SEISMIC IMAGES OF THE GRENVILLE OROGEN IN ONTARIO

In 1990, Lithoprobe acquired 240 km of seismic-reflection data across parts of the Central Gneiss Belt (CGB) and the Central Metasedimentary Belt (CMB) within the western Grenville Province of southern Ontario. Interpretation of these data in conjunction with geological constrain ts provided by bedrock mapping supports a model of northwest-directed thrusting and crustal shortening for the Grenville Orogen. Within the CGB, the Parry Sound shear zone is imaged as a 3 km wide zone of refle ctions dipping southeastward at 20-25-degrees and soling at depths < 7 km in the north and < 3 km in the south beneath Parry Sound domain. P arry Sound domain and the immediately adjacent domains are underlain b y a gently southeast-dipping reflective zone at 4.5-12.0 km depth inte rpreted as a detachment surface, likely associated with the central Br itt shear zone. This zone may have accommodated northwesterly transpor t of Parry Sound, southern Britt, and northwestern Rosseau domains ove r Britt domain during Grenvillian thrusting. Within the CMB, the seism ic data indicate that crustal shortening and imbrication have not been confined to domain and terrane boundaries, as presently defined. A 6 km wide band of reflections dips south at approximately 20-degrees fro m the surface within Bancroft terrane, soling into a mid-crustal decol lement beneath Elzevir terrane. Beneath and to die north of this plana r reflective zone is a complex pattern of strong, south-dipping (10-40 -degrees) reflections that extends from the near surface to the lower crust above a less reflective wedge-shaped zone. The zone of complex r eflectivity projects updip to the CMB boundary zone and into the CGB; together with the linear band of reflections affiliated with Bancroft terrane, they form the tectonized boundary between the CGB and the CMB . To the south of the linear reflective zone, prominent reflective pac kages are restricted to the middle and upper crust. The generally nonr eflective uppermost crust beneath Elzevir terrane is underlain by a se ries of gently southeast-dipping, antiformal reflections that appear t o sole into the mid-crustal decollement beneath Mazinaw terrane. These observations suggest that the collision between the CMB and the CGB r esulted in a sequence of relatively thin (15 - 20 km thick) allochthon ous terranes within the CMB being transported along a regional decolle ment and thrust northwestward over footwall rocks of the CGB along a p enetratively deformed tectonic zone, while a lower crustal wedge may h ave delaminated the CMB lower crust. Crustal thickness where defined b y the seismic data is 42.0-43.5 km in both the CGB and the CMB.