Releveling and other geophysical data for the Imperial Valley of southern California suggest the northern section of the Imperial-Brawley fault system, which includes the Mesquite Basin and Brawley Seismic Zone, is much younger than the age of the valley itself. A minimum age of 3000 years is calculated for the northern segment of the Imperial fault from correlations between surface topography and geodetically observed seismic/interseismic vertical movements. Calculation of a maximum age of 100,000 years is based upon displacements in the crystalline basement along the Imperial fault, inferred from seismic refraction surveys. This young age supports recent interpretations of heat flow measurements and the evolution of geothermal systems, which also suggest that the current patterns of seismicity and faulting in the Imperial Valley are not long lived. The current fault geometry and basement morphology suggest a northwestward growth of the Imperial fault and a northwestward migration of the Brawley Seismic Zone. If this localized process if representative of more regional tectonic processes along the extent of the Salton Trough, we suggest that this migration is a manifestation of the propagation of the Gulf of California rift system into the North American continent. ¿1991 American Geophysical Union