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The mechanics of phyllosilicates-bearing faults: insights from field examples and rock deformation experiments
Full text linkFaults 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
The Role of Anisotropy in Fault Mechanics: Experimental Insights from Deformation Experiments in Triaxial Saw-cut Configuration
No full textThe hydro-mechanical properties of sealing horizons consisting of mechanical multilayers
No full textThe role of pre-existing anisotropies in fault mechanics: experimental insights from triaxial saw-cut experiments
No full textFault orientation and fault rock mechanical properties exert a major control on fault reactivation. Slip along faults that lie at high angle to the maximum compressive stress requires restrictive conditions, such as very low friction coefficient and/or near-lithostatic pore fluid pressure. However, the mechanism allowing a misoriented fault to initiate is still a matter of debate. Nevertheless, misoriented faults, such as the San Andreas fault, low-angle normal faults and sub-horizontal décollements, are recognized throughout the world. On a smaller scale, field observations show that mesoscale faults in mechanical multilayers often propagate within clay-rich layers at high angle to the maximum principal stress. These observations suggest that the presence of pre-existing anisotropies, not only resulting from faulting but also due to inherited foliation or sedimentary layering, plays a key role in determining fault geometry. To investigate this problem, we present a suite of triaxial experiments in which we deformed cylindrical samples of sandstone with pre-imposed saw-cuts with variable orientations respect to the axial stress. A layer of powdered marl or shale was placed within the saw-cut to simulate a pre-existing anisotropy. We performed: 1) conventional triaxial experiments to evaluate the failure envelope of the sandstone, and 2) biaxial experiments to evaluate the frictional strength of the marl and shale. Then, 3) we conducted triaxial experiments at constant confining pressure with saw-cuts oriented at different angles to the axial stress, from 30° (favourably oriented) up to 80° (severely misoriented). Microstructural observations on reactivated saw-cuts were conducted to investigate the deformation processes with increasing fault deformation. Finally, we compared the resulting deformation mode, i.e. saw- cut reactivation vs. new fracture development, with theoretical predictions based on sandstone strength and marl/shale friction. Our results show a complex mechanical behaviour that results from the interplay between the stress field orientation and the contrast in mechanical properties between the gouge and the surrounding. Reactivation occurs only in saw-cuts at angles to the axial stress that are lower than 60°-70°, consistently with theoretical predictions. Preliminary laboratory data show that under dry conditions the reactivation is a multi-stage process: after an initial compaction phase and a linear-elastic phase, yielding of the fault gouge occurs. Given the same boundary conditions, yielding occurs at the same differential stress independently of saw-cut orientation. After the yielding, a second linear-elastic phase precedes stick-slips followed by stable sliding. Integrating mechanical data and microstructural investigation we suggest that the first inelastic yielding occurs when the Mohr circle is tangent to the frictional reactivation criterion, obtained from friction experiments (2). Then, stick-slip and stable sliding are associated to localization of deformation along shear planes. We infer that they occur only when the stress path of the saw-cut intersects the frictional reactivation criterion
Frictional behavior of talc-calcite mixtures
Full text linkFaults involving phyllosilicates appear weak when compared to the laboratory-derived strength of most crustal rocks. Among phyllosilicates, talc, with very low friction, is one of the weakest minerals involved in various tectonic settings. As the presence of talc has been recently documented in carbonate faults, we performed laboratory friction experiments to better constrain how various amounts of talc
could alter these fault’s frictional properties. We used a biaxial apparatus to systematically shear different mixtures of talc and calcite as powdered gouge at room temperature, normal stresses up to 50 MPa and under different pore fluid saturated conditions, i.e., CaCO3-equilibrated water and silicone oil. We performed slide-hold-slide tests, 1–3000 s, to measure the amount of frictional healing and velocity-stepping tests, 0.1–1000 μm/s, to evaluate frictional stability. We then analyzed microstructures developed during our experiments. Our results show that with the addition of 20% talc the calcite gouge undergoes a 70% reduction in steady state frictional strength, a complete reduction of frictional healing and a transition from velocity-weakening to velocity-strengthening behavior. Microstructural analysis shows that with increasing talc content, deformation mechanisms evolve from distributed cataclastic flow of the granular calcite to localized sliding along talc-rich shear planes, resulting in a fully interconnected network of talc lamellae from 20% talc onward. Our observations indicate that in faults where talc and calcite are present, a low concentration of talc is enough to strongly modify the gouge’s frictional properties and specifically to weaken the fault, reduce its ability to sustain future stress drops, and stabilize slip
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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