Preseismic ambient temperature and inferred formation depth of earthquake pseudotachylytes

Leila Honarbakhsh, Eric C. Ferre, & John Geissman

Published November 2025, SCEC Contribution #14797

Tectonic pseudotachylytes, formed through frictional melting of fault rocks during seismic slip, are unambiguous "earthquake fossils," providing direct evidence of fault-zone processes under extreme deformation conditions. The depths at which these melts form have been estimated to range from near the surface to approximately 80 km and perhaps even deeper in subduction zones, yet their precise depth of formation remains poorly determined. This uncertainty arises primarily from the lack of reliable geobarometers for melts that crystallize out of equilibrium. In this study, we present a novel approach to estimate the host rock ambient temperature during pseudotachylyte formation by combining purely conductive 1-D heat transfer models with an independent measurement of the heat transfer from the melt to the host rock using a magnetic method. By applying the baked-contact test to a well-characterized case study in Alpine Corsica, where the pressure-temperature-time history of the host rock is well-established, we deduce the depth of seismic rupture. The inferred shallow depth, consistent with the presence of vesicles in the pseudotachylytes (a signature of quenching at depths <5 km), provides a revised and contrasting estimate of seismic rupture conditions in this tectonic region. Our innovative approach admittedly relies on a well-constrained P-T-t history of the host rock and offers significant potential for broader application across regions, provided the host rock is capable of recording a characteristic remanent magnetization. Our findings suggest that the commonly reported depths of pseudotachylyte formation, especially those associated with intermediate depths, may require reevaluation. This study not only refines our understanding of seismic rupture processes but also introduces a powerful new tool for future investigations into fault-zone dynamics.

Key Words
Pseudotachylyte, seismic rupture, heat transfer models, depth estimation, fault zone dynamics

Citation
Honarbakhsh, L., Ferre, E. C., & Geissman, J. (2025). Preseismic ambient temperature and inferred formation depth of earthquake pseudotachylytes. Earth and Planetary Science Letters, 669. https://doi.org/10.1016/j.epsl.2025.119564.