EVOLUTION OF THE GUERRERO COMPOSITE TERRANE ALONG THE MEXICAN MARGIN: NEW GEOLOGIC AND DETRITAL ZIRCON DATA FROM THE COASTAL TAHUE AND ZIHUATANEJO TERRANES

The western margin of Mexico is ideally suited for testing two opposing models for the growth of continents along convergent margins: accretion of exotic island arcs by the consumption of entire ocean basins vs. accretion of fringing terranes produced by protracted extensional processes in the upper plate of a single subduction zone. We present geologic and detrital zircon evidence that the Tahue and Zihuatanejo Terranes of the Guerrero composite terrane originated by the latter mechanism. Although they may have been rifted off the Mexican margin by Jurassic extension, the evolution of the Tahue/Zihuatanejo Terranes can be explained by shifting between extensional and contractional settings, operating entirely within the upper plate of a long-lived subduction zone that dipped east under the Mexican margin. Prior to this study, all Mesozoic rocks in the western part of the Guerrero Composite Terrane were considered part of a single arc. However, we divide it into six distinctive tectonostratigraphic assemblages: I) Paleozoic North America-derived turbidites (continent-slope assemblage). II) Triassic-Early Jurassic accretionary complex (Arteaga Complex). III) mid- to late Jurassic volcanic arc assemblage. IV) Early Cretaceous extensional arc assemblage. V) Santonian-Maastrichtian contractional arc assemblage. The western Zihuatanejo Terrane records a more protracted history of arc magmatism than is yet dated in other terranes of western mainland Mexico, but closely matches those dated in Baja California to the west.