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. 40 Ar/ 39 Ar 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.

Abstract The West Sudetes (NE margin of the Bohemian Massif) consist of a complex mosaic of several tectonometamorphic units juxtaposed during the Variscan orogeny. The polyphase Variscan tectonothermal development of the West Sudetes was determined by 40 Ar/ 39 Ar ages of single grains and mineral concentrates. Late Famennian (359 Ma) mica ages from the high-grade Góry Sowie Block suggest continuous uplift after a Late Devonian high temperature-low pressure (HT-LP) event contemporaneous with the end of subduction-related high pressure-low temperature (HP-LT) metamorphism in the East Krkonoše Complex. Mid-Late Devonian high pressure events in the Krkonoše-Jizera Terrane and Orlica-Śnieżnik Dome are followed by coeval high temperature events between 345 and 335 Ma (Viséan). The latter are interpreted as consequence of uplift, and decompression during overthrusting of both complexes on their forelands. Subsequent small- to large-scale shear movements dated at around 325–320 Ma (early Namurian) affected the Orlica-Śnieżnik Dome, Krkonoše-Jizera Terrane, including the Intra-Sudetic Fault, and also the eastern Lusatian Granitoid Complex. They were accompanied by contemporaneous emplacement of the Krkonoše-Jizera pluton. The upper limit of the tectonometamorphic and magmatic activity is dated at 314–312 Ma (Namurian/Westphalian boundary). The final juxtaposition of the diversified tectonometamorphic units, which constitute the West Sudetes, took place in early Namurian times.