<p>It is well known that faults, once formed, become permanent weaknesses in the crust, localizing subsequent brittle strain increments. The case of repeated brittle reactivations localized along pre-existing plastic shear zones is less recognized, although this situation is frequently observed in many geologically old terranes.</p><p>We have studied the prolonged deformation history of the Himdalen&#8211;&#216;rje Deformation Zone (H&#216;DZ) in SE Norway by combining K&#8211;Ar and <sup>40</sup>Ar&#8211;<sup>39</sup>Ar geochronology with structural analysis. The H&#216;DZ consists of a large variation of deformation products from mylonites and cataclasites to pseudotachylites and fault gouge. Several generations of mylonites make up the ductile part of H&#216;DZ, called the &#216;rje shear zone, a km-think SW-dipping shear zone within the late Mesoproterozoic Sveconorwegian orogen. <sup>40</sup>Ar&#8211;<sup>39</sup>Ar dating of white mica from one of these mylonites give a plateau age of c. 908 Ma, interpreted to constrain the timing of late-Sveconorwegian extensionial reactivation of the &#216;rje shear zone.</p><p>This mylonitic fabric is extensively reworked in a brittle fashion along the SW-dipping Himdalen fault, a 10&#8211;25 m thick fault zone of cataclasite, breccia, fault gouge and, in places, abundant pseduotachylite veins. <sup>40</sup>Ar&#8211;<sup>39</sup>Ar dating of pseduotachylite material gives several small plateaus between c. 375 and 300 Ma, whereas K-feldspar clasts from the cataclasitically deformed host rock carry a Caledonian signal (plateau at c. 435 Ma). K&#8211;Ar dating of three fault gouges constrain the timing of gouge development at c. 270 and 200 Ma. Two of the fault gouges also contain protolithic K-bearing mineral phases that overlap in age with the c. 375 Ma pseudotachylite <sup>40</sup>Ar&#8211;<sup>39</sup>Ar plateau age, consistent with field observations of the former reworking the latter.</p><p>In sum, the H&#216;DZ records multiple Paleozoic and Mesozoic brittle reactivations of the early Neoproterozoic (and older) mylonitic &#216;rje shear zone. Most of the brittle deformation is interpreted to have accumulated during development of the Permian Oslo rift and its subsequent latest Triassic evolution. The suggested late Devonian (c. 375 Ma) initiation of brittle deformation does not have a clear tectonic association, but we speculate that it relates to strike-slip displacements caused by the Variscan orogen, as also suggested for the sub-parallel Tornquist zone to the south.</p>
Post-Caledonian brittle deformation in the Bergen area, West Norway: results from K–Ar illite fault gouge dating
