North China Craton Destruction:constraints From Surface Gravity And Topography

As parts of the old continents,cratons are characterized by low surface heat flow,low density,high viscosity and fast seismic velocity anomalies.After its formation,Cratons remain stable and complete without the influence from global tectonic events.However,not all cratons are indestructible,recent researches of North China Craton(NCC)shows NCC undergone thinning and destruction during Cenozoic and Mesozoic.NCC consists of two distinctly different blocks(WNCC and ENCC),they amalgamated and collided with each other along Trans-North China Orogen(TNCO)1.85 billion years ago.Then,the whole craton was formed and stayed stable until Mesozoic.The lower parts of ENCC were then decratonization and modified in contrast to the longevity stable WNCC.In this paper,global gravitational model EGM2008,surface relief model ETOPO1 and Moho depth data derived from previous works of receiver function and deep seismic soundings are used to calculate the free air gravity anomaly,complete bouguer gravity anomaly,Airy isostatic gravity anomaly,residual gravity anomaly,surface topography,residual topography.The topography is high at 1.5-2km in the WNCC and TNCO instead of only several hundred meters in ENCC,both WNCC and ENCC are relatively flat,but the TNCO is composed of high mountains and deep valleys.The free air anomaly in WNCC and ENCC is between-50 and 10mgal,while it's much higher in TNCO.The bouguer anomaly is reversely proportional to the topography,besides,the gravity lineament lies just at the east borders of Taihang Mountain.The Airy isostatic anomaly suggest that WNCC and ENCC is mainly in isostatic equilibrium,but the isostatic anomaly in TNCO is relatively higher.When removing the gravity anomaly and topographic isostatic reduction produced by crustal thickness variation from bouguer gravity anomaly and surface topography,the obtained residual gravity and residual topography indicate that there are density deficits and excess buoyancy in most parts of ENCC.After the destruction and enrichment of ENCC,the newly-formed lithosphere mantle should have high chemical density,however the low residual gravity and high residual topography of this region may suggest that temperature effect weighs more dominantly in density than that of chemical composition.TNCO could still keep its thick lithosphere root according to previous seismology and geochemistry researches.So the extremely low residual gravity and high residual topography are determined by high temperature below TNCO.Combined the residual topography,residual gravity together with the 3-d residual density structure distribution inverted from it,detailed analysis can be conducted that approximately 50-60%of old depleted craton mantle remains in ENCC lithosphere from the perspective of chemical composition,thus supporting the viewpoint that new lithosphere in ENCC might be a mixture of the relics of old craton mantle and the normal mantle.