Crustal structure and evolution of the Trans‐Hudson orogen: Results from seismic reflection profiling

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Abstract
A 400‐km‐long deep seismic reflection transect across northeastern Montana and northern North Dakota reveals the crustal‐scale structural fabric of the Early Proterozoic Trans‐Hudson orogen beneath the Williston basin. Comparison with deep seismic reflection data across the Canadian portion of the same orogen ∼700 km to the north reveals first‐order similarities in crustal architecture but documents significant along‐strike variation in orogenic evolution. Both transects display a broad crustal‐scale antiform axial to the orogen. In the north, geologic data suggest that this antiform is cored by an Archean microcontinent. In the south, west dipping reflections on the western flank of the antiform extend from the upper crust to the uppermost mantle and truncate prominent subhorizontal lower crustal reflections of the Archean Wyoming craton. Within the Wyoming craton, the eastern limit of east dipping midcrustal reflections coincides with the subsurface age boundary between the craton and the Early Proterozoic Trans‐Hudson orogen as interpreted from potential field and drill core data. On the basis of subsurface geochronologic data from the crystalline basement and by analogy with the Glennie domain within the exposed Trans‐Hudson orogen in Canada, we suggest that the southern antiform is cored by an Archean crustal fragment that was caught up in the terminal collision of the Wyoming and Superior cratons during Hudsonian orogeny. The eastern side of the Trans‐Hudson orogen is characterized on both seismic transects by predominantly east dipping crustal penetrating reflections. We interpret the easterly dip of these reflections as evidence that the Superior province was thrust westward over the interludes of the orogen during terminal collision. Although juvenile Early Proterozoic terranes characterize the exposed segment of the Trans‐Hudson orogen in Canada, limited drill core information within the Dakota segment of the orogen shows a predominance of granulitic Archean age crust. This difference in basement lithologies along strike within the orogen may indicate the following: either juvenile crust comparable to that exposed in the northern Trans‐Hudson was never present in the south, or it was removed by progressive over thickening, erosion, and/or faulting. Postorogenic extensional collapse may be responsible for preservation of juvenile terranes in the north.