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In this study, the directional semivariogram is deployed to investigate the spatial variability of map-scale fracture network attributes in the Paradox Basin, Utah. The relative variability ratio ( R ) is introduced as the ratio of integrated anisotropic semivariogram models, and R is shown to be an effective metric for quantifying the magnitude of spatial variability for any two azimuthal directions. R is applied to a GIS-based data set comprising roughly 1200 fractures, in an area which is bounded by a map-scale anticline and a km-scale normal fault. This analysis reveals that proximity to the fault strongly influences the magnitude of spatial variability for both fracture intensity and intersection density within 1–2 km. Additionally, there is significant anisotropy in the spatial variability, which is correlated with trends of the anticline and fault. The direction of minimum spatial correlation is normal to the fault at proximal distances, and gradually rotates and becomes subparallel to the fold axis over the same 1–2 km distance away from the fault. We interpret these changes to reflect varying scales of influence of the fault and the fold on fracture network development: the fault locally influences the magnitude and variability of fracture network attributes, whereas the fold sets the background level and structure of directional variability. [ABSTRACT FROM AUTHOR]

Abstract: Growing macroscale fold structures are one of the most dynamic and complex systems in geology, where during fold growth pore fluid pressure, fluid temperature, and fluid composition (PTX) conditions do not remain static but instead vary continually at all points within the deforming fold such that no two points within the fold will have the same deformation or fluid history. These PTX conditions in turn directly affect fluid storage and mobility as well as rock strength, which has direct feedback on the mechanical development of the fold. In this review, we first outline the research methodologies that have yielded significant insights, and review four examples of well-constrained fold-related fluid systems. We then discuss the fluid-related, fold-related, and mesoscopic deformation processes that are relevant to fold-related fluid systems. Finally, we close by presenting several conceptual models for fold-related fluid system structure, and highlighting key areas for future work. [Copyright &y& Elsevier]

Abstract: Many basement-involved fault-related folds are thought to form by reactivation of pre-existing faults. Although oblique slip is expected on such structures it is often difficult to identify because the underlying faults are blind, paleomagnetic records are ambiguous, and mesoscopic deformation patterns in the associated monoclines are often incompletely exposed or difficult to interpret. This study seeks to establish a relationship between oblique slip on a basement reverse fault and the deformation patterns in the overlying folded rock layers. Using scaled physical models of wet clay overlying a rigid basement, we create a suite of Laramide-style folds over basement faults with variable obliquity. Precise displacements and strains on the surface of the clay are recorded using close-range photogrammetry. We use these measurements to predict deformation patterns that we would expect to find in analogous natural structures. The results of our models show that there are three general strain zones that form in monoclines that form above oblique-slip faults. The upper-hinge region of the monocline is dominated by extension, the lower-hinge region by contraction, and the middle of the fold limb is dominated by shear strains. The boundaries of these three zones, as well as the magnitude of strain in each of the zones vary with the amount of oblique slip and fault throw. Deformation should be dominated by joints and extensional faults in the extensional zone, by contractional faults and cleavage in the contractional zone, and may contain extensional, contractional, or strike-slip faults along with joints and cleavage arranged in Riedel style geometries in the shear strain dominated zone. At fault obliquities above 45° there is significant overlap of the different strain zones and deformation patterns in these regions can consequently be very complex, involving reactivation or overprinting of earlier structures, or the formation of mixed-mode structures. Displacements on the surface of our model monoclines suggest that previous paleomagnetic interpretations of vertical axis rotations on natural monoclines may need to be re-evaluated. Although such rotations increase with increasing fault obliquity, the magnitude of these rotations will vary significantly with position across a monocline. [Copyright &y& Elsevier]

