Geometry, kinematics, and roughness properties of segmented normal fault zones: insights from the Monte Capo di Serre carbonate fault zone, central Italy

The Monte Capo di Serre normal fault zone of central Italy is a ∼8 km-long, 110 m-offset active structure displacing Mesozoic-Tertiary platform carbonates and Pleistocene slope debris. The fault zone strikes ca. NW-SE, dips SW, and it is continuously exposed along-strike at surface for ca. 400 m forming four overlapping segments including multiple main slip surfaces striking mainly N130-160E solving pure dip-slip extension. Within relay ramps, the attitude of single slickensides is dependent upon overstep geometry. Dextral oversteps include NNW-SSE to N-S striking slickensides, whereas sinistral oversteps ESE-WNW to E-W striking slickensides. Relative to the main slip surfaces, stress inversion analysis shows ∼40° clockwise stress rotations under a vertical axis within the former relay ramps, and σ2-σ3 permutations within the latter ones. Spectral analysis of surface roughness indicates that the main slip surfaces include slickensides with a smooth morphology (root mean square roughness, Rq, ∼1 mm), differently relay ramps contain slickensides with rougher topographies (Rq = 2.6-6.2 mm). Combining these data with those after microstructural analysis of fault rock samples collected in the field, we infer that the main slip surfaces predominantly accommodated seismic slip, as shown by truncated clasts and multiple ultracataclasite layers. Conversely, the relay ramps localized diffuse post-seismic deformation and aftershock-related fracturing, as recorded by diffuse brecciation of both carbonates and slope debris. Applying this knowledge to extensional regimes, results of this work might better elucidate the role exerted by fault geometry on the spatiotemporal distribution of seismicity at shallow crustal depths.