Description
Mid-Tertiary, extensional deformation is widespread in the detachment terrane of Arizona, California, Nevada, and Sonora. Detachment-related, upper-crustal folding and faulting have been described in ranges throughout this region, so that a general model of mid-Tertiary extension based on the presence and geometry of detachment faults has become generally accepted. Upper-crustal folding, or doming, without the presence of a distinct detachment fault, is also present in the region but has been largely ignored. The Dome Rock Mountains of western Arizona are perhaps the best examples of large-scale doming without the production of a visible detachment fault. Tertiary doming appears to have been deformed by penetratively distributed shear through the well-foliated rocks of the "lower-plate" of the Dome Rock Mountains Mesozoic clastic rocks. "Upper-plate" extension is clearly expressed by normal faulting and tilting of a distinctive Tertiary sequence of redbeds, tuffs, and volcanic rocks. Basin-and-range type normal faulting has also affected the area, creating a major repetition of both Tertiary units, and truncation of the Mesozoic clastic rocks of the Dome Rock Mountains along its steep western margin. Tilting of the "upper-Plate" indicates relative west to southwest transport of the "upper-plate" over inferred detachment faults at depth. Late Miocene alluvial units and the Miocene-Pliocene Bouse Formation are deposited unconformably over the rotated redbed, volcanic, and metamorphic rocks of the Dome Rock Mountains. Deformation by basin-and-range style faulting has rotated the Late Miocene alluvial fan sediments, indicating that extension occurred up to Late Miocene time. Mid-Tertiary crustal extension as displayed in the Dome Rock Mountains may be representative of a hitherto poorly recognized style of detachment-related deformation. Similar styles of deformation appear to have affected other regions containing thick sequences of well-foliated rocks, such as the Pelona-Orocopia Schist terrane of southern California and western Arizona.
