Large Influence of Subduction Geometry on Extreme Exhumation in Orogen Syntaxes

Abstract:

Some of the highest and most localized rates of lithospheric deformation in the world are observed at the transition between adjacent plate boundary subduction segments (syntaxes). The initiating perturbation of this deformation has long been attributed to vigorous, climate driven, erosional processes as observed at Nanga Parbat and Namche Barwa in the Himalaya and at Mt. St. Elias in Alaska. However, an erosion-dominated mechanism ignores the 3D geometry of curved subducting plates and changes in paleoclimate. Here we present an alternative explanation for rapid exhumation at these locations based on the 3D thermo-mechanical evolution of collisions between plates with nonplanar geometries. A set of numerical solutions and thermochronometer observations are presented for the interaction between a rigid indenter between subduction segments with an overriding viscous material. The model setup is intentionally simplified to identify how the 3D geometry of a subducting plate influences upper plate deformation, and therefore omits many of the complexities of collision interfaces, such as the transfer of mass between the downgoing and overriding plates. Temperatures are calculated using the 3D advection diffusion equation with radiogenic heat production and shear heating. The thermal and Stokes flow solutions are fully coupled and evolve throughout the simulation. Model predicted cooling ages are compared to observed cooling ages from published bedrock and detrital thermochronometer studies by calculating cooling rate dependent thermochronometer ages. Comparison of model predictions with existing thermochronometer data reproduces the defining characteristics of these mountains such as a localized “bulls-eye” pattern of rapid exhumation and young cooling ages above the rigid indenter between subduction segments. These results demonstrate a ‘bottom up’ tectonic rather than ‘top down’ erosional initiation of feedbacks between erosion and tectonic deformation. While the role of enhanced precipitation as an instigator of enhanced deformation has been suggested, we demonstrate that the geometry of subducting plates is important for producing many of the observations previously used to demonstrate a strong influence of climate on tectonic processes.