OCGC Seminar - Contrasting styles of (ultra-)high-pressure metamorphism and exhumation in small-cold and large-hot orogens: examples from the Alps and Caledonides


Dr. Rebecca Jamieson
Department of Earth Sciences, Dalhousie University

Contrasting styles of (ultra-)high-pressure metamorphism and exhumation

 Thursday, October 30th, 2014
11:00 a.m.

Harrison Hall
Geological Survey of Canada
601 Booth Street 


(Ultra-)high-pressure ((U)HP) metamorphic terranes record deep burial and/or subduction of continental crust to depths ≥90 km (≥2.5 GPa). Using 2-D upper-mantle-scale numerical models, we have tested a range of hypotheses concerning the processes leading to metamorphism and exhumation of these terranes by comparing specific case histories with quantitative model predictions. Here we compare the dynamics of subduction and (U)HP rock exhumation in simplified end-member cases of small-cold (“Alpine-style”) and large-hot (“Caledonian-style”) orogens. Successful models should replicate key constraints on (U)HP terrane exhumation and general orogen-scale evolution from the Western Alps or Norwegian Caledonides, the natural equivalents of the style examples. Results from models that “work” can be used to inform interpretation of the lithosphere-scale evolution of these and similar orogenic belts. The model results show how differences in the strength of the subducting crust, the duration and scale of orogeny, and driving plate motions can lead to marked contrasts in the nature of burial and subsequent exhumation-related extension. Most differences in the size, pressure-temperature history, and structural evolution of Alpine and Caledonian (U)HP terranes can be explained by these factors, with crustal strength of particular importance. In the Western Alps, rapid (1-3 cm/a) exhumation of small, relatively cold (~500-650°C) (U)HP terranes during the early stages of collision was accompanied by localized syn-convergent upper crustal extension. The model results show that this exhumation style can be driven by detachment and rapid ascent of buoyant crust from the subduction channel. In contrast, the much larger Western Gneiss Region (WGR) of Norway underwent high-temperature (~700-800°C) (U)HP metamorphism during the final Scandian (Devonian) phase of the Caledonian orogeny, followed by slow exhumation (~0.5-1 cm/a) during post-convergent extension. These features can be explained by crustal subduction beneath a previously assembled hot orogenic wedge, followed by protracted residence at (U)HP conditions. Subsequent slow exhumation of the large (U)HP terrane results from orogen-scale extension during the waning stages of convergence and later plate divergence. These examples can be viewed as end-members in which (U)HP terranes were exhumed at different times during orogenesis, with timescales determined by crustal strength and internal structure by contrasting modes of detachment and ascent. Where lower-plate continental crust is weak, it is detached and exhumed as small slices during the early stages of convergence. Where it is strongly coupled to the downgoing plate it retains some coherence and is mainly exhumed by post-convergent extension. This concept can be used to predict whether or not (U)HP rocks might be found in other orogenic belts, such as the Grenville and Appalachian Orogens.