Geological interpretation of gravity profiles through the Karlovy Vary granite massif (Czech Republic) |
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Authors: | Vratislav Blecha Miroslav ?temprok Tomá? Fischer |
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Institution: | (1) Institute of Hydrogeology, Engineering Geology and Applied Geophysics, Faculty of Science, Charles University in Prague, Albertov 6, 128 43 Praha 2, Czech Republic;(2) Institute of Petrology and Structural Geology, Faculty of Science, Charles University in Prague, Albertov 6, 128 43 Praha 2, Czech Republic;(3) Institute of Geophysics, Acad. Sci. Czech Republic, Boční II/1401, Praha 4, Czech Republic |
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Abstract: | We examined the shape of the Late Variscan Karlovy Vary granite massif located south of the Ohre/Eger graben in Northern Bohemia
by reinterpretation of existing gravity data on two perpendicular profiles. The granite body of about 360 km2 total outcrop size has the elongation ratio 0.35 with the major axis trending NE-SW. The SW part of the body was crossed
in the nineties by the seismic profile 9HR which localized the bottom of granites in a depth of about 10 km. We used this
value as a reference datum in our gravity profiles. We positioned one of our profiles along the seismic profile 9HR and the
other one perpendicularly, i.e. parallel with the elongation of the outcrop surface. We interpret the shape of the main granite
body in the vicinity of Karlovy Vary as a continuous desk whose floor is horizontal (or subhorizontal) and varies along its
whole extension about a depth of 10 km. This thickness is approximately identical with that of the Saxothuringian nappes imaged
by seismic reflection. The near surface upper contact of the granite body is mildly inclined, and outward dipping. It changes
to steep sides or inward inclined contacts in deeper levels. The Lesny-Lysina (Kynžvart) massif is a separate granite body
about 324 km thick, not continuously connected with the main Karlovy Vary massif. The gravity curve suggests that granites
often enclose in their endocontact large blocks of country metasediments or metabasites the existence of which is partly evidenced
by their outcrops outside the line of the profile. The granite body is found density-homogenous. Minor density differences
between granite varieties are caused mainly by more intense hydrothermal alterations in younger suite granites. We interpret
vertical conduits for the ascent of granitic magmas to be parallel to the Jáchymov-Gera and Ohře (Eger) lineaments or the
Mariánské Lázně fault zone as indicated by the elongation of some outcrops. However, they are not clearly imaged from the
gravity data. The effect of the depression of the Sokolov basin along the faults parallel with the Ohře (Eger) lineament is
shallow and it is not indicated by any change in the floor depth of the granite body. Comparison of the seismicity distribution
suggests that the hypocenters occur mostly outside of the granite bodies or near their contact with the country rock. |
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Keywords: | Variscan granites gravity modeling Karlovy Vary Massif Western Bohemia Saxothuringian zone Ohř e/Eger graben |
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