Dynamic evolution of porosity in lower crustal faults during the earthquake cycle

Fractures derived from earthquakes can create permeable conduits for fluids to flow, enhancing fluid-rock interactions, and potentially altering the strength and rheology of fault systems. In the dry lower crust, numerous field examples show mutually overprinting pseudotachylytes (solidified melts produced during seismic slip) and mylonitized pseudotachylytes (produced during the post- and interseismic viscous creep). The mylonites contain hydrous mineral assemblages, suggesting episodic pulses of fluids infiltration and rheological weakening triggered by the earthquake. Here, our aim is to understand the porosity generating mechanisms during the earthquake cycle and characterize the intermittent evolution of porosity.

The Nusfjord East shear zone network (Lofoten, Norway) is an exhumed lower crustal section composed largely of anhydrous anorthosites that contain mutually overprinting pseudotachylytes and mylonitized pseudotachylytes. We present a microstructural analysis focusing on the mechanisms generating, maintaining, and destroying porosity from an exhumed network of lower crustal coeval pseudotachylytes and mylonites using synchrotron X-ray microtomography (SμCT), focused ion beam scanning electron microscopy (FIB-SEM) nanotomography, electron backscatter diffraction (EBSD) analysis, and SEM imaging.

In the pristine pseudotachylyte, SμCT data show that porosity is concentrated within the pseudotachylyte vein (0.16 vol% porosity), especially around framboidal garnet clusters and single garnet grains. SEM observations reveal that garnets within the vein often contain an asymmetric rim of barium-enriched K-feldspar. The damage zone of the host anorthosite on the other hand is efficiently healed (0.03 vol% porosity) primarily with the growth of plagioclase neoblasts nucleated from pulverized fragments of the host anorthosite, and secondly with the precipitation of barium-enriched K-feldspar found lining intragranular microfractures. A FIB-SEM transect along one of these microfractures shows a myrmekite microstructure formed during fluid-rock interaction that completely sealed the porosity.

In the mylonitized pseudotachylyte, SμCT data show a porosity of 0.03 vol%, mainly concentrated within monomineralic domains of plagioclase, which are interpreted as recrystallized, sheared survivor clasts of wall-rock fragments. EBSD analyses indicate that deformation in these monomineralic domains was accommodated by diffusion creep and grain boundary sliding. Polymineralic domains along the mylonitic foliation, which primarily derive from the overprint of the original pseudotachylyte veins, also deformed by diffusion creep and grain boundary sliding. However, unlike in the monomineralic domains, they lack detectable porosity. We interpret these observations to reflect the efficient precipitation of hydrous phases into the pores during creep cavitation.

Dynamic fracturing during earthquakes is the primary mechanism for porosity generation in the lower crust. Our study shows that porosity is further reduced by up to 90% when a pristine pseudotachylyte is viscously re-worked under deformation conditions promoting grain-size sensitive creep and grain boundary sliding. We suggest that such porosity reduction eventually results in shear zone hardening, which may evolve in the development of new pseudotachylytes overprinting the mylonites, as frequently observed in Nusfjord. Thus, earthquake-induced rheological weakening of the lower crust is intermittent, and occurs only as long as the fluid can infiltrate in the shear zone, thereby facilitating diffusive mass transfer.