A first palaeomagnetic study of Jurassic formations from the Qaidam basin, Northeastern Tibet, China-tectonic implications.

International audienceWe present a new palaeomagnetic study on Upper Jurassic red beds collected at nine sites near Huatugou (38.46°N, 90.75°E) in the Qaidam basin. Thermal demagnetization up to 690 °C shows both low- and high-temperature components (LTC and HTC, respectively). LTC seems to be a recent overprint, HTC, carried principally by haematite and presenting a sole reverse polarity, passes the fold test at the 95 per cent confidence level. The palaeomagnetic pole calculated from the tilt-corrected overall mean direction (Ds= 220.4°, Is=-34.8°, 95= 7.5°) is situated at 50.1°N, 198.0°E (dp/dm= 5.0/8.6) . The comparison of this result with coeval palaeomagnetic poles from Tarim, the North China Block (NCB) and the apparent polar wander path of Eurasia reveals a large-scale tectonic evolution of the Asian continent since the late Jurassic. This study suggests that the northward convergence of the Qaidam block is statistically negligible with respect to Tarim (4.5°± 8.2°) since Late Jurassic times. However, a significant relative clockwise rotation of the studied area with respect to Tarim seems to have occurred (16.2°± 11.2°) . Whether this rotation is representative of the whole Qaidam basin or is of a local character is yet to be confirmed. The important feature of this study resides in the difference of northward displacement of the Qaidam block with respect to Eurasia on one hand and with respect to the NCB on the other hand. The overall NS convergence absorbed between Qaidam and Eurasia is 19.3°± 9.2° , whereas that absorbed between the Qaidam and the NCB is 8.0°± 6.2° . We suggest that the latter value is the result of the India-Asia collision, while its difference with respect to the former would be absorbed during the Mongol-Okhotsk ocean closure at the end of the Jurassic or at the Jurassic-Cretaceous boundary at the latest