Recent Reactivation of Variscan Tectonic Zones: A Case of RodlKapliceBlanice Fault System (Bohemian Massif, Austria/Czech Republic)
Abstract
The Rodl-Kaplice-Blanice fault system (RKB) of Variscan shear origin, repeatedly active since the Late Paleozoic to the Recent, is expressed by a number of lithological contacts, distinct geophysical gradients and many landforms. A general trend of the RKB as well as linear configuration of its internal architecture is fairly similar to those of topical near Rhine Graben and Alpine-Carpathian transition area as the two other consistent recently reactivated large-scale tectonic structures in the extended (thinned) crust of central Europe. In middle part of the RKB, the occurring linear topographic and geological features parallel to the main RKB sections point to the existence of a wide tectonic zone in the crust following the fault system. Our multidisciplinary study includes a summary of corresponding basic geological data, overview of seismic, regional geophysical and geomorphological conditions, primary model of recent kinematic activity in the RKB area derived from the space (Global Navigation Satellite System-GNSS) monitoring and terrestrial (repeated high precision levelling) geodetic data and comparison of these various information. The obtained knowledge indicates that the RKB is active up to similar to 1.0 mm horizontally and > 0.5 mm vertically. The fault system area in the Bohemian Massif can be subdivided into the three parts of diverse tectonic structure and block kinematics. Sinistral horizontal movements are highest near the southern surface sections (Rodl-Kaplice, Rudolfov and Drahotesice faults), whereas noticeable vertical differentiation is going on mainly along the Blanice and Kourim faults in the north where the RKB activity is gradually decreasing towards the extensive Elbe shear zone with transverse movements. The middle part of the RKB is dislocated by a large active transverse tectonic structure of the South Bohemian Basins (SBB) with variable horizontal velocity vectors of surface GNSS stations. Most of the weak regional earthquakes have been recorded west of the RKB. Besides faults of the SBB, these were mainly associated with the RKB-subparallel Lhenice fault. Based on the earthquake distribution and foci depths, the latter fault can have similar structural position as the RKB related to lower part of the Variscan level in the similar to 10-12 km depth.
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Vol.:(0123456789) Surveys in Geophysics (2024) 45:609–661 https://doi.org/10.1007/s10712-023-09811-x 1 3 Recent Reactivation ofVariscan Tectonic Zones: ACase ofRodl‑Kaplice‑Blanice Fault System (Bohemian Massif, Austria/Czech Republic) PavelRoštínský1 · LubomilPospíšil2,3 · OtakarŠvábenský3 · AnastasiiaMelnyk4 · EvaNováková1 Received: 10 March 2023 / Accepted: 30 September 2023 / Published online: 22 February 2024 © The Author(s) 2023 Abstract The Rodl-Kaplice-Blanice fault system (RKB) of Variscan shear origin, repeatedly active since the Late Paleozoic to the Recent, is expressed by a number of lithological contacts, distinct geophysical gradients and many landforms. A general trend of the RKB as well as linear configuration of its internal architecture is fairly similar to those of topical near Rhine Graben and Alpine-Carpathian transition area as the two other consistent recently reactivated large-scale tectonic structures in the extended (thinned) crust of central Europe. In middle part of the RKB, the occurring linear topographic and geological features parallel to the main RKB sections point to the existence of a wide tectonic zone in the crust following the fault system. Our multidisciplinary study includes a summary of corresponding basic geological data, overview of seismic, regional geophysical and geomorphological conditions, primary model of recent kinematic activity in the RKB area derived from the space (Global Navigation Satellite System—GNSS) monitoring and terrestrial (repeated high precision levelling) geodetic data and comparison of these various information. The obtained knowledge indicates that the RKB is active up to ~ 1.0 mm horizontally and > 0.5mm vertically. The fault system area in the Bohemian Massif can be subdivided into the three parts of diverse tectonic structure and block kinematics. Sinistral horizontal movements are highest near the southern surface sections (Rodl-Kaplice, Rudolfov and Drahotěšice faults), whereas noticeable vertical differentiation is going on mainly along the Blanice and Kouřim faults in the north where the RKB activity is gradually decreasing towards the extensive Elbe shear zone with transverse movements. The middle part of the RKB is dislocated by a large active transverse tectonic structure of the South Bohemian Basins (SBB) with variable horizontal velocity vectors of surface GNSS stations. Most of the weak regional earthquakes have been recorded west of the RKB. Besides faults of the SBB, these were mainly associated with the RKB-subparallel Lhenice fault. Based on the earthquake distribution and foci depths, the latter fault can have similar structural position as the RKB related to lower part of the Variscan level in the ~ 10–12km depth. Keywords Rodl-Kaplice-Blanice active shear zone· Geophysics· Geomorphology· Global navigation satellite system· Recent kinematics· Foci Extended author information available on the last page of the article
610 Surveys in Geophysics (2024) 45:609–661 1 3 Article Highlights • Integration of various geoscience data links surface movements to deeper crustal structures and seismicity • Global Navigation Satellite System data can be used for compilation of geodynamic model of slowly deformed areas • Major Variscan deep-seated faults in the Moldanubian unit of the Bohemian Massif manifest recent tectonic activity • Movement along the study fault system can be considered a potential geodynamic risk for significant state infrastructure 1 Introduction Many deep-seated fault systems in the Variscan crust of Europe, characterised by juxtaposed different structural levels, have been reactivated in various ways during the later Alpine orogeny and currently represent significant features of extension (thinning) or transtension. The repeated tectonic activity occurred in a consequence of spatial similarity of both the evolutionary phases largely forming outer Earth’s layers of the continent, inherited crustal anisotropy and changing stress conditions exerted by moving lithospheric blocks including compressional, extensional or strike-slip regimes (e.g. Ziegler 1990; Scheck etal. 2002; Schumacher 2002; Michon etal. 2003; Uličný etal. 2009; Vauchez etal. 2012; Egli etal. 2017). A number of these important tectonic zones have been developing until the present time (Peterek etal. 2011; Coubal etal. 2015; Štěpančíková etal. 2019; Špaček etal. 2022) and some faults are even inducing the recent seismicity (Edel etal. 2007; Roštínský etal. 2013; Špaček etal. 2015; Babuška etal. 2016; Pospíšil etal. 2019). Hence, they can represent a hazard for humans including fundamental infrastructure and are significant objects of multidisciplinary research. The same priority currently exists in the area of the Bohemian Massif (easternmost Variscides) fragmented by large tectonic zones into many crustal blocks (e.g. Röhlich and Šťovíčková 1968; Šťovíčková 1973; Blížkovský etal. 1975; Zeman 1978; Dvořák 1985; Matte etal. 1990; Brandmayr etal. 1995; Alexandrowski etal. 1997; Pitra etal. 1999; Cháb etal. 2010; Badura and Rauch 2014), where a number of geodynamic studies have focused on Quaternary activity of such fault systems. As outlined in Sect.1, these works investigated diverserelated phenomena or applied different methodological approaches. Hence, they represent well the extensive scope of research of recently reactivated tectonic zones worldwide. Both direct and indirect evidence of ongoing tectonic development were addressed. Several data pointing at strong influences of external (exogenous) processes on present-day geodynamic conditions in the central European crust (e.g. mass wasting or post-glacial isostatic rebound), frequently outweighing manifestations of movements along the fault systems coming from the Earth’s interior, have been published as well. The highest attention in the Bohemian Massif has been paid to research of the significant Western Bohemia / Vogtland seismic area (western part of the province; Fig.1a) with frequent earthquake swarms (depths mostly 5–14km; max. instrumentally recorded magnitude ML ~ 4.4; Neunhöfer and Hemmann 2005; Fischer etal. 2014; Babuška etal. 2016; WEBNET 2022). The majority of earthquakes are related there to the N–S trending sinistral fault zones (Bankwitz etal. 2003), based on focal mechanisms conjugated with the WNW–ESE dextral faults (Havíř 2000; Vavryčuk etal. 2013). Holocene movements dated
