Late Miocene topographic inversion in southwest Tibet based on integrated paleoelevation reconstructions and structural history

Abstract

Investigations of the deformation history of the Himalayan orogen support interpretations of rapid and striking changes in the landscape of the Tibetan Himalaya and High Himalaya. We examine this issue by integrating oxygen isotope-based paleoelevation reconstructions of the Zada basin in southwestern Tibet with information on the structural evolution of the High and Tibetan Himalaya between 79°E and 84°30′E. δ 18O psw values were calculated from δ 18O cc values from pristine fluvial Miocene gastropod shells. Analyses comparing the most negative δ 18O sw and reconstructed δ 18O psw values to Δδ 18O sw versus elevation relationships based on both thermodynamic models and an empirical data set suggest a decrease in the mean watershed elevation of 1 to 1.5 km since the Late Miocene. Geologic mapping and structural data from crustal scale fault systems in the Zada region and regions to its east indicate a phase of arc-normal shortening and vertical thickening since the Middle Miocene, followed by ongoing arc-parallel extension and vertical thinning. These results suggest that regions in this part of the orogen transitioned from undergoing arc-normal shortening to arc-parallel extension in the Late Miocene, and that arc-parallel extensional structures root deeply within the Himalayan thrust wedge. When combined with data on the distribution, age, and provenance of sedimentary basins, our geologic mapping, structural data, and paleoelevation results suggest that this transition from shortening to extension was accompanied by a topographic inversion from mountains to basins in < 4 m.y. These observations can be explained by a foreland propagating fault system that accommodates outward radial expansion of the Himalayan orogen.