Numerical simulation of subcontinent lithosphere dynamics : craton stability, evolution and formation

• Main Author: Wang, Hongliang
• Published: Durham University 2015
• Subjects: 550
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.650246

Through geodynamical modelling, two hypotheses about the craton stability and evolution were revisited and an important process of cratonization is investigated. Unlike most previous, related numerical studies, non-Newtonian rheology with composition dependence was used in these studies, and the rheological parameters are thus directly comparable with laboratory experiment of mantle. The first hypothesis, that the cratonic lithosphere is “isopycnic”, is found to be not strictly necessary for craton stability and longevity. The high viscosity of the cratonic litho- sphere due to compositional effects on the mantle rheology is found to be essential to maintain a thickness difference between cratonic and non-cratonic lithosphere for over billions of years and it allows a modest negative buoyancy of the cratonic root, depending on the strengthening factor due to the compositional effects. The second hypothesis to be tested is that mantle plume im- pingements cause rapid, significant removal of subcontinental lithosphere. The results presented in this thesis show that the erosion caused by a plume impact on a continent that is strong enough to have survived billions of years of Earth’s history is rather limited. A special weaken- ing mechanism of such highly viscous and buoyant roots is required to reactivate this cratonic lithosphere and thus cause significant thinning within 10s of Myrs. The fluid/melt-rock interac- tion during mantle metasomatism is probably the most likely mechanism to modify and weaken depleted cratonic lithosphere. Therefore, metasomatic weakening is essential for the significant thinning of subcontinental lithosphere observed, e.g.at North China Craton and Namibia, south- ern African, no matter whether caused by a plume impact or another tectonic event. Using the reasonable compositional effects on the buoyancy and rheology of mantle rocks from the above studies, numerical experiments are performed to study the formation of thick cratonic lithosphere from a layered, depleted mantle material. In this scenario, substantial tec- tonic shortening and thickening of previously depleted material seems to be an essential ingre- dient to initiate the cratonization process. Afterwards, gravitational self-thickening will cause further thickening. Compositional buoyancy resists Rayleigh-Taylor instability collapse and stabilizes the thick cratonic root, while the secular cooling also has a stabilizing effect on the cratonic root by reducing the thermal buoyancy contrast between lithosphere and asthenosphere and increasing mantle viscosity. The presented numerical results are consistent with the vertical movement of cratonic peridotite as suggested on petrological grounds.