Long-lagged response of accelerated erosion to surface uplift on the northern margin of the Tibetan Plateau

Abstract:

The northern margin of the Tibetan Plateau is defined by the ~1600-km long Altyn Tagh fault system (ATFS) that has accommodated ~350 km horizontal offset and generated 2-4-km topographic relief between the plateau margin and the Tarim Basin. Quantifying the timing and magnitude of the uplift associated with the ATFS is crucial to the evolutional models describing the northward plateau growth. Seismic reflection data reveal that, due to the fault-perpendicular thrusting of the ATFS, a fold-and-thrust belt has developed in the SE Tarim Basin during the Cenozoic and involves the North Altyn Fault, the Cherchenhe Fault and the wide region in between, although few folds or faults are observed at surface. Most deformation within this belt has been localized on the North Altyn Fault that lifted the Altyn Shan to be ~2 km higher than the SE Tarim Basin.We herein used the low-relief, high-elevation surfaces (LRHESs) that has been well preserved in the Altyn Shan, and the combined bedrock and detrital apatite (U-Th)/He (AHe) analyses to investigate the uplift and erosion histories of the Altyn Shan. These LRHESs were interpreted as elevated paleo-surfaces that served as a reference to quantify the amount of surface uplift and erosion. Bedrock AHe analysis of samples collected from the mountain flank revealed rapid cooling (~100 m/Ma) since ~39 Ma, likely resulting from isothermal perturbations related to activity of the North Altyn Fault. Detrital AHe analysis and associated numerical simulation, however, suggested that the accelerated erosion (~120 m/Ma) occurred as late as ~17 Ma in the interior of the Altyn Shan merely ~10 km away from the North Altyn Fault. Taken together, they implied a long-lagged (~22 million years) response of accelerated erosion to tectonic-driven surface uplift possibly due to the extremely arid climate that led to inefficient erosion there. The reconstructed uplift and erosion histories based on the above results suggested that the Altyn Shan has almost reached the present elevation by Early Miocene. Our findings question the feasibility of using the fast exhumation event to directly represent the tectonic-driven surface uplift. Geological, geomorphological and even paleoclimatic constraints should be incorporated to obtain more reliable interpretations.