Tectonic Implications of Alternative Cenozoic Reconstructions for Southern Mexico and the Chortis Block

摘要

Most current Eocene reconstructions juxtapose the Chortis block of northern Central America against southern Mexico, and invoke approx 1100 km Cenozoic sinistral displacement on the Acapulco-Motagua-Cayman fault zone, the inferred northern margin of the Caribbean plate. Such a hypothesis is incompatible with the presence of undeformed Upper Cretaceous-Recent sediments that cross the projected trace of the Motagua fault zone in the Gulf of Tehuantepec, minimal offset of the Permian Chiapas batholith, and the absence in Honduras of several major features in southern Mexico. These problems may be overcome if the Chortis block is back-rotated anticlockwise about a pole near Santiago, Chile, i.e. approx 1100 km along the Cayman transform faults during the Cenozoic. Such a reconstruction when combined with reconstructions of features in the Pacific Ocean, suggests that Middle Miocene collision of the Tehuantepec aseismic ridge with the Acapulco Trench led to: (1) asymmetric flattening of the subduction zone; (2) an anticlockwise rotation of the Mexican mag-matic arc to its present location by the Middle Miocene; (3) the development of a vplcanic arc gap in southeastern Mexico, in which the late Middle Miocene Chiapas fold-and-thrust belt developed: as the Tehuantepec Ridge swept westward, arc volcanism was re-established in the gap. Eocene collision of the Chumbia Seamount Ridge (inferred mirror image of the Moonless Mountains-unnamed seamount ridge between the Molokai and Clarion fracture zones) with the Acapulco Trench followed by its ESE migration during the Oligocene led to: (a) flattening of the subducting slab inducing subduction erosion and exhumation of the southern Mexican margin; (b) anticlockwise rotation of the volcanic arc; and (c) sinistral strike-slip faulting in the Sierra Madre del Sur. This contrasts with the region north of the projected Molokai fracture zone where the dip of the subduction zone appears to have steepened, producing extension. Eocene(-Late Cretaceous) subduction along the southern coast of" Mexico explains the remnants of a Late Cretaceous arc in the Gulf of Tehuantepec and neighboring Guatemala.