# At the border of peri-Gondwana and Baltica: the structure of the eastern termination of the Variscan belt

Stanislaw Mazur, Karel Schulmann

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Stanislaw Mazur, Karel Schulmann. At the border of peri-Gondwana and Baltica: the structure of the eastern termination of the Variscan belt. Journal of the Geological Society, 2025, 182, 10.1144/jgs2024-075. insu-05082259

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# At the border of peri-Gondwana and Baltica: the structure of the eastern termination of the Variscan belt

Stanisław Mazur<fnref n="1" />* and Karel Schulmann<fnref n="2" />,<fnref n="3" />

**Abstract:** The termination of the eastern Variscan belt has long been a matter of debate because it is deeply concealed beneath younger sediments. Its geometry has been addressed by the two competing hypotheses of the Variscan orocline and right-lateral strike-slip tectonics. We compiled a set of new gravity and magnetic maps for the Czech Republic, Poland and eastern Germany to track Variscan structures in the subsurface. These maps clearly show that the Rheno-Hercynian Suture turns by almost 90° east of the Harz Mountains in Germany and continues into Poland towards the ESE. Our results therefore favour the concept of a semi-orocline that terminates in the east against the Brunovistulian Block. The pattern of magnetic and gravity anomalies, combined with geological evidence, indicates two superimposed stages of accretion in the eastern Variscan belt. The initial west-east convergence led to the formation of the NNE-SSW-trending structures that dominate the southern Bohemian Massif. These structures were overprinted by an important north-south-shortening event that shaped the WNW-ESE-oriented features. The latter run parallel to the Baltica margin and dominate the area NE of the Elbe Fault. This is consistent with seismic data showing large-scale underthrusting of the Baltica crust beneath the Variscan belt over a distance of at least 100 km.

**Thematic collection:** This article is part of the Processes of Pangea construction collection available at: https://www.lyellcollection.org/topic/collections/processes-of-pangea-construction

**Received** 14 April 2024; revised 9 August 2024; accepted 25 September 2024

The Variscan belt was created as a result of the convergence between Laurussia and Gondwana and some smaller intervening terranes (e.g. Behr et al. 1984; Lorenz and Nicholls 1984; Matte 1986, 1991, 2001; Franke 1989, 2000; Ziegler 1989). Regardless of the various subdivisions of the Variscan terranes, it is generally accepted that they docked to the section of the southern Laurussia margin occupied by Avalonia (e.g. Eckelmann et al. 2014; Franke et al. 2017). The Baltica part of Laurussia has never been considered to be a Variscan foreland because the Baltica crust was thought to have occurred further east beyond the Teisseyre-Tornquist Zone (TTZ; Fig. 1; e.g. The Pace TMR Network Team and Winchester 2002; Guterch and Grad 2006; Nawrocki and Poprawa 2006; Narkiewicz et al. 2015).

The area NE of the Bohemian Massif, corresponding to the foreland of the eastern Variscides, is concealed under a thick cover of the Permian-Mesozoic sediments of the Polish Basin (Dadlez et al. 1995; Van Wees et al. 2000; Stephenson et al. 2003) and has therefore always been poorly explored. The basement of the Polish Basin has been considered to be Avalonia (e.g. Moczydłowska 1997; Unrug et al. 1999) or, most often, the Trans-European Suture Zone (e.g. Pharaoh 1999; The Pace TMR Network Team and Winchester 2002; Pharaoh et al. 2006; Fig. 1), an enigmatic zone composed of hidden, unrecognized terranes. Only in the last two decades have new data and interpretations begun to emerge suggesting that the basement of the NE German-Polish Basin is the attenuated margin of the Baltica palaeocontinent (Tanner and Meissner 1996; Berthelsen 1998; Bayer et al. 2002; Malinowski et al. 2005; Mazur et al. 2015; Smit et al. 2016), presently forming the core of the East European Craton (EEC).

The PolandSPAN and POLCRUST deep reflection seismic profiles (Malinowski et al. 2013; Krzywiec et al. 2014), integrated

with potential field data, showed that the TTZ does not constitute a sharp boundary of the EEC (Mazur et al. 2015, 2016). This is also indicated by the reinterpretation of seismic refraction data (Smit et al. 2016), especially because earlier refraction profiles suggested an extension of the EEC basement to the edge of the Bohemian Massif and the adjacent Brunovistulian Block, at least at the level of the lower crust (Grad et al. 2003, 2008; Malinowski et al. 2005). The discovery that the Variscan fold-thrust belt (Variscan externides) does not bend oroclinally in the basement of the Polish Basin, but instead runs WNW-ESE, up to the border with Ukraine, was also important (Krzywiec et al. 2017a; Mazur et al. 2020; Fig. 2). In its eastern section, within the Lublin Basin, this fold-thrust belt enters the slope of the EEC (Fig. 2), an area previously considered as part of the Precambrian Platform, providing direct evidence for the interaction between the Variscan orogen and the former Baltica (Krzywiec et al. 2017a; Mazur et al. 2020).

In recent years, attempts have been made, using various geophysical modelling techniques, to better understand the basement structure underneath the Polish Basin (Grad and Polkowski 2016) and to determine the boundary between the Variscan crust and the thinned margin of Baltica (Maystrenko and Scheck-Wenderoth 2013; Mikołajczak et al. 2019; Mazur et al. 2021). This research showed that the basement architecture between the Bohemian Massif and the TTZ was significantly modified as a result of an early Permian continental rifting event and the geophysical signature of the pre-Permian structures was partially blurred (Mazur et al. 2021; Jóźwiak et al. 2022). We present here a summary of the arguments for the direct contact of the Variscan and Baltica crust and discuss the possible nature of this contact.

The paper is partly of a review character, referring to research results from the last decade and providing a synthesis of the

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<footnote n="1">¹ Institute of Geological Sciences, Polish Academy of Sciences, Senacka 1, 31-002 Kraków, Poland</footnote>

<footnote n="2">² Institut Terre et Environnement de Strasbourg, Université de Strasbourg, UMR 7063, 1 Rue Blessig, 67084 Strasbourg, France</footnote>

<footnote n="3">³ Center for Lithospheric Research, Czech Geological Survey, Klárov 3, 11821, Prague 1, Czech Republic
* SM, 0000-0002-6560-1048; KS, 0000-0002-5814-9161</footnote>

<footnote>*Correspondence: ndmazur@cyf-kr.edu.pl</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** The European Variscan belt with palinspastic restoration of Iberia prior to the opening of the Bay of Biscay. CDF, Caledonian Deformation Front; CZ, Spanish Central Zone; CIZ, Central Iberian Zone; EF, Elbe Fault; FBF, Franconian-Bavarian Fault; GF, Grójec Fault; GTMZ, Galicia-Tras os Montes Zone; ISF, Intra-Sudetic Fault; M, Moldanubian Zone; NPZ, Northern Phyllite Zone; OF, Odra Fault; S, Schwarzwald; ST, Saxo-Thuringian Zone; TB, Teplá-Barrandian Zone; TTZ, Teisseyre-Tornquist Zone; V, Vosges Mountains; WALZ, West Asturia-Leonese Zone. Source: modified from various sources, including *Martinez Catalán* (2011), *Shaw et al.* (2012) and *Franke* (2014).</figcaption></figure>

relevant data. However, the innovative element of this contribution is the presentation of new maps of the potential fields for the border zone between the Variscides and Baltica and their interpretation. For the first time, we present good quality integrated magnetic and gravimetric data for Poland, the Czech Republic and eastern Germany.

