Paleomagnetism and Its Tectonic Implication of the Red Beds of Oligocene Kangtuo Formation in the Qinghai-Tibetan Plateau

Abstract:

Available paleomagnetic data show that after the collision, additional convergence of ~3000 km took place between the Indian Craton and Eurasian plate, of which ~1800 km was between the Indian Craton and the Lhasa terrane. According to the “fill-the-gap” solution, this shortening is interpreted as the evidence for the existence of the “Greater India”. Intraplate shortening takes place often in the forms of folding and thrusting, and the major thrusts in the Himalayan orogen remained active after the Oligocene. Estimating the amount of the shortening quantitatively is of great importance for understanding the tectonic modeling for the evolution of this magnificent collision zone. To better understand the continental shortening after the India-Eurasia collision, 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, whereas a high temperature component (HTC) unblocks at ~665-690°C. The HTC distributions show a clear east-west elongated distribution, which is considered as reflecting inclination flattening. After inclination calibration using the E/I method, the HTC could pass both a reversal test and a fold test at 95% confidentce level, showing the mean direction at Ds=340.3°, Is=44.2°, with k=54.9, and α₉₅=3.3°, corresponding to a paleopole at 71.7°N, 340.7°E (A₉₅=3.3°), and the paleolatitude of the sampling site at 25.9±3.3°N.

Comparing our new data with the apparent polar wander paths of East Asian blocks, Europe, and India, we have reached the following conclusions. (1) There is no significant paleolatitudinal difference between the Lhasa terrane and other central and northern Asian terranes at ~30 Ma. (2) The observed paleolatitude of the Kangtuo Fm is 8.0±5.0° lower than the expected paleolatitude deduced from the data of stable Europe, but is 4.6±5.2° higher than that deduced from the data of the India Plate, likely presenting a ~506±572 km shortening between the India and Lhasa terrane since the Oligocene.