Deformation and healing processes in the damage zone of a lower-crustal seismogenic fault

In anhydrous, strong, and metastable lower-crustal rocks, coseismic fracturing is an effective mechanism for creating pathways for fluids to infiltrate and interact with the host rock, ultimately resulting in metamorphism and rheological weakening. In this study, we have characterized the damage zone flanking a lower-crustal pseudotachylyte (solidified frictional melt produced during seismic slip) to understand the fracture generating and fluid-assisted healing processes operating during and immediately after a seismic event.

The Nusfjord East shear zone (Lofoten, Norway) is a network of coeval pseudotachylytes and mylonitized pseudotachylytes that formed at lower-crustal conditions within anhydrous anorthosites. We present a micro- and nanostructural analysis focusing on plagioclase in the damage zone of a pseudotachylyte using focused ion beam (FIB) prepared scanning transmission electron microscopy (STEM), Fourier Transform Infrared (FTIR) Spectroscopy, electron backscatter diffraction (EBSD) analysis, electron microprobe analysis (EMPA), and SEM-cathodoluminescence (CL) imaging.

The damage zone of the host anorthosite is characterized by a network of comminuted primary plagioclase (plagioclase₁) grains with minimal offset. Very fine (<15 mm) plagioclase₁ grains and secondary plagioclase neoblasts (plagioclase₂), differentiated from each other by SEM-CL and EBSD observations, fill the fractures along with a minor amount of K-feldspar. Plagioclase₁ and plagioclase₂ have the same major element compositions (average: An₅₂) and are not zoned aside from a small increase in anorthite along the grain boundaries. Away from the pseudotachylyte margin, plagioclase₂ grains filling the fractures show a host-controlled crystallographic preferred orientation (CPO) governed by plagioclase₁ grains. With decreasing distance toward the vein margin, the CPO is weakened as a result of minor amount of solid-state deformation by grain-boundary sliding after the coseismic event. Plagioclase₁ grains often exhibit a diffuse CL intensity zonation from bright grain cores to a dark grey in healed cracks, while plagioclase₂ have a uniform mid-tone grey CL intensity with dark grain boundaries. CL zonation in the plagioclase₁ does not correlate with EMPA major element maps nor EBSD misorientation maps. TEM foils across the dark CL grain boundaries reveal microfractures filled with nanograins of plagioclase₂ containing few dislocations. FTIR maps transecting the thin section do show the presence of molecular water trapped along fractured plagioclase₁ grain boundary regions. At the thin section scale, there is no measurable gradient of molecular water with increasing or decreasing distance toward the pseudotachylyte margin.

In summary, these observations suggest that (1) fracturing was in accordance to a pulverization-style fragmentation process, (2) water is from a local source; presumably coseismic fracturing released fluid inclusions enclosed within plagioclase₁ and the frictional heating caused the melting of primary biotite, and (3) the little amount of molecular water freely available did not diffuse within the plagioclase grains, and did not promote hydrolytic weakening in the damage zone. Strain localization is primarily determined by repeated occurrences of extreme grain-size reduction and phase mixing, in addition to some amount of fluid wetting the grain boundaries. Therefore the “wet and weak” structure, preferential for further ductile deformation, is often the pseudotachylyte vein when present and not the surrounding damage zone.