Deep seismic crustal studies in Manitoba

Abstract

abstract The results of 84 deep seismic sounding recordings scattered over an area of 250,000 km 2 (average spacing, 25 km) with each recording yielding up to 3 depth points for each interface mapped, are compiled on maps giving crustal thickness and depth to the transition between an upper and a lower crustal layer. Ranges are from a few kilometers up to about 700 km. A velocity model is derived as follows: upper crustal layer, α = 6.05 km/sec, β, = 3.46 km/sec; lower crustal layer, α = 6.85 km/sec, β, = 4.00 km/sec (with the strong possibility that these values are averages, with a small vertical velocity gradient in the lower layer); uppermost part of upper mantle, α = 7.92 km/sec, β, = 4.60 km/sec. A sub-Mohorovicic event, showing triplication, is found. It may represent a boundary in a layered medium, or a second-order discontinuity in a medium with a vertical gradient. If it is the former, a velocity transition to a velocity in the range 8.3 to 8.4 km/sec is indicated at an average depth of 50 km. Lateral velocity variation was found to be small. A method of calculating velocities from wide-angle reflections for a 3-layer medium overlying a uniform half-space is given. The derived deep crustal structure yields important geological information, showing downward extension of geological features visible at the surface. These include the boundary between the Churchill and Superior geological provinces of the Canadian shield, and a trough-like thickening of the upper crustal layer (accompanied by a thinning of the total crustal section) which has been traced 350 km from the English River area of Ontario, across the south basin of Lake Winnipeg, Manitoba. It lies beneath a geological unit known as the English River gneissic belt. In addition, faulted boundaries of crustal blocks are shown in the deep seismic structure. The derived seismic structure shows correlation with regional gravity and magnetic data.