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

Frictional behavior of talc-calcite mixtures

Faults 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

Representaciones sociales sobre alimentación saludable en los cuidadores de niños preescolares de Barrio Chingolo, Córdoba, en el año 2017

Introducción. En la población infantil, el acto de comer, las preferencias y el rechazo a determinados alimentos se encuentran fuertemente condicionados por el contexto familiar, y es en este período cuando se adoptan la mayoría de los hábitos y prácticas alimentarias; las cuales están influencias por las representaciones que se tengan de ellas. Objetivo: Indagar las representaciones sociales sobre alimentación saludable que tienen los cuidadores de niños preescolares de Barrio Chingolo durante el año 2017. Metodología: Estudio cualitativo, basado en la Teoría Fundamentada. Se realizaron 12 entrevistas a docentes y madres de alumnos del jardín y 2 observaciones participantes. El análisis comprendió un nivel descriptivo y otro interpretativo a través del Método Comparativo Constante. La información fue validada mediante la técnica de triangulación. Resultados: Tanto las madres como las docentes definieron a la alimentación saludable como aquella que aporta variedad de alimentos, considerando a las frutas y verduras, seguidos por los lácteos, carnes y cereales como alimentos saludables. Se observa que la fuente de información predominante en las madres es el equipo de salud del barrio. Con respecto a los principales alimentos preferidos por los niños emergen el yogur, las frutas, las golosinas y gaseosas, y los productos cárnicos procesados, mientras que los más rechazados fueron las verduras y la leche. Si bien los cuidadores reconocen cuáles son los alimentos saludables, refieren no conocer acerca de formas de preparación atractivas para incorporarlos. La alimentación que provee la escuela suele reproducir estas comidas y formas de preparación, reforzando los hábitos del hogar. Los docentes y profesionales de salud del barrio reconocen al sobrepeso y obesidad como una problemática en aumento, y consideran oportuna la intervención en edades tempranas Las madres, en cambio, tienen poco conocimiento de los conceptos y consecuencias de una mala nutrición. Conclusión: Las comidas habituales del hogar difieren de los conocimientos sobre alimentación saludable. La alimentación infantil es un fenómeno complejo, determinado por múltiples condiciones, entre ellas culturales, familiares, económicas, sociales. La manera en que los niños preescolares se alimentan, determina la formación de hábitos alimentarios que persistirán en la vida adulta y contribuyen a su estado nutricional, por lo que resulta necesario que las intervenciones en alimentación no solo se orienten a divulgar las propiedades de los alimentos, también deben propiciar un mayor acceso a los mismos.Fil: Alderete, Melina. Universidad Católica de Córdoba. Facultad de Ciencias de la Salud; ArgentinaFil: Giorgetti, Ana Carolina. Universidad Católica de Córdoba. Facultad de Ciencias de la Salud; Argentin

The influence of roughness on experimental fault mechanical behavior and associated microseismicity

Fault surfaces are rough at all scales, and this significantly affects fault-slip behavior. However, roughness is only occasionally considered experimentally and then often in experiments imposing a low-slip velocity, corresponding to the initiation stage of the earthquake cycle. Here, the effect of roughness on earthquake nucleation up to runaway slip is investigated through a series of dry load-stepping biaxial experiments performed on bare rock surfaces with a variety of roughnesses. These laboratory faults reached slip velocities of at least 100 mm/s. Acoustic emissions were located during deformation on bare rock surfaces in a biaxial apparatus during load-stepping experiments for the first time. Smooth surfaces showed more frequent slip instabilities accompanied by slip bursts and larger stress drops than rough faults. Smooth surfaces reached higher slip velocities and were less inclined to display velocity-strengthening behavior. The recorded and localized acoustic emissions were characterized by a greater proportion of large-magnitude events, and therefore likely a higher Gutenberg-Richter bGR-value, for smoother samples, while the cumulative seismic moment was similar for all roughnesses. These experiments shed light on how local microscopic heterogeneity associated with surface topography can influence the macroscopic stability of frictional interfaces and the associated microseismicity. They further provide a laboratory demonstration of roughness' ability to induce stress barriers, which can halt rupture, a phenomenon previously shown numerically

Microglial cells are involved in the susceptibility of NADPH oxidase knockout mice to 6-hydroxy-dopamine-induced neurodegeneration

