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Journal of the Geological Society

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Dating fault reactivation by Ar/Ar laserprobe: an alternative view of apparently cogenetic mylonite–pseudotachylite assemblages

^{1 1}Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK (e-mail: s.sherlock@open.ac.uk)
^{2 2}Reactivation Research Group, University of Durham, Durham, DH1 3LE, UK
^{3 3}The Geological Survey of Norway, Leiv Eirikssons vei 39, 7491 Trondheim, Norway
^{4 4}Present Address: Shell UK Exploration and Production, 1 Altens Farm Road, Aberdeen AB12 3FY, UK

Apparently cogenetic mylonite–pseudotachylite assemblages are commonly reported in major fault zones. They represent two very incompatible modes of deformation, and complex formation mechanisms have been proposed to explain this paradox. We report here one such assemblage from Central Norway in which apparently synchronous mylonite and pseudotachylite formation is separated by 100 Ma-mean muscovite Ar/Ar laserprobe spot ages from mylonite, and pseudotachylite matrix, are 406 ± 11 Ma and 290 ± 10 Ma respectively. In preference to a complex cogenetic model we invoke fault reactivation as a viable alternative, and suggest that assumed contemporaneity in such assemblages may be invalid in many cases.

KEYWORDS: Møre–Trøndelag Fault Complex Ar/Ar, pseudotachylite, reactivation,

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