Discovery of the first Quaternary maar in the Bohemian Massif, Central Europe, based on combined geophysical and geological surveys

Abstract

Based on results of previous investigations of tephra-tuff volcaniclastic deposits and a geophysical survey in the surroundings of the Železná hůrka Quaternary volcano, West Bohemia, we performed detailed geophysical surveys using gravimetry, magnetometry and electrical conductivity techniques. Striking anomalies were revealed in a morphological depression near Mýtina, West Bohemia, as a strong evidence of the assumed maar-diatreme structure. The sharp isometric gravity low of − 2.30 mGal, as well as the corresponding positive magnetic anomaly of 200 nT with a negative rim on its northern side indicate a steeply dipping geological body of low density and containing magnetic rocks/minerals. Magnetic survey also showed pronounced local anomalies outside the depression that can reflect relicts of the tephra rim of the maar.

This geophysical evidence was then proven by an exploratory drilling near the centre of the gravity anomaly. Macroscopic on-site evaluation of the core, and more detailed sedimentological, petrochemical, palynological and microbiological laboratory analyses further confirmed the existence of a maar structure filled by 84 m of lake sediments reflecting a succession of several warm and cold climatic periods. Results of palynological analyses confirm the presence of a continuous palaeoclimate archive, with at least three successive warmer periods of most probably interstadial character from the upper Quaternary Saalian complex. Therefore, the recovered sediment sequence holds strong potential for in-depth paleoclimate reconstruction and deep biosphere studies.

At the bottom of the Mýtina-1 (MY-1) borehole (84–85.5 m), country rock debris was found, containing also volcanic bombs and lapilli.

The discovered volcanic structure is considered to be the first known Quaternary maar-diatreme volcano on the territory of the Bohemian Massif. Because of hidden active magmatic processes in combination with earthquake swarm seismicity ca. 20–30 km north of the Mýtina maar, reconstruction of the palaeovolcanological evolution is important for evaluation of hazard potential of the NE and E part of the Cheb Basin.

Introduction

Maar-diatreme volcanoes are the most common volcano type following scoria cones. The Westeifel, Germany, is the classical area for maar-diatreme volcanoes worldwide with more than 68 of such structures of Quaternary age (Büchel and Pirrung, 1993). Maar-diatreme volcanoes develop when magma rises in dykes and intersects with water causing thermohydraulic explosions (Lorenz et al., 2003). Up to now several Tertiary maar-diatreme volcanoes are known from the northern part of the Bohemian Massif (Brus and Hurnik, 1984, Lindner et al., 2006, Kopecky, 1968, Krutsky, 1994, Malasek et al., 1980, Rutsek, 1973, Suhr and Goth, 1996, Suhr and Goth, 1999, Wiedemann, 1961), which are related to volcanic activity of the Eger Rift. The Eger Rift (Kopecký, 1978, Mrlina, 1980) is part of European Cenozoic Rift System (ECRIS; Ziegler, 1992). Within ECRIS Quaternary maar volcanoes are well known from the French Massif Central and the western Eifel (Fig. 1, inset map).

The area under investigation (Fig. 1, Fig. 2) is located at the NW corner of the Bohemian Massif. According to Fiala and Vejnar, 1997, Fiala and Vejnar, 2004 and Richter and Stettner (1993), the crystalline schists of the local Variscan basement comprise a succession of psammo-pelitic, carbonatic, and volcanogenic rock sequences of an Upper Cambrian to Ordovician age. The crustal segment of the surroundings shows large areas with Variscan intrusives dominated by granites (eastern surroundings of the area: Karlovy Vary and Zandov plutons, western surroundings: Mitterteich and Flossenbürg Granites).

During Late Eocene the Eger Rift started to evolve. According to Kämpf et al. (2005 and citations therein), the following steps of Cenozoic alkaline volcanic activities were recognized in the western part of the Eger Rift:

• Early Oligocene–Early Miocene (31–20 Ma): volcanic activity of the Rift in (i) the eastern and central part — the České Středohoří Mts. and the Doupovské Hory Mts., (ii) the western Eger Graben and the adjacent Krušné Hory/Erzgebirge Mts. and the Smrčiny/Fichtelgebirge area (24–20/15 Ma), (iii) the westernmost continuation of the rift as far to the Frankonian Lineament/northern Oberpfalz area (29–19 Ma).

• Middle to Late Miocene (16.5–8.3 Ma): volcanic activity synchronous with the graben formation dated by its pre-Middle Miocene? (> 11.7 Ma) up to Late Pliocene sedimentary fill.

• Middle Pleistocene (0.7–0.3 Ma): two volcanoes (scoria cones) in the Cheb Basin area, Komorní hůrka/Kammerbühl and Železná hůrka/Eisenbühl.

The Cheb Basin, a small (c. 265 km²) intracontinental basin, is located in the western Eger Rift. The basin was formed during the Late Tertiary as a result of the reactivation of three fault zones (N–S trending Regensburg–Leipzig–Rostock zone, the ENE–WSW trending Eger Graben faults and the NNW–SSE trending faults of the Cheb-Domažlice Graben) by continental rifting (Bankwitz et al., 2003 a,b and citations therein).

Presently, the area is characterized by remarkable geodynamic activity and possibly magmatic unrest. It is known as one of the most active earthquake swarm regions in continental rifts worldwide with thousands of small and intermediate magnitude swarm earthquakes (ML < 5) (Fischer and Horálek, 2003, Neunhöfer and Hemmann, 2006, Ibs-von Seht et al., 2008). Primarily, earthquake swarms are a peculiarity of volcanic regions and mid-ocean rifts; however at present, the western Eger Rift is classified as a non-volcanic region in Central Europe. The earthquake activity seems to be connected with small, but active surface movements related to tectonic stress evolution (Mrlina and Seidl, 2008). The seismicity is furthermore correlated in space and time with significant CO₂ dominated degassing at the surface in mofettes and mineral springs (Bräuer et al., 2003, Kämpf et al., 2007). For the first time, a hidden magmatic activity in the eastern part of the Cheb Basin has been traced by ³He/⁴He characteristics from repeated gas sampling between 1993 and 2005 (Bräuer et al., 2005, Bräuer et al., 2008a, Bräuer et al., 2008b). Various lithospheric studies indicate an updoming of the lithosphere–asthenosphere boundary (LAB) to approximately 80–90 km depth, a velocity anomaly at the base of the lithosphere between 50 and 65 km depth, an updoming of the Moho (up to 26 km from approx. 31 km in the surroundings) or thickening of the lower crust (rift pillow?) at the base of the crust (Mrlina, 1980, Hofmann et al., 2003, Hrubcová et al., 2005, Geissler et al., 2005, Geissler et al., 2007, Heuer et al., 2006, Heuer et al., 2007, Babuška and Plomerová, 2008).

The working area is located at the southern periphery of the Cheb Basin between the Mýtina village (north) on territory of the Czech Republic and the Neualbenreuth village (south) on the German side (Fig. 2). From NW to SE the crystalline basement consists of phyllites, quartz phyllites and quartzite lenses bearing mica schists of Upper Cambrian age belonging to the Cheb–Dyleň Crystalline Unit (Fiala and Vejnar, 1997, Tonica et al., 1998). The NNW–SSE trending valley between the depression of the assumed maar and the Kozly village (Fig. 2) is marked by a fault zone/quartz vein structure, belonging to the Tachov fault of late Hercynian age. The Železná hůrka scoria cone is also located at the Tachov fault (ca. 700 m SE of the proposed maar).