Abstract

The seismic structure, distribution of normal-sense shear zones, and peak pressures of Ottawan (~1090-1020 Ma) metamorphism in the hinterland of the Grenville Orogen collectively define a crustal-scale horst and graben architecture inferred to have resulted from collapse of a former orogenic plateau. Domal-shaped horsts are large core complexes 100 km or more across that expose Ottawan, high-grade mid crust (peak P-T conditions ~1000 ± 100 MPa, 850-900 °C) with sub-horizontal gneissic foliations. Basinal-shaped grabens are underlain by the remnants of an orogenic lid in which sub-vertical, pre-Ottawan foliations are preserved and peak Ottawan P-T conditions were ≤ 400 MPa, 500 °C. Intervening crust, with peak Ottawan pressures between ~1000 and 400 MPa, exhibits steep foliations and is under-represented at the erosion surface.

Timing of Ottawan metamorphism exhibits a progression from granulite-facies in the orogenic mid crust at ~1090-1050 Ma, through amphibolite-facies in the upper crust at ~1050-1020 Ma, to heating to ≤ 500 °C in the uppermost crust surrounding the orogenic lid at ~1020-980 Ma. This temporal and spatial sequence is inferred to result from conductive heat transfer as hot mid crust was exhumed against successively higher crustal levels during collapse. The age data imply collapse lasted for ≥ 50 Ma, comparable to the duration of Ottawan thickening, and that it overlapped with important crustal thickening at the orogen margin during the ~1000-980 Ma Rigolet phase.

In addition to normal-sense displacement on major shear zones separating the core complexes and orogenic lid, the structural signal of collapse in the migmatitic mid crust includes orogen-normal stretching lineations defined by retrograde mineral assemblages, and evidence for pervasive vertical thinning (flattening), which resulted in a component of orogen-parallel stretching. Collapse thus involved gravity-driven horizontal extension of the ductile mid crust, which in turn led to wholesale boudinage of the overlying brittle upper crust, with core-complex formation initiated by rise of ductile mid crust into boudin necks.

In contrast to the mid crust, approximately coeval fabrics and structures in the under-represented ductile upper crust are steep to upright and record vertical thickening driven by sub-horizontal plate tectonic traction forces. Coeval vertical thinning / horizontal spreading of the weak mid crust driven by gravity, and vertical thickening / horizontal shortening of the stronger upper crust driven by traction forces, implies that deformation in the two levels was decoupled, strain was triaxial at both crustal levels, and that collapse was a fundamentally 3-D process. An important conclusion from these observations is that orogenic collapse cannot be realistically modelled in a 2-D, plane-strain framework.

That collapse also affected igneous and hydrothermal processes is indicated by the temporal and spatial association of normal-sense shear zones at the margins of several core complexes with leucogranite emplacement, and with fenetization (alkali metasomatism) and the formation of IO(CG) deposits.

In summary, viewing the seismic, structural, metamorphic and igneous features of the Grenville Orogen through the prism of orogenic collapse yields a profoundly revised image of its architecture and appears to provide a coherent context for its tectonic evolution.

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