The Eidsfjord shear zone, Lofoten–Vesterålen, north Norway: An Early Devonian, paleoseismogenic low-angle normal fault

Abstract

We report structural and 40Ar/39Ar isotopic information on the Eidsfjord shear zone that document it to be a seismogenic, tops-west (hinterland directed), Devonian ductile low angle (25–30° dip, shallowing locally) normal detachment fault. Anorthosite/migmatitic gneiss in the detachment’s upper plate, mangerite in the lower plate, and detachment mylonites are all cut by generations of abundant pseudotachylyte occurring over approximately 150 km2. The mean of four laser 40Ar/39Ar plateau ages for single crystals of recrystallized muscovite from mylonites defining the Eidsfjord shear zone indicates an age of 403.6 ± 1.1 Ma (2σ) for deformation and recrystallization. 40Ar/39Ar step-heating analyses are reported for muscovite from mylonitized rocks of the Fiskefjord shear zone, a nearby tops-east Caledonian thrust that was reactivated as a tops-west normal fault, documenting cooling of the upper plate through the ∼350 °C isotherm at ∼457 Ma. Together with Middle-Ordovician tectonothermal relics found farther west in Lofoten, tops-down-to-the-west normal-slip movement on these extensional shear zones explains maintenance of high-crustal levels throughout the Siluro-Devonian Scandian event. Potassium feldspar 40Ar/39Ar results document a pulse, or multiple pulses, of uplift and cooling between ca. 235 Ma and 185 Ma, consistent with formation of Triassic–Jurassic rift basins flanking the Lofoten Ridge.The Eidsfjord detachment appears to mark the northern terminus of the Early Devonian detachment system traceable 800 km southward to the Nordfjord–Sogn detachment and westward across the North Atlantic to detachments of roughly the same age on the conjugate side of the orogen in East Greenland. The timing, geometry, kinematics, and rheological development of Eidsfjord detachment are grossly similar to the Nordfjord–Sogn detachment but the former contrasts in that it presently lacks exposed deposits of Devonian sedimentary rocks, has smaller magnitudes of displacement, a more prolonged exhumation history, is severely chopped by later high angle normal faults related to rifting and final Eocene continental separation, and it has abundant pseudotachylite occurrences in the upper plate. We interpret the Eidsfjord detachment to be a rare example of an ancient eroded seismogenic low-angle normal fault.