Paleomagnetic data bearing on the Mesozoic deformation of the Qiangtang Block: Implications for the evolution of the Paleo- and Meso-Tethys

Abstract

Paleomagnetism has played an important role in quantifying the Mesozoic evolution of “Proto-Tibet”. In this paper, we present new paleomagnetic data from five Middle-Upper Jurassic sedimentary sequences (Quemo Co, Buqu, Xiali, Suowa and Xueshan Fms.) of the eastern North Qiangtang Terrane (QT) at Yanshiping (33.6°N, 92.1°E). The new paleomagnetic results form a large dataset (99 sites, 1702 samples) and reveal a paleopole at 79.1°N/306.9°E (dp=3.9°, dm=6.3°) for the Quemo Co Fm., at 68.9°N/313.8°E (dp=2.1°, dm=3.7°) for the Buqu Fm., at 66.1°N/332.1°E (dp=2.7°, dm=4.6°) for the Xiali Fm., at 72.4°N/318.6°E (dp=3.9°, dm=6.7°) for the Suowa Fm., and at 76.9°N/301.1°E (dp=7.9°, dm=13.2°) for the Xueshan Fm. These results indicate clockwise (CW) rotations of ~19.8±9.4° between ~171.2 and 161.7Ma and counterclockwise (CCW) rotations of ~15.4±13.4° between ~161.7 and <157.2Ma for Yanshiping. We attribute the change in rotation sense at approximately ~161.7Ma to the initial collision of the Lhasa and Qiangtang terranes. Using this and other paleomagnetic data from the Lhasa, Qiangtang and Tarim terranes, as well as other geological evidence (e.g., tectonism-related sedimentary sequences, volcanism, and HP metamorphism), we propose a new conceptual evolution model for the Mesozoic QT and Tethyan Oceans. The Longmo Co-Shuanghu oceanic slab was subducted before 248Ma, followed by continental collision of the North-South Qiangtang subterranes between ~245 and 237Ma. The Qiangtang Terrane experienced post-collisional exhumation between ~237 and 230Ma during subduction of the Jinsha oceanic slab. The collision of the Qiangtang and Songpan-Ganzi terranes occurred between ~230 and 225Ma. The QT experienced post-collisional relaxation from ~225 to ~200Ma, followed by subsidence and extension-related exhumation between ~200 and 162Ma in association with subduction of the Bangong-Nujiang oceanic slab. Finally, these events were followed by the scissor-like diachronous collisions of the Lhasa and Qiangtang terranes between ~162Ma and the mid-Cretaceous.