611 Surveys in Geophysics (2024) 45:609–661 1 3 Fig. 1 a Location of the Rodl-Kaplice-Blanice fault system (RKB). BCB—Bohemian Cretaceous Basin; UMB—Upper Morava Basin. Other main recently active shear zones in the southern Bohemian Massif: DO—Donau fault system; PL—Pfahl Line fault system; VP—Vitis-Přibyslav fault system; DB—DiendorfBoskovice fault system. Important fault-related phenomena in the northern Bohemian Massif: ER—Eger Rift; ML—Mariánské Lázně fault system; ESZ—Elbe Shear Zone; HP—Hronov-Poříčí fault system; SM—Sudetic Marginal fault system; BF—Bělá fault system. Main recent seismic regions in the Bohemian Massif: WBV—Western Bohemia / Vogtland (NK—most active Nový Kostel focal zone); Sudetes Mts. with adjacent areas. b Main tectonic elements in the study area along the RKB. The selected plutonic phenomena are visualised according to Klomínský etal. (2010): CBP—Central Bohemian Plutonic Complex; MPC—Moldanubian (South Bohemian) Plutonic Complex; KM—Klenov Massif; ŠM—Ševětín Massif. KHC—Kutná Hora Crystalline unit. Faults, basins and horsts: RKf—Rodl-Kaplice fault; Rf— Rudolfov fault; Df—Drahotěšice fault; Bf—Blanice fault; Kf—Kouřim fault (all as sections of the RKB); DO—Donau fault; PL—Pfahl Line fault; Lhf—Lhenice fault; Ksf—Karlstift fault; South Bohemian Basins: Duf—Dubné fault; HDf—Haklovy Dvory fault; Zbf—Zbudov fault; Hf—Hluboká fault; Dof—Dobřejovice fault; Lif—Líšnice fault; Sf—Stropnice fault; BB—Budějovice Basin; LH—Lišov Horst; TB—Třeboň Basin. Macroseismic and instrumental earthquake foci were added after Špaček etal. (2011). KTB—location of superdeep well (up to 9101m) drilled within the German Continental Deep Drilling Programme
612 Surveys in Geophysics (2024) 45:609–661 1 3 inpaleoseismic trenches crossing the deep-seated Mariánské Lázně fault (Moldanubian / Teplá-Barrandian unit structural boundary) indicated there ongoing activity also of this NNW–SSE-oriented tectonic zone limiting the uplifting Erzgebirge Mts. (also referred to as Krušné hory Mts. in the Czech territory) against the Cheb Basin (Štěpančíková etal. 2019; max. estimated earthquake magnitude Mw ~ 6.3–6.5). The Quaternary activity of the latter zone was indirectly supported by anomalies in the regional fluvial network southwards: vertical (up to 20m high) differences of analogous Ohře river terraces on the opposite Mariánské Lázně fault strands (Balatka etal. 2019) or places of young river piracy and increased stream incision into uplifting blocks along the middle fault section (Flašar and Štěpančíková 2022a). A dextral strike-slip regime along the fault in the Plio-Pleistocene period was mostly considered (e.g. Švancara etal. 2000; Schenk etal. 2012; Andreani etal. 2014; Štěpančíková etal. 2019). However, several authors supposed non-negligible sinistral movements instead (cf.Špičáková etal. 2000; Peterek etal. 2011). Additionally, a large part of the Western Bohemia / Vogtland seismic area is located within the WSW–ENE striking Eger Rift as a component of the European Cenozoic Rift System expressed by extensive volcanism and many related phenomena (e.g. Ziegler and Dèzes 2007; Ulrych et al. 2013). The > 30-km-wide rift structure extends across the northern Bohemian Massif along the reactivated Teplá-Barrandian (Bohemicum) / Saxothuringian unit boundary. The youngest volcanic occurrences (middle Quaternary basalt hills and maar-diatreme volcanoes; Wagner et al. 2002; Mrlina et al. 2009; Rohrmüller etal. 2017) exist near the western margin of the Cheb Basin at the crossing of both mentioned deep-seated boundary structures. The persistent tectonic activity of the western Eger Rift is evidenced also by high heat flux associated with a number of thermal springs (max. temperature > 73°C in Karlovy Vary) and places of CO2 degassing, specifically with increased radon concentration (Hanzlík 1998; Geissler et al. 2005; Weinlich etal. 2006). The Eger Rift was active in the Quaternary also in its middle part, as supported by slickensides found there in middle Pleistocene loess and indirectly by a few fault deformations known in late Pliocene fluvial accumulations (Coubal and Adamovič 2000) or by a few existing thermal springs near the volcanic České středohoří Mts. As indicated by the results of morphotectonic analyses (Peterek etal. 2011; Andreani etal. 2014), several crustal blocks alongside the Eger Rift have been uplifted during the Quaternary against a morphological graben in axial part of the structure. The second important recently active area in the Bohemian Massif involves the Sudetes Mts. (NE part of the province; Fig.1a), oriented fairly consistently with a general trend of the Elbe-Oder fault system which extends across a large part of central Europe (e.g. Arthaud and Matte 1977; Scheck et al. 2002; Špaček et al. 2015). The corresponding boundaries between several structural units (crystalline Lugian, Teplá-Barrandian, Moldanubian and sedimentary Rhenohercynian) occur within this zone (Dvořák 1985; Cháb etal. 2010; Coubal etal. 2015). The prominent Upper Morava Basin system along the Haná fault, evolving in the regional dextral transtensional regime, also belongs to the active area (Grygar and Jelínek 2003; Špaček etal. 2015). A subsidence in middle parts of this graben structure has been associated with deposition of Plio-Pleistocene fluvial and lacustrine sediments, whose preserved thickness reaches up to 320m (Špaček etal. 2015). The possible long-lasting landsliding detected in paleoseismic trench across the Kosíř fault in western part of the basin has obscured unequivocal young internal tectonic effects (Špaček et al. 2017). Close geodynamic relations of the Upper Morava Basin crossing the Bohemian Massif / Western Carpathians boundary to both the Bělá fault system (east) and the Nectava fault system (west) in adjacent segments of the Bohemian Massif were indicated by similar NW–SE trending belts of weak earthquakes (common
613 Surveys in Geophysics (2024) 45:609–661 1 3 depths 9–18 km; max. instrumentally recorded magnitude ML ~ 3.8; Havíř 2002, 2004; Jelínek 2008; Špaček etal. 2015; Pospíšil etal. 2017, 2019) as well as by occurrences of numerous carbonated springs and a regionally increased CO2 flux (Špaček etal. 2006). The youngest, presumably Bělá fault-related basaltic rocks in a small Pliocene to Early Pleistocene volcanic field near Bruntál (eastern Sudetes Mts.) were dated to 0.8–1.0Ma (Šibrava and Havlíček 1980; Ulrych etal. 2013). Another significant regional structure is the deep-seated Sudetic marginal fault, expressed on the surface as a prominent ~ 140-km-long NE front of the middle Sudetes Mts. with several generations of facets (Badura etal. 2007). It truncates middle and late Pleistocene terraces of the transverse Nysa Kłodzka river; 10–25 m high scarps have developed within them (Krzyszkowski etal. 2000). A number of reverse, normal or strikeslip deformations evolved along this fault in the Quaternary, as revealed by structural conditions in transverse and longitudinal paleoseismic trenches (Štěpančíková etal. 2010). Although dextral movements prevailed during the earlier fault history, sinistral strike-slips related to the Late Pleistocene post-glacial unloading appeared to be the last documented kinematic events (Štěpančíková et al. 2022). The ongoing regional tectonic activity is indicated by thermal springs in the adjacent Góry Złote Mts. (referred to as Rychlebské hory Mts. in the Czech territory) near the SE fault section (temperature up to 44 °C; Dowgiałło 2002). The third main active part of the Sudetes Mts. includes the Hronov-Poříčí fault and adjacent structures (earthquake depths mostly 5–15km; max. inferred historic magnitude Mw ~ 4.7; Schenk etal. 1989; Valenta etal. 2008; Špaček etal. 2015). The largest cluster of cold carbonated or thermal springs in the Polish Sudetes Mts. is located in the SE continuation of the Hronov-Poříčí fault (Duszniki area; Dowgiałło 2002). However, within a near > 1600-m deep boreholes in the northern Sudetes Mts. close to the eastern Eger Rift the water temperature reaches nearly 98°C. A high heat flux was detected in the region as well. Both phenomena can be related to subsurface Quaternary volcanism (Dowgiałło 2002; Ulrych etal. 2013). Additionally, deformations of speleothems were measured in several caves in the Sudetes Mts. area (Briestenský etal. 2014; Bábek etal. 2015; Szczygieł etal. 2021). The existing damages, several dated by various methods, were indirectly attributed to the Late Quaternary seismicity associated with an activity of the particular near main fault systems. Much less features related to the active tectonics have been evidenced in the central and southern Bohemian Massif. The Quaternary movements accompanied by the locally increased seismicity (max. estimated magnitude Mw ~ 3.5; Špaček et al. 2022) were considered along deeper levels of the Diendorf-Boskovice fault (Roštínský etal. 2013). This fault separates at the surface the Bohemian Massif from the Western Carpathian foreland basin (Roštínský and Roetzel 2005), whereas in the footwall the Moravo-Silesian unit is limited against the Moldanubian and Brunovistulian units (Roštínský etal. 2013 and references therein). The recent subsidence in the Budějovice Basin was supported by Homolová etal. (2012; sedimentological study) and Popotnig etal. (2013; morphotectonic study). However, presumable transverse fault offsets or intact sedimentary bodies deposited at the end of the Pleistocene above the fault traces seemingly contradicts an ongoing activity of both mentioned tectonic structures (Špaček etal. 2017, 2022). A distribution of the existing stress field in central Europe, induced by the continuing northward collision of Africa with the Eurasian plate, a transmission of the MidAtlantic Ridge push and locally by orogenic factors in the Alps or the Carpathians, drove there the recent dominant normal and strike-slip fault kinematics. While in the northern Bohemian Massif the NW–SE to NNW–SSE direction of SH similar to that
