Cenozoic tectonic patterns and their controls on growth strata in the northern Tianshan fold and thrust belt, northwest China

Abstract

Field-based mapping and subsurface interpretations of high-resolution seismic images, as well as other geophysical data, have been integrated to investigate tectonic patterns and growth strata in the northern Tianshan fold and thrust belt (NTFTB). The NTFTB formed as a result of intense deformation and uplift of the northern Tianshan region that was triggered by the far field effects of the Indo-Eurasia plate collision. The structural styles of the NTFTB vary, and are primarily classified as thick-skinned thrust nappes that involve basement rocks and thin-skinned fault-related fold structures with multiple décollement layers in the sedimentary strata. The overall NTFTB can be divided into three segments based on its structural geometry. The western segment is characterized by an intense backthrust in the northern Tianshan piedmont, with little displacement transfer into the southern Junggar Basin. In the central segment, the emergence of three rows of anticlines reveals a progressive deformation sequence and displacement transfer into the foreland through multiple décollements. In the eastern segment, the tectonic pattern is characterized by steep faults that branch forward in a narrow zone to generate intense uplift. Evidences indicate that major basement faults, paleo-uplifts, and inheritance of former structures play key roles in driving the segmentation of the NTFTB, and that secondary fault assemblages may control regional tectonic styles within each tectonic unit. Cenozoic growth strata have been observed in the NTFTB, the geometry and deformation mechanism of which are subject to the structural style in a given structure. Growth strata can also provide constraints for time of deformation in the northern Tianshan, and the Dushanzi Formation has been shown to represent clear growth in the piedmont, which indicates rapid uplift of the northern Tianshan as well as intense tectonic activity in the NTFTB in the late Miocene. The growth strata become younger within structures to the north, towards the foreland. In general, the Cenozoic evolutional process has been relatively uniform in the northern Tianshan, generally showing a forward propagation of fault splays into the foreland, and the geometry and distribution of sedimentary strata have been controlled by tectonics.