Constraints on deformation kinematics across the Yumu Shan, NE Tibetan Plateau, based on fluvial terraces

摘要

In compressive ranges, the kinematics of thrusts and related folds are the key to understanding mountain building processes; however, relatively little attention has been paid to folding on the hanging-wall of a thrust fault which has broken the surface. On the hanging-wall of the Yumu Shan thrust fault, which lies on the north side of the Yumu Shan at the NE edge of the Tibetan Plateau, we found that significant folding deformation had occurred simultaneously with faulting at the surface. To determine the kinematics of the folding and faulting, we surveyed deformed fluvial terrace surfaces across the fold and fault. The geometry of the surface of terrace T4 of the Liyuan River exhibits a faulting displacement of 55 +/- 11 m at the fault trace, while in the hinterland, there is an active anticline with a wave length of similar to 4000 m. Limb rotation on the forelimb is recorded by the terrace geometry. Based on terrace geometry and the deformation of bedrock strata, we infer the geometry of the fault and fold, and conclude that the forelimb was formed by simple-shear deformation above the fault ramp, and that the backlimb was formed by the down-lowering of the fault dip angle. Using the geometry of the fault and fold and previous age control for the terraces, an average rock uplift of 1.2 +/- 0.1 mm/a is estimated across the range, which produces a total fault slip rate of 1.8 +/- 0.4 mm/a below the Yumu Shan. This new uplift rate indicates an onset age of 2.5 +/- 0.5 Ma for the uplift of the Yumu Shan, suggesting that this is a young mountain range. This result suggests that the outward accretion of the NE Tibetan Plateau occurred at similar to 3 Ma, and it provides an insight into the kinematics of deformation within relatively recently deforming structures, suggesting that folding is a significant process in accommodating the crustal shortening where the thrusting has broken the surface.