614 Surveys in Geophysics (2024) 45:609–661 1 3 in the Western European stress domain largely occurs (Peška 1992, 1993; Havíř 2004; Ziegler and Dèzes 2007; Vavryčuk etal. 2013; Špaček etal. 2015; Stemberk etal. 2019; Jarosiński etal. 2021), in its southern part the N–S to NNE–SSW stress was inferred related to movements of frontal segments in the Eastern Alps and adjacent foreland basins (Reinecker etal. 2010; Levi etal. 2019). Various indicators have been used in the Bohemian Massif for the determination of stress direction, mostly focal mechanisms and borehole parameters. In several areas, a few different stress orientations were calculated (e.g. Havíř 2004; Jarosiński etal. 2021). A highly reliable information about recent regional horizontal movement tendencies of crustal blocks provide data from the Global Navigation Satellite System (GNSS) monitoring. A construction of the GNSS networks on all continents began in late 1980s. Soon, the results allowing to identify hazardous active structures were obtained (e.g. Grenerczy etal. 2000; Grenerczy 2002; Hearn etal. 2010; Tong etal. 2013; Kierulf 2017; Tamay etal. 2021). After the regional permanent EPN network was built in Europe, epoch networks focused there on study of particular active fault systems (Hefty 2007; Ziegler and Dèzes 2007; Devoti etal. 2014; Lyros etal. 2021). The increasing number of GNSS monitoring stations enabled to unify the surface data for larger areas also in the Bohemian Massif (Kontny etal. 2004; Schenková etal. 2007; Švábenský etal. 2012; Roštínský etal. 2020), many along tectonically active faults (Wendt and Dietrich 2003; Schenk et al. 2009; Schenková etal. 2009; Roštínský etal. 2013; Kapłon etal 2014; Švábenský etal. 2014; Pospíšil etal. 2017). During the recent time, we processed and unified the data from permanent GNSS stations in southern part of the province focusing on geodynamics of the large-scale sinistral Rodl-Kaplice-Blanice fault system (RKB; Fig.1b). Although only low seismicity occurs in the adjacent area (ML < 2.5; Kárník et al. 1981; Lenhardt etal. 2007; WEBNET 2022), our results combined with the earlier estimation of regional Recent Vertical Movements (RVM; Vyskočil 1996) indicated non-negligible ongoing horizontal and vertical movements also along this tectonic zone. The middle part of the RKB runs near (up to < 20km) the Nuclear Power Plant Temelín (ETE) as a significant component of the Czech energy infrastructure. Hence, the research of recent regional geodynamics should follow the valid International Atomic Energy Agency Safety Standards for protecting people and the environment, primarily the ‘Seismic Hazards in Site Evaluation for Nuclear Installations’ (International Atomic Energy Agency Safety Standards Series 2022; Specific Safety Guide No.SSG-9) in order to relate its results to the knowledge from similar studies in the surrounding areas. In our paper, we additionally evaluated and compared selected data from the disciplines recommended in the mentioned Standard. Our review study extends the current RKB-related knowledge by a: 1. summary of basic information about this fault system provided by geology, yet subdivided in a lot of previous studies; 2. overview of regional geophysical and geomorphological conditions based on newly processed and visualised existing data as a significant additional information about the RKB structure; and 3. compilation of regional kinematic model, primarily based on the GNSS monitoring data as a piece of information not yet considered in assessment of the recent RKB geodynamics; it took into account also selected repeated high precision levelling, seismological, topographic and geophysical data. The main kinematically different areas were distinguished.
615 Surveys in Geophysics (2024) 45:609–661 1 3 2 RKB Setting, Evolutionary History andInternal Structure 2.1 General Setting The RKB is located mostly in the Moldanubian unit of the Bohemian Massif. It extends on the surface > 200km in the SSW–NNE direction between the Cenozoic Alpine Molasse Basin and the Bohemian Cretaceous Basin (Fig.1a; Rajlich 1990; Wallbrecher etal. 1993; Brandmayr etal. 1995; Chábet al. 2010; Špaček etal. 2022). We assessed a ~ 200 × 80km large belt along the fault system (RKB study area; Fig.1b). The southernmost section of the tectonic zone ~ 50-km-long is hidden beneath the foreland basin close to the presentday Alpine orogenic front (Wagner 1998; Pfleiderer et al. 2016; Hintersberger et al. 2017). To date, the RKB and transverse faults have been studied in detail only in selected subareas, mostly in the Temelín region (primarily for the Nuclear Plant safety assessment; e.g. research reports by Decker et al. 2010a, b; Špaček et al. 2011; Prachař 2014; Navrátilováand Nol 2018). The investigation of the fault system as a whole has focused mainly on its geological aspects. Works mutually evaluating outputs from more disciplines have been scarce there and devoted only to smaller structures or areas. Besides the RKB, the following generally SSW–NNE trending sinistral fault systems (shear zones) developed in the southern Bohemian Massif during final stages of the Variscan orogeny in the Late Paleozoic (Fig.1): Vitis-Přibyslav (Veselá 1976; Lenhardt et al. 2007; Špaček et al. 2022), Diendorf-Boskovice (Schermann 1966; Figdor and Scheidegger 1977; Roetzel 1996; Roštínský and Roetzel 2005; Roštínský et al. 2013; Pospíšil etal. 2017; Paoletti etal. 2022) and smaller Lhenice (Mísař etal. 1983; Rajlich etal. 1986; oriented approximately N–S), Karlstift (Brandmayr etal. 1999) or Freyenstein (Griesmeier et al. 2020). In the south, the RKB terminates the transverse WNW–ESE to NW–SE striking Pfahl Line and possibly also Donau dextral strike-slip fault systems (Thiele 1961; Führer 1978; Brandmayr etal. 1995; Wagner 1998; Pitra etal. 1999; Büttner 2007; Žák etal. 2014) as two significant components of a wide subparallel tectonic zone currently extending from the Rhine Graben in the Alpine foreland to the front of the Alpine orogen (e.g. Ziegler and Dèzes 2007; Pfleiderer etal. 2016), yet poorly constrained. Both the orthogonal sets of Late Palaeozoic fault systems, primarily arranged as kinematically conjugated, played a role during regional geodynamic evolution in the subsequent periods until the Recent (Malecha and Pícha 1963; Malkovský 1979, 1980; Schröder 1987; Brandmayr etal. 1995; Popotnig etal. 2013). Brittle elements (strike-slip, normal and reverse faults) have developed within the RKB (Malecha 1964; Malkovský 1979; Vrána and Bártek 2005; Kadlec 2017) besides ductile deformations (Brandmayr et al. 1995; Büttner 2007; Iglseder 2013). During the ~ 303–280Ma period (Late Paleozoic), deformations were related to the three main kinematic regimes: compressional, sinistral strike-slip (transtensional) and extensional (Zachariáš and Hübst 2012; Kadlec 2017; Bárta etal. 2021). At that time, sinistral horizontal displacement of the Bohemian Massif blocks along the RKB could reach up to 35km (Hintersberger etal. 2017). However, lower displacement values are commonly considered based on the present-day distance of corresponding rock bodies on the opposite fault strands or other indicators (cf.Fuchs etal. 1968; Brandmayr etal. 1995; Vrána etal. 2005; Vrána and Bártek 2005; Büttner 2007; Iglseder 2013). An elongated depositional space filled with a thick sequence of Permo-Carboniferous deposits had evolved along the RKB in the main transtensional regime (Zachariáš and Hübst 2012). The formation of the SSW–NNE trending shear zones and corresponding elongated Permo-Carboniferous
