Geometry and kinematics of Mesozoic and Cenozoic structures in northern Tianshan foreland basin: insights from quantitative structural analysis of 2-D and 3-D seismic reflections and balanced restoration

In central Asia, the Tianshan mountains have undergone a series of subduction-collision-accretion processes during Paleozoic times that resulted in forming basement structures later reactivated during the Cenozoic rejuvenation of the range. In the northern Tianshan-South Junggar foreland basin, en échelon W-E / WNW-ESE folds constitute the large-scale fold-and-thrust belt (FTB). Using recent industrial 2-D and 3-D seismic surveys carried out in the western area of the FTB, we have analyzed outcropping structures (Dushanzi and Xihu anticlines) and buried structures (Gaoquan, Kadong, Kayindike, and Dunan anticlines). Observing that the strike of these structures changes from W-E in the east to NW-SE in the west, we investigated the parameters that controlled this lateral variation along the FTB. We particularly analyzed the structural and kinematic relationships between deep Mesozoic and shallow Neogene-Quaternary structures. We support our investigation by using seismic interpretation and balanced restoration.

Our results provide new insights into the structural and kinematical history of the Mesozoic-Cenozoic tectonic evolution of the northern Tianshan foreland basin, and into the record of deformation propagation. We first demonstrated that the tectonic and sedimentary evolution during the Triassic and Jurassic is characterized by NW-SE and NNW-SSE strike-slip faults that controlled the development of pull-apart and restraining bend systems. These features were partially reactivated during the Neogene-Quaternary contractional deformation, depending on their position relative to the mountain front. Second, we quantified that about 7 km of total S-N shortening has been accommodated across the western area of the FTB. We also quantified the displacement rate accommodated on every single structure. It is ~0.13-0.2 mm/yr in the Gaoquan anticline since Quaternary, ~0.19-0.30 mm/yr in the Dunan anticline since Quaternary, ~0.36 mm/yr in Dushanzi anticline since Pliocene and ~0.30-1.2 mm/yr in the Xihu, Kadong and Kayindike anticlines during Quaternary times. Additionally, the rate of thrusting above the reactivated strike-slip faults in the Gaoquan anticline is 3 to 6 times lower than that of thrusting above the inactive strike-slip faults of the Kadong and Kayindike anticlines. This suggests that the reactivation of the basement strike-slip fault zone partitioned the contractional strain during Quaternary times. Finally, we integrate our results in a 3-D model of the western area of the FTB that illustrates the spatial pattern of structures at depth. Notably, it emphasizes how deep Mesozoic structures exerted a primary control on the growth of Cenozoic thrust-related folds by localizing the nucleation of thrust ramps during compression. This pattern of deep vs. shallow deformation interaction might be applied in some other areas of the Tianshan foreland basin.