Numerical Simulations On Gravitational Instability Of Cratonic Lithosphere And Its Dynamic Consequences

As oldest part of continents, cratons are generally considered to remain stable to resist mantle convective motions for billions of years. Cratons are characterized by low surface heat flux and thick and cold mantle lithosphere, which has melt-depleted compostions with respect to the normal mantle and thus has low intrinsic density and is chemically buoyant. The melt-depletion together with its low temperature lead the cratonic mantle lithosphere to have high strength (viscosity). The compositional buoyancy and high viscosity help to stabilize cratons and thus play important roles in craton evolution. However, not all cratons are always stable, and there are also some cratons that have been destructed and modified. Therefore, the dynamics of the stability and destruction of cratons are keys to understand the evolution of continents.The North China Craton (NCC) is a typical example of craton destruction. Large-scale magmatic activities had developed during Mesozoic to Cenozoic in the eastern part of the NCC, and geochemical and geophysical studies indicate that not only the thickness and thermal state of the NCC lithosphere have changed, but also the compostion of the lithospheric mantle has been modified. Although abundant field observations have indicated that the lithosphere of the eastern part of the NCC has been modified and destructed, there are also many issues that remain unclear, such as the mechanism and timing of the NCC destruction. To explain different geochemical observations, many destruction models have been proposed such as delamination, thermal-chemcial erosion, melt-peridotite interaction and water weakening, etc. However, there is no geodynamical model that can directly support these mechanisms. The main purpose of this paper is to investigate the dynamics of NCC destruction through geodynamical modellings on gravitational instability of cratonic lithosphere.Previous works on gravitational instability of cratonic lithosphere has been limited in Newtonian rheology in laboratory experiments. In this paper, by setting up two dimensional thermochemical convection models with non-Newtonian rheology, we study the dynamics of gravitationally instability of cratonic lithosphere and also its surface responses, including the style and temporal and spatial scales of lithospheric instabilities and also the responses of topography and heat flux. Numerical results suggest that the non-Newtonian rheology can efficiently reduced the viscosity of cratonic lithosphere and play an important role in lithospheric instabilities. With non-Newtonian rheology, the lithospheric instability of cratonic lithosphere is episodic and multistaged and may last for-100 Myr. The magmatic activities from Mesozoic to Cenozoic in the NCC are also episodic and had continued for 100 Myr. Thus the instability process revealed from our study explains the charateristics of magmatic activities that associated with the NCC destruction. We also find that the instability starts from the cold and shallower part of the lithosphere, which allows the hot asthenospheric materials rising to the shallower part of the lithosphere, thus provides efficient heat to the base of lower crust which required for the formation of eclogite. This provides a possible explanation for the existence of eclogitic exponent found in volcanic rocks. Meanwhile, some of the sinking cratonic lithospheric materials would move back to lithospheric depth and mix with the rising asthenospheric material in our numerical simulations. This may explain the hybridized magmatic sources of cratonic lithosphere and oceanic mantle found in xenoliths in the NCC. Therefore, the lithospheric instabilities revealed from our numerical simulations provide good explanations for the dynamics of the NCC destruction.The dynamic process lithospheric instabilities should be directly related to the surface expression such as topography and heat flux, which are mainly controlled by the thermal and chemical structure of the lithosphere. Therefore, the topography and heat flux can be used as constraints on the dynamic process of craton destruction. The numerical results shows that, the lithsopheric instability of cratonic lithosphere would increase the topography and heat flux of the destabilized area, which is consistent with the field observations of residual topography and heat flux for the eastern and western part of the NCC. This has provided a feasible method to quantify the relicts of ancient cratonic materials after craton destruction. Using present day residual topography and heat flux of the NCC as constraints, the numerical results suggest that there are 20%-50% of the ancient cratonic lithospheric materials preserved beneath the eastern part after NCC destruction.