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In southwestern Mexico, Late Cretaceous to Early Tertiary deformation has been generally associated with the Laramide orogeny of the Cordillera. Several alternative models consider the deformation to result from the accretion of the Guerrero terrane, formed by the Zihuatanejo, Arcelia, and Teloloapan intraoceanic island arcs, to the continental margin of the North American plate. Here, we present a detailed geologic and structural study and new 40Ar/39Ar and U-Pb ages for a broad region in the central-eastern part of the Guerrero terrane that allow the accretion model to be tested. In the Huetamo–Ciudad Altamirano part of the region, an almost complete Cretaceous-Paleogene succession records the transition from an early Cretaceous shallow-marine environment to continental conditions that began in Santonian times, followed by the development of a major continental Eocene magmatic arc. Folding of the marine and transitional successions signifies a shortening episode between the late Cenomanian and the Santonian, and a subsequent, out-of-sequence, coaxial refolding event in Maastrichtian-Paleocene time amplified the previous structures. A major left-lateral shear zone postdates the contractional deformation, and it passively controlled the geographic distribution of Eocene silicic volcanism. Minor transcurrent faulting followed.

Our results indicate that the Huetamo–Ciudad Altamirano region, which has been considered part of the Zihuatanejo subterrane, was in proximity to a continent during most of the Mesozoic. We found continental recycled material at various stratigraphic levels of the Huetamo Cretaceous succession and Grenvillian inherited ages in zircons from the ca. 120 Ma Placeres del Oro pluton. More importantly, detrital zircon ages from the pre-Cretaceous basement of the Huetamo succession (Tzitzio metaflysch) and the pre–Early Jurassic basement of the Arcelia subterrane (Tejupilco suite) yield very similar Late Permian and Ordovician age peaks. These ages are typical of the Acatlán complex, onto which the Guerrero terrane has been proposed to have been accreted in the Late Cretaceous. Similarly, Paleozoic and Precambrian ages are reported in detrital zircons from the volcano-sedimentary successions of the Zihuatanejo, Arcelia, and Teloloapan subterranes. Models considering this part of the Guerrero terrane as having formed by intraoceanic island arcs separated by one or more subduction zones cannot explain the ubiquitous presence of older continental material in the Mesozoic succession. We favor a model in which most of the Guerrero terrane consisted of autochthonous or parautochthonous units deposited on the thinned continental margin of the North American plate and where the Mesozoic magmatic and sedimentary record is explained in the framework of an enduring west-facing migrating arc and related extensional backarc and forearc basins.

The results presented here exclude the accretion of allochthonous terranes as the cause for Laramide deformation and require an alternative driving force to explain the generation of the Late Cretaceous–early Tertiary shortening and shearing on the southern margin of the North American plate.

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