Hydromechanical Behavior of Rocks in Shallow Tectonic Settings: Implications for Natural and Engineered Systems
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2025
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Doctoral Thesis
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Abstract
The hydromechanical behavior of rocks in shallow tectonic settings is essential to understand key processes in both natural deformation systems and engineered subsurface applications. Collisional tectonic settings, including subduction zones and fold-and-thrust belts in the foreland of mountain ranges, are of significant interest because they host large and devastating earthquakes. Attention has been given to phyllosilicate-rich rocks because the internal structure of phyllosilicate minerals causes unique hydromechanical properties, such as low frictional strength, low permeability, and the ability to swell. These properties play a key role in localizing deformation and controlling fluid flow. They also make phyllosilicate-rich formations relevant to engineered systems, such as carbon capture and storage (CCS) facilities and radioactive waste repositories. However, many aspects of how phyllosilicate minerals influence the hydromechanical behavior of rocks in shallow tectonic settings are not fully understood. This thesis addresses some of these knowledge gaps through three studies.
The first study investigates the factors that control the occurrence and spatial distribution of different slip modes - such as seismogenic slip, slow slip events (SSEs), and aseismic creep - within an accretionary wedge, as well as how these slip modes are influenced by rock type and physical conditions. To address this, the exhumed McHugh Accretionary Complex in Alaska was used as an analog for the shallow subduction interface. Field observations, compositional analyses, and microstructural data were combined with laboratory friction experiments using representative fault materials and host rocks. Friction experiments were performed under dry and water-saturated conditions at effective normal stresses representative of shallow subduction zones. The results show that increased phyllosilicate content - especially when combined with increased organic matter and water-saturated conditions - reduces the frictional strength and enhances velocity-strengthening behavior, favoring aseismic creep. These findings align with field observations of strain localization in argillitic lithologies. Conversely, frictionally stronger rocks, such as some altered basalts and cherts, may host seismic slip or SSEs depending on their degree of deformation. Lithological contrasts mapped across the McHugh Complex may produce stress and fluid pressure concentrations, which may also influence slip initiation.
The second study examines how rock composition and fabric influence seismic properties and the interpretation of subsurface geophysical data, particularly in fold-and-thrust belts. To accomplish this, core samples were analyzed from the second borehole of the "Collisional Orogeny in the Scandinavian Caledonides" project (COSC-2). This borehole intersects various lithologies of an exhumed, inactive fold-and-thrust belt, including turbidites, black shale, and basement rocks. Ultrasonic P- and S-wave velocity measurements were performed on six representative lithologies under different confining pressures. Rocks rich in phyllosilicates, such as graywacke and black shale, show strong seismic anisotropy of orthorhombic or higher symmetry. This challenges the common assumption that phyllosilicate-rich rocks are transverse isotropic. The organic matter in the black shale reduces seismic velocities and further amplifies anisotropic behavior. This may make major thrust faults, which often localize in weak shales, prominent reflectors. In contrast, phyllosilicate-poor samples are weakly anisotropic or isotropic, but seismic velocities can be significantly affected by microcracks, depending on the degree of water saturation.
The third study focuses on how polar fluids alter the frictional behavior of phyllosilicate-rich rocks, as well as the effect of swelling stress on the overall stress state. Triaxial friction experiments were performed on Opalinus claystone-Berea sandstone interfaces under non-wetted conditions and with the injection of polar and non-polar fluids (water and decane, respectively). Macroscopic mechanical data were complemented by distributed strain sensing using optical fibers. The results of these experiments show that water reduces the frictional strength, likely due to phyllosilicate lubrication and the transition of the Opalinus claystone into an incohesive mud. Notably, slip occurred during the initial injection of water, even when the apparent stress state was below the yield stress. These findings emphasize the importance of incorporating swelling-induced stress into models of fault reactivation and highlight implications for the integrity of clay-based sealing formations in engineered storage facilities.
Together, these three studies advance our understanding of the hydromechanical properties of rocks in shallow tectonic settings, offering new insights into slip behavior, seismic imaging, and fluid-driven fault activity.
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Examiner : Behr, Whitney M.
Examiner : Madonna, Claudio
Examiner : Ruh, Jonas B.
Examiner : Marone, Chris J.
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ETH Zurich
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09636 - Behr, Whitney / Behr, Whitney
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197029 - Coupling mechanics and fluid flow of evolving fault zones (SNF)