Structural characteristics of epicentral areas in Central Europe: study case Cheb Basin (Czech Republic)

Journal of Geodynamics 35 (2003) 5–32 www.elsevier.com/locate/jog

Structural characteristics of epicentral areas in Central Europe: study case Cheb Basin (Czech Republic) P. Bankwitza,*, G. Schneiderb, H. Ka¨mpf c, E. Bankwitza a

Gutenbergstr. 60, 14467 Potsdam, Germany b Gerokstr 58, 70184 Stuttgart, Germany c GeoForschungsZentrum, Telegrafenberg, 14473 Potsdam, Germany

Abstract The earthquake distribution pattern of Central Europe differs systematically from the neighbouring areas of NW and southern Europe regarding the fault plane kinematics. Within a belt between the French Massif Central and the northern part of the Bohemian Massif (1000 km) sinistral faulting along N-S zones dominates on the contrary to the Alps and their foreland with common bookshelf shears. One of the prominent N-S structures is the Regensburg-Leipzig-Rostock Zone (A) with several epicentral areas, where the main seismic center occurs in the northern Cheb Basin (NW Bohemia). The study demonstrates new structural results for the swarm-quake region in NW-Bohemia, especially for the Novy´ Kostel area in the Cheb Basin. There the N-S-trending newly found Pocˇatky-Plesna´ zone (PPZ) is identical with the main earthquake line. The PPZ is connected with a mofette line between Hartusˇ ov and Bubla´k with evidence for CO2 degassing from the subcrustal mantle. The morphologically more prominent Maria´nske´ La´zneˇ fault (MLF) intersects the PPZ obliquely under an acuate angle. In the past the MLF was supposed to be the tectonic structure connected with the epicentral area of Novy´ Kostel. But evidence from the relocated hypocentres along the PPZ (at 7–12 kms depth) indicate that the MLF is seismically non-active. Asymmetric drainage patterns of the Cheb Basin are caused by fault related movement along Palaeozoic basement faults which initiate a deformation of the cover (Upper Pliocene to Holocene basin filling). The PPZ forms an escarpment in Pliocene and Pleistocene soft rock and is supposingly acting as an earthquake zone since late Pleistocene time. The uppermost Pleistocene of 0.12–0.01 Ma deposited only in front of the fault scarp dates the fault activity. The crossing faults envelope crustal wedges under different local stress conditions. Their intersection line forms a zone beginning at the surface near Novy´ Kostel, dipping south with increasing depth, probably down to about 12 km. The intersection zone represents a crustal anomaly. There fault movements can be blocked up and peculiar stress condition influence the behaviour of the adjacent crust. An ENE-WNW striking dextral wrench fault was detected which is to expect as kinematic counterpart to the ca. N-S striking sinistral shear zones. Nearly E-W striking fracture segments were formerly only known as remote sensing lineaments or as joint density zones. The ENE shear zone is characterized by a set of compressional m-scale folds and dm-scale faults scattered within a 20 m wide wrench * Corresponding author. Tel.: +49-331-292009; fax: +49-331-292009. E-mail address: [email protected] (P. Bankwitz). 0264-3707/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0264-3707(02)00051-0

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zone. It is built up of different sets of cleavage-like clay plate pattern of microscopical scale. The associated shear planes fit into a Riedel shear system. One characteristic feature are tiny channels of micrometer scale. They have originated after shear plane bending and are the sites of CO2 mantle degassing. # 2002 Elsevier Science Ltd. All rights reserved.

1. Central European belt with dominant seismic N-S strike-slip faults 1.1. The main N-S structures (A–E) In central and western Europe areas of earthquake activities are preferentially connected with N-S zones. They are arranged within a belt of 1000 km length trending from the French Massif Central in the SW to the Bohemian Massif in the NE (Fig. 1). Within this belt geological N-S strike-slip faults are also confirmed by GPS measurements and focal solutions of epicentres. The seismically active faults are segments of large narrow zones (100 km and longer). The individual fault segments can vary in direction between NNW, N and NNE, the complete zones form bands striking about N-S. The seismic activity within these zones occurs only at some of the segments,

Fig. 1. Seismic active areas in Central Europe. Ellipses: Epicentres of single events or areas of earthquake swarms and connected movements. Preferentially sinistral strike-slip occur in a central field concentrated within N-S zones (A–E; see the text). In the northern part of the studied area dextral strike slip and normal faulting were recorded (F). Bookshelf movements dominate in the adjacent southern region, including the Alps (G). The encircled numbers present the velocity of recent vertical movements (in mm/a; Iouanne and Me´nard, 1994; Jeanrichard, 1981; Senftl and Exner, 1973; Zippelt and Ma¨lzer, 1987). Number in the hatched circle: Velocity of vertical movements during the period 1996–1997 (Mrlina, 2000).

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other parts behave aseismically (Ahorner, 1975). The seismic activity can migrate from one segment to another during decades. The seismically active structures in the northern and southern surrounding of the studied area differ from that within the belt where sinistral lateral movements along N-S zones dominate. In the Alps and their foreland bookshelf shear zones are common. In the northern part of Central Europe dextral lateral movements along WNW shear zones (Brabant) or normal faulting at NWto NNW-trending faults (Lower, middle and northern part of the Upper Rhine Graben; Schneider, 1988) occur. The four most prominent N-S zones within the belt are: A, The Regensburg-Leipzig-Rostock Zone (length: 700 km and width: 40 km) is seismically active in its middle part (about ca. 150 km between Cheb and Leipzig, Fig. 1). This zone is, especially in the German part, dominated by numerous N-S faults which are composed of en echelon segments measuring a few kilometers. They are partly inherited from Hercynian time, as indicated by a 100 km long N-S elevation zone in SW Saxony (Sokolovski et al., 1975). This zone represents the western part of the prominent N-S zone ‘‘A’’, whereas the eastern part is associated with depression forms as the Cheb Basin. Some earthquake clusters in the Vogtland area (Neunho¨fer and Gu¨th, 1990) occur within the zone ‘‘A’’, and especially north and south of Novy´ Kostel in the Cheb Basin focal areas are aligned in N-S direction (Hora´lek et al., 2000; Figs. 2a and 3). The Novy´ Kostel area has a high frequency of earthquakes, where up to 10.000 events during the period from August to December 2000 have been observed. B, The 9
E earthquake zone Stuttgart-Albstadt (Fig. 2c) east of the Rhine Graben (Schneider, 1993). C, Minor extended areas of earthquake occurrence appear also connected to N-S structural elements within the eastern part of the Upper Rhine Graben (Schneider, 1988). D, The earthquake area at the western margin of the Vosges (Remiremont-Eloyes; Haessler and Hoang-Trong, 1985) with a linear distribution of the epicentres (Fig. 2b) along an escarpment. This N-S striking earthquake zone appears to continue towards the area of Saarbru¨cken. E, Several epicentre areas of the French Massif Central (Dorel et al., 1995) are arranged in N-S direction: (1) Mont Dore region; (2) region NW of Clermont Ferrand and its northern prolongation associated with a N-S fault (length of both areas together: 200 km); (3) St. Flour region and its northern and southern prolongation (length: 175 km); (4) the Rhone region. But until now related N-S striking faults appears to be rarely known. The ellipses in Fig. 1(E) mark clusters with dense epicentres which were more or less arranged in N-S direction. 1.2. Geological and geophysical data from the Regensburg-Leipzig-Rostock (RLR) Zone A Zone A in Fig. 1 represents a noteworthy N-S zone in Central Europe. In the middle part (Fig. 2a) it consists of a set of subparallel faults which were partly detected from satellite images and from geomorphological negative forms, and which were observed in numerous mining induced exposures (including boreholes and prospecting trenches), especially in Germany. Some of the N-S striking faults have caused offsets in Variscan granites. Within one of these faults (length: 15 km) clay mining of caolinized phyllitic basement (Tannhaus fault near Scho¨neck, Vogtland; Lower Palaeozoic; Fig. 3) was active 100 years ago. This fault is the continuation of the Skalna fault (Cheb Basin) with a total length of about 120 km.

