COCORP (Consortium for Continental Reflection Profiling) deep-reflection profiles crossing the northern Williston basin show (1) dipping reflections within the crust of the underlying Trans-Hudson orogen that extend downward into the upper mantle and (2) no clearly defined reflection Moho. In contrast, Lithoprobe (Canada's national geoscience project) profiles crossing the Trans-Hudson orogen north of the basin show dipping reflections within the crust that terminate at a sharply defined subhorizontal reflection Moho—a reflection character typical of the interior of collapsed Phanerozoic orogens. We suggest that a crustal root produced during Hudsonian collision was incompletely removed by “normal” postorogenic extension in the region now underlain by the Williston basin. This remnant crustal keel much later underwent eclogite-facies metamorphism, which overprinted a new nonreflective Moho across the root and induced the subsidence of the Williston basin.

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