The Significance Of Crustal Structure And Its Dynamical Implication Inferred From A Local Tomography In The Dabie Orogenic Belt

A number of geological and geophysical studies have demonstrated that the traction force of high-density oceanic lithosphere leads to the subduction of continental lithosphere,which can reach down to depths of 80-120 km and generate ultrahighpressure(UHP)metamorphic rocks in there.Then,the exhumation of UHP rocks from great depths is aided by the low-density of the subducted plate and buoyancy of the continental crust.The Dabie Orogenic Belt,which involved the deep subduction and continental crust collision in the Triassic,is one of the major continental collision zones on the globe.The Yangtze Block subducted to mantle depths of ca.200 km beneath the North China Block,which resulted in the formation of UHP metamorphic terranes.In addition,the area underwent extensional tectonic collapse and post-orogenic magmatism/volcanism in the early Cretaceous.In order to study the formation mechanism and deep processes of the Dabie orogenic belt,a number of geophysical studies has been carried out in this area since last century,such as deep seismic reflection/refraction,tomography,receiver function and Magnetotelluric(MT)sounding,etc.However,these works didn't involve in magmatism/volcanism in this area.In other words,the relationship between the crustal structure and magmatism/volcanism of the post-orogen need to be further defined or discussed.Here,we collected abundant local earthquakes and performed a doubledifference tomographic study to constructed crustal velocity structure in the Dabie Orogenic Belt(N29-34°,E114-119°)and study the relationship between crustal structure and post-orogenic magmatism/volcanism.In order to ensure the credibility of the inversion results and improve inversion precision,the seismic data are strictly selected.In the data processing,we picked up the P-and S-wave first arrival times,controlled the magnitude and travel time residuals of seismic events(ML?1.0,travel time residuals?0.5s),and removed anomaly data by using the time-distance analysis.Finally,the inversion data set includes 13890 P-wave and 13799 S-wave first arrival times from 1603 seismic events,and every event is observed by at least 4 stations.In the process of event pairs matching,the 164129 differential P wave times and 156980 differential S wave times from 1584 seismic events were inferred from event pairs that separated smaller than 30 km and greater than 0.1 km.We set up 0.5°×0.5° and 0.3°×0.3° inversion grid nodes to obtain the 3-D velocity structures in the Dabie Orogenic Belt by using above absolute arrival times and differential arrival times.Results show there is a low-velocity structure(Lv1)beneath the Dabie Orogenic Belt,which might be melting material of mantle source which might be associated with magmatism and/or volcanism in the early Cretaceous.The upwelling asthenosphere can be generated by break-off of subduction slab or low crustal/lithospheric delamination in the Mesozoic in the Dabie Orogenic Belt.The upwelling asthenosphere might invade into the crust and form the melting material.Finally,magmatism and volcanism was likely triggered by the melting material in the crust.Meanwhile,there is another large-scale lowvelocity structure(Lv2)located at Hefei Basin,which might be generated by the sedimentary layer in the shallow.There is a dipping vestige of high velocity beneath the Dabie Orogenic Belt,which might be the Xiangfan-Guangji fault or northward subduction relic.