Fault zone geometry of a mature active normal fault: A potential high permeability channel (Pirgaki fault, Corinth rift, Greece) (original) (raw)
Related papers
Permeability of fault-related rocks, and implications for hydraulic structure of fault zones
Journal of Structural Geology, 1997
The permeability structure of a fault zone in granitic rocks has been investigated by laboratory testing of intact core samples from the unfaulted protolith and the two principal fault zone components; the fault core and the damaged zone. The results of two test series performed on rocks obtained from outcrop are reported. First, tests performed at low confining pressure on 2.54-cm-diameter cores indicate how permeability might vary within different components of a fault zone. Second, tests conducted on 5.1-cm-diameter cores at a range of confining pressures (from 2 to 50 MPa) indicate how variations in overburden or pore fluid pressures might influence the permeability structure of faults. Tests performed at low confining pressure indicate that the highest permeabilities are found in the damaged zone (10−16–10−14 m2), lowest permeabilities are in the fault core (< 10−20–10−17 m2), with intermediate permeabilities found in the protolith (10−17–10−16 m2). A similar relationship between permeability and fault zone structure is obtained at progressively greater confining pressure. Although the permeability of each sample decays with increasing confining pressure, the protolith sustains a much greater decline in permeability for a given change in confining pressure than the damaged zone or fault core. This result supports the inference that protolith samples have short, poorly connected fractures that close more easily than the greater number of more throughgoing fractures found in the damaged zone and fault core. The results of these experiments show that, at the coreplug scale, the damaged zone is a region of higher permeability between the fault core and protolith. These results are consistent with previous field-based and in-situ investigations of fluid flow in faults formed in crystalline rocks. We suggest that, where present, the two-part damaged zone-fault core structure can lead to a bulk anisotropy in fault zone permeability. Thus, fault zones with well-developed damaged zones can lead to enhanced fluid flow through a relatively thin tabular region parallel to the fault plane, whereas the fault core restricts fluid flow across the fault. Although this study examined rocks collected from outcrop, correlation with insitu flow tests indicates that our results provide inexact, but useful, insights into the hydromechanical character of faults found in the shallow crust.
The where and how of faults, fluids and permeability –
Fault stepovers are features where the main trace of a fault steps from one segment to the next in either an underlapping or overlapping manner. Stepovers exert a critical influence on crustal permeability and are known to control phenomena such as the migration of hydrocarbons and the location of geothermal fields. In the Kalgoorlie-Ora Banda greenstone district, Western Australia, we demonstrate a spatial association between stepovers and gold deposits. It is shown that although underlapping stepover geometries are typically rare in fault systems, they are anomalously associated with gold deposits. Further, the along-strike and across-strike dimensions of both underlapping and overlapping fault stepovers fit, to a first-order approximation, the same self-similar trend. Boundary element modelling of Coulomb failure stress changes is used to explain these observations in terms of damage generated by rupture events on the bounding fault segments and associated aftershock sequences. Our models indicate that a larger region of damage and permeability enhancement is created around underlapping stepovers than around overlapping stepovers. By taking into account both the enhancement and decay of permeability during the seismic cycle, it is estimated that a 5 Moz goldfield could feasibly form in 1–16 earthquake-aftershock sequences, potentially representing durations of just 10–8000 years. The existence of supergiant gold deposits is evidence that crustal permeability attains transiently high values on the order of 10 12 m2. It should be expected that transient and time-integrated permeability values have a distinct threedimensional structure in continental crust due to stepover-related channels.
non-seismic surface faulting - Extra Materials
2007
During the last months of 2005 a surface rupture started to form in Peraia suburb southwest of Thessaloniki city, located at the southern shore of Thermaikos bay in northern Greece. The rupture is concave, with one branch trending WSW -ENE, while the other one trends WNW -ESE. It dips towards the N and is located along the scarp that defines two morphological plateaus. These two plateaus are the top surfaces of the hangingwall and the footwall respectively of the well known, mapped and studied Anthemountas fault. This is a large active normal fault that bounds the southern margin of Anthemountas valley and controls the shoreline at its western extension. It is believed to be associated with certain large historical earthquakes, while its recent activity is documented by microseismicity. It is one of the most hazardous earthquake sources for the city of Thessaloniki. Four boreholes (G1-G4) have been drilled on both sides of the rupture, in order to examine the stratigraphical and geotechnical properties of the faulted geological substratum as well as possible water level fluctuations. Furthermore, correlation of boreholes G1 and G2 shows that there is a vertical displacement of at least 35 m during the Quaternary. Trenching along the fault has also proved that the surface rupture coincides with the main fault zone, showing a very large vertical displacement, non measurable in the trench. Periodic measurements of the water level in a number of boreholes used for irrigation have shown that there has been a significant fall of the water level during recent years. In conclusion, it seems that the Peraia rupture has been formed along the already existing Anthemountas fault and at least a part of the total displacement is attributed to overpumping.
Fault zone architecture and permeability structure
Geology, 1996
Fault zone architecture and related permeability structures form primary controls on fluid flow in upper-crustal, brittle fault zones. We develop qualitative and quantitative schemes for evaluating fault-related permeability structures by using results of field investigations, laboratory permeability measurements, and numerical models of flow within and near fault zones. The qualitative scheme compares the percentage of the total fault zone width composed of fault core materials (e.g., anastomosing slip surfaces, clay-rich gouge, cataclasite, and fault breccias) to the percentage of subsidiary damage zone structures (e.g., kinematically related fracture sets, small faults, and veins). A more quantitative scheme is developed to define a set of indices that characterize fault zone architecture and spatial variability. The fault core and damage zone are distinct structural and hydrogeologic units that reflect the material properties and deformation conditions within a fault zone. Whether a fault zone will act as a conduit, barrier, or combined conduit-barrier system is controlled by the relative percentage of fault core and damage zone structures and the inherent variability in grain scale and fracture permeability. This paper outlines a framework for understanding, comparing, and correlating the fluid flow properties of fault zones in various geologic settings.
The Internal Structure of Fault Zones: Implications for Mechanical and Fluid-Flow Properties
Geological Society, London, Special Publications, 2008
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The thickness of faults: From laboratory experiments to field scale observations
Tectonophysics, 2006
To assess the role of the fault thickness on its mechanical behavior, we first present the results of an experimental modeling of a thick fault core. Our laboratory setup consists in an annular simple shear apparatus in which we can apply very large shear displacements (50 m) to 100 particle thick granular samples. Thanks to a window in the apparatus, pictures of the microstructures can be continuously taken during shear. We observe from a Correlation Image Velocimetry technique that a significant strain field exists outside of the observable shear band. This strain field, though of small magnitude compared to that existing inside the shear band, is very structured and extends in a region much wider than expected from individual static observations (i.e. wider than the directly observable shear band). Moreover, this strain field controls most of the evolution of the shear strength of the fault. We then propose plausible comparisons of our experimental results to geological observations of fault cores in the region of Aigion (Corinth Gulf, Greece). The studied faults indeed display spectacular indurated fault planes lying on weakly cohesive material. Signatures of cementation, clay mineral distribution and porosity profile of one of the studied fault cores are included and discussed in the light of the experimental results. Our observations suggest that the maximum shear strain during earthquakes might occur not in the center, but on the border of the fault cores. It is presumably localized in a transition zone which exhibits a significant cementation owing to a process of mechanical smearing by fine particles. This zone may also act as a very low permeability layer responsible for a channeling of the fluid flow. Such a scheme of progressive multi sub-localizations, is different from classical descriptions of faults and consistent with a layering of the core consisting of separated zones of high strains or large cataclastic flows.
Recent advances in the understanding of fault zone internal structure: a review
Geological Society, London, Special Publications, 2008
It is increasingly apparent that faults are typically not discrete planes but zones of deformed rock with a complex internal structure and three-dimensional geometry. In the last decade this has led to renewed interest in the consequences of this complexity for modelling the impact of fault zones on fluid flow and mechanical behaviour of the Earth's crust. A number of processes operate during the development of fault zones, both internally and in the surrounding host rock, which may encourage or inhibit continuing fault zone growth. The complexity of the evolution of a faulted system requires changes in the rheological properties of both the fault zone and the surrounding host rock volume, both of which impact on how the fault zone evolves with increasing displacement. Models of the permeability structure of fault zones emphasize the presence of two types of fault rock components: fractured conduits parallel to the fault and granular core zone barriers to flow. New data presente...
Damage and permeability around faults: Implications for mineralization
Mineral deposits are commonly hosted by small-displacement structures around jogs in major faults, but they are rarely hosted by the major fault itself. This relationship may be explained by time-dependent fracturing and healing in and around major faults and associated permeability evolution. A damage mechanics formulation is used here to explore the spatial-temporal evolution of damage in and around a fault following a fault-slip event. We show that regions of increased damage rate correspond to the location of mineral deposits and that these areas correspond to areas of aftershocks predicted by stress-transfer modeling. The fault itself enters a healing regime following the slip event; hence, it is expected to become less permeable than the fracture network outside the fault. Our results support the hypothesis that mineralization occurs in a fracture network associated with aftershocks; this may be due to the higher time-integrated permeability of the fracture network relative to the main fault.
Journal of Structural Geology, 2003
We studied the nucleation and growth of cataclastic fault cores from fractured damage zones in extensional and strike-slip fault zones in carbonate rocks. Analysed fault zones have similar protolith lithology and sedimentary fabric, but different geometry, kinematics, size, tectonic environment and deformation history. Orthorhombic rock lithons, a few decimetres in size, characterise the structural fabric of damage zones. Lithons derive from the intersection of a dominant fracture/cleavage set with bedding and/or joints. At the damage zone–fault core transition, orthorhombic lithons reduce in size and approach an isometric shape. Their cross-sectional aspect ratio has an average value of 1.4. Analysed fault cores have similar rock textures, sorting and comminution degree. Particle-size distributions of fault core rocks show linear trends in log-log graphs and average fractal dimension of 2.5. Our results on rock fabrics suggest that fault core development initiates from rock masses in damage zones, where the shape anisotropy of orthorhombic lithons favours additional fracturing at high angle to their long axes. Eventually, smaller, nearly isometric lithons generate from repeated fracturing of orthorhombic lithons. When the aspect ratio of these lithons approaches the threshold value of about 1.4, particle rotation is favoured and cataclastic flow starts. Owing to the granular nature of the damage zone-fault core transitions in carbonate rocks, analogies with the nucleation of deformation bands in sandstones can be established. Our results may be of use to the industry for quantitative characterisation of fault zone permeability. According to the proposed model, radical changes on the permeability properties are expected during the growth of fault cores.