Rates of crustal deformation in the southern Basin and Range (SBR) and Colorado Plateau (CP) provinces are relatively low in the context of the Pacific-North America plate boundary (PA–NA); however, the accumulation of small amounts of strain over long periods of time can lead to large earthquakes such as the Mw 7.5 1887 Sonoran earthquake in northern Mexico. SBR and CP rates of deformation are difficult to quantify due to a dearth of young faulting and seismicity. Moreover, strain accumulation and release related to the adjacent, more active San Andreas and Gulf of California fault systems to the west and southwest can mask the background strain rates associated with SBR and CP tectonics. With data from an enhanced continuous GPS network, we estimate crustal surface velocities of the SBR and CP, after removing coseismic and postseismic displacements, and elastic loading effects arising from major fault zones to the (south)west. We use cluster analysis and geologic data to separate the GPS velocity field into regions and calculate distinct block rotation and uniform strain rates for each region. We find the highest strain rate region includes southwestern Arizona; an area with sparse Quaternary faults, relatively low seismicity, and a relatively large discrepancy between geodetic and geologic rates of deformation. This anomalous strain rate may reflect residual, unmodeled PA-NA strain seeping into the Arizona study area from the west. Alternatively, it may represent the potential for one or more rare, future, large-magnitude earthquakes or indicate strain is being released through other process(es).