| The Tibetan Plateau is generally acknowledged to belong to the eastern Tethyan domain, its formation and evolution have long been a research focus for geologists. The Triassic was one of the critical periods in the evolution of the Tibetan Plateau during which a series of major historical geologic events had happened, such as the north-south migration of the main Terranes, the accompanied contraction and closure of the Paleo-Tethys, and the gradual opening of the Meso-Tethys. The Lhasa and Qiangtang Terranes, main terranes consist of the Tibet Plateau, are located at hinterland of the Tibet Plateau. Kinematic characteristics of the two terranes are closely related to the geological event such as the closure of the Paleo-Tethys and the gradual opening of the Meso-Tethys. Paleomagnetism is an effective approach to the past tracks of plates and terranes. In order to better constrain the evolution of the Bangong-Nujiang Tethyan, a combined paleomagnetic and rock magnetic study was conducted on the Triassic strata of the Lhasa and Qiangtang Terranes in Tibetan Plateau, China.Orientated samples were collected from 32 sites (327 samples) on the side of the Dibu Co Lake, Coqen County, in the western region of the Lhasa terrane. Rock magnetic data revealed that the most of the samples were dominated by magnetite and/or pyrrhotite. The stepwise demagnetization curves illustrated three-components:a low temperature component nears the present-day field (PDF), a secondary remanent magnetization that may be from the Cretaceous Period, and a high-temperature component (Component C). The Component C of the Early-Middle Triassic rocks passed a reversal test (B class,95% confidence level) and a fold test (99% confidence level), that of the Late Triassic rocks passed a fold test (95% confidence level). The corresponding paleopoles for the Early-Middle and Late Triassic periods of the Lhasa Terrane were at 19.3°N.209.3°N with A95= 4.0° and 19.6°N.211.8°E with A95= 10.7°, the paleolatitudes are 16.9±4.0° S and 18.4±10.7° S, respectively.Orientated samples were collected from 26 sites (249 samples) on the north part of the Qiangtang Terrane. Rock magnetic data revealed that most of the samples were dominated by magnetite and/or pyrrhotite. Two components were isolated as a low and high temperature component (LTC and HTC) from the Early Triassic and the Late Triassic rocks. Owing to the effect of re-magnetization, we failed to isolate the reliable HTC from the Middle-Late Triassic rocks. The HTC of the Early Triassic and the Late Triassic positively passed fold tests at high confidence level suggest primary origin of the remanent magnetization. The corresponding paleopoles are 23.8°N,210.3°E, with A95= 10.9° and 70.6°N,275.6°E with A95= 6.1 The corresponding paleolatitude are 11.4±10.9°S and 14.8±6.1°N, respectively.Our new paleomagnetic results, together with previously published paleopoles, demonstrate that the Lhasa Terrane was located at mid-low latitudes in the southern hemisphere from the Paleozoic to the Mesozoic, then amalgamated with Eurasia in the Cretaceous. From the Paleozoic to the Early Triassic, the Qiangtang Terrane located at mid-low latitudes in the southern hemisphere, massive scale northern displacement (about 2880km) taken place from the Early to the Late Triassic, and reached 14.8±6.1°N in the Late Triassic. Then northern moved continually but the speed and the scale are less than the Triassic period. So we speculate that the Tethys Ocean, as partly represented by the Bangong-Nujiang suture zone (BNS) between the Lhasa and Qiangtang Terranes, gradually opened up in the Early-Middle Triassic and rapidly expanded during the whole Triassic period. The quick expansion of the Bangonghu-Nujiang Tethyan during the Triassic period may have droven the Qiangtang Terrane to move rapidly northwards, prompting its amalgamation with the main body of the Eurasia. |
PDF Full Text Request