To investigate the behavior of the lithosphere undergoing extension, we use a simple rheological model broadly consistent with experimental data on rock creep and with the nature of the brittle/ductile transition. A plastic surface layer overlies a substrate that deforms by power law creep with a stress exponent n=3 and an effective viscosity that decreases with depth. In extension this model shows a strong necking instability provided that the thermal gradient is sufficiently large: otherwise, stable uniform extension is indicated. The predicted structures display uniformly spaced necks or regions of enhanced extension (basins) alternating with regions of reduced extension (ranges). If the depth to the brittle/ductile transiton is roughly 10 km as suggested by the maximum depth of seismic faulting the model yields spacings for the incipient Basin and Range structures of about 25-60 km in excellent agreement with observation.