THE RIGOLET PHASE: A RECORD OF LOWER PLATE SECONDARY SUBDUCTION, EXTRUSION AND UNDERTHRUSTING AT THE NORTHWESTERN MARGIN OF THE GRENVILLE OROGEN

The ~500km long, 30km wide Grenville Front Tectonic Zone (GFTZ), western Grenville Province, is a crustal-scale high-strain zone formed at the orogen margin on a pre-existing boundary in Laurentian lithosphere in the terminal Rigolet phase of the Grenvillian Orogeny. Field observations integrated with published data suggest it developed by subduction of upper and mid continental crust that was sheared off the Laurentian foreland, shortened by reclined folding and tectonically buried to ≤30km depth before extrusion back toward the surface in the northwest. Coeval tectonism in the Laurentian lower crust and subcontinental lithospheric mantle (SCLM) involved underthrusting (flat subduction) towards the southeast beneath crust thickened in the earlier Ottawan phase, where it underwent partial melting, forming magma sources for small dyke suites. Along strike to the east of the GFTZ, metamorphic fold-thrust belts define linear accretionary prisms, in places with compensatory crustal roots, that are similarly compatible with an origin by burial of the Laurentian margin followed by tectonic extrusion, and together imply significant differential uplift and erosion along the length of the northern Grenville Province. In summary, it is proposed that Rigolet tectonism is an example of lower-plate secondary subduction and underthrusting following slab breakoff in the primary Ottawan subduction system. Available constraints suggest Rigolet subduction lasted ~20 My and involved shortening of Laurentian upper and mid crust by ~100-300km, implying a subduction rate of <1.5cm.a-1. It ceased when depleted Archean SCLM from Interior Laurentia reached the Grenville Front, choking the subduction system. This scenario is compatible with interpretations of secondary subduction in the Himalaya-Tibet Orogen, the archetype large hot long-duration orogen.