The Late Triassic Longmenshan lateral foreland thrusting: New insights from geological evidence and 3-D particle discrete-element simulation

This work proposes a new lateral foreland thrusting model based on geological evidence and 3-D particle discrete-element simulation to explain the Longmenshan southeastward thrusting during the closure of the Songpan-Ganze basin. The Late Triassic NE-SW compression caused by the northward movement of the Qiangtang Block and the resulting differential shortening within the wedge-shaped Songpan-Ganze terrane produced south-eastward topographic gradient. The thick sedimentary pile, driven by the horizontal tectonic force and the deviatoric stress generated from gravitational effect, and decoupled from the subducting basement by the low-strength decollement, was laterally extruded and resulted in the southeastward Long-menshan thrusting. Therefore, the Longmenshan thrust belt is a lateral foreland thrust belt of the Songpan-Ganze terrane. For the first time, 3-D particle discrete-element simulation was used for the geological study of the Longmenshan area, and it clearly reproduces the dynamical process of the Long-menshan southeastward thrusting and well predicts the Xiaojin Arcuate Zone. The particle discrete-element simulation results verify the new model and reveal that two key factors facilitate the lateral foreland thrusting: the wedge-shaped geometry that produces differential shortening and lateral topographic gradient, and the low-strength decollement that decouples the extruded sedimentary pile from the basement. The lateral foreland thrust belt, which is unique in its tectonic location and dynamic behavior, is a new kind of foreland thrust belt that is different from the pro-foreland and retro-foreland thrust belts, and it provides new insight into the tectonic evolution of collisional orogens.