Coeval thrusting and extension during lower crustal ductile flow – implications for exhumation of high‐grade metamorphic rocks

Abstract

AbstractNumerical models of tectonic processes in large hot orogens offer insight into the styles, controls and consequences of ductile flow of middle and lower orogenic crust. We present results from crustal‐scale thermal‐mechanical models illustrating contrasting styles of syn‐ and post‐convergent ductile flow. Crustal viscosity in the models is controlled by temperature‐dependent flow laws, scaled to represent lithological strength variations. An additional viscosity decrease at high temperature can be imposed to represent weakening during incipient partial melting. Two end‐member tectonic styles, in which lower crustal flow is driven either by gravity or tectonics, develop in response to the initial properties and evolving strength of the model crust. Homogeneous channel flow, driven by the gravitational potential energy difference between a plateau and its foreland, is characteristic of Himalayan–Tibetan (HT‐series) models with relatively weak, laterally homogeneous crust and moderately high heat production. In contrast, Grenville Orogen (GO‐series) models with relatively strong, laterally heterogeneous crust do not reach the low effective viscosity (ηeff≤1019 Pa.s) required for gravity‐driven channel flow. Instead, tectonically driven lateral flow and explusion of lower crust is triggered by underthrusting of a strong indentor. The dominant structures are ductile fold nappes that may be transported hundreds of kilometres towards the foreland. When tectonic convergence is turned off, the model orogens undergo gravitational spreading, in which ductile middle and lower crust flow laterally from the plateau towards the foreland, driving thrusting on the orogenic flanks and ductile thinning in the orogenic core. The resulting geometry can resemble that produced by syn‐convergent ductile flow, except for the development of regional‐scale normal‐sense shear zones in the post‐convergent stage. Model feasibility is tested against data from the western Grenville Orogen in Ontario. The comparison suggests that high‐grade metamorphic rocks in the orogenic core record the effects of protracted syn‐ and post‐convergent ductile flow, whereas those on the flanks record mainly post‐convergent thrusting and extension. The models suggest a number of ways to distinguish among different styles of syn‐ and post‐convergent ductile flow; metamorphic P–T–t paths alone are not diagnostic of tectonic process without additional information on crustal‐scale structure and timing. In combination with erosion, both types of flow may contribute to exhumation of regionally extensive high‐grade gneiss terranes.