The mechanics of phyllosilicates-bearing faults: insights from field examples and rock deformation experiments

Faults in the brittle crust are zones of weakness, whose reactivation depends on their friction and stress field acting on them. Over the last few decades, increasingly attention has been paid to characterize the frictional properties of phyllosilicates. These layer-structured minerals are indeed particularly weak, if compared with earliest laboratory experiments conducted on a vast gamut of crustal rocks showing friction almost independent of rock type and in the range of μ = 0.6-0.85. Phyllosilicates are not only inherently weak, but also unable to re-gain strength during inter-seismic period and to host earthquake nucleation. Moreover, previous studies have reported that even small amounts of phyllosilicates can drastically affect the overall frictional properties of fault rocks. These observations have strong implications for natural faults that involve different lithologies, including phyllosilicates-bearing rocks, and thus develop geometrical and lithological heterogeneities along dip and strike. The influence of these heterogeneities on fault mechanics is still poorly constrained. Here, I integrate field observations and laboratory experiments on phyllosilicate-bearing faults to address different aspects regarding the role of phyllosilicates in fault mechanics. I examine questions such as: what is the minimum amount of phyllosilicates that drastically affects fault frictional properties? What is the mechanics of incipient faults within phyllosilicate-rich mechanical multilayers? What is the role of stress field orientation in the reactivation of phyllosilicate-bearing faults? In Chapter 1, I experimentally investigate the frictional properties of talc-bearing faults in carbonates. Although talc has been found along carbonate-bearing faults, little is known about the amount of talc able to effectively weaken calcite fault gouges. In Chapter 2, I integrate field data and laboratory deformation experiments to study fault initiation and growth within clay-rich mechanical multilayers. Thus I give insight into the mechanics of clay-rich multilayers that is still poorly understood. In Chapter 3, I report on laboratory deformation experiments designed to evaluate the reactivation of pre-existing clay-bearing faults depending on their orientation within the stress field. The reactivation of pre-existing faults can be theoretically predicted assuming a zero-thickness fault. I attempt to validate frictional reactivation for a finite-thickness fault. This dissertation provides insight into the mechanics of phyllosilicate-bearings faults. Firstly, I show that small amounts of talc fully weaken calcite-rich faults, developing an interconnected network of talc lamellae, and that even minor amounts of talc result in the evolution from velocity-neutral to velocity-strengthening behavior and in the reduction of 50% in frictional healing. Secondly, I demonstrate that the complex geometry of faults affecting mechanical multilayers results from the interplay between the mechanical properties of the involved lithologies and the presence of pre-existing discontinuities. Finally, I show that misoriented faults of finite thickness are weaker than theoretically predicted and that the assumption of a zero-thickness plane provides an upper bound for the stress required for the reactivation of a finite-thickness fault.