RELATIONS BETWEEN THRUST BELT EVOLUTION, GRAIN-SCALE DEFORMATION, AND METAMORPHIC PROCESSES - CAPE SMITH BELT, NORTHERN CANADA

An important step towards understanding how continental thrust belts evolve is to study the relationship between the map-scale structures and the associated grain-scale deformation mechanisms and metamorphic processes. This relationship is examined in the early Proterozoic Cape Smith Thrust Belt, in the context of the belt's P-T-t history and fluid-rock interactions. Overprinting deformation-metamorphism relationships allow the thrust belt's evolution to be described in terms of four events: 1. (1) piggyback thrusting above a basal décollement localized at the basement-cover contact; 2. (2) greenschist to amphibolite grade metamorphism and development of a ductile basal shear zone; 3. (3) re-imbrication (out-of-sequence faulting) of the internal zone at pre- to syn-thermal peak conditions; 4. (4) post-thermal peak out-of-sequence thrusting and retrograde metamorphism. Out-of-sequence thrusting was accommodated by the development of ductile shear zones. In general, relatively broad ductile shear zones developed adjacent to thrusts during prograde metamorphism, while relatively narrow shear zones developed adjacent to thrusts during retrograde metamorphism. The width of the shear zones was controlled by factors such as the competition between softening and hardening processes (prograde shear zones), and the extent of fluid influx (retrograde shear zones). Four processes were primarily responsible for accomplishing penetrative deformation and accommodating thrust faulting during the belt's history: 1. (1) partitioning of strain into pre-existing weak zones or into relatively weak layers in a compositionally-stratified sequence; 2. (2) growth and/or rotation of metamorphic products into orientations of maximum resolved shear stress; 3. (3) transitions in dominant deformation mechanism from brittle or semi-brittle to crystal-plastic; 4. (4)retrograde metamorphism, involving fluid influx and hydration reactions producing weaker products than reactants. © 1990.