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Fig. 2. N-S strike-slip zones in Central Europe. (a) Central part of the N-S zone ‘‘A’’ between the Vogtland and Leipzig (Sokolovski et al., 1975). Ellipse: Epicentral area of Novy´ Kostel (Hora´lek et al., 2000). Stars: Selected earthquakes acc. to Seismol. U¨bersichtskarte Sachsen (1996). (b) N-S zone ‘‘D’’: Earthquakes near Eloyes, western Vosges (Haessler and Hoang-Trong, 1985). (c) 9
E earthquake zone ‘‘B’’, Swabian Jura: Distribution of seismic segments and creep segments within the N-S-trending strike-slip zone (length: 100 km). Square: Site of the quarry with liquefaction. (d) Liquefaction structure in Jura limestone near Illingen (see c) probably due to earthquake motion.

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Fig. 2. (continued)

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Fig. 3. The Tertiary Cheb Basin and the surrounding Hercynian basement structures. Ellipses (1) to (7): Earthquake swarms (Gru¨nthal et al., 1990; Fischer and Vavrye`uk, 2000). Dark grey ellipse (1): Area of 10.000 swarm earthquakes (August to December 2000). Thick line (fault 1): Newly detected sinistral N-S-fault zone that appears to be seismic active (PPZ: Poe`atky-Plesna´ Zone). Star (fault 2): Open pit with exposed ENE-trending dextral wrench fault zone (30 m width) in uppermost Pliocene sediments (Bankwitz et al., 2000, 2001). Insets: Area of this study.

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The prominent RLR zone was detected as beginning in the area of Regensburg (Germany) striking northward through the NW-corner of Bohemia (Czech Republic), passing through the Vogtland area and the region of Leipzig in Saxony. It can be traced in satellite images further to the north, up to the area of Rostock at the Baltic Sea (Ka¨mpf et al., 1992). Earthquakes occurred near Leipzig (Engelsdorf) and Werdau (near Gera, Fig. 2a). The current deformation of one associated fault zone was proved by GPS measurements as a sinistral movement (Scho¨neck fault; Wendt, 1999; southern part of Fig. 2a). The fault pattern of the Vogtland basement shows a high density. Besides the N-S fracture system faults striking NW-SE are frequent. Numerous about E-W-trending linear features (10–50 km length) were extracted from satellite images. In the basement areas around the Cheb Basin such E-W faults are rare, what is in contrary to the large amount of remote sensing lineations. Field evidence for NW-SE orientation of the maximum horizontal stress axis comes from horizontal slickensides and Riedel shears within a newly detected dextral ENE shear zone and from fault plane solutions of earthquakes distributed along the sinistral PPZ (N-S). The active fault pattern in the Vogtland and the Cheb Basin appears to reflect a re-arrangement of the stress field into the recent maximum horizontal normal NW-SE directed stress, causing sinistral strike-slip movements along reactivated N-S basement faults, and dextral strike-slip above reactivated about E-W striking basement faults, and supposingly extension and normal faulting along NW-SE faults as the Maria´nske´ La´zneˇ fault. 1.3. Geological problems regarding seismic active structures In many epicentral areas the event-related geological fault systems are uncertain or not exposed. Mostly, the related geological fault can only be supposed even in places where fault plane solutions have been made, for instance at Albstadt-Ebingen within the 9
E earthquake zone (Fig. 1, B) and in the western Vosges for the earthquakes near Epinal (Fig. 1, D). During field work in some of the areas in Fig. 1 several active faults presented by geomorphological anomalies were found. On the base of this data a correlation of various geological structures and the above mentioned recently seismic active N-S zones was possible: (1) A frequent causative relationship between N-S-trending structures (faults or zones) and earthquake occurrence in Central Europe. At the 9
E earthquake zone Stuttgart-Albstadt microgravimetric measurements (Schneider, 1996) across the expected N-S-trending earthquake zone supported the assumption of a N-S-trending active shear zone (main events 1911, 1943, 1979). Earthquakes occurred only in restricted sections (Fig. 2c). The zone is obviously in parts aseismically active with sinistral strike-slip (Zippelt, 1988). Liquefaction structures in limestones may be the result of Palaeo-earthquakes (Fig. 2d). (2) In all of the above mentioned earthquake regions there are wells and CO2 degassing found, but evidence for CO2 degassing from the subcrustal mantle only in the French Massif Central, the Eifel area and the Cheb Basin (Weinlich et al., 1999). Fluids indicate a stress release along the fault zones. (3) Within the zones A–D the earthquakes are located on reactivated basement wrench faults which are in favourable position to the recent stress field. The stress on such shear zones (of possibly Variscan age) can also be lowered by aseismic creep. Such a shear zone acts seismically, if the creeping process is blocked by cross-cutting active faults (Schneider, 1993). Evidence for

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intersection of N-S faults with sinistral strike-slip movements by NW striking faults was recognized in the Vosges (D), at the 9
E zone (B) and within the Regensburg-Leipzig-Rostock zone (A), probably at all sites caused by intersecting NW faults. The earthquakes of Eloyes (D) occurred near the crossing area with the NW-SE Jarmenil fault (Fig. 2b), the events around Albstadt (B) at a crossing depth with the Hohenzollern Graben, the earthquake swarms near Novy´ Kostel (A) where the Maria´nske´ La´zneˇ fault is interfering (Fig. 13).

