THE EIDSFJORD SHEAR ZONE: AN EARLY DEVONIAN, PALEOSEISMOGENIC LOW-ANGLE NORMAL FAULT EXPOSED IN LOFOTEN-VESTERÅLEN, NORTH NORWAY

Structural and ⁴⁰Ar/³⁹Ar isotopic information on the Eidsfjord shear zone document a seismogenic, tops-west (hinterland directed), Devonian, low-angle (25-30^o dip, shallowing locally) normal detachment fault. Anorthosite/migmatitic gneiss in the 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 shear zone indicates an age of 403.6±1.1 Ma (2s) for deformation and recrystallization. ⁴⁰Ar/³⁹Ar step-heating analyses 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, document upper-plate cooling 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 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 marks 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 appears to lack associated Devonian basins, 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 suggest that the Eidsfjord detachment is a rare example of an ancient eroded seismogenic low-angle normal fault.