Abstract
Evidence has been obtained for the presence of a Precambrian rift in southern Alberta beneath 2.5 km of flat-lying Paleozoic and Mesozoic sediments. The feature has been studied by reflection seismology, gravity and magnetics. Several deep reflecting horizons appear to be continuous over at least 100 km and these outline a major east-west graben in the 2.0 · 10 9-year old basement. A reflecting horizon at an average depth of 30 km shows dips up to 20° and faults with vertical displacements as much as 5 km. Reflected energy from depths of 38–47 km probably correlates with the Mohorovičić discontinuity on the basis of reversed refraction in-line and broadside refraction profiles. This horizon also appears to be involved in the faulting. A combination of seismic velocities and Bouguer anomalies is used to develop a model of the crust and this indicates that the rift on the Precambrian surface is filled with lighter density material. The feature has been traced by gravity and magnetic trends for several hundred kilometers across Alberta and under the Rocky Mountains into British Columbia.
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