As the North American crust interacts with the migrating Mendocino triple junction, the crust is first significantly thickened and then equivalently thinned over a distance of a few hundred kilometers (within a time frame of 5 m.y. or less). This process of ephemeral crustal thickening is proposed to result from viscous coupling between the northward-migrating Gorda slab and the base of North America south of the triple junction. A time-dependent, thermal-mechanical finite-element model is developed to test this hypothesis of plate-boundary tectonics. Results of the numerical simulations show patterns of crustal deformation consistent with the mapped sequence of folding and faulting in the area, the observed crustal structure and triple junction regional seismicity, and localized regions of crustal extension coincident with the position of a hypothesized lower-crustal melt zone.