Seismic data and drilling in the Iron Springs area of southwestern Utah show that the Three Peaks pluton is a laccolith and overlies a large anticline in Paleozoic and Mesozoic rocks. The Iron Springs area is in the structural transition zone between the Colorado Plateau and the Basin and Range provinces and has been structurally high since the Early or Late Cretaceous. The Iron Springs thrust fault formed during Sevier deformation and shows ∼3.5 mi of eastward displacement. Crustal thickening during Sevier deformation caused rapid maturation of organic material, and hydrocarbons migrated to the east along the Iron Springs thrust fault. The Iron Springs thrust apparently prevented hydrocarbon migration into the anticline beneath the laccolith, but allowed hydrocarbon migration to the surface at Iron Springs, forming a large surface-soil hydrocarbon anomaly.