LAKE SEDIMENTS AS NATURAL SEISMOGRAPHS: EARTHQUAKE-RELATED DEFORMATION IN CENTRAL CANADIAN LAKES

Central Canada experiences recurring low to moderate intraplate earthquakes linked to reactivation of poorly- understood Precambrian basement structures. Unfortunately such structures are deeply buried below Paleozoic cover rocks, Pleistocene glacial and postglacial sediments and large lake bodies. Unconsolidated fine-grained sediments in lake basins are ‘natural seismographs’ with the potential to record ancient earthquakes during the last 10,000 years since the retreat of the Laurentide Ice Sheet. Many lake basins are cut into bedrock and are structurally-controlled by the same Precambrian basement structures (shear zones, terrane boundaries and other lineaments) implicated as the source of ongoing mid-plate earthquake activity. A multi-year seismic sub-bottom survey of some 17 lakes consisting of more than 2000 kilometres of high-resolution track line data, supplemented by multibeam and sidescan sonar survey records, reveals a consistent sub-bottom stratigraphy of relatively-thick lowermost lateglacial facies composed of interbedded semi-transparent mass flow facies (debrites, slumps) and rhythmically-laminated (varved) silty clays. Mass flows together with cratered (‘kettled’) lake floors and associated deformations reflect a dynamic ice-contact glaciolacustrine environment. Sub-bottom profiles identify sediment faulting, diapiric deformation and slumping on many lake floors located above Precambrian terrane boundaries. Exceptionally thick mass flow successions and extensive faults in Lake Timiskaming located along the floor of the Ottawa-Timiskaming Graben within the seismically active Western Quebec Seismic Zone, suggest a higher frequency of earthquakes and slope failure during deglaciation and rapid glacio-isostatic rebound, but faulting and failure of lakefloor sediments is ongoing confirming the hypothesis that the graben is a weak zone within the North American plate. Mapping of debris flows in nearby Lake Kipawa identifies a systematic distribution related to the epicenter of the 1935 Timiskaming earthquake (M6.2). Results support the model that lakes are natural seismographs and that ongoing mid-plate earthquake activity is a consequence of brittle deformation of the fractured and faulted upper crust of the North American plate.