Influence of grain size heterogeneity on strength and microcracking behavior of crystalline rocks

published_or_final_versio

Linking between Multi-scale Behaviours of Brittle Rocks at Deep Underground Excavation

Rock fracturing is a hot issue in rock engineering. The macroscopic fracture development is associated with the microscopic damage evolution. Therefore, this research investigated the failure mechanism based on multi-scale analysis methods, including micro-scale (grain scale), meso-scale (laboratory scale), and macro-scale (field scale). This research combined the microscopic data and laboratory-scale mechanical properties to create a valid field-scale model based on the comparison with the in-situ data

3D-modelling of fault-induced small-scale secondary fracturing in crystalline rocks

The objective of this Thesis was to develop new methods to model the microstructures within the bedrock, as these models improve the understanding of the properties of the micro-scale fracture networks and could be further applied to improve the interpretation of kinematics in the deformation zones and micro-scale hydrological properties of the different rock types. The second aim of the study was to compare datasets generated from the same samples by two alternative methods: X-ray Ct-scanning and the new 3D- grinding method. The study area is located in the municipality of Geta, in the northern parts of the Åland Islands, southern Finland. The Geta fault is a sub-vertical NE-SW trending dextral strike-slip fault. The 3D character of the fault and its well-developed damage zone allows studying the fault and its deformation zone in various scales and methods. Setting for the 3D-samples was defined by field observations, 2D fracture and fault mapping from orthophotographs and 3D-photogrammetry models, which allow correlation of the 3D-fracture network characteristics in variable scales and with regional 2D-datasets from recent and ongoing investigations (Orrengrund and other MIRA-3D project targets). 3D-samples were drilled into 50*50/60mm sized drill cores for the 3D-grinder. Two of the 3D-samples were first Ct-scanned and grinded afterwards with 3D-grinder for making possible the micro-scale topology analyses in different sample depths and 3D-modelling on microstructures. Results show that grinding tomography images are accurate and many different details can be viewed from them. The grinding tomography method allows generating data based on which microstructures can be modeled and observed with micrometer accuracy. The results of the 3D-modelling indicate that the orientations and dips of the micro-scale secondary fracturing corresponds to the macro-scale fracturing within the damage zones. However, fracture intersection affects the fracture geometries in micro-scale but not in larger scale. Micro-scale topology analyses show very little variations compared to macro-scale analyses

Influence of dissolution/reprecipitation reactions on metamorphic greenschist to amphibolite-facies mica ⁴⁰Ar/³⁹Ar ages in the Longmen Shan (eastern Tibet)

Linking ages to metamorphic stages in rocks that have experienced low to medium‐grade metamorphism can be particularly tricky due to the rarity of index minerals and the preservation of mineral or compositional relicts. The timing of metamorphism and the Mesozoic exhumation of the metasedimentary units and crystalline basement that form the internal part of the Longmen Shan (eastern Tibet, Sichuan, China), is, for these reasons, still largely unconstrained, but crucial for understanding the regional tectonic evolution of the eastern Tibet. In‐situ core‐rim 40Ar/39Ar biotite and U‐Th/Pb allanite data show that amphibolite‐facies conditions (~10‐11 kbar, 530 °C to 6‐7 kbar, 580 °C) were reached at 210‐180 Ma and that biotite records crystallization, rather than cooling, ages. These conditions are mainly recorded in the metasedimentary cover. The 40Ar/39Ar ages obtained from matrix muscovite that partially re‐equilibrated during the post peak‐P metamorphic history comprise a mixture of ages between that of early prograde muscovite relicts and the timing of late muscovite recrystallization at c. 140‐120 Ma. This event marks a previously poorly documented greenschist facies metamorphic overprint. This latest stage is also recorded in the crystalline basement, and defines the timing of the greenschist‐overprint (7 ± 1 kbar, 370 ± 35 °C). Numerical models of Ar diffusion show that the difference between 40Ar/39Ar biotite and muscovite ages cannot be explained by a slow and protracted cooling in an open system. The model and petrological results rather suggest that biotite and muscovite experienced different Ar retention and resetting histories. The Ar record in mica of the studied low to medium grade rocks seems to be mainly controlled by dissolution‐reprecipitation processes rather than by diffusive loss, and by different microstructural positions in the sample. Together, our data show that the metasedimentary cover was thickened and cooled independently from the basement prior to c. 140 Ma (with a relatively fast cooling at 4.5 ± 0.5 °C/Ma between 185 and 140 Ma). Since the Lower Cretaceous the metasedimentary cover and the crystalline basement experienced a coherent history during which both were partially exhumed. The Mesozoic history of the Eastern border of the Tibetan plateau is therefore complex, polyphase, and the basement was actively involved at least since the Early Cretaceous, changing our perspective on the contribution of the Cenozoic geology

