Program > Browse abstracts by speaker > Eberhard Lisa

Fluid-rock interactions critically influence the seismogenic potential of fault zones. For example, some segments of the San Andreas fault system lack strong seismic events and strain is accommodated by creep, possibly due to fluid-rock interactions. Infiltration of CO2-bearing fluids in the ultramafic fault segments may cause the formation of frictionally weak talc from serpentine (Klein, et al. 2021). Thermodynamic calculations confirm a shift from serpentine to talc+magnesite (soapstone) and quartz+magnesite (listvenite) assemblages upon infiltration of CO2-bearing fluids. The strength and seismogenic potential of an ultramafic fault zone may thus dynamically change during progressive carbonation.

To test this hypothesis, we performed rotary-shear experiments on gouge layers with compositions ranging from serpentinite to soapstone and listvenite at 300 °C, 250 MPa normal stress and 100 MPa pore pressure, and velocities from 0.002 μm/s to 10 μm/s.

Serpentinite gouges are found to be relatively strong and slightly velocity-weakening, with friction coefficients dropping from 0.45 (0.002 μm/s) to 0.42 (10 μm/s). Soapstone gouges exhibit similar velocity-dependence, although at significantly lower frictional strength (µ = 0.22 - 0.30). Listvenite gouges show the opposite trend and friction coefficients increase from 0.25 (0.002 μm/s) to 0.48 (10 μm/s).

Microtextural observations indicate grain-size reduction through cataclasis in serpentinite gouges. In soapstone and listvenite gouges shear is accommodated mainly in fine-grained monomineralic talc layers, which formed due to the buffer capacity of the system tending to increase the silica activity of the fluid. However, the velocity dependence of the friction parameters dramatically changes from quartz-undersaturated conditions (soapstone) to quartz-saturated conditions (listvenite).

Our results show that serpentinized fault zones have the potential to nucleate seismic events. CO2-fluid-rock interaction in ultramafic fault gouges may promote creep. However, the results also suggest that creep-efficiency, i.e., dissolution-precipitation kinetics, depends strongly on the fluid composition.

References:

Klein, F., et al. (2022). GRL, e2022GL099185.