METAMORPHIC CORE COMPLEXES, OROGENIC LID, MEGABOUDINAGE AND REGIONAL CROSS-FOLDING: SIGNALS OF PROFOUND OROGENIC COLLAPSE AND DUCTILE GRAVITATIONAL SPREADING IN THE WESTERN GRENVILLE PROVINCE

Building on previous tectonic interpretations that the exposed crust in the hinterland of the Grenville Province consists of large high-grade, mid-crustal metamorphic core complexes (orogenic infrastructure) surrounded by the orogenic lid (remnants of the lower grade, upper crustal orogenic superstructure), this presentation examines the record from the western Grenville Province in Ontario, where the Ottawa River Gneiss Complex (ORGC; formerly Central Gneiss Belt; orogenic infrastructure) is in tectonic contact with the Composite Arc Belt (orogenic superstructure). Specifically, we focus on the evolution of the Muskoka domain, the highest structural level in the ORGC that is principally composed of post-peak amphibolite-facies straight gneiss derived from retrogressed, peak Ottawan, granulite-facies precursors. We present evidence that retrogression and high strain occurred during orogenic collapse and that the Muskoka domain acted as the ductile detachment zone between adjacent stronger crustal units, the underlying granulite-facies metamorphic core known as the Algonquin domain and the overlying lower grade cover comprising the Composite Arc Belt.

The core complex architecture developed in a regime of gravitationally-driven, crustal-scale extensional collapse, in which rheological contrast between the cool brittle-ductile upper crust and hot ductile mid crust resulted in megaboudinage and extensional flow into the neck regions between megaboudins respectively. Extension in the Muskoka domain began under suprasolidus conditions, was facilitated by fluid ingress and widespread retrogression, and continued under subsolidus conditions as the mid crust was exhumed and cooled. High-temperature ductile extensional structures include a regional system of extension-dominated transtensional cross-folds, and multi-order boudinage and associated extensional bending folds. Brittle-ductile fault propagation folding associated with the emplacement of late-tectonic pegmatite dykes took place at higher crustal level after the gneiss complex was substantially exhumed and cooled. Collectively the data record ~60 M.y. of post-peak extension on the margin of an exceptionally large metamorphic core complex in which the ductile detachment zone has a true thickness of ~7 km.