Cainozoic evolution of the state of stress and style of tectonism of the Basin and Range province of the western United States

Cainozoic evolution of the modern plate boundary along the western United States from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western United States. Extensions tectonism responsible for the modern Basin and Range province appears to represent a unique late-stage episode of a much longer period of extension initiated in an ‘in tra -arc ’ setting contemporaneously with calc-alkaline magmatism. Basin-range extension is distinguished from early extension on the basis of angular unconformities, differences in fault trends and spacing, and associated magmatism (basaltic). Prebasin-range extension (i.e. extension preceding the break-up of the region into ranges resembling the modern ones) was under way locally by at least 30 Ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks, by large regions of the crust underlain by passively emplaced subvolcanic batholiths, and by the thickness and distribution of stratigraphic units. Locally, high strain rates that accompanied early extensions of as much as 50-100 % are implied. Data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform WSW -ENE least principal stress orientation in the period ca. 20-10 Ma, during this early extension. The change from early extension to basin-range style faulting of the upper 15 km of crust, which resulted in broadly spaced ranges (25-35 km crest-crest spacing), was time-transgressive and probably not abrupt; locally both types occurred concurrently. Southern Basin and Range block faulting occurred largely in the period 13-10 Ma, in response to a stress field orientated similarly to that responsible for the early extension. In contrast, northern Basin and Range block faulting developed after 10 Ma and continues to the present in response to a stress field orientated approximately 45° clockwise to the earlier stress field. This modern stress field, with a WNW -ESE to E-W directed least principal stress, characterizes the entire modern Basin and Range province and Rio Grande rift region. The 45° change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the San Andreas transform fault. Inclusion of pre-basin-range extension may help resolve the discrepancy between estimates of 15-30% for basin-range block faulting and total extension estimates of 100-300 % for the Basin and Range province.