India-Asia collision is one of the most profound geologic events in the Cenozoic. It marks the termination of Neo-Tethys evolution and the beginning of the continental-continental collisional orogeny that resulted in the rise of Tibetan Plateau, exerting dramatic impacts on the terrestrial environment and biodiversity in Asia. However, the timing, position and the mode of India-Asia collision are still hotly debated. One way toward resolving these controversies is to delineate the tectonic background of the southern margin of Asia in Late Cretaceous prior to the India-Asia collision. Specifically, was the convergence between the northward drift of India plate and Asia plate accommodated by one or two subduction systems? The two different views arose from the different interpretations of the supra-subduction zone (SSZ) ophiolite and the related metamorphic and sedimentary records along the thousands of kilometers of the Indus-Yarlung suture zone (Fig. 1). In the eastern part of the suture zone, paleomagnetic data of the Lower Cretaceous and Eocene rocks in the Xigaze forearc basin (~86° E) show a paleolatitude of 16-20° N, suggesting that it was close to the southern margin of Asia and there was an oceanic subduction system south of Asia. The suture widens westward and contains the Kohistan-Ladakh intra-oceanic arc (KLA), which is bounded by the Indus suture to the south and Shyok suture to the north (Fig. 1), indicating that there were two subduction systems, one close to the southern margin of Asia as indicated by the Shyok suture and the other further south as indicated by the Indus suture. Interestingly, sedimentary provenance analysis reveals Asia-sourced sediments in KLA at ~90 Ma, suggesting that it was close to the southern margin of Asia then, but paleomagnetic data of ~66 Ma rocks in KLA show that it was situated near the equator, i.e., ~2000 km south of the southern margin of Asia, at ~66 Ma. To reconcile these datasets, Kapp and DeCelles (2019) proposed a back-arc spreading model in which back-arc spreading in southern margin of Asia in Late Cretaceous led to KLA-Xigaze forearc rifted and drifted to the equator, and the back-arc basin was subsequently closed along the Shyok suture and its eastern counterpart suture that coincided with the Gangdese thrust (GT). This model has two prominent features: (1) KLA-Gangdese arc underwent large-scale southward displacement to the equator in Late Cretaceous (~90-66 Ma); (2) the convergence between the northward drifting India plate and Asia plate along the Indus-Yarlung suture zone was evenly partitioned.
The straightforward approach to test this “back-arc spreading” model is to determine the paleolatitude of the Xigaze forearc basin in Late Cretaceous. The research group led by Prof. Yong-Xiang Li from School of Earth Science and Engineering and State Key Laboratory of Critical Earth Material Cycling and Mineral Deposits carried out a paleomagnetic study of the Upper Cretaceous Padana Formation in the Xigaze forearc basin (Fig. 1). The purpose of this study is to constrain the paleolatitude of the Xigaze forearc in Late Cretaceous and compare it with that of coeval KLA to reconstruct the kinematics of blocks in the southern margin of Asia and resolve the controversy regarding the subduction system south of Tibet.
Fig. 1 Maps showing the geology of the study area. (A) Indus-Yarlung suture zone separates the northern margin of India plate from the southern margin of Asia plate. The Sangsang section (1) and Cuojiangding section (2) of Xigaze forearc basin and the KLA (3)； The geology of the Sangsang section and its vicinity (C, D).
The paleomagnetic results of the Upper Cretaceous Padana Fm are shown in Fig. 2. The red siltstone and sandstone in the upper part of the section yield characteristic remanent magnetization (ChRM) that passed reversal and fold tests, and are interpreted as primary remanence. Together with available biostratigraphic and detrital zircon U-Pb geochronological constraints, the magnetostratigraphic results constrain the red siltstone and sandstone in the upper part of the Padana Fm to ~71.4 Ma—69.3 Ma (Fig. 2). After correction for inclination shallowing, the paleomagnetic data show that the Xigaze forearc basin was located at 18.4° ± 3.6°N at ~70 Ma, which is only <5° (i.e., <500 km) lower than the coeval paleolatitude of ~22°N of Lhasa block. The Late Cretaceous paleolatitude is similar to the paleolatitudes obtained from the Early Cretaceous and Eocene rocks in the Xigaze forearc basin, indicating that the Xigaze forearc did not undergo large-scale southward displacement, that is, no significant back-arc spreading in Late Cretaceous, thus supporting the view that there was only one subduction system along the southern margin of Asia.
Fig. 2 Paleomagnetic results of the Sangsang section in the Xigaze forearc basin. （A）Magnetostratigraphic results; (B) representative demagnetization data; (C, D) Correction for inclination shallowing.
Integrating the paleolatitude of 18.4° ± 3.6°N for the Xigaze forearc (~86° E) with provenance analysis data of the ~90 Ma strata and the ~66 Ma paleolatitude of 8.1° ± 5.6°N for the KLA (~77° E), the researchers proposed an “asymmetric back-arc spreading” model for Late Cretaceous southern margin of Asia, which is modified after the “back-arc spreading” model of Kapp and DeCelles (2019) (Fig. 3). In brief, the southern margin of Asia experienced differential back-arc spreading in Late Cretaceous. The magnitude of spreading in the western part was large, but decreased eastward, producing a triangular back-arc basin in western part of the suture in Late Cretaceous, which was subsequently closed. The KLA-Xigaze forearc formed by such as an asymmetric back-arc spreading is similar in nature to the southern Alaskan-Aleutian arc today. The asymmetric back-arc spreading was probably driven by the roll-back of oceanic sub-slab of the Neo-Tethys Ocean and associated with toroidal mantle flow, a process probably resembling that lead to the formation of the triangular Lau basin in northeastern New Zealand where the Pacific plate is subducting westward. Additionally, this model suggests that the convergence between India and Asia plates along the suture zone was not laterally evenly distributed as previously thought, but laterally partitioned unevenly.
Fig. 3 The Late Cretaceous “asymmetric back-arc spreading” model proposed in this study
This research is recently published (online on February 8, 2025) in the Geological Society of America (GSA) journal Geology (Nature Index Journal). PhD student Shihua Xu is the first author, Prof. Yong-Xiang Li is the corresponding author, other authors include Prof. Douwe van Hinsbergen from Utrecht University in the Netherland, Prof. Xianghui Li, PhD students Xinyu Liu, Binchen Li and Northwestern University PhD student Teng Wang. Special thanks are due to National Natural Science Foundation of China and China Scholarship Council for supporting the research.
The article information:
Xu, S., Li, Y-X.*, van Hinsbergen, D. J., Liu, X.Y., Li, B.C., Li, X.H., Wang, T., 2025, An asymmetric Late Cretaceous back-arc basin south of Tibet? Geology, https://doi.org/10.1130 /G52634.1.
