Lithospheric Thinning Of Cratons: Implications From Craton Comparison And Geodynamic Numerical Modeling

Cratons are early consolidated shallow units since the Earth's formation and cover more than 50% of its continental surface at present.The cratonic lithosphere is hard to lose stability even during later geological events,because of its large area,big thickness,low geothermal gradient,depletion in melt component,low density and large viscosity.However,the lithosphere of some cratons is not as stable as it was understood,instead,it can be reactivated and even be destructed(e.g.,eastern part of the North China Craton,the North Atlantic Craton,and the Tanzania Craton).But the mechanisms and driving forces for the reactivation and destruction are still controversial.The North China Craton(NCC),as one of the cratons having been studied in the highest degree,its lithospheric destruction has been a world famous phenomenon for long time.During the destruction stage,the NCC suffered from complex geological events,including subductions,collisions,and mantle upwelling or mantle plumes.The related lithospheric thinning mechanism includes extension,delamination,convective removal,hydroweakening,melt-peridotite reaction,etc.These make the researches controversial in assessing the main driving force and mechanism of craton lithospheric thinning / destruction.Comparing with the NCC,the geological evolution of the North Atlantic and Tanzanian cratons during their lithospheric thinning stage is relatively simple,and is mainly dominated by mantle upwelling or mantle plume related thermal-mechanical erosion and extensional thinning.The geological history,lithosphere modification history and geophysical features of these three cratons and other cratons are compared in this thesis to qualitative analysis the similarities and differences of lithospheric thinning in cratonic interiors,cratonic margins,and different places within continental unit.The qualitative analysis and comparison result shows that:(1)Marginal thinning is mainly caused by subduction-,collision-and repeated extension-related metasomtism,magmatism,delamination,metacratonization,melt-peridotite reaction,fluid actions,etc.The thinning level can reach lower crust depth.(2)The interior thinning is mainly caused by large-area extension,large-scale dehydration and hydration,convective removal,and large-scale delamination.Large-scale lithospheric thinning is also influenced by the stability of lithosphere itself.For instance,the weak coupling layer along the mid lithosphere depth(MLD,80-120 km deep)can enhance the lateral offset or vertical foundering of the lower lithosphere below this layer during continental drifting and can form lithospheric thinning.(3)The active margin is mainly influenced by subduction related melt,fluid actions and related convective removal.(4)Passive margin is influenced by secondary convective removal enhanced by lithospheric steps between thick and thin lithosphere or mantle plumes.Besides,it is also influenced by lateral offset along the MLD in(2).(5)Lithospheric weakening mechanism is rarer and more important than destruction mechanism.Lithospheric weakening is the basis of lithospheric destruction.Based on these comparison work and qualitative analysis,some mechanisms for large scale lithospheric thinning in cratonic interiors are chosen to implement quantitative analysis using numerical modelling methods(including large scale dehydration,hydroweakening,and convective removal model;large scale delamination model;and lateral offset and delaying of lithospheric bottom model).The quantitative analysis indicates that:(1)Large scale flat-lying slab penetrating into a strongly hydrous mantle transition zone(water content ?0.6 wt %)can cause large scale upwelling and dehydration of it and can lead to hydration of the overlying asthenosphere and lithosphere,enhance their convection and deformation,which advantages to large scale cratonic lithosphere thinning.(2)The water from the strongly hydrous mantle transition zone(water content ?0.6 wt %)is mainly derived from multiple subducting slabs,bending,stacking,avalanching or penetrating into the lower mantle and leading to dehydration of slabs and hydration of mantle transition zone.(3)Lower crust scale delamination mainly takes place near cratonic margins or the interior of continental plateaus owing strongly thickened lower crust.For cratonic interiors,MLD scale(80-120 km)delamination is much easier than lower crust scale(20-40 km)delamination.(4)For cratons with buoyant roots,the roots can be left behind during continental drifting along the weak coupling MLD(80-120 km).The offset is horizontal and can make the cratonic root displaced underneath lithosphere of oceanic basins linking to the trailing said of the drifting continent and can make the cratonic shallow parts near the front side of the drifting continent lost their root.The quantitative analysis using numerical modeling methods are comparable with geological and geophysical case studies in the craton comparison work.The combination of geodynamic,geological and geophysical studies supplies great supporting to the applicability of all the ideas in this thesis in cratons' evolution.