The well-exposed Acatlan Complex of southern Mexico records the closure of a major Paleozoic ocean. Exposing Mexico's largest area of Paleozoic geology (27,500 km 2 ), the complex comprises an assemblage of repeatedly deformed clastic metasedimentary sequences, locally eclogite-facies mafic-ultramafic suites, and arc-related granitoid bodies that together record a protracted history of Paleozoic orogenesis. Juxtaposed against granulite facies gneisses of late Mesoproterozoic age with an Amazonian affinity (the Oaxacan Complex), the nature of the tectonostratigraphic record of the Acatlan Complex is not only central to the accretionary history of Mexico, but also bears upon wider issues of Paleozoic continental reconstructions. Despite its importance, however, a targeted structural-geochronological study of the complex of the type needed to unravel its tectonic history has yet to be undertaken. As a result, significant aspects of its geologic evolution are uncertain and existing models for its tectonic development are in conflict. Thus, the age and number of tectonothermal events the complex records are uncertain, key aspects of its kinematic history are unknown, its initial deformation has been variably correlated with the Taconic, Salinic and Acadian events of the Appalachians, and its evolution has been attributed to closure of both the Iapetus and Rheic oceans. To help resolve these controversies and clarify the complex's tectonic history, The PI's propose to conduct an investigation that combines detailed field mapping, structural-kinematic analysis and geochemistry with precise laserprobe 40 Ar/ 39 Ar, conventional U-Pb and ion-microprobe age dating. The investigation will use a targeted approach aimed at determining (1) the precise age, inheritance and kinematic significance of the complex's granitoid bodies, (2) the depositional age, provenance and tectonothermal history of its metasedimentary sequences, and (3) the precise age and tectonothermal evolution of its mafic-ultramafic units. These data are essential to understanding the tectonic history of the complex and will be used to establish its role in Mexico's accretionary history and to evaluate its linkages to the Appalachians. Intellectual Merit: Structural-kinematic analysis in combination with geochemical fingerprinting and precise geochronology provides a powerful tool for unraveling the tectonic history of regions of complex geology. The proposed project will greatly clarify the evolution of this important Paleozoic terrane and, in so doing, provide new insight into key aspects of the Paleozoic accretionary history of Mexico, its relationship to the development of the Appalachians, and its role in Laurentia-Amazonia connections for the Neoproterozoic and Paleozoic.
Broader Impact: Preliminary analysis of the tectonostratigraphic record of the Acatlan Complex coupled with its close association with the late Mesoproterozoic Oaxacan Complex, the Paleozoic cover of which contains fossils of Gondwanan affinity, suggests that the complex records the closure of the Rheic Ocean. If so, its tectonothermal evolution promises new insight into the history of this important but poorly understood ocean, the closure of which produced the climactic Ouachita-Alleghenian-Variscan collision between Gondwana and Laurussia during the assembly of Pangea. The project also represents an international collaboration between the United States and Mexico that will promote joint research and educational partnerships and provide both undergraduate and graduate students with unique opportunities for research and international exchange.
