Spatial Pattern of Late Quaternary Shortening Rate in the Longmen Shan Foreland, Eastern Margin of the Tibetan Plateau

Abstract

Abstract As the eastern boundary of the Tibetan Plateau, Longmen Shan possesses a narrow thrust belt with steep topography but lacks matching Cenozoic foreland basin. Multiple kinetic models have been proposed to debate on the dominant mechanism of developing such range–foreland system. Crustal shortening rate is a feasible approach to test different tectonic models and estimate structural patterns. In this study, we focused on the deformation pattern and shortening rate of the complex foreland area of the southern Longmen Shan, which is comprised of the Xiongpo, Sansuchang, and Longquanshan anticlines. By the means of net-based RTK measurement and Quaternary chronology, we measured and dated the six-level terraces of the Qingyi River, which flows southeastward across this region. Excess area method was applied to calculate shortening rate. The results indicate that the Late Quaternary shortening rates of the Xiongpo anticline, Longquanshan anticline, and Sansuchang anticline are 1.01 mm/yr, 0.89 mm/yr, and 0.16 mm/yr, respectively. The total shortening rate of the foreland in southern Longmen Shan is 2.06 mm/yr. Consequently, a mechanical model was presented to show the tectonic pattern: the southern Longmen Shan is an actively expanding edge of the plateau, and the shortening is distributed to the three anticlines dominated by the foreland detachment system. This model supports that crustal shortening is the dominating force in the current orogenesis of the Longmen Shan. In addition, the along-strike variation of the Longmen Shan was further specified from the perspective of crustal shortening distribution. We propose that the southern Longmen Shan and its foreland basin are in a state of compression, while the northern Longmen Shan has both thrust and strike-slip characteristics.