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  (iv) The calcites exhibit zoned development characterised by cyclic and pulsed evolution of precipitation temperatures and fluid compositions as a result of varying mixing ratios of the two fluid end-members.    (v) The high temperature fluid end-member formation water, may be connate, and  has evolved under low fluid:rock ratios and is in apparent equilibrium with the silici-clastic basin fill.   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, hydrocarbon generation and silicate diagenesis 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 the pore fluid pressure increases. These periods are punctuated by short duration episodes of fluid release. This resembles very closely a seismic valve type process in which rising pore fluid pressure reduces the effective stress on fault surfaces, ultimately leading to failure and the formation of a high permeability path along which the pore fluid is released and the pressure dissipated \citep{Sibson_1981}.