MIRROR-LIKE FAULTS AND EARTHQUAKE SCARS IN DOLOSTONES

Light-reflective (so called “mirror-like”) faults are widely observed geological features, especially in carbonate-bearing faults of the shallow crust, but their origin has never been established. We report the occurrence of mirror-like faults cutting dolostone gouges in the Italian Alps. Field observations and rotary-shear friction experiments demonstrate that such faults (i) represent scars of ancient earthquakes, and (ii) can be used to estimate power dissipation during seismic slip.

The Foiana Line (FL) is a transpressive fault zone exhumed from ~2 km depth containing pulverized dolostones cut by faults with a mirror-like finish. The faults formed at normal stresses of ~30-50 MPa and have (i) mean surface roughness of ~2 µm (White Light Interferometry [WLI] over areas ~200×300 µm²) and, (ii) displacements of 0.04-0.5 m.

To understand the origin of the mirror-like faults, low- to high-velocity friction experiments using SHIVA (INGV, Rome) were performed on layers of dolostone gouge collected from the FL. Tests were at slip rates of 0.0001-1 m/s, normal stresses up to 26 MPa and displacements of 0.02-3.5 m.

At seismic slip rates of ~1 m/s the dolostone gouges showed a dramatic reduction of the friction coefficient (µ) from a peak value of ~0.7 to a steady-state value of ~0.25. During the tests the instantaneous frictional power density (shear stress*slip rate) dissipated in the samples reached values of 4-10 MW/m², comparable to natural earthquakes. Above a power density threshold of ~1 MW/m² shiny mirror-like slip surfaces were progressively formed. WLI revealed that mean surface roughness of the experimental slip surfaces was ~4 µm, similar to the FL faults. Moreover, microstructures of both natural and experimental mirror-like surfaces were characterized by strong strain localization.

At sub-seismic slip rates (0.0001-0.001 m/s) µ was constant at ~0.7, power density ~0.008-0.04 MW/m², and no mirror-like surfaces were formed. The slip surfaces at sub-seismic slip rates had mean roughness (from WLI) of ~30 µm.

We conclude that small-displacement mirror-like faults in dolostones gouges result from extreme power dissipation (1-10 MW/m²) during seismic slip. Since active faults can also be aseismic, the presence of mirror-like faults (i.e. earthquake ruptures) may be relevant in seismic hazard assessment.