Channel flow of the lower crust and its relation to large-scale tectonic geomorphology of the eastern Tibetan Plateau

Abstract

The Tibetan Plateau is a large-scale tectonic geomorphologic unit formed by the interactions of plates. It has been commonly believed that convective removal of the thickened Tibetan lithosphere, or lateral flow of the lower crust beneath the Tibetan plateau plays a crucial role in the formation of the large-scale tectonic geomorphologic features. Recent geological and geophysical observations have provided important evidence in support of the lower crustal channel flow model. However, it remains unclear as how the geometry of lower crustal channel and the lateral variation of crustal rheology within the lower crust channel may have affected spatio-temporal evolution of the tectonic geomorphologic unit of the Tibetan Plateau. Here, we use numerical methods to explore the mechanical relations between the lower crustal channel flow and the tectonic geomorphologic formation around the eastern Tibetan plateau, by deriving a series of governing equations from fluid mechanics theory. From numerous tests, our results show that the viscosity of the channeled lower crust is about (1−5)×10¹⁸ to (1−4)×10²⁰ Pa s (Pa·s) beneath the margin of the eastern Tibetan Plateau, and increases to about 10²² Pa s beneath the Sichuan Basin and the southern region of Yunnan Province. Numerical tests also indicate that if channel flows of the lower crust exist, the horizontal propagation and the vertical uplifting rate of the eastern Tibetan Plateau margin could be accelerated with the time. Thus, the present results could be useful to constrain the rheological structure of the crust beneath the eastern Tibetan plateau, and to understand the possible mechanics of rapid uplift of the eastern Tibetan Plateau margin, especially since its occurrence at 8Ma as revealed by numerous geological observations.