UPPER-MANTLE REFLECTIVITY BENEATH THE WILLISTON BASIN, PHASE-CHANGE MOHO, AND THE ORIGIN OF INTRACRATONIC BASINS

COCORP (Consortium for Continental Reflection Profiling) deep-reflecti on profiles crossing the northern Williston basin show (1) dipping ref lections within the crust of the underlying Trans-Hudson orogen that e xtend downward into the upper mantle and (2) no clearly defined reflec tion Moho. In contrast, Lithoprobe (Canada's national geoscience proje ct) profiles crossing the Trans-Hudson orogen north of the basin show dipping reflections within the crust that terminate at a sharply defin ed 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 Wi lliston basin. This remnant crustal keel much later underwent eclogite -facies metamorphism, which overprinted a new nonreflective Moho acros s the root and induced the subsidence of the Williston basin.