HR: 0800h
AN: T41C-1237    [Abstracts]
TI: Range Front Faulting and Ancestral Cascades Arc Magmatism in the Central Sierra Nevada at 10 Ma: Onset of Basin and Range Extension or Sierran Root Delamination?
AU: * Rood, D H
EM: dylan@crustal.ucsb.edu
AF: Department of Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Busby, C J
EM: busby@geol.ucsb.edu
AF: Department of Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Putirka, K
EM: kputirka@csufresno.edu
AF: Department of Earth Sciences, California State University, Fresno, CA 93740 United States
AU: Gans, P
EM: gans@geol.ucsb.edu
AF: Department of Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AB: The volcanic rocks of Sonora Pass, Central Sierra Nevada, California, occupy a transition between Cascadian subduction and Basin and Range extension; these rocks thus provide a key to understanding the evolution of these provinces. Our preliminary work includes detailed mapping, 40Ar/39Ar geochronology on 15 mineral separates, and a geochemical study of 110 samples. Volcanic units are from the Mehrten and Stanislaus Formations. Within our study area, small shifts in alkalinity are apparent, and occur within a larger temporal pattern of increasing alkalinity with time. A shift from subalkalic to mildly alkalic, then back to subalkalic volcanism matches similar relative trends (of larger magnitude) in the southern Sierra, which have been interpreted as representing lithosphere delamination. On a regional scale, Sonora Pass volcanics are also intermediate in alkalinity compared to Pliocene-age volcanics from the southern Sierra Nevada and volcanic rocks from Lassen; on the basis of K2O vs. SiO2, three very distinct magmatic suites are evident: a relatively low K2O Lassen series, an ultrapotassic Pliocene series, and an intermediate series represented by Sonora Pass. But, the origin of these K2O variations is unclear. At Lassen, Sr/P ratios are a positive index of slab-derived fluids (Borg et al., 2002), and such fluids have been called upon to explain elevated K2O contents of Pliocene Sierra volcanics (Farmer et al., 2002). But our data show an inverse correlation between K2O and Sr/P. Also, a granite xenolith from the Little Walker Caldera indicates that the crust might not provide an alternative reservoir to explain the K2O-Sr/P systematics. Isotope and trace element analyses are being undertaken to improve our understanding of the major element geochemistry. Miocene strata of the Sonora Pass area record the onset of faulting both during and after eruption of the andesitic Early (?)-Late Miocene Mehrten Fm, with continued faulting after eruption of the latitic Late Miocene Stanislaus Fm (Busby et al., 2003; Rood et al., 2004). At Sonora Peak, an angular unconformity exists between the Mehrten Fm and the overlying Table Mountain Latite flows of the Stanislaus Fm. Exposures of subvertical fault planes restricted to the Mehrten Fm show shallowly-plunging lineations and Riedel shears that suggest dextral deformation. Such dextral strike-slip motion on faults as old as 10 Ma may record the birth of the Walker Lane fault zone in the region. Our new field, 40Ar/39Ar, and geochemical data indicate a rapid change in the structural and geochemical setting of the Sonora Pass area at 10.25+/-0.06 Ma coeval with eruption of the Table Mountain Latite flows. This was followed by large-volume eruptions of high-K quartz latite (Eureka Valley Tuff) between 9.30+/-0.03 Ma and 9.16+/-0.03 Ma. Latitic volcanism was both preceded by andesitic arc volcanism, as young as 10.10+/-0.06 Ma, and followed by andesitic arc volcanism at 7.12+/-0.06 Ma. Latitic volcanism at 10.25 Ma cannot be attributed to a slab window because the triple junction was located well to the south of Sonora Pass at that time. We speculate that the latites erupted during a phase of rapid extension in an arc otherwise dominated by andesitic volcanism, perhaps indicating the initiation of Basin and Range faulting or the delamination of the Sierra Nevada batholithic root.
UR: http://www.geol.ucsb.edu/faculty/busby/VSSAC-FR.html
DE: 8109 Continental tectonics--extensional (0905)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting