PROTEROZOIC CRUSTAL TRANSITION BENEATH THE WESTERN CANADA SEDIMENTARYBASIN

New deep seismic reflection data outline a major crustal transition, f rom thick craton on the east to thin basinal crust on the west, that i s buried beneath the Phanerozoic rocks of the Western Canada sedimenta ry basin. The transition formed prior to, or during, deposition of wes tward-thickening Mesoproterozoic sedimentary rocks between about 1.85 and 0.9 Ga and appears to be the southward continuation of the Fort Si mpson structural trend, which is crossed by reflection data nearly 700 km to the north. It may be analogous to crustal ramps that are now co mmonly observed in many Phanerozoic orogens and, along with an associa ted decrease in Moho depth, has been preserved at this location for mo re than 1.0 b.y.