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Geologic relations and data from strontium and oxygen isotopic studies are used together to speculate on the paleogeographic positions of various crustal blocks or terranes in southwestern North America with respect to the middle Paleozoic continental margin. The southwestern margin of the middle Paleozoic continent, based on these data, is quite irregular in its present configuration, but most of the irregularities can be accounted for by strike-slip displacement of continental fragments during several post-middle Paleozoic tectonic events. We conclude that large-scale late Paleozoic sinistral displacements and Mesozoic and Cenozoic dextral displacements have been the major events affecting the configuration of this margin. Our preferred model includes the following elements: (1) 500 km of left-lateral displacement of a large part of the margin during a late Paleozoic truncation, (2) slivering of the margin by right-lateral faulting during the Mesozoic, amounting to about 275 km of total displacement, and (3) further deformation of the margin during the Cenozoic, primarily by right-lateral faulting, oroclinal bending, and crustal extension.