San Gaspar ignimbrite, western Mexico: Magnetic fabric and Paleomagnetic studies - constraints on emplacement mode and source vents

HR: 0830h
AN: GP41A-09 [Abstracts]
TI: San Gaspar ignimbrite, western Mexico: Magnetic fabric and Paleomagnetic studies - constraints on emplacement mode and source vents
AU: * Caballero, C
EM: cecilia@geofisica,unam.mx
AF: Laboratorio de Paleomagnetismo y Geofisica Nuclear, Instituto de Geofisica, UNAM, Circuito Institutos s/n Ciudad Universitaria, D. Coyoacan, Mexico, DF 06140 Mexico
AU: Alva-Valdivia, L M
EM: lalva@geofisica.unam.mx
AF: Laboratorio de Paleomagnetismo y Geofisica Nuclear, Instituto de Geofisica, UNAM, Circuito Institutos s/n Ciudad Universitaria, D. Coyoacan, Mexico, DF 06140 Mexico
AU: Rosas-Elguera, J
EM: lalva@geofisica.unam.mx
AF: Centro de Ciencias de la Tierra, U de G., Universidad de Guadalajara, Guadalajara, Jalisco, MＹico, Guadalajara, Jal Mexico
AB: Paleomagnetic and magnetic fabric data for the 4.8 Ma San Gaspar ignimbrite, one of the largest in western Mexico, are used to investigate on the source vents, emplacement mechanism and tectonics. The within-site consistency of directional features of magnetic fabric supports that the ignimbrite welded and cooled in situ and was probably emplaced in NW-SE rift zone from a single eruption center located south or east of a younger Pleistocene caldera. Paleomagnetic, rock-magnetic and ore microscopy data support the hypothesis that the extensive and widespread ignimbrite deposits in western Mexico correspond to a major explosive volcanic phase in the Pliocene Rock-magnetic properties are similar, suggesting relatively homogeneous mineralogy of the unit. Isothermal remanence and continuous susceptibility-temperature experiments point to low to medium-Ti titanomagnetite as the main ferromagnetic mineral. Hysteresis parameters of most samples fall in the pseudo-single-domain grain size region; wasp-waisted hysteresis loops were identified corresponding to high Hcr/Hc values. Stepwise thermal and alternating field demagnetization shows that secondary components are negligible, and always completely removed below 20 mT or 400° C. The characteristic component is isolated in all sites with small within-site dispersion of mean direction in most of them (alfa 95 less than 10 at 7 out of 10 sites).
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
DE: 1520 Magnetostratigraphy
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly