Numerical modelling of lithosphere evolution in the North China Block: Thermal versus tectonic thinning

摘要

In an attempt to understand the dynamics of lithospheric thinning in the North China Block (NCB), two-dimensional finite-difference numerical models have been constructed to simulate the lithosphere thinning process in the NCB. The models consider the theory of conductive thermal transfer on earth and explore how lithospheric thickness will vary in response to the changes in the thermal conditions of the lithosphere. As a process independent of thermal thinning, the effect of mechanical tectonic extension on lithospheric thickness has been explored by incorporating a constant extensional displacement rate at the lateral edges of the model. The results of a model involving only mechanical extension show that the thinning of the lithosphere is still quite limited to not more than 50 km even under a 25% extension. In contrast, the results of the thermal model quantitatively demonstrate that for a given thermal conductive rate and radioactive heating condition, the increase of the mantle thermal flux at the base of the lithosphere can trigger a great deal of changes in both the thermal state and the lithospheric thickness. The lithospheric thickness in the NCB could be thinned from an original thickness of 200 km to the new thickness of < 100 km, when the mantle thermal flux was increased to 35-40 mWm~(-2). Therefore, thermal perturbation seems to be the predominant mechanism responsible for the significant lithosphere thinning in the NCB. The effects of mechanical tectonic extension may become more profound on lithosphere thinning at a later stage when the lithosphere had already been heterogeneously thinned. A temporary and genetic link between the NCB lithospheric thinning and the Cretaceous global event is put forward to interpret the lithospheric evolution throughout the eastern China in late Mesozoic.