Paleotectonic restoration of Miocene extensional strain of crystalline rock provinces and terranes in southwestern North America (Arizona, California)

The crust of southwestern North America records diverse deformational events that took place during Mesozoic and Tertiary time. Among these are: two periods of extension since Oligocene time, contraction during Cretaceous time and Late Jurassic strike-slip faulting. This dissertation focuses upon the extensional episodes during Tertiary time.; A retrodeformable model of reconstruction, based upon restoration of north-trending pull-apart basins connected by northwest-striking right-lateral strike-slip faults, is proposed for Late Miocene and later displacements east of the San Andreas in southwestern North America. Simple shear strain is assumed within this area. Both reconstructions account for distributed deformation since about 25 Ma when transforms became active along the plate margin. Restoration of Late Miocene extension in the field area shows that southwestern North America has expanded approximately 9–10% westward over the past 12 Ma.; Although the older Cretaceous and Jurassic deformations are not restored, the distribution of Precambrian rock provinces and terranes, in part resulting from dislocations along the hypothetical Mojave-Sonora megashear (a Late Jurassic transform), provides important constraints upon extensional strain. Study of the Precambrian rocks in the Quitobaquito Hills reveals the relationship of rock units, including quartzofeldspathic gneiss intruded by aplite dikes, that are comparable to exposures in the San Gabriel Mountains of California. These rocks are considered part of the San Gabriel terrane and suggest that the Quitobaquito thrust is a Late Jurassic tranpressional structure related to the megashear. Basement rocks in the Mohawk, and Agua Dulce Mountains, Sierra Pinta, and northern O'Neill Hills are correlated with units comprising the Yavapai and Mazatzal provinces. Precambrian exposures within Sierra el Choclo Duro in Sonora, Mexico, and Quitobaquito Hills of Organ Pipe Cactus National Monument, Arizona, reveal rocks similar to both the San Gabriel and Caborca terranes. Field mapping, the distribution of fault displacements, geophysical data, and elementary remote sensing analysis of Landsat imagery also constrain the trace of the megashear, along which the composite San Gabriel and Caborca terranes were juxtaposed against autochthonous Mojave, Yavapai, and Mazatzal/Pinal province rocks of North America.