The area under investigation (
Figs. 1, 2
) is located at the NW corner
of the Bohemian Massif. According to
Fiala and Vejnar (1997, 2004)
and
Richter and Stettner (1993)
, the crystalline schists of the local
Variscan basement comprise a succession of psammo-pelitic, carbo-
natic, 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:
1. 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 Kru
šné Hory/Erzgebirge Mts. and the Smrčiny/Fichtelge-
birge area (24
–20/15 Ma), (iii) the westernmost continuation of the
rift as far to the Frankonian Lineament/northern Oberpfalz area
(29
–19 Ma).
2. Middle to Late Miocene (16.5
–8.3 Ma): volcanic activity synchro-
nous with the graben formation dated by its pre-Middle Miocene?
(
N11.7 Ma) up to Late Pliocene sedimentary fill.
3. Middle Pleistocene (0.7
–0.3 Ma): two volcanoes (scoria cones) in
the Cheb Basin area, Komorní h
ůrka/Kammerbühl and Železná
h
ůrka/Eisenbühl.
The Cheb Basin, a small (c. 265 km
2
) 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-
Doma
žlice 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
b5) (
Fischer and Horálek, 2003; Neunhöfer and Hemmann, 2006;
Ibs-von Seht et al., 2008
). Primarily, earthquake swarms are a pecu-
liarity of volcanic regions and mid-ocean rifts; however at present, the
western Eger Rift is classi
fied 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 signi
ficant CO
2
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
3
He/
4
He characteristics from repeated
gas sampling between 1993 and 2005 (
Bräuer et al., 2005; 2008a, b
).
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; 2007; Heuer et al., 2006; 2007; Babu
ška
and Plomerová, 2008
).
Fig. 1. Location map of the western Eger Rift area (after
Geissler et al., 2004
and Ibs-von Seht et al.,
2008
). Black full and dashed lines: fault zones; triangles: Quaternary volcanoes of
Železná hůrka (ZH) and Komorní hůrka (KH); red circle: location of Mytina Maar; white circles: seismicity (ML N0.5) from 1962 to 1999 (
Neunhöfer, 2000
); stars: locations of main
focal zones; yellow ellipse: main area of active CO
2
emissions sites; full and dashed lines: fault zones; Upper right: position of the study area (red square) within the European
volcanic province (after
Wilson & Downes, 1991
). Abbreviations are as follows: MLFZ, Mariánské Lázn
ě fault zone; PPZ, Pocatky–Plesna zone; TFZ, Tachov Fault zone; Cheb–Dom.
Graben, Cheb
–Domažlice Graben; MC, Massif Central; AM, Armorican Massif; BM, Bohemian Massif; MN, Moldanubian; RH, Rhenohercynian; ST, Saxothuringian.
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J. Mrlina et al. / Journal of Volcanology and Geothermal Research 182 (2009) 97
–112
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).
2. Statement of problem
Our paper comprises the third step of our study of Quaternary
volcanism at the southern rim of the Cheb Basin, western part of the
Eger Rift.
In the
first step,
Geissler et al. (2004)
studied an app. 4 m thick
pro
file of volcaniclastic deposits in a temporary trench near Mýtina.
The only known nearby volcano at that time was
Železná hůrka/
Eisenbühl (
Reuss, 1852; Proft, 1894; Lochmann, 1961; Kämpf et al.,
1993; Schwarzkopf, 1997
) in about 1.5 km distance from the trench.
Geissler et al. (2004)
found geological indications for the possible
existence of another source of volcanic material surrounding Mýtina
than just
Železná hůrka — they assumed an unknown maar structure.
A morphological depression of ca. 500 m in diameter and 50 m in
depth was proposed as a potential maar structure (see
Fig. 10
in
Geissler et al., 2004
).
