Structural complexity and fault roughness properties of carbonate relay ramps

The geometry and kinematics of small faults within relay ramps are affected by local stress perturbations due to coeval propagation of laterally overstepping normal faults with negligible separations. Independent of their stepping sense and relative fault slip rates, previous studies documented the high structural complexity of carbonate relay ramps cropping out in central Italy, where Mesozoic carbonates of the Lazio-Abruzzi Platform are crosscut by an active extensional fault system. Notably examples include the ~400 m-wide, ~900 m-long relay ramp within the Tre Monti fault dissected by small faults with variable attitudes and kinematics showing values of fault and fracture density peaks in its middle portion. Similarly, the ~3 km-wide, ~7km-long relay ramp bounded by the Venere-Gioia dei Marsi and Pescina-Parasano normal faults exhibits the highest amount of extensional strain within its central portion.

In order to gain new insights on the possible role of surface geometry of the main slipping planes on the spatial distribution of fault-related damage, we focus on the ca. 8 km-long, 110 m-offset, NW-SE striking and SW-dipping Monte Capo di Serre fault. This fault displaces Mesozoic-Tertiary platform carbonates and Pleistocene slope debris, and it is continuously exposed along a ~800 m-long along-strike outcrop. Studying a ~60 m-long and 32 m-wide relay ramp bounded by 100’s m- long fault segments forming a sinistral overstep, and at smaller scale a 9 m-long, 5 m-wide relay ramp bounded by 10’s of m-long dextral overstepping slip surfaces we first conduct field and digital structural analyses and then fault roughness analysis.

Results show that the slickenside attitude and kinematics are controlled by overstep geometry. In fact, dextral oversteps are associated with NNW-SSE to N-S striking high-angle slickensides recording pure-dip slip extension, whereas sinistral oversteps are characterized by ESE-WNW to E-W striking, moderate-angle slickensides recording left-lateral transtension. Independently of the overstep geometry, results of spectral analysis of the outcropping slickensides indicate they are significantly rougher (root mean square roughness, R_q≈25-68 mm) within relay ramps than along the main slip surfaces (R_q≈1 mm). Integration with microstructural observations suggests that the relay ramps localized diffuse post-seismic deformation and aftershock-related fracturing, as recorded by diffuse host rock brecciation and widespread fracturing. Conversely, the main slip surfaces predominantly accommodated seismic slip, as shown by truncated clasts and multiple generation of cataclasite and ultracataclasite layers. We argue that these results support the interpretation that fault-surface roughness within carbonate relay ramps might exert a primary control on local stress perturbations, thereby contribution to their complex structural and kinematics complexities.