An extensional collapse model for the Lhasa–Qiangtang orogen in Central Tibet

Abstract

Abstract Large-scale geodynamic processes not only result in magmatism, but also generate corresponding tectonic and sedimentary evidence. Therefore, magmatic, sedimentary, and tectonic evidences can be used to investigate lithospheric geodynamics. This study reports Late Cretaceous magmatic–sedimentary–tectonic evidence from the Tari Co area in central Tibet of post-collisional extensional collapse of the lithosphere. Zircon U Pb dating shows that the Tari Co Late Cretaceous igneous rocks formed during ca. 90–75\xa0Ma, including (1) thickened lower crust-derived adakitic granodioritic porphyry (90.4\xa0±\xa01.0\xa0Ma and 90.7\xa0±\xa01.0\xa0Ma) with high Sr/Y (34.4–53.8) ratios but low Y (5.48–9.14\xa0ppm), Cr (10.2–14.4\xa0ppm), and Ni (4.33–6.63\xa0ppm) contents; (2) delaminated lower crust-derived dioritic porphyry dike (80.5\xa0±\xa01.6\xa0Ma) with high Mg# (48–53) and moderate SiO2 (57.9–63.6\xa0wt%), Cr (40.3–54.1\xa0ppm) and Ni (43.5–56.4\xa0ppm) contents; and (3) mafic dikes (74.8\xa0±\xa01.0\xa0Ma) derived from different magma sources. According to sedimentary facies, gravel composition, and depositional age, the Upper Cretaceous Jingzhushan Formation in study area is divided into the lower subunit and upper subunit. Different from the typical lower subunit of the Jingzhushan Formation with sub-rounded limestone gravels as the main components, the upper subunit of the Jingzhushan Formation comprises slump breccias and coarse conglomerates. The gravel clasts in the upper subunit are mostly adakitic granodioritic porphyry (90.1\xa0±\xa01.4\xa0Ma), indicating a sedimentary response to the crustal uplift associated with lithospheric delamination. In terms of structural deformation, the study area gradually entered the post-collisional setting in the late Late Cretaceous time (ca. 79-75\xa0Ma), forming an east-west trending extensional tectonic system comprising extensional and step faults, grabens and horsts, and mafic dikes. Based on the Late Cretaceous geological records and previous studies, we suggest a three-stage post-collisional model including melting of thickened crust, lithospheric delamination, and gravitationally driven ductile collapse. Our results provide new insights into the evolution of continental orogens.