A GEOPHYSICAL OVERVIEW OF THE TRANSITION FROM THE ROCKY MOUNTAINS TO THE GREAT PLAINS: IMPLICATIONS FOR THE LINK BETWEEN PRECAMBRIAN STRUCTURE AND THE MODERN CRATON

The Rocky Mountain Front divides the high orogenic plateau of the western United States, a region that has been subjected to crustal addition and multiple episodes of deformation in Phanerozoic time, and the Great Plains, which has undergone relatively little tectonism. Yet, both areas share a Precambrian history that included accretion of crust from the south in early to middle Proterozoic time and episodic magmatism at ca. 1.3-1.4 Ga and 1.1 Ga. In recent years, the accumulation of new geophysical results from the Rocky Mountain–Great Plains transition in combination with existing data has led to a new understanding and new questions about the structure of that region. Seismic refraction data show that the crust shows little change in thickness at transition, that high-velocity material, suggestive of mafic underplating, occurs across the boundary, and that upper mantle velocity are relatively low, suggestive of low density and higher temperatures of the modern mantle. Deep crustal reflection data near the Rocky Mountain Front document the character of early Proterozoic accretion, whereas barely 50 km to the east, these data image sills and basins associated with craton stabilization and contemporaneous mild extension. Inexplicably, gravity models show little evidence for crustal density changes across the transition, even though rocks that are 1.3-1.4 Ga in age are exhumed from 10 to 15 km depth in the Rockies, whereas contemporaneous rhyolites are found at the basement surface beneath the Great Plains.