Numerical Modeling Of Lithospheric Delamination In Tibet

Tibetan Plateau is the highest and largest orogenic plateau in the world,and the study on Tibet has always been the hot topic of continental dynamics.The lithospheric mass balance is one of the main problems in the Tibetan Plateau evolution.The horizontal shortening from north to south in the Tibetan Plateau is at least 1630 km,and the ground surface rises by more than 4 km.Geophysical studies indicate 60%shortening has been absorbed by crustal thickening,but the remaining 40%is not well explained.Therefore,the studies about lithospheric mass balance in Tibetan Plateau is focused on explaining the missing lithosphere material.Delamination is one of the important explanations for the material deficiency in Tibet,which proposes that partial thickened lithosphere sinks in convective mantle due to gravitational instability.Seismic images show that three seismic high velocity anomalies are observed respectively in upper mantle,transition layer and lower mantle beneath the central Tibetan Plateau.However,because of the complex tectonic background and the long term of Tethyan oceanic subduction at the Tibetan Plateau margin,explanations for the observed seismic high velocity anomalies are intensively debated.Some studies interpret these high velocity anomalies are the delaminated lithosphere,but other studies suggest that these are the remnants of subducted oceanic plates.In this thesis,we perform two-dimensional finite element models to study including（1）the dynamics of delaminated lithosphere in convective mantle,and analyze the main factors affecting the sinking rate of the delaminated lithosphere,（2）explain the above seismic high velocity anomalies combined with geological observation in the Tibetan Plateau.Our results show that（1）the sinking rate of the delaminated lithosphere decreases along with time and depth.The average sinking rate of delaminated lithosphere in upper mantle is～2+0.5 cm/year and～1+0.2 cm/year in transition layer,which is mainly depending on the density of lower crust and the viscosity of lower mantle.When the viscosity of lower mantle is higher than 3×10²² Pa s,the delaminated lithosphere is difficult to sinking cross660 km and into lower mantle by 65 Ma.（2）Based on the sinking rate of delaminated lithosphere in the mantle,this thesis proposes that the seismic high velocity anomaly in upper mantle may delaminated lithosphere during 10-20 Ma,and the high velocity anomaly in transition layer may delaminate lithosphere during 25-30 Ma,and the high velocity anomaly in lower mantle may the subducted ocean plate before India-Asia plate collision.