Evolution of the foreland basins on either side of the Tian Shan and implications for lithospheric architecture

Abstract

The previous understanding of Tian Shan's lithospheric architecture can be summarized into three models: unidirectional underthrusting, bidirectional underthrusting, and mantle upwelling. These models make distinctive predictions regarding the migration and subsidence histories of the foreland basins on either side of the range. Seismic profiles crossing both foreland basins document their migration and subsidence history. We analyze a seismic profile across the Southern Junggar foreland basin (SJFB) to the north of the range, which demonstrates that the basin migrated northward at ∼3.1 mm/yr during 22–16 Ma. Combining this with previously known migration rates of the Northern Tarim foreland basin (NTFB) to the south of the range, we constrain the migration of these two foreland basins relative to their basements. We then employ an elastic plate model to simulate the subsidence profiles across these two basins at ∼20 Ma, 13–16 Ma, ∼5 Ma, and 0 Ma constrained by back stripping well log data and seismic records of the basin onlaps. Our modelling reveals the propagation rates of the southern and northern Tian Shan thrust wedges and the evolution of their topographic loads. Our analysis and modelling demonstrate that the NTFB has the characteristics of a retro-foreland basin, whereas the SJFB resembles a pro-foreland basin during early-middle Miocene times. This suggests that the Tian Shan's growth during this period was dominated by underthrusting of the Junggar lithosphere. Furthermore, our analysis and modelling reveal a convergent pattern between the Tarim lithosphere and the Tian Shan changed from lithospheric indentation to lithospheric subduction after the middle Miocene, as the plate setting of the NTFB and SJFB changed. The Tian Shan had begun to uplift before the middle Miocene, with the orogen's height reaching half of its present elevation before the late Miocene.