A RECORD OF MID-MIOCENE EXPLOSIVE VOLCANSIM AND RIFT BASIN DEVELOPMENT IN THE SANTA ROSA-CALICO VOLCANIC FIELD, NEVADA

The Santa Rosa-Calico volcanic field (SC) is situated at the intersection of the southwestern extension of the eastern Snake River Plain-Yellowstone volcanic trend and the northwestern end of the Northern Nevada Rift. This complex volcanic field is characterized by mafic through silicic dikes, plugs, and domes and their associated effusive products, as well as pyroclastic flow and fall deposits from local dome-related and regional explosive activity. Throughout the SC, silicic fallout deposits are found in three dominant contexts: 1) interstratified with basaltic to andesitic lava flows, 2) directly underlying locally erupted, small-volume ash-flow tuffs and rhyolite lava flows, and 3) as discrete horizons within a sedimentary basin sequence. Identified mafic through silicic vents in the SC are concentrated along two sub-parallel, NNW-SSE trending zones corresponding to the general trend of the Northern Nevada Rift. These zones bound a topographic low (Goosey Lake Depression) where a well-preserved 18 m section (base not exposed) of predominantly clastic, volcanogenic sediment, presumably a remnant of a more widespread depositional system, is observed. Four distinct 8 to 180 cm thick, fine to medium grained vitric tephra horizons are preserved within these sediments. All four tephra are subalkaline rhyolites as revealed by bulk analyses of purified glass shards. Overall, the major and trace element signatures of these shards are suggestive of either local SC or regional southeastern Oregon/northern Nevada sources (e.g., Perkins and Nash, 2002). Preliminary geochemical correlations are suggested between the section-capping tephra horizon and the ca. 15.6 Ma Virgin Valley 1 ash bed, and between a tephra horizon 9 m from the base of the exposed sedimentary sequence with a ca. 15.8 Ma unnamed Buffalo Canyon type ash bed. These correlations provide first-order constraints on the age of the sedimentary sequence and on the formation of the basin into which these sediments were deposited. We suggest that this basin is the product of rift-related subsidence along faults responsible for the sub-parallel, NNW-SSE trending SC vent alignments. The >15.8 Ma formation of this basin may reflect a renewed period of magmatic and extensional tectonic activity associated with injection of basaltic magma into the lithosphere.