Abstract: This study uses fluid inclusions in quartz and calcite veins to characterize the fracture-controlled paleohydrology of a map-scale, evaporite-cored detachment fold in the Sierra Madre Oriental of northeastern Mexico. Field observations indicate that the veins are tectonic in origin, and that they formed in a general sequence that corresponds to four broad stages of progressive folding. We collected samples from each vein stage in various structural and stratigraphic positions across the fold. After determining the mineral paragenesis and origin of inclusions in each sample, we used standard microthermometric techniques to measure the homogenization temperatures (Th ), salinities and eutectic temperatures of the available two- and three-phase aqueous inclusions. Neither Th , salinity nor eutectic temperature varied systematically with inclusion origin or mineral type. The data are also not significantly correlated with structural position or vein type, but are strongly partitioned by stratigraphy, suggesting that the area was a vertically stratified hydrologic system consisting of three regional paleohydrostratigraphic units. An upper unit comprised the Indidura Formation through Difunta Group, and is characterized by Th near 150°C and salinity <5wt% NaCl equivalent. A middle unit comprised the Taraises and Cupido Formations, and is characterized by Th near 150°C and salinities near 12 or 22wt% NaCl equivalent. The lower unit comprised the Zuloaga and La Casita Formations, and is characterized by an average Th near 225°C and salinities in three groups near 12, 22 or 36wt% NaCl equivalent. The preservation of multiple, distinct fluid types in many veins suggests that fracture development during folding and uplift created a second paleohydrologic system that overprinted the vertically stratified system. Fracture-controlled fluid migration in this second system occurred in periodic, repeated, and spatially variable pulses. [Copyright &y& Elsevier]

Abstract: Geologists now recognize the presence and abundance of detachment folds in various contractional settings. Several two-dimensional geometric and kinematic models exist to describe the development of such structures. These models typically are area-balanced, are interpreted to develop by hinge migration, limb rotation, or a combination of the two processes, and possess a constant regional level outside the fold itself. We present two new two-dimensional geometric and kinematic models for detachment folds that incorporate hinge migration and limb rotation as their deformation mechanisms. These area-balanced models differ from previous models, however, in that they allow ‘hinterland inflation’ to occur, thereby creating a higher local regional level in the hinterland relative to the foreland. This local regional difference across the detachment fold may reflect hinterland uplift due to thickening of the incompetent unit, foreland deflation of the incompetent unit as material migrates into the fold core during fold growth, and/or migration of material out of the fold core as the fold tightens. We illustrate the utility of these models by constructing pseudo-three-dimensional representations of the western termination of the Nuncios Fold Complex in the Monterrey Salient, northeastern Mexico. Our results suggest that the limb rotation model more accurately portrays the overall three-dimensional fold geometry for the Nuncios Fold Complex, matches the observed hinterland inflation, and predicts more reasonable detachment depths for the structure along its length. These results agree with interpreted kinematics based on field observations and illustrate how this modeling approach may help constrain interpretations of detachment folds with hinterland inflation in areas lacking sufficient data. [Copyright &y& Elsevier]

This review paper examines deformation adjacent to salt stocks and walls and beneath salt sheets, with a focus on passive salt rise or emplacement and structures ranging from large-scale folds and faults to small-scale folds, fractures, and shear zones. The analysis begins with a summary of the existing literature on physical and numerical modeling and empirical subsurface and outcrop data, which offer conflicting interpretations and models. The emphasis here, however, is on exposed diapirs and sheets in a variety of salt basins. These demonstrate that near-salt deformation during passive diapirism is less common and less pronounced than is typically thought. In most cases, diapir rise and sheet emplacement do not directly shear and fracture adjacent strata. Instead, salt movement leads to drape folding of a thin roof, which in turn may cause associated fracturing, just as with folding of any origin. There can be exceptions, with the most common being regional extensional, contractional, or strike-slip deformation that is coeval with or postdates diapirism. This is especially the case if the salt becomes welded and there is ongoing weld-parallel slip or if fault damage zones formed away from diapirs subsequently become juxtaposed against the salt by ongoing slip. • Outcrop data are critical for testing models of near-salt deformation. • Diapir growth and sheet emplacement do not cause shear/drag of adjacent rock. • Passive diapirism generates drape folding of roof strata and associated fracturing. • Excess deformation is caused by regional stresses or other anomalies. [ABSTRACT FROM AUTHOR]