HR: 16:55h
AN: V24A-03    [Abstracts]
TI: Late Neogene Volcanic Stratigraphy in the Southern Puertecitos Volcanic Province of Baja California: Time Constraints and Vent Source Location
AU: * Garcia-Carrillo, P
EM: pgarcia@cicese.mx
AF: Departamento de Geologia, CICESE, Km 107, Carretera Tijuana-Ensenada., Ensenada, BC 22860, Mexico
AU: Martin, A
EM: amartin@cicese.mx
AF: Departamento de Geologia, CICESE, Km 107, Carretera Tijuana-Ensenada., Ensenada, BC 22860, Mexico
AU: Lopez-Martinez, M
EM: marlopez@cicese.mx
AF: Departamento de Geologia, CICESE, Km 107, Carretera Tijuana-Ensenada., Ensenada, BC 22860, Mexico
AU: Cañon, E
EM: ecanon@cicese.mx
AF: Departamento de Geologia, CICESE, Km 107, Carretera Tijuana-Ensenada., Ensenada, BC 22860, Mexico
AB: Late Neogene syn-rift explosive volcanism occurred in the Puertecitos Volcanic Province along the western margin of the Gulf of California. This volcanic episode is possibly related to extension during opening of the lower Delfin basin in mid-late Pliocene time. The volcanic stratigraphy in the southern Puertecitos Volcanic Province comprises three main groups: group 1 is a mid-Miocene, arc-related volcanic and sedimentary apron. Groups 2 and 3 are syn-rift volcanic units interstratified within alluvial conglomerate. Group 2 includes a non-welded, crystal-rich pyroclastic flow deposit, and a dark glassy dacite lava flow. Distinctive mineralogy of the crystal tuff is augite, sanidine-microcline and quartz. Two ³⁹Ar/⁴⁰Ar laser step-heating experiments on sanidine grains yielded an 6.18 ± 0.03 Ma isochron age, consistent with a 6.1 ± 0.4 Ma plateau age obtained in the dacite lava. Thickness of the crystal tuff varies from 35 m in the northeast to 10 m in the southwest along 5 km of distance. Group 3 is characterized by the lack of quartz and potassic feldspar phenocrysts. Three laser step heating experiments on groundmass samples constrain this pulse of explosive volcanism between 2.9 ± 0.1 and 2.3 ± 0.03 Ma. Thicknesses of individual units increase to the northeast and collectively reach up to 150 m. Isopath maps for distinctive flow-units indicate consistent dispersion direction to the SW (average azimuth 210° ± 15°). This inferred flow direction is similar to the orientation of magnetic susceptibility axes measured in 20 oriented samples that yield a mean azimuth of 214°± 24°. In group 3 flow-units eutaxitic foliation is concordant and dips 8-20° to the ENE. Tilting of the volcanic sequence is produced by a series of NNW-trending, west-dipping, high-angle normal faults with less than 40 m of throw. Balanced cross- sections in the southern Puertecitos Volcanic Province indicates that post-2.8 Ma extension is less than 15% suggesting that major deformation during the opening of the Lower Delfin basin has been accommodated to the east. Our data support multiple source vents located offshore the central Puertecitos Volcanic Province. These pyroclastic flows may constitute useful marker horizons in marine seismic lines for reconstructing the timing and amount of extension across conjugate margins in the Lower Delfin basin.
DE: 1135 Correlative geochronology
DE: 1518 Magnetic fabrics and anisotropy
DE: 8408 Volcano/climate interactions (1605, 3309)
DE: 8428 Explosive volcanism
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly