Collapse of the Songpan-Ganze Orogenic belt and Mesozoic mid-crustal ductile channel flow: Evidence from the Longmenshan foreland thrust belt

Abstract

The early Mesozoic orogeny in the Songpan-Ganze region, SW China produced crustal thickening (~50–60 km), followed by regional extension and crustal thinning. The regional extension involved ductile deformation and metamorphism of the Sinian-Paleozoic strata around extensional domes within the Longmenshan area. This extensional deformation removed or selectively thinned the strata. The deformed rocks are marked by ductile shear zones, bedding penetrative foliation and mineral lineation. The kinematic markers indicate SSE- or south-ward motion in the northern part of Longmenshan and SE-ward motion in the middle and southern sections. The mylonitic rocks have greenschist to low amphibolite facies mineral assemblages and P-T conditions in the Silurian Maoxian group are estimated to have been 265-405°C and 0.31-0.48 GPa, consistent with a mid-crustal ductile channel flow. Previous studies placed the age of metamorphism at 190–150 Ma using 39Ar/40Ar and SHRIMP U-Pb methods. We conclude that the thickened crust was thinned by mid-crustal ductile channel flow, which then led to collapse of the Mesozoic Songpan-Ganze orogenic belt.