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

Coseismic fracturing of the lower crust is an effective mechanism for creating permeable pathways for fluids to infiltrate and interact with the host rock, thus effectively altering the rheology of otherwise anhydrous and strong lower-crustal rocks. Most of the fracturing and fragmentation that facilitate fluid infiltration occurs in the damage zone of seismogenic faults. In this study, we have focused on characterizing the damage zone adjacent to a lower-crustal pseudotachylyte (solidified frictional melt produced during seismic slip) to understand the fracture generating and healing processes during a seismic event.

The Nusfjord East shear zone network (Lofoten, Norway) contains coeval pseudotachylytes and mylonitized pseudotachylytes that formed at lower-crustal conditions within anhydrous anorthosites. We present a micro- and nanostructural analysis of plagioclase grains in the damage zone of a natural pseudotachylyte using focused ion beam (FIB) prepared scanning transmission electron microscopy (S/TEM), 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 fractures with minimal offset, consistent with a pulverization-style fragmentation process. CL intensities differentiate primary plagioclase (plagioclase₁), from secondary plagioclase neoblasts (plagioclase₂) filling some of the fractures. 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 targeted key microstructure domains characterized by CL: (1) bright CL plagioclase₁ core, (2) dark CL plagioclase₁ in healed cracks, (3) transitional CL from bright to dark across a healed crack, and (4) plagioclase₂ neoblast. Results from S/TEM show that the dark CL spanning the healed cracks is associated with a high concentration of crystalline nanoparticles. In contrast, bright CL is associated with a few scattered dislocations, no nanoparticles, and numerous dispersed Ba-Ti-oxide nanograins. The mid-tone grey CL plagioclase₂ neoblast have the lowest dislocation density. Follow-up Transmission Kikuchi Diffraction (TKD) and NanoSIMS analyses on the nanoparticles in the healed cracks and the plagioclase₁ grains immediately next to these cracks will help further elucidate the origins of the nanoparticles and the CL intensity zonation.