MAPPING PRECAMBRIAN BASEMENT DOMAINS BENEATH THE WESTERN CANADA SEDIMENTARY BASIN

The Western Canada Sedimentary Basin (WCSB) extends from the Canadian Shield in Manitoba to the Rocky Mountains of British Columbia. The general shape of the sedimentary section in the WCSB approximates a westward thickening wedge that attains a thickness of more than 5 km in western Alberta. The sedimentary rocks occupying the basin are only weakly magnetic, so the observed magnetic field is primarily due to variations in magnetization of the underlying basement (crystalline) rocks. Many geological units and structures in the Canadian Shield are associated with distinctive geophysical anomaly patterns and trends, observed in gravity and magnetic maps. These Precambrian geophysical signatures extend beyond the confines of the Shield over the adjacent Phanerozoic cover, often retaining a strong expression, in spite of attenuation caused by depth of burial. Calibrating gravity and magnetic signatures with Shield geology, augmented by extrapolation of signatures away from the Shield margin, provides one of the few available means for shedding light on the composition and structure of hidden Precambrian basement. Magnetic and, to a lesser extent, gravity maps are used here to map the extensions of first-order Precambrian domains under the WCSB. Furthermore, interpretations are made in the context that the basement architecture is the result of the amalgamation of many separate domains by the plate tectonic processes of subduction and collision. Several of the domains with the highest amplitude anomalies are interpreted as magmatic arcs, consisting of plutonic rocks such as calc-alkaline granites, which were formed over subduction zones. Domains with intermediate amplitude anomalies suggest the presence of metasedimentary, basic volcanic rocks and granitoid rocks, and may indicate accreted terranes. Some moderately magnetic domains contain areas of weakly magnetic lithologies with associated weak magnetic fields. Narrower regions of magnetic lows are interpreted as resulting from demagnetization effects accompanying collision. Since demagnetization zones are limited in areal extent, the wider, more extensive magnetic lows may reflect a combination of collision-related demagnetization and lower bulk magnetizations of local crustal lithologies.