The extent and complexity of Laramide foreland basement deformation in the Wyoming province adjacent to the Idaho-Wyoming thrust-fold belt have led to numerous tectonic hypotheses, including tangential crustal compression (manifest as thrusting), vertical uplift, and strike slip; actually, all three types of deformation have occurred. With the advent of plate tectonics, tangential compression related to subduction has been mentioned most frequently as the ultimate cause of foreland deformation. Tangential compression is presumably operative in all subduction-related orogenic belts, but because most orogenic belts do not have foreland basement deformation comparable with that of the Wyoming province, compression alone does not seem sufficient to account for foreland basement structuring. Apparently a more specific condition is necessary. It is here proposed that in addition to tangential compression and strike slip, a lithospheric slab subducted well beneath the foreland is a fundamental requisite for deformation there, providing the buoyancy necessary for uplift of basement blocks. Recent work based on K2O/SiO2 ratios of Cenozoic andesites suggests that paleosubduction did occur far east within the Wyoming province (Lipman and others, 1971).

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