Progressive Damage Mechanism of Rocks Subjected to Cyclic Loading

This thesis investigates the effect of cyclic loading conditions on the strength and deformation behaviour of rock materials under uniaxial compression. Two microstructurally and mineralogically different rocks are considered in this experimental investigation. Variations in loading stress level, stress amplitude and frequency are considered to derive novel conclusions on the damage mechanism and fatigue strength. The investigation revealed the greater susceptibility of hard rocks to cyclic loading compared to soft rocks

Earthquakes: from chemical alteration to mechanical rupture

In the standard rebound theory of earthquakes, elastic deformation energy is progressively stored in the crust until a threshold is reached at which it is suddenly released in an earthquake. We review three important paradoxes, the strain paradox, the stress paradox and the heat flow paradox, that are difficult to account for in this picture, either individually or when taken together. Resolutions of these paradoxes usually call for additional assumptions on the nature of the rupture process (such as novel modes of deformations and ruptures) prior to and/or during an earthquake, on the nature of the fault and on the effect of trapped fluids within the crust at seismogenic depths. We review the evidence for the essential importance of water and its interaction with the modes of deformations. Water is usually seen to have mainly the mechanical effect of decreasing the normal lithostatic stress in the fault core on one hand and to weaken rock materials via hydrolytic weakening and stress corrosion on the other hand. We also review the evidences that water plays a major role in the alteration of minerals subjected to finite strains into other structures in out-of-equilibrium conditions. This suggests novel exciting routes to understand what is an earthquake, that requires to develop a truly multidisciplinary approach involving mineral chemistry, geology, rupture mechanics and statistical physics.Comment: 44 pages, 1 figures, submitted to Physics Report

Fault-Adjacent Damage at the Base of the Seismogenic Zone and Seismic Anisotropy of Fold Structures

