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Retrograded eclogite-facies pseudotachylytes as deep-crustal paleoseismic faults within continental basement of Lofoten, north Norway

¹ Department of Geology and Geography, Auburn University, Auburn, Alabama 36849, USA
² Department of Geosciences, University of Oslo, Box 1047, Blindern, Oslo 3, Norway

Field observations and electron micro-probe analyses indicate that pseudotachylytes discovered on the Lofoten island of Flakstadøy, north Norway, represent rare examples of deep-crustal paleoseismic faults. The pseudotachylyte occurrences are restricted to the margins of eclogite-facies shear zones that sharply cut pristine granulite-facies continental basement rocks. Generally, pseudotachylyte veins are sharply truncated by the eclogite shears, but some have been sheared and folded into them, documenting prekinematic to synkinematic injection. Textures preserved in the pseudotachylyte matrix document crystallization directly from the frictional melt; for example, dendritic garnets, similar in appearance, size, and composition to those from eclogite pseudotachylytes of the Bergen Arcs and lesund (Austrheim and Boundy, 1994; Lund and Austrheim, 2003), reflect rapid (likely in terms of tens of seconds) crystallization, and distinct fining of grains toward the margins of the pseudotachylyte veins indicates quenching textures. Electron microprobe analysis and backscattered-electron imaging document that the pseudotachylyte matrix is composed of microlites of garnet (Gr_25–30, Py_15–19, and Al_54–58), orthopyroxene (En_61–64), low-Na clinopyroxene (Jd₆), amphibole (ferroan pargasite), with or without K-feldspar, quartz, biotite, various Fe opaques and Fe-Ti opaques, kyanite, dolomite, and calcite. The cogenetic eclogite-facies shear zones and pseudotachylytes were variably retrograded during Caledonian amphibolite-facies metamorphism. Omphacite is replaced by clusters or symplectites of low-Na clinopyroxene (Jd₆) and oligoclase/andesine (An_20–36); kyanite, orthopyroxene, Na-Ca clinopyroxene, amphibole, and dolomite occur as inclusions in garnet. The Flakstadøy pseudotachylytes indicate that the rocks exposed in Lofoten were rigid and resilient parts of the lower crust of an ancient continent from ca. 1.8 Ga until the Middle Ordovician. Subduction to deeper-crustal levels (depths >45 km) caused the stiff, nonreacted granulite to accommodate aseismic, steady-state flow in fluid-mediated, eclogite shear zones by concomitant, brittle, seismogenic failure and pseudotachylyte formation. Later in the Middle Ordovician, these deep-crustal rocks were exhumed to middle-crustal levels, where they were retrograded under amphibolite-facies conditions. Our results help to explain how deep-crustal earthquakes form in modern continent-continent collisional zones like the Himalayas.

Keywords: eclogites • Caledonian • deep crust • paleoseismicity • Lofoten • Norway