The Role of Lateral Rheology Contrasts in the Evolution of Mountain Ranges: Insights from South East Tibet

Controversy surrounds the rheology of the continental lithosphere and how this controls the evolution and behaviour of mountain ranges. We address this question using numerical modelling and recently published results from stable-isotope palaeoaltimetry. Stable-isotope palaeoaltimetry provides the constraints on vertical motions required to distinguish between competing models for lithosphere rheology in South East Tibet. Such results suggest that parts of South East Tibet have been at or near their present-day elevations since the late Eocene, meaning that uplift rates are likely to be much lower (<0.2 mm/yr) than has previously been suggested based on river incision. We use numerical modelling of the temporal evolution of a gravity-driven fluid to investigate the effect of lateral rheology contrasts on the shape and evolution of mountain ranges. We find that lateral rheology contrasts, analogous to the contrast between the relatively undeforming Sichuan Basin and Central Lowlands of Myanmar and the rapidly deforming south-eastern margin of Tibet, result in deformation that mirrors the main features of the present-day topography, GPS velocity field and earthquake-derived strain rate in South East Tibet, without the need for the low-viscosity, lower-crustal channel which has previously been proposed. The first-order similarity between this simple model and key features of the deformation and topography of South East Tibet suggests that lateral rheology contrasts play a first-order role in determining the shape and temporal evolution of topography. Such contrasts are likely to be important in the development of other mountain ranges, such as the Andes and the Zagros, and this modelling provides an explanation for the correlation between cratonic regions and the steep range fronts of mountain ranges. In addition, our modelling suggests that sediments deposited at or near the surface may be transported hundreds of kilometres, an effect which is important in interpreting palaeoaltimetric results.