John W. Cosgrove

Development of internal structures in deformed anisotropic rocks

Abstract In this paper, the behaviour during deformation of composite foliated rocks is discussed in terms of the “average” rheological properties of the rock, not the properties of the individual components. It is shown that the behaviour of many such rocks can be explained theoretically using an analysis, first put forward by Biot (1965b), of the behaviour of a homogeneous, anisotropic body. The meaning of this anisotropy, and its importance in governing the way in which the material deforms, are discussed. The initial development of internal structures within a block of homogeneous, anisotropic material is examined. The results of this analysis have been used to account for the initiation of the following structural features of foliated rocks deformed by natural processes: symmetric and asymmetric multilayer buckles, normal and reverse kink-bands, box-folds, similar folds, boudin-like structures, and faults which are obtuse to the principal compressive stress direction. Sinusoidal buckles and conjugate kink-bands are suggested to be end-members of a range of possible internal fold structures. In experiments with plasticine models, the initial form of the structures which appeared is shown to be compatible with the theoretical predictions. The subsequent development of these structures is beyond the scope of the incremental theory, but has been investigated experimentally. As a result of the comparison of experimental results with field examples, two mechanisms are proposed for the formation of chevron folds. It is concluded that rocks which have anisotropic stress/strain properties may become unstable internally, when compressed. This instability, which does not necessarily involve a competence contrast, leads to the development of internal structures whose form is mainly governed by the degree of anisotropy of the rock, rather than by other rheological properties such as elasticity or viscosity.

Hydraulic fracturing during the formation and deformation of a basin: A factor in the dewatering of low-permeability sediments

The geological expression of hydraulic fracturing is varied and is controlled primarily by the magnitude of the differential stress and the intrinsic properties of the rock. The orientation and type of fractures that develop within a basin are determined by the state of stress, which in turn is controlled by the geological boundary conditions. During the early stages of burial and diagenesis the formation of hydraulic fractures is thought to be an important factor in the movement of fluids through and out of low-permeability, semilithified sediments. Unfortunately, these fractures are not generally preserved and are presumed to heal once the fluid pressure is relieved. The low-permeability Mercia Mudstones of the Bristol Channel Basin, southwest England, however, contain bodies of sand that, during the opening of the basin, were injected along some of the hydraulic fractures in the mudstones, preserving them as sedimentary dikes and sills. Field observations indicate that fluid pressures within the Mercia Mudstones were also very high during basin inversion and that hydraulic fracturing provided a transient permeability that relieved this excess pressure. The fractures are not visible in most of the mudstones but have been preserved within evaporite-rich horizons as a network of satin spar veins. Thus, the chance preservation of the sedimentary dikes and satin spar veins shows that at different times during the evolution of the basin, fluids migrated through low-permeability units along transient networks of hydraulic fractures. In addition, the orientation and spatial organization of these fractures reflect the boundary conditions operating at various stages in the basin history.

The interplay of faulting and folding during the evolution of the Zagros deformation belt

Abstract In this short paper satellite images, aerial photographs and seismic sections are used to show that pure buckle folds, pure forced folds and folds intermediate between the two have all formed, and are still forming, in association with the compression tectonics currently occurring in the Zagros deformation belt which is situated along the northeastern margin of the Arabian plate. The type of folding and its distribution can be linked directly to the distribution of ancient basement faults and to the rheological profile of the cover sequence.

A comparison of the geometry, spatial organization and fracture patterns associated with forced folds and buckle folds

Abstract In this paper the three-dimensional geometry and spatial organization of folds (both buckle folds and various types of forced folds) are considered, together with their associated fracture patterns, in an attempt to determine if these features can be used in regions of poor exposure or in areas where the geologist must rely on seismic data to indicate the type of folding that has occurred. This study of the relationship between the various fold types and associated fracture patterns draws on theoretical considerations of ideal conceptual models of folds, analogue models and field studies.

The interplay between fluids, folds and thrusts during the deformation of a sedimentary succession

Abstract In this paper, field evidence is presented to support the idea that fluid pressures in excess of lithostatic pressure were operating during the initiation and reactivation of large thrusts and folds in the Wenlock shales of Llangollen, North Wales. It is also argued that as major thrusts are reactivated and as major folds amplify into finite structures they in turn exert an influence on the subsequent migration and concentration of fluids within the deforming sedimentary succession.

