Structure Feature And Formation Mechanical Of Dabashan Arcuate Orogenic Belt

Dabashan arcuate orogenic belt,lociated the north margin of Yantzeblock,belongs to transiztional zone of Sichuan basin and Qinling orogenic belt.It’s aideal area for understanding basin-range coupling and intracontinentorogenesis.Arcuate orogenic belts are widespread in most of huge orogenicbeltaround the wrold.Strucutre analysis and paloemagnetic are most power methodsof studying arecutate orogenic belt.This thesis is based on structuregeology,combined with low-temperature theromochronology,geophysics andbalanced cross section,to research structure characteristics of Dabashan arcuateorogenic belt.Dabashan arcuate orogenic belt is bounded by Tiexi-Wuxi fault(TWF) to southand by Chengkou-Fangxian fault (CFF) to north, connectting with Micang-Hannanuplift to west and with Shennongjia-Hanling upliftto east.This arc belt is divided tonorth fold-thrust belt and south detachment belt by Zhenba-Jichang fault(ZJF).Faultsare common in north fold-thrust belt,including imbricate faults,duplex,popup andback thrust.Tight and colsed folds are main fold styles,and chervron folds aresecondary.But foldsare more popular than faults in south detachment belt.Closedfold are primary,and box foldsare secondary. CFF plays an important in formationof Dabashan arcuate orogenic belt.It may be formed under extension movementbefore Yangtze and North blocks collision.Then it’s reactivated in the process ofcollision and intracontinent orogenism by thrusting and strike-slip.A lots of plungefolds suggest that strike-slip movement is very strong.But right-handed slip occurs inthe western part of Dabshan arcuate orogenic belt,and left-handed slip are present inthe central and eastern part. Blanced cross section shows that shortening decreasefrom two ends to the central part.Microtectonis of Shiquan-Ankang fault(SAF) and CFF indicate the deformationmechanism and temperature-pressure conditions. Quartz gain with preferenceoriented is undulating and banded extinction in SAF. Sub-grain rotation is the maindynamic recrystallization. All evidence suggest ductile deformation occurred nearSAF.Quartz c axis slip is basial or(and) diamond sliding system, indicating the deformation temperature is under350-450℃.Flinn index is larger than1,so interstrain is extension ellipsoid. But dynamic recrystallization did not occur in quartzgarin of CFF. Brittle-ductile deformation happen because basial slip is main slipsytem.Flinn index are less1,so inter strain is flatten ellipsoid. The long axis ofellipsoid of SHF is horizontal, but it is vertical from Chengkou to Zhengpin. It’concluded that CFFwere reactivated again during thrusting.Kinematic vorticityshows simple and pure shear are coexist.Magnetic fabric have similary characteristics along Dabashan arcuate orogenicbelt. Magnetic foliation are more development than magnetic lineation alongDabashan orogenic belt.The western is best in magnetic folation,but the eastern issecdonary and the central is last. Flinn index indicate flatten ellipsoid. Pj is less than1.20.Dabasahn arcuate orogenic belt is medium-low strain. Magnetic lineationorients to150°and60°and Magnetic foliation is NW and NNW in western part.Magnetic lineation orients to140°and65°and Magnetic foliation is NW and ENE incentral part. Magnetic lineation orient to90°and40°and Magnetic foliation is E-W ineastern part. The magnetic fabric show that at least two deformations areoccurred.Kmax-Strike slop is0.63±0.03.So it’s a conclusion tha Dabashan arcuateorogenic belt is progressive arc with primary and secondary arc feature and the bendis bigger than50%.Multiple low-tempreature thermochronology indicate that the most importanttectonic event happen in early Cretaceous and Miocen.All thermochronology datasuggest that two massifs on the two ends of Dabshan arcuate orogenic belt upliftearlier than the central Dabashan.Dabashan arcuate orogenic belt deformation are controlled by many factors. Atfirst, Permian riftting before the collision between South and North China blocksresulted in an arc normal fault which becomes more bend by later NW compressionstress.Two massifs uplift earlier than central part and obstruct during thrusting fromNE to SW.There are four detachment layers and different basement depth,so themovement rate in central part is quicker than western and eastern part.All factors areprone to form so perfect arcuate orogenic belt. As for asymmetry, there are somedifference in thickness of detachment, basement and lithosphere depths andlithosphere temperature along the strike of arcuate orogenic belt that result indifference in structure feature.Evolution history of Dabashan arcuate orogenic belt are build up based on ourstudy.N-S compression were occurred during collision between South and NorthChina blocks.NE compression followed form late Jurassic to early Cretaceous,and thrustingandfolding propagate toward forebasin form NE to SW. A short extensionhappenedunder Qinlin orogenic belt collapes on late Cretaceous. CFF reactivatedwith dextral-slip by NNW compression from the far field of Tibet uplift to east.