616 Surveys in Geophysics (2024) 45:609–661 1 3 basins, as well as an origin of many magmatic bodies in the Bohemian Massif surrounding the RKB (see Sect.2.3 and Fig.1b) are considered the main manifestations of gravitational collapse of the Variscan belt (e.g. Bárta etal. 2021). Besides the corresponding longitudinal faults of N–S, SSW–NNE, SW–NE or WSW–ENE directions related to common RKB trends, a number of transverse (to oblique) sets of mostly NW–SE to NNW–SSE and less W–E to WNW–ESE faults have developed within the fault system area in the Late Paleozoic afterwards and during the following periods (Wallbrecher etal. 1993; Brandmayr etal. 1995; Wagner 1998; Zachariáš and Hübst 2012; Hintersberger etal. 2017). The largest transverse deformation system crossing the middle RKB is that of the South Bohemian Basins (SBB; Figs.1,2; Malecha etal. 1962, 1964; Malkovský 1980; Špaček etal. 2011; Popotnig etal. 2013) with preserved remnants of Late Cretaceous, Oligocene to Neogene and also Quaternary deposits. The SBB are composed of the NW–SE trending Budějovice Basin (BB) and the SSW–NNE elongated Třeboň Basin (TB), separated by the crystalline Lišov (or Rudolfov) Horst. The most significant discontinuities within the basins are the Hluboká fault along the NE rim and the Dubné fault along the SW rim of the BB and the Stropnice fault associated with a NW–SE graben along the SW margin of the TB. The combination of RKB and SBB structural trends has led in the basin system to development of a dense crustal fragmentation with numerous blocks arranged in a little regular ‘parquet-like’ system (Malkovský 1980). The basement of the generally W–E elongated Alpine Molasse Basin near the RKB is cross-cut by numerous transverse faults as well, including the NW–SE to NNW–SSEoriented ones at the southwestern margin of the Bohemian Massif and the SSW–NNE, SW–NE to WSW–ENE striking ones at its south-eastern margin; several of them reach surface in outcropping part of the Massif (Kröll etal. 2006; Pfleiderer etal. 2016). The southern RKB is located in an intersection space of both fault groups (Wagner 1998). The Bohemian Cretaceous Basin near the northern RKB is considered to be developed in a frame of the RKB-transverse WNW–ESE to NW–SE striking Elbe shear zone, a part of the reactivated Late Variscan Elbe-Oder fault system (Malkovský 1987; Uličný etal. 2009; Coubal etal. 2015). The basin was filled with up to > 1000m (currently preserved thickness) Cenomanian to Santonian continental and marine clastic strata. The original basin size and thickness have been heavily reduced by erosion related to the subsequent tectonic movements (e.g. Malkovský 1980; Chlupáč etal. 2002; Danišík etal. 2012). At the present time, most of the Late Cretaceous deposits are preserved in the Labe Lowland area largely Fig. 2 Significant SW–NE to SSW–NNE fault-related lithological contacts dominating the RKB structure with added important lithological boundaries in the SBB. Background: overview 1:500,000 scale Geological map after Kodym etal. (1967). Main RKB sections: 1—Rodl fault; 2—Kaplice fault; 3—Rudolfov fault; 4—Drahotěšice fault; 5—Blanice fault; 6—Kouřim fault; HG—Haselgraben structure (outcropping part). Blue dotted lines—geological sections across the RKB. AMB—Alpine Molasse Basin; BB—Budějovice Basin; LH—Lišov Horst; TB—Třeboň Basin; LG—Lhotice Graben; ČBG—Český Brod Graben; BCB—Bohemian Cretaceous Basin. ČB—České Budějovice; TA—Tábor; VL—Vlašim. ETE—Nuclear Power Plant Temelín. Brief lithology along the RKB-related boundaries: Basement rocks, Moldanubian unit: m—wide belt of mylonised crystalline rocks; 1—granite; 2—paragneiss; 3—granite; 4—paragneiss; 5—granodiorite; 6—paragneiss to migmatite; 7—granite; 8—orthogneiss, 9—paragneiss to migmatite; 10—granulite; 11—migmatite; 12—orthogneiss; 13—crystalline limestone; 14—amphibolite; 15—orthogneiss to metagranite; 16—migmatite; 17—granite to quartz diorite; 18—granite, Kutná Hora Crystalline unit (KHC): 19—mica-schist; 20—orthogneiss to migmatite. Cover rocks: 21—conglomerate, sandstone, siltstone, claystone, locally with limestone (Permian-Carboniferous); 22—conglomerate, sandstone, siltstone, claystone (Cretaceous of the Bohemian Cretaceous Basin); 23—conglomerate, sandstone, siltstone, claystone (Cretaceous of the South Bohemian Basins); 24—sand, silt, clay, diatomite (Neogene of the South Bohemian Basins) ▸
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624 Surveys in Geophysics (2024) 45:609–661 1 3 attributed to various parts of the Oligocene—Miocene period (Přibyl 1999; Chábera and Huber 2000; Krenmayr etal. 2006; Nehyba and Roetzel 2010). 3 Data Sources andMethods 3.1 Regional Geophysics The gravity data, allowing to study density structure of the crust, are available in a high detail for the whole Czech Republic (e.g. Blížkovský and Novotný 1981; Lenhardt etal. 2007). The fault-related density boundaries could be traced in the basic Complete Bouguer Anomaly map, derivative maps or visualisations of vertical density contrasts (Linsser 1967; Lenhardt etal. 2007). Additionally, the interpreted gravity profiles allowed to quantify density boundary parameters. The density contacts in the profiles were confronted with the Stripped Gravity map of the Bohemian Massif (Blížkovský and Novotný 1981), providing a structural image of its crystalline basement also beneath sedimentary basins. In this paper, we used the Complete Bouguer Anomaly map for 2.67g.cm−3 reduction density (100m grid; Geofyzika Brno / Czech Geological Survey—Geofond) and visualised the derivative maps of Vertical Gravity Gradient for Radii (R) of 500m and 4km (Fig.6), Horizontal Gravity Gradient for R of 500m and 3km (Fig.7) and Linsser indications (added in several figures). We created them in the Oasis Montaj Software (Version162; GeosoftLtd. 2015), same as the presented overview magnetic and radiometric maps. Most of the resulting regional figures visualising geophysical, geomorphological or geodetic data were made in the ArcGIS software (EsriInc.). The magnetic data are suitable to detect faults mostly in volcanic or strongly metamorphosed structural units in those places, where high magnetic susceptibility contrasts occur between adjacent rock complexes. They covered the whole RKB area (Geofyzika Brno / Czech Geological Survey—Geofond; Šalanský1995; Aric etal. 1997). In this paper, we present the basic Magnetic map of ΔT (Fig.8), providing an image of distribution of magnetic structures (sources) and their boundaries in the crust. We also considered information in the derivative maps of Vertical Magnetic Gradient, Horizontal Magnetic Gradient, Structural Lines and Euler Deconvolution. The radiometric data, coming from the Radiometric map of the Czech Republic (Mannová and Matolín 1995), reflect natural radioactivity of rocks. The parameters characterising this radioactivity are gamma-ray dose rate (Da[nGy.h−1]) and mass concentration of potassium (%K), uranium (ppmeU) and thorium (ppmeTh). The radioactive field is expressed in Da 1m above the land surface. However, the radionuclide concentration in near-surface autochthonous covers wellcharacterises properties of basement rocks in the depth. Hence, the isoline map constructed with 10nGy.h−1 basic interval highlights large geological objects, whereas the radioactive image of local objects is suppressed. In this paper, we visualise a part of the Radiometric map for the RKB area (Fig.9). Finally, we reviewed archive vertical electric sounding data, reflection seismic data and complex geophysical studies obtained in the SBB and the Bohemian Cretaceous Fig. 6 Map of Vertical Gravity Gradient (mGal/m) in the RKB area with added possibly active faults (Špaček etal. 2011; Navrátilováand Nol 2018), earthquake foci (Špaček etal. 2011) and horizontal velocity vectors of GNSS stations. BG—Blanice Graben. Faults in the SBB area: Tes.f—Těšínov fault; Vlf— Vlhavy fault. Explanation of other fault abbreviations see in Fig.1 ▸