# Geology of the contact zone between the Variscides and Baltica

## Rheic and Rheno-Hercynian sutures

The Rheic and Rheno-Hercynian sutures (Franke 2000; McKerrow et al. 2000; Matte 2001; Linnemann et al. 2007; Von Raumer and Stampfli 2008; Nance et al. 2010) mark the southern limit of Laurussia (Laurentia-Baltica-Avalonia) and its tectonic contact with the peri-Gondwana terranes of the Variscan belt. The Rheic Ocean between Avalonia and the peri-Gondwana terranes was closed before or during the Emsian (Early Devonian) in Europe (e.g. Nance et al. 2010, 2012; Franke et al. 2017). However, it was soon replaced by the narrow and short-lived Rheno-Hercynian Ocean, the opening and closure of which controlled the evolution of the Rheno-Hercynian fold-thrust belt (Franke et al. 1995, 2017). The outcrops of the Rheic and Rheno-Hercynian sutures nearly overlap in the present-day intersection level. The Rheic Suture Zone is represented by Silurian arc rocks within the Northern Phyllite Zone, which were partly underthrust under the Mid-German Crystalline Rise (MGCR; Franke et al. 1995, 2017). The Rheno-Hercynian Suture is set between the southernmost flysch and/or mid-ocean ridge basalt (MORB)-type volcanics of the Northern Phyllite Zone to the north and the MGCR to the south (e.g. Oncken et al. 1999; Franke 2000). Eifelian (c. 390 Ma) and Emsian (c. 400 Ma) condensed pelagic sediments deposited on MORBs limit the minimum age of the Rheno-Hercynian Ocean (Franke et al. 2017). Despite their similar location, the Rheic and Rheno-Hercynian sutures represent subduction zones with mutually opposite polarity: the early Mesozoic Rheic Ocean and the Devonian Rheno-Hercynian Ocean were subducted towards the north and south, respectively (e.g. Franke 2000, 2024).

Although the Rheno-Hercynian Suture is well exposed and documented in the German part of the Variscides, its extension NE

of the Harz Mountains remains unclear. There is little drilling data available due to the thick pile of Permian-Mesozoic sediments and the maps of potential fields published so far are not conclusive on this issue (e.g. Banka et al. 2002; Pharaoh et al. 2006) or do not cover the key area (Schulmann et al. 2022). There is also a lack of significant seismic data available. This leaves a large margin of uncertainty for various interpretations and speculations (see Franke and Żelaźniewicz 2023 for a review).

# NE termination of the Variscan belt: strike-slip tectonics v. orocline

The concept of a Variscan orocline was proposed because the Variscan belt sharply changes its trend in easternmost Germany and western Poland with respect to that prevailing further west (Fig. 1). On many overview maps, the Variscan belt is depicted as making almost a U-turn, encircling the Bohemian Massif from the north (e.g. Kossmat 1927; Lorenz 1976; Ellenberger and Tamain 1980; Engel et al. 1983; Lorenz and Nicholls 1984; Franke 1989; Tait et al. 1997; Narkiewicz 2007). According to this view, the outermost tectonostratigraphic zone of the Variscan belt, the Rheno-Hercynian Zone, continues eastward in the deep substratum of the Permian-Mesozoic German-Polish Basin and reappears at the surface along the eastern rim of the Bohemian Massif as the Moravo-Silesian Zone in the eastern Czech Republic and southern Poland. An extensive critical review of the orocline concept is presented in Mazur et al. (2020).

An alternative view that postulates a dextral transpressional regime during the final accretion of the Variscan terranes has gained increasing attention since the 1970s (Dewey and Burke 1973; Arthaud and Matte 1977; Badham 1982; Behr et al. 1984; Matte 1991). At the eastern termination of the orogen, the Variscan shortening was thought to be accommodated by prominent NW-SE-oriented dextral faults (e.g. Edel et al. 2018). This transpressional tectonic regime was believed to have resulted from sublateral dextral displacements between the Gondwana and Laurussia margins during the Carboniferous to early Permian (Arthaud and Matte 1977; Eckelmann et al. 2014). Arthaud and Matte (1977) proposed that a series of NW-SE-trending major strike-slip faults interrupt the continuity of the Variscan-Appalachian orogenic belt along a c. 6000 km long and c.
<figure><img src="image_4.png" /></figure>

**Fig. 2.** Simplified tectonic map of Central Europe showing the tectonic sutures and strike-slip faults discussed in the text. The coordinate system used in Figures 2, 5, 6, 8–10, 12 and 13 is based on the WGS84 datum and Universal Transverse Mercator projection for zone 32 N. BH, Bielawy High; BV, Brunovistulian Block; CDF, Caledonian Deformation Front; CTF, Carpathian Thrust Front; DF, Dolsk Fault; EF, Upper Elbe Fault; HCF, Holy Cross Fault; HCM, Holy Cross Mountains; ISF, Intra-Sudetic Fault; KFZ, Kock Fault Zone; KLF, Kraków–Lubliniec Fault; MS, Moravian Suture; MSB, Moravo-Silesian Belt; NPZ, Northern Phyllite Zone; OF, Odra Fault; RHS, Rheno-Hercynian Suture; STS, Saxo-Thuringian Suture; TMB, Teplá–Barrandian-Moldanubian boundary; VDF, Variscan Deformation Front; WH, Wolsztyn High. Source: base map modified from several sources, including Franke *et al.* (1995), Pharaoh (1999) and Banka *et al.* (2002). Orientation of the Variscan deformation front in Poland after Mazur *et al.* (2020).

1000 km wide semi-brittle megashear zone. As a consequence, dismembered fragments of several Variscan tectonostratigraphic units can be found in the Sudetes Mountains (SW Poland) on the NE margin of the Bohemian Massif. These units have been dextrally displaced parallel to the SW margin of the EEC, although the scale of this displacement remains disputable (e.g. Matte *et al.* 1990; Aleksandrowski 1995; Aleksandrowski *et al.* 1997; Aleksandrowski and Mazur 2002; Franke and Żelaźniewicz 2002, 2023; Mazur *et al.* 2020).

