The South Wales Coalfield is a Variscan foreland basin extensively deformed by both linked and isolated thrusts in response to regional NW-SE compressive stress. Thrusts normally strike NE-SW, but transpression has caused a variable dextral rotation of the thrusts to strike E-W and even NW-SE. The range in orientation has allowed thrust-related fractures locally to be opened within the neotectonic stress field in which 0'1 is oriented NW-SE. It is argued that similar thrust-related permeability should be developed in other coal-bearing foreland basins both associated with the Late Carboniferous Variscan/ Appalachian orogeny and with younger compressional tectonic systems. The dominant meso- to major-scale structures formed during the compression of coalbearing sequences are thrusts and folds. Strains at leading and trailing tip-lines of isolated thrusts and in the immediate hanging wall and footwall generate tension cracks which may act as methane conduits. Unsealed, these allow permeability parallel to thrust strike. Decollements form as bed-parallel detachments within coals, developing as pervasive shear zones, characterized by cleavage duplexes and C-S fabrics in which a penetrative new hinterland-dipping fabric is formed. This fabric, under changing regional stress conditions, may be opened to form a highly effective gas migration pathway. Coal-bearing strata develop chevron folds with flexural slip in fold limbs and tension gashes in competent strata, generating porosity and permeability parallel to the strike of fold limbs. Incompetent coal seams are strongly sheared, producing cleavage duplexes with contrasting vergence in opposing limbs.

We present a finite-element study of stress perturbation in evolving compressive and extensional strike-slip fault bridges. The results are compared with a fracture study of a compressive bridge at St Donats, UK. Horizontally interbedded calcareous mudstone and bioclastic calcilutite at St Donats have a distinct vertical permeability anisotropy. This sedimentary sequence behaves as a set of horizontal aquifers. The fluid flow in these aquifers is sensitive to mean stress gradients. Paleostress analysis of field fracture data, verified by finite-element modelling, indicates a rotation of σ1 towards parallelism with boundary faults inside the growing compressive bridge. Boundary faults and bridge faults recorded numerous fluid flow events. The modelled mean stress pattern shows a regional maximum within the bridge and local maxima/minima pairs at boundary fault tips. Finite-element modelling of an extensional bridge indicates that σ3 rotates towards parallelism with boundary faults. The mean stress pattern is similar to the pattern in compressive bridge but with maxima and minima locations interchanged. The stress patterns are reestablished by each stress build-up preceding the rupturation of the boundary faults throughout the development stages of strike-slip fault bridges. Mean stress gradients developed pre-failure control the fluid flow in fractures of the strike-slip fault system at and after the end of each stress build-up and the fluid flow in boundary faults post-failure. Fracture reactivation and new fracture generation within an evolving bridge is a process consisting of multiple successive events that retain the storage capacity of the bridge. Rupture and sealing of the main bounding-faults is a step-wise process that opens and closes fluid conduits between areas with different pressures.

Analysis of two examples of obliquely convergent external orogenic zones, the western part of the Western Carpathians and the northern Variscan margin in southwest Britain, indicates the operation of two dominant stress rotation mechanisms in the transpressionally deformed thrust wedge: (1) the rotation of an inferred stress field; (2) the rotation of a deforming body within a constant stress field. In the thinnest, external parts of the thrust wedge, σ1 stress trajectory rotations of up to 90° occur with deformation having a relatively small component of pure shear. Towards the hinterland, σ1 stress trajectories in thicker parts of the wedge are progressively less rotated but develop a larger component of pure shear. Resultant σ1 trajectories are curvilinear, lying parallel to the orogenic convergence vector in the hinterland but diverging progressively from this direction towards the foreland, where they lie at high angles to the external margin in frontal parts of the thrust wedge. It is argued that balanced cross-sections should be constructed parallel to the curved trace of the σ1 stress trajectories.

Variscan compressional deformation in the western and central coalfield of South Wales is dominated by thrust faulting. Opencast extraction records and field data have shown the existence of distinct levels of thrust development as isolated structures and basal detachments within imbricate systems. Two areas have been studied where the thrust faults exhibit similar features which have formed by different propagation mechanisms. At Nant Helen opencast site major thrusts are separated by smaller isolated structures, which apparently do not link to a basal décollement below the site. The isolated thrusts are adduced to have formed due to fluid over-pressuring and mechanical anisotropies in the rock sequence. In the Ffyndaff district deformation at the Ffyndaff opencast site is characterised by a series of foreland propagating thrusts linked by a detachment below the Nine Foot Seam, whereas to the west at Rhigos and Dunraven the detachment rises to a higher level and is interpreted as a gentle eastwards dipping lateral ramp.

A review of the shear strength characteristics of Coal Measures lithologies suggests that major structurally controlled discontinuities in mudrock or coal are likely to show near-residual shear strength values, with minimal cohesion and a friction angle of approximately 12°. The principal structurally controlled discontinuities are those associated with tilting, folding and faulting. The former result from bedding plane slip during flexural folding that smoothes the bedding planes and reduces cohesion. The latter are produced by fault movements that weaken the rock and produce a structural anisotropy in the fault surface. Both are likely to confine the movement of water through the rock, enhancing weathering and further weakening the surface. In open pit design, the stability of the highwall is discussed in relation to the orientation of the various types of structurally controlled discontinuity. A case study from Ffyndaff Opencast Coal Site in the South Wales Coalfield is analysed, and shows that the major normal faults that cut the site intersect the highwall with a stable orientation. Regional dip in the site also has a stable orientation, but the site is affected by several major north-verging thrusts that have been investigated by computer analysis of borehole and coal extraction data. The results indicate that the thrusts have a ramp-flat geometry and are likely to imbricate upwards from a bed-parallel detachment in a coal seam. The thrusts have generally developed in a piggy-back sequence, although at least one formed as a break-back thrust. The thrust displacement (slip separation) variation along the thrusts is described and suggests a normal displacement gradient of 0.16-0.18. These geometrical characteristics are used to interpret the thrust structure of a potential site from exploratory borehole logs, where the predicted geometry limits the location of the bounding highwall.