Paleoelevations in the Jianchuan Basin of the southeastern Tibetan Plateau based on stable isotope and pollen grain analyses

The southeastern margin of the Tibetan Plateau (TP) is a key region for understanding the region's surface uplift mechanisms. This study focused on the Jiuziyan and Shuanghe Formations (Fms) in the Jianchuan Basin, both of which include lacustrine calcareous mudstones and marls. Ostracods of the genus Austrocypris found within the Jiuziyan and Shuanghe Fms constrain the age of strata to the Late Eocene. This study used two different proxies, i.e. fossil pollen coexistence and the δ¹⁸O (VPDB) values of carbonate (δ¹⁸O_c), to reconstruct paleoelevation and the extant paleoenvironment from lacustrine calcareous mudstones and marls preserved in the Eocene stratigraphy of the Jianchuan Basin. The coexistence approach (CA) using pollen data from the Shuanghe Fm indicates a paleoelevation of 1.3–2.6 km above sea level (asl), which would most probably have been associated with a vegetation cover consisting of tropical-subtropical, deciduous, coniferous, broadleaf forests. The reconstructed mean annual air temperature (MAAT) had a value of 16.8–21.7 °C, warmer than today's MAAT (~6 °C). Oxygen isotope results from the Jiuziyan Fm, with/without modification between Eocene and modern Myanmar sea level, suggested that the surface of the Jianchuan Basin was at a paleoelevation between 0.5^+0.8 _–0.5 km asl and 2.5 ± 0.7 km asl (δ¹⁸O_mw: −8.9 ± 1.3‰ 2σ). During the Shuanghe Fm sedimentation the paleoelevation was between 0.9 ^+0.7 _–0.7 km asl and 2.9 ± 0.6 km asl (δ¹⁸O_mw: −9.5 ± 1.1‰ 2σ). Our results suggest that a stepwise uplift of Jianchuan Basin and crustal thickening initiated during the Eocene was the cause of passive surface uplift of the southeastern Tibetan Plateau (TP) rather than Miocene lower crustal flow.