An attempt was also made to combine both hypotheses, the strike-slip and oroclinal theories, in the conceptual model of the Bohemian Arc. In this modified version of the oroclinal concept, Franke and Żelaźniewicz (2000, 2002, 2023) correlated the area SW and NE of the Odra Fault (Fig. 2) with the MGCR and the Northern Phyllite Zone, respectively. Nevertheless, they did not suggest a continuation of the main part of the Rheno-Hercynian belt along the Baltica margin. These researchers acknowledged the existence of important WNW–ESE dextral dislocations, which were included in their model of the dissected Variscan orocline (Franke and Żelaźniewicz 2000, 2002, 2023). A similar solution including a combination of semi-orocline and strike-slip faults was adopted by Martínez Catalán *et al.* (2020, 2021).

## Major Variscan strike-slip faults

Several late Carboniferous to Permian NW–SE-oriented major strike-slip faults are recognized in the Bohemian Massif (Figs 1, 2). They are all characterized by a similar orientation and at least two phases of activity: first, the main dextral displacement, followed by the minor stage of sinistral displacement (e.g. Aleksandrowski *et al.* 1997; Franke and Żelaźniewicz 2000; Edel *et al.* 2018; Mazur *et al.* 2020). The age of fault activity was estimated as late Carboniferous for the dextral movements and the latest Carboniferous–early Permian for the sinistral movements (e.g. Aleksandrowski *et al.* 1997; Franke and Żelaźniewicz 2000, 2002; Aleksandrowski and Mazur 2002; Edel *et al.* 2018; Mazur *et al.* 2020). The main structures belonging to this family of faults are, from south to north: the Franconian–Bavarian,

Elbe, Intra-Sudetic, Odra, Kraków–Lubliniec and Dolsk faults (Figs 1, 2). For many of these faults, the kinematic interpretations were based on field observations (Aleksandrowski *et al.* 1997; Aleksandrowski and Mazur 2002; e.g. the Elbe and Intra-Sudetic faults) or the analysis of core material (Buła *et al.* 1997; Żaba 1999; the Kraków–Lubliniec Fault). Some other faults have been assigned strike-slip kinematics by analogy with the others (the Odra and Dolsk faults; Mazur *et al.* 2020). At least some of these faults have a significant vertical throw component in addition to the strike-slip component, especially the Odra and Dolsk faults (e.g. Mazur *et al.* 2020). Assessment of the magnitude of horizontal displacements has always been difficult due to the lack of appropriate markers. Even in the case of the Elbe Fault, where the maximum displacement has been estimated at 120 km based on the regional geology (Rajlich 1987, 1990), this remains difficult to verify.

## Deep crustal structure in the NE Variscan foreland

Far to the NE, the Variscan foreland is undeniably Baltica, which presently corresponds to the East European Precambrian Platform. The latter includes undeformed to weakly deformed Neoproterozoic–Phanerozoic sediments resting on the thick cratonic crust of the EEC. The Precambrian Platform is roughly bounded to the SW by the TTZ (Fig. 2), a feature variously interpreted over the past decades (e.g. Brochwicz-Lewiński *et al.* 1984; Berthelsen 1998; Dadlez *et al.* 2005), but recently compared with a lithospheric necking zone produced during the break-up of Rodinia (Mikołajczak *et al.* 2019; Mazur *et al.* 2021). According to this concept, the attenuated EEC crust continues further SW in the basement of the Permian–Mesozoic German–Polish Basin and the Variscan external fold–thrust belt (Fig. 3), the area formerly described as the Trans-European Suture Zone (e.g. Pharaoh 1999). However, unlike the Precambrian Platform, the putative EEC crystalline basement SW of the TTZ is overlain by the pre-Permian sediments that were folded and thrust during the Caledonian (Pomerania; Figs 1, 2) and Variscan cycles (e.g. Dadlez *et al.* 1994; Mazur *et al.* 2020). This area is therefore conventionally defined as
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Regional cross-sections through the boundary zone between the eastern Variscan belt and Baltica coinciding with (a) seismic refraction profile POLONAISE’97 P4 (Grad et al. 2003) and (b) seismic refraction profile CELEBRATION’2000 CEL02 (Malinowski et al. 2005), respectively. For location, see Figure 2. EEC, East European Craton; HCF, Holy Cross Fault; KFZ, Kock Fault Zone; KLF, Kraków–Lubliniec Fault; OF, Odra Fault; TTZ, Teisseyre–Tornquist Zone; VDF, Variscan Deformation Front. Abbreviated borehole names (numbers omitted): Ab, Abramów; Bie, Bielawy; Br, Brenno; Bro, Brodnica; By, Byczyna; Ce, Celejów; DB, Dolina Będkowska; Do, Donatowo; Du, Dubie; GD, Górka Duchowna; Ka, Karniowice; Kal, Kaleje; Kar, Karnkowo; Kat, Katarzynin; KM, Kalina Mała; Ko, Kolechowice; KoM, Kostki Male; Kr, Krajkowo; Mi, Mirów; Mil, Miłosław; Na, Nasiechowice; OL, Opole Lubelskie; Pa, Parczew; Pi, Piotrowice; Pł, Plusy; Po, Podedwórze; Pr, Przeciszów; PW, Polanka Wielka; PŻ, Poręba Żegoty; Ro, Rozkochów; Si, Siciny; St, Słomniki; Sła, Sława; Sp, Spytkowice; St, Straszniów; Śr, Śrem; Ta, Tarkawica; Tr, Trzonów; Wę, Węchadłów; WK, Wola Kalinowska; Wr, Września; Ws, Wschowa; Za, Zator; Zal, Zalas; Ża, Żakowo. Source: after Mazur *et al.* (2020).</figcaption></figure>

the Paleozoic Platform. This division is not exact because the Variscan fold–thrust belt enters the area of shallow EEC basement in SE Poland, traditionally classified as part of the Precambrian Platform (Figs 2, 3).