While earthquakes represent a major hazard to life and property, there are a number of open questions about how earthquake faults operate at depth, and how the energy released by earthquakes travels as elastic waves through Earth’s complexly deformed crustal rocks. The aims of my dissertation are to explore (a) the extent of co-seismic damage in an ancient earthquake fault exhumed from great depths, (b) the deformation processes and mechanics of the fault at depth during earthquake cycles, and (c) the role of different rock structures in determining the velocities of seismic waves. When tectonic plates collide, deformation tends to localize into narrow zones: frictional faults in the upper crust and high-temperature viscous shear zones in the lower crust. The transition in material behavior from the upper to lower crust is known as the frictional-to-viscous transition (FVT; ~10–20 km deep). During earthquake cycles, the FVT experiences transient brittle deformation followed by long-term viscous processes. Owing to this complex behavior over the earthquake cycle, the FVT is the most important horizon for understanding earthquake mechanics. Rareness of exposures of ancient earthquake faults at FVT depths has hindered studying of their brittle co-seismic damage structures and rheology of their deep portions during earthquake cycles. From the Sandhill Corner shear zone, a strand of the Norumbega fault system (an ancient seismogenic strike-slip fault at the FVT), I analyze fluid inclusion abundance in quartz as a proxy for transient co-seismic damage using secondary electron image and optical observation, and collect quantitative data of quartz across the shear zone such as grain-size, grain-shape, crystallographic orientation, misorientation, and fabric intensity through electron backscatter diffraction. The results indicate that brittle co-seismic damage occurs up to at least ~90 m in width at the FVT, and the inner shear zone (~40 m wide) experienced cycles of co-seismic microfracture-assisted grain-size reduction followed by post-seismic viscous deformation dominated by grain-size-sensitive processes, whereas the outer shear zone was deformed dominantly by grain-size-insensitive processes during earthquake cycles. My findings have important implications for the strength, or mechanics, of the fault/shear zone system, and may help determine 3-D volume of brittle damage zone. Measuring the extent of damage zone is critical for estimating the potential energy that an earthquake releases because the co-seismic damage zone acts as a dissipative energy sink by creating fracture surface areas. Earthquakes not only represent hazards but radiate energy as seismic waves. Since the direction-dependent nature of wave propagation velocities (called “seismic anisotropy”) changes in response to rock flow due to preferred orientation of elastically anisotropic minerals, the seismic anisotropy has been used to investigate Earth’s interior structure and deformation processes in tectonically active regions. However, this is a challenge for waves passing through the crust because their anisotropies are profoundly modified by macroscale folds, which are very common structures in ancient and current orogenic belts and shear zones. To evaluate the modification of seismic anisotropy by the deformation structures, I develop a new mathematical methodology for calculating bulk elastic tensors and seismic anisotropy of macroscale folds, assuming the seismic waves are much larger than the fold heterogeneity. The results show that the velocities of seismic waves propagating through macroscale folds in three dimensions are systematically related to fold shape and orientation. Because fold orientations are related to flow directions, it is now possible for real seismic observables to provide information on the directions of flow for actively deforming rocks at depth

Fractures, Fluids, and Metamorphism: Shear Zone Initiation in the Marcy Anorthosite Massif, Adirondacks, New York, USA

Localized shear zones are important rheological features that influence deformation behavior throughout the Earth’s middle-to-lower crust. Therefore, the processes through which shear zones initiate and localize remains an important geologic question. The study of strain localization and shear zone initiation is made difficult due to continued deformation overprinting the microstructures which lead to initiation and obfuscating the context in which localization occurred. The Marcy anorthosite in the Adirondack Highlands, New York, is a nominally granulite-facies, plagioclase-rich massif cut by centimeter-to-meter scale shear zones which provides a natural example of shear zone localization within the middle-to-lower crust. My work focuses on the microstructural examination of shear zones at Bennies Brook within the Marcy massif to construct the sequence of geologic events which lead to shear zone initiation. I used field observations combined with optical and electron microscope observations and electron probe geochemistry to investigate how microstructural conditions changed over time as the sequence progressed, as well as explore the tectonic implications of shear zone development within the massif. My results suggest that the initiation of viscous shearing was facilitated by a combination of physical and chemical weakening during exhumation. The country rock anorthosite was pervasively fractured and/or crushed which increased permeability sufficiently to allow for the infiltration of chlorine-rich hydrothermal fluids, primarily along centimeter-wide brittle fault zones where permeability was greatest. These fluids triggered the retrograde replacement of plagioclase feldspar to scapolite and pyroxene to amphibole and quartz. This weakened the rock through the introduction of the relatively weak minerals as well as an associated reduction in grain size. In the planar zones of greatest metasomatism, this weakening was sufficient for viscous shearing to initiate. The relative timing and orientation of the shear zones supports previous work suggesting that the Adirondack Highlands underwent exhumation associated with orogenic collapse during the Ottawan orogeny (ca. 1080–1000 Ma)

Geologic Extremes of the NW Himalaya: Investigations of the Himalayan Ultra-high Pressure and Low Temperature Deformation Histories

