| dc.description.abstract | Pseudotachylytes, also called “fossil earthquakes,” are the remains of solidified melt that formed along fault planes. If produced by brittle, seismic slip, they are important indicators of seismicity. If they originate in the lower crust, they can be used to examine the seismic behavior of this much-debated layer of the lithosphere. This study focuses on such lower-crustal pseudotachylytes from the Nusfjord region of Lofoten, Norway. The goals were twofold; to investigate the potential for cyclical exploitation of the same weakness between multiple generations of pseudotachylyte, and to determine their formation mechanism, whether it was brittle or ductile. A brittle-origin pseudotachylyte would be the result of frictional melting and dynamic rupture propagation, while a ductile-origin pseudotachylyte would be the product of thermal runaway, the end result of a feedback loop of shear heating and viscous creep that can form in shear zones. A combination of field measurements and observations, optical microscopy, and scanning electron microscopy work was used to examine two shear zones in the area. Both shear zones contain at least two generations of pseudotachylyte, one mylonitized and the other pristine. The pristine pseudotachylyte is the younger of the two. This younger pseudotachylyte exploits the weakest and finest-grained portion of the mylonite, an ultramylonite band that runs through the extent of the outcrop until it eventually branches and exits into the host anorthosite rock. It also has a highly asymmetric damage zone, with more damage appearing in the footwall of the fault, which is an indicator of a brittle origin, along with other brittle features. Together, these indicate that the fault experiences a cycle of brittle and ductile deformation, and the younger pseudotachylyte is exploiting the weakest portion of the previous generation of pseudotachylyte. This also shows that brittle failure must have occurred in the lower crust. | eng |