HR: 1340h
AN: T33A-1145    [Abstracts]
TI: Geologic Evidence for Eruption of Voluminous High-K Magmas at the Onset of Walker Lane Transtensional Faulting, Central Sierra Nevada: Birth of a Plate Margin, Not Root Delamination
AU: * Busby, C J
EM: busby@geol.ucsb.edu
AF: Dept. of Earth Science, University of California, Santa Barbara, CA 93106, United States
AU: Hagan, J C
EM: hagan@umail.ucsb.edu
AF: Dept. of Earth Science, University of California, Santa Barbara, CA 93106, United States
AU: Putirka, K
EM: kputirka@csufresno.edu
AF: Department of Earth and Environmental Sciences, California State University, Fresno, 2345 E. San Ramon Ave., MS/MH24, Fresno, CA 93720, United States
AB: Two major NSF projects have been using dominantly geophysical techniques to consider the role of lithospheric foundering in the tectonic evolution of the Sierra Nevada over the past 10 My. Yet some of the best constraints on our understanding of Sierran landscape evolution have come from field studies of dateable Cenozoic strata in the Sierra, largely preserved as volcanic-volcaniclastic fill of paleochannels. Our work, supported by two new NSF grants, is providing detailed stratigraphic, structural, geochronogical and geochemical constraints on the Cenozoic evolution of the central Sierra Nevada and adjacent areas. These provide a test of the high- K/delamination link. We infer that three episodes of Cenozoic uplift occurred in the central Sierra, in addition to Cretaceous uplift. These correspond to: (1) weak uplift associated with the onset of arc magmatism (at 15 Ma), (2) major uplift during the onset of Walker Lane transtension (at 10 Ma), and (3) renewed uplift at the arrival of the triple junction and cessation of arc volcanism (at about 6 Ma). We present evidence that the onset of Walker Lane transtension was accompanied by eruption of 10 - 9 Ma voluminous high-K volcanics at the Little Walker Center, the largest Miocene volcanic center in the Sierra Nevada. We propose that the Little Walker Center formed at a right (releasing) stepover in a dextral transtensional fault system that includes the longest fault in the central Sierra. Our 40Ar/39Ar dates and field data show that this fault zone became active for the first time only 270 - 30 Ka before the onset of high-K volcanism. Transtensional activity in this short time frame included tilting of Oligocene to Miocene strata 40 degrees toward the range front, with offsets up to 700 m on multiple faults, as well as widespread landsliding, with slide blocks up to several kilometers long being shed onto downthrown blocks. The immediately ensuing 10 - 9 Ma high-K volcanism was accompanied by continued transtension. We interpret the Little Walker Center as a pull-apart basin, with faults that penetrated a lithospheric plate with a thick crustal section, thereby tapping low degree partial melts generated at relatively great depth (Putirka and Busby, Geology, in press). In contrast, the 15 - 6 Ma andesites and trachyandesites that enclose the 10 - 9 Ma high-K volcanic rocks were vented along shorter faults, with normal to weakly transtensional offset, thereby tapping magmas at shallower depths. High-K volcanism in the Sierra Nevada thus records the birth of the transtensional plate boundary, rather than lithosphere delamination.
UR: http://www.geology.ucsb.edu/faculty/busby
DE: 8104 Continental margins: convergent
DE: 8109 Continental tectonics: extensional (0905)
DE: 8178 Tectonics and magmatism
DE: 8486 Field relationships (1090, 3690)
DE: 9604 Cenozoic
SC: Tectonophysics [T]
MN: 2007 Fall Meeting