THE STRATIGRAPHIC RECORD OF SYN- AND POST-VOLCANIC EXTENSION ADJACENT TO THE SIERRA MADRE OCCIDENTAL SILICIC LARGE IGNEOUS PROVINCE, NORTHERN MEXICO (Invited Presentation)

Continental arc volcanism and crustal extension through the middle Eocene to earliest Miocene produced a landscape of rhyolitic ignimbrite plateaux disrupted by half-graben part-filled with lavas, pyroclastic, volcaniclastic, and siliclastic sedimenatary rocks. The central core of the SMO is not significantly extended, however it is bound to the west by the Miocene - Recent Gulf of California extensional province, and the east by the Eocene - Pliocene southern Basin and Range. The relative timing of extension and arc flare-up supervolcanism are being investigated. The loci of silicic volcanism and extension in the SMO migrated westward through the Oligocene in response to opening of a slab window through the subducted Farallon plate, slab roll-back of the Farallon plate, or a combination of both. In the west, half-graben opened through basement and distal Oligocene SMO ignimbrite sheets before being filled by proximal, latest Oligocene and earliest Miocene volcanic and volcaniclastic deposits, including silicic and mafic lavas erupted from dikes intruded along active normal faults; Miocene extension and volcanism eventually evolved into the Gulf of California rift system. To the east, however, the pattern is more complex and less well understood. Coeval rifting and alkaline volcanism initiated together in the middle Eocene, and then waned. Renewed volcanism along the length of the SMO arc developed to an ignimbrite-flare-up in the early Oligocene (30 - 36 Ma) without a pronounced period of pre-volcanic extension. This formed a broad ignimbrite plateau that began to be extended by at latest 32 Ma; new half-graben are filled by coarse fluvial conglomerates intruded by small silicic bodies and buried by small lava domes. We will present a new Ar/Ar and U-Pb geochronology data-set collected across a traverse across the SMO. These data constrain the onset of SMO volcanism and the onset of graben formation across the region.