The older units are complexly faulted and steeply tilted by several generations of normal faults. Early closely-spaced subhorizontal to moderately-dipping, NW-striking normal faults repeat tilted sections and document large magnitude extension oriented ~N50¡E. North of a NE-trending accommodation zone, tilts are consistently to the northeast and normal faults have mainly top-to-the-southwest displacement. South of this zone, the polarity of faulting and tilting is reversed. Younger, widely spaced NS- to NNW- trending, high-angle normal faults cut the previously faulted and tilted sections and are associated with modest ~E-W extension. Kinematic indicators on some of the steep faults suggest late-stage oblique-slip or strike-slip displacement. Palinspastic reconstruction of a section north of the accommodation zone indicates cumulative extension of ~90%. Angular unconformities and growth fault relations within the Neogene sequence indicate that large-magnitude NE-SW extension occurred between 26 and ~20 Ma, and that extension in the northern tilt domain is slightly older than that in the southern one. Subsequent E-W extension of ²10-15% and associated basin formation is largely bracketed between 20 and 17 Ma.
The magnitude of Neogene extension in southeastern Sonora is substantially greater and the timing is older than previously recognized. Reconnaissance of selected areas between the unextended Sierra Madre Occidental and the coast at this latitude, together with previously published data, suggests that most extension in Sonora occurred between ~27 and 12 Ma, while remnants of the Farallon plate were still being subducted and the magmatic arc shifted westward. The inception of extension was often associated with mafic volcansim, following a period of relative tectonic and magmatic quiescence during the Eocene.
Plate-tectonic models that require substantial extension in Sonora during proto-Gulf (~10 - 5 Ma) transtensional deformation may need to be re-evaluated. An alternative model, presented here, is that Baja California began moving with close to Pacific plate motion coincident with the ~11 Ma termination of subduction at this latitude. In this model, post-11 Ma northwest motion of Baja relative to mainland Mexico would total ~500 km and would be accomplished by both proto-Gulf (10-5 Ma) and modern Gulf (5-0 Ma) distributed right lateral shear and oblique rifting within the previously extended and thermally weakened Mid-Miocene magmatic arc. This speculative reconstruction has the advantage of obviating the need for rapid, large magnitude post-11 Ma NE-SW extension in Sonora and may provide a better match of the pre-Late Miocene geology on either side of the Gulf.