Paper No. 4
Presentation Time: 1:55 PM


FONDRIEST, Michele1, SMITH, Steven A.F.2, CANDELA, Thibault3, NIELSEN, Stefan2, MAIR, Karen4 and DI TORO, Giulio5, (1)Geosciences Department, University of Padova, via G. Gradenigo 6, Padova, 35137, Italy, (2)Istituto Nazionale di Geofisica e Vulcanologia (INGV), Via di Vigna Murata 605, Rome, 00143, Italy, (3)Department of Geosciences, Pennsylvania State University, University Park, PA 16802, (4)Physics of Geological Processes, Department of Geosciences, University of Oslo, Oslo, 0371, Norway, (5)Dipartimento di Geoscienze, University of Padova, via G. Gradenigo, 6, Padova, 35131, Italy,

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.