Two rift systems, one of late Mesozoic age and the other of Tertiary age, in the southern Cordillera of North America formed along the inner flanks of former continental 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 stratigraphy and geochemistry preceding and during crustal extension of each rift system suggest a three-phase magmatic-depositional model for the formation of passive continental rifts above a foundering subducted slab. Continental arc magmatism associated with normal subduction weakens the continental crust during phase 1. Phase 2 involves incipient retrograde motion of the slab, or slab foundering, initiating mantle return into the wedge-shaped volume between slab and overlying continental lithosphere. This causes crustal extension, lithospheric melting, and deposition of conglomerate in nascent rift basins stratigraphically above and adjacent to the extinguished arc. Caldera-related silicic volcanism defines an ignimbrite flare-up, accompanied by extrusion of lithosphere-derived basalt. Mafic volcanism, block faulting, and extensional sedimentary-basin formation continue after the end of silicic volcanism. During phase 3, decompression partial melting of convecting asthenosphere creates basalts with ocean-island chemical affinities intercalated with alluvial or marine sedimentary rocks in extensional basins.

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