Paleomagnetic results from Jurassic-Cretaceous strata in the Chuangde area of southern Tibet constrain the nature and timing of the India-Asia collision system

Abstract

The Tethyan Himalaya sits between the India craton and the Lhasa block, the contact being marked by the Indus-Tsangpo suture zone (ITSZ). The amalgamation of the Tethyan Himalaya with the Lhasa block has been traditionally regarded as representing the India-Asia collision. Recently, however, a dual-collision model has been proposed, in which Tethyan Himalayais was considered as a separate block that rifted from Greater India’s northern margin in Late Cretaceous. The Tethyan Himalaya subsequently underwent two episodes of collision, i.e., the first with Lhasa block to the north ~50 Ma and the second collision with India to the south ~23 Ma. The dual-collision model hinges on the speculated rifting of Tethyan Himalaya from Greater India in Late Cretaceous, which, however, has not been paleomagnetically constrained because the Late Cretaceous paleomagnetic data from Tethyan Himalaya are lacking. Clearly, elucidating the Mesozoic, particularly the Late Cretaceous, tectonic evolution of Tethyan Himalaya is crucial to understanding the India-Asia collisional processes. To better constrain the paleogeographic position and drift history of Tethyan Himalaya during the Mesozoic, particularly in Late Cretaceous, a paleomagnetic investigation was conducted of the Xiare and Weimei (Jurassic) and Chuangde (Cretaceous) formations in the Chuangde areas of southern Tibet. Paleomagnetic samples were collected from 39 sites, among which 5 sites were from grey andesite of the Xiare Formation, 25 sites were from siltstone and sandstones of the Weimei Formation, and 9 sites were from limestones of the Chuangde Formation. A total of 118 paleomagnetic specimens were subjected to stepwise thermal or alternating field demagnetization. The demagnetization results show that samples from the Xiare Formation and the Weimei Formation did not yield characteristic remanent magnetization (ChRM). Specimens from the Chuangde Formation yield stable ChRMs upon demagnetization that display both normal and reversed polarities, and passed a reversal test. The anisotropy of magnetic susceptibility (AMS) data of samples from the Chuangde Formation show that the minimum axes are sub-vertical and the intermediate and maximum axes are subparallel to the bedding, indicative of a depositional-type fabric. Temperature-dependence magnetic susceptibility ( k - T curves) and stepwise thermal demagnetization of the composite isothermal remanent magnetization (IRM) indicate that the remanence is carried principally by hematite and magnetite. Taken together, the sedimentological, rock magnetic, and paleomagnetic results suggest that the Chuangde ChRMs likely represent primary remanence that was acquired during 86.3–74.0 Ma, the age being constrained based on the foraminera data. The mean of the ChRMs of specimens from the Chuangde Formation is D s=152.0°, I s= - 52.9°, κ =18.0, α 95=5.9° after tilt correction. The paleomagnetic results are interpreted to represent remanence of a normal polarity based on geological constraints and thus indicate that Tethyan Himalaya was located at 33.1°±5.6°S in Late Cretaceous (80.1±6.2 Ma). The paleolatitude of Tethyan Himalaya is similar to the coeval paleolatitude of India (reference site: 28.0°N, 88.5°E), suggesting that Tethyan Himalaya and India were not separated by oceans, but likely formed a single crustal block in the Campanian. It is possible that rifting took place afterwards. However, provenance studies of sediments in the middle Eocene (~45 Ma) Bhainskati Formation in Lower Himalaya have components sourced from the Tethyan Himalaya, suggesting that the Tethyan Himalya and India was then in immediate proximity. If the Tethyan Himalaya rifted from India after 74 Ma, it must have collided with India by ~45 Ma. This would require the development of and elimination of a ~1700-km-wide ocean within ~30 Myr, which is tectonically highly unlikely. Therefore, the paleomagnetic data obtained from the Chuangde Formation do not support the dual-collision model, but instead support the traditional model, that is the collision between Tethyan Himalaya and Lhasa block represents the India-Asia collision.