ONGOING NEOTECTONIC ACTIVITY EXPRESSED AS FAULTS AND SLUMPS IN LAKE FLOOR SEDIMENTS: TIMISKIMING GRABEN, WESTERN QUEBEC SEISMIC ZONE, CANADA

The Timiskaming Graben straddles the Ontario/Quebec border and lies within a belt of heightened seismic activity within the Western Quebec Seismic Zone. The graben is a conspicuous 50 km wide morphotectonic depression partly filled by Lake Timiskaming (100 km long, 209 m max depth) and intersects the Grenville Front Tectonic Zone (a major crustal boundary) near Lake Kipawa (65km long, 98m max depth). These lakes are postglacial successors to glacial Lake Barlow which drained about 8000 years ago leaving an extensive clay plain. High resolution seismic sub-bottom data reveals that both late-glacial Barlow and Holocene sediments are extensively deformed by neotectonic horst and graben structures in Lake Timiskaming and by earthquake-triggered slump structures in Lake Kipawa. Graben subbasins in Timiskaming are identified recording ongoing postglacial subsidence between parallel bounding faults, one of which is expressed offshore as a scarp and can be mapped onshore as a 20 km-long, 10m high scarp that offsets the surrounding clay plain. Thirty-six slumps identified from the floor of Lake Kipawa reveal a clear relationship with the epicenter of the 1935 Timiskiming Earthquake (M6.2). The floors of these Lakes indicate ongoing neotectonic activity on a scale not recognized anywhere else across intracratonic North America. Data confirm the Timiskaming Graben as an intraplate ‘weak zone’ that may contain a long Late Cenozoic sediment record suitable for deep drilling.