Collapse of Songpan-Garzê Orogenic Belt Resulted from Mesozoic Middle-crustal Ductile Channel Flow: Evidences from Deformation and Metamorphism Within Sinian-Paleozoic Strata in Hinterland of Longmenshan Foreland Thrust Belt

The early Mesozoic orogeny in the Songpan-Garzê area resulted in a thickened crust (∼50–60 km) and was followed by a regional subsequent extensional tectonics and crustal thinning. However, the process and mechanism to form this subsequent tectonics are still unclear. This article describes the ductile deformation and metamorphism of the Sinian-Paleozoic strata in the hinterland, especially around the extensional domes within the hinterland of the Longmenshan overthrust belt, the easternmost of the Songpan-Garzê orogenic belt. The structural styles include ductile shear zone, bedding penetrative foliation and mineral lineation, mylonitic rocks formed at greenschist to low amphibolite facies. This extensional deformation removed or selectively thinned the strata. The kinematic senses indicate SSE- or south-ward motion in the northern section, and SE-ward senses in the middle and southern sections of Longmenshan. The p-T conditions of the Silurian Maoxian group are estimated to be 265–405 °C, and 0.31–0.48 GPa, respectively, and are consistent with and thus define a middle crust ductile channel flow. Previous studies constrained the metamorphic ages of the ductile deformation at 190–150 Ma using 39Ar/40Ar and SHRIMP U-Pb methods, and thus proved to be subsequent tectonic episodes of the early Mesozoic orogeny. It was thus concluded that the thickened crust was largely thinned by a middle crust ductile channel flow, which probably has major contribution for the collapse of the Mesozoic Songpan-Garzê orogenic belt.