Can Pseudotachylytes Form via Fracture-Induced Decompression Melting Under Hydrous Conditions?

Frictional rock sliding and resultant shear heating along a fault plane are proposed as the necessary conditions to generate earthquake-related pseudotachylytes. However, frictional melting alone is energy expensive, requiring large temperature increases of several hundreds to thousands of degrees. Using the example of the pseudotachylyte structures of the Balmuccia Peridotite Massif (Ivrea-Verbano Zone, Alps, Italy), a minimum temperature increase of up to ~540°C for frictional melting has been proposed under isobaric and anhydrous conditions. Such conditions are however inconsistent with the moderate temperature increases, of up to~400°C, and diminishing pressures, less than 0.7 GPa, required to form the observed A-type pseudotachylyte structures, with ultramafic composition corresponding to the melting of specific mineral phases as a function of Carboniferous Period pressure and temperature conditions. Here we show that pseudotachylytes could be produced by fracture-induced decompression melting under hydrous conditions, that favor the formation of immiscible liquids derived from melting Al-Cr spinel and orthopyroxene with suspended clinopyroxene minerals in this immiscible melt. We propose thermodynamic calculations that constrain phase stability in the pressure-temperature space of the Balmuccia lherzolite under both anhydrous and hydrous conditions (0.5 to 1 wt.% H2O). We 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 favored by frictional heating but also by pressure drop (0.3--0.9 GPa) following dilation related to rock fracturing. Our results call for a reassessment of the origin of many pseudotachylytes formed in the lower crust and upper mantle. We show that fracture-induced decompression melting under hydrous conditions can be a viable 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 in the deep crust and upper mantle.