Cretaceous-Eocene magmatism and Laramide deformation in southwestern Mexico: No role for terrane accretion

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 40 Ar/ 39 Ar and U-Pb ages for a broad region in the centraleastern 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 CretaceousPaleogene 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 signifi es a shortening episode between the late Cenomanian and the Santonian, and a subsequent, out-of-sequence, coaxial refolding event in Maastrichtian-Paleocene time amplifi ed 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.