2. Study case: the Cheb Basin 2.1. Topic of the study The study demonstrates new structural results for the swarm-quake region in NW-Bohemia, especially for the Novy´ Kostel area in the Cheb Basin (zone A; Fig. 3). The tectonical crustal structure to which earthquake activity is connected was hitherto not known. Fault plane solutions show various types of earthquakes (Hora´lek et al., 2000), one of them is characterized by sinistral strike-slip movements on planes trending about N-S. The relocated hypocentres of the Novy´ Kostel swarm events of 1985/86, 1994, 1997 and 2000 with chain-like N-S distribution don’t show any connection with the Maria´nske´ La´zneˇ fault, which intersects the epicentre area (ellipse) in NW direction. Geological and geomorphological field observations and measurements, information from topographic maps, satellite images on a scale 1:25.000 and digital terrain models, give evidence of yet unknown fault zones. One of them forms an escarpment of more than 20 m height in the Pleistocene/Pliocene beds of the Cheb Basin. This detected fault zone runs parallel to the N-S extension of the Novy´ Kostel epicentres. A second N-S fault zone was recognized about 1 km further to the west by anomalies of the drainage pattern and the distribution of sedimentary units. The two N-S fault zones with geomorphological evidence are not unique in the Cheb Basin. About five km west of the PPZ the next mapped N-S fault zone, passing through the village of Skalna, was seismically active during the last decades. The Skalna fault enters the Vogtland area, where six smaller epicentre areas occur (Fig. 3). The investigated area of the Cheb Basin with the main N-S earthquake-line of Novy´ Kostel is part of the prominent N-S zone A (Fig. 1). 2.2. Tectonic setting of the Cheb Basin During the Upper Tertiary the small intracratonic Cheb Basin (Malkovsky´, 1987) developed at the NW corner of the Bohemian Massif. The basin was initiated by reactivation of basement faults. It covers granites and Lower Palaeozoic series (Fig. 3). The asymmetric basin is deepening towards east and limited there by the Maria´nske´ La´zneˇ fault, at the other sides Tertiary sediments overlapp basement series. To the north the basin covers the southern slope of the Erzgebirge Mountains. The sedimentation was not continuous. The filling of the basin consists mostly of Miocene lignite, clay and sand, and after a gap of 12 Ma there follow Upper Pliocene sand, gravel and kaolinitic clay formations (Malkovsky´, 1987; Sˇpicˇa´kova´ et al., 2000). Pleistocene fluvial sands and gravels occur along river valleys. The thickness of the Neogene is less than 300 m. The roughly

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N-S-trending basin originated at the crossing area between the older NE striking Ohøe (Eger) Graben and the NW directed pre-Neogene Maria´nske´ La´zneˇ fault (Pitra et al., 1999). This fault forms an escarpment over a length of nearly 100 km. Non-tectonic and tectonic deformation features of the Pliocene clays and sands are exposed in open mines. Recent crustal activities are furthermore characterized by mineral springs, CO2 mofettes and by swarm earthquakes (Me <3.0). Especially from isotopic studies (Weinlich et al., 1999) it is obvious that the CO2 and He indicate degassing from the of subcrustal mantle (d13C, He3; Ka¨mpf et al., 1999). Two Quaternary cinder cones hint at a renewed mobility of the lithosphere, possibly due to the European Plate movement.

Fig. 4. Results from topographic map and field observations. (a) Theoretically expected drainage system of a basin. (b) Real drainage system of the northern half of the Cheb Basin. M. L. Fault: Maria´nske´ La´zneˇ fault. Plesna´ r.: River valley ahead of the escarpment of the PPZ (Pocˇatky-Plesna´ Zone). Hatched: Dry valleys. (c) The Luzˇni creek cuts off (arrows) by a N-S-trending uplift zone, assumed to be a fault zone. On the high eastern flank of the Plesna´ river the run off of a former creek was cut off (arrow) due to the uplift of this flank that coincides with the PPZ. , uplift side of the fault.

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3. Results 3.1. Drainage pattern within the Cheb Basin The drainage system of the Cheb Basin does not simply reflect the relief of the surrounding basement structures (eastern rim: slope of the elevated basement bridge along the Maria´nske´ La´zneˇ fault; northern rim: slope of the Erzgebirge Mountains; western rim: slope of the Fichtelgebirge granite) running then towards the center of the basin (Fig. 4a). The brooks and their tributaries indicate disturbances of the main run-off direction. The discharge from the western and northern part goes to the eastern third part of the basin into a N-S valley ahead of an escarpment within soft rock that accompanies the epicentre alignement. At the other hand, at many places the drainage from the eastern slope does not reach the receiving stream of the basin, although the slope, formed in pre-Neogene solid rocks along the Maria´nske´ La´zneˇ fault, is the largest one (Fig. 4b). The main drainage of the northern half of the basin follows predominantly the PPZ line, formed during late Pleistocene to Holocene. This newly mapped Poe`atky-Plesna´ fault zone (PPZ) and its prominent scarp are partly adopted by the Plesna´ River. The erosion of the low level footwall—in front of a geomorphological step—deepened the fault trace and formed the valley depression. Elevation of the eastern flank and erosion of the sediments ahead of the fault exposed the high scarp of the hanging wall. Some peculiarities of the drainage pattern are: First, the E-W- to ESE-trending valleys are straight indicating post-Pliocene reactivated basement faults. Second, at the western flank of the PPZ, one of the ESE flowing tributaries (Luzˇni river) changed the flow off direction to the south and its former NW-SE valley section became dry (hatched in Fig. 4b, c). Third, on the eastern high flank of the PPZ there are some now dry brooks mapped with blind beginnings and blind ends. 3.2. Geomorphological indications of current processes The peculiarity of the drainage system due to changes of the relief is caused by neotectonic and current processes, because the age of the deformed sediments is less than 0.12 Ma. The primary ESE drain of the Plesna´ river stopped at the N-S fault scarp of the PPZ that forms a step of more than 20 m height. From there the Plesna´ river turns to the south parallel to the escarpment, gathering all other ESE running brooks as tributaries. The fault scarp limits the eastern higher flank (or tectonic terrace) of the PPZ, that forms a plain between the PPZ and the Maria´nske´ La´zneˇ fault (see Fig. 5b). Because this topographic step was formed in Plio/Pleistocene soft rock (clay, sand and gravel), its relatively steep slope indicates actual process of thrusting on of the PPZ and upward motion of the eastern flank, otherwise the step should be much more eroded. Height differences along the N-S valley give evidence for current uplift of the eastern flank (Fig. 4c). The true level difference between the both flanks amounts only 5–15 m, but the slope along the valley reaches locally 29 m maximum height. The alignment of the epicentres are closely associated with the scarp of the PPZ. Focal solutions of numerous earthquakes show that strike-slip dominates and confirm that the main horizontal stress orientation SH is NW-SE.

