Tectono-thermal evolution of deep crust in a Mesoproterozoic continental collision setting: the Manicouagan example

High-pressure metamorphic rocks in continental collision belts provide impo rtant information on the orogenic evolution of deep crust. In the Grenville Province, a prime example is the Manicouagan Imbricate Zone (MIZ), a Mesop roterozoic crustal section that was metamorphosed up to 1800 MPa and 850-95 0%C at similar to 1050 Ma. High temperatures and the presence of synmetamor phic within-plate tholeiitic gabbro intrusion in the MIZ attest to thermal perturbation during burial and emplacement of mantle-derived melts, consist ent with thinning of lithospheric mantle. Subsequently, the MIZ was extrude d by a combination of northwest-directed thrusting over a crustal-scale ram p and extension at higher crustal levels. In the middle MIZ, extension was coeval with thrusting, but perpendicular to the thrust direction, and was a pparently controlled by ductility contrasts between slices. In contrast, th e highest levels show evidence of southeast-directed extension, i.e., of op posite sense to the thrust direction, that postdated thrusting within the M IZ. This episode was followed by renewed northwest-directed thrusting of me dium-pressure Mesoproterozoic units over the MIZ, and then by final southea st-directed extension. Overall configuration of the MIZ was achieved betwee n similar to 1040-990 Ma and is consistent with outward propagation of the orogen. Alternating periods of thrusting and extension displacement are a l ikely result of adjustments permitting maintenance of dynamic equilibrium w ithin the advancing stack. The tectonothermal characteristics of the MIZ al low comparison between the Grenville Province and younger, thermally pertur bed orogens (i.e., Variscan Belt) and may be the final product of a Tibetan Plateau style evolution.