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626 Surveys in Geophysics (2024) 45:609–661 1 3 Basin (research reports by Kadlec etal. 1975; and those in Vacek ed. 1983) to improve the knowledge of location and dip direction of fault planes and support the transverse geological section in the SBB by geophysical curves. Noteworthy, seismic data (distribution of crustal velocities) from the significant regional refraction profiles (CEL09, S04; Hrubcová etal. 2005, 2010) do not offer a useful information for detailed research of the RKB structure. 3.2 Geomorphology The new schematic overview map (Fig.10) illustrates surface expressions of the RKB, SBB and Lhenice fault structures and indicates a linear plan of their surroundings; such a geomorphological view of the study area has not yet been presented. For the figure creation, we used a merged digital elevation model based on airborne scanning (light detection and ranging method) with 10-m grid in the Austrian territory (Land Oberösterreich 2021) and with 5-m grid in the Czech territory (CUZK 2022). The elevation model was partly compared to the satellite Landsat7 ETM + (colour syntheses 321, 531 or753) and SRTM (1arc-second resolution) images, both provided by the NASA/USGS (2023). In the map, we drew over the regional topography manually extracted representatives (elements) of selected significant geomorphological phenomena. Hence, the approach involved a generalisation of all landforms included. We applied various visualisation modes of the data used (contours, digital elevation model, hillshade) during delineation of the elements. The following steps led to indication and assessment of the phenomena spatially concordant or discordant to the main regional faults, primarily of (straight-)linear character. 1. Basic geographical directions were categorised as longitudinal (N–S, SSW–NNE, SW– NE and WSW–ENE) and transverse (W–E, WNW–ESE, NW–SE and NNW–SSE) with regard to the general strike of the RKB. 2. Slope sections (outside valleys), elongated ridges and valley sections as the three basic types of distinct linear landforms at the local scale were searched for in the whole RKB area and drawn in the map according to their prevailing trend within one of the two direction categories mentioned in the step (1). The slopes were delineated as lines at their feet, where the highest changes in gradient commonly occur. We distinguished the two types of slope landforms (length > 500m): scarps (steep surfaces of a high lateral continuity; max. height > 20m, max. gradient > 20°) and significant topographic gradients (more complex higher slopes separating gently inclined lower surfaces from higher surfaces at boundaries of adjacent topographic segments; max. height mostly > 50m). The distinct elongated ridges are represented in the map by their longitudinal axes (length > 500m, max. relative height > 30m). The (straight-)linear valley sections, including both simple landforms and elongated belts of meanders, were drawn as valley floor axes (length > 500m, max. depth > 30m). 3. The three other related phenomena of point character at the given scale were added in the map: distinct saddles, (commonly associated with two counter-directed linear valley heads together forming larger elongated landforms of linear nature; relative Fig. 7 Map of Horizontal Gravity Gradient (mGal/m) in the RKB area with added possibly active faults, location of geological sections across the RKB, earthquake foci (Špaček etal. 2011; IGCAS—Institute of Geophysics of the Czech Academy of Sciences: WEBNET 2022) and horizontal velocity vectors of GNSS stations. Explanation of the fault abbreviations see in Figs.1, 6 ▸
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628 Surveys in Geophysics (2024) 45:609–661 1 3 depth > 25m), abrupt changes (bends) in valley (or stream) direction into trend of a significant fault (RKB section or the Lhenice fault; change in orientation > 30°) and facets (roughly planar triangular or trapezoidal surfaces in a sense by Fairbridge 1968; Keller and Pinter 2002; or Burbank and Anderson 2011). The first type of phenomena was marked only in a vicinity of the significant fault structures (RKB, SBB, Lhenice), the second type only at large rivers near these structures and the third type in the whole area. The features of all three types could be also distinguished as RKB-longitudinal or transverse ones based on their geometric properties, either own (strike of facet surface, trend of valley section subparallel to the fault down the distinct bend) or of supplementary landforms (the case of saddles). 4. Selected similarly oriented linear landforms were spatially grouped in the two ways, based on their relation to the RKB trend and different style of their spatial arrangement: (i) oriented in longitudinal directions and frequently fairly concentrated in narrow linear zones of common 5–15km width; and (ii) oriented in transverse directions and mostly distributed only in wider subareas with one prevailing surface trend. Hence, both the types of groups were marked and termed differently in the map (Fig.10) as (i) longitudinal linear landform zones and (ii) transverse linear landform sets. Additionally, the former groups (longitudinal landform zones) were classified based on a strike similarity to the significant fault structures into the sub-categories I (SW–NE; strike of the Rodl fault); II (SSW–NNE; strike of the Rudolfov, Drahotěšice, Blanice and Kouřim faults); III (approximately N–S; strike of the Lhenice fault); IV (purely N–S; strike of the Haselgraben); and V (WSW–ENE; strike of the Závist-Clay fault system in its NE section). 5. Finally, spatial relations between longitudinal and/or transverse elements were assessed. We schematically highlighted (i) in the whole RKB area abrupt terminations of the longitudinal landform zones against another linear zones or transverse landform sets in those places where the particular linear surface trends do not continue on the opposite strands of the crossing landform groups, and (ii) in the marginal areas of the SBB the main topographic segments (blocks) of a low crustal disruption and relative high surface related to their lower surroundings commonly associated with an increased density of linear landforms. As to the recent tectonic activity, facets and distinct scarps can be the results of the related processes (cf.Keller and Pinter 2002; Burbank and Anderson 2011). However, the geometry of these landforms does not directly indicates the date or the way of their origin. In the slowly deformed areas, strong landform modifications by synand post-genetic surface erosion or significant influences of different rock resistance on the opposite fault strands used to be involved in their development. 3.3 Seismology The regional seismicity was taken into account based on the data in the catalogues of instrumentally recorded earthquakes covering large areas of the Czech Republic (Kárník etal. 1981; Sýkorová etal. 2019; WEBNET 2022). We visualised the earthquake foci in the maps of Fig. 8 Magnetic map of ΔT (nT) in the RKB area. Possibly active faults, location of geological sections across the RKB, earthquake foci (Špaček etal. 2011; IGCAS: WEBNET 2022) and horizontal velocity vectors of GNSS stations are added in the image. Explanation of the fault abbreviations see in Figs.1, 6 ▸
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630 Surveys in Geophysics (2024) 45:609–661 1 3 regional geophysical fields as phenomena related to deeper parts of the crust. Data presented in the significant regional studies or reports (Lenhardt etal. 2007; Špaček etal. 2011; Prachař 2014) were considered as well, including interpretations of earthquake properties. Additionally, we compiled three cross-section models transverse to the RKB with orthogonally projected earthquake foci categorised according to the magnitudes to illustrate a different seismic activity west and east of the RKB, and north and south of the SBB. 3.4 Geodesy 3.4.1 Repeated High Precision Levelling The Czech Republic is covered by the State Levelling Network. Based on the results of remeasurement of the levelling lines of orders I and II in a 30-years interval, Vyskočil (1996) compiled a Recent Vertical Movements map, later reambulated (estimated vertical accuracy L × 0.04 mm yr−1, where L is a distance in km between the particular points). We used a part of the map for assessment of vertical movement tendencies in the RKB area (Fig.11). 3.4.2 GNSS Monitoring The procedures used for the processing of the GNSS geodetic data to assess kinematic behaviour of individual crustal blocks were described in our earlier works (Pospíšil etal. 2017; Roštínský etal. 2020). In this paper, we present the analysis of recent horizontal movement tendencies of 20 suitably spaced active permanent stations of the EPN, CZEPOS, GEONAS, TOPNET, VESOG and VRS-NOW networks in an area of southern Bohemia surrounding the RKB. However, the GNSS data from a larger region were considered. The processed data volume involved for each year two consecutive days (2 × 24h) in the spring as well as in autumn period for available parts of the considered time span (2005–2021; totally 6.1–13.0years for the individual stations). The evaluation parameters included elevation mask angle 10º, CODE precise satellite orbits and Earth’s rotation parameters, CODE absolute offsets and variations of antenna phase centres, QIF ambiguity solution strategies and tropospheric parameters estimated in 1-h intervals. The resulting solution was obtained using an iono-free combination of the phase data on both carrier frequencies (L1,L2). The assessment of the data was done by individual days. The coordinate solution of each day in IGS14 reference frame we obtained by a connecting to the reference (fiducial) GNSS points WTZR, GOPE (IGS network) and CLIB, CPAR, CTAB, LINZ and POUS (EPN network) by minimally constrained adjustment. We used the Bernese Software5.2 (Dach etal. 2015) and reprocessing support products (CODE REPRO 2015). Fig. 9 Part of the Radiometric map of the Czech Republic (Mannová and Matolín 1995; Da values in nGy/h), with added possibly active faults, location of geological sections across the RKB, earthquake foci (Špaček etal. 2011; IGCAS: WEBNET 2022) and horizontal velocity vectors of GNSS stations. Explanation of the fault abbreviations see in Figs.1, 6 ▸
