A Late Cambrian True Polar Wander And Widespread Early Mesozoic Remagnetization In South China:Paleomagnetic Evidence From The Cambrian Marine Strata In South China

Great changes happened to the Earth during the Ediacaran-Cambrian when Rodinia fragmented followed by the aggregation of Gondwana.This period also witnessed dramatic changes of global climate and ecosystem.An intriguing explanation to these changes is the Early Cambrian true polar wander(TPW)hypothesis.However,by far none consensus has come to this hypothesis due to different opinions on selecting the paleomagnetic data during that interval.Testing the TPW hypothesis needs to acquire high-quality and well time-constrained paleomagnetic data from as more plates as possible.The Cambrian strata in South China have been subject to intensive investigation for decades,and,especially,several established Cambrian global standard stratotype-sections and points(GSSPs)and Cambrian GSSP candidates are located in South China,thus providing excellent time constraints on the strata.A detailed paleomagnetic study was conducted on the Cambrian succession of the Duibian section,in the Jiangshan county,western Zhejiang,to examine the Cambrian TPW hypothesis,and to provide tight constraints on the timing of the TPW,if any.Then,to further examine the Late Cambrian TPW discovered in Jiangshan,a detailed paleomagnetic investigation was carried out on the Upper Cambrian of the Wa ergang section,in the Taoyuan county,western Hunan.The whole Cambrian succession of the Duibian section outcrops very well.The succession is dominated by carbonates.Pre'vious biostratigraphic studies have established continuous trilobite zones,thus providing excellent chronological controls for the investigated succession.Three natural magnetization remanence(NRM)components are isolated by stepwise thermal demagnetization from the paleomagnetic samples.The low-and intermediate-temperature components fail to pass the fold test and represent secondary overprints.The high-temperature component passes the fold test and reversals rest.Rock magnetic results indicate that the remanence carriers are pseudo-single-domain magnetite or low-Ti magnetite grains.Scanning electron microscopic(SEM)and transmission electron microscopic(TEM)results show a detrital origin for magnetite grains.These results strongly suggest primary nature for the high-temperature component.Further analyses reveal that a directional shift of the high-temperature component occurs between the Lower-Middle Cambrian and the Upper Cambrian.This directional shift is tentatively interpreted to indicate a?570 polar wander from?500.5 to?494 Ma.The rate of polar wander is?8.8 ° Myr-1.Because the rate of polar wander is too fast to be a tectonic origin,a preferred explanation for this polar wander is a Late Cambrian TPW,which is distinct from the previously proposed Early Cambrian TPW.This TPW appears to be coeval with the Steptoean positive carbon isotope excursion(SPICE)and the major trilobite mass extinctions,suggesting a potential link between the TPW and the Late Cambrian climatic and biotic changes.This Late Cambrian TPW event is exceptionally well-dated,thus it could be testable through examination of the sections elsewhere.To further examine the authenticity of the Late Cambrian TPW that was discovered in Jiangshan,a detailed paleomagnetic survey was carried out on a continuous Upper Cambrian succession of the Wa'ergang section.This section contains well-defined trilobite zones correlative with those from the Jiangshan section.Additionally,the biostratigraphic data show that the succession of the Wa'ergang section appears to have faster sedimentation rates than that of the Jiangshan section and thus has the potential to reveal the internal processes of the Late Cambrian TPW.The sampled interval belongs to the Upper Cambrian and consists of limestone and argillaceous limestone.Stepwise thermal demagnetization uncovers three remanence components from the Upper Cambrian limestone.The low-and intermediate-temperature components fail to pass fold tests,thus representing secondary magnetizations.The high-temperature component passes fold tests,but its corresponding pole is insignificantly different from the Middle Triassic pole of South China,indicating a Middle Triassic remagnetization.Rock magnetic and SEM results indicate that the high-temperature component resides in small framboidal magnetite aggregates that were formed via alteration and replacement of pyrite and thus represents a chemical remagnetization.SEM results also imply that the chemical remagnetization was likely caused by external fluids.This Middle Triassic remagnetization and previously recognized Triassic remagnetization exhibit a younging trend from the east to the west of South China.The widespread Triassic remagnetization in South China seems linked to the Indosinian orogeny during which the scissor-like collision processes between the South and North China Blocks and the so-called squeegeed orogenic fluids led to progressively westward younging remagnetization in South China.Although thorough remagnetization of the Wa'ergang section precludes us from effectively examining the Late Cambrian TPW,the new paleomagnetic results provide important insights into understanding the Early Mesozoic remagnetization and associated geological processes in South China.