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Blind drains at the eastern flank of the PPZ zone indicate a changing drainage at the eastern slope of the basin (Fig. 4b). Due to the upward motion of the eastern flank of the PPZ, the former eastern tributaries of the lower Plesna´ valley finished the runoff to the Plesna´ valley. Two small creeks, sited upon the high flank of the PPZ, drain recently with blind draughts parallel to the escarpment of the Plesna´ river. The exception of this peculiar drainage pattern of the eastern Cheb Basin are four tributaries in the most northeastern part of the basin, where the ongoing uplift of the Erzgebirge superimposes the fault-related uplift and causes the run-off of NE tributaries into the Plesna´ river (Fig. 4b). Another geomorphological feature is the Luzˇni valley, that bends from ESE to the south at a distance of 1 km west of the PPZ (Fig. 4c). The prolongation of the formerly used part of the ESE directed Luzˇni valley, downwards to the Plesna´ valley, is now a dry depression. The bending point of the drain is marked by a crest-like chain of hillocks that interrupt the former Luzˇni valley and forced it to drain southward. The ridge-like heights are observed over several kilometers supporting the assumption of an unknown recently active N-S fault zone (Luzˇni fault), that creates uplift of the eastern flank, or of an small fault-related ridge, which cuts off the former Luzˇni valley. 3.3. New fault patterns (N-S and ENE-WNW) In the mentioned cases the drainage was modified by the movement along N-S structures that were unknown before. The PPZ, the Luzˇni fault and the Skalna fault appear to be terminated to the south at the Ohøe (Eger) river valley that runs across the basin in E-W direction. But, south of it and south of the Jesenice reservoir several straight N-S-trending steep valleys indicate the prolongation of adequate fault zones. The asymmetric relief along these southern faults is due to the same fault tectonics as in the area north of the Ohøe river line. Each of them is characterized by a steep eastern and a flat western slope. When interpreting the geomorphological features, the domino-model for extensional areas seems to provide the easiest explanation for the asymmetric dipping slopes (Fig. 5a) and the different levels of the flanks. But this model is unlikely because the region is under NW-SE compression and horizontal strike slip dominates. All known active N-S structures of the prominent

Fig. 5. Fault pattern of the Cheb Basin. (a) Hypothetical E-W cross section with ‘‘domino’’ tectonics due to tension. (b) Schema of an E-W field cross section: Real scenario of strike-slip movements with uplift component under compressional conditions; M. L. Fault, Maria´nske´ La´zneˇ fault. (c) Plane view: Fault traces.  H and arrow, max. horizontal stress axis.

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zones A-E (Fig. 1) are strike-slip faults with a vertical component (thrusts). The fault mechanisms (Fig. 5b) become obvious from the plane view (Fig. 5c) and the illustrated position of the faults in the current stress field. During field work in 2001, additionally a large ENE striking shear zone was detected, temporarily exposed at the bottom of the kaolinite mine Nova´ Ves II, east of the Skalna´ village. The strike-slip zone is about 30 m wide and could be followed over a length of about 300 m. It consists of thousands of small shear planes with slickensides and Riedel shears in pure clay. The shear planes are polished by the gas jets caused by strong CO2 mantle degassing. 3.3.1. Sinistral N-S wrench fault with seismic activity Most of the recent earthquakes (80%), recorded since 1985 in the Vogtland/NW-Bohemia area (Neunho¨fer and Gu¨th, 1990; Nehybka and Skacelova´, 1995; Hora´lek et al., 2000; Klinge and Plenefisch, 2001; Sˇpicˇa´k and Hora´lek, 2000), occurred within a narrow N-S band north and south of the Novy´ Kostel village (Fig. 6c, d). None of the hitherto mapped geologic faults is related to this pattern of hypocentres (at depths dominantly between 9 and 11 km; Hora´lek et al. 2000). Field observations (Bankwitz et al., 2000, 2001) gave evidence of a prominent high-angle escarpment (more than 20 m high) in the soft rock landscape, extending over 10 kms (Fig. 6b) trending N-S in upper Pliocene/Pleistocene sediments. The scarp reflects a fault, reaching the hypocentres at depths between 7 and 11 km (Fig. 6a). This Poe`atky-Plesna´ fault zone (PPZ) is partly the reason for the position of the Plesna´ valley, and its escarpment accompanies the observed alignement of epicentres. The freshness of appearance gives evidence of actual movements along this fault, that sometimes can be a discontinuous process. The height difference of 25–30 m between the higher level of the eastern flank and the lower level of the western flank, and the relative steepness of the slope indicate that the uplift is still active. The fault plane solutions show sinistral strike-slip movements along N-S planes connected with a vertical shear component. Towards the north the fault crosses the national boundary to Germany. Its total length can be assumed to be 35 kms, but the fault is segmented. Between Vackovec and Novy´ Kostel the fault zone consists of two sets of scarp en echelon segments (015
and 160–150
), coinciding with sinistral strike-slip movement, in addition with degassing and spring activity. This N-S system is interacting with dextral NW faults and sinistral NE faults which are the cause for stress accumulation along the 015
faults and for earthquake activity at the intersection areas. The morphologically prominent Maria´nske´ La´zneˇ fault, which had displaced the Variscan basement and forms the boundary of the Tertiary Cheb Basin and of the uplifted eastern shoulder, appears recently to be seismically non-active, but block up the supplementary motion of the recent active N-S fault system at Novy´ Kostel. From a geophysical point of view Sˇvancara et al. (2000) and Havirˇ (2000) supposed a N-S zone, but only to the north of Novy´ Kostel, that could be associated with the epicentres of Novy´ Kostel. However, they did not know any field indication for that supposed northerly fault section between two NW-trending geologically kown faults (Maria´nske´ La´zneˇ fault in the south and Kraslice-Chodov fault to the north) which limit the expected fracture. They named the supposed fault ‘‘Pocˇatky-Novy´ Kostel-Zwota line’’. Novy´ Kostel is the southernmost point of this hypothetic structure. But just from Novy´ Kostel further to the south, the detected Pleistocene fault scarp is best developed (Bankwitz et al., 2000, 2001). The generation of the PPZ scarp started

