Pseudotachylyte Veins in Accretionary Complexes: Melt or Mechanical Wear?

Abstract

\n Whether seismic rupture propagates over large distances to generate mega-earthquakes or is rapidly arrested mainly depends on the slip processing within the fault core, including in particular frictional melting or intense grain-size reduction and amorphization. The record of seismic slip in exhumed fault zones consists in many instances in Black Faults Rocks, dark and glass-like-filled aphanitic veins that have been interpreted as resulting from quenching of frictional melts, i.e., pseudotachylytes. Such interpretation has nevertheless been questioned as similar macro to nano-textures have been observed either on intensely comminuted natural fault rocks or on slow creep experiments on crustal rocks, where melting is absent. Here, we report a new dataset of Raman Spectroscopy of Carbonaceous Material analyses, aimed at discriminating the slip weakening processes operating in the fault core during slip. Using high spatial resolution profiles on natural Black Fault Rocks from accretionary complexes and an experimentally calibrated modelling of Raman intensity ratio evolution with temperature, we assessed different scenarios of temperature evolution during fault slip. In the three studied Black Fault Rocks interpreted so far as natural pseudotachylytes, Raman Spectroscopy of Carbonaceous Material results are not consistent with a molten origin and therefore should reflect a mechanical wear during deformation. These results bear major consequences on the dynamics of faulting, as the slip-weakening processes that occur during seismic slip rely on the extreme grain-size reduction and fluidization rather than melting.