Crustal structure beneath the Western Canada Sedimentary Basin: constraints from gravity and magnetic modelling

Abstract

Regional gravity and magnetic anomalies that originate from crystalline basement rocks extend over many parts of the Western Canada Sedimentary Basin. Although these potential-field anomalies provide a basis for tectonic subdivisions of the basement crust, most previous interpretations of these features have been largely qualitative in nature. This study focuses on numerical simulation and quantitative interpretation of five regional potential-field anomalies in Alberta, Canada, for which independent constraints on crustal structure are available from Lithoprobe seismic and electromagnetic studies. The Kimiwan High (~55°N, 116°W) is a roughly 250 km long linear magnetic high. Seismic profiles across this anomaly provide evidence for a crustal-scale extensional fault system that offsets the Winagami reflection sequence, a series of mid-crustal sills. We find that the magnetic anomaly can be modelled as either a 15–40 km wide zone of moderate positive susceptibility (4.5 × 10–3 SI units) in the hanging wall of the detachment (5–17 km depth), or as a narrower (5–10 km), steeply dipping zone of high susceptibility (2.5 × 10–2 SI units) in the footwall (16–32 km depth). We interpret the former scenario as indicative of an extensive zone of alteration above the fault, whereas the latter could represent a decapitated granitic pluton that correlates with magmatic rocks farther north. To the southeast, the Thorsby Low (~53°N, 114°W) is a sinuous, 400 km long magnetic low and gravity gradient trend that appears to be a splay of the much more extensive Snowbird tectonic zone. Previous seismic interpretations across the Thorsby Low indicate that it coincides with a 10 km offset in the Moho. Our results show that this abrupt change in crustal thickness is consistent with, but not required by, the gravity signature of this feature. The northeast-striking Red Deer High (~53°N, 112°W) is a narrow magnetic anomaly in central Alberta with variable intensity along strike. Previous magnetotelluric studies suggest that the Red Deer anomaly is closely associated with a linear, highly conductive body in the upper basement. Our modelling results confirm the shallow depth of the causative body and suggest an eastward dip that is consistent with published seismic interpretations. Near the eastern border of Alberta, the 300 km long Eyehill High (~52°N, 110°W) is a prominent north-striking magnetic anomaly adjacent to the western hinterland of the Trans-Hudson Orogen. Combined gravity and magnetic modelling show that this feature occurs near the western boundary of a large block of dense material in the lower crust. The magnetic anomaly can be simulated by a near-vertical dyke-like body in the upper crust. Lastly, the Vulcan structure (~50.5°N, 112°W) forms the ~400 km long, northern boundary of the Archean Medicine Hat block. It is defined by a sinuous east-trending magnetic anomaly and gravity low. The wavelength and polarity of the magnetic anomaly (positive in the north), coupled with the coincident gravity low, are most simply explained by a mid-crustal low-density body with significant remanent magnetization oriented antiparallel to the present-day field.