New structural and 40Ar/39Ar data from the Santa Rosa area, southeastern Sonora shed light on the magnitude and timing of extensional tectonism. Episodic magmatic activity from the Late Cretaceous to the Miocene is recorded by a 3--4 km thick structural/stratigraphic section that includes a composite Laramide granodiorite batholith and its andesitic wall rocks, early to middle Tertiary rhyolite ignimbrites and silicic domes, and late Oligocene to middle Miocene mafic lavas interbedded with tuffaceous sediment and fanglomerate. The older units are complexly faulted and steeply tilted by several generations of normal faults. Palinspastic reconstructions indicate cumulative extension of ~90%. Early gently dipping, NW striking normal faults document major extension oriented ~N50 ¿E. Angular unconformities and growth fault relations within Oligo-Miocene sequences bracket this extension between 26 and ~20 Ma. Younger, widely spaced NS to NNW trending, high-angle normal faults cut the previously faulted and tilted sections and produced modest (10--15%) E-W extension that is largely bracketed between 20 and 17 Ma. The magnitude of Neogene extension in this region is greater and the timing is older than previously recognized. Reconnaissance of other areas between the Sierra Madre Occidental and the coast at this latitude suggests that most extension in Sonora occurred between ~27 and 12 Ma, while remnants of the Farallon plate were still being subducted. Plate-tectonic models that predict substantial extension in Sonora during proto-Gulf (~10-5 Ma) transtensional deformation may need to be reevaluated. An alternative model, presented here, is that Baja California began moving with Pacific plate motion shortly after the ~11 Ma termination of subduction and that northwest motion of Baja relative to mainland Mexico might total 500 km, distributed across the previously extended Oligo-Miocene magmatic arc.¿ 1997 American Geophysical Union