Late Triassic to Middle Jurassic tectonic evolution of the South China Block: Geodynamic transition from the Paleo-Tethys to the Paleo-Pacific regimes

Abstract

Multi-plate convergence around a nuclear block and the geodynamic transition occurred generally after the onset of plate tectonics. Late Triassic to Middle Jurassic geodynamics of the South China Block has been under hot debate. Late Triassic to Middle Jurassic basins and provenance, magmatism and geochemistry, structural deformation and geochronology were reevaluated and a geodynamic model was proposed for the South China Block. Late Triassic to Early Jurassic peripheral foreland basins and retro-arc foreland basins occurred along the western and northern margins and the southeastern margin, respectively. Sandstone components, paleocurrent directions and detrital zircon UPb ages indicate that clastic rocks in these foreland basins were derived mainly from adjacent Indosinian orogens around the South China Block. Late Triassic to Middle Jurassic magmatism in the South China Block mainly includes weakly to strongly peraluminous granites. Both NW-SE and NE-SW granitic sections show that Late Triassic granites have high whole rock (87Sr/86Sr)i ratios, accumulational whole rock ε Nd (t) values and the same sources of global sediments whereas Early and Middle Jurassic granites have low whole rock (87Sr/86Sr)i ratios, dispersive whole rock ε Nd (t) values and different sources of global sediments and ocean island basalts. But all granites have similar zircon ε Hf (t) values and TDM2 ages. Late Triassic and Middle Jurassic granites occurred in a syn-collisional environment whereas Early granites along the southeastern margin in a volcanic arc setting or Early granites along the E-W Nanling granitic belt in a within-plate setting. NE-SW and NW-SE structural sections present NW-striking folds and thrust faults and NNE-striking folds, thrust faults and ductile strike-slip shearing, respectively. The NNE-striking structures show younger from SE to NW direction whereas NW-striking folds and thrusts along the southern and northern margins occurred synchronously. The NW-striking folds and thrust faults were superimposed by the NNE-striking folds and thrust faults. Lines of evidence indicate that the Paleo-Tethys and Paleo-Pacific tectonic regimes co-existed in the South China Block during Late Triassic to Early Jurassic and the geodynamic transition from the Paleo-Tethys to Paleo-Pacific tectonic regimes was completed during Middle Jurassic.