Pressure-temperature-time deformation history of the exhumation of ultra-high pressure rocks in the Western Gneiss Region, Norway
Published:January 01, 2004
L. Labrousse, L. Jolivet, T.B. Andersen, P. Agard, R. Hébert, H. Maluski, U. Schärer, 2004. "Pressure-temperature-time deformation history of the exhumation of ultra-high pressure rocks in the Western Gneiss Region, Norway", Gneiss Domes in Orogeny, Donna L. Whitney, Christian Teyssier, Christine S. Siddoway
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The Nordfjord area, north of the Hornelen Devonian basin in Western Norway, is the southernmost part of the Ultra-High Pressure (UHP) Province, defined by the occurrence of coesite-bearing and diamond-bearing continental rocks. Compilation of structural, petrological, and chronological data from the area leads to a model for the formation of dome structures at the crustal scale and the behavior of the continental crust during its exhumation from mantle depths. The Nordfjord area appears as a 100 × 50 km dome-shaped boudin affected by at least two deformation stages. A stage of E-W stretching and top-to-west shearing produced several envelopes of migmatitic gneisses bounded by narrow high-strain zones over a core preserving the Precambrian granulite protolith. This dome is affected by the west-vergent Nordfjord Mylonitic Shear Zone on its southern limb during late exhumation under the Nordfjord-Sogn Detachment Zone. The first stage of deformation is coeval with reequilibration from maximum pressure conditions around 2.8 GPa, 650 °C (THERMOCALC multiequilibrium method) in the coesite stability field to higher temperature and lower pressure conditions (1.8 GPa, 780 °C). Subsequent retrogression was recorded in the amphibolite facies (0.7 GPa, 580 °C) and in the greenschist facies (0.4 GPa, 420 °C). Dates for these stages yield exhumation velocities higher than 2 mm/yr. 40Ar/39Ar ages in the area, compared to a spectrum of cooling ages along a 500-km-long N-S profile, show that cooling of the northern part of the Western Gneiss Complex is at least 20 Ma younger than in the south. The Western Gneiss Complex is therefore the result of the late juxtaposition of two complexes, the Northwestern Gneiss Complex, characterized by UHP relics, constrictive stretching, partial melting, and doming during a multi-stage exhumation from the deep parts of the orogen, and the Southwestern Gneiss Complex with Devonian basins, a well-developed detachment system, and distinct high pressure to medium pressure units stacked together during a single and rapid exhumation stage. The two complexes may represent deep subduction channel dynamics (north) and shallower wedge circulation (south) in the Caledonian orogen. The Nordfjord Mylonitic Shear Zone appears as a major tectonic in the Western Gneiss Complex. Partial melting in the Northwestern Gneiss Complex may have favored the late exhumation of E-W elongated domes such as the Nordfjord crustal-scale boudin and their juxtaposition to the Southwestern Gneiss Complex during top-to-west shearing.