CRUSTAL-SCALE SHORTENING AND EXTENSION ACROSS THE GRENVILLE PROVINCE OF WESTERN QUEBEC

A deep seismic reflection survey shot in 1993 crosses almost the full width of the Grenville Province in western Quebec. The seismic transec t provides a very clear image of the crust-mantle boundary and the mos t precise definition to date of the various Grenvillian terranes. The crust is around 44 km thick beneath the Grenville Front but thins rapi dly to 36 km some 60 km to the southeast; it is also notable that the greatest crustal thickness of 50 km occurs at the southeast end of the transect, far from the inferred location of the main Grenvillian coll ision. The Grenville Front zone, in which NW directed thrusting at abo ut 1 Ga was followed by SE directed extension, is defined by discontin uous, SE dipping reflections, which extend down to the Moho. To the so utheast, the overlying migmatitic Archean parautochthon, almost half o f the transect, is characterised instead by NW dipping reflectors exte nding into the lower crust. These reflectors are in turn truncated to the southeast by a 12-km-thick zone of intense SE dipping reflections, the Baskatong crustal ramp. The base of the allochthonous terranes (A llochthon Boundary Thrust) is likely located at this ramp, which flatt ens out at around 30-km depth into the base of the relatively transpar ent intermediate crust. A highly reflective upper crustal deck corresp onding to rocks of the Mont-Laurier terrane was thrust over the Baskat ong crustal ramp and is represented further to the northwest by the kl ippelike Cabonga allochthon. The synformal, transparent Morin anorthos ite-charnockite complex belongs to the same upper crustal level. Ramp anticlines and the overriding basal thrust of the upper allochthons de monstrate the NW directed propagation of tectonic transport during the Grenvillian orogeny, involving deformation and displacement along the Baskatong ramp with a relay into the Grenville Front zone. Postaccret ional extension appears to have been primarily accommodated along the latter two crustal discontinuities thinning the crust immediately sout h of the Grenville Front and affecting the crust up to 350 km away fro m the front.