Crustal extension in the Basin and Range Province, southwestern United States

Abstract

Summary Cenozoic extension of areally varying ages and amounts has on average doubled the width of the Basin and Range Province. Extensional structures that formed at all depths down to 20 km, and which range in age from Oligocene to Holocene, are widely exposed and are here interpreted in terms of a model of depth-varying deformation. The middle crust is extended by discontinuous ductile shear as internally underformed lenses slide apart along gently dipping zones of mylonite. The tops of these lenses are undulating detachment faults, the composite area of which increases with time as deep lenses slide out from underneath shallower ones. Brittle blocks of upper-crust bedrock above the detachments respond first by rotating between range-front faults, the same direction of rotation being maintained across a series of lenses, and then by pulling completely apart, while basinal strata fill the gaps and are dragged directly on detachment faults. Some faults rise gently from the main detachment zones and surface as range-front faults. Most tilted-block ranges are isolated atop detachments. Detachment faults cut out crust. Beneath them are mid-crustal rocks of any age and type and above them are mostly upper-crustal rocks, including extensive syndeformational basin sediments rotated to steep or moderate dips. As attenuation proceeds and components rise, detachment faults evolve from ductile to brittle, develop splays, and are themselves broken by steep brittle structures related to new, deeper detachments. Parts of detachment faults remain active even after exposure at the surface, but slip on them is then limited to the down-dip direction. It is inferred from seismic reflection profiles and rock-mechanic considerations that the unexposed lower crust is extended by more pervasive ductile flattening.