GEOPHYSICAL MAPPING OF THE ARCHEAN BENEATH THE NORTH AMERICAN CENTRAL PLAINS (Invited Presentation)

Knowledge of the architecture of Archean lithosphere helps in understanding the formation and destruction of cratons and how tectonic processes evolved through Earth history. The North American Central Plains are underlain by a collage of Archean cratons stitched together by Paleoproterozoic mobile belts during the formation of supercontinent Columbia (2.1 -1.8 Ga). Concealed beneath sedimentary rocks of the Western Interior Seaway, fundamental questions related to the extent and internal makeup of these cratons are frustrated by limited age dating and a paucity of drill holes intercepting basement.

We present a 3D electrical resistivity model derived from more than 1000 stations of EarthScope and LithoProbe magnetotelluric data. The model covers Archean components of Laurentia including the entire Wyoming Craton and parts of several adjacent cratons. The eastern and southern margins of the Wyoming craton are clearly delineated by deep-crustal conductive belts diagnostic of modern-style subduction along the Trans-Hudson orogen and Cheyenne Belt, respectively. The western margin marks the edge of the Basin and Range extensional province, while the northwest corner of the craton shows evidence of lithospheric modification by Yellowstone-Snake River Plain magmatism.

Internal to the Wyoming craton, Laramide uplifts dominate the geophysical signature of the crust and are recognized to >20 km depth. This character is distinct from that of neighboring cratons largely unaffected by Laramide deformation. In the lithospheric mantle, geophysical subdomains of the Wyoming Craton are recognized and loosely reflect its geologic division into the Montana Metasedimentary Belt, the Beartooth-Bighorns Magmatic Terrane and the Southern Accreted Terranes. Spatial correlation between a lithospheric-scale resistivity boundary, the Neoarchean Oregon Trail structure and a series of aligned Laramide uplifts is consistent with repeated reactivation of an ancient zone of crustal weakness.

Finally, the geophysical signatures of the internal Neoarchean boundaries are distinct from the Paleoproterozoic craton boundaries. The absence of linear high-conductivity belts, a product of continental-margin sediments transported to depth and metamorphosed during subduction, hints at fundamental changes to the process of continental growth at the end of the Archean. Such changes may relate to the rapid emergence of the continents during this time and accompanying increases in subaerial weathering and the growth of continental margins.