Paleomagnetic Study On Middle-late Neoproterozoic Strata Of South China And Its Paleogeographic Implications

The Neoproterozoic is an anomalous period in Earth's history.Postulated extreme climatic,geological and biological fluctuations are unparalleled in the other geological record.These climatic and biological transformations occurred against a backdrop of a disintegrating Rodinia supercontinent.Reconstructing the paleogeography,one of the most important factors in climate control,is essential for understanding these unusual conditions.The Neoproterozoic paleogeography,however,is a matter of considerable debate that is fuelled by discordant paleomagnetic data.Recently,due to more debates,the paleogeographic position of the South China block in the Rodinia had been identical to reconstruct the framework of the Rodinia.The center of debates is the relationship of the South China and the Australia.Although a series of propositions for the close proximity between South China and Australia are suggested,their relative positions are varied.To better constrain the paleoposition of South China,we have carried out a new systemic paleomagnetic study from the Middle-Late Neoproterozoic strata at Three Gorges area,in Hubei province,China.Paleomagntism,rock magnetism,Scanning Electronic Microscope and the SHRIMP U-Pb Zircon dating are used to confirm the quality of the results.Three magnetic components were isolated by stepwise thermal demagnetization from the Qinglinkou section Liantuo Formation Member 1.The low temperature component falls close to the local current Earth's field direction.The VGP derived from the intermediate temperature component coincides with the Triassic poles from South China,suggesting a pervasive Triassic remagnetization.In addition,a high temperature component 'H' with both reversed and normal polarity was revealed up to 680?,and some unblocked at 660?,with a mean direction of Dg=77.9°,Ig=34.2°,kg=69.5,?95=5.0° in situ;and Ds=89.8°,Is=64.5°,ks=45,?95=6.2°after tilt correction.A change of magnetic polarity,with 4 magnetozones defined,is revealed in our sampling section,demonstrating the primary nature of the 'H'component.Stepwise thermal demagnetization reveals that a late Cretaceous remagnetization(component 'L',350-500?)was removed from Liantuo Formation in Yichang area.A medium component('M')isolated mainly between 350? and 600 ?,but in some cases with an upper limit of 640 ?,yields a mean direction of Dg=59.5°,Ig=64.6°,?95=3.7° in situ;and Ds=81.6°,Is=61.5°,?95=4.1° after tilt correction.In addition,a high temperature component 'H'(Ds=102.5°,Is=62.4°,?95=3.7° after tilt correction)with both reversed and normal polarity was revealed up to 690 ?.A change of magnetic polarity,with 5 magnetozones defined,is revealed in our sampling section,in which a reversed-normal polarity succession in the upper part of the section may be correlative with that obtained in a section 6.5 km away by Evans et al.(2000),demonstrating the primary nature of the 'H'component.Three magnetic components were isolated by stepwise thermal demagnetization from the Ediacaran strata.The low temperature component falls close to the local current Earth's field direction.The intermediate temperature component coincides with the Jurassic poles from South China,suggesting a pervasive Jurassic remagnetization.In addition,a higher temperature component was isolated between 300? and 480?540?.By comparing the available Phanerozoic poles from South China,the higher temperature component from the Qinglinkou(QLK)section may reflect a late Triassic remagnetization.However,the results from the Tianjiayuanzi(TYZ),Jiulongwan Member2(JLWE)and Sanxiarenjia(SXRJ)sections were afected by a Silurian remagnetization.The remagnetization events revealed by our study can be explained by the multiple generations of oil and gas in the Ediacaran and Cambrian strata(Zhu et al.,2015).In contrast,the result from the Jiulongwan Member3(JLWS)section may have escaped late remagnetization.A more reliable Ediacaran pole(JLWS:31.3°N,169.8°E,dp/dm=3.5°/4.7° tilt corrected)was calculated using the results from the JLWS section,the unaffected results of Zhang et al.(2015)and QLK section.AMS of the Liantuo Formation in both Zigui and Yichang are sedimentary fabric,suggesting that there were no affections of stress and strain on the strata in this area.Together the results of previous studies on the Liantuo Formation in the Three Gorges area with our own SHRIMP U-Pb dating age,we suggest that the Liantuo Formation Memberl in the Three Gorges area deposited during 780-750Ma,while the Member2 deposited during 750-700Ma.In addition,the upper most 20m should be younger than 735Ma.Due to the big age range of the Liantuo Formation,we use the results of Memberl and Member2,respectively.Mean directions are Ds=92.9°,Is=64.7°,N=15,ks=49.7,?95=5.5°for Memmberl;Ds=99.7°,Is=63.4°,N=11,ks=135.9,?95=3.9°for Member2(after tilt correction).Both pass the reversal test(McFadden and Lowes,1981)and the fold tests(McElhinny,1964;McFadden,1990).An age of 720 Ma was assigned to Member2 results and its corresponding pole,while 760Ma to the Member1.Their poles are 20°N,157.8°E,dp/dm=7.1°/8.8° for Memberl(LTM1);15°N,157.3°E,dp/dm=4.9°/6.2°for Member2(LTM2).After carefully reevaluating the Neoproterozoic poles and apparent polar wander paths(APWPs)from Australia and South China,we suggest that the South China block may be not far from the northwest of Northern Australia at 760Ma.And then moved to the Northern Australia and collide with it before 635Ma.During Ediacaran,south China block was located to the northwest of Northern Australia.Subsequently,the South China block moved to the northwest of Western Australia through counter-clockwise rotation of the Northern Australian craton at the later time of 550 Ma,or in the middle Cambrian,which may relate to the activating of the Darling Fault of the West Australia.