Semi-Brittle Deformation of Talc at the Base of the Seismogenic Zone

Talc is commonly found in the cores of exhumed faults and may be important to the dynamics of slip in active fault zones. To understand the rheology of talc at conditions relevant to subduction zones, we conducted torsional deformation experiments at high pressure (1 GPa) and temperatures (450-500 degrees C). Scanning Transmission Electron Microscope imaging revealed a marked decrease in grain size with increasing strain, in addition to the development of grain kinking and nanoporosity. The similarity of these microstructures to talc deformed in natural faults and low-pressure experiments indicates that the dominant deformation mechanisms of talc are similar across a wide range of depths. We conclude that frictional processes remain an important control on talc rheology even under high normal stresses. However, deformation-induced porosity could enhance the percolation of high-pressure or reactive fluids through talc-rich lithologies. Plain Language Summary Talc, an extremely weak mineral commonly observed in natural faults, likely plays an important role in controlling how earthquake-generating faults slip. We performed high pressure deformation experiments on a natural talc sample to help understand the role of talc at depth in large faults. Many minerals will undergo a change in deformation style as pressure and temperature increase, from a mechanism controlled by friction to a mechanism controlled by defects within the crystal lattice. Conversely, our findings indicate that talc does not experience a change in deformation mechanism at high pressure and temperature, with friction remaining an important control on talc deformation across all our experiments. This suggests that talc remains brittle throughout the seismogenic zone, including the region of tremor and slow slip. However, we also observed that as strain in the talc increased so did the generation of pore space in the samples. This could increase permeability, allowing fluids to migrate more readily through deformed talc.