Study On The Fine Lithospheric Structure Of The Northeastern Tibetan Plateau From Joint Inversion Of Seismic And Gravity Data

As a growth front of the continental collision,the northeastern Tibetan Plateau(NETP)is one of the best places to investigate the uplift and expansion mechanism of the Tibetan Plateau.In order to better explore the growth and deformation mechanism of the northeastern Tibetsn Plateau,more accurate underground physical parameters,such as crustal thickness,velocity structure and Vp/Vs ratios,are needed to deduce the composition and state of the matter of the lithospheric materials.The stations include 673 broadband stations in a temporary seismic network of ChinArray(PhaseⅡ)was included in the dataset,combined with the WGM2012 gravity model.Crustal thickness was obtained using the improved H-κ method(H-κ-c),joint inversion with P wave constraint and the gravity inversion with seismic constraints.The joint inversion method with independent P wave model also revealed high resolution lithospheric S wave structure,crustal Vp/Vs ratios and the sedimentary structure.In addition,the residual density map was calculated by the joint inversion of surface waves and gravity data.The crustal thickness obtained using the H-κ-c method is in agreement with that from the joint inversion of receiver functions and surface wave dispersions with P wave model constaint.The crustal thickness gradually thinning from the Tibetan Plateau.The Qilian orogenic belt in the northwest possess the thickest Moho～66 km,while the Ordos Basin shows the thinnest crust with 36 km.The crustal thickness also shows significant lateral variations.The Qilian orogen,the Alxa block and the western Qinling orogen are all gradually thinning from west to east.The thick sedimentary layers mainly appear in the Ordos Basin and Sichuan Basin,and the northern Qilian orogen also receivered some sediment.The gravity inversion with seismic constraints(surface waves and receiving functions)obtains the Moho depth of the NETP.The smooth model mainly reflects the regional large-scale structural features,and has limited resolution for local small-scale features.A remarkable low-velocity zone(LVZ)in the mid-lower crust of the SPGZ were detected by our joint inversion method.The appearance of high Vp/Vs ratio indicates the existence of weak materials(partial melts)in the lower crust beneath SPGZ.Furthermore,a prominent LVZ(Vs<4.2 km/s)residing in the uppermost mantle is present beneath the SPGZ at depths between 70～100 km,while the Qinling-Dabie orogen shows high velocities in both the crust and the uppermost mantle.Given that there is few LVZ beneath the Qinling-Dabie orogeny,the channelized extrusion of ductile mantle material beneath Tibet through the Qinling Belt would be feasible only at lithospheric depths.The velocity-density structure in the study area was obtained by joint inversion of surface wave dispersions and Bouguer gravity data.The velocity-density shows an obvious relation with the blocks.The northeastern Tibetan Plateau,consists of the Songpan-Ganzi,the Qilian orogen and the West Qinling shows the characteristics of low velocity and lo w density,while the stable Alxa block,the Ordos block and the northern Sichuan Basin exhibit significant high velocity and high density.The velocity-density relationship shows a positive linear correlation overall,whereas negative correlation exist in the shallow crust and the deep mantle.The Ordos block shows thick deposits and low Vs velocity,but high residual density,which may be related to the sedimentary components and their porosity.There is a significant gravity anomaly between the Qaidam Basin and the Qilian orogen.The anomaly mass shows high Vs(4.5 km/s),which is significantly higher than the surroundings.This remarkable low-density anomaly begins at a depth of 80 km and continues to a depth of 120 km.It is speculated that there may be a low density,basalt-riched composition,highly refractory craton lithospheric mantle here.Based on the different crustal thickness,crustal Vp/Vs,lithospheric Vs structure and residual density distribution obtained by the mentioned methods above,we believe that the northern Sichuan Basin,Ordos and Alxa blocks are characterized by thin crustal thickness,low Vp/Vs ratios and high velocity and high density,thus strong strength lithosphere.However,massive mid-lower crust and upper mantle low velocity anomalies are discovered beneath the Qilian orogen and the SongpanGanzi terrane,accompanied by high Vp/Vs ratios and low residual density,which indicates the existence of partial melting.We suggest that mantle upwelling resulted from the removal of the lithospheric mantle contributes to the isolated partial melting materials in NETP.The crust and the upper-most mantle velocity of the Tibetan Plateau are significantly correlated,implying the consistency of the crust and mantle structure and deformation in this region.The seismic anisotropy also reveals the consistency of the fast polarization directions between the crust and mantle,thus we suggest that the NETP is dominated by the coupling deformation of the whole lithosphere.