Paleomagnetism And Its Tectonic Implication Of The Red Beds Of Oligocene Kangtuo Formation In The Qinghai-Tibetan Plateau

The collision of the India Craton and Eurasia plate was one of the most important eventsin the cenzoic. the collision caused the formation of the Tibet Plateau and extensive developingdeformation in the Eurasia. The time of the collision and the uplifting progress of the TibetPlateau are still confusing us. The disappearing progress of the "Greater India" is a key tounderstand the questions. Based on the studies, the faults and thrusts in the Himalayan regionactuated since the Oligocene. By estimating the shortening we can know the scale of theGreater India in the Oligocene, and provide a way for understanding the evolution of the TibetPlateau.A paleomagnetic study on the red beds of the Oligocene Kangtuo Formation (Fm) wascarried out in the Gerze Basin of the Lhasa terrane. A total of700samples were collected from37sites. Stepwise thermal demagnetization revealed that the main magnetic carrier is hematite.The natural remnant magnetization （NRM） consists of two components. A low-temperaturecomponent (LTC) is identified below300°C and is interpreted to be a recent viscous overprint,whereas a high temperature component (HTC) unblocks at665-690°C and is interpreted to bethe primary magnetization. The HTC distributions show a clear east-west elongateddistribution, which is considered as reflecting inclination flattening of deposited magneticremanence carriers. After inclination calibration using the E/I method, the HTC could passboth a reversal test and a fold test at95%confidentce level, showing the mean direction atDs=340.3°, Is=44.2°, with k=54.9, and α95=3.3°, corresponding to a paleopole at71.7°N,340.7°E （A95=3.3°）, and the paleolatitude of the sampling site at25.9±3.3°N. Thepaleolatitude is consistent with that expected from the coeval pole of the Qiangtang terraneobtained from volcanic rocks, suggesting that there has been no paleomagnetically-discernablelatitudinal motion between the Qiantang and Lhasa terranes since~30Ma.Comparing our new data with the apparent polar wander paths (APWPs) of East Asianblocks (Cogne et al.,2013), Europe, and India （Besse and Courtillot,2002）, we have reachedthe following conclusions.(1) There is no significant paleolatitudinal difference observedbetween the Lhasa terrane and other central and northern Asian terranes at~30Ma.(2) Theobserved paleolatitude of the Kangtuo Fm is8.0±5.0°lower than the expected paleolatitudededuced from the data of stable Europe, highlighting the ‘Asian inclination anomaly’phenomenon, but is4.6±5.2°higher than that deduced from the data of the India Plate, likelypresenting a506±572km shortening between the India and Lhasa terrane since the Oligocene.It is inferred that this amount of shortening has been absorbed mainly by the HimalayaOrogenic Belt.