This work proposes a new lateral extrusion dynamic model based on
geological evidence, mathematical modeling and 3D discrete element
simulation to explain the 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 southeastward topographic gradient. The thick sedimentary pile,
decoupled from the subducting basement by the low-strength decollement,
was driven by gravity and laterally extruded, leading to the
southeastward Longmenshan thrusting. Therefore, the Longmenshan thrust
belt is a lateral foreland thrust belt of the Songpan-Ganze terrane. The
mathematical modeling provides detailed description of the differential
shortening occurred within the Songpan-Ganze terrane. The 3D discrete
element simulation, which is first time used for the geological study of
the Longmenshan area, clearly reproduces the dynamical process of the
Longmenshan southeastward thrusting and well predicts the Xiaojin
Arcuate Zone. Both mathematical modeling and discrete element simulation
verify the new lateral extrusion dynamics and reveal that two key
factors facilitate the lateral extrusion and foreland thrusting, which
are 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 has specializations in its tectonic location
and dynamic behavior, is a new kind of foreland thrust belt after
pro-foreland thrust belt and retro-foreland thrust belt, and provides a
new insight into the tectonic evolution of collisional orogens.