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632 Surveys in Geophysics (2024) 45:609–661 1 3 Fig. 10 Significant geomorphological features in the RKB area. The main linear landforms are categorised as longitudinal (N–S, SSW–NNE, SW–NE and WSW–ENE) or transverse (W–E, WNW–ESE, NW–SE and NNW–SSE) with regard to the general trend of the complex RKB fault system. The uppermost river reaches are missing in the figure because of spatial ambiguity of several major streams. The main RKB sections are the same as in Fig.2. Main general trends (sub-categories) of the longitudinal linear landform zones: I—SW–NE; II—SSW–NNE; III—approximately N–S; IV—purely N–S; V—WSW–ENE. Alpine Molasse Basin: EB—Eferding Basin; LB—Linz Basin. NH—Nové Hrady Mts. SBB—South Bohemian Basins (phenomena indicated by light green map elements, descriptions or masks): BB—České Budějovice Basin; TB—Třeboň Basin, both separated by the Lišov Horst; smaller abbreviations indicate basin segments without Cretaceous sedimentary infill but with preserved larger bodies of Cenozoic deposits; green double side arrows—fairly continuous trend of the southwestern SBB marginal zone against the Bayerischer Wald–Weinsberger Wald Mts. BG—Blanice Graben. ČB—České Budějovice; TA—Tábor. ETE— Nuclear Power Plant Temelín
633 Surveys in Geophysics (2024) 45:609–661 1 3 4 Results 4.1 Geophysical Fields 4.1.1 Gravity The regional gravity field mostly reflects granitoid bodies of the Central Bohemian and Moldanubian plutons inducing large negative anomalies (e.g. Figs.3,5). Many anomalies are related to higher depths of rock bodies reaching 5–8km. A large gravity minimum extends from the middle RKB area towards the south, where it is amplified by Mesozoic and Cenozoic sedimentary cover in front of and within the northern Alps (Wessely etal. 2006; Lenhardt etal. 2007; Pfleiderer etal. 2016). The RKB is reasonably traceable in both presented images of Vertical Gravity Gradient (Fig. 6) and Horizontal Gravity Gradient (Fig. 7). A sharp gradient between paragneiss and granitoids has developed along the Kaplice fault (Fig.5). A zone of distinct gradients occurs also along the southern and middle BG. In the SBB, the gravity field is influenced by transverse horsts and grabens. However, two individual blocks along the Rudolfov and Drahotěšice faults are associated with the SSW–NNE elongated positive anomalies consistent with the dominant RKB trend. In the north, the WNW–ESE to NW–SE-oriented gravity anomalies correspond to the Elbe shear zone geometry. 4.1.2 Magnetics In the magnetic field, a number of consistent anomalies have developed along large parts of the RKB (Fig. 8). The highly fragmented SBB and several surrounding areas are characterised by mostly negative ΔT values. Local positive anomalies reflect metamorphic rocks in the basement, sometimes of ultrabasic character (e.g. the western margin of the TB near the Rudolfov and Drahotěšice faults). The northern Rodl-Kaplice fault sharply separates a positive area in the NW from a smaller negative area in the SE, the latter also limited against another positive anomaly along the SW–NE striking Karlstift fault. The Lhenice fault is followed by a subparallel positive anomaly belt. A trend of the transverse Železné hory fault within the Elbe shear zone corresponds well to the WNW–ESE orientation of another positive anomaly belt north of the Kouřim fault. 4.1.3 Radiometry The variable radioactivity in the RKB area (Fig.9) reflects changing rock lithology and different evolution of the individual crustal units (e.g. Mannová and Matolín 1995). In the Monotonous Group of the Moldanubian unit, biotite and sillimanite paragneiss to migmatite (Da55–75) and mica-schist (Da60–90) are characteristic rocks, frequently with a fairly uniform field of radioactivity values. The metamorphosed rocks of the Moldanubian Variegated Group, represented by quartzite, quartz gneiss, crystalline limestone and amphibolite, form local radioactivity minima (Da35–75). The granulite bodies are associated with a low radioactivity (Da35–60), mainly caused by a low abundance of uranium and thorium (K:2–4%, U:1ppm, Th:1–8ppm). Orthogneiss occurs in smaller bodies of various radioactivity; to the most active ones belong Choustník orthogneisses at the southern BG (Da70–110). The Kutná Hora Crystalline unit, mostly comprising orthogneiss and mica-schist, provides intermediate radioactivity values (Da60–90). The Moldanubian and
640 Surveys in Geophysics (2024) 45:609–661 1 3 Fig. 13 Evaluated annual baseline change rates (mm/yr) and directions (horizontal velocity vectors) of permanent GNSS stations in the RKB area. Explanation of the fault abbreviations see in Figs.1, 6
641 Surveys in Geophysics (2024) 45:609–661 1 3 phenomena primarily considered during the compilation of the regional kinematic model (Sects.5.2,5.3). Although the geomorphological knowledge was based only on the schematic map output, it seems to complete the information about near-surface crustal levels. The indicated dislocation zones are not proposed solely along the main known faults but also in the surrounding RKB area, sometimes in those places where ruptures have not yet been considered in its geological structure. The prevailing SW–NE trend characteristic of the Rodl-Kaplice fault occurs frequently also in case of longitudinal landform zones east of the TB, where it combines with the SSW–NNE striking longitudinal zone elements characteristic of the RKB sections in the SBB and the BG (Fig.10). Hence, the distribution of features of both directions points to a different structural geometry in the wider southern RKB area compared to its middle and northern parts. The significant changes in an arrangement of drainage network just near the Kaplice fault, where a transition between both orientation styles is located, Flašar and Štěpančíková (2022b) attributed to the Pliocene–Quaternary tectonic activity. Additionally, the density of the proposed longitudinal landform zones and facets (as indicators of significant late Cenozoic vertical tectonic activity) is the highest in the Bayerischer Wald Mts. / Weinsberger Wald Mts. boundary area in the Rodl-Kaplice fault vicinity (southern RKB). On the contrary, the most apparent change in trends of the transverse landforms sets occurs in the northern RKB area, where the general NW–SE directions characteristic of the SW part of the Bohemian Massif (Bayerischer Wald–Weinsberger Wald Mts., SBB area) are replaced with the WNW–ESE directions prevailing in the Elbe shear zone (Uličný etal. 2009). The SSW–NNE trending middle RKB is followed in a ~ 30km distance by another belt of subparallel longitudinal zones extending from the eastern TB towards the Sázava river valley in the north, consistently with trends of near gravity or magnetic gradients (Figs.6, 7 and 8). It indicates there existence of a structural zone > 100-km long and > 30-km wide, whose flanks sharply limits Cretaceous deposits preserved in central part of the TB (cf.geological conditions in Fig.2). These spatial parameters approximate a little to those of other large deep-seated tectonic zones within the Bohemian Massif: Elbe shear zone (length > 350 km, width > 50 km; Uličný et al. 2009; Špaček et al. 2015), Eger Rift (length > 300km, width > 30km; Mlčoch and Konopásek 2010; Ulrych etal. 2013; Andreani etal. 2014), West Bohemian shear zone including the Mariánské Lázně fault (length > 150km, width > 20km; Pitra etal. 1999; Zulauf etal. 2002; Peterek etal. 2011) or the Sudetic Marginal fault–Bělá fault system (length > 200km, width > 30km; Buday etal. 1995; Badura etal. 2007; Pospíšil et. al. 2019). The existing terminations of longitudinal landform zones just at the BB, TB or BG margins indicate a block character of all three graben-like structures; the corresponding boundaries possibly run along the recorded faults or their continuations (Hluboká— termination No.3, Dubné—terminations No.4–6, Stropnice—termination No.7, Blanice— terminations No.11,12). Several terminations correspond to geophysical gradients, mainly at the SW limits of the BB and TB (Figs.6, 7, 8 and 9), which points at the deeper character of these boundaries. Both geological (tectonic, sedimentological) and frequently spatially corresponding geomorphological indicators (dense block segmentation) evidence that the SBB areas is much larger (min. 100km × 100km) than that covered with the existing basins with fairly continuously preserved Cretaceous, Tertiary and locally Quaternary sediments (BB and TBs.s.). As to the general RKB trend and high linear complexity of the region surrounding this fault system, the two topical similar large-scale structures with the locally increased seismicity partly related to the ongoing sinistral strike-slip activity occur in near central
