PALEOTECTONIC RECONSTRUCTIONS OF TERTIARY EXTENSION IN SOUTHWESTERN NORTH AMERICA

Tertiary extension in California and Arizona south of the Garlock fault, east of the San Andreas fault, and north of the Pinacate volcanic field may be restored by applying a two-step process. (1) West-directed Late Miocene extension within a broad zone of simple shear is reconstructed by restoring fault displacements along right-lateral strike-slip faults that close north-trending pull-apart basins at releasing steps. Cumulative Late Miocene and later displacements of ~84 km along right-lateral faults in southern California accommodate part of the ~52 km (8.8%) of westward extension from the Santa Rosa Valley westward to the western Mojave Desert. (2) Early Miocene southwest-directed extension is reconstructed by closing basins in detached hanging walls and restoring offset on detachments perpendicular to normal faults striking 330° and parallel to lineations trending 060°. Northeast-striking accommodation zones may be the principal structures that bound panels of tilted strata. Detached strata west of the core complexes are restored northeastward 20 km (Spencer and Reynolds, 1991). Pure shear strain (~100% ?) in core complexes (interpreted as boudins of ductile crust) at the southwestern margin of the Colorado Plateau is not restored. Large rafts of crust west of the core complexes that record little extension suggest the absence of high strain in these areas. Aligning offset segments of the Mojave-Sonora megashear across the Gila River line results in partial restoration of extensional strain in southwestern Arizona. Restoration of Early Miocene strain in the Mojave Desert does not align regional pre-Miocene structures such as the Independence dike swarm. A paleogeographic bulge of Precambrian basement and cover west of the Colorado Plateau protrudes westward and coincides with curved belts of ductile and brittle thrusts (e.g., Maria fold and thrust belt and Santa Rosa mylonite zone), many of which are west-directed (Silver, 1983; May, 1989).