A broad region of late Paleozoic tectonism along the Laurentian margin from central Nevada (United States) to Sonora (Mexico), including offshore western terranes, is herein termed the Southwestern Laurentian Borderland (SLaB). Carboniferous–middle Permian sinistral translation within the SLaB explains a wide range of apparently disparate geologic observations, including (1) diachronous spatial distribution of regional structures and continental-margin sedimentary basins; (2) sub-regional unconformities and associated structures in central Nevada and Sonora; (3) displacement of the southwest part of the early Paleozoic passive margin, which now comprises the Caborca block in Sonora; and (4) distribution from central Nevada to Sonora of allochthonous lower Paleozoic deep-water sandstones with distinctive detrital zircons derived from northern Laurentia. Northward latitudinal translation of Laurentia by ∼3000 km proposed by recent plate-tectonic models provides a mechanism for displacement of the continent relative to outboard western lithospheric domains. The SLaB was a sinistral transpressive plate boundary active from Mississippian to middle Permian time. As defined here, it directly followed the Antler collisional event and ended prior to the Sonoma orogeny.