In the second step,
Mrlina et al. (2007)
tested the hypothesis from
Geissler et al. (2004)
by geophysical (gravity and magnetic) pro
filing of
an N
–S directed area of 200 m×1200 m, which crossed the mor-
phological depression. The gravity map showed a striking isometric
gravity low of about
−2.3 mGal in the centre of the morphological
depression, whereas the magnetic map exhibited a positive anomaly up
to 200 nT related to the same source. These distinct geophysical
anomalies were the
first in-situ proof of the assumed maar. The age of
the Mýtina tephra was dated on phlogopite and olivine-nephelinite rock
matrix by
40
Ar
–
39
Ar laser dating. The average Ar
–Ar age is 288±17 Ka
(Mid Pleistocene). In the trench, the magmatic (juvenile) component
increases from about 12% to 32% in the tuff layer at the base, whereas it is
about 60% in the tephra layer at the top without systematic trend in time.
In summary,
Mrlina et al. (2007)
concluded that a Quaternary maar
structure is most likely, but without drilling the
final evidence was still
lacking.
In order to provide such evidence for a maar structure near Mýtina,
we started a systematic aerial geophysical (gravimetric, magnetic and
geoelectric) mapping of the area approximately 1 km × 1 km with the
assumed maar structure in the centre. Finally, an exploratory drilling
near the centre of the gravity anomaly by wire line technique was
conducted in 2007 (borehole MY-1). The geophysical and drilling
results represent the principal content of this paper. Additional to our
common interest in the neovolcanic evolution of the western Eger
Rift, a strong motivation was directed to palaeoclimate reconstruc-
tions, if maar sediments were present.
3. Geophysical investigations
In the last years the effectiveness of gravity and magnetic inves-
tigations of maar-diatreme structures in Central Europe, namely in the
Eifel, Bavaria, Saxony, W and N Bohemia, was demonstrated among
others by
Mrlina et al. (1989)
;
Büchel and Pirrung (1993)
;
Gabriel
(2003)
;
Kroner et al. (2006)
; and
Mrlina and Cajz (2006)
. In other
regions, for example the explosive craters in the Coastal Mts., Syria, a
successful localization of volcanic channels was also based on gravity
and magnetic measurements (
Mrlina and Rychtar, 1990
). Similar
geophysical surveys were applied to study maar structures in New
Zealand by
Cassidy et al. (2007)
. Based on previous experience, gravity,
magnetics and geoelectrics were selected for detailed mapping of the
assumed maar structure near Mýtina.
3.1. Geomorphology of the area
In order to demonstrate the position of the investigated struc-
ture and the character of the geomorphology, we built a detailed
DEM (Digital Elevation Model) based on SRTM90 data with approx.
90 m × 90 m sampling, supplemented by elevation data of all
geodetically observed points that we measured during geophysical
surveys. Detailed editing was necessary to the data as the SRTM
values were randomly offset by up to 20 m.
In
Fig. 2
the unique shape of the morphological depression under
study can easily be recognized. This almost circular structure is located
on top of the
first hilly block to the south of the limit of the Cheb Basin.
Further to the SE (
Fig. 1
) there is the massif of Dyle
ň (940 m a.s.l.) as
part of the
Český Les morphological unit prolonged to the south along
the Czech-German border.
The diameter of the depression between Mýtina and Neualben-
reuth is about 500 m and the relative depth is 50 m (
Figs. 2, 3
). The
depression opens to NNW into the valley of the Kozly brook. The valley
may have its origin due to a fracture/fault within the Tachov Fault
Zone (TFZ),
Fig. 1
. The TFZ was de
fined and discussed e.g. by
Mahel
et al. (1984)
,
Tonica et al. (1998)
, and
Bankwitz et al. (2003)
.
Three principal geomorphologic directions can be recognized in
the surrounding region (
Fig. 2
). Firstly, the NNW
–SSE drainage system
Fig. 2. Two views of topography (DEM) of the Mýtina surrounding region. Left: 3D view from south, view inclination 77°, sunlight from west. Morphological features show general
drainage trends with prevailing direction NNW
–SSE, perpendicular ENE–WSW, and N–S. Mýtina depression is in the centre, on top of a hilly massif (orange to ochre colour). Right:
top view of gray scale DEM (sunlight from NW) with drainage system indicated by blue, red and green dashed lines. 1
— Železná hůrka volcano, 2 — CO
2
emmission site, 3
—
exploratory trench in Mýtina (
Geissler et al., 2004
), 4
— morphological depression of the assumed maar.
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J. Mrlina et al. / Journal of Volcanology and Geothermal Research 182 (2009) 97
–112