Geological interpretation of a gravity low in the central part of the Lugian Unit (Czech Republic, Germany and Poland)

A new detailed map of Bouguer gravity anomalies covers an area of c. 13 500 km2 of the Lugian Unit in the Bohemian Massif (Czech Republic, Germany and Poland). It is limited approximately by the towns of Bautzen (NW), Zlotoryja (NE), Červený Kostelec (SE) and Roudnice nad Labem (SW). An extensive gravity low about 120 km long (W–E) and 40 km wide (N–S) is the most remarkable feature in the Bouguer gravity map and the maps derived from it. This central Lugian gravity low (CLGL), is adjacent to the Saxothuringian low in the West but it disappears in the gravity high of the Kaczawa Complex and the Intra Sudetic Basin in the East. Five partial gravity lows, i.e., (A) – Česká Kamenice Low, (B) – Zittau Low, (C) – Jizerské hory Mts. Low, (D) – North Krkonoše/Karkonosze Mts. Low and (E) – Stráž pod Ralskem Low are identified within the CLGL. The lows (C), (D) and (E) were interpreted quantitatively using a gravity modelling, whereas the lows (A) and (B) were evaluated without modelling. The presented gravity cross-sections show that the Cambrian Rumburk granite is the main source of the lows (A), (B) and (E). On the other hand, the Variscan Krkonoše-Jizera granites together with the Cambrian Jizera orthogneisses are the main sources of the lows (C) and (D). Besides that, the sediments of the Czech Cretaceous Basin play an important additional role in the formation of lows (A) and (E) and the Zittau Basin Tertiary sediments are markedly involved in the low (B) amplitude. The gravity crosssections imply that the granite bodies influencing gravity lows can reach a depth of 7 to 10 km. A hidden mafic body situated in the basement of the South Krkonoše Piedmont Late Palaeozoic Basin, producing the gravity high near the town of Vrchlabí, was interpreted to reach a depth of 5 to 6 km beneath the current surface.