HR: 14:50h
AN: T23C-05    [Abstracts]
TI: Exhumation of the Western Gneiss Region Ultrahigh-Pressure Terrane
AU: * Hacker, B R
EM: hacker@geol.ucsb.edu
AF: Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Young, D
EM: davey@umail.ucsb.edu
AF: Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Johnston, S
EM: johnston@umail.ucsb.edu
AF: Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Root, D
EM: root.david@nmnh.si.edu
AF: Mineral Sciences, National Museum of Natural History, Smithsonian Institution,, Washington, DC 20560-0119 United States
AU: Walsh, E
EM: walshe@union,edu
AF: Geology, Union College, Schenectady, NY 12308 United States
AU: Andersen, T
EM: t..b.andersen@geologi.uio.no
AF: Earth Sciences, University of Oslo, Oslo, N-0316 Norway
AU: Gehrels, G
EM: ggehrels@geo.arizona.edu
AF: Geosciences, University of Arizona, Tucson, AZ 85721 United States
AU: Mattinson, J
EM: mattinson@geol.ucsb.edu
AF: Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Mattinson, J
EM: mattinson@geol.ucsb.edu
AF: Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567 United States
AU: Grove, M
EM: marty@argon.ess.ucla.edu
AF: Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567 United States
AB: Coordinated structural, petrological, and geochronological analysis of the Norwegian HP-UHP terrane, the world's largest, lends new insight into these remarkable rocks. New UHP eclogites, new HP eclogites, and new 40Ar/39Ar ages define three discrete ultrahigh-pressure (UHP) domains that are separated by distinctly lower pressure, but still eclogite-facies, rocks. 40Ar/39Ar mica and K-feldspar ages show that this outcrop pattern is the result of gentle regional-scale folding younger than 380 Ma, and that the UHP domains comprise antiformal culminations beneath a HP veneer. The shapes of the UHP antiforms overturn the long-held idea that P and T increase monotonically toward the coast. The sizes of the UHP antiforms range from 2500 km2 to 100 km2; if the UHP culminations are part of a continuous sheet at depth, the UHP terrane underlies a minimum of 11,000 km2 and is overlain by a HP veneer of 60,000 km2. No recognized structure separates the UHP and HP domains, and the transition in the Nordfjord area is defined by a gradual increase in P and T from the amphibolite to the coesite-eclogite facies. Metapelites in the UHP core of the orogen indicate that the (U)HP metamorphism was followed by near-isothermal decompression to 1.2-1.7 GPa/750 C and then to 0.5 GPa/750 C; metapelites in the HP area show a supra-Barrovian overprint at 1.0-1.2 GPa/750 C and then 0.5 GPa/750 C. New U/Pb zircon, U-Th/Pb monazite, and 40Ar/39Ar ages, combined with extant geochronology, require decompression from eclogite-facies conditions at 410-405 Ma to mid-crustal depths in a few million years. The short timescale and consistently high temperatures imply adiabatic exhumation of a UHP body with a minimum dimension of 30-40 km. 40Ar/39Ar muscovite ages of 397-380 Ma show that this extreme heat advection was followed by rapid cooling (30 C/m.y.) at mid-crustal levels caused by large-scale extension.
DE: 8100 TECTONOPHYSICS
DE: 3660 Metamorphic petrology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting