Paleomagnetism of the Oligocene Kangtuo Formation red beds (Central Tibet): Inclination shallowing and tectonic implications

Abstract

A paleomagnetic study on the red beds of the Oligocene Kangtuo Formation (Fm) was carried out in the Gerze Basin of the Lhasa terrane. A total of 700 samples were collected from 37 sites. Stepwise thermal demagnetization revealed that the main magnetic carrier is hematite. The natural remnant magnetization (NRM) consists of two components. A low-temperature component (LTC) is identified below 300°C and is interpreted to be a recent viscous overprint, whereas a high temperature component (HTC) unblocks at ∼665–690°C and is interpreted to be the primary magnetization. The HTC distributions show a clear east–west elongated distribution, which is considered as reflecting inclination flattening of deposited magnetic remanence carriers. After inclination calibration using the E/I method, the HTC could pass both a reversal test and a fold test at 95% confidence level, showing the mean direction at Ds=340.3°, Is=44.2°, with k=63.0, and α95=3.1°, corresponding to a paleopole at 71.7°N, 339.3°E (A95=3.1°), and the paleolatitude of the sampling site at 25.9±3.1°N. The paleolatitude is consistent with that expected from the coeval pole of the Qiangtang terrane obtained from volcanic rocks, suggesting that there has been no paleomagnetically-discernable latitudinal motion between the Qiantang and Lhasa terranes since ∼30Ma. Comparing our new data with the apparent polar wander paths (APWPs) of East Asian blocks (Cogné et al., 2013), Europe, and India (Besse and Courtillot, 2002), we have reached the following conclusions. (1) There is no significant paleolatitudinal difference observed between the Lhasa terrane and other central and northern Asian terranes at ∼30Ma. (2) The observed paleolatitude of the Kangtuo Fm is 8.0±4.9° lower than the expected paleolatitude deduced from the data of stable Europe, highlighting the ‘Asian inclination anomaly’ phenomenon, but is 4.6±5.1° higher than that deduced from the data of the India Plate, likely presenting a ∼506±561km shortening between the India and Lhasa terrane since the Oligocene. It is inferred that this amount of shortening has been absorbed mainly by the Himalaya Orogenic Belt.