Where is the northernmost boundary of the southeastward extrusion in the southeastern Tibetan Plateau?

As the response to the India–Eurasia collision during the Cenozoic, the southeastern Tibetan Plateau has been widely regarded as having experienced significant clockwise rotation and southeastward extrusion since the Late Eocene. However, recent paleomagnetic studies have revealed pronounced north–south differences in the tectonic response of this region: southeastward extrusion may be confined to the southern part (e.g., the Lanping–Simao Terrane), whereas the northern part (e.g., the eastern Qiangtang Terrane) may have undergone northward motion. To date, the magnitude and onset timing of this northward motion, its dynamic coupling with the southeastward extrusion in the south, and especially the location of the tectonic boundary controlling such opposing north–south motions remain unclear.

To address these issues, this research selected the Eocene Dongwang Formation in the Deqin area, located in the central part of the southeastern Tibetan Plateau, and conducted a systematic paleomagnetic study. Through integrated rock magnetic and petrographic analyses, combined with thermal demagnetization, reversals and fold tests, the characteristic remanent magnetization (ChRM) directions were confirmed to be primary. Based on regional lithological correlations and detrital zircon provenance analyses, the depositional age of the studied strata was further constrained to be older than the Late Eocene. The paleomagnetic directions indicate that the Deqin area was situated at a paleolatitude of approximately 34.4°N before the Late Eocene, which is close to the expected paleolatitude derived from the Eurasian reference pole, suggesting that the Deqin area has experienced no significant latitudinal displacement relative to Eurasia since the Late Eocene.

Using the new paleolatitude data from the Deqin area, together with paleolatitude data obtained from Late Eocene volcanic rocks in Mangkang, and incorporating crustal shortening estimates from the northern region, this study proposes that, in contrast to the significant southeastward extrusion in the southern part, the northern part of the southeastern Tibetan Plateau likely experienced only limited northward displacement after the Late Eocene. The Deqin area probably constitutes the boundary separating the opposing north–south motions along the southeastern Tibetan Plateau after the Late Eocene. Accordingly, this study proposes a deformation model involving opposing displacements on either side of Deqin, whose dynamic mechanism may be regional clockwise rotation around an Euler pole located near Deqin since the Late Eocene.

Xu W, Yan M, Guan C, Fu Q, Yu L, Li B, Zhang D, Shen M, Xu Z, Zhang Y. 2026. Paleomagnetic constraints on the northernmost boundary of the southeastward extrusion in the southeastern Tibetan Plateau. Science China Earth Sciences, 69(5): 1856–1871, https://doi.org/10.1007/s11430-025-1855-2