Crustal subduction and the exhumation of (ultra)high-pressure terranes: contrasting modes with examples from the Alps and Caledonides

Abstract

The widespread recognition of (ultra)high-pressure ((U)HP) metamorphic rocks in orogens worldwide suggests that subduction and exhumation of crustal rocks from mantle depths are normal processes at convergent plate margins. However, the dynamics of these processes, in particular the comparative roles of erosion and crustal extension, and the driving forces of extension during (U)HP rock exhumation, remain controversial.

This thesis presents numerical modeling and field/analytical studies that address the geodynamics of crustal subduction and exhumation in two intensely studied orogens, the Alps and the Caledonides. The 2D numerical models show how different scales and durations of orogeny and plate motions can lead to marked contrasts in the style of orogenic growth, crustal subduction, and (U)HP exhumation. In the Western Alps, rapid exhumation (1-3 cm/a) can be explained by local, syn-orogenic extension driven by the buoyant ascent of deforming (U)HP crust from the subduction channel. Later trans-crustal exhumation probably resulted from the combined effects of syn-convergent thrusting, local extension, and erosion. The low temperatures (500-700°C) of Alpine (U)HP metamorphism are attributable to the small size of the orogen and short duration of subduction/exhumation. Contrary to recent suggestions, neither erosion nor absolute extension is required to explain (U)HP exhumation in the Alps.

The Western Gneiss Region (WGR) (Norwegian Caledonides), in contrast, can be explained by subduction to (U)HP conditions followed by plate divergence. Gravitational spreading of a thick, hot orogenic wedge leads to a short period of coeval thrusting and extension. Exhumation of (U)HP crust from the subduction channel is achieved by normal-sense shearing along the top of the (U)HP terrane, with minor associated shortening. Trans-crustal exhumation by vertical thinning of the orogenic wedge results from continued absolute extension and erosion. The comparatively high temperatures (700-800°C) achieved by Caledonian (U)HP rocks reflect the orogen's greater size, slower exhumation rates, and possible stalling of the (U)HP terrane at depth.

These contrasting models underscore the variety of possible mechanisms responsible for (U)HP exhumation, and represent new benchmarks in the understanding of Alpine and Caledonian tectonics and (U)HP rock exhumation in general.