Our limited knowledge of the structure of the deep substratum of the NE Variscan foreland mostly relies on the results of the seismic wide-angle reflection and refraction (WARR) experiments (e.g. Perchuć 1984; Guterch et al. 1986; Guterch and Grad 2006). The crust under the NE margin of the Bohemian Massif (the Sudetes Mountains) is characterized by a two-layer structure with low P-wave velocities down to the Moho (6.6 km s⁻¹; Fig. 4; Grad et al. 2002, 2003, 2008). This velocity structure extends NE beyond the Odra Fault, conventionally considered to be a margin of the Bohemian Massif, and terminates at the Dolsk Fault c. 100 km further NE (Figs 2, 4). The crustal structure between the Dolsk Fault and the TTZ is strikingly different, showing a three-layer velocity structure typical of cratons. This includes: (1) a thick upper crust with the low P-wave velocities (<6.2 km s⁻¹) down to c. 20 km; (2) a thin mid-crust with a moderate P-wave velocity of 6.4 km s⁻¹; and (3) a lower crust with a velocity of 6.5–7.2 km s⁻¹ and high Vp velocity gradient (Grad et al. 2002, 2003; Guterch and Grad 2006). The depth to the Moho ranges between 38 and 31 km beneath central and SW Poland, respectively (Guterch and Grad 2006; Mazur et al. 2021).

The nature of the thick upper crust is enigmatic and was previously interpreted as a low-grade metamorphic volcano-sedimentary sequence (Puziewicz 2006). This is consistent with the view that the thick upper crust represents the sedimentary fill of the Ediacaran rift preceding the fragmentation of Rodinia (Pharaoh et al. 2006; Mikolajczak et al. 2019; Mazur et al. 2021). The geometry of the contact between the two crustal domains NNE and SSW of the Dolsk Fault, as well as the kinematics of the fault itself, remain unknown due to the limitations of the WARR method. The fault is most often depicted as steep to subvertical at the level of the upper and mid-crust (e.g. Grad et al. 2002, 2003). However, the high velocity lower crust is imaged to extend SW up to the Odra Fault (Grad et al. 2002, 2003, 2008). The WARR data therefore do not allow us to clearly determine whether the Variscan crust is truncated by a steep strike-slip fault or thrust 100 km over the EEC margin.

## Variscan external fold–thrust belt

The Variscan external fold–thrust belt, in the basement of the Paleozoic Platform of western and central Poland (Fig. 2), is entirely buried below the Permian–Mesozoic to Cenozoic infill of the Polish Basin. The fold–thrust belt consists of a Carboniferous synorogenic clastic sedimentary succession with a minimum thickness exceeding 2.5 km, which was deposited in an extensive foreland basin.
<figure><img src="image_6.png" /></figure>

**Fig. 4.** Two-dimensional P-wave velocity model for the POLONAISE '97 P4 profile obtained by ray tracing (Grad *et al.* 2003). A 360 km long section is shown from the German–Polish border (0 km) to a point immediately beyond the Teisseyre–Tornquist Zone. The thick solid lines are layer boundaries and the thin grey lines are isovelocity contours in kilometres per second. Model obtained by forward ray tracing modelling using the SEIS83 package (Červeny and Pšenčik 1983). The yellow and red dotted lines represent the base of the Permian and the top of the basement horizons, respectively, adopted from Mazur *et al.* (2021). Numbers indicate $V_p$ velocity in km s⁻¹. EEC, East European Craton; TTZ, Teisseyre–Tornquist Zone. Source: modified from Schulmann *et al.* (2022).

This basin formed part of the Variscan foreland basinal system extending from England to Poland along the passive margin of Laurussia. The substratum of Carboniferous sediments in western and central Poland is only known from two WNW-ESE-oriented structural highs, the Wolsztyn and Bielawy highs, located within the Wolsztyn Block between the Odra and Dolsk faults (Fig. 2). Low-grade metamorphic slates were drilled there by several boreholes beneath the reduced cover of Carboniferous strata. The metamorphic age of these low-grade rocks is constrained at 358.6 ± 1.8 Ma (earliest Carboniferous; Mazur *et al.* 2006), but their protolith age remains unknown. Carboniferous clastic rocks occur in a transgressive position on top of the metamorphic basement, marking a back-stepping of the foreland basin onto the hinterland. This suggests a connection between the basement of the Wolsztyn Block and the Variscan internides (Fig. 3).

Carboniferous flysch has been drilled region-wide below the base Permian unconformity. Moderate to steep (30–80°) dips predominate across the Carboniferous subcrop, with no clear steepening/shallowing trend (Mazur *et al.* 2020). The predominance of moderate to steep dips may be an expression of thin-skinned folding and/or imbricate stacking of the Carboniferous series (Fig. 3a). The borehole data leave a broad margin of uncertainty for the position of the Variscan deformation front because a large gap exists in borehole coverage across the deepest depocenter of the Permian–Mesozoic Polish Basin (Fig. 3a). Nevertheless, statistical analysis of the structural dips allowed the subsurface position of the Variscan deformation front that separates the external fold–thrust belt and the mostly undeformed section of the Variscan foreland basin to be delimited (Fig. 2; Mazur *et al.* 2020). The orientation of the Variscan deformation front beneath the Permian–Mesozoic Polish Basin is WNW–ESE (Fig. 2). An exception is the NNE–SSW-oriented side-branch of the Variscan front in Moravia (the Czech Republic; Fig. 2), which was formed as a result of the interaction of the Bohemian Massif with the rigid buttress of the Brunovistulian Block (e.g. Schulmann and Gayer 2000; Mazur *et al.* 2020). However, the Moravian section of the Variscan front terminates against the Kraków–Lubliniec Fault at the northern boundary of the Brunovistulian Block (Fig. 2).

The occurrence of Variscan thin-skinned compressional deformation east of the Holy Cross Mountains, over the Radom–Kraśnik Block and the Lublin Basin (Fig. 2) – that is, in areas located far beyond the then generally accepted eastern extent of the Variscan

fold–thrust belt (Jubitz *et al.* 1986; Pożaryski *et al.* 1992) – was first postulated by Antonowicz *et al.* (2003) and Antonowicz and Iwanowska (2004). The combined POLCRUST and PolandSPAN deep seismic reflection surveys (Malinowski *et al.* 2013; Krzywiec *et al.* 2014), along with newly acquired industrial seismic data (Krzywiec *et al.* 2017a; Tomaszczyk and Jarosiński 2017), imaged a Variscan thin-skinned fold–thrust belt entering a little deformed basement slope of the EEC (Fig. 3b; Krzywiec *et al.* 2017a, b). This belt of thin-skinned deformation involves a number of sub-Permian tectonic units of the Paleozoic Platform, overprinted by Variscan shortening (Krzywiec *et al.* 2017a; Tomaszczyk and Jarosiński 2017; Kufrasa *et al.* 2020). The seismic data show a NE-vertent, thin-skinned thrust stack overthrust towards the craton and imbricating a 10–12 km thick pile of Neoproterozoic–Devonian sediments (Fig. 3b; Krzywiec *et al.* 2017a). Its leading edge is a triangular zone related to the jump of the basal detachment from a basement–cover interface to Silurian shales (Krzywiec *et al.* 2017a).

## Methodology for preparing potential fields maps

The magnetic data (total magnetic intensity) for Central Europe outside Poland were obtained from the Getech Group's European magnetic data compilation (Fletcher *et al.* 2011). The data were gridded at a resolution of 0.01° (c. 1 km) at a common elevation of 1 km above sea-level. This elevation was chosen to provide the seamless merging of different magnetic surveys acquired with various parameters. The reduction-to-pole (RTP) transform was applied to the magnetic anomaly data. The RTP transform attempts to simplify the magnetic field by rotating the magnetic vector to vertical, thereby cantering the magnetic anomalies above their causative bodies (MacLeod *et al.* 1993).

The regional gravity data for Central Europe outside Poland were obtained from the Getech Group's Multi-Satellite altimetry-derived gravity product (Green *et al.* 2019). The original data were gridded at a resolution of 0.02°, corresponding to c. 2.2 km in the y-dimension and c. 1.3 km in the x-dimension. The gravity anomaly map is a compilation of the free-air gravity offshore and the Bouguer gravity onshore. A complete Bouguer correction was calculated using a rock density of 2.67 g cm⁻³.

The gravity and magnetic data for territory of Poland were obtained from the Central Geological Database (Central Geological Database 2019) managed by the Polish Geological Institute (http://
baza.pgi.gov.pl/). The gravity database consists of measurements derived from 513 800 ground stations uniformly distributed over the study area. The gravity data were processed by a complete Bouguer correction using a digital terrain model and a slab density of 2.67 g cm⁻³. The gravity datum used was the IGSN71 international gravimetric standard (The International Gravity Standardization Net 1971) and the theoretical gravity was based on the Geodetic Reference System '80 (GRS80) reference ellipsoid. The Bouguer gravity data were gridded at an interval of 2000 m using a minimum curvature algorithm. The magnetic database included 519 600 measurements of the total magnetic intensity compiled from a number of ground surveys. These data were gridded at an interval of 500 m using a minimum curvature algorithm and continued upward to 500 m mean terrain clearance.

The magnetic and gravity data for Poland and Central Europe were merged as seamlessly as possible using grid knitting. Some minor imperfections are located outside the area of interest. Although the resolution of the gravity data is comparable in both datasets, the resolution of the magnetic data for Poland is twice as good as for the rest of the area.

The tilt derivative (TDR) was calculated for the gridded data using Oasis montaj software (Sesequent 2024) with the tilt derivative option (TILTDRV GX) from the 2D Filtering menu. The TDR is useful for mapping shallow basement structures and mineral exploration targets. The derivative is defined as $TDR = tan^{-1}$ (VDR/THDR), where VDR and THDR are first vertical and total horizontal derivatives, respectively, of the total magnetic intensity or Bouguer gravity.

## Qualitative interpretation of potential field maps

Magnetic anomaly maps are particularly helpful when investigating the structure of the deep basement because the magnetic susceptibility of sedimentary rocks is negligible compared with that of igneous and plutonic rocks. They are especially useful in the study of the contact zone between the Variscan belt and the former Baltica margin because this area is concealed beneath thick Carboniferous sediments of the Variscan fold-thrust belt and foreland basin, as well as the Permian-Mesozoic sediments of the Polish Basin. Gravity anomaly maps represent the cumulative signal from all rock layers, so the influence of thick sediments may mask the gravimetric signature of the deep basement. Hence the magnetic maps most clearly illustrate the basement features associated with the accretion of the Variscan orogen.

## Reduced-to-the pole magnetic anomaly maps

The magnetic anomaly map is presented in two versions: (1) for the entire border zone of the Variscan orogen and the former Baltica (Fig. 5) to highlight the regional context; and (2) in an enlarged version for the area containing the Variscan tectonic sutures to emphasize the details (Fig. 6). On both maps, the Rheno-Hercynian Suture is clearly visible in its German section as a strong and extended positive magnetic anomaly (Figs 5, 6). The anomaly is associated with the outcropping or shallow subcropping metamorphic rocks (Northern Phyllite Zone) and the presence of Devonian MORB-type metabasalts. Immediately to the SE, the MGCR contains numerous intrusions. In the area between the Rhenish Massif and the Harz Mountains, the Northern Phyllite Zone (containing the Rheno-Hercynian Suture) and the MGCR form one large anomaly c. 50 km wide. The position of the suture based on the magnetic anomaly does not always exactly coincide with that determined on the basis of geological data. This is because the source of the anomaly at depth does not have to match precisely with the trace of the suture on the sub-Permian surface.

The position of the Rheno-Hercynian Suture is not geologically constrained NE of the Harz Mountains. However, the associated magnetic anomaly is clearly visible. It bends by almost 90°, adopting a WNW-ESE orientation in Poland (Figs 5, 6). The anomaly representing the Rheno-Hercynian Suture is located directly NE of the Odra Fault, but not coinciding with it and remains slightly oblique. By contrast, the anomaly nearly completely overlaps with the Bielawy High in the Wolsztyn Block. The anomaly ends NE of Wrocław at the NE extension of the Moravian Suture separating the Bohemian Massif from the Brunovistulian Block (Figs 5, 6).

North of the Harz Mountains and the Elbe Fault, a single anomaly representing the Rheno-Hercynian Suture and the MGCR together splits into two branches (Figs 5, 6). The stronger northern branch, described in the preceding paragraph, corresponds to the Rheno-Hercynian Suture, whereas the southern, less pronounced branch is the NE continuation of the MGCR. In Poland, this anomaly mostly overlaps with the Odra Fault, but deviates from it eastward and terminates against the major magnetic anomaly of the Ślęża Ophiolite (Fig. 6).

Although the course of the Rheno-Hercynian Suture is continuous, the Saxo-Thuringian Suture is segmented into three parts by dextral strike-slip faults (Fig. 6). South of the Elbe Fault, the suture follows the SE margin of the large magnetic high over the Erzgebirge Mountains. Further to the NE, it surrounds the Karkonosze-Izera Massif and reaches a clear positive anomaly associated with the Leszczyniec Complex (Figs 6, 7). After further dextral displacement, the suture reappears immediately west of a distinct positive anomaly associated with the Ślęża Ophiolite (Fig. 6). Directly south of the ophiolite, the Saxo-Thuringian Suture is hidden beneath the Góry Sowie allochthon.

