Many Tertiary normal faults in the southwestern United States dip gently (<30°). Some of these normal faults apparently were initiated with gentle dips, in conflict with the predictions of elementary dynamic analysis. Close correspondence between low-angle normal faulting and intense hydrothermal alteration in the Mojave Desert, western Arizona, and the Rio Grande rift is consistent with the hypothesis that hydrothermal systems play an important role in the genesis of these faults. We present a mechanical model to demonstrate that low-angle normal faults could form in response to stress trajectories that become reoriented in a periodically sealed geothermal system beneath a sloping surface.

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