642 Surveys in Geophysics (2024) 45:609–661 1 3 Europe, mostly in front of the Alpine-Carpathian orogen: Rhine Graben (~ 400km towards the west) and Alpine-Carpathian transition area (~ 200km towards the east), loosely limited by the Diendorf-Boskovice shear zone and the western Danube Basin (Schumacher 2002; Cloetingh etal. 2006; Decker etal. 2005; Edel etal. 2007; Ziegler and Dèzes 2007; Roštínský et al. 2013; Pospíšil et al. 2017). Although developed in different geological settings, both structures are also characteristic of increased concentration of SW–NE, SSW–NNE and even N–S linear features predisposed by extensive (continental-scale) fields of consistent linear ruptures andgeometric configurations analogous to those of the RKB have been frequently formed within them. Similar geological and geomorphological linear elements are abundant also within the SSW–NNE zone running from the Cheb Basin area towards frontal parts of the Alps (~ 200kmto the west; cf.Špičáková etal. 2000;or Štěpančíková etal. 2019). Such a regular structural geometry created by large tectonic zones may exist in other regions of extended crust worldwide. 5.2 Recent Kinematic Conditions The data used for compilation of the presented kinematic model indicated that the RKB is not evolving uniformly and allowed to loosely divide the fault system area into the following three main parts of different structural character and style of recent tectonic activity. 5.2.1 Rodl‑Kaplice Fault Area The complex Rodl-Kaplice fault is composed of a number of subparallel deformations, forming sinistral shear zone (Brandmayr etal. 1995; Dallmeyer etal. 1995). It is manifested by lithological boundaries, mylonite belts, several elongated grabens, geophysical gradients (most distinct in the radiometric field), a number of consistent landforms nearly along the whole fault trace in a belt up to > 5-km wide including several facets and scarps or boundaries of RVM anomalies (Figs.2, 6, 7, 8, 9, 10, 11 and 15). Based on the foliation parameters of mylonite, the Rodl fault is steeply inclined towards the NW (~ 80°; Wallbrecher etal. 1993). The fault plain of the Kaplice fault is similarly inclined (dip 70–80° towards the WNW; Melnyk 2017; Melnyk etal. 2022). Figure5 wellillustrates the relation between paragneisses of the Kaplice unit (west) and the Moldanubian granitoid complex (east). The Pliocene to Pleistocene tectonic activity of the Kaplice fault is indicated by young changes in river network of the surrounding area, deduced from the morphostratigraphic correlation of moldavite-bearing and related fluvial deposits (Flašar and Štěpančíková 2022b). As an important evidence for its recent sinistral strike-slip fault activity, we consider the detected consistent counter movement of the near GNSS stations located on its opposite strands, reaching > 0.6mm yr−1 (Fig.13; CZBO / CKAP and LINZ). Based on processed GNSS data not included in this paper, movements of the GNSS stations in the Fig. 14 Residual Gravity Anomaly map (R = 500m; values in mGal) in the RKB area. Structural lineaments derived from the gravity data are added in the image, as well as possibly active faults, location of geological cross sections, earthquake foci (Špaček etal. 2011; IGCAS: WEBNET 2022) and horizontal velocity vectors of GNSS stations. Explanation of the fault abbreviations see in Figs.1, 6 ▸
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644 Surveys in Geophysics (2024) 45:609–661 1 3 adjacent Alpine Molasse Basin (hidden RKB section area) are decreased and their directions more variable. Numerous subparallel N–S structures in the Rodl-Kaplice fault surroundings, on the surface mostly expressed by streams creating incised valleys and by a few slopes systems Fig. 15 Significant geological and geomorphological features along middle part of the Rodl-Kaplice fault and SE part of the Pfahl Line. The areal geological scheme was created primarily based on the map by Krenmayr etal. (2006). All listed crystalline rocks belong to the Moldanubian unit of the Bohemian Massif. Note a number of differences in surface elevation (ma.s.l.) along nearly the whole length of visualised part of the RKB
645 Surveys in Geophysics (2024) 45:609–661 1 3 with facets (Haselgraben, middle Vltava river valley), can represent rejuvenated Paleozoic phenomena (Iglseder 2013, 2014a, b; Pfleiderer etal. 2016) as Riedel shears related to this section of the RKB. Taking into account the absence of GNSS-derived counter movements in the SE block of the Rodl-Kaplice fault, the subparallel N–S valley structures have likely developed in a W–E extensional regime associated with late Cenozoic evolution of the broader Bayerischer Wald–Weinsberger Wald Mts. region where a complex horst-andgraben structure occurs (Fischer 1965; Flašar and Štěpančíková 2022b). Foci of significant earthquakes do not occur close to the Rodl-Kaplice fault. However, the existence of young morphotectonic features also along ruptures transverse to the trend of this fault (most distinct within the Donau fault system; Fig.10) indicates a recent tectonic activity of the wider area surrounding the southern RKB with diminishing strike-slip movements of the RKB towards the SW. 5.2.2 South Bohemian Basins Area The RKB-related zone crossing the SBB is up to > 20-km wide, as shown by the geological data (Figs. 2, 4). Scarps, elongated ridges and several small valleys belong to the corresponding geomorphological features. The Rudolfov and Drahotěšice faults are also expressed as a consistent trend of gradients in all evaluated regional geophysical fields (Figs.6, 7, 8 and 9). Based on the GNSS monitoring data (CBUD and TCBU stations), the eastern BB between the Hluboká and Dubné faults moves (up to 0.4mm yr−1) towards the north fairly consistently with the dominant trend detected further east to the RKB (~ 1.0mm yr−1; surroundings of the CJHR station; Fig.13). It indicates in the SBB a local suppression of the dominant RKB sinistral geodynamic regime. The spatial continuity of the RKB has been in the upper crust of the basin area strongly disturbed by an activity of the transverse NW–SE to NNW–SSE faults. Hence, the verification of recent kinematics in the middle RKB, where the fault system root is located in a higher depth beneath them, is complicated. The earlier movements along the SBB faults induced a development of smaller grabens limiting Middle Miocene, Late Miocene and Pliocene deposits; the activity of marginal ruptures lasted at least until the end of Neogene (Špaček etal. 2011; Prachař 2014). The morphometric comparison of the Hluboká and Rudolfov faults (Popotnig et al. 2013) pointed even to a higher recent activity of the former and a continuing moderate Lišov Horst uplift along the latter fault, although movements were not directly proved in the crossing trenches (Špaček etal. 2017). The young fault activity could be associated with the Late Quaternary aggradational regime in the eastern BB; the oldest fluvial sediments were there dated to MIS5 or even a later period. Either recent BB floor surface subsidence or stability related to the near NE basin margin uplifted along both the marginal faults was thus considered for evolution of their depositional setting (Homolová etal. 2012). A subsidence of this part of the basin is supported also by the RVM data (Fig.11). The GNSS-derived movements, RVM and properties of the regional seismicity generally indicate a variable character of the recent geodynamic activity in the SBB, as well as the differences between eastern and western parts of this transverse tectonic structure. The prevailing NW–SE to NNW–SSE-oriented gradients and anomalies occur there in the RVM map (Fig.11). A newly highlighted narrow NNW–SSE striking extensional zone with a relative subsidence up to 1mm yr−1, extending from the Závist-Clay fault system along the TB axis up to the SE margin of this basin, appears to be the most significant such feature. It spatially fits to the largest discontinuity in the RKB trace between the Drahotěšice and
