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

Abstract

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), Cerveny 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., (<i>A</i>) - Ceska Kamenice Low, (<i>B</i>) - Zittau Low, (<i>C</i>) - Jizerske hory Mts. Low, (<i>D</i>) - North Krkonose/Karkonosze Mts. Low and (<i>E</i>) - Straz pod Ralskem Low are identified within the CLGL. The lows (<i>C</i>), (<i>D</i>) and (<i>E</i>) were interpreted quantitatively using a gravity modelling, whereas the lows (<i>A</i>) and (<i>B</i>) were evaluated without modelling. The presented gravity cross-sections show that the Cambrian Rumburk granite is the main source of the lows (<i>A</i>), (<i>B</i>) and (<i>E</i>). On the other hand, the Variscan Krkonose-Jizera granites together with the Cambrian Jizera orthogneisses are the main sources of the lows (<i>C</i>) and (<i>D</i>). Besides that, the sediments of the Czech Cretaceous Basin play an important additional role in the formation of lows (<i>A</i>) and (<i>E</i>) and the Zittau Basin Tertiary sediments are markedly involved in the low (<i>B</i>) amplitude. The gravity cross-sections 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 Krkonose Piedmont Late Palaeozoic Basin, producing the gravity high near the town of Vrchlabi, was interpreted to reach a depth of 5 to 6 km beneath the current surface

Multistage magma emplacement and progressive strain accumulation in the shallow‐level Krkonoše‐Jizera plutonic complex, Bohemian Massif

“…Recent gravimetric interpretation [ Sedlák et al ., ] suggests that the shape of the complex at depth is thickly tabular, not exceeding 8–9 km below the present‐day erosion level in its western half (Figures c and d). The Bouguer gravity anomaly map (Figure c; Sedlák et al .…”

“…(d) Interpretation of the gravity data in vertical cross sections along lines A–A′ and B–B′ (locations shown in Figure c) suggests a thickly tabular shape of the plutonic complex at depth with a root zone corresponding to the gravity minimum; redrafted from Sedlák et al . [].…”

Gravity response of igneous rocks in the northwestern part of the Bohemian Massif

“…On the contrary, the main feature of the gravity field of the hidden part of the Lusatian Anticline (S of the Lusatian Thrust Fault) is a large gravity low reaching up to −45 mGal. At least, two partial gravity lows can be distinguished in this area, caused by concealed southern part of the Lusatian Pluton and by the Late Palaeozoic and Cretaceous basins (Sedlák et al 2007a).…”

“…The shallower part of the Gravity Low of the Erzgebirge between the Karlovy Vary/Eibenstock extreme and the Altenberg-Teplice minimum is supposed to be caused by a large hidden plutonic complex spread there below several kilometres thick metamorphic sequences (Hejtman 1984; DEKORP Research Group B 1994;Siebel et al 1997).The Gravity Low of the Erzgebirge finally turns to the E and continues as the W part the Gravity Low of the Lugian Unit (Sedlák et al 2007a).…”

“…A new map of Bouguer anomalies of the eastern (Lugian) part of the Saxothuringian Unit covering large neighbouring areas of E Saxony (Germany), SW Lower Silesia (Poland) and N Bohemia (Czech Republic) was compiled and interpreted by Sedlák et al (2007a).…”