Garnet stability in high-grade (deep-seated) pseudotachylytes may be obscured by the peculiar thermomechanical properties of garnet. Within garnet-rich high-grade mylonites, the supposedly coeval pseudotachylytes are commonly free or poor of garnet. Based on a study of amphibolite-facies mylonite-pseudotachylyte association of the Mont Mary unit (western Alps) we suggest a general process for garnet disappearance due to thermal shock fragmentation during coseismic frictional heating. We show that garnet has the lowest thermal shock resistance between the host-rock minerals (garnet, plagioclase, quartz, and sillimanite, in an increasing sequence of resistance), and thus underwent extreme comminution leading to total melting within the frictional melt, which was deprived of nuclei for growth of garnet microlites. Our analysis highlights the critical role of thermal shock as a general process in mineral comminution during the initial stages of coseismic slip preceding (and promoting) extensive frictional melting, though the process remains active during these later stages. Our finding has general implications for establishing the coeval development of pseudotachylytes and mylonites and, therefore, for recognizing the evidence of deep earthquakes in exhumed rocks.