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Fig. 6. (a–c) N-S-trending Pocˇatky-Plesna´ fault zone (PPZ) of the Cheb Basin with 10.000 events during the last earthquake swarm period, August to December 2000 (ellipse of the plane view: Fischer and Vavrye`uk, 2000). (a) Scheme of the estimated dip of the PPZ. The hypocenters occurred at depths between 8.7 and 10.0 km (circles, Hora´lek et al., 2000). (b) Sections across the prominent scarp of the PPZ (vertical exaggeration 1:10; Bankwitz et al., 2000). Scarp height: 25–30 m in unlithified sediments. The level difference between the two flanks of the PPZ: 5–15 m. (c) The modification of the escarpment by bending of its strike in the middle part of the PPZ indicates that the main vertical displacement occurred on R1 shears and P shears (R1, P). Hatched lines: The supposed boundaries of the PPZ shear zone system. Dots: Location of mofettes and mineral springs. (d) Map of epicenters around Novy´ Kostel (1) and Skalna´ (7; Hora´lek et al., 2000).

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where the assumed section of Sˇvancara et al. (2000) ends. Havirˇ (2000) assumed hypothetically a southern prolongation. Many of the seismic events occurred south of Novy´ Kostel along the Pleistocene scarp. In the elevation contour map (Sˇvancara et al., 2000) the Pleistocene PPZ fault scarp is reflected as the boundary of an elevated eastern block, but this feature was formerly not recognized as a seismically active fault. The color intervals of this map documents a height difference between 20 and 40 m, respectively, across the PPZ (and the Plesna´ valley), that agrees very well with the detected scarp height. 3.3.2. Evidence for the age of the seismically active PPZ fault Between Novy´ Kostel and Hartousˇ ov the Pocˇatky-Plesna´ Zone is morphologically developed as a slope. The height difference between the eastern higher flank and the western lower flank amounts 5–30 m. The escarpment dips with angles up to 45
steeply to the W. Geologically the Plesna´ brook cuts into Quaternary sediments. Sˇkvor and Sattran (1974) have given the following succession (ages acc. to Ru˚zˇie`ka and Tyra´cˇek, in Klominsky´, 1994, Table Pleistocene) with an age between 800 000 and 10 000 years (Fig. 7). In some places Quaternary sand overlays Pliocene clay in the Cheb Basin. The distribution of the Quaternary sediments along the Plesna´ brook reveals a specific pattern: mainly at both flanks Q2 is exposed. The drainage itself of the Plesna´ and its tributaries is characterized by Q3 fluviatile sediments. Q4 occurs only along the western slope of the Plesna´ and partly of the Luzˇni river. Obviously, the escarpment was formed before or during the sedimentation of the Q4 sands, nearly since 120000 years b.p. This observation supports the assumption, that the Novy´ Kostel earthquake swarms, with their epicentres distributed parallel to the escarpment, is active since 10000 years as minimum age (nearly at the beginning of the Holocene) or since 120000 years as maximum age (Wu¨rm glacial). From this time the site of seismic activity around Novy´ Kostel seems to be connected with the PPZ. During an earthquake-swarm in 1997 an elevation of 4–6 mm of the eastern shoulder near Novy´ Kostel was recorded by geodetical relevelling (Mrlina, 2000) of the area with epicentres, which occur at the higher flank of the PPZ. A sigmoidal bending of some eastern brooks running from the hills east of Novy´ Kostel, indicates wrench faulting during Holocene time along NW-faults, striking parallel to the Maria´nske´ La´zneˇ fault (Fig. 8).

Fig. 7. Field evidence for the initiation age of a fault (PPZ). (a) Ages of the Pleistocene sedimentary succession Q3 to Q4 (modified from Sˇkvor and Sattran, 1974). (b) Scheme of a cross-section across the seismic active PPZ. Fluvial sand deposits of the Q4 period are exposed only at the western side of the river, Q3 at both sides. The fault was formed after sedimentation of Q3 that dates the fault initiation (0.12 Ma).

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Fig. 8. Northeastern Cheb Basin and adjacent basement in uplifted position, limited by the Maria´nske´ La´zneˇ fault. Units including Pleistocene formations according to the Geological Map (Sˇkvor and Sattran, 1977), improved by first time recognized faults (black thick lines). PPZ, Poe`atky-Plesna´ fault zone, reflected by a > 20 m high escarpment at the eastern side of the lower Plesna´ valley. The youngest Pleistocene formation Q4 was only deposited in front of the scarp of the PPZ, and on the western lower side of the Luzˇni valley, in both cases there dating the formation of the scarps. Obviously, the Maria´nske´ La´zneˇ fault (MLF) consists southeast of Novy´ Kostel of four parallel trending faults which were active recently, crossing Holocene deposits. The MLF subplanes were stopped along N-S segments. Arrows mark locations where the water changed the flow direction; some drainage valleys are recently dry due to the uplift of the scarp shoulder.

