The Precambrian Geologic Framework and Crustal Structure of the North-Central United States: What We Know and What We Need to Learn

A combination of geologic mapping, geophysical studies, and geochronologic investigations over the last few decades has greatly improved the geologic framework of the Precambrian rocks of the north-central United States. The area includes the southernmost extension of the Neoarchean (ca 2800-2600 ma) Superior Province, an accreted complex of greenstone-granite belts that essentially forms the core of the North American craton. The Minnesota River Valley gneiss terrane forms the southern terminus of the Superior Province, and is thought to represent an accreted block of Mesoarchean (ca 3500-3400 ma) crust. A series of Paleoproterozoic orogens bound the Superior Province, including the ca 1900-1800 ma Trans-Hudson Orogen to the west, the ca 1880-1830 ma Penokean orogen to the east, and the ca 1800-1730 ma Yavapai orogen to the south. The Archean and Paleoproterozoic crust of the region is cut by the ca 1100 ma Midcontinent Rift System, which was associated with extensive volcanism and sedimentation at the surface, and extensive plutonism at depth. In short, the Precambrian geology of the north-central United States presents a 2500 my window that records several of the major formative events of the North American craton.

Unfortunately, many significant gaps persist in our geologic knowledge of the region. Relatively little is known regarding the crustal structure of the southern extension of the Superior province, or of the Paleoproterozoic orogens that surround it. The Precambrian geology of the Trans-Hudson Orogen and the adjoining Superior Province in the Dakotas is still poorly understood, owing at least in part to poor quality gravity and aeromagnetic coverage. Although projects like the Transportable Array of the EarthScope initiative will tremendously improve our understanding of the sub-crustal lithosphere in the region, a complete 4-dimensional perspective of the lithosphere will not be possible until more geophysical investigations are conducted with a focus on crustal structure.