Lithosphere thermal-rheological structure and geodynamic evolution model of the Nansha trough basin, South China Sea

The Nansha trough is a critical foreland oil-gas basin located in the deepwater area to the south of the South China Sea. The mechanism of continental crust thinning and deformation of the deepwater fold-thrust tectonics in the Nansha trough has become an important research subject in geothermics and geodynamics studies. Based on seismic, gravity, magnetic and heat flow data, this paper analyzes the sedimentary strata structure, Moho depth, mantle-flow stress field and thermal-rheological structure of the Nansha trough basin. The results show that the Moho depth is between 18 and 26 km in the Nansha sea area, with the smallest Moho depth lying on the groove center that quickly falls from the trough center southeast to the continental slope by 20-26 km. One possible explanation for this observation is that there had been strong crustal tectonic movement in this region in the geological era. The continental crust appears to be one whole piece with a FC/FL ratio (FC-crust rheological strength, FL-lithosphere rheological strength) of less than 80% in the South China Sea area. Since the reef area has a QC/Q0 ratio (QC-crust heat flow, Q0-heat flow of ocean floor) of more than 60% and the trough area of less than 40%, the thermal structures of the reef and trough areas correspond to the 'hot-crust-cool-mantle' and 'cool-crust-hot-mantle' types, respectively. The Nansha trough is a low temperature basin with a Moho surface temperature between 300-700℃. There is a stratified distribution of high and low temperature gradients along the vertical direction of the stratigraphic section, where the temperature gradients are 15-30℃/km and greater than 45℃/km in the upper and lower crust, respectively. The characteristic of stress distribution resembles that of a accordion bellow structure that the north side is squeezed while south side stretched, the shrinkage formation reduced from deep to shallow in the north squeezed zone, and the tension formation increased from deep to shallow in the south stretching zone. Viscosity coefficient and rheological strength are high in the north of the trough, about 2-3 orders of magnitude higher than that in the south; therefore, the tectonic deformation is more likely to occur in the south of the trough. Based on the calculation results, we reason that the thinning mechanism of the Nansha trough continental crust is related to the 'crustal gravity balance effect' and 'hot mantle uplift effect'. Overall, the deformation mechanism of the Nansha trough deepwater fold-thrust tectonics is largely due to the 'crustal shortening' followed by 'gravitational decollement' effects. It is noted that relationships involving 'Moho rolling', 'crustal isostatic' or 'hot mantle uplift' in the continental crustal evolution process, or 'elastic-deflection' or 'fold-thrust' in the Nansha trough basin sedimentary basement deformation, are not discussed in this paper. © 2017, Editorial Office of Earth Science Frontiers. All right reserved.