TRANSECT ACROSS THE NORTHWESTERN GRENVILLE OROGEN, GEORGIAN BAY, ONTARIO - POLYSTAGE CONVERGENCE AND EXTENSION IN THE LOWER OROGENIC CRUST

The Grenville orogenic cycle, between similar to 1190 and 980 Ma, invo lved accretion of magmatic arcs and/or continental terranes to the Lau rentian craton. A transect across the western Central Gneiss Belt, Geo rgian Bay, Ontario, which crosses the boundary between parautochthonou s and allochthonous units at an inferred orogenic depth of 20-30 km, o ffers some insights on the thermal and mechanical behavior of the lowe r crust during the development of the Grenville orogen. Prior to Grenv illian metamorphism this part of Laurentia consisted largely of Mesopr oterozoic (similar to 1450 Ma) granitoid orthogneisses, granulites, an d subordinate mafic and supracrustal rocks. Grenvillian convergence al ong the transect began with transport of the previously deformed and m etamorphosed (similar to 1160 Ma) Parry Sound domain over the craton s ometime between 1120 Ma and 1080 Ma. This stage of transport was follo wed by out-of-sequence thrusting and further convergence along success ively deeper, foreland-propagating ductile thrust zones. A major episo de of extension at similar to 1020 Ma resulted in southeast directed t ransport of allochthonous rocks along the midcrustal Shawanaga shear z one. The final stage of convergence involved deformation and metamorph ism in the Grenville Front Tectonic Zone at similar to 1000-980 Ma. Pe ak metamorphism along most of the transect at 1065-1045 Ma followed in itial transport of allochthonous rocks over the craton by 15-35 m.y. R egional cooling, which postdated peak metamorphism by >70 m.y., was pr obably delayed by the combined effects of late-stage extension and con vergence. Transport of allochthons at least 100 km over the craton was accomplished along a weak, migmatitic decollement; further propagatio n of the orogen into the craton followed partial melting and weakening of parautochthonous rocks below this decollement. Extensional deforma tion was associated with distributed ductile flow, the formation of re gional transverse folds with axes parallel to the stretching direction , and reactivation of the allochton-parautochthon thrust boundary as a n extensional decollement. The extensional lower crustal flow was like ly the primary cause of the subhorizontal attitude of many structures and seismic reflectors in this part of the Central Gneiss Belt.