ABSTRACT: Basement Fault Reactivation beneath Sedimentary Platforms: Constraints from Alberta and Southern Ontario
David W. Eaton, Dept. of Earth Sciences, University of Western Ontario, London, ON, Canada, and Gerry M. Ross, Geological Survey of Canada, Calgary, AB, Canada
Canada's LITHOPROBE project recently has completed the Alberta Basement Transects (ABT), which included 2000-km of deep seismic profiles used to investigate the nature and extent of basement fault reactivation and tectonic inheritance. Excellent examples of this process, and lack thereof, are from the Peace River Arch district, and parts of central Alberta. Several Devonian reef chains, as well as Carboniferous normal faults and overlying Cretaceous depositional trends, are co-located with linear magnetic anomaly trends that reflect basement structural fabric. Vertical stacking and spatial coincidence of these linear features have prompted suggestions that ancient faults in western Canada have been intermittently reactivated during the Phanerozoic. The seismic profiles revealed spectacular basement structures and produced some surprises, such as a recognition of the important role of extensional forced folding, but have failed to provide compelling evidence for reactivation of discrete basement structures. This paucity of direct seismic evidence limits the scale of basement fault reactivation, if any, to less than a few tens of meters. Mechanical considerations indicate that thermal age of the mantle lithosphere, and hence its strength, plays a critical role in the reactivation of basement faults. In addition, although potential-field lineaments often correlate well with basement structures, strain hardening processes may have rendered these features mechanically stronger than adjacent crustal blocks. Prominent linear magnetic anomalies are also evident in parts of southern Ontario. Potential earthquake hazards associated with basement faults in this region remains a contentious issue. Recent (weak) seismicity exhibits at least one linear trend, but the relationship of this trend to the magnetic anomalies is uncertain.
Search and Discovery Article #90907©2000 AAPG Eastern Section Meeting, London, Ontario, Canada