HR: 0800h
AN: T21A-0510    [Abstracts]
TI: Exhumation of Norwegian Ultrahigh-Pressure Rocks: Microstructural Evolution of the Nordfjord-Sogn Detachment Zone, Hornelen Region, Norway
AU: * Johnston, S M
EM: johnston@umail.ucsb.edu
AF: Uiversity of California, Santa Barbara, Department of Geology, Building 526, Santa Barbara, CA 93106
AU: Hacker, B R
EM: hacker@geol.ucsb.edu
AF: Uiversity of California, Santa Barbara, Department of Geology, Building 526, Santa Barbara, CA 93106
AB: The Nordfjord-Sogn Detachment Zone (NSDZ) of Western Norway juxtaposes the eclogite bearing Western Gneiss Complex with the low-grade Hornelen Basin, and is widely cited as one of the primary extensional structures responsible for the exhumation of the Norwegian ultrahigh-pressure (UHP) rocks. In the Hornelen region, the NSDZ is exposed as a 2-6 km thick section of intensely deformed allochthonous rocks that were isoclinally folded and buried to depths of 50-60km at ~435-465 Ma during early contraction. They then remained at or near the base of the crust until after the UHP event at 410-405 Ma, passing through the muscovite closure temperatures at ~400 Ma during orogen-wide extension. Major questions regarding the kinematics of this extension, however, remain problematic: does the contact between the Western Gneiss Complex and the deformed allochthonous rocks represent a crustal scale extensional fault, and what is the role of coaxial versus non-coaxial deformation during extension and exhumation of high and ultrahigh-pressure rocks? Here, as a first step toward answering these questions, quartz lattice preferred orientations and microstructural analysis provide insight into the development of the NSDZ and the evolution of extensional deformation. Quartz $<$c$>$ and $<$a$>$ axis orientations on quartzites were measured using electron back scatter diffraction and yielded strong lattice preferred orientations from all structural levels within the allochthons and throughout the Hornelen region. Samples from middle to high structural levels give lattice preferred orientations that suggest greenschist to lower-amphibolite facies deformation. While lower temperature samples generally yield asymmetric patterns that indicate non-coaxial, top-W shearing, higher temperature samples predominantly yield symmetric patterns suggesting coaxial shearing. Lattice preferred orientations from the lowermost structural levels, and closest to the contact with the Western Gneiss Complex, exclusively indicate coaxial, amphibolite facies shearing. Extension crenulation cleavage and S-C fabrics implying non-coaxial, top-W sense of shear were observed in the field at all structural levels. However, petrographic observations reveal the growth of chlorite in association with these fabrics suggesting that they represent a lower temperature, later stage of extension, and that they overprint earlier higher temperature quartz fabrics. The lack of high temperature non-coaxial fabrics near the base of the allochthons indicates that the contact between the allochthons and the Western Gneiss Complex does not represent a major extensional fault that was active throughout the thickness of the crust. Furthermore, this new data set suggests that coaxial shear played an important role in the earlier, high temperature stages of extension, while non-coaxial shear became more important in later, low temperature stages of extension.
DE: 8030 Microstructures
DE: 8109 Continental tectonics--extensional (0905)
DE: 5120 Plasticity, diffusion, and creep
DE: 3660 Metamorphic petrology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting