HR: 17:15h
AN: T32F-06    [PDF]
TI: Exhumation of Norwegian Ultrahigh-Pressure Rocks: Zircon Geochronology and Tectonostratigraphy of the Hornelen Region
AU: * Johnston, S M
EM: johnston@umail.ucsb.edu
AF: University of California, Santa Barbara, Department of Geology University of California, Bldg 526, Santa Barbara, CA 93106 United States
AU: Hacker, B R
EM: hacker@geol.ucsb.edu
AF: University of California, Santa Barbara, Department of Geology University of California, Bldg 526, Santa Barbara, CA 93106 United States
AU: Gehrels, G
EM: ggehrels@geo.Arizona.edu
AF: University of Arizona, Department of Geosciences The University of Arizona, Tucson, AZ 85721 United States
AB: The Norwegian UHP province is thought to have been exhumed in the footwall of the Nordfjord-Sogn Detachment Zone (NSDZ). However, the presence of high pressure eclogites in the both the hanging wall and footwall, in addition to a lower-crustal supra-Barrovian overprint observed across the Western Gneiss Complex, suggest that the NSDZ exhumed rocks from lower-crustal levels, but not from UHP depths. In order to fully understand the significance of the NSDZ and the extent to which it was responsible for the exhumation of the Norwegian UHP provinces, we must describe the kinematic evolution of the NSDZ, as well as the regional deformation prior to the onset of extension. Here, we employ zircon geochronology as a tool to differentiate key tectonostratigraphic units, characterize the various stages of deformation, and determine the time of deposition in extension-related basins. In the Hornelen region, the NSDZ consists of a 2-6 km thick section of pervasively top-W sheared allochthonous rocks that separate an eclogite-bearing footwall from low-grade Devonian sedimentary basins. Previous mapping divided the allochthons into two tectonostratigraphic units: Dalsfjord Suite orthogneisses overlain by Hoyvik Group paragneisses. We used zircon geochronology to unravel the complex deformation of these two units: detrital zircons from Hoyvik Group quartzites yielded two distinct age populations of 1100 and 1600 Ma while Dalsfjord Suite crystalline gneisses and schists gave single zircon age populations between 1600 and 1700 Ma. These zircon age populations support new field observations that indicate that the Hoyvik Group and Dalsfjord Suite represent a basement-cover unconformity that was isoclinally folded into E-vergent map-scale nappes during early contraction. Thermobarometry on coarse garnet-muscovite schists across the region indicates tectonic burial to peak pressures of 12-17 kb and 550-625 C. During late stage orogenic collapse, these nappe structures were rotated and transposed parallel to the stretching direction as the region experienced widespread extension and the Devonian Basins opened. The Devonian sedimentary basins are much less deformed than the underlying allochthons. Distal sandstones from the western Hornelen Basin yielded three distinct zircon age populations of 420, 1200 and 1700 Ma, while proximal conglomerate matrix from the eastern Hornelen Basin yielded two distinct zircon age populations of 1700 and 1000 Ma. The presence of 420 Ma zircons within the basins indicates that Caledonian metamorphic rocks must have been exhumed prior to the opening of the extensional basins.
DE: 1035 Geochronology
DE: 3660 Metamorphic petrology
DE: 8102 Continental contractional orogenic belts
DE: 8109 Continental tectonics--extensional (0905)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting