Distribution and Nature of Fault Architecture in a Layered Sandstone and Shale Sequence: An Example from the Moab Fault, Utah (original) (raw)
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Tectonophysics, 2003
The deformation mechanisms producing the Chimney Rock normal fault array (San Rafael Swell, Utah, USA) are identified from detailed analyses of the structural components of the faults and their architecture. Faults in this area occur in four sets with oppositely dipping fault pairs striking ENE and WNW. The ENE-striking faults initially developed by formation of deformation bands and associated slip surfaces (deformation mechanism 1). After deformation band formation ceased, three sets of regional joints developed. The oldest two sets of the regional joints, including the most prominent WNW-striking set, were sheared. Localized deformation due to shearing of the WNW-striking regional joints formed WNW-striking map-scale normal faults. The formation mechanism of these faults can be characterized by the shearing of joints that produces splay joints, breccia, and eventually a core of fault rock (deformation mechanism 2). During this second phase of faulting, the ENE-striking faults were reactivated by shear across the slip surfaces and shearing of ENE-striking joints, producing localized splay joints and breccia (similar to deformation mechanism 2) superimposed onto a dense zone of deformation bands from the first phase. We found that new structural components are added to a fault zone as a function of increasing offset for both deformation mechanisms. Conversely, we estimated the magnitude of slip partitioned by the two mechanisms using the fault architecture and the component structures. Our analyses demonstrate that faults in a single rock type and location, with similar length and offset, but forming at different times and under different loading conditions, can have fundamentally different fault architecture. The impact by each mechanism on petrophysical properties of the fault is different. Deformation mechanism 1 produces deformations bands that can act as fluid baffles, whereas deformation mechanism 2 results in networks of joints and breccia that can act as preferred fluid conduits. Consequently, a detailed analysis of fault architecture is essential for establishing an accurate tectonic history, deformation path, and hydraulic properties of a faulted terrain.
Geological Society of America Bulletin, 2005
Faults in sandstone are frequently composed of two classes of structures: (1) deformation bands and (2) joints and sheared joints. Whereas the former structures are associated with cataclastic deformation, the latter ones represent brittle fracturing, fragmentation, and brecciation. We investigated the distribution of these structures, their formation, and the underlying mechanical controls for their occurrence along the Moab normal fault in southeastern Utah through the use of structural mapping and numerical elastic boundary element modeling. We found that deformation bands occur everywhere along the fault, but with increased density in contractional relays. Joints and sheared joints only occur at intersections and extensional relays. In all locations, joints consistently overprint deformation bands. Localization of joints and sheared joints in extensional relays suggests that their distribution is controlled by local variations in stress state that are due to mechanical interaction between the fault segments. This interpretation is consistent with elastic boundary element models that predict a local reduction in mean stress and least compressive principal stress at intersections and extensional relays. The transition from deformation band to joint formation along these sections of the fault system likely resulted from the combined effects of changes in remote tectonic loading, burial depth, fl uid pressure, and rock properties. In the case of the Moab fault, we conclude that the structural heterogeneity in the fault zone is systematically related to the geometric evolution of the fault, the local state of stress associated with fault slip, and the remote loading history. Because the type and distribution of structures affect fault permeability and strength, our results predict systematic variations in these parameters with fault evolution.
