Oligocene-Miocene back-thrusting in southern Mexico linked to the rapid subduction erosion of a large forearc block

Both the timing and mechanism for the removal of a ∼150-250 km wide forearc block from southern Mexico during the Cenozoic are controversial. Principal competing hypotheses are (1) removal due to sinistral strike-slip shear, in which slow, diachronous removal of the Chortis Block throughout the Cenozoic is inferred, and (2) removal due to subduction erosion, in which rapid removal of a large forearc block during the late Oligocene/early Miocene is inferred to be synchronous with the rapid landward migration of the southern Mexican arc. New data indicate northeast-directed back-thrusting in (1) the Chacalapa shear zone west of-96.5E, with the timing of shear deformation bracketed by a 25.5±0.5 Ma U/Pb zircon age and a 20.7±0.6 Ma Ar/Ar biotite age, and (2) in an unnamed shear zone to the south, with the timing of deformation bracketed by a 27.5±0.5 Ma U/Pb zircon age and a 25.1±0.2 Ma Ar/Ar biotite age. Zircon and biotite ages date the emplacement and cooling of deformed plutons, respectively. The observed back-thrusting is consistent with a model of forearc removal due to subduction-erosion processes because it is evidence for subduction-orthogonal shortening occurring within the upper plate just before the landward migration of the southern Mexican arc. Rapid subduction of the southern Mexican forearc could have recycled continental lithosphere into the upper mantle at a rate up to half the global average rate of subduction erosion during the late Oligocene/early Miocene.