Are faults preferential flow paths through semiarid and arid vadose zones (original) (raw)
Related papers
Geology, 2001
The Sand Hill fault is a steeply dipping, large-displacement normal fault that cuts poorly lithified Tertiary sediments of the Albuquerque basin, New Mexico, United States. The fault zone does not contain macroscopic fractures; the basic structural element is the deformation band. The fault core is composed of foliated clay ftanked by strncturally and lithologically heterogeneous mixed zones, in turn flanked by damage zones. Structures present within these fault-zone architectural elements are different from those in brittle faults formed in lithified sedimentary and crystalline rocks that do contain fractures. These differences are reflected in the permeability strncture of the Sand Hill fault. Equivalent permeability calculations indicate that large-displacement faults in poorly lithified sediments have little potential to act as vertical-flow conduits and have a mucb greater effect on horizontal How than faults with fractures.
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.
Deformation along faults in the shallow crust (b 1 km) introduces permeability heterogeneity and anisotropy, which has an important impact on processes such as regional groundwater flow, hydrocarbon migration, and hydrothermal fluid circulation. Fault zones have the capacity to be hydraulic conduits connecting shallow and deep geological environments, but simultaneously the fault cores of many faults often form effective barriers to flow. The direct evaluation of the impact of faults to fluid flow patterns remains a challenge and requires a multidisciplinary research effort of structural geologists and hydrogeologists. However, we find that these disciplines often use different methods with little interaction between them. In this review, we document the current multidisciplinary understanding of fault zone hydrogeology. We discuss surface-and subsurface observations from diverse rock types from unlithified and lithified clastic sediments through to carbonate, crystalline, and volcanic rocks. For each rock type, we evaluate geological deformation mechanisms, hydrogeologic observations and conceptual models of fault zone hydrogeology. Outcrop observations indicate that fault zones commonly have a permeability structure suggesting they should act as complex conduit-barrier systems in which along-fault flow is encouraged and across-fault flow is impeded. Hydrogeological observations of fault zones reported in the literature show a broad qualitative agreement with outcrop-based conceptual models of fault zone hydrogeology. Nevertheless, the specific impact of a particular fault permeability structure on fault zone hydrogeology can only be assessed when the hydrogeological context of the fault zone is considered and not from outcrop observations alone. To gain a more integrated, comprehensive understanding of fault zone hydrogeology, we foresee numerous synergistic opportunities and challenges for the discipline of structural geology and hydrogeology to co-evolve and address remaining challenges by co-locating study areas, sharing approaches and fusing data, developing conceptual models from hydrogeologic data, numerical modeling, and training interdisciplinary scientists.
Environmental Earth Sciences, 2017
In this study, the main hydromechanical characteristics of a portion of the Oriente fault, in the valley of Aguascalientes, Mexico, are analyzed and discussed. This discontinuity corresponds to a normal fault that was originated before the formation of the Aguascalientes graben. However, its recent activity and potential reactivation may be directly linked to the land subsidence currently occurring in the center of the valley. A 1-year campaign to monitor deformation in two transects perpendicular to this fault indicates continuous deformation activity with maximum vertical descending values of 43 mm in the hanging wall. On the other hand, a fieldwork procedure to determine the geologic structure of the fault zone and to establish the infiltration patterns of a selected point in the fault was performed as well. This field test consisted of analyzing the resistivity patterns of profiles across and along the fault, implemented before, during and after pouring groundwater into the fault and monitoring the fluids movement through the fault zone. Results suggest that the fault contains three perpendicular fault branches in a distance of 80 m, portraying a potential domino fault system. The high infiltration capacity of the fault opening allowed the rapid intrusion of water into the subsoil, and the distinguishable changes in electrical resistivity in the profiles parallel to the fault plane suggest that the infiltrating water goes down into the soil through the fault plane or adjacent to it.
Field investigation into unsaturated flow and transport in a fault: model analyses
Journal of Contaminant Hydrology, 2004
Results of a fault test performed in the unsaturated zone of Yucca Mountain, Nevada, were analyzed using a three-dimensional numerical model. The fault was represented as a vertical fracture and the surrounding rock was treated as a dual-continuum system in the model. Model calibration against the seepage and water travel velocity data suggests that the lithophysal cavities connected to fractures can considerably enhance the effective fracture porosity and therefore retard water flow in fractures. Comparisons between simulation results and the tracer concentration data also indicate that the matrix diffusion is an important mechanism for solute transport in unsaturated fractured rock. We found that an increased fault-matrix interface area was needed for matching the observed tracer data, which is consistent with previous studies. The study results suggest that the current site-scale model for the unsaturated zone of Yucca Mountain may underestimate radionuclide transport time within the unsaturated zone, because effects of an increased fracture-matrix interface area and the lithophysal cavities are not considered in the current site-scale model.
Geofluids, 2005
Mineralization of brittle fault zones is associated with sudden dilation, and the corresponding changes in porosity, permeability and fluid pressure, that occur during fault slip events. The resulting fluid pressure gradients cause fluid to flow into and along the fault until it is sealed. The volume of fluid that can pass through the deforming region depends on the degree of dilation, the porosity and permeability of the fault and wall rocks, and the rate of fault sealing. A numerical model representing a steep fault cutting through a horizontal seal is used to investigate patterns of fluid flow following a dilatant fault slip event. The model is initialized with porosity, permeability and fluid pressure representing the static mechanical state of the system immediately after such an event. Fault sealing is represented by a specified evolution of porosity, coupled to changes in permeability and fluid pressure, with the rate of porosity reduction being constrained by independent estimates of the rate of fault sealing by pressure solution. The general pattern of fluid flow predicted by the model is of initial flow into the fault from all directions, followed by upward flow driven by overpressure beneath the seal. The integrated fluid flux through the fault after a single failure event is insufficient to account for observed mineralization in faults; mineralization would require multiple fault slip events. Downward flow is predicted if the wall rocks below the seal are less permeable than those above. This phenomenon could at least partially explain the occurrence of uranium deposits in reactivated basement faults that cross an unconformity between relatively impermeable basement and overlying sedimentary rocks.
Sandstone-filled normal faults: A case study from central California
Journal of Structural Geology
Despite the potential of sandstone-filled normal faults to significantly influence fluid transmissivity within reservoirs and the shallow crust, they have to date been largely overlooked. Fluidized sand, forcefully intruded along normal fault zones, markedly enhances the transmissivity of faults and, in general, the connectivity between otherwise unconnected reservoirs. Here, we provide a detailed outcrop description and interpretation of sandstone-filled normal faults from different stratigraphic units in central California. Such faults commonly show limited fault throw, cm to dm wide apertures, poorly-developed fault zones and full or partial sand infill. Based on these features and inferences regarding their origin, we propose a general classification that defines two main types of sandstone-filled normal faults. Type 1 form as a consequence of the hydraulic failure of the host strata above a poorly-consolidated sandstone following a significant, rapid increase of pore fluid over-pressure. Type 2 sandstone-filled normal faults form as a result of regional tectonic deformation. These structures may play a significant role in the connectivity of siliciclastic reservoirs, and may therefore be crucial not just for investigation of basin evolution but also in hydrocarbon exploration.