DEEP-STRUCTURE BENEATH LAKE-ONTARIO - CRUSTAL-SCALE GRENVILLE SUBDIVISIONS

Lake Ontario marine seismic data reveal major Grenville crustal subdiv isions beneath central and southern Lake Ontario separated by interpre ted shear zones that extend to the lower crust. A shear zone bounded t ransition between the Elzevir and Frontenac terranes exposed north of Lake Ontario is linked to a seismically defined shear zone beneath cen tral Lake Ontario by prominent aeromagnetic and gravity anomalies, eas terly dipping wide-angle reflections, and fractures in Paleozoic strat a. We suggest the central Lake Ontario zone represents crustal-scale d eformation along an Elzevir-Frontenac boundary zone that extends from outcrop to the south shore of Lake Ontario. Seismic images from Lake O ntario and the exposed western Central Metasedimentary Belt are domina ted by crustal-scale shear zones and reflection geometries featuring a rcuate reflections truncated at their bases by apparent east-dipping l inear reflections. The images show that zones analogous to the interpr eted Grenville Front Tectonic Zone are also present within the Central Metasedimentary Belt and support models of northwest-directed crustal shortening for Grenvillian deep crustal deformation beneath most of s outheastern Ontario. A Precambrian basement high, the Iroquoian high, is defined by a thinning of generally horizontal Paleozoic strata over a crestal area above the basement shear zone beneath central Lake Ont ario. The Iroquoian high helps explain the peninsular extension into L ake Ontario forming Prince Edward County, the occurrence of Precambria n inlier outcrops in Prince Edward County, and Paleozoic fractures for ming the Clarendon-Linden structure in New York.