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(v) The fluids have evolved under low fluid:rock ratios.    Simple thermal considerations indicate that fluid flow was episodic and highly focussed along the fault plane. We conclude that rising pore fluid pressures as a result of rapid sedimentation, coupled with increasing tectonic stress as the basin inverted during the Variscan orogeny led to fault movement and release of the pore fluid pressure as the sediment pile dewaters. As with the example of the upper Mississippi Valley sedimentary basins the dewatering is episodic with extended periods during which pore fluid pressure increases punctuated by short periods of fluid release. This process resembles very closely a seismic valve \citep{Sibson_1981}.   \section{Geological setting of the Variscan faults and vein fill}  We studied samples of hydrothermal vein calcite from three areas of Carboniferous limestone in the British Isles: (i) The Peak District of the southern Pennines in Derbyshire and Staffordshire; (ii) The Gower Peninsula, South Wales, and (iii) The Burren, County Clare, Ireland, Figure 1. All the locations lie to the north of the Variscan front in the British Isles with faulting and vein fill in each area associated with foreland deformation during the late Carboniferous and early Permian periods.  There are distinct similarities in the general geologic setting of the three areas. During mid-Devonian to the end of the lower Carboniferous Britain was subject to a north-south back arc-extension as a result of subduction of the Rheic ocean to the south. Extension was largely accommodated by the development of a series of graben and half graben deep water basins bounded by normal growth faults and with foot wall topographic highs. Amongst the basins are: the Edale Gulf, Goyt Trough and Widmerpool Gulf in the Peak District; the Culm basin to the south of the Gower Peninsula, and; the Clare Basin along the axis of the Shannon estuary in the west of Ireland. Dinantian shallow water platform and ramp carbonates accumulated on the topographic highs whilst deep water facies limestones and shales were deposited in the basins (Figure 1). Both extension and carbonate sedimentation ended at the end of the Dinantian and was succeeded by Namurian shales and later a range of Westphalian river and deltaic sandstone facies. The total thickness of post Dinantian sediment accumulation on the platform limestones ranged from a 1 to 2km, with correspondingly thicker sequences of 3-4km accumulating in the basins.   Variscan shortening resulted in basin inversion and in the foreland reactivation of extensional faults with both significant strike slip and reverse components of movement. There is evidence for high pore fluid pressures during the period of faulting with the development of mode I fractures and in places a pervasive mesoscale fracture network. Fracture dimensions range from sub-mm to fault widths of several metres. Fracture and fault vein fill at all the locations is dominated by white, sparry calcite often showing a syntaxial growth pattern with varying degrees of complexity as a result of repeated episodes of movement, brecciation and renewed hydrothermal mineral growth.  Details of the sample localities are given below.  (i) Peak District. The Derbyshire platform is largely composed of shallow water Dinantian shelf limestones that show little evidence of deformation. The structural evolution of the area is summarised by Quirk (1991). During the lower Carboniferous NE-SW extension, accompanied by normal faulting produced a series of WNW-ESE trending deep water basins that border the platform. The basins are infilled with thick sequences of Dinantian basin limestones, shales and sands. Rotation of the stress field during the upper Carboniferous as a result of the Varsican orogeny to the south resulted in fault re-activation with a significant strike-slip component of movement. It is these re-activated faults that host the largest mineral veins exposed on the North Derbyshire platform.  The thickness of overburden on the platform is not well constrained. Estimates based on reconstruction of post Dinantian upper Carboniferous stratigraphy range up to 2km, with maximum sediment thickness in the basins of 3km (Colman, 1989). Similarly the temperature conditions during vein formation are not well constrained. Fluid inclusion temperatures obtained from hydrothermal calcite suggest temperatures ranging from 50° to as high as 240°C (Rogers, 1977; Masheder and Rankin, 1988; Rodger, 1996; Kendrick et al., 2002).  We sampled hdrothermal calcite from five sites in the Peak District.  (i) Dirtlow Rake (GR) is a major WSW-ENE trending strike slip fault lying just to the south of Castleton. The width of the exposed fault is greater than 10m and it has been extensively worked over a greater than 10km length for Pb (galena) and Zn (sphalerite). Hydrothermal calcite occurs as large syntaxial and elongate, sparry crystals. The growth form often exhibits dog-tooth terminations indicative of growth into a void.  (ii) Pindale  (iii) Hucklow Edge (GR)  (iv) Blakelow Lane quarry (GR)   (v) Ecton Mine.  (ii) Gower Peninsula. The Gower Peninsula lies to the south of the Wales-Brabant Massif and just north of the Variscan Front which is marked by the Bristol Channel Fault Zone, a major south dipping thrust (refs). The peninsula is composed of moderately deformed Devonian and Carboniferous rocks, Figure 2. The deformation is consistent with a north-south compression associated with the Variscan orogeny. Structural elements include folded open, ESE-WNW trending folds that gently plunge to the east. Associated with the folding are a series of contemporaneous SSW dipping thrust faults (e.g. Bishopston thrust) that verge towards the north and the Port Eynon thrust, a NNE dipping backthrust (Wright et al., 2009). A transition in the stress regime from vertical σ3 to vertical σ2 is marked by the development of SSW-NNE trending, vertical wrench faults with development of strike slip and oblique slip slickensides. Fault displacements range from several tens to hundreds of metres. Where these faults cut the lower Carboniferous limestones they are characterised by mineral filled fissures several metres wide with a wide range of infill textures (Wright et al., 2009). These include dendritic haematite, elongated syntaxial calcite growth, spar ball and cockade breccia formation, sediment infill and void collapse breccias (Wright et al., 2009, Frenzel and Woodcock, 2014).   The overburden thickness and P-T conditions during deformation and mineralisation are poorly constrained. There are no available estimates of temperature based on either isotope geothermometry or fluid inclusion homogenisation temperatures. Reconstruction of sediment stratigraphy by comparison with adjacent areas where upper Carboniferous sediments are preserved suggests maximum burial depths of 3 to 4km (Sibson, 1981, Wright et al., 2009) at the onset of Variscan inversion. These values are somewhat greater than estimates of Cenozoic denudation which range between 0.5 and 1.5km for South Wales and the Bristol Channel (Jones et al., 2002, Hills et al., 2008). Though not directly comparable because of the lack of data for Mesozoic cover and erosion the estimates place lower and upper bounds on the depth at the time of deformation of 0.5 to 4km.  We sampled hydrothermal calcites from three vertical wrench faults exposed on the foreshore at Limeslade (GR), Oxwich East (GR) and Oxwich West (GR).   (iii) Burren, County Clare. The Burren is composed of a sequence of relatively undeformed, shallowly dipping (2° to the south) Asbian and Brigantian limestones that are uncomformably overlain by the condensed Clare Shales of Namurian age. To the south lies the Clare Basin filled with a thick sequence of Namurian and Westphalian shales and sandstones. Exposures of the Brigantian Slievenaglasha limestones at Fisherstreet (GR) comprise 3-20m thick cyclic units of crinoidal grainstones with abundant corals, cherty layers and a well developed joint system. In addition the limestones are cut by vertical veins ranging in width from a few mm to tens of cm. The veins are persistent and, unlike the jointing, cross several limestone beds and have a near north-south strike (Gillespie et al., 2001). Most of the veins are mode I hydraulic fractures with little or no apparent displacement. The veins are typically filled with white sparry calcite though locally both fluorite and galena can be found.   Estimates of the depth of burial at the time of vein formation are poorly constrained. Gillespie et al. (2001) have attempted to estimate the burial depth by: (i) reconstructing the missing Silesian overburden prior to the culmination of Variscan activity, and; (ii) using the mechanical characteristics of veins crossing discontinuities between beds of similar elastic properties. They determine a likely minimum and maximum depth of burial of 1.25km and 2km respectively. Previous fluid inclusion homogenisation temperatures obtained from fluorite in the veins range from 85 to 200°C (O’Connor et al., 1993) .  We sampled hydrothermal calcite from fractures in the Slievenaglasha Limestones that are exposed on the coastal section to both the north and south of Fisherstreet (GR).