Crustal Thickness Of The Northeastern Tibet And Adjacent Regions Revealed By Virtual Deep Seismic Sounding

Since ~50 Ma,the collision between the Indian plate and the Eurasian plate has resulted in an intense uplift of the Tibetan plateau,making it the highest,largest and youngest plateau in the world.The collision of continental plates not only led to the formation of Tibetan plateau,but also had a significant impact on the tectonic and climate around the plateau.The northeastern margin of the Tibetan plateau is located in the junction region between the Alxa block,Ordos block and the Tibetan plateau.It is an important component of the plateau and the most leading edge of the plateau northeastern growing to the inland.The border of the Tibetan plateau represents the latest deformation and thickening process of the plateau,so it is the key region to understand the process of the Tibetan plateau extension and the crustal thickening.At present,deep seismic sounding and receiver function studies show that the crust in the northeastern margin of the Tibetan plateau is thicker,but there are still some controversies about the leading edge position of the crustal thickening and the thickening mechanism.For example,the morphology of crustal thickening front in the northeast margin of the Tibetan plateau that obtained by different receiver function studies is quite different.The crustal structure of this region needs to be further constrained.Virtual deep seismic sounding(VDSS)is an effective method to constrain the crustal thickness.In this paper,we obtained the crustal thickness distribution of the northeastern Tibetan plateau and adjacent regions by using the virtual deep seismic sounding based on dense seismic arrays data of “ChinArray” project.The main results and conclusions in this paper are as follows:(1)In the virtual seismic profiles constructed by VDSS method,the SsPmp phase has relatively strong energy and can be continuously observed in both vertical and radial components.Compared with the radial component profile,SsPmp phase in the vertical component profile is easier to identify.Moreover,the crustal thickness variation trend obtained by different component waveform modeling has a good consistency.(2)The crustal thickness changes dramatically in different tectonic units.There are systematically west-east variation in the crustal thickness within the Qilian and western Qinling orogenic belts.The crust east of ~103°E is about 45~50 km,in contrast,the crust in west region has significantly thickened,more than 55 km.This result is basically consistent with the results by received function,indicate that crustal thickening mainly occurs in the western region of Qilian and western Qinling blocks.Combined with other results,we speculate that the outward expansion of the northeastern Tibetan plateau has evolved by distributed crustal thickening before a removal of the lower crust.The crust is also thicker(nearly to 55km)under the transitional region between the Alxa block and the Qilian orogenic belt,while it is about 45~50 km within the central Alxa block.Combined with previous research,we speculated that the leading edge of the plateau northeastern growth has already arrived in the southern margin of the Alxa block.(3)In comparison,the Ordos block has a thinner crust and the crustal thickness tends to increase from northern and southern parts(~45 km)to the central region(~50 km).In addition,we observed complex SsPmp phase under the Liupanshan fault zone,which suggests a double Moho.