West of the Mesozoic foreland Sevier orogenic belt in northern Utah and southern Idaho, a regional detachment of inferred mid-Jurassic age occurs in Upper Mississippian shale and can be traced westward to the margin of the coeval magmatic arc, represented by spaced calc-alkalic intrusions. The detachment may be the oldest and structurally highest thrust of the Idaho-Wyoming thrust belt. If so, it bridges the apparent spatial and temporal gap between magmatism and thrusting, thus suggesting a direct genetic link. Thin-skinned deformation is interpreted here to be driven from the heated magmatic arc toward the colder craton by lateral tectonic compression created by lateral volume increase due to intrusion, elevated crustal geotherms, and possible thickening in the arc. The metamorphic complexes occur at the crustal interface between these two thermal-tectonic regimes and may themselves be allochthonous.