The SE periphery of the Teplá-Barrandian Zone is characterized by a strong magnetic anomaly referred to in this paper as the Teplá-Barrandian Anomaly (Figs 5, 6). The location of the anomaly does not exactly coincide with the Teplá-Barrandian-Moldanubian boundary determined from surface outcrops (Fig. 7). Instead, the source of the anomalies is correlated with igneous rocks belonging to the Central Bohemian Plutonic Complex, which are the remnants of a Variscan volcanic arc (the Saxo-Thuringian Arc; e.g. *Zák et al.* 2005; *Schulmann et al.* 2009). The Teplá-Barrandian-Moldanubian boundary is considered to be a steep, normal-slip shear zone that produced a significant regional-scale downthrow of the Teplá-Barrandian rocks relative to the Moldanubian rocks during the mid-Carboniferous (e.g. *Zulauf* 1994; *Zulauf et al.* 2002). The timing of this event corresponds to the age of displacements along the major West Bohemian shear zone, which was confined to the time interval 350–320 Ma, with a culmination at c. 340 Ma (*Zulauf et al.* 2002).

Regardless of the nature of the contact between the Teplá-Barrandian and Moldanubian zones, which is an issue widely discussed in the literature (e.g. Matte 1991, 2001; Franke 2000; Schulmann et al. 2014; Franke et al. 2017; Maierová et al. 2021; Faryad et al. 2024), arc-related intrusions, primarily tonalites and gabbros, generate a distinct magnetic anomaly that can also be traced beneath the Czech Cretaceous Basin (Figs 6, 7). In the area of this basin, the anomaly turns first to the east and then to the north (Figs 6, 7). Shifted slightly along the Elbe Fault, the Teplá-Barrandian Anomaly continues north into the Polish Sudetes Mountains along the western margin of the Orlica-Śnieżnik Massif (Figs 6, 7). There, it bends again around the Góry Sowie Massif, heading north along its eastern margin. In this area, the Teplá-Barrandian Anomaly almost coincides with the Moravian Suture, although neither can be traced further north beyond the Ślęża Ophiolite (Figs 6, 7).

The Moravian Suture, the position of which is well constrained by geological data (Fig. 6), separates the Bohemian Massif from the large magnetic high over the Brunovistulian Block. Despite local complexities produced by collisional tectonics, the trace of the suture
<figure><img src="image_7.png" /></figure>

**Fig. 5.** Reduced-to-the pole magnetic anomaly map for the boundary zone between the Variscan belt and the East European Craton (Baltica) overlain with the key structural elements (UTM 32N). See text for details. BH, Bielawy High; BM, Bohemian Massif; BV, Brunovistulian Block; CDF, Caledonian Deformation Front; CTF, Carpathian Thrust Front; DF, Dolsk Fault; EF, Elbe Fault; GF, Grójec Fault; GG, Glückstadt Graben; HCM, Holy Cross Mountains; Hz, Harz Mountains; KLF, Kraków– Lubliniec Fault; LEL, Lower Elbe Lineament; MGCR, Mid-German Crystalline Rise; MM, Małopolska Massif; MPT, Mid-Polish Trough; MS, Moravian Suture; MSB, Moravo-Silesian Belt; Od, Odenwald; OF, Odra Fault; ISF, Intra-Sudetic Fault; NPH, Northern Phyllite Zone; RHS, Rheno-Hercynian Suture; RM, Renish Massif; RT, Rheinsberg Trough; Ru, Ruhla; SO, Ślęża Ophiolite; Sp, Spessart; STS, Saxo-Thuringian Suture; STZ, Sorgenfrei-Teisseyre Zone; TBA, Teplá-Barrandian Anomaly; TTZ, Teisseyre-Tornquist Zone; VDF, Variscan Deformation Front; WH, Wolsztyn High.

remains generally north-south (Figs 5, 6). It is therefore almost perpendicular to the Baltica margin and virtually all the major structural elements of the Bohemian Massif. This geometry must have important kinematic implications, which will be discussed later.

The Brunovistulian Block is a distinct domain characterized by a high magnetic response over most of its area (Fig. 5). The western boundary of this strongly magnetic domain coincides with the Moravian Suture. Neither the Rheno-Hercynian Suture nor any of the WNW-ESE-oriented faults (the Odra, Intra-Sudetic and Elbe faults) penetrates the Brunovistulian magnetic domain. The NE boundary of the domain lies on the SE extension of the Dolsk Fault (Fig. 5). The strong susceptibility gradient there may be related to either the contact of the basement blocks or to the rapid deepening of the magnetic basement. The edge of the magnetic plateau located 55 km north of the Kraków–Lubliniec Fault coincides with the point where the margin of the EEC is wedged out on the CEL-02 seismic refraction profile (Malinowski et al. 2005). The high magnetic domain covers two geological units, the Brunovistulian Block and the southern part of the Małopolska Block, previously considered to be two individual geological units separated by the Kraków– Lubliniec Fault (e.g. Bula et al. 1997). The latter is clearly coupled with an elongated anomaly associated with igneous bodies emplaced at the Carboniferous–Permian transition (Słaby et al. 2010). Neither the fault nor the anomaly extends towards the NW, with both ending inside the domain of high magnetic susceptibility (Fig. 5).

## Tilt derivative of the magnetic anomaly

The tilt derivative is one of the most useful image enhancement methods that can be applied to gridded potential field data. Because it acts as a type of automatic gain control, it is particularly useful for

revealing low-amplitude, short-wavelength features in data (Salem et al. 2008; Stewart and Miller 2018). Indeed, some features visible on the RTP magnetics map are highlighted by the tilt derivative (Fig. 8). This applies primarily to the anomaly representing the Rheno-Hercynian Suture. The suture is terminated in the ESE by an oblique local anomaly, probably corresponding to a fault running NW-SE (Fig. 8). This fault ends against the north-south boundary of the highly magnetic domain in the east. The derivative reveals an area of slightly increased susceptibility extending directly to the NE of the anomaly and correlated with the Rheno-Hercynian Suture. However, the Dolsk Fault is not visible at all in the magnetic data (Figs 5, 6 and 8). Only further NE is there a WNW-ESE-elongated negative anomaly (Figs 5, 8), which extends towards the WNW into the southern slope of the Ringkøbing-Fyn High. The position of the Variscan thrust front roughly coincides with this magnetic low (Fig. 8). The north-south western boundary of the Brunovistulian Block is also revealed well by the tilt derivative. The area of the Brunovistulian Block appears as a coherent magnetic domain in the data (Figs 6, 8).

