Basement Tectonics and Fault Reactivation in Alberta , Canada

Injection-induced seismicity in western Canada and elsewhere in North America has drawn considerable recent interest. Current models indicate that induced earthquakes occur on reactivated basement faults, which can be challenging to detect using seismic-reflection data. Here we use regional gravity and magnetic datasets, together with LITHOPROBE crustal seismic profiles, to investigate basement tectonics and crustal structure in an area of western Canada that is prone to induced seismicity. Previously mapped basement faults that were active during the Paleozoic can be recognized on the basis of pronounced curvature, truncations and/or offsets of stratigraphic marker horizons. Within the Precambrian crystalline basement, however, brittle faults are poorly imaged by seismic data due to various factors such as the obscuring effect of multiples. Regional potential-field fabrics are critical to establish the tectonic setting of basement domains, with complementary information provided by magnetic, Bouguer and isostatic residual gravity anomalies. Based on 2D modelling constrained by seismic profiles, however, individual faults appear to lack diagnostic expression in regional potential-field anomaly data since the anomalies are dominated by the effects of larger-scale crustal structures. We show evidence that large-scale basement faults can potentially be recognized on the basis of truncation and offset of distinct horizons within the Winagami Reflection Sequence (WRS), which is interpreted as a regionally-extensive mid-crustal sill complex emplaced during a Proterozoic magmatic pulse. An abundance of caution is necessary to interpret these features, due to complications arising from out-ofplane reflections at long reflection times.