A paradox exists between the great number of intermediate-depth earthquakes occurring along active subduction interfaces worldwide and the extreme scarcity of paleo-seismic events recorded in exhumed metasediments from ancient subducted slabs. Recrystallization associated with exhumation-related overprinting generally contributes to the nearly-complete erasing of markers of unstable slip events in metamorphic rocks. We herein focus on a sample from an ancient deep thrust from a Cretaceous High-Pressure paleo-accretionary complex in Chilean Patagonia. A representative, moderately foliated micaschist exhibits broken garnet crystals that host a dense network of healed micro-fractures. While garnet fragments appear thoroughly disaggregated along the main foliation, the rock matrix that completely recrystallized has lost the record of brittle deformation. We employ a 2D visco-elasto-plastic numerical modelling approach in order to investigate the mechanical conditions that enable the fracturing of isolated garnet grains in a relatively weak matrix. The rupture of these stiff grains is achieved in our models at strain rates faster than 10 -10 /s to 10 -12 /s for elevated pore fluid pressures (80 to 99 % of the lithostatic value, respectively). Since high pore fluid pressures prevail in deep subduction interface settings, it is suggested that the rupture of these garnet crystals occurred through cataclastic deformation via (transient) slip rate acceleration, perhaps as a consequence of localized slip associated with slow to conventional earthquakes. Upon slip rate deceleration, viscous disaggregation of the broken garnet clasts occurred along with the erasing of the matrix cataclastic fabric.