New Mexico Geological Society Annual Spring Meeting — Abstracts

Two rift systems, the Late Mesozoic Borderland rift that includes the Bisbee and McCoy basins and the Cenozoic Rio Grande rift, in the southern Cordillera of North America formed along the inner flanks of former continental-margin arcs. Both rift systems were initiated when arc magmatism abandoned its former inboard extent as a result of retrograde motion of the subducted slab. Similarities in the tectonic and geochemical stratigraphy preceding and during crustal extension of each rift system suggest a three-phase magmaticdepositional model for the formation of passive continental rifts above a foundering subducted slab. Phase 1: Continental margin arc magmatism during normal subduction weakens the continental crust. This phase is represented in the Borderland rift system by the Mount Wrightson Formation
and related calc-alkaline volcanic successions, and in the southern Rio Grande rift by the Rubio Peak and Palm Park formations. Phase 2: Incipient retrograde motion of the slab, or "slab foundering," initiates mantle return into the wedgeshaped volume between slab and overlying continental lithosphere, causing crustal extension, lithospheric melting, and deposition of conglomerate in nascent rift basins stratigraphically above and adjacent to the extinguished arc. Caldera-related silicic volcanism and coeval extrusion of lithosphere-derived basalt and basaltic andesite define a bimodal "ignimbrite flare-up." This phase is represented in the Borderland rift system by an assemblage of outflow and intracauldron tuffs informally known as the "quartz porphyries" (for example, the Cobre Ridge Tuff and Canelo Hills Volcanics) and in the southern Rio Grande rift by the Bell Top Formation. Mafic volcanism, represented in southern New Mexico by the Uvas basaltic andesite, block faulting, and extensional sedimentary-basin formation continue after the end of silicic volcanism. Phase 3: Decompression-induced partial melting of convecting asthenosphere in the mantle wedge creates basalts with ocean-island chemical affinities intercalated with alluvial or marine sedimentary rocks in extensional basins. This phase is represented in southern Arizona by the Glance Conglomerate, Crystal Cave Formation, and interbedded volcanic rocks, and in southeastern New Mexico by newly discovered marine and volcanic strata beneath the Lower Cretaceous Hell-to-Finish Formation. In the Rio Grande rift system, phase 3 is recorded by the Santa Fe Group, including the Hayner Ranch, Rincon Valley, and Camp Rice formations.