We have estimated patterns and rates of crustal movement across 800 km of the Basin and Range at ~39¿ north latitude with Global Positioning System surveys in 1992, 1996, 1998, and 2002. The total rate of motion tangent to the small circle around the Pacific-North America pole of rotation is 10.4 ¿ 1.0 mm/yr, and motion normal to this small circle is 3.9 ¿ 0.9 mm/yr compared to the east end of our network. On the Colorado Plateau the east end of our network moves by ~1--2 mm/yr westerly with respect to North America. Transitions in strain rates delimit six major tectonic domains within the province. These deformation zones coincide with areas of modern seismicity and are, from east to west, (1) east-west extension in the Wasatch Fault zone, (2) low rate east-west extension centered near the Nevada-Utah border, (3) low rate east-west contraction between 114.7¿W and 117.9¿W, (4) extension normal to and strike-slip motion across the N10¿E striking Central Nevada Seismic Zone, (5) right lateral simple shear oriented N13¿W inside the Walker Lane Belt, and (6) shear plus extension near the Sierra Nevada frontal faults. Concentration of shear and dilatational deformation across the three westernmost zones suggests that the Walker Lane Belt lithosphere is rheologically weak. However, we show that linear gradients in viscosity and gravitational potential energy can also effectively concentrate deformation. In the Basin and Range, gradients in gravitational potential are spatially anticorrelated with dilatational strain rates, consistent with the presence of horizontal variations in viscosity of the lithosphere.