Abstract. Organic and clay indicators were measured to characterise the burial and thermal history of Devonian through Upper Carboniferous siliciclastic and carbonate rocks in the Rheno-Hercynian zone of the Variscan orogeny in Moravia (eastern Czech Republic). The very low-grade metamorphism is documented by the illite crystallinity (IC) and vitrinite reflectance (R r ) in the inner part of the thrust and fold belt in the NNW. Application of forward thermal modelling suggests maximum palaeo-temperature of 240-360 • C and burial depth of 4-9 km. In the Variscan foreland in the SSE the IC and R r values are typical of diagenetic conditions with maximum palaeo-temperature of 80-130 • C. The distribution of both clay and organic maturity parameters is interpreted as a result of pre-and syn-tectonic thermal exposure of the rocks due to burial by a wedge-shaped body of thrust sheets thinning towards the Variscan foreland where only sedimentary burial was effective. The amount of uplift and denudation increases from the foreland to the RhenoHercynian thin-skinned thrust and fold belt.
Vertical tectonic movements often change the structural style and physicochemical habitat of sedimentary basins. Changes in pressure, temperature and salinity of the groundwater caused by tectonic uplift may result in the release of previously dissolved gas. This process of gas exsolution from groundwater is shown to be an important mechanism in the formation of gas accumulations in uplifted basins. Two principal types of gas release are discussed. A hydrodynamic type is active when groundwater flows into areas of lower pressure or mixes with water of different temperature or salinity. It is anticipated that this effect is more of local importance, but over long periods of groundwater flow large volumes of gas may be exsolved. The hydrostatic type of gas release can occur in any sequence of sedimentary rocks where uplift causes a drop in pressure and temperature. This phenomenon may act basin-wide. Mass balance calculations show that the largest gas accumulations on Earth, such as the Urengoy field in West Siberia, could have been formed by this process.
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