Abstract

High-pressure metamorphic rocks in continental collision belts provide important information on the orogenic evolution of deep crust. In the Grenville Province, a prime example is the Manicouagan Imbricate Zone (MIZ), a Mesoproterozoic crustal section that was metamorphosed up to 1800 MPa and 850–950°C at ∼1050 Ma. High temperatures and the presence of synmetamorphic within-plate tholeiitic gabbro intrusion in the MIZ attest to thermal perturbation during burial and emplacement of mantle-derived melts, consistent with thinning of lithospheric mantle. Subsequently, the MIZ was extruded by a combination of northwest-directed thrusting over a crustal-scale ramp and extension at higher crustal levels. In the middle MIZ, extension was coeval with thrusting, but perpendicular to the thrust direction, and was apparently controlled by ductility contrasts between slices. In contrast, the highest levels show evidence of southeast-directed extension, i.e., of opposite sense to the thrust direction, that postdated thrusting within the MIZ. This episode was followed by renewed northwest-directed thrusting of medium-pressure Mesoproterozoic units over the MIZ, and then by final southeast-directed extension. Overall configuration of the MIZ was achieved between ∼1040–990 Ma and is consistent with outward propagation of the orogen. Alternating periods of thrusting and extension displacement are a likely result of adjustments permitting maintenance of dynamic equilibrium within the advancing stack. The tectonothermal characteristics of the MIZ allow comparison between the Grenville Province and younger, thermally perturbed orogens (i.e., Variscan Belt) and may be the final product of a Tibetan Plateau style evolution.

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