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Dalradian metasediments in the Cur district of north-east Connemara have had a complex history of deformation and metamorphism. The earliest schistosity present formed during biotite-grade metamorphism and may be mimetic after a compaction fabric. Subsequently, the rocks attained garnet grade and major D 2 folds developed. This deformation involved an initial buckling followed by a coaxial homogeneous flattening and the structures produced were probably initially upright. After D 2 , staurolite, and locally kyanite, grew under moderately high pressure conditions. Continued increase in metamorphic grade was, however, apparently accompanied by regional uplift and erosion, for staurolite breakdown occurred at lower pressures than those required to form kyanite. Furthermore, the highest grade metamorphism was accompanied by a steepening of the thermal gradient, since breakdown of the assemblage staurolite + muscovite + quartz produced andalusite at high structural levels, but sillimanite deeper down. D 3 deformation began after the initial uplift at the peak of metamorphism and produced two major northward-facing nappes thrust over a basal fold-nappe. The nappes root to the south of the Corcogemore Mountains but continued uplift in south Connemara faulted out the root zones as later nappes developed. After cooling, broad open D 4 folds were formed and the Connemara Schists were thrust up and to the south over lower grade rocks. Uplift of the Connemara region may have been complementary to the subsidence of the Mayo Trough to the north, in which case the oldest Ordovician rocks in South Mayo may have been deposited at the same time that the peak of metamorphism was attained in Connemara.
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Fluid pressure in the crust may be controlled by different mechanisms according to depth, temperature, and the mineralogy of the host rocks. Where rocks are fluid‐saturated, fluid pressure may approach lithostatic or hydrostatic pressure depending on the ductility of the wall rocks and the connectivity of pores and fractures. However, if the host rocks contain minerals formed at temperatures higher than those currently prevailing, they will react with fluids to produce hydrated (or carbonated) retrograde minerals, and the fluid pressure will be limited by thermodynamic equilibrium between high‐grade reactant minerals and retrograde products. The thermodynamically constrained parameter, water fugacity, may have a value of tens to hundreds of bars in the lower crust. In practice, this means that for typical igneous or high‐grade metamorphic rocks now occurring in stable lower crust, notional fluid pressures are substantially (1 to 3 orders of magnitude) lower than lithostatic. No free, connected fluid phase can be present in deep stable crust, and alternative explanations must be sought for the relatively high electrical conductivity of such rocks. The proposal that high lower crustal conductivity is due to thin grain boundary films of graphite is also unlikely to be generally true because films of sufficient thickness would be readily visible on broken surfaces of hand specimens. An alternative explanation of the discrepancy between laboratory and field measurements of the conductivity of high‐grade rocks is that laboratory measurements are not normally made under appropriate conditions of rock‐buffered fluid pressure.
This chapter contains sections titled: Overview, Theme 1: What are The Controls on Fluid-Rock Chemical Interaction in and Adjacent to Fault Zones?, Theme 2: How Does Fluid Flow Change Before, During, and After Earthquakes?, Theme 3: What are the Magnitudes of Fluid Flux Throughout the Lithosphere in Different Tectonic Environments?, Summary, References
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