A Conceptual Model for the Origin of Fault Damage Zone Structures In High-Porosity Sandstone
Journal of Structural Geology, 2003
We present a conceptual model to explain the development of damage zones around faults in high-porosity sandstones. Damage zone deformation has been particularly well constrained for two 4-km-long normal faults formed in the Navajo Sandstone of central Utah, USA. For these faults the width of the damage zone increases with fault throw (for throws ranging from 0 to 30 m) but the maximum deformation density within the damage zone is independent of throw. To explain these data we modify a previously published theoretical model for fault growth in which displacement accumulates by repeated slip events on patches of the fault plane. The modi®cations are based on ®eld observations of deformation mechanisms within the Navajo Sandstone, the throw pro®les of the faults, and inferences concerning likely slippatch dimensions. Zones of enhanced stress are generated around the tips of each slipping patch, raising the shear stress on adjacent portions of the fault as well as potentially causing off-fault damage. A key ingredient in our model for off-fault damage accumulation is the transition from strain hardening associated with deformation band development, to localised strain softening as a slip-surface develops. This transition occurs at a critical value of deformation density. Once a new slip-surface develops at some distance from the main fault plane and it starts to accumulate throw it can, in turn, generate its own damage zone, thus increasing the overall damage zone width. Our approach can be applied to interpret damage zone development around any fault as long as the host-rock lithology, porosity and deformation mechanisms are taken into consideration. q
Overprinting faulting mechanisms in high porosity sandstones of SE Utah
Journal of Structural Geology, 2003
Normal faults in sandstone of the Jurassic Entrada Formation, SouthEast Utah formed by two mechanisms: (1) deformation band faulting overprinted by (2) jointing and subsequent shearing along joints. Fundamental structural elements of deformation band faults are single deformation bands, zones of deformation bands, and slip surfaces. Joint-based faults are composed of joints, sheared joints, splay fractures, fragmentation zones, breccia, and fine-grained fault rock. We demonstrate that both mechanisms contribute to slip in a single fault zone with joint-based faulting consistently postdating deformation band faulting at a given location along a fault. The occurrence, distribution, and geometric arrangement of structures formed by the two mechanisms resulted in faults with distinct fault architecture. This fault architecture is related to the relative contributions of each deformation mechanism to the total offset and to their relative timing. Overprinting of a deformation band-based fault by a joint-based mechanism introduces extensive localized structural heterogeneity with a distinct hydraulic signature. Whereas deformation bands tend to act as fluid baffles, joints may act as preferred fluid conduits. Therefore, fluid flow properties such as the permeability of the faults with overlapping mechanisms are expected to change over time accompanying the overprinting process.
Journal of Structural Geology, 2002
We analyze displacement pro®les measured from a population of normal faults that cut across layered clastic rocks, in order to investigate the controls of mechanical layering on fault growth. Abundant fault tips and displacement minima are found at lithologic contacts, and in some cases are associated with relay structures, suggesting that lithology is responsible for controlling the location of vertical, along-dip segment linkage. Based on the locations and distributions of displacement minima and maxima within the stratigraphic section, as well as the distribution of small faults, we conclude that: (1) most faults initiate within shale beds, and (2) lithologic contacts restrict fault growth at a variety of scales. One consequence of fault restriction is the development of high displacement gradients at fault tips. Because fault tips are only temporarily pinned at bed boundaries, the degree of restriction will¯uctuate as faults propagate through the section. In general, maximum displacement (D max) across the faults correlates with cross-sectional trace length (L). The D max /L ratio decreases as a function of percent shale offset by a fault, and increases as a function of near-tip displacement gradient. An empirically-derived equation relates D max /L to rock composition and fault tip displacement gradients, thereby providing a mechanism to predict fault dimensions in the subsurface from limited data.
The internal architecture and permeability structures of faults in shale formations
Clay Minerals Society Workshop Lecture Series, 2016
The evaluation of fluid flow through fractures and faults is of primary importance for the long-term performance assessment of radioactive waste repositories in shale formations and could be used to assess CO 2 storage security or the integrity of caprocks and reservoir capacity. This study focuses on the structural evolution within brittle to ductile shear zone in order to understand permeability enhancement and sealing processes affecting a strike-slip fault system within the Toarcian shale formation from the Tournemire Underground Research Laboratory (URL), southern France. A combination of quantitative field measurements and laboratory and in situ experiments was used to estimate the fluid-flow properties of a fractured shale formation. Results indicate that microfractures govern the matrix porosity in the damage zone and play an increasingly dominant role in fluid flow along the boundary between fault core and fault damage zone.
Journal of Structural Geology, 2005
Deformation structures in the Jurassic Moab Member of the Entrada Sandstone have been studied in the Courthouse area where two major fault segments (Segments A and B) of the Moab Fault are connected. Field data show that Segment A developed from an early stage of (thick) deformation band formation and that distinctively thinner deformation bands and fractures were subsequently added to its damage zone at a later stage. Only the second stage is expressed along Segment B. Geometric and kinematic evidence indicates that Segment B linked with Segment A at the time when Segment A (and its thick deformation bands) was already present in the Courthouse area. We attribute the transition from thick deformation bands to thin deformation bands to pore-space reduction caused by syn-faulting quartz dissolution and precipitation that changed the mechanical properties of the rock. In this model, thin deformation bands formed as porosity was reduced during quartz diagenesis. The observations underscore the importance of syn-kinematic diagenetic changes and the variation in small-scale structures along faults that apparently formed during the same faulting event in porous sandstones. q