Presented June 1, 2000 at GeoCanada2000 (Calgary, Alberta, Canada)
R.A. Jamieson (Department of Earth Sciences, Dalhousie University, Halifax, N.S. B3H 3J5; beckyj@is.dal.ca), C. Beaumont (Department of Oceanography, Dalhousie University), O. Vanderhaeghe (Géologie et Gestion des Ressources Minérales et Energétiques, Université Henri Poincaré Nancy 1, France), and P. Fullsack (Department of Oceanography, Dalhousie University)
The formation of synorogenic granulites and migmatites requires temperatures in excess of 700° C to be reached within the orogenic crust during convergence. Orogenic thermal structure, and hence peak temperature, is controlled by the competition between heat sources and sinks. In collisional orogenic settings not associated with magmatic arcs, the main heat sources are the mantle and radiogenic heat generated by crustal material accreted within or beneath the orogen. Heat is lost by surface cooling and by the effects of subduction beneath, and/or accretion of cold material within, the orogen during crustal thickening.
The role of the mantle in heating the lower crust has been the subject of considerable speculation. Heat can be transferred from the mantle to the crust by underplating or intraplating of mantle-derived magmas; in order to heat the lower crust to the temperatures required to generate granulite or migmatite, large volumes of magma are required. Although this is likely in magmatic arc settings, many orogens characterised by regional-scale high-grade metamorphism lack evidence for voluminous syn-metamorphic mafic magmatism. Replacement of cool mantle lithosphere by hot asthenosphere as a result of delamination or subduction rollback should heat the lower crust, although the tectonic controls, geological consequences, and time- and length-scales of this process are not well understood. In contrast, convective delamination of thickened mantle lithosphere is predicted to leave a layer of relatively cool mantle lithosphere beneath the orogen, and is therefore unlikely to produce high temperatures within an orogen unless accompanied by other processes. more work is required on the thermal and mechanical behaviour of the mantle lithosphere during convergence before quantitative predictions of the contribution of mantle heat to lower crustal meta morphism can be made.
Accretion of heat-producing upper crustal material within an orogen during convergence can lead to orogenic self-heating on timescales of 10-50 My. This is a very effective long-term heat source, providing that a sufficient volume of heat-producing material is accumulated. The influence of tectonically accreted radioactive material ("tarm") on orogenic thermal structure has been investigated using coupled thermal-mechanical models of convergent orogens that are driven by subduction of sub-orogenic mantle lithosphere. Temperatures above 700° C can be be generated at mid- to lower crustal levels within 30 My of accretion for a range of crustal heat production values and convergence velocities. The crustal temperature field in these models is closely linked to the distribution of heat-producting material. Deformation styles that incorporate "tarm" at mid- to deep crustal levels are particularly effective, since the "tarm" is protected from erosion and remains in the crust as a long-term heat source. Following the end of convergence, when cold material is no longer being advected into the orogen, self-heating of buried "tarm" can produce termperatures well in excess of crustal melting temperatures on timescales of tens of millions of years. However, both the distribution of heat-producing material within real orogens and the corresponding heat-production values are relatively poorly known.
In summary, mantle heat could contribute substantially to heating the lowermost crust but its influence on the mid-crust is limited and the geological evidence for a mantle heat source is typically cryptic. Crustal heat production is an effective heat source for the lower and middle crust if a significant volume of "tarm" is accumulated and protected from syn-orogenic erosion. However, these factors are difficult to evaluate in most natural orogens; the Central Gneiss Belt of the western Grenville Orogen represents a case in point.