We explored the impact of Nox-2 in modulating inflammatory-mediated microglial responses in the 6-hydroxydopamine (6-OHDA)-induced Parkinson’s disease (PD) model. Nox1 and Nox2 gene expression were found to increase in striatum, whereas a marked increase of Nox2 expression was observed in substantia nigra (SN) of wild-type (wt) mice after PD induction. Gp91phox-/- 6-OHDA-lesioned mice exhibited a significant reduction in the apomorphine-induced rotational behavior, when compared to wt mice. Immunolabeling assays indicated that striatal 6-OHDA injections reduced the number of dopaminergic (DA) neurons in the SN of wt mice. In gp91phox-/- 6-OHDA-lesioned mice the DA degeneration was negligible, suggesting an involvement of Nox in 6-OHDA-mediated SN degeneration. Gp91phox-/- 6-OHDA-lesioned mice treated with minocycline, a tetracycline derivative that exerts multiple anti-inflammatory effects, including microglial inhibition, exhibited increased apomorphine-induced rotational behavior and degeneration of DA neurons after 6-OHDA injections. The same treatment also increased TNF-α release and potentiated NF-κB activation in the SN of gp91phox-/--lesioned mice. Our results demonstrate for the first time that inhibition of microglial cells increases the susceptibility of gp91phox-/- 6-OHDA lesioned mice to develop PD. Blockade of microglia leads to NF-κB activation and TNF-α release into the SN of gp91phox-/- 6-OHDA lesioned mice, a likely mechanism whereby gp91phox-/- 6-OHDA lesioned mice may be more susceptible to develop PD after microglial cell inhibition. Nox2 adds an essential level of regulation to signaling pathways underlying the inflammatory response after PD inductionFAPESPCNPqApplied Neuroscience Nucleus (NAPNA, University of São Paulo

The influence of loading path on fault reactivation: a laboratory perspective

Despite natural faults are variably oriented to the Earth's surface and to the local stress field, the mechanics of fault reactivation and slip under variable loading paths (sensu Sibson, 1993) is still poorly understood. Nonetheless, different loading paths commonly occur in natural faults, from load-strengthening when the increase in shear stress is coupled with an increase in normal stress (e.g., reverse faults in absence of the fluid pressure increase) to load-weakening when the increase in shear stress is coupled with a decrease in normal stress (e.g., normal faults). According to the Mohr-Coulomb theory, the reactivation of pre-existing faults is only influenced by the fault orientation to the stress field, the fault friction, and the principal stresses magnitude. Therefore, the stress path the fault experienced is often neglected when evaluating the potential for reactivation. Yet, in natural faults characterized by thick, incohesive fault zone and highly fractured damage zone, the loading path could not be ruled out. Here we propose a laboratory approach aimed at reproducing the typical tectonic loading paths for reverse and normal faults. We performed triaxial saw-cut experiments, simulating the reactivation of well-oriented (i.e., 30° to the maximum principal stress) and misoriented (i.e., 50° to the maximum principal stress), normal and reverse gouge-bearing faults under dry and water-saturated conditions. We find that load-strengthening versus load-weakening path results in clearly different hydro-mechanical behavior. Particularly, prior to reactivation, reverse faults undergo compaction even at differential stresses well below the value required for reactivation. Contrarily, normal faults experience dilation, most of which occurs only near the differential stress values required for reactivation. Moreover, when reactivating at comparable normal stress, normal faults (load-weakening path) are more prone to slip seismically than reverse fault (load-strengthening path). Indeed, the higher mean stress that normal fault experienced before reactivation compacts more efficiently the gouge layer, thus increasing the fault stiffness and favoring seismic slip. This contrasting fault zone compaction and dilation prior to reactivation may occur in different natural tectonic settings, affecting the fault hydro-mechanical behavior. Thus, to take into account the loading path the fault experienced is fundamental in evaluating both natural and induced fault reactivation and the related seismic risk assessment

Load-strengthening versus load-weakening faulting in gouge-bearing faults: insights from triaxial saw-cut experiments

Brittle reactivation of pre-existing faults is theoretically constrained by their friction, the stress field orientation, and magnitude. Thus, following the Mohr-Coulomb failure criterion, the increase in tectonic shear stress leads to the reactivation of the fault whenever the shear stress achieves its frictional strength. Moreover, the increase in shear stress can occur following different fault loading paths: commonly, reverse faulting is accompanied by an increase in normal stress (load-strengthening path), while normal faulting is accompanied by a decrease in normal stress (load-weakening path). However, how the loading of the fault influences its frictional strength, i.e. its reactivation as well as its seismic behavior, is still poorly understood. To investigate this, we conducted triaxial saw-cut experiments under different loading conditions. We simulated pre-existing fault zones placing a layer of quartz gouge in a saw-cut through a cylinder of Rothbach sandstone. We investigated faults favorably and unfavorably oriented for reactivation, i.e. oriented at 30. and 50. to the maximum principal stress. These experimental faults were subjected to load-strengthening and load-weakening paths. Different confining pressures (i.e. minimum principal stress) were tested in such a way as to reactivate faults, that undergo different loading paths, at comparable shear and normal stresses. In addition, acoustic emissions were monitored during fault deformation. Consistently with previous studies, our results show that increasing the angle to the maximum principal stress from 30. to 50., the frictional strength (i.e. the apparent friction) of gouge-bearing faults decreases. Furthermore, the frictional strength of unfavorably oriented faults (50.) is affected by the loading path, showing slightly lower frictional strength for load-strengthening than for load-weakening path. These observations suggest that the potential for reactivation of thick, gouge-bearing faults depends on the loading conditions and it cannot be properly assessed assuming a zero-thickness fault plane