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

Abstract

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.

Frictional Melting in Hydrothermal Fluid‐Rich Faults: Field and Experimental Evidence From the Bolfín Fault Zone (Chile)

“…Experimental evidence testing the preservation potential of pseudotachylyte’s primary microtextures in the presence of hydrothermal fluids ( P p ≥ 150 MPa, T ≥ 300°C), documents their rapid (days to months) alteration (Fondriest et al., 2020 ). Experimental pseudotachylytes were heavily altered with dissolution of the matrix, neo‐formation of clay aggregates and generation of a clastic microtexture (Fondriest et al., 2020 ).…”

“…Experimental evidence testing the preservation potential of pseudotachylyte’s primary microtextures in the presence of hydrothermal fluids ( P p ≥ 150 MPa, T ≥ 300°C), documents their rapid (days to months) alteration (Fondriest et al., 2020 ). Experimental pseudotachylytes were heavily altered with dissolution of the matrix, neo‐formation of clay aggregates and generation of a clastic microtexture (Fondriest et al., 2020 ). The alteration of pseudotachylytes driven by hydrothermal fluids is consistent with earthquake faulting being responsible for the rapid infiltration and redistribution of large volumes of fluids in the upper crust (Sibson, 1981 ) and the generation of hydrothermal mineralization; a condition which sensu lato can apply to the Bolfín Fault Zone case.…”

“…Hence, the natural pseudotachylytes were produced by the melting of hydrous-and carbonate-rich minerals during seismic faulting, most likely under the presence of infiltrated hydrothermal fluids, giving to the pseudotachylytes (both natural and experimental) a dark gray and brownish-red color (Figures 2c,2d,and 6), where in the field the brownish-red correspond to the oldest, most altered ones that resemble a cataclastic rock (either a cataclasite or an ultracataclasite), making them hard to distinguish. Experimental evidence testing the preservation potential of pseudotachylyte's primary microtextures in the presence of hydrothermal fluids (P p ≥ 150 MPa, T ≥ 300°C), documents their rapid (days to months) alteration (Fondriest et al, 2020). Experimental pseudotachylytes were heavily altered with dissolution of the matrix, neo-formation of clay aggregates and generation of a clastic microtexture (Fondriest et al, 2020).…”

Focal Mechanisms of Intraslab Earthquakes: Insights From Pseudotachylytes in Mantle Units

“…Post‐seismic magnetite formation is ruled out for three reasons: (1) the skeletal shapes indicate that these grains are microlites, unrelated to post‐seismic devitrification (spherulites); (2) there is a well‐documented systematic grain size decrease toward the cooler margins of the veins; (3) the lack of serpentine minerals in these pseudotachylytes further indicates that magnetite did not form through serpentinization, although a few serpentinized clasts are present in some veins (COR24‐05; Deseta et al., 2014). Furthermore, some studies suggest that pseudotachylytes may alter rapidly, in a few weeks (Fondriest et al., 2020), and therefore be missed from the geologic record. Yet, these rocks are thermally sintered and have a low permeability in comparison with their fractured host rock (Ferré et al., 2014, their Figure 4), which may explain the exceptional preservation at Cima di Gratera.…”

Experimental Plastic Reactivation of Pseudotachylyte‐Filled Shear Zones

“…Pseudotachylytes often include small amounts subreenschist facies alteration minerals (mainly epidote) precipitated after frictional heat has dissipated (see Di Toro & Pennacchioni, 2004) for a complete description of the altered pseudotachylytes studied). While these altered pseudotachylytes are not expected to present exactly the same rheology than the fresh one made of amorphous glass, a recent study highlighted that alteration processes in greenschist facies can occur within months only after their formation (Fondriest et al., 2019). Because of the rapid onset of alteration, studying the rheology of altered pseudotachylytes is relevant to constrain the strength of active faults in the continental crust.…”