Placement of Basin and Range Faulting
Extensional forces of the Basin and Range province of Western North America are responsible for the linear north-south trending valleys partitioned by parallel mountain ranges that characterize the western United States. This large-scale extension has formed classic horst and graben topography resulting from brittle deformation of continental crust. As the crust is stretched normal faults develop as compensation for stresses caused by extensional forces. Fault blocks become detached and move downward as a result of gravity. The resulting flat-bottom basins form the grabens of the Basin and Range province. The remaining crust that does not settle downwards remains high relative to the basins. These topographic highs are the horsts of the Basin and Range.
Tectonic forces have been at work on the margins of North America since the continent was formed. These forces pulse through time and are more active during some periods of geologic history than others. Extension of the Basin and Range began during the Miocene Epoch (23 ma) and continues to the present day. Because of active faulting abrupt mountains characterize much of the region. Over time, as extension slows and stops, erosion will bevel off steep ridges and cliffs filling in basins and flatten the topography of the region.