Abstract
The Red River Fault zone (RRF) is the major geological discontinuity that separates South China from Indochina. Today it corresponds to a great right-lateral fault, following for over 900 km the edges of four narrow (< 20="" km="" wide)="" high-grade="" gneiss="" ranges="" that="" together="" form="" the="" ailao="" shan-red="" river="" (asrr)="" metamorphic="" belt:="" the="" day="" nui="" con="" voi="" in="" vietnam,="" and="" the="" ailao,="" diancang="" and="" xuelong="" shan="" in="" yunnan.="" the="" ailao="" shan,="" the="" longest="" of="" those="" ranges,="" is="" fringed="" to="" the="" south="" by="" a="" strip="" of="" low-grade="" schists="" that="" contain="" ultramafic="" bodies.="" the="" asrr="" belt="" has="" thus="" commonly="" been="" viewed="" as="" a="" suture.="" a="" detailed="" study="" of="" the="" ailao="" and="" diancang="" shan="" shows="" that="" the="" gneiss="" cores="" of="" the="" ranges="" are="" composed="" of="" strongly="" foliated="" and="" lineated="" mylonitic="" gneisses.="" the="" foliation="" is="" usually="" steep="" and="" the="" lineation="" nearly="" horizontal,="" both="" being="" almost="" parallel="" to="" the="" local="" trend="" of="" the="" gneissic="" cores.="" numerous="" shear="" criteria,="" including="" asymmetric="" tails="" on="" porphyroclasts,="" c-s="" or="" c'-s="" structures,="" rolling="" structures,="" asymmetric="" foliation="" boudinage="" and="" asymmetric="" quartz="" (c)="" axis="" fabrics,="" indicate="" that="" the="" gneisses="" have="" undergone="" intense,="" progressive="" left-lateral="" shear.="" p-t="" studies="" show="" that="" left-lateral="" strain="" occurred="" under="" amphibolite-facies="" conditions="" (3-7="" kb="" and="" 550-780="" degrees="" c).="" in="" both="" ranges="" high-temperature="" shear="" was="" coeval="" with="" emplacement="" of="" leucocratic="" melts.="" such="" deformed="" melts="" yield="" u/pb="" ages="" between="" 22.4="" and="" 26.3="" ma="" in="" the="" ailao="" shan="" and="" between="" 22.4="" and="" 24.2="" ma="" in="" the="" diancang="" shan,="" implying="" shear="" in="" the="" lower="" miocene.="" the="" mylonites="" in="" either="" range="" rapidly="" cooled="" to="" approximate="" to="" 300="" degrees="" c="" between="" 22="" and="" 17="" ma,="" before="" the="" end="" of="" left-lateral="" motion.="" the="" similarity="" of="" deformation="" kinematics,="" p-t="" conditions,="" and="" crystallization="" ages="" in="" the="" aligned="" ailao="" and="" diancang="" shan="" metamorphic="" cores,="" indicate="" that="" they="" represent="" two="" segments="" of="" the="" same="" tertiary="" shear="" zone,="" the="" ailao="" shan-red="" river="" (asrr)="" shear="" zone.="" our="" results="" thus="" confirm="" the="" idea="" that="" the="" asrr="" belt="" was="" the="" site="" of="" major="" left-lateral="" motion,="" as="" indochina="" was="" extruded="" toward="" the="" se="" as="" a="" result="" of="" the="" india-asia="" collision.="" the="" absence="" of="" metamorphic="" rocks="" within="" the="" 80="" km="" long="" ''midu="" gap''="" between="" the="" gneissic="" cores="" of="" the="" two="" ranges="" results="" from="" sinistral="" dismemberment="" of="" the="" shear="" zone="" by="" large-scale="" boudinage="" followed="" by="" uplift="" and="" dextral="" offset="" of="" parts="" of="" that="" zone="" along="" the="" quaternary="" red="" river="" fault.="" additional="" field="" evidence="" suggests="" that="" the="" xuelong="" shan="" in="" northern="" yunnan="" and="" the="" day="" nui="" con="" voi="" in="" vietnam="" are="" the="" northward="" and="" southward="" extensions,="" respectively,="" of="" the="" asrr="" shear="" zone,="" which="" therefore="" reaches="" a="" length="" of="" nearly="" 1000="" km.="" surface="" balance="" restoration="" of="" amphibolite="" boudins="" trails="" indicates="" layer="" parallel="" extension="" of="" more="" than="" 800%="" at="" places="" where="" strain="" can="" be="" measured,="" suggesting="" shear="" strains="" on="" the="" order="" of="" 30,="" compatible="" with="" a="" minimum="" offset="" of="" 300="" km="" along="" the="" asrr="" zone.="" various="" geological="" markers="" have="" been="" sinistrally="" offset="" 500-1150="" km="" by="" the="" shear="" zone.="" the="" seafloor-spreading="" kinematics="" in="" the="" south="" china="" sea="" are="" consistent="" with="" that="" sea="" having="" formed="" as="" a="" pull="" apart="" basin="" at="" the="" southeast="" end="" of="" the="" asrr="" zone,="" which="" yields="" a="" minimum="" left-lateral="" offset="" of="" 540="" km="" on="" that="" zone.="" comparison="" of="" cretaceous="" magnetic="" poles="" for="" indochina="" and="" south="" china="" suggests="" up="" to="" 1200="" +/-="" 500="" km="" of="" left-lateral="" motion="" between="" them.="" such="" concurrent="" evidence="" implies="" a="" tertiary="" finite="" offset="" on="" the="" order="" of="" 700="" +/-="" 200="" km="" on="" the="" asrr="" zone,="" to="" which="" several="" tens="" of="" kilometers="" of="" post-miocene="" right-lateral="" offset="" should="" probably="" be="" added.="" these="" results="" significantly="" improve="" our="" quantitative="" understanding="" of="" the="" finite="" deformation="" of="" asia="" under="" the="" thrust="" of="" the="" indian="" collision.="" while="" being="" consistent="" with="" a="" two-stage="" extrusion="" model,="" they="" demonstrate="" that="" the="" great="" geological="" discontinuity="" that="" separates="" indochina="" from="" china="" results="" from="" cenozoic="" strike-slip="" strain="" rather="" than="" more="" ancient="" suturing.="" furthermore,="" they="" suggest="" that="" this="" narrow="" zone="" acted="" like="" a="" continental="" transform="" plate="" boundary="" in="" the="" oligo-miocene,="" governing="" much="" of="" the="" motion="" and="" tectonics="" of="" adjacent="" regions.="" 700="" and="" 200="" km="" of="" left-lateral="" offset="" on="" the="" asrr="" shear="" zone="" and="" wang="" chao="" fault="" zone,="" respectively,="" would="" imply="" that="" the="" extrusion="" of="" indochina="" alone="" accounted="" for="" 10-25%="" of="" the="" total="" shortening="" of="" the="" asian="" continent.="" the="" geological="" youth="" and="" degree="" of="" exhumation="" of="" the="" asrr="" zone="" make="" it="" a="" worldwide="" reference="" model="" for="" large-scale,="" high-temperature,="" strike-slip="" shear="" in="" the="" middle="" and="" lower="" crust.="" it="" is="" fair="" to="" say="" that="" this="" zone="" is="" to="" continental="" strike-slip="" faults="" what="" the="" himalayas="" are="" to="" mountain="">