Middle Jurassic Palaeomagnetism Of The Qaidam Basin,China,and Their Implications

Palaeomagnetic study was carried out on the Middle Jurassic strata near Delingha city of the Qaidam Basin.Stepwise thermal demagnetization successfully isolated stable higher temperature component?HTC?,which passed reversal test in classification C(?=1.5°<?_critic=16.4).The tilt-corrected site mean direction of this HTC is Ds=20.3°,Is=49.3°,ks=25.2,?₉₅=7.2°,corresponding to a pole at 71.8°N,202.9°E with A₉₅=7.1°and a paleolatitude of 30.2?6.3°N for the Qaidam Block during the Middle Jurassic.The anisotropy of magnetic susceptibility results suggest that our samples have not experienced significant strain due to tectonic stress or compaction.Corrected results of the elongation/inclination model indicate that no significant inclination shallowing present in our samples.Comparison with coeval palaeomagnetic results from the North China Block?NCB?suggests that insignificant northward motion and relative rotation have occurred between the Qaidam Block and the NCB after the Middle Jurassic.Combining reported lithofacies and UP-b evidences,we suggest that the closure of the Zongwulong Ocean,or collision between the Qaidam and Alashan-NCB blocks,might have happened during the early Late Triassic and finished before the Middle Jurassic.