Temperature-dependent frictional and healing behaviour in serpentinite shear zones: implications for subduction zone seismicity

Serpentinites are “weak" rocks common in several geodynamics settings, including mid-ocean ridges, transform faults and subduction zones. Because of this, serpentinites play a pivotal role in the nucleation and propagation of slow and regular earthquakes.

We studied serpentinites pertaining to the exhumed Monte Fico shear zone (Elba Island, Italy) that reached greenschist facies conditions during deformation. The shear zone, tens to hundreds of m thick, is made of 10-100 cm lenses of metaperidotite, mainly composed by the lizardite and chrysotile, wrapped by foliated serpentinites. Bulk deformation is accommodated by anastomosing and pervasive S/C foliations. The lenses are bounded by 1-3 cm thick brittle faults decorated by slickenfibers composed of chrysotile and polygonal serpentine.

To determine the frictional and healing properties of the serpentinite-bearing shear zones and faults under realistic ambient shallow-subduction conditions, we performed 39 slide-hold-slide experiments at σ’n=20 MPa, Pf=6 MPa, V_shear= 10 µm/s and at temperatures from 25°C to 400°C. We sheared with a rotary shear apparatus equipped with a hydrothermal vessel (ROSA-HYDROS, Dept. of Geosciences, UniPD, Italy) the powders obtained from the metaperidotite and the slickenfibers.

Regarding the frictional properties of the metaperidotite, when the water is in a liquid state, the friction coefficient increases from 0.3 at 25°C to 0.5 at 300°C; when water is in vapour and supercritical states, the friction coefficient is strain hardening (0.6-0.89 at 300°C-400°C). Stick-slip behaviour (i.e., seismic slip) is observed only at 400°C. In the case of slickenfibers, when the water is in liquid state, the friction coefficient increases from 0.23 at 25°C to 0.34 at 300°C. When water is in vapour conditions the friction coefficient is 0.47 at 300°C and 0.57 at 400°C. In conclusion, the metaperidotite can deform by aseismic creep or seismic slip and the slickenfibers deform by aseismic creep.

Regarding the frictional healing properties, it differs between metaperidotite and slickenfibers. Frictional healing of the metaperidotite is positive and increases with temperature, independently of the physical state of water. Instead, frictional healing of the slickenfibers is negative for nearly all the conditions, with a maximum positive healing between 150°C and 250°C.

Our results show that the frictional response of low-grade serpentinites sheared in the laboratory at shallow-subduction hydrothermal conditions is controlled by the mineral assemblage and temperature. As a consequence, in nature, the combination of frictional and healing behaviour is highly heterogeneous and becomes the driver for enhanced instabilities on the weak but fast-healing slickenfibers in a narrow temperature window between 150°C and 250°C.