## Bouguer gravity anomaly map

The gravity data are generally consistent with the magnetic data and determine a broad arc of positive anomalies from Germany through western Poland up to the Carpathian Thrust (Fig. 9). The arc includes the Saxo-Thuringian Zone, the Northern Phyllite Zone, the Sudetes Mountains, the Brunovistulian Block and the southern part of the Małopolska Block (Fig. 9). The surrounding negative anomalies are related to the Mid-Polish Trough in the NE, the Outer Carpathians in the SE and the Eger Graben in the south (Fig. 9). Smaller scale negative anomalies are associated with the North
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Reduced-to-the pole magnetic anomaly map for the eastern termination of the Variscan belt overlain with the key structural elements (UTM 32N). Enlargement of the SW part of Figure 5. LC, Leszczyniec Complex. For key to remaining abbreviations, see Figure 5.</figcaption></figure>

Sudetic and Opole Cretaceous basins. A conspicuous feature is a negative gravity anomaly directly north of the Dolsk Fault, between the Variscan arc and the Mid-Polish Swell, which coincides with a magnetic anomaly (Figs 5, 6 and 8). The coincidence of the gravity and magnetic lows suggests a significant basement depression that is not compensated by any uplift of the Moho.

The common gravitational effect of the basement and sedimentary cover means that gravity data do not allow the tracking of

individual structures of the Variscan basement, such as the Rheno-Hercynian Suture. Nevertheless, they allow us to recognize general trends in the structure of the basement. Primarily, a general continuity of the Saxo-Thuringian and Northern Phyllite Zones from Germany into the Sudetes Mountains is revealed, in addition to the Orlica-Śnieżnik and Góry Sowie massifs. Furthermore, a similar gravity signature is shared by the Brunovistulian Block, the southern part of the Małopolska Block and most of the Sudetes

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Geological sketch of the northern part of the Bohemian Massif (Franke *et al.* 1995) indicating the location of the main tectonic sutures. GSM, Góry Sowie Massif; ISB, Intra-Sudetic Basin; ISF, Intra-Sudetic Fault; KIM, Karkonosze-Izera Massif; LC, Leszczyniec Complex; MS, Moravian Suture; OSM, Orlica-Śnieżnik Massif; SBF, Sudetic Boundary Fault; SO, Ślęża Ophiolite; STS, Saxo-Thuringian Suture; TBA, Teplá-Barrandian Anomaly. Age assignments: Pt₃, Lower Proterozoic; Cm, Cambrian; Or, Ordovician; S, Silurian; Pz₁, Lower Paleozoic; D, Devonian; C, Carboniferous; Cr, Cretaceous. Source: modified from Pin *et al.* (2007).</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Tilt derivative of magnetic data for the eastern termination of the Variscan belt overlain with the key structural elements. For key to symbols and abbreviations, see Figure 5.</figcaption></figure>

Mountains further west (Fig. 9). The Dolsk Fault limits a major gravity high from the NE, although it was undetectable in the magnetic data. This is probably related to the increase in the thickness of low-density Permian and Mesozoic sediments NE of the fault. Moreover, according to the results of deep refraction sounding (Grad *et al.* 2003; Fig. 4), there is a density contrast of

0.4 g cm⁻³ between the upper crust of the Wolsztyn High and that underlying the Polish Basin further to the NE. A similar contrast is observed further to the ESE along seismic refraction profile CEL02 (Fig. 3b; Malinowski *et al.* 2005). Malinowski *et al.* (2005) correlate this contrast with the Kraków–Lubliniec Fault located c. 35 km south of the edge of the positive gravity anomaly (Fig. 9).

<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Bouguer gravity anomalies map for the boundary zone between the Variscan belt and East European Craton (Baltica) overlain with the key structural elements (UTM 32N). See text for details. For key to abbreviations, see Figure 5.</figcaption></figure>
## Tilt derivative of the Bouguer gravity anomaly

The tilt derivative adds some interesting details to our interpretation of the gravity anomalies. There are positive anomalies in the Wolsztyn Block that nearly overlap with the magnetic anomalies representing the Rheno-Hercynian Suture (Fig. 10). Furthermore, the tilt derivative reveals the same NW-SE-oriented lineament that is visible in the magnetic data and which is interpreted to cut the magnetic anomaly associated with the Rheno-Hercynian Suture (Figs 6, 8). The lineament, presumably representing a fault, is terminated against the western margin of the Brunovistulian Block (Fig. 10). The Moravian Suture, marking the western limit of the latter, is clearly visible from the tilt derivative map, but it does not continue further north than the Odra Fault and the southeastern end of the oblique fault (Fig. 10). Neither the tilt derivative nor any other previously discussed potential field data shows any connection between the anomalies and the position of the Variscan deformation front. This confirms the shallow, thin-skinned nature of this structure and the entire Variscan external fold-thrust belt.

## Post-Variscan tectonics

The Paleozoic Platform was affected by early Permian continental rifting soon after the end of the Variscan Orogeny (e.g. Dadlez et al. 1995; Van Wees et al. 2000; Stephenson et al. 2003). At that time, the TTZ acted as a mechanical buffer that prevented the penetration of extensional stresses further NE (Mazur et al. 2021). The main rift depocentre, the Mid-Polish Trough, was therefore localized directly SW of the TTZ and the EEC margin underlying the Paleozoic Platform was further attenuated (Fig. 4; Mazur et al. 2021). A regional early Stephanian hiatus is observed over much of the Variscan foreland, with Stephanian and lower Permian red beds unconformably overlying truncated Westphalian series (e.g. McCann 1996).

This regional uplift coincides with the onset of voluminous magmatism across the region, raising the possibility that the uplift could have been related to the presence of a widespread thermal anomaly within the upper mantle (e.g. Wilson et al. 2004). Widespread volcanic activity produced c. 70 000–80 000 km³ of volcanic rocks (Benek et al. 1996; Paulick and Breitkreuz 2005). The main volcanic centre was in the NE German Basin, where a

thickness of volcanic rocks of up to 2300 m has been found in places (Geißler et al. 2008). The thickness of the volcanic horizon gradually decreases eastwards and does not exceed a few hundred metres in western Poland (Geißler et al. 2008). Incipient rifting was accompanied by the deposition of lower Permian Rotliegend clastics and was followed by post-rift thermal subsidence and the sedimentation of upper Permian Zechstein salt (e.g. Kiersnowski et al. 1995; Bayer et al. 1999; Scheck and Bayer 1999).

Gradual thermal subsidence from the late Permian to the Late Cretaceous caused widening of the German–Polish Basin over the entire Paleozoic Platform and the SW slope of the EEC. The Polish Basin was inverted in the Late Cretaceous–early Paleogene as a far-field effect of the collision between northern Europe and the Alpine–Carpathian orogen (Krzywiec 2002). At that time, the Mid-Polish Trough was transformed into the Mid-Polish Swell, adjoining the TTZ from the SW (Krzywiec 2002).

