LARAMIDE INVERSION OF LATE JURASSIC BASINS IN SOUTHWESTERN NORTH AMERICA: THE RECORD IN THE EAST POTRILLO MOUNTAINS, SOUTHERN NEW MEXICO

Among Cordilleras of western North America, Cretaceous orogenesis commonly is marked by folds and thrusts. In much of the United States and central Mexico shortening in sedimentary cover has been accommodated by movements along gently dipping thrust faults that may be tectonically driven in the north but are mainly gravity-driven in the south. Ranges within an intervening region encompassing New Mexico and Arizona in the US and the northern tier of Mexican states record a different style of Phanerozoic orogenesis. The deformational history incorporates diverse events that began in Jurassic time when the Proterozoic craton and overlying Paleozoic and Early Mesozoic platform-like cover were systematically fractured during Late Jurassic transtension. In the East Potrillo Mountains, breccia and conglomerate initially accumulated in the pull-apart basins. Overlying sandy strata contain pencontemporaneous folds that record transport down the basin flanks. Eastward migration of Cretaceous magmatic centers between ca. 95 to 65 Ma correlates with dislocation of large masses and related debris from Early Cretaceous carbonate, commonly reefs, from the rims and flanks of the Late Jurassic basins. Folds and microstructures within ductile shear zones show that the coarse debris moved southeastward along the basin axis pushing and overriding sediment at the toe. In northern Mexico volcanic units from Late Cretaceous volcanoes (~ 70 Ma) buried debris flows containing carbonate olistoliths. Subsequent northeast-directed contraction during Latest Cretaceous time resulted in inversion of the basins. Basin closure involved left-lateral and/or reverse oblique movement along reactivated east-striking normal faults and formation of northwesterly trending folds in the basin fill. Shortening also involved uplift along steep thrust faults formed along reactivated Late Jurassic (?) left-lateral faults coincident with the basin flanks. Detachments at the bases of slides also probably served as slip surfaces. Most ranges were tilted above listric normal faults during Tertiary extensional events.