Paleomagnetical Research On Qiangtang Block Of Late Triassic To Early Cretaceous

Qiangtang block is the main block of Qinghai-Tibet plateau belonging to estern Tethys thetonic domain. Research on Qiangtang block whose formation and evolution involve close of paleo-Tethys ocean and open of new Tethys ocean is very impotant to problems including tectonic evolution, kinematics and dynamics of Qinghai-Tibet plateau. In the paper, Mesozoic layer in Tanggula Mountain region north to Qiangtang block is the target stratum, for the profile’s stratigraphic age is clear and the stratum’s contact relation is positive. We hope that paleomagnetic research on Mesozoic layer of the region can provide quantitative constraint to tectonic evolution of Qiangtang block and formation of new Tethys ocean.15points in which 9 points belonged to Jiezha goup of Late Triassic and 6 points belonged to Cuojuri group of early Cretaceous were setted in Tanggul a Mountain region, and 105 directed samples were gotten tatally. They were m achined into guide sample to be experimented with rock magnetism and demag netizm in Paleomagnetic and chronology laboratory of CAS.According to rock magnetism result of Jiezha goup in Late Triassic, the main magnetic minerals contain hematite, pyrrhotite, a small amount of titanium magnetite and little magnetite. While he main magnetic minerals of Cuojuri group in early Cretaceous contain hematite and little goethite.In terms of mineral types, appropriate demagnetizing methods of thermal demagnetization, alternating demagnetization and mixed demagnetization and demagnetizing steps are formulated. Through the analysis with kirschvink and statistics with Fisher, high temperature components in which the component of Jiezha goup undergoes the fold test and the component of Cuojuri group pass the reverse test can be seperated, so high temperature components are thought to be original residual magnetism.Through the above analysis and calculate, high temperature component’s a verage direction of Jiezha goup is:Ds=4.2°, Is=-29.7°, Ks=24, α95=10.7°; P aleomagnetic pole position calculated is:φp=259.4°, λp=71.5°, dp/dm=8.8°; P aleolatitude is 15.9°N. The direction of Cuojuri group is: Ds=154.8°, Is=-43.2°, Ks=19.2, a95=15.7°, Paleomagnetic pole position calculated is:λp=66.3°, λp=34 6.2°, dp/dm=15.4°, Paleolatitude is 25.2°N.According to paleomagnetic reults in the paper combing previous research on Lhasa, graphs of APWP and paleolatitude are drew. It is concluded that ne w Tethys ocean represented by Bangonghu-Nujiang suture zone spreaded most widely in late Triassic, and the maximum differrence value of the paleolatitude s between Qiangtang (Φ古=15.9) and Lhasa (Φ古=-16.6) is 32.5°at least, which shows that scale of Bannu ocean which began to shrink after late Triassic is 3600km at least. Based on APWP, Qiangtang block showed apparent phenomen on of "pole shift" and rapid northern drift from late Triassic to middle-late Jur assic. From middle-late Jurassic to early Cretaceous, Qiangtang block drifted sl owly and rotated deasil. APWP and paleolatitude analysis of Qiangtang block a nd Lhasa block indicate that Bangonghu-Nujiang ocean closed because of Lhas a block’s northern drift in middle Jurassic.