TECTONICS OF LATE JURASSIC-EARLY CRETACEOUS EXTENSIONAL BASIN DEVELOPMENT IN THE VIZCAINO-CEDROS REGION, BAJA CALIFORNIA AND PROVENANCE LINKAGE TO THE GUERRERO TERRANE OF MAINLAND MEXICO

Ongoing stratigraphic and geochronologic analysis provides evidence for development of a Tithonian-Neocomian intra-arc rift basin in the Vizcaino Peninsula-Cedros Island region of west-central Baja California. Rift basin strata are bounded by unconformities with an underlying substrate of Late Triassic-Middle Jurassic suprasubduction zone rocks and overlying Albian-Maastrichtian forearc basin deposits of the Valle Group. Rift basin strata include two units, the Eugenia and Coloradito Formations, previously interpreted as separate stratigraphic elements however similarities in depositional style and composition of sandstone and distinctive olistostrome blocks suggest to us that there they are facies of the same basin fill sequence. Steep paleoslopes, intercalated alkaline pillow lava, and the absence of contractional structures are consistent with an extensional or transtensional rift basin. The central Magdelena terrane of Blake et al. (1984) ~300 km farther south in the Baja Pacific margin is probably a correlative unit. The Baja margin rift basin sequences may correlate to the Late Jurassic-Early Cretaceous extensional Border rift system in southern Arizona, southern New Mexico, northern Sonora and Chihuahua as described by Lawton & coworkers which formed in response to a major reorganization of the Cordilleran margin. Detrital zircons from quartz-rich sandstone olistolith blocks in the Coloradito Formation were analysed by LA-ICP-MS and contain populations dominated by early Mesozoic, Pan-African and Grenville age grains. The zircon age spectra closely match those from quartz rich sandstone in the Guerrero terrane Ateaga Complex on mainland Mexico. The detrital zircon ages link these regions to North American continental basement sources and conflict with recent models that propose that Jura-Cretaceous intra-oceanic arc terranes were accreted to Mexican segment of the western North America margin from late Early Cretaceous to Late Cretaceous time.