Structure and mechanics of carbonate-hosted fault zones: insights from lab, field, and virtual outcrop models

Carbonate-hosted fault zones have been extensively studied in the recent past, due to their high socio-economic importance. In fact, they often host seismic sequences characterized by destructive earthquakes with shallow hypocentres (< 10 km) and high magnitude (MW > 5). Studies dealing with carbonate-hosted faults can therefore help a better assessment of the seismic risk of regions where seismicity occurs within thick carbonate successions. Moreover, since more than the half of the current hydrocarbon reserves are held within carbonate reservoirs, carbonate-hosted faults play a key role in hydrocarbon migration and storage. Being intimately related to fault mechanics and fluid flow, fault zone structure has been studied at different scales and using different techniques. Segmentation and/or bending of faults at sub- regional scales (1-10 km) can have important consequences on seismic rupture propagation and arrest. The outcrop-scale (10 m – 1 km) structure has fundamental implications for the fluid flow and fault mechanics. On one hand, the fault core vs. damage zone arrangement, and particularly fracture distribution within the latter, define the fault permeability structure. On the other hand, the deformation style (i.e., localized or/and distributed) can offer insights on the frictional properties of the fault zone (i.e., strong or/and weak fault). Microstructures collected on the principal slip zones (mm-thick zones that accommodates most of the displacement in faults) can shed light on the deformation mechanisms accommodating slip on faults. Finally, natural microstructures in principal slip zones can be associated with a specific mechanical behaviour and physical-chemical conditions (e.g., normal stress, slip velocity, saturating fluid, temperature) leveraging on their comparison with structures observed in principal slip zones retrieved from friction experiments. Although many studies focused on carbonate-hosted faults structure during the last two decades, there is still a lot of work to do in order to accomplish the complete characterization of their structure and mechanics. One of the most challenging goal is to understand factors controlling fault zone structure. In this thesis I investigate the factors controlling the geometry, kinematics, mechanics, and distribution of various components of fault zones: principal slip zones (chapter 2), subsidiary faults (chapter 3), and fractures (chapter 4). In chapter 2, “Strength evolution of simulated carbonate-bearing faults: The role of normal stress and slip velocity”, I present the results obtained from a series of rock mechanics experiments, conducted to evaluate the friction of simulated carbonate-bearing faults in water- saturated conditions and for a wide range of normal stresses (from 5 to 120 MPa) and slip velocities (from 0.3 to 100 μm/s). Since the coexistence of structures related to pressure dependent (i.e., cataclastic) and pressure independent deformation processes (i.e., pressure- solution and granular plasticity) is common within natural carbonate-hosted faults exhumed from shallow seismogenic depths (< 6 km), I simulated the slip nucleation on simulated carbonate-bearing faults in order to constrain the boundary conditions (normal stress and slip velocity) that are necessary to activate pressure independent processes. The comparison between the mechanical results and the obtained microstructures allowed me to evaluate the effect of the activation of pressure independent deformation processes on friction. At low normal stresses (σ" ≤ 20 MPa) the deformation is accommodated by localized cataclastic grain size reduction, and friction is high (μ = 0.64). Pressure independent processes, especially pressure-solution, increase their contribution in accommodating slip with increasing normal stress and decreasing slip velocity. The activation of such processes produces an anastomosed foliation, accompanied by cementation, grain indentations, grain folding, and the formation of striated surfaces coated with nanograins. Friction decreases with an increasing contribution of pressure independent processes, reaching very low values (μ = 0.47) at the highest normal stress (σ" = 120 MPa) and lowest slip velocity (v = 0.3 μm/s) tested conditions. The results suggest that the activation of fluid assisted diffusion mass transfer (i.e., pressure-solution) and grain plasticity can significantly reduce the frictional strength of carbonate-bearing faults, facilitating the onset of fault slip. In chapters 3, “Complex geometry and kinematics of subsidiary faults within a carbonate- hosted relay ramp”, and 4, “Lithological and structural control on fracture distribution within a carbonate-hosted relay ramp”, I investigate an exceptional exposure of a portion of a carbonate- hosted relay ramp damage zone, pertaining to the Tre Monti normal fault in the Central Apennines (Italy). The studied outcrop is located immediately at the footwall of the front fault segment. The relay ramps are zones of slip transfer between overlapping normal faults and represent a very challenging and interesting case-study. In fact, the mechanical interaction between the two faults promotes an increase in damage, representing a potential preferential pathway for fluids. Moreover, relay ramps can represent zones of stress field rotation. For both the chapters I leveraged on the integration of traditional field techniques and interpretation of virtual outcrops. Three-dimensional digital reconstructions of outcrops (VOM: Virtual Outcrop Models, or DOM: Digital Outcrop Models) can be obtained from terrestrial laser scanner and/or photogrammetric surveys. Virtual outcrops are increasingly used in structural geology because they enhance our ability to collect data, allowing the exploration of inaccessible portions of the outcrop and the collection high-precision georeferenced dataset. In chapter 3, the geometry and the kinematics of the subsidiary faults have been investigated. Minor faults show complex geometry and kinematics, having multiple attitudes each one characterized by highly variable kinematics. The fault slip analysis highlights that minor faults geometry and kinematics are not compatible neither with the overall dip-slip kinematics of the Tre Monti fault nor with the active regional extension occurring in the central Apennines. Conversely, a local stress field, retrieved from the kinematic inversion of the locally occurring right lateral slickenlines on the front fault segment, is able to explain most of the minor faults geometry and kinematics. Such a stress field is likely caused by the mechanical interaction between the fault segments bordering the relay ramp. The results obtained in this chapter highlight that the local stress field plays a key-role in the complex minor faults geometry and kinematics. Further complexity can be provided by the local scale temporal interaction with other stress. Finally, in chapter 4, the fracture distribution within the relay-ramp damage zone is imaged through the integration of classical field techniques (i.e., scanlines), fracture counting on oriented rock samples, and interpretation of a virtual outcrop derived from an aero- photogrammetric survey. Fracture density increases with distance from the front segment of the relay ramp. The results also highlight a control of carbonate facies on fracturing, with supratidal and intertidal facies showing higher fracture density than subtidal limestones. This apparent anomalous pattern of fracture density, that increases moving away from a main fault segment, is related to two main factors. (1) Since moving away from the front segment (i.e., toward the centre of the relay ramp), also the number of subsidiary faults increases, the damage is likely related to the activity of subsidiary faults accompanying the development of the relay ramp. (2) The supratidal/intertidal facies content increases toward the centre of the relay ramp leading to an increase in fracture density. This thesis furtherly emphasizes the importance of friction experiments and virtual outcrops in structural geology studies dealing with fault zone structure and mechanics. Friction experiments allowed to establish the effect of pressure insensitive deformation processes on the carbonate- faults mechanics through a direct comparison between microstructures and the mechanical behaviour. The employment of virtual outcrops enabled a very detailed mapping of subsidiary faults and fracture density within a carbonate-hosted damage zone, allowing the investigation of the factors that controls subsidiary faults geometry and kinematics, and fracture distribution.