Dynamic Evolution of Porosity in Lower-Crustal Faults During the Earthquake Cycle

Earthquake-induced fracturing of the dry and strong lower crust can transiently increase permeability for fluids to flow and trigger metamorphic and rheological transformations. However, little is known about the porosity that facilitates these transformations. We analyzed microstructures that have recorded the mechanisms generating porosity in the lower crust from a pristine pseudotachylyte (solidified earthquake-derived frictional melt) and a mylonitized pseudotachylyte from Lofoten, Norway to understand the evolution of fluid pathways from the coseismic to the post- and interseismic stages of the earthquake cycle. Porosity is dispersed and poorly interconnected within the pseudotachylyte vein (0.14 vol%), with a noticeably increased amount along garnet grain boundaries (0.25-0.41 vol%). This porosity formed due to a net negative volume change at the grain boundary when garnet overgrows the pseudotachylyte matrix. Efficient healing of the damage zone by fluid-assisted growth of feldspar neoblasts resulted in the preservation of only a few but relatively large interconnected pores along coseismic fractures (0.03 vol% porosity). In contrast, porosity in the mylonitized pseudotachylyte is dramatically reduced (0.02 vol% overall), because of the efficient precipitation of phases (amphibole, biotite and feldspars) into transient pores during grain-size sensitive creep. Porosity reduction on the order of >85% may be a contributing factor in shear zone hardening, potentially leading to the development of new pseudotachylytes overprinting the mylonites. Our results show that earthquake-induced rheological weakening of the lower crust is intermittent and occurs when a fluid can infiltrate a transiently permeable shear zone, thereby facilitating diffusive mass transfer and creep. Plain Language Summary Earthquakes create fractures and increase the porosity in crustal rocks. These fractures can help transport fluids to newly accessible regions in the crust, which in turn may kickstart metamorphic reactions, and potentially alter the rheology. However, very little is known about the mechanisms, the microstructural context, and the morphology of this increased porosity. We analyzed ancient earthquake-generated frictional melts (pseudotachylytes) and their immediate damage zone in the host rock, as well as plasticly deformed pseudotachylytes, that have since been exhumed from depth and are now exposed at the surface in Lofoten, Norway. We analyzed these rocks to determine the processes that create porosity and how this porosity evolves with increasing plastic deformation. The pseudotachylyte hosts more porosity than the damage zone immediately flanking the vein, in particular there is a high concentration of porosity around garnets. We interpret this porosity to have formed as a result of the metamorphic growth of garnet. Much of the fracture-related porosity created during the initial earthquake has been efficiently sealed. Porosity is greatly reduced in the sheared pseudotachylytes because of solution-precipitation processes that operated during ductile deformation. Porosity reduction may reflect fluid consumption, leading to shear zone hardening and possibly new pseudotachylyte formation.

Automated summary

This study analyzed pseudotachylytes and mylonitized pseudotachylytes from Lofoten, Norway to understand the evolution of fluid pathways and porosity. It was found that porosity is dispersed within the pseudotachylyte vein, but increased along garnet grain boundaries. The porosity is mainly formed due to metamorphic growth of garnet. In the mylonitized pseudotachylyte, porosity is dramatically reduced due to solution-precipitation processes. Porosity reduction may lead to shear zone hardening and new pseudotachylyte formation.