The Development of Pseudotachylites within Heier's Shear Zone, Lofoten-Vesterålen, North Norway, and Implications for Late- to Post-Caledonian Tectonics

We report initial field and petrographic studies of ‘Heier's zone', an enigmatic high strain zone within Archean and Proterozoic, granulite-facies continental basement rocks of Lofoten-Vesterålen, north Norway (latitude 68.5^oN). Heier's zone comprises crystal-plastic shear zones, a >200 km² area of pseudotachylite (PST) veins, and numerous later formed brittle faults. The zone ‘front' is located ~25 kilometers west of and ~12 km structurally beneath what is traditionally considered the basal Caledonian thrust - the A-type subduction zone boundary that emplaced far-traveled exotic and suspect terranes as well as imbricated horses of the telescoped Baltic margin. All PST veins are restricted to the hanging wall block and mylonites of a major shear zone we call Heier's shear zone (HSZ). PST veins generally are planar, tabular features that commonly branch or wedge off of thicker ‘feeder' veins that may be up to 15 cm thick. Where we have measured them, PSTs are generally steep to moderate dipping with widely variable strikes. Within mylonites of HSZ, we have identified both sheared and post-kinematic PSTs. Our kinematic and geometric observations document HSZ to be a shallow, west-dipping, tops-east, crystal-plastic shear zone that was reactivated as a tops-west ductile-brittle fault, and then was orthogonally segmented by even later high-angle brittle faults. Timing of earliest movement along HSZ is not yet directly established. Limited existing ⁴⁰Ar/³⁹Ar dates hint at a late- to post-Caledonian age for mylonitization, but we will present new results that should better constrain this timing. HSZ is cut by Jurassic age faults, related to rifting from Greenland, thus placing a minimum age on movement. The tremendous area of PST exposed in Heier's zone points to derivation due to coseismic activity along a shallow-dipping (thrust or normal detachment). The scale of PST development in Heier's zone is comparable to rupture zones determined for modern thrust earthquakes.