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The main volcanic centre of the České støedohoøí Mts. in North Bohemia is located at the crossing of the Ohøe/Eger Rift (NE-SW) and the Labe tectonic zone (NW-SE), two of the major tectonic features within the Bohemian Massif. Active Tertiary volcanism is documented by the existence of a large trachytic breccia body, several essexite/monzodiorite stocks and feeders of superficial volcanics of basaltic chemistry. Dyke swarm of high lithologic variety reaches several kms far from the centre. The complex geophysical methods were used to recognize the shape and position of individual subvolcanic bodies in the subsurface.
Geophysical investigations comprised gravity, magnetics, shallow seismic, gamma-ray spectrometry and geoelectrics. Rough topography was a challenge to all methods applied. Gravity surveys delimited the extent of trachytic breccia in the filling of the youngest crater vent and its approximate depth. Some hidden essexite bodies were indicated in a NNW-SSE trending zone crossing this structure. However, the main subvolcanic body is expected on the southern part of the whole volcanic centre, as a positive gravity anomaly has its pronounced maximum in that area. This result was confirmed by magnetic measurements that showed the contour of the collapse structure as well, but first of all the topography of particular elevations of the main subvolcanic body. The connection between the central and southern part is unclear, but more likely there are separated intrusions along the zone rather than a single prolonged body. In the southern part of the complex, shallow seismic together with magnetometry displayed detailed relief of the top of the intrusion. VLF electromagnetic survey indicated zones of low resistivity, which may correspond to tectonic features. Indications of mineralization were obtained from induced polarisation measurements in larger area of a formerly known occurence of a hydrothermal deposit.
Geophysical data provided a new insight into the whole neovolcanic structure, especially by indications of hidden bodies and their morphology. This case history can serve as an example of a geophysical multi-disciplinary study in a volcanic environment.
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