Westward underthrusting of thick North American crust: The dominant thickening process that built the Cordilleran orogenic plateau

Quantification of the crustal thickening processes that construct orogenic plateaus is essential for interpreting their genesis. In the North American Cordillera, a 2.75−3.5-km-elevation, 200−250-km-wide plateau was constructed to the west of the Cretaceous−Paleogene Sevier fold-and-thrust belt (SFTB). The SFTB deformed a Mesoproterozoic to Mesozoic sedimentary package that thickened westward from a 2−3-km-thick platform section that was deposited above the ∼40-km-thick craton to a 15−25-km-thick continental margin section that was deposited above middle to lower crust that had been significantly thinned during Neoproterozoic rifting. Shortening in the SFTB translated this thick sedimentary package as much as 265 km eastward, which resulted in the relative westward underthrusting of an equivalent length of thick cratonic basement beneath the hinterland region. Measurement of components of thickening with respect to the initial and final crustal thickness above and below the basal thrust décollement demonstrates that thickening accommodated by underthrusting outweighed thickening in the overlying SFTB by a factor of 1.5−3 and was likely the dominant thickening mechanism that constructed the broad hinterland plateau. In eastern Nevada, the reconstructed western edge of the underthrusted craton underlies the western limit of 2.75−3.5 km paleoelevations, which supports this interpretation. This analysis provides an important case study for underthrusting as a first-order thickening process in fold-and-thrust systems that deform sedimentary packages with a high pre-orogenic taper.