A SEISMIC-REFLECTION-BASED REGIONAL CROSS-SECTION OF THE SOUTHERN ABITIBI GREENSTONE-BELT

Seismic reflection profiles from the southern Abitibi greenstone belt reveal four first-order subdivisions: (1) Between 0 and similar to 4.5 s, the upper crust is weakly reflective, with prominent local to late rally extensive reflections. (2) Between similar to 4 and similar to 9 s, the crust is strongly and heterogeneously reflective with laterall y continuous reflections. (3) From similar to 9 to similar to 13 s, th e crust is more homogeneously reflective and displays downward decreas ing reflectivity. (4) Below similar to 13 s (Moho?) the upper mantle i s weakly reflective. The upper layer may correspond to subgreenschist - greenschist-facies supracrustal rocks cut by low-angle shear zones a nd intruded by regional tabular batholiths; the middle layer, to ducti ley deformed amphibolite-facies gneisses, granitoids, and (or) metased iments; and the lower layer, to more homogeneously deformed granulite- facies rocks. North-dipping, low-angle reflections extending beneath b oth diverse supracrustal assemblages and regional batholiths may repre sent structural detachments upon which both the supracrustal assemblag es and batholiths were imbricated and translated southward. However, t he preservation of regional low-pressure metamorphic rocks and the com mon para-autochthonous relationships between assemblages suggest that thrust-related vertical separations and the magnitude of crustal thick ening were not large. Steeply dipping regional shear zones within the greenstone belt appear to disrupt subhorizontal reflections down to - similar to 15 km and may represent late-tectonic strains, which were p rogressively concentrated into linear zones during continued north-sou th shortening. The crustal-scale structure determined from the seismic reflection profiles, combined with surface geology, is compatible wit h post-2.70 Ga north-south shortening accommodated by south-directed(? ) thrusting in a thermally softened mid crust and by upright folding i n the upper crust. This scenario is comparable to recently proposed mo dels for the Paleozoic, high-temperature, low-pressure Lachlan fold be lt of Australia.