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NE of Frantisˇ kovy Lazneˇ (Franzensbad) the mofette field of Soos is situated. Above Tertiary sediments 7 m thick Holocene diatom beds have been deposited, starting at 10 250 years b.p. (Rehakova´, 1988), initiated by tectonic movements and a permanent mineral water supply. Maybe that the initiation of the depression was synchronously to activity at the Plesna´ escarpment. 3.3.3. Dextral ENE wrench fault associated with basement strike-slip faults In addition, faults striking about E-W belong to the recent active N-S fracture system with strike-slip movements. They are not outlined in geological maps, but morphotectonic and drainage patterns indicate their presence. One newly detected ENE wrench fault is the surface expression of a basement fault in the hidden Hercynian granite beneath the Cheb Basin. The granite underlies the basin at a depth of 300 m and crops out at the western border of the basin. Strike-slip movements along this fault cause deformation features in the Pliocene clay formation. In one of the kaolinite clay mines this dextral Nova´ Ves shear zone (NVZ; see Fig. 3, star) is exposed in the lowest level of the quarry (40 m below surface) over a length of 300 m (azimuth 070–080
). In the western prolongation of this shear zone two linear depressions in the relief at the western rim of the Cheb Basin occur in granite and appear to be part of the strike-slip fault. The ENE strike of this fault zone and its position hints at a relationship to the northern boundary of the ENE striking Ohrˇ e Graben. It appears to be part of the graben boundary system. The wrenching along the Nova´ Ves zone occurred supposingly intra-Pliocene, because comparable deformation features are missing in the younger beds prograding over the deformed strata (or were not yet recognized). Within this shear zone numerous mofettes occur. The gas jets escape through fine cracks at the surface connected to deeper fault planes of the NVZ (Fig. 9). The gas jets have polished the mostly dark surfaces of such primary shear planes. The clay appears to be able to react by shear and to move along planes (Arch et al., 1988) which are densely arranged like lamellae. 3.3.3.1. Microfabrics of the shear zone. From SEM micrographs a complex inventory of shear planes becomes evident; it correlates with planes measured at the faces of the mine. At least four sets of shear planes can be distinguished. All of them dip sub-vertical and their space-relationship hints at the same shear plane geometry found in the macro-cale. The shear plane system gives evidence for fractal structures within the zone as described by Maltman (1994). SEM micrographs reveal the analogy of the clay shear plane structure with cleavage planes in hard rock as slate or shale. The lamellae-like dense planes appear to correspond with R-, R0 - P- and Y-shears (Figs. 10 and 11a). R0 -anti Riedel shear planes are assumed to be latest formed. They were opened by a wrench movement and by bending of the R-shear; they are characterized by sub-microscopical degassing channels. The fault zone (Fig. 9) consists of thousands of small, narrow standing single shear planes (Fig. 10), mostly of decimetre-size, occasionally up to 2 m, often covered with traces of intersecting cleavage-like shear planes (Fig. 10b) and with fine or coarse horizontal slickensides (Fig. 10c). Within the ca. 30 m wide dextral shear zone (Bankwitz et al., 2000, 2002) the numerous discrete shear planes of various strike and dip (dominantly subvertical) are arranged in ENE direction. In its structure this fault zone differs from shear zones found in hard rock since it

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21

exposes hundreds of single planes of
Fig. 9. Cheb Basin: Open pit in kaolinic clay. Hatched: Boundaries of a strike-slip zone. The zone is not fault-bounded. White dots: Locations of clay samples for SEM investigations; numbers (10 and 11) indicate the location of the sub-fault planes demonstrated in Figs. 10 and 11. Grey ellipses and black arrows: Main degassing locations. Numbers (1) to (4): Four samples of degassing CO2 vents (Table 1). Length of the exposed strike-slip zone in view direction: 300 m.

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The shear deformation occurs in discrete disc-like domains, not distributed over the whole volume of the shear zone. Anastomosing faults develop from the coalescence of minor faults. The thickness of the disc-like shear planes is between 100 mm and 1 cm. They are darker than the non-sheared clay and therefore easily to be detected. 3.3.3.2. ‘‘Secondary porosity’’and degassing channels in clay. SEM investigations made obvious that the gas escapes through vertical micro-tubes showing a diameter down to 1–2 mm (Fig. 11). The micro-tubes originated from opening of Riedel-anti shears. Studies of the kaolinite clay in the environment of the gas vents number 1 and 2 (Fig. 10) gave insights into the interior of small-size shear planes as shown in Fig. 11b–d. Such a shear plane is in reality enriched by countless sub-planes

Fig. 10. (a) Fabric of the dextral ENE shear zone based on field measurements and micro-scale investigations. The scheme was reconstructed from several SEM micrographs with different orientation (e.g. Fig. 11a). The fabric correlates with the meso- and macro-fabrics of the shear zone (Fig. 9). (b) Subvertical ENE shear plane with (primary) vertical traces of steps possibly due to intersecting R-shears as shown in the sketch. (c) Most frequent horizontal slickenside. Strike-slip fault within Fig. 9.

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which are documented in the example of the subvertical gas jet plane of Fig. 11a. The sample was taken from the gas vent 2, that contains the highest amount of CO2 within the studied shear zone (Table 1). The R-shear planes have a distance of 0.02 mm. The micro-channels were opened with diametres of tubes from 0.001 to 0.05 mm (Fig. 11a–d). Such tiny channels should be closed for water, but not for gas (CO2). Through tube-like pores the mantle derived CO2 (Ka¨mpf et al. 1999; Weinlich et al., 1999) is discharged under high pressure (and with noise). On cross-sections of shear planes the micro-tubes occur in a high areal density (per mm2). The walls of the degassing tubes are polished and occasionally coated by individual clay mineral sheets. It may be assumed that the holes were widened by the fluid jet stream. 3.4. Evidence for mantle fluid flow through the Wildstein Clay Formation The Nova´ Ves wrench fault is a site of permanent strong degassing: At many points the exit of CO2 can be seen, heared and felt, mainly within small water filled mud holes of several centimetres to decimetres in diametre. There is not any doubt that the mentioned deformation of clay at micro-dimension was the prerequisite for the initiation of degassing event through the clay.

Fig. 11. Microscale inventory of the dextral ENE shear zone Nova Ves II (Cheb Basin). Particles from the subvertical surface of the gas jet fault plane at the gas vent 2 in Fig. 9 (view is perpendicular to the fault plane). (a) Traces on the plane indicate further shear plane systems and degassing channels (arrows). (b–d) Details from (a): (b) Degassing channels on R0 -anti Riedel shears as dark stripes, (c) traced along the trace of a R-Riedel shear, (d) enlarged channel from (c), 0.002 mm in diameter. SEM micrograph: J. Kasbohm, Univ. Greifswald, M. Sto¨rr, Usedom.

