ALBERT

Description: This chapter documents the fracture process associated with the early cooling stage of felsic magma. Characteristics of pre-exhumation joints include their spatial distribution in granite bodies, their fracture surface morphology, and geological and petrological evidence for the depth of fracture initiation. These characteristics allow inferences about the depth and the time of joint origin in the South Bohemian Pluton. The intrusion levels of currently exposed granites of the pluton were 7.4 km in the northern part and 14.3 km in the southern part. Within the northern Mrakotin Granite (Bor[s]ov) early NNE joints propagated while the granite was at a temperature near the solidus, and, in part, magma was still being injected, post-dated by thin granite dykes along NNE joints. Evidence for the pre-exhumation initiation of these joints comes from the geochronological dating of these late-granite dykes (1-2 cm thick) at 324.9 Ma in age, which were creating their own rupture in the rock. The timing of the pluton emplacement at 330-324 Ma and the cooling ages of 328-320 Ma have been given by previous studies. From fluid inclusions within the late-granite dykes that occupy joint surfaces, the trapping depth of the analysed inclusions was calculated to be 7.4 km. Near the solidus H2O separates during the crystallization of anhydrous phases. The associated increasing H2O pressure can initiate the first cracks and can generate a small portion of new granitic melt, which forces the cyclic fracture propagation together with mobile, low-viscosity residual melt' input into the fracture. The determination of the intrusion level and time at which the dykes began cooling provide evidence for the joint initiation at a depth of 7.4 km, which was connected with the level and process of final emplacement and early cooling of the Mrakotin Granite long before the main exhumation. At the earliest, the erosion of the upper rock pile, 7.4 km in thickness, started significantly after generation of the early joint sets. The NNE-trending joints are persistent in orientation throughout the South Bohemian Pluton, but the joint-surface morphology varies in all subplutons and occupies all sections of the stress intensity v. crack-propagation velocity curve (Wiederhorn-Bahat curve).

Description: The small intracratonic Cheb (Eger) Basin in NW Bohemia (Central Europe) is characterized by swarm earthquakes, many mineral springs and mofettes with upper mantle CO2 degassing and by neotectonic graben and basin structures. Especially in non-lithified Upper Pliocene clay formations of the basin, a variety of deformation patterns is exposed. They include non-tectonic and tectonic activity and comprise faulting and folding from {micro}m- to km-scale. Previously unrecognized N-S- and ENE-striking faults are sites of mantle degassing and seismic activities. Confined-layer deformation and liquefaction structures hint to palaeoseismic events and gas escape activity. Cleavage-like arranged clay mineral plates represent the microfabric of clay within fault zones. For the first time the degassing channels of Upper Mantle fluids/gases through the Pliocene clay sediments can be documented: {micro}m-scale micro-tubes were produced by the opening of Riedel shear planes induced by fault movements.