EVIDENCE OF CO-MAGMATIC DEFORMATION IN THE CRETACEOUS ARC-RELATED SUCCESSIONS OF THE ZIHUATANEJO TERRANE, WESTERN PORTION OF THE GUERRERO COMPOSITE TERRANE, MEXICO: PRELIMINARY RESULTS

The Guerrero composite terrane of western Mexico contains large volumes of Upper Jurassic-Lower Cretaceous volcanic-sedimentary rocks of arc affinity. The basement of these arc-related successions is heterogeneous in composition, and some of its terranes are floored by previously accreted terranes. The Guerrero composite terrane, has been divided into five terranes: the Tahue terrane, the Zihuatanejo terrane, the Arcelia terrane, the Teloloapan terrane, and the Guanajuato terrane. The basement of the Zihuatanejo terrane is the Arteaga Complex, which is made up of accreted Triassic ocean-basin-continent slope assemblages that were deformed by Early Jurassic time. The rocks that overlie the Arteaga complex have been interpreted by previous authors as a continuous succession all related to one single magmatic event. This has been described as an alternation of basaltic to andesitic lava flows, interbedded with rudist reefal limestone, volcanic conglomerates, and other epiclastics deposited on shallow marine and subaerial environments. However our preliminary mapping suggest that the Cretaceous succession has a complex depositional archetecture, with abrupt lateral variations in facies and stratal thicknesses, and internal disconformities. We have identified that the lower part of the stratigraphy is formed by localized marine basins, filled with black shale and detrital limestone, that change laterally to non marine and shallow marine facies that contain rhyolites to andesites with island-arc geochemical signatures. The lower succession shows rapid lateral thickness variations and is missing in areas that were basement highs. It also contains clasts derived from pre-Cretaceous units suggesting partially exposures of the basement during the arc activity. The age of this succession has not been well constrained, but it is in part Berriasian to Albian. It is all covered by Albian to Cenomanian reefal limestone and basaltic lava flows, some with shoshonitic affinities. These last are also deposited on the basement in the area of Arteaga. The changes in litofacies suggest that the arc was under an extensional setting, with associated normal faults that exposed the basement. The stratigraphic change in the Albian-Cenomanian may record an evolution from intra-arc to back-arc volcanism and sedimentation.