Utilizing Frequency-Bessel Multimodal Ambient Noise Surface Wave Tomography Method To Study The Process Of Continental Reworking In The Eastern South China Block

The South China Block(SCB),which is located in the southeastern part of the Eurasian plate,was amalgamated by the Yangtze Craton(YC)to the northwest and the Cathaysia Block(CB)to the southeast during the early Neoproterozoic era.During the Triassic Indosinian movement,the SCB collided with the North China Craton(NCC),generating the well-known Qinling-Dabie(QD)high and ultrahigh-pressure(HP and UHP)metamorphic orogen.Since the late Mesozoic Yanshanian movement,the Pacific plate subducted towards the SCB,inducing widespread magmatism.With this complex evolutionary history,the SCB,similar to other ancient blocks,such as the North American continent,the South American continent,and the NCC,has clearly experienced drastic reworking process.Continental reworking refers the process in which the composition,structure,and physicochemical properties of the continental lithosphere(crust and lithospheric mantle)change when original or ancient geological systems were reworked by drastic tectonothermal events,such as continental collisions and ocean plate subduction.With no doubt,this process would affect the seismic velocity structure of the continental lithosphere(e.g.,the formation of anomalous high-or low-velocity zones).Thus,seismic velocity imaging is an effective tool for studying the process of continental reworking.Compared with the teleseismic tomography,ambient noise surface wave tomography does not rely on earthquakes and can measure shorter periods of surface wave,thus effectively constraining the shallower continental lithosphere.With simpler data processing procedure,ambient noise surface wave tomography has been widely applied to image the seismic velocity structure of continental lithosphere in the past 20 years.As much as previous ambient noise tomography studies in the eastern SCB,most of them just utilized the fundamental-mode surface wave dispersion curves,which can not overcome the severe nonuniqueness of inversion and thus can not reveal the detailed velocity structure.The Frequency-Bessel(F-J)transform method can be used to extract the multimodal dispersion spectrum containing multimodal dispersion curves.When higherorder modes dispersion curves are incorporated into the inversion,the nonuniqueness of inversion would be effectively reduced and thus the subsurface shear-wave velocity structure can be better constrained.Here,the F-J multimodal ambient noise surface wave tomography refers the process in which one firstly adopts the F-J method to extract the multimodal dispersion spectrum,then conducts the joint inversion of multimodal dispersion curves,and finally images the subsurface shear-wave velocity structure.In order to further study the process of continental reworking in the eastern SCB and its adjacent areas,this study collected～10-year continuous seismic ambient noise data within this area and then utilized the F-J multimodal ambient noise surface wave tomography method to image the crustal and upper-mantle shear-wave velocity structure.Model comparison and evaluation show that our acquired velocity model generally overpasses previous models.Benefit from the incorporation of higher-order modes of dispersion curves,our velocity model reveals the more detailed structure,especially improves the imaging result in the crust,and thus provides new and reliable evidence on the continental reworking in the eastern SCB and its adjacent areas.Compared with previous studies,our tomography results reveal that there are widespread low-velocity zones(LVZs)distributing within both the middle crust(～10-20 km)and the upper mantle(～60-120 km).The mid-crustal low-velocity zones(MCLVZs)are more continuous in the areas north of the QD orogen and west of the North-South Gravity Lineament(NSGL),but have clear interruptions near the QD orogen.Moreover,the MCLVZs in the CB and the lower YC are generally fragmented.Compared with the MCLVZs,the upper-mantle low-velocity zones(UMLVZs)are more continuous and widespread.In the east of the NSGL,the average top depth of the UMLVZs is～60 km,consistent with the depth of lithosphere-asthenosphere boundary(LAB)within this area.What’s more,the UMLVZs present convex morphology variation near the suture line(Shaoxing-Jiangshan-Pingxiang fault zone,SJPFZ)between the YC and the CB,and near the Tanlu fault zone,indicating the asthenospheric upwelling.In the west of the NSGL,however,the top depth of the UMLVZs almost keeps～70 km,far less than the LAB depth(>150 km)revealed by previous studies.In result,the UMLVZs within this area are thought to be the mid-lithospheric discontinuity(MLD).The spatial distribution of these two LVZs has a good correlation with the distribution of magmatic rocks and terrestrial heat flow in the study region.Where the top depth for the UMLVZs is shallower corresponds to the more fragmented MCLVZs,the more widespread magmatic rocks,and the higher heat flow.According to previous studies,we think,widespread MCLVZs formed due to the compositional and structural layering of the continental crust.What’s more,a long and quiet tectonic period,that is,from the late Neoproterozoic to the early Phanerozoic,provided enough time for the MCLVZs to be generated.The two LVZs jointly shaped the distribution of magmatic rocks and terrestrial heat flow in the study region.The shallower top depth of the UMLVZs indicates that the asthenosperic upwelling is more drastic.When upwelling hot materials crowded into the crust along the large-scale fault zones,they would mix and react with those materials from the MCLVZs,thus destroying the MCLVZs.The mixed hot materials would further move towards the surface,generate large amounts of magmatic rocks,and result in higher terrestrial heat flow.By analyzing the characteristics of two LVZs and their correlations with other regional tectonics including magmatic rocks and terrestrial heat flow,we provide a generalized description of the process of continental reworking in the eastern SCB and its adjacent areas.During the long tectonically quiet period,that is,from the late Neoproterozoic to the early Phanerozoic,compositional and structural layering in the crust led to continuous and widespread MCLVZs.During the Triassic Indosinian movement,the SCB collided with the NCC,resulting the formation of the Qinling-Dabie UP and UHP metamorphic orogen.Meanwhile,this destroyed the MCLVZs near the Qinling-Dabie orogen in which low-velocity and-density rocks are metamorphosed into high-velocity and-density rocks.Since the late Mesozoic Yanshanian movement,the subduction of the Pacific plate induced widely asthenospheric upwelling,which made the crust and lithosphere clearly thin in this area.Meanwhile,the subduction reactivated some ancient and lithosphere-scale suture lines or fault zones including the SJPFZ and the Tanlu fault zone,thus providing convenient channels for upwelling hot materials to be transported into the crust.Upwelling hot materials would mix and react with materials from the MCLVZs,which destroyed the continuity of the MCLVZs.Furthermore,mixed and hot materials moved towards the surface,formed larger amounts of late Mesozoic magmatic rocks,and caused higher terrestrial heat flow.