New Early Cretaceous Paleomagnetic Results From The Zenong Group Volcanic Rocks In Cuoqin Area, Tibet And Their Paleogeographic Implications

Because the Lhasa terrane was the southern margin of Eurasia during late Mesozoic and early Cenozoic, its paleoposition is essential to reconstruct pre-collisional paleogeography. This is, in turn, is key to understanding the India-Asian collision process and to estimating the magnitude of intracontinental deformation within Asia. However, Cretaceous paleomagnetic data from the Lhasa terrane is quite insufficient and most results came from red beds which might be affected by inclination shallowing due to compaction. Furthermore, the distribution of cretaceous paleomagnetic results in the Lhasa terrane is quite uneven, particularly in Midwest of the Lhasa terrane. And there is some difference in the age.We carried out paleomagnetic and rock magnetic studies on the early Cretaceous volcanic rocks (110-130Ma) from the Cuoqin, Lhasa terrane. The rock magnetic experiments consisted of acquisition of isothermal remanent magnetization (IRM), stepwise thermal demagnetization of the three-axis IRM andχ-T curve. The thermal demagnetization and rock-magnetic results mentioned above indicate that the unstable VRM (viscous remanent magnetism) is carried by goethite and pyrrhotite, and the HTC (high temperature component) is carried by magnetite and hematite. The site-mean directions for 18 sites is Dg=318.5°, Ig=-2.1°, kg=22.7,α95=7.4°in geographic coordinates. After bedding corrected, the site-mean direction is Ds=327.1°, Is=35.6°, ks=59.4, a95=4.5°, corresponding to a paleopole at 58.3°N, 342.0°E, A95=4.8°, plat=20.1°±4.8°N. The HTC directions pass both McEIhinny (1964) and McFadden (1990) fold tests at 99% confidence level. In addition, the reversal test has been proved positive at the 95% confidence level according to McFadden and McElhinny (1990). Thus, the HTC is interpreted as a primary magnetization, and its corresponding paleopole should represent early Cretaceous pole of the studied area.Comparing our observed directions with the mean expected directions from stable Eurasia between 120-130 Ma, there is the difference of the paleolatitudes (7°). Our result supports that the paleolatitude of the southern Tibet was stabilized between 17°～20°N during the Paleogene and Cretaceous. Therefore we can infer that the north-south crustal shortening was caused by the collison and extrusion of the Lhasa terrane and stable Eurasia which had caused strong intracontinental deformation between north of the Lhasa block to the south of Siberia. Abundant geological and geophysical evidence indicate that frequent tectonic activities in Asia are often cited as the consequence of continental collision between India and Eurasia. The tectonic activities include extensive orogenies, subduction belts and strike-slip faults. Until now, acquirable geologic evidence supports that the crustal shortening had been absorbed by fold belts and strike-slip faults within Eurasia. Comparing the paleolatitudes between stable India and the Lhasa terrane indicates the width for the Neotethys ocean was 7159.5±266.4 km in the early Cretaceous.