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 Juárez mylonitic complex, located in southern Mexico, is a polyorogenic north-northwest–trending structure. Here we report U-Pb mylonitization dates of 165 ± 20 Ma for igneous zircon from the syntectonic San Felipe granite, and an integrated 40Ar/39Ar age of 169.3 ± 1.7 Ma from synkinematic muscovite, both of which indicate a Middle Jurassic age for the strike-slip event along the Sierra de Juárez 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.

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