FTB, northern Tian Shan with an overall vergence to the north. The results of structural interpretations of seismic reflection profiles combined with the age constraints provided by previous magnetostratigraphic studies demonstrate that: 1) The kinematics of the SJT vary along strike from purely structural wedging to classical fault-bend folding. Besides these end-member models, intermediate cases exist between them, in which fault slip is partitioned by two-stage structural deformation, i.e., fault-bend folding subsequent to structural wedging at the range front. As a result, part of the displacement is transferred northward, forming the Tugulu anticline. 2) Along-strike variations in displacement along the Tugulu fault (frontal fault of the SJT) also affect the geometry and kinematics of the Tugulu anticline, varying from detachment folding, trishear fault-propagation folding with a pure-shear fault-bend folding back-limb, and classical trishear fault-propagation folding to fault-bend folding with increasing displacement. 3) The SJT has an average Quaternary slip rate of 3.9 ± 0.4 mm/yr, serving as a principal structure accommodating tectonic shortening in the southern Junggar FTB. The results of this study are helpful for seismic hazard assessments of the study area, which may also shed light on structural evolution of fault-related folds in other FTBs.