POSTGLACIAL FAULTING OF THE FLOOR OF LAKE TEMISKAMING IN THE TEMISKAMING GRABEN, ONTARIO

Lake Temiskaming is a long (100 km), deep (220 m) lake on the Canadian Shield lying within the Temiskaming Graben, the northern arm of the Ottawa-Bonnechere Graben and St. Lawrence Rift System. A high-resolution, subbottom seismic reflection survey of the sediment infill of the lake identifies a twofold seismic stratigraphic succession below the lake floor consisting of late glacial and Holocene sediments deformed by high-angle normal faults that define horst and graben structures and appear on the modern lake floor as scarps. The downfaulted Holocene sediment, the lack of sediment cover on the lake floor scarps (in an area with high sedimentation rates) and the presence of open tension gashes suggest a very recent age of faulting. The continuation of faults onshore is marked by a prominent 7 m high, 20 km long scarp north of the lake boundary that is found to have the same general trend. The area is characterized by frequent, moderate to large magnitude earthquakes (at least a single M>3 earthquake every year) such that the possibility of active neotectonic faulting should be considered.