646 Surveys in Geophysics (2024) 45:609–661 1 3 Blanice faults, mutually located ~ 10km apart (Fig.2; CGS 2023). The extensional zone is associated also with consistent geophysical indications (Figs.6, 7, 8 and 9) and many landforms (topographic gradients, elongated flat-top ridges, valley sections; Fig.10). At the northern zone end, a significant earthquake cluster has been recorded (Figs.6, 7, 8 and 9; Špaček etal. 2011). The NE margin of the extensional zone corresponds to the most important change in GNSS-derived horizontal movement tendencies in the RKB area. The velocity vectors of the stations west of the Blanice fault (GOPE, TBEN and CTAB) point to an increase in the surface velocity from the north towards the SBB in the south (Fig.13), but in the opposite direction as compared to the velocity vectors in the eastern BB (CBUD and TCBU stations). The transverse NNW–SSE extensional zone can serve just as a compensation structure of both counter-directed trends. The two stations located at its northern rim move approximately in opposite ways, TMIL significantly towards the NNW and CZUS towards the SE (> 0.9 mm yr−1 relative difference), which indicates there an additional dextral kinematic component. Hence, the extensional zone appears to separate a more uniform crust segment in the north from the much more fragmented SBB structure composed of blocks moving in variable, from the RKB trend frequently different directions. In the western BB, GNSS stations are absent. However, both the main transverse marginal faults (Hluboká, Dubné) are limited or dislocated there by the N–S Lhenice fault or accompanying subparallel ruptures (Špaček etal. 2011; CGS 2023; geophysical maps in Figs.6, 7 and 8). West of the Lhenice fault, the two GNSS stations detected significantly counter-directed movements (CPRA towards the SW / CZST much faster towards the N; the difference > 1.3mm yr−1). Supported by an existence of spatially corresponding Linsser indications, a belt of preserved small Miocene to Pliocene sedimentary remnants (Fig.2) or many geomorphological features (Fig.10), the NW–SE Dubné fault could continue also to this area. Additionally, this region is characterised by the increased seismicity (Fig.16a; Špaček etal. 2011), mostly in the Strakonice-Písek-Orlík surroundings and near the Lhenice fault where the earthquake foci are mostly located in the < 10km depths. As proposed already by Melnyk etal. (2022), the latter tectonic zone can thus be at floor level of the Variscan crust interconnected with the subparallel RKB; normal and listric (to the depth) geometry was considered for the possible uniting deep structure. While west of the RKB a fairly high earthquake density with the foci located mostly in < 10km depths has occurred, east of the fault system the seismic events have been only scarce and generally induced in higher depths (Fig.16b). This finding can be related to different thickness of the particular Moldanubian blocks in the complex Variscan structure (e.g. Stackebrand and Franzke 1989; Dudek etal. 1991; Vránaand Štědrá 1997) with a common floor of the Variscan level in the 10–12km depth (Hrubcová et. al.2008). A difference in the foci distribution exists also between the northern and southern RKB area (Fig.16c,d). In the former part, the foci located < 10km have not been recorded near the RKB, whereas in the latter part, a few such phenomena have appeared near the fault system (ML ~ 1–2). 5.2.3 Blanice Graben Area The RKB width in the BG area reaches 15–20km (Figs.2,10). This part of the fault system is characterised by the highest concentration of remnant grabens with Permo-Carboniferous sediments (Fig.17). Besides the surface topography (subparallel elongated graben structure with distinct marginal slopes and consistent valley network on its floor, a few
647 Surveys in Geophysics (2024) 45:609–661 1 3 Fig. 16 Foci of instrumentally recorded regional earthquakes (Špaček etal. 2011) orthogonally projected into selected WNW–ESE striking vertical cross section perpendicular to the RKB axis (view from the south). a Spatial distribution of all foci included (national S-JTSK kilometric coordinate system used in the map) along with the considered RKB axis and the selected cross section (bold light grey line) approximately separating northern and southern part of the fault system area. Selected faults are added in the image for its location. Points of the places mentioned in the text: Stra—Strakonice; Pis—Písek; Orl—Orlík. b Modelled projection of all foci in the delimited area. c Selected foci in northern part of the area, including the South Bohemian Basins and Blanice Graben structures. d Selected foci in southern part of the area, including the Rodl-Kaplice fault structure. A depth indication of the main faults added in the b–d images is schematic
648 Surveys in Geophysics (2024) 45:609–661 1 3 Fig. 17 Selected geological features in the Blanice Graben area (CGS 2023) drawn over the RVM image (after Vyskočil 1996; arranged by the authors; values in mm/yr), with added vertical density boundaries (Linsser indications compiled for 4-km depth level), earthquake foci (WEBNET 2022) and horizontal velocity vectors (mm/yr) of GNSS stations. BG—Blanice Graben; Bf—Blanice fault; Kf—Kouřim fault. CGS—Czech Geological Survey
649 Surveys in Geophysics (2024) 45:609–661 1 3 distinct scarps), the BG is well-visible also in the gravity and partly in magnetic images (Figs.6, 7 and 8). A significant change in geodynamic regime occurs along the BG. Based on the GNSS velocity vectors, a uniform trend of horizontal movements exists in middle and southern parts of the graben structure (Fig. 13). Although the GNSS stations on the eastern BG strand are located further from the RKB, it is possible to assume a sinistral recent movement along the driving Blanice fault and related ruptures. On the contrary, along the northern BG non-negligible vertical movements were primarily recorded (Fig. 11). At the northern termination of the RKB, this extensional regime is replaced with the NW–SE-oriented dextral movement activity within the Elbe shear zone (e.g. Uličný etal. 2003; see also the corresponding significant velocity vector of the CZKO station). Hence, the change in kinematic regime is there similar to the southern RKB termination. 5.3 Regional Kinematic Model The resulting preliminary kinematic model of the RKB area is visualised in Fig.18. The non-negligible horizontal movements of blocks along the RKB presumably continue even nowadays, although its structure is in shallow crustal levels significantly modified by the active transverse SBB rupture zone. The kinematics consistent with the fault system is gradually changed towards the north, where the current activity of the significant Elbe shear zone plays the dominant role. In several parts of the investigated area, e.g. near Tábor and Strakonice, the existent data are yet insufficiently interpretable and several geodynamic solutions are possible. In the future, more voluminous data should be incorporated into the analysis to increase reliability of the output expressing the regional kinematic conditions. The knowledge presented in the model is in line with the results from earlier geodynamic studies in the Bohemian Massif: The kinematic conditions in southern and middle part of the Diendorf-Boskovice fault area involve blocks moving along the approximate SSW–NNE axis of that fault system (Roštínský etal. 2013; 2020), but in its northern part, WNW–ESE to NW–SE movements subparallel to the Elbe shear zone trend become similarly prevailing (Švábenský etal. 2014; Špaček etal. 2015; Pospíšil etal. 2017). The significant structures in the western Bohemian Massif surrounding the Mariánské Lázně fault (Vavryčuk et al. 2013; Štěpančíková et al. 2019) have the different NNW–SSE to N–S dominant geometric arrangement and represent another group of active tectonic phenomena within the Eastern Variscides, presumably combined there with a generation of young volcanic rocks (cf. Wendt and Dietrich 2003; Weinlich etal. 2006). The RKB, as well as other significant subparallel fault systems in the southern Bohemian Massif (Vitis-Přibyslav, Diendorf-Boskovice), is located in a transition area between the two different recent stress domains in the northern Eastern Alps and northwestern Europe. Hence, faults of a wide strike spectrum could be reactivated in its region during alternating pushes from the south and the northwest: SW–NW, SSW–NNE, N–S, NNW–SSE or NW–SE. This tentative assumption is fairly consistent with the obtained results from the geodetic (RVM, GNSS) and geomorphological research along the RKB. Its data supported in this area mostly the strike-slip or normal regimes. Both longitudinal and transverse tectonic zones thus appear to be active simultaneously (Figs.11,18), as suggested earlier already by Popotnig et al. (2013) for the eastern BB. The increased number of facets and steep scarps in the southernmost Bohemian Massif (Fig. 10) is consistent with the ongoing extensive regional relief rejuvenation in front of the Eastern Alps (Wetzlinger etal. 2023). Additionally, the RKB ends near the Alpine front. Hence,
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