Paleomagnetism of Middle Triassic Lavas From Northern Qiangtang (Tibet): Constraints on the Closure of the Paleo-Tethys Ocean

We present results from a paleomagnetic study of Middle Triassic lavas (similar to 242-240 Ma) from the northern Qiangtang block to improve our understanding of the timing and kinematics of the closure of the eastern Paleo-Tethys Ocean. Characteristic remanent magnetization directions carried by magnetite and hematite formed during high-temperature oxidation during initial cooling of the lavas are successfully isolated by progressive thermal and alternating field demagnetizations. We bin 28 site mean directions into 20 independent direction groups that pass the fold test, average secular variation, and show no rock magnetic or petrographic evidence of weathering. These data establish the first lava-based paleomagnetic pole of Middle Triassic age for the northern Qiangtang block (62.2 degrees N, 196.4 degrees E, A(95) = 5.6 degrees). We compare this pole to coeval poles from the North China Block and Tarim and estimate that the remnant Paleo-Tethys or IIoh-Xil-Songpan-Ganzi Ocean could have been up to 700 km at similar to 240 Ma. A comparison of the Triassic latitudinal history between the northern Qiangtang block and the North China Block and Tarim shows that the closure of the Paleo-Tethys Ocean most likely occurred during the latest Triassic. Our review of published Permian-Triassic poles from the northern Qiangtang block show that the average south-north plate velocity of the terrane decelerated from similar to 8.5 cm/yr during similar to 300-240 Ma to similar to 3.6 cm/yr during similar to 240-210 Ma. We suggest that an arc-continent collision between the northern Qiangtang block and a Palco-Tethys or Yidun arc around or before 240 Ma contributed to this change in plate velocity.