Pseudotachylyte Alteration and the Rapid Fade of Earthquake Scars From the Geological Record.

Tectonic pseudotachylytes are solidified frictional melts produced on faults during earthquakes and are robust markers of seismic slip events. Nonetheless, pseudotachylytes are apparently uncommon fault rocks, because they are either rarely produced or are easily lost from the geological record. To solve this conundrum, long‐lasting (18–35 days) hydrothermal alteration tests were performed on fresh pseudotachylytes produced by sliding solid rock samples at seismic slip rates in the laboratory. After all tests, the pseudotachylytes were heavily altered with dissolution of the matrix and neo‐formation of clay aggregates. Post‐alteration products closely resemble natural altered pseudotachylytes and associated ultracataclasites (i.e., fault rocks affected by fracturing in the absence of melting), demonstrating that the preservation potential of original pseudotachylyte microstructures is very short, days to months, in the presence of hydrothermal fluids. As a consequence, pseudotachylytes might be significantly underrepresented in the geological record, and on‐fault frictional melting during earthquakes is likely to occur more commonly than generally believed. Plain Language Summary: Tectonic pseudotachylytes are solidified melts produced by rapid sliding of faults during earthquakes. A long‐lasting unsolved dispute in earthquake physics regards the abundance of pseudotachylytes in nature and the relevance of frictional melting as a seismic‐related process. Although experimental and theoretical arguments indicate that frictional melts are easily generated at seismic deformation conditions, pseudotachylytes are apparently rare in the geological record and are often related to specific structural settings (i.e., water‐deficient environments and high shear stresses). Such a discrepancy poses the problem whether pseudotachylytes are rarely generated or only rarely preserved in a recognizable form. Here we investigated the preservation potential of pseudotachylytes in the presence of fluids, by performing long‐lasting (18–35 days) hydrothermal tests on fresh pseudotachylytes produced in the laboratory. After all tests, the pseudotachylytes were heavily altered with the generation of microstructures resembling those of very common fault rocks called cataclasites, which are unrelated to frictional melting. We suggest that pseudotachylytes are easily produced during earthquakes but they fade from the geological record in few weeks at most, in the presence of altering fluids percolating along the faults. This implies that frictional melting is a relevant process occurring on faults during earthquakes rupturing crystalline basement rocks. Key Points: We investigated the preservation of pseudotachylytes in the presence of fluids with long‐lasting (18–35 days) hydrothermal experimentsAfter the experiments, pseudotachylytes were heavily altered with dissolution of the matrix and neo‐formation of clay aggregatesHydrothermal alteration can determine the rapid (days to months) fade of pseudotachylytes from the geological record [ABSTRACT FROM AUTHOR]