Is the Wyoming Craton Being Decratonized?

The Wyoming craton is one of the Archean blocks that form the core of Laurentia. It is relatively small compared to most of the other blocks that collided in the Paleoproterozoic (1.8-2.0 Ga) to form the Canadian Shield. The Wyoming craton is bounded on the east by the Trans-Hudson. Its original boundary on the west is indeterminate because of the Neoproterozoic rifting along the western margin of Laurentia. Finally, its southern boundary was a passive rifted margin at 2.4-2.1 Ga as evidenced by the presence of the miogeoclinal strata of the Snowy Pass Supergroup in southern Wyoming. The recent DEEPROBE and CD-ROM seismic experiments produced considerable new insight on the deep structure of the Wyoming craton and the nature of its southern boundary. The DEEP PROBE experiment showed that the Wyoming craton has very thick crust (~50-60 km) that has an unusually thick mafic lower layer. Xenolith data from the Sweet Grass Hills near the U. S.-Canadian border indicate that at least part of this lower crust was underplated at about 1.8 Ga. From that time until the Laramide, the Wyoming craton was stable and avoided tectonism and significant subsidence beyond having the Williston basin form along its northeastern margin. However, Laramide deformation produced a widespread series of basins and uplifts that are bounded by deep-seated thrust faults with typical dips of 45 deg. These structures form large arches that gravity data shown produce a network that crosses the State of Wyoming. Today various tomographic images show that the deep cratonal root of North America does not fully extend beneath the Wyoming craton. In addition, the Yellowstone hotspot is modifying the lithospheric structure drastically. These data suggest that tectonism along the western margin of North America has been decratonizing the Wyoming region for the better part of 100 million years.