Feldspar reduces fault frictional healing rate under hydrothermal conditions.

Fault restrengthening after earthquakes enables country rocks to restore elastic strain energy for subsequent seismic cycles. It originates from the contribution of several processes resulting in fault sealing and fault frictional healing, the latter arising from contact area growth and formation of new chemical bonds between grains. Laboratory studies demonstrate that fault frictional healing typically increases logarithmically with time during quasi-stationary contact (i.e., positive healing rate). Here we show that this logarithmic healing does not hold under certain (hydrothermal) conditions for feldspar, the most abundant mineral in the Earth's crust, and for common feldspar-rich rocks. The healing rate switches from positive to negative with increasing quasi-stationary contact time at temperatures ≥ 200 °C in the presence of pressurized water. This unusual healing behavior suggests that thermally-activated friction weakening processes may exist at feldspar interfaces. This hypothesis is further validated through nano-scale friction experiments by sliding single-feldspar crystals on a feldspar substrate. Given the abundance of feldspars in the Earth's crust, their peculiar healing behavior under hydrothermal conditions can lead to unexpected variations in fault strength, changing our understanding of fault mechanics and the seismic cycle, and challenging the direct extrapolation of laboratory-derived healing parameters to natural earthquake recurrence intervals.