SEISMIC IMAGES OF A TECTONIC SUBDIVISION OF THE GRENVILLE-OROGEN BENEATH LAKES ONTARIO AND ERIE

New seismic data from marine air-gun and Vibroseis profiles in Lake On tario and Lake Erie provide images of subhorizontal Phanerozoic sedime nts underlain by a remarkable series of easterly dipping reflections t hat extends from the crystalline basement to the lower crust. These re flections are interpreted as structural features of crustal-scale subd ivisions within the Grenville Orogen. Broadly deformed, imbricated, an d overlapping thrust sheets within the western Central Metasedimentary Belt are succeeded to the west by a complex zone of easterly dipping, apparent thrust faults that are interpreted as a southwest subsurface extension of the boundary zone between the Central Metasedimentary Be lt and the Central Gneiss Belt. The interpreted Central Metasedimentar y Belt boundary zone has a characteristic magnetic anomaly that provid es a link from the adjacent ends of lakes Ontario and Erie to structur es exposed 150 km to the north. Less reflective. west-dipping events a re interpreted as structures within the eastern Central Gneiss Belt. T he seismic interpretation augments current tectonic models that sugges t the exposed ductile structures formed at depth as a result of crusta l shortening along northwest-verging thrust faults. Relatively shallow reflections across the boundary region suggest local, Late Proterozoi c extensional troughs containing post-Grenville sediments, preserved p ossibly as a result of pre-Paleozoic reactivation of basement structur es.