The Purcell anticlinorium in southwestern Canada and the northwestern United States is situated at the transition from deformed North American rocks of the foreland thrust and fold belt to accreted rocks of the Kootenay arc. Nearly 1000 km of industry seismic reflection data were processed with extended VIBROSEIS correlation and coherency filtering and then combined with LITHOPROBE reflection data to outline the three-dimensional crustal structure of the anticlinorium. Reflection Moho is visible at ∼12.0 s (∼36 km) beneath the anticlinorium and deepens eastward to ∼15.0 s (∼45 km) beneath the Rocky Mountain thrust and fold belt. The transition from thick to thin crust coincides with westward deepening of North American basement and with the southern Rocky Mountain Trench, a basin or series of basins formed during Tertiary extension. Regionally extensive reflections in the upper crust correlate with ca. 1468 Ma gabbroic sills in the Aldridge Formation of the Mesoproterozoic Belt/Purcell Supergroup. Features outlined by the sills support an interpretation that the anticlinorium formed during Mesozoic contraction when imbricate thrust faults carried up to 15 km of Belt/Purcell and Paleozoic margin sedimentary rocks, and basement in some areas, eastward over a prominent basement ramp.