Alteration and deformation of fault-generated pseudotachylytes
Mineralogy, Petrology and Geochemistry
Geophysics
Final Report Abstract
Field reports on fault-generated pseudotachylytes, solidified melts that formed due to frictional heating during an earthquake, seem rare in the light of the number of earthquakes expected to yield a sufficient temperature rise. The question underlying this project, whether pseudotachylytes are rarely generated or rarely preserved in a form recognizable in the field, is, therefore, important to evaluate current earthquake models. To establish the macroscopic and microscopic alteration characteristics and kinetics of pseudotachylyte alteration, we conducted hydrothermal alteration experiments on natural pseudotachylytes from the Adamello area, Italian Southern Alps, and synthetic samples spanning temperatures of 200– 500 °C and alteration durations up to 100 days. For fluid-poor conditions, no significant effect of alteration on microstructure was observed, whereas the presence of fluid leads to dissolution of the pseudotachylyte, porosity formation and growth of secondary minerals, comparable with natural examples of altered pseudotachylytes. The porous microstructure of altered pseudotachylytes will likely collapse due to high stresses at the depth where earthquake sources are active, such that they will resemble cataclasites rather than pseudotachylytes. As fluid availability governs pseudotachylyte alteration, we investigated the hydraulic properties of pseudotachylytes as well as their strength at the relevant temperature and pressure conditions, which will govern the potential for brittle fracture, i.e., the development of fluid pathways. The permeability and hydraulic diffusivity of pseudotachylytes is at least two orders of magnitude lower than that of the surrounding rocks. Therefore, infiltration of the pseudotachylytes appears unlikely. Deformation experiments conducted at a temperature T = 500°C showed that pseudotachylytes are relatively strong and brittle deformation is concentrated in the host rock, while only few shear fractures form in the pseudotachylyte. These results explain why hydrothermal alteration of pseudotachylytes was limited in the Adamello area. The experimental study showed that pseudotachylytes may be lost from the rock record on geologically short time scales in the presence of hydrous fluids. Their preservation in fault zones where intense hydrothermal alteration occurred is due to the high degree of permeability and potentially strength contrast between host rocks and pseudotachylytes, limiting the access of fluids to the pseudotachylytes.
Publications
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(2019). Hydrothermal Alteration of Natural Pseudotachylytes and Synthetic Glasses. AGU Fall Meeting 2019, abstract MR51B-0046
Rempe, M., Renner, J., Di Toro, G.
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(2020). Permeability evolution of pseudotachylytes during hydrothermal alteration experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8269
Rempe, M. and Renner, J.
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(2020). Pseudotachylyte alteration and the rapid fade of earthquakes scars from the geological record. Geophysical Research Letters, 47(22)
Fondriest, M., Mecklenburgh, J., Passelegue, F., Artioli, G., Nestola, F., Spagnuolo, E., Rempe, M., Di Toro, G.