The deformation of multilayers by layer-normal compression; an experimental investigation

Abstract In this paper, experiments are described which were carried out to investigate the initiation and amplification of structures which form when a multilayer is compressed normal to the layering. The multilayers were made up of either putty or paraffin wax and an attempt is made to relate the structures that form to those predicted by the various theories which consider the deformation of either a single layer, a multilayer or an anisotropic material when compressed normal or at a high angle to the layering or fabric. The experiments give an insight into the temporal and spatial relationships between the various types of structures that form by showing clearly how the initiation and growth of one type of structure such as a boudin can provide the local perturbation necessary for the initiation of other structures such as internal pinch-and-swell structures or normal kink bands.

The influence of mechanical anisotropy on the behaviour of the lower crust

Abstract The aim of this paper is to determine what can be deduced about the rheology of the lower crust undergoing extension, the state of stress existing there and how the finite strain pattern is developed, from a consideration of the theoretical analyses of layer-normal compression, analogue models and outcrops of rocks from the lower crust. It is concluded that the extended lower crust contains units with a significant competence contrast, has a high mechanical anisotropy and experiences bulk rheological changes during deformation. Based on a consideration of hydraulic fracturing in anisotropic materials it is argued that, depending on the magnitude differential stress, fluids will move through and out of the lower crust by fluid fracturing along the planar, horizontal stretching fabric and gently inclined normal kink-bands (extensional shears) or by the formation of vertical fractures.

Core and seismic observations of overpressure-related deformation within Eocene sediments of the Outer Moray Firth, UKCS

Cores from the Eocene Alba Field of the Outer Moray Firth, UKCS contain sandstone injections and bedding-parallel fibrous ‘beef’ veins. Both of these features are associated with high fluid pressure and the process of hydraulic fracturing. The orientation of the hydraulic fractures (along which sand injection occurred, or calcite precipitated) was controlled by the interplay of the stress field and the intrinsic anisotropy of the sediments. Seismic sections indicate that sand injection occurred on a larger scale than is apparent from the cores. Interpreted dykes (400 m long by 30 m wide) emanate from the margins of the Alba channel sandstone along fault planes. An analogy is drawn between these dykes, and the peripheral dykes formed at the margins of laccoliths as a result of the flexing and subsequent fracturing of the overlying strata. ‘Decompacting’ of ptygmatically folded dykes suggests that the process of hydraulic fracturing and sandstone intrusion initiated between burial depths of 40 to 400 m below seabed.

Prediction of static and dynamic fluid pathways within and around dilational jogs

Abstract The distribution of stresses around a fault controls the development of second order fractures and influences fluid migration pathways. This paper demonstrates how the regions of enhanced fracture-induced permeability and fluid migration pathways, in both static and dynamic situations, can be predicted for dilational jogs subjected to simple shear. Mean stress is one of several key factors influencing the migration of fluids through a system. By combining mean stress data with second order fracture predictions obtained from experimentally determined stress trajectory and differential stress data around fault geometries, it is possible to derive predictions for the fluid pathways associated with a static (homogenous) system. Superimposing the mean stress states obtained for the same jog at different applied loads enables the changes in mean stress (and hence fluid migration) associated with the cyclical loading of a fault during stick-slip faulting to be studied. In addition, the mean stress patterns associated with successive stages in the jog’s evolution have been studied and the variations in these patterns, and therefore of the associated fluid migration pathways, mapped in order to track the change in these pathways that occurs as the overlap of the jog-defining faults increases.

Thickness and spatial distributions of clastic dykes, northwest Sacramento Valley, California

Abstract The geometry and distribution of the clastic dykes of the Ono district, North Sacramento Valley are examined within stream sections. Five traverses along dry stream beds provide good exposure allowing the spacing, thickness and geometry of the dykes to be recorded. The spatial and thickness distribution of the dykes are considered using cumulative frequency plots, allowing a visual estimation of a best fit distribution. Dyke thickness conforms best to a log-normal distribution. There is also a characteristic minimum dyke thickness in a traverse and this is attributed to the minimum aperture that a fluid with sand clasts is able to exploit. Dyke spacing, however, shows a good correlation with a power-law distribution for four traverses, suggesting that there is a mechanistic control on the spatial distribution. Plotting dyke thickness against minimum dyke spacing reveals that thin dykes do not generally intrude in isolation. Unlike veins and igneous dykes, clastic dykes continue to provide preferential pathways for fluid flow, subsequent to their intrusion, thus inhibiting intrusion in the area surrounding a pre-existing dyke. A combination of this process and dyke branching provides the best model for the observed spatial and thickness distribution of clastic dykes seen in the Ono district, California.