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Fig. 11. (continued)

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Table 1 Gas composition of air-free fraction and 13 CCO2 isotopic composition of gases from the ENE strike-slip zone in the kaolinite clay mine (locations of the gas vents in Fig. 9) Gas vent (from ENE to WSW)

Sample Sample Sample Sample

2 1 3 4

He

N2

Ar

CH4

CO2

d13C

(ppmv)

(vol.%)

(ppmv)

(ppmv)

(vol.%)

(%)

10 11 20 34

0.11 0.19 0.34 0.57

19 36 77 125

2 3 6 9

99.89 99.80 99.65 99.41

2.39 2.06 2.25 2.18

(Figs. 9 and 11). The data of geochemical und isotopic analyses from four main degassing locations (of ca. 50) within the shear zone confirmed the mantle source of the CO2 (Table 1). Gas vents emanate CO2 at about 50 locations within the ENE shear zone through the kaolinite clay beds. At four sites (Fig. 9; Table 1) gas samples were collected. The CO2 content was determined volumetrically, and other components such as N2, Ar, CH4, and He, by gas chromatography after CO2 absorption in KOH solution. The isotopic analysis of carbon was carried out in a Finnigan MAT Delta-S mass spectrometer. d13C is related to PDB. The gas composition and the carbon isotope data from four main degassing locations within the shear zone is given in Table 1. The content of N2, Ar, CH4, and He increases from ENE to WSW and the CO2 content decreases in the same direction. This is in agreement with the gas flux, which also decreases from ENE to WSW. The 13CCO2 isotopic data of all samples are similar and comparable with results from other mofettes in the surroundings (Hartousˇ ov: d13C=2.1%, Bubla´k: d13C=2.0%, Bubla´k north: d13C=1.7%, Deˇvin: d13C=1.8 %, Weinlich et al. 1999). In addition, the strong degassing process produces compressional driving forces that may deform the clay recently. This assumed micro-deformation superimposes the intra-Pliocene formed structures at distinct places where the gas stream escapes. The striking features are polished wavy fault planes with dark surfaces, super-fine degassing channels, mm-wide mesoscopic fluid traces on fault planes with light mud rim. It is uncertain, which micro-features (e.g. fault parallel cleavage) were the result of the strata-bounded Pliocene deformation and what arises recently by ongoing processes.

4. Discussion In the Cheb Basin field evidence for neotectonic and recent processes is common. Within Upper Pliocene and Pleistocene sediments, anomalies of geomorphology and drainage indicate an unbalanced state of crustal behaviour. The anomalies can be dated by the age of the sediments. The regular arrangment of scarp segments (PPZ) in soft rock reflects a strike-slip structure that fit at depth to the hypocentre N-S striking distribution of the Novy´ Kostel swarm, e.g. during the period 1991–1993 (Nehybka and Skacelova´, 1995) and 1991–1998 (Nehybka, in: Sˇvancara et al., 2000). The fault structure reveals indications of the possible fault mechanism. From the geological point of view the threedimensional configuration of two deep-seated fault zones (PPZ and

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MLF) may play a role concerning the initiation and distribution of seismic events. Evidence for the deep reaching structures comes from CO2 subcrustal mantle degassing related to these faults. The new found ENE shear zone (NVZ) has produced short-time deformation features in upper Pliocene sediments, that hint possibly at seismic activities already in pre-Pleistocene time. The selected three seismic-related structures will be discussed below. 4.1. Interpretation of the PPZ fault segments Generally, the PPZ fault zone strikes N-S (see Fig. 6c), but partly 170
, its northern section is slightly curved. The escarpment does not form a straight line, but appears to be composed of two directions: NNE and NNW. Five 015
segments, partly arranged enechelon-like, and five NNWtrending sections (azimuth 160–150
) occur in between. All of them are of km-scale. The complete structure of the PPZ covers a width of about 1 km. The mofette line between Hartousˇ ov and Bubla´k is situated on a NNW striking fault segment, Nova´ Ves and Deˇvin are located westwards of the PPZ border line. According to Riedel’s model (1929) the 150–160
fault segments are the R1-shears as 1st order sinistral strike-slip planes, whereas the segments with a trend 015
appear to be the P-shears as 2nd order sinistral strike-slip within the N-S zone (Fig. 12). The segments of the escarpment are comparable with results from sand box experiments simulating strike-slip movements in sediments above a basement fault (Mandl, 1988). From sections of the escarpment it can be supposed that additional conjugate R2-shears are developed and also NE-trending thrust faults. The described inventory of the strike-slip model appears to be reflected by the relief (PPZ) and the drainage patterns of the Cheb Basin, and partly by fault structures in hard rock exposures

Fig. 12. (a) Riedel’s model of active shear planes within a strike-slip fault zone. (b) Segments of the detected N-S escarpment of the PPZ shear zone in the epicentral area Novy´ Kostel of the Tertiary Cheb Basin, NW Bohemia.

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(north of Novy´ Kostel). Similarities exist between the structure of the Pocˇatky-Plesna´ fault zone and other seismically active N-S zones with sinistral shear sense in Central Europe. 4.2. Threedimensional interaction of fault structures with seismic events The detected new faults (PPZ and NVF) are of importance for the understanding of the crustal structure and its discontinuities which appears to be associated with the stress release during seismic events. The geometry of the PPZ coincides with the distribution of the epi- and the hypocentres. But of further importance is the configuration of both prominent fault zones within the seismic active region: the PPZ and the MLF. Both faults intersect each other and form wedges (triangles) south and north of their intersection point at the surface. The intersection point near Novy´ Kostel is the end of an hidden intersection zone between the two faults, that arises more and more to north and reaches the surface near Novy´ Kostel (Fig. 13). They dip towards east and SSW, respectively, so they cross another at greater depth, but not along their whole length, because the angle and the distance between both faults increase to the south (Fig. 13a).

Fig. 13. Scheme of the geometric configuration of the PPZ and the Maria´nske´ La´zneˇ fault. (a) Schematically illustrated hypocenter clusters along the PPZ are added at depths between 6 and 12 km, to the south and north of Novy´ Kostel according to Hora´lek et al. (2000). (b) Crustal wedges enveloped by the two faults with various local stress conditions south and north of Novy´ Kostel. A-A’, Intersection zone between PPZ and MLF. (a) and (b) out of scale.

