Basic Features of the Crustal Structure in the Lower Yangtze and Its Neighboring Area in the Chinese Mainland： Review of Deep Seismic Sounding Research

TheDeepSeismicSounding(DSS)projectscarriedoutfromthe1970sinthelowerYangtzeregionanditsneighboringareawerereviewedinthispaper,thenthebasicwavegroupfeaturesofthosewideanglereflection/refractionrecordsections,andofthecrustalstructurearesummarized.Itshowsthattherewereintotalfiveclearwavegroupsontherecordsections,whichincludethefirstarrivalPg,thereflectionP1fromthebottominterfaceoftheuppercrust,thereflectionP3fromthebottominterfaceofthemiddlecrust,thestrongreflectionPmfromtheMohoboundary,andtherefractionPnfromuppermostmantle.Ingeneral,thesephasesareeasilyconsistentlytracedandcompared,despitesomefirstarrivalsbeingdelayedorarrivingearlierthannormalduetotheshallowsedimentarycoverorbedrocks.Inparticular,intheDabieMountainregiontheseismiceventsofafewgatheredshotsalwayshaveweakreflectionenergy,aretwisted,orexhibitdisorganizedwaveforms,whichcouldbeattributedtothedisruptionvariationsofreflectiondepth,thebrokenMoho,andthediscontinuityofthereflectionboundarywithincrust.Theregionalcrustalstructuresarecomposedoftheupper,middleandlowercrust,ofwhichthemiddleandlowerlayerscanbedividedintotwoweakreflectionones.ThecrustalthicknessoftheNorthChinaandYangtzeplatformare30km-36km,andtheMohoexhibitsaflatgeometrydespitesomelocaluplifts.Theaveragepressurevelocityinlowercrustbeneaththistwotectonicareais6.7±0.3km/s.Nevertheless,beneaththeDabieshanareathecrustalthicknessis32km-41km,theMohobendsdownsharplyandtakesanabrupt4km-7kmdislocationintheverticaldirection.TheaveragepressurevelocityinthelowercrustbeneaththeDabieshanareais6.8±0.2km/s.