Early-middle Triassic Paleomagnetic Result From The Hexi Corridor, North China And Its Tectonic Implications

The eastern Asia is composed of three major continental blocks:the North China Block (NCB), the South China Block (SCB) and Tarim Block, which contained different ages of Precambrian basement. The NCB and Tarim block were considered as a single tectonic unit since long time as named as Sino-Korean-Tarim block. However, the paleomagnetic results indicate a significant different of paleolatitudes during the Carboniferous and Permo-Triassic between the NCB and Tarim block. It is now more and more geological data that are inclined to separate these two blocks by the Jurassic, even if the geological boundary is not clear in between. The Alashan-Hexi Corridor (AHC) is a juncture between the NCB and Tarim block, which is considered as an independent terrane. The Mesozoic paleomagnetic results from the AHC will therefore provide a further constraint on the relationship between the NCB and Tarim block.We sampled the early-middle Triassic sediments from eastern of Hexi corridor, and carried out rock magnetic and paleomagnetic measurements. The Xidagou group of Hexi corridor had well-exposed and complete sedimentary sequence. A total of 25 sites were collected from early-middle Triassic sediments along three separated sections and the Xidagou group consists largely of red, purple, light gray or grayish green sandstones and siltstones. The characteristic higher temperature component (HTC) (Ds=304.1°, Is=38.4°, ks=33.2, A95=5.2°) isolated by stepwise thermal demagnetization exhibits dual polarity, which passes the fold and reversal tests. Compared with paleomagnetic coveal results from the NCB, this result suggests that the paleolatitude differences between the observed at Jingtai area and those expected from the NCB reference early and/or middle Triassic poles are Pλ=-0.9°±7.3°and Pλ=4.9°±5.9°, respectively. This implies that the Hexi Corridor was the extension of the NCB by the middle Triassic. However, in comparison with early and middle Triassic results from the NCB, an anticlockwise rotation of studied area relative to the NCB with Rd=22.8°±4.9°or Rd=24°±4.4°, is detected, which might be illustrated by the ball-bearing model.