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

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