This dissertation focuses on two extremes of orogenic development in the Himalaya: the timing of early ultra-high pressure related tectonics and the subsequent emplacement of the high grade Greater Himalayan Crystallines. The Himalayan orogeny is one of if not the best example of ongoing collisional systems, marked by the ongoing convergence of the Indian and Asian continents. The presence of coesite in the Tso Morari complex respresents subduction of the Indian continental crust to ultrahigh-pressure (UHP) conditions. However, the timing of UHP metamorphism is debated, creating an uncertainty in the calculation of subduction and exhumation rates. Petrologic and geochronologic analyses of eclogitic zircon and rutile from two samples—a kyanite bearing white mica schist and a garnet-biotite schist were conducted to constrain the timing and duration of the UHP metamorphic event. Titanite analyses from quartzofeldspathic gneiss constrain timing of exhumation and record the regional amphibolite-facies metamorphism. Petrology and U-Pb analyses and observations reveal peak metamorphic histories in rutile-bearing metapelites. Ages in rutile bearing samples are 50.3 ± 0.85 Ma and 47.60 ± 0.52 Ma. This geochronology provides insight into the suite of geochronometers already published in previous Tso Morari studies, but additional thermobarometry is needed to strongly correlate potential UHP ages with UHP conditions. How the Greater Himalayan Crystalline was emplaced; and how much shortening was accomplished since the initial collision are debated. Three models; the Channel Flow model, Tectonic Wedging and Wedge extrusion models all have specific predictions of the spatial setting of the Greater Himalayan Crystalline relative to overlying stratigraphy. Here we show that the emplacement of the Greater Himalayan Crystalline is consistent with the predictions of Tectonic Wedging through structural mapping and sampling. The shortening budget of the Himalaya experiences approximately a 2,000 km deficit between plate convergence and crustal shortening recorded in the geologic record. It has recently been proposed in the Greater Indian Basin model that this can be attributed by 2675 ± 700 km of North-South extension of the Greater Indian margin in the late Mesozoic. The extension resulted into creating a Tethyan subcontinent that collided with Asian around 50 Ma followed by India around 25 Ma. Our field mapping is inconsistent with the two collision sequence predicted by the Greater Indian Basin hypothesis. Additionally the amount of shortening predicted by the Greater Indian Basin hypothesis greatly exceeds the shortening recorded by our line-length balancing of the Tethyan stratigraphy

Voronoi-Based discrete element analyses to assess the influence of the grain size and its uniformity on the apparent fracture toughness of notched rock specimens

The fracture toughness reflects the rock resistance to crack propagation, and therefore represents an important parameter for rock fracture assessments. From a strict point of view, the real fracture toughness ( KIC ) corresponds to a cracked situation in which the notch radius is theoretically equal to zero. However, most of the defects in rocks have a finite radius and, therefore, should be studied as notch-type defects. Here, the notch effect is numerically studied together with the influence of the grain size and the sorting coefficient (grain size uniformity) on the apparent fracture toughness ( KIN ). To this end, several four-point bending tests with different U-shaped notch radii, mean grain sizes and degrees of uniformity in grain size and shape have been simulated using the Discrete Element Method. In order to represent the grains of the rocks, the Voronoi tessellation is used to create randomly sized and distributed polygonal blocks. These Voronoi polygons have been defined, on the one hand, by an average edge length of 1, 2 and 3 mm, and, on the other hand, by a different number of iterations ( n ) in the relaxation process during the generation of the polygons, which defines the grain size uniformity. The numerical analyses performed and the interpretation of the results show a clear notch effect in all the studied cases, as the apparent fracture toughness ( KIN ) increases with notch radius. Finally, the obtained stress fields at the notch tip have been compared to those obtained from the traditional finite element method.The authors of this work would like to express their gratitude to the Spanish Ministry of Economy and Competitiveness for financing the National Plan Project (Ref. BIA2015-67479-R) under the name of “The Critical Distance in Rock Fracture”, to the Department of Universities and Research, Environment and Social Policy of the Government of Cantabria, for financing the Project “Characterization of the fracture process in rocks for geothermal applications”, and to the DAAD for the short-term research grant given to J. Justo for his research visit at TU Bergakademie Freiberg