Three-dimensional mechanical modeling of the GPS velocity field around the northeastern Tibetan plateau and surrounding regions

Abstract

The northeastern corner of the Tibetan plateau is a complex tectonic region with different fault mechanisms from left-lateral thrusting of the Qilian Shan, left-lateral strike slipping of the Haiyuan fault along the plateau edge, and right-lateral extension across the Alashan and the Ordos out of the northeastern Tibetan plateau. Here, we use 3D finite element models incorporating fault as Coulomb-type friction zone to investigate mechanical relation between crustal rheology and long-term deformation of the main active fault systems. Models are constrained with GPS velocity field and available geological slip rates. Crustal rheology is simplified as an elastobrittle upper part, underlying with viscoelastic crust. We test models with fault frictions (μ) from 0.4 to 0.02 on different fault systems, and mean viscosities (η) of the lower crust from 1019Pa.s to 1021Pa.s in the Tibetan plateau and 1021Pa.s to 1023Pa.s out of the Tibetan plateau. A common feature from the numerical experiments is that the Haiyuan fault reflects a low fault friction (μ<0.1–0.08). The predicted low fault friction associated with the mean viscosities of the lower crust of ~1019Pa.s in the Tibetan plateau and ~1021Pa.s out of the Tibetan plateau can fit the geological slip rates well for the active faults. This suggests that slip partitioning around northeastern boundary of the Tibetan plateau is related mechanically to the low fault friction. Numerical experiments also show that after strain rates are absorbed by the main fault systems, the rest are strongly affected by crustal rheology. Finally, even the fault friction decreases to ~0.05, the mean viscosities of lower crust attain to 1021Pa.s and 1023Pa.s in and out of the Tibetan plateau, the northeastern Tibetan plateau and the Alashan still deforms diffusively, except that the Ordos behaves more like a rigid block.