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The depth of intersection of both faults depends on the dip angles which are not known exactly, but can be estimated from surface exposures. With supposed dip angles of about 80
an intersection is possible: at 12 km depth below Bubla´k-Kacerˇ ov, at about 7 km depth below Milhostov-Hluboka´, at 5 km depth below Mly´nek-Kopanina, at 1.5 km near Lesna´, and zero near Novy´ Kostel. The hypocentres occur at depths dominantly between 6 and 12 km (Hora´lek et al., 2000). In the earthquake swarm periods 1997 and 2000 the epicentres were arranged N-S in a very narrow stripe, approaching at the hypocentre depth the PPZ shear zone (Fig. 13a). Both faults envelope crustal wedges of several kilometer extension. The shape of the wedges north and south of the crossing point is different. The southern wedge narrows towards north correlating with decreasing depth. The northern wedge is extremly narrow below Novy´ Kostel, interfingering there with the southern wedge, and is widening to the north and with increasing depth (Fig. 13b). Specific stress conditions can arise within the wedges. Taking into account the possible displacements along these faults, different local stress conditions south and north of Novy´ Kostel are to be expected. Within the recent stress field (main principal stress axes  1 > 2 >  3; max. horizontal stress axis SH NW) preferentially the MLF should react by extension, and the PPZ by compression and sinistral slip. The result may influence the local stress regime and the mechanisms of small swarm earthquakes. The intersection zone A-A0 could react as an additional crustal discontinuity if both of the intersecting fault zones are recently active. This crustal feature can interact with recent processes superimposing the crustal behaviour. Within the wedges gas emanating from the mantle can be gathered. 4.3. Forced folds along the dextral ENE shear zone, probably connected to seismic events Shear folds with southeastwards vergency within a pure clay succession (thickness: 5 m) are restricted to the Nova´ Ves shear zone. The NW limbs of the m-scale folds are partly thinned by shearing to the south, accompanied by clay injections into the shear bands, that can be detected by the different colours of the clay layers (Fig. 14a, b). The antiform axial surface traces are north dipping (50–25
; Fig. 14c). The strong and regular southern vergency of the sheared, partly flame- or injection-like folds along shear planes (thrust faults) have developed independently of compaction processes, confined above the mentioned large-scale ENE transcurrent fault in the Nova Ves II mine (Fig. 9). The eastern quarry-face of the mine, where the shear folds are exposed, cross-cuts the ENE shear zone nearly in perpendicular direction. The asymmetric inclined folds disappear apparently to the north at a distance of ca. 15 m from the fault, to the south at a distance of ca. 100 m (referring to the conditions of exposure within the mine in 2001). Consequently, the folding is asymmetric distributed above the transcurrent fault. The folds are developed in a broader stripe at the southern flank. The shear fault-related nature of the folds is evident by its occurrence only above the dextral ENE fault. Shear along a basement wrench fault induces compression in the cover rocks and may produce en echelon folds (Sattarzadeh et al., 2000) as in the Nova Ves II mine. The deformation of the Upper Pliocene clay layer is probably confined to a short time interval under minor burial, during an earthquake period (Allen, 1986) or wrench movement, since the pattern was truncated by erosion and buried by undeformed younger strata that had not yet been deposited at the time of the mentioned short event.

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Fig. 14. (a) Strata-bounded folding: Forced folds restricted to a clay sequence (ca. 5 m thickness); a brighter horizon of sandy clay is intercalated. Section: 50 m long, from a face of 120 m length. (b) Offsets (20 to 80 cm) of the upper dark clay due to normal faulting; detail from the northern prolongation of the face in (a). Arrows: Faults and injections of darker into lighter clay and vice versa. (c) Scheme of the Nova´ Ves shear zone (70
–80
) and forced folds (a) that were formed within in a relatively short time interval, confined by a layer succession. The folded horizon is overlain by horizontal strata which are not deformed by folding. Shear movement along a basement wrench fault induces compression in the cover rocks and may produce en echelon folds. Due to the recent mining stage the inclined folds were visible along the eastern exposed section of the shear zone. Fold axial surfaces traces northwest dipping (50–25
). Folds not to scale.

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5. Conclusions Recent crustal movements in Central Europe are the result of collision between the North Atlantic-European and Adriatic Plates, respectively. The main reaction to this compression results in wide-spanned bending of basins and vertical movements of massifs. (1) Due to the collisional compression the relative displacements within adjacent and remote areas are concentrated to systems of shear zones. Some N-trending left-lateral strike slip zones are associated with regionally important earthquake activity. These fracture zones are composed of many individual faults of several km length, independent of each other. (2) The four strike slip zones ‘‘Western Vosges’’, ‘‘Eastern Upper Rhinegraben’’, ‘‘Western Swabian Jura’’ and ‘‘Western Bohemia/Vogtland’’ are similar considering the fault plane solution and the depth distribution of foci. But there exists a remarkable difference between the ‘‘Western Bohemia/Vogtland’’ area and the other mentioned seismically active areas: This concerns the magnitude distribution, i.e. their ‘‘swarm’’ character caused by an interaction between shear zone motion and CO2 transport from the subcrustal mantle through the crust to the Earth’s surface. The earthquakes occur in all mentioned N-S zones at intersection areas between N-S and crossing faults. (3) In the Cheb Basin (western Bohemia) several new fault zones were detected. They have formed escarpments and modified the drainage pattern. One of them (PPZ) coincides with the main epicentral area of the Western Bohemia/Vogtland region at Novy´ Kostel. (4) Evidence for the age of the beginning of the fault activity in the epicentral area of Novy Kostel comes from the development of the PPZ escarpment in Late Pleistocene time (Wu¨rm period). This escarpment is the site of most of the recent earthquakes in the NW Bohemia/Vogtland region. The scarp hints at the beginning of the earthquake activity in the epicentral area of Novy´ Kostel with 120000 years in the maximum and 12000 years in the minimum. (5) Also other faults indicate young crustal activities. A dextral shear zone of the Cheb Basin reveals a deformation fabric rich in shear indications along individual wrench fault planes including degassing channels of micrometer scale diameter, where CO2 from the uppermost mantle emanates.

Acknowledgements This study has been supported by the Deutsche Forschungsgemeinschaft (DFG), grants BA 1184/8-2, SCHN 251/2-2 and KA 902/7-1. We thank K. Bra¨uer (UFZ Halle) and J. Tesarˇ (Frantisˇ kovy La´zneˇ) for analyzing the gas samples, M. Sto¨rr (Usedom) for SEM investigations, the Wismut GmbH (Chemnitz), especially A. Hiller and M. Schauer, for kind assistance regarding to the use of the archives of this institution. We are grateful to T. Plenefisch, K. Klinge, (Seismolog. Observatory Erlangen), Z. Ska´celova´ and J. Nehybka (Inst. of Physics of the Earth, Brno University), J. Horale´k, A. Bousˇ kova´, L. Sˇpicˇakova´ (Geophys. Inst. Acad. Sci. Praha) for fruitful discussion. We thank KEMAT, Skalna´, for the permission to work in their open mines.

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