Paleomagnetism of Late Triassic Volcanic Rocks From the South Qiangtang Block, Tibet: Constraints on Longmuco-Shuanghu Ocean Closure in the Paleo-Tethys Realm

The South Qiangtang block of the Qinghai-Tibet Plateau represents an area critical to understanding the late Paleozoic and early Mesozoic history of the Tethyan realm, but its drift history remains poorly constrained. Here we report a new quantitative paleogeographic constraint for the South Qiangtang block from a paleomagnetic study of Late Triassic volcanic rocks of the Xiaoqiebao Formation. A characteristic remanent magnetization isolated from 25 sites passes both fold- and reversal tests, and likely represents a primary magnetization. On the basis of these data, we estimate that the South Qiangtang block occupied a paleolatitude of 30.1 +/- 4.6 degrees N at ca. 222 Ma. When combined with existing paleomagnetic constraints, these new results indicate that the South Qiangtang block (and other "Cimmerian" blocks) moved rapidly northward (in true latitude) between the middle Permian and Late Triassic. Our new data further suggest that the southern branch of the Paleo-Tethys (Longmuco-Shuanghu Ocean) likely closed by the mid-Late Triassic. The Paleo-Tethys was a major eastward-widening oceanic domain that separated eastern Gondwana and eastern Laurasia during Carboniferous-Permian time. The eventual disappearance of this ocean coincided with the amalgamation of the terranes comprising the modern Qinghai-Tibet Plateau. However, the plate kinematic history that led up to this suturing remains poorly constrained. In particular, the South Qiangtang block, which is thought to have formed the southern margin of the system, is a key area in need of additional constraints. In this work, we present new paleomagnetic results which indicate that the South Qiangtang block drifted rapidly northward between the middle Permian and Late Triassic (at an average south-north speed of similar to 13.4 cm/yr) to arrive to a paleolatitude of 30 degrees N by 222 Ma. Such a position suggests that the southern branch of the Paleo-Tethys (Longmuco-Shuanghu Ocean) may have closed by this time. The South Qiangtang block was located at similar to 30 degrees N in the Late TriassicIn the Permo-Triassic the South Qiangtang block drifted rapidly northward at similar to 13.4 cm/yrThe Longmuco-Shuanghu Ocean closed no later than ca. 222 Ma