CAN PSEUDOTACHYLYTES FORM VIA FRACTURE-INDUCED DECOMPRESSION MELTING UNDER HYDROUS CONDITIONS?

Frictional rock sliding and resultant shear heating along the fault plane are proposed as the necessary ingredients to generate earthquake-related pseudotachylytes. However, frictional melting alone is energy expensive and does not yield some observed pseudotachylyte compositions at specific pressure-temperature conditions. Using the example of the pseudotachylytes of the Balmuccia peridotite (Ivrea-Verbano Zone, Western Alps, Italy), we show that pseudotachylytes can be produced by fracture-induced decompression melting under hydrous conditions, that favour the formation of immiscible liquids derived from melting Al-Cr spinel and orthopyroxene with suspended clinopyroxene minerals, which solidify as droplets observed at nanoscale. Thermodynamic calculations that constrain phase stability in the pressure-temperature space of the Balmuccia lherzolite under anhydrous and hydrous conditions (0.5 to 1 wt.% H₂O), illustrate that the Al-Cr spinel+orthopyroxene composition of the pseudotachylyte is consistent with lower pressure conditions than those of the initial peridotite prior to fracturing. These thermodynamic calculations help determine the pressure-temperature path of pseudotachylyte formation, not only favoured by frictional heating but also by pressure drop (0.3-0.9 GPa) following rock fracturing. Our results call for a reassessment of the origin of many pseudotachylytes, and show that fracture-induced decompression melting under hydrous conditions is a key mechanism that assists frictional melting by reducing the temperature rise from ambient temperature to melting temperature by 18% to 74%. A similar process may be significant in producing other pseudotachylytes during tectonic movement of lithospheric blocks.