HR: 0800h
AN: GP21A-0112    [Abstracts]
TI: Paleomagnetism of the Stanislaus Group, CA reveals revised stratigraphy, Walker Lane kinematics, and radio-isotopic constraints on C5 magnetic subchrons
AU: * Pluhar, C J
EM: cpluhar@csufresno.edu
AF: California State University, Fresno, Dept. of Earth and Enviro. Sciences 2576 East San Ramon Ave M/S ST24, Fresno, CA 93740, United States
AU: Wright, T J
EM: bowlryder@hotmail.com
AF: University of California, Earth Science Dept. 1156 High St., Santa Cruz, CA 95064-1077, United States
AU: Fischer, C P
EM: fobash@gmail.com
AF: University of California, Earth Science Dept. 1156 High St., Santa Cruz, CA 95064-1077, United States
AU: Busby, C J
EM: busby@geol.ucsb.edu
AF: University of California, Earth Science Dept. Webb Hall, Bldg 526, Santa Barbara, CA 93106-9630, United States
AB: Paleomagnetic study of the c.a. 9.2-10.3 Ma Stanislaus Group of intercalated latite (trachyandesite) lavas, ignimbrites and accessory sediments at three localities in Mono county California reveals: 1) a detailed, revised stratigraphy for the Stanislaus group, 2) kinematic constraints on the part of the Walker Lane since Stanislaus group emplacement, and 3) two age-constrained magnetic subchrons during chron C5N recorded by latites that had previously only been identified in seafloor magnetic anomalies. The revised stratigraphy results from detailed magnetostratigraphy combined with previous ⁴⁰Ar/³⁹Ar geochronologic constraints and stratigraphic studies. We find the lowermost unit, Table Mountain Latite, to consist of 23 or more individual lava flows falling into 5 magnetic (mostly normal) polarity zones, indicating that these rocks span at least 40,000 years of geologic time, based on the expected duration of magnetic reversals. Overlying Table Mountain Latite is the reversed-polarity Tollhouse Flat member of the Eureka Valley Tuff as described by previous authors. In the Sweetwater Roadless Area, thought to be proximal to the Stanislaus eruptive center, latite lava of both normal and reversed polarity are emplaced ontop of the Tollhouse Flat Member. Normal-polarity By Day member and normal-polarity Upper Member lie at very top of entire sequence. We find no field evidence for the normal polarity Dardanelle Formation latite flow at the top of the Group as had been previously reported by other workers. Instead, the Dardanelle formation member likely corresponds to the latite lava(s) between the By Day and Tollhouse Flat Eureka Valley Tuff. Based on previous ⁴⁰Ar/³⁹Ar dating, the two reversed zones within our magnetostratigraphy correspond to two of the proposed reversed subchrons/excursions during chron C5N. Direct dating of these reversed units may lead to future improvements to the magnetic polarity timescale for C5N. Our paleomagnetic results from three study localities also reveals kinematic constraints for the boundary between the Sierra Nevada and central Walker Lane belt. The section at Sonora Peak, CA, within the stable Sierra Nevada microplate, yields a locality mean direction for these rocks indistinguishable from the expected direction for the late Miocene. The two other study localities, Grouse Meadow and Burcham Creek, lie within the western Walker Lane Belt, east of the Sierra Nevada range front fault zone. Results from these latter localities reveal 10°-20° of vertical axis rotation of fault-bounded blocks of the western Walker Lane since emplacement of the Stanislaus Group. These results are obtained relative to a variety of datums – by averaging secular variation, by comparing locality mean directions from one place to another, and by comparing unit mean directions for the extensive e ignimbrites from one place to another. These results allow explicit delineation of the kinematic boundary between stable Sierra Nevada and the Walker Lane and geometrical calculation of total strain accommodated by these rotations.
DE: 1513 Geomagnetic excursions
DE: 1520 Magnetostratigraphy
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 8111 Continental tectonics: strike-slip and transform
DE: 8428 Explosive volcanism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting