ABSTRACT

A mosaic of basement blocks beneath the Montana plains east of longitude 108° west, has been mapped from 12 Landsat multispectral scanner images. Concentrations of individual linear features are interpreted as lineament zones which reflect block boundaries; blocks and boundaries have expression on maps of geophysical, stratigraphic and structural data. The basement blocks influenced deposition of Phanerozoic rocks, controlled Laramide tectonism, and are the sites of fluid migration. Block boundaries are interpreted to correspond with zones of weakness in basement rocks which were produced by Precambrian plate tectonic interactions of lithosphere plates.

Block boundaries have expression on a map of magnetic anomalies and a map of crustal thickness. Although most Laramide displacement on the block boundaries was dip-slip, assemblages of geologic structures suggest there has been at least some left-lateral movement along northeast zones and right-lateral movement along northwest zones. This sense of strike-slip displacement is in contrast with north-trending right-lateral displacement and northwest left-lateral displacement found to the south in Wyoming. But, the two strike-slip domains are consistent with the idea that the Colorado Plateau constitutes a microplate which has interacted with the North American plate along a broad margin. Fluid migration on block boundaries is documented by geothermal anomalies and by patterns of initial gas pressure in the Phillips Sandstone (Cretaceous) on Bowdoin Dome.