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One of the least-known aspects of the evolution of the Gulf of Mexico is the nature and location of shear zones along which the relevant continental fragments were displaced. The Sierra de Juarez mylonitic complex, located in southern Mexico, is a poly-orogenic north-northwest-trending structure. Here we report U-Pb myionitization dates of 165 [+ or -] 20 Ma for igneous zircon from the syntectonic San Felipe granite, and an integrated 40Ar/39Ar age of 169.3 [+ or -] 1.7 Ma from synkinematic muscovite, both of which indicate a Middle Jurassic age for the strike-slip event along the Sierra de Juarez mylonitic complex. This event therefore occurred during the opening of the Gulf of Mexico, and we propose that the shear zone was kinematically related to the southeast displacement of the Yucatan block.

The middle to late Cenozoic tectonic-magmatic evolution of the Sierra Madre Occidental volcanic province south of the Tropic of Cancer is summarized and analyzed for the first time, based on new geologic and structural work and published information. In the eastern part of the study region (Mesa central physiographic province) silicic volcanism occurred in a short-lived episode culminating at ca. 30 Ma and was followed by crustal-scale extension between 30 and 27 Ma. In the western part of the study area (Sierra Madre Occidental physiographic province) a voluminous episode of ignimbrite volcanism at 24-21 Ma was succeeded by east-west extension that produced regularly spaced grabens affecting only the upper crust. In the westernmost part of the study region, an andesitic to rhyolitic are, formed between 17 and 12 Ma, was affected by crustal-scale, north-northwest-trending, extensional faulting, leading to the formation of the Gulf of California. In the Mesa central the maximum extension was oriented approximately east-west and amounted to [approximately]20%. In the eastern Sierra Madre Occidental physiographic province extension was only 8% and oriented approximately east-west. We observe that trenchward shifting of the climax of subduction volcanism and extension occurred during late Oligocene, early Miocene, and late Miocene time. Comparison with the offshore tectonics indicates that the first two tectonic-magmatic pulses coincide with periods of fast spreading at the Pacific-Farallon boundary, south of the Shirley fracture zone. We propose that increases in the spreading rate are related to periods of high subduction rate, which in turn correspond to episodes of retreating subduction. A retreating slab may have generated a flux of hotter asthenospheric material into the mantle wedge, producing widespread melting at the base of the crust as well as intraarc extension in the overriding plate. Boundary conditions (i.e., plate tectonics) ultimately determined timing, magnitude, and orientation of extension, whereas volcanic and tectonic styles are controlled by the internal structure of crustal blocks and by the gravitational and thermal effects of magmatism.