The depth to base Permian structural map for Poland (Fig. 11) clearly shows that all the important faults discussed here (the Odra, Kraków–Lubliniec, Intra-Sudetic and Elbe faults) are the boundaries of important structural uplifts. This implies that these faults were reactivated by post-Variscan tectonics and their best-documented activity was associated with this period. Of course, the reactivation of earlier Variscan and pre-Variscan structures is not excluded and is even probable. However, these structures do not necessarily coincide exactly with the boundaries of the Variscan basement blocks and, due to a lack of better data, have been used in various reconstructions and interpretations. Only the Dolsk Fault is associated with a minor base Permian elevation, although the Wolsztyn High was active as a structural uplift in the early Permian (Kiersnowski et al. 2010) and Mesozoic (Mazur et al. 2006). Its subvertical geometry is inferred from a combination of seismic and well data that do not reach below the Carboniferous (Kiersnowski et al. 2010). The correlation of the Dolsk Fault with the results of deep refraction sounding (Grad et al. 2003) is therefore not well constrained.

## Discussion

The potential field data show the continuity of the Rheno-Hercynian Suture, which makes a 90° bend, changing its trend from NE–SW in Germany to WNW–ESE in Poland. Such a geometry has been

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Tilt derivative of Bouguer gravity data for the eastern termination of the Variscan belt overlain with the key structural elements. For key to symbols and abbreviations, see Figure 5.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Depth to the base of Permian–Cenozoic sediments, including Permo-Carboniferous volcanics, within a boundary zone between the Variscan belt and East European Craton (Baltica) overlain with the key structural elements. The coordinate system is Poland 1992, based on the ETRS89 datum, GRS80 spheroid and the Transverse Mercator projection with 19° E as a central meridian. See text for details. BT, Brunovistulian Block; CF, Caledonian Front; CaF, Carpathian Front; DF, Dolsk Fault; EEC, East European Craton; EF, Elbe Fault; GF, Grójec Fault; HCM, Holy Cross Mountains; ISF, Intra-Sudetic Fault; KLF, Kraków–Lubliniec Fault; MPT, Mid-Polish Trough; OF, Odra Fault; TTZ, Teisseyre–Tornquist Zone; VF, Variscan Front. Source: modified from Mazur *et al.* (2021).</figcaption></figure>

postulated for a long time by Franke (2000) and Franke and Żelaźniewicz (2000, 2002, 2023) and was considered admissible by Mazur *et al.* (2020, their fig. 12b). At the same time, the magnetic data show that the Rheno-Hercynian Suture ends NE of Wrocław at the northward extension of the Moravian Suture. The suture is truncated by a NW–SE-oriented oblique fault. The analysis of the potential fields data therefore does not confirm another bend in the Rheno-Hercynian Suture and its connection with the Moravian–Silesian Zone, which was postulated or considered possible in many interpretations (e.g. Kossmat 1927; Lorenz 1976; Engel *et al.* 1983; Lorenz and Nicholls 1984; Franke 1989; Tait *et al.* 1997; Franke and Żelaźniewicz 2000, 2002, 2023). By contrast, the magnetic data show that the Variscan belt ends with a semi-orocline, changing its direction by 90°. The ending of the Rheno-Hercynian Suture at the northern extension of the Moravian Suture resembles the concept of the ‘Moravian line’ (The Pace TMR Network Team and Winchester 2002), although we sustain the opinion of Mazur *et al.* (2020) that there is no evidence for the continuation of this line north of the Dolsk Fault.

The Rheno-Hercynian Suture is located between the Odra and Dolsk faults, overlapping neither of them. This shows that the large WNW–ESE strike-slip faults at the outskirts of Baltica are upper crustal structures that formed after the accretion of the Variscan belt and were also active in post-Variscan times (Fig. 11). None of the faults cuts nor disturbs the continuity of the Rheno-Hercynian Suture. This excludes the right-lateral strike-slip displacements of the order of 300 km (e.g. Aleksandrowski 1995; Aleksandrowski and Mazur 2002). Furthermore, our data do not even confirm more moderate estimates: 100 km each on the Elbe and Intra-Sudetic faults (Mazur *et al.* 2020) or 70 km on the Intra-Sudetic Fault (Franke and Żelaźniewicz 2002, 2023). Shifts in the Saxo-Thuringian Suture in the Bohemian Massif indicate lateral displacements of the order of 30–40 km on the Elbe and Intra-Sudetic faults (Fig. 6). There is no marker to estimate displacements on the Odra, Dolsk and Kraków–Lubliniec faults. It is worth noting, however, that none of these faults displaces the western boundary of the magnetic high associated with the Brunovistulian Block.

Three tectonic sutures, the courses of which were determined based on magnetic data, converge at a point east of Wrocław at the junction between Baltica and the Brunovistulian Block (Fig. 12). These structures are the Rheno-Hercynian, Saxo-Thuringian and Moravian sutures, as well as the boundary between the Teplá–Barrandian and Moldanubian Zones emphasized by the position of the Saxo-Thuringian Arc. Their convergence at a single point suggests that this was where the Euler pole was located during the accretion of the Variscan terranes to Laurussia. A similar location of the Euler pole during the accretion of the Variscan terranes was postulated earlier by Edel *et al.* (2003) and Kroner *et al.* (2016). Because the drift of the terranes docking at the southern edge of Laurussia had to respect the convergence along the small circles centred on the Euler pole, the collision could have had a predominantly frontal character, both towards the Baltica margin in Poland and the Avalonia margin in Germany (Fig. 13). Of course, the convergence also generated right- and left-lateral components along the margins of Baltica and Avalonia, respectively (Fig. 13). A left-lateral transpression has indeed been confirmed along the Avalonian part of the Rheno-Hercynian Suture (Stephan *et al.* 2016), whereas right-lateral transpression has been documented along several faults parallel to the Baltica margin (e.g. Mazur *et al.* 2020). The position of the Euler pole and the wedging of the Variscan terranes towards its locus demonstrate a high non-cylindricity of the Variscan belt near its eastern ending.

These observations have two important implications. First, the Brunovistulian Block must have been previously docked to the margin of Baltica. This agrees with the stratigraphic data (e.g. Valverde-Vaquero *et al.* 2000; Belka *et al.* 2002), which suggest a common history of the Brunovistulian Block and the adjacent Małopolska Massif since the Mid-Devonian. Moreover, the coherent magnetic high over the Brunovistulian Block indicates that it was not fragmented by strike-slip faults (Fig. 5). Resolving whether the Brunovistulian Block extends northward beyond the Kraków–Lubliniec Fault and belongs to the Teisseyre–Tornquist Terrane Assemblage of Franke and Żelaźniewicz (2023) is beyond the scope of the data used in this study. Second, the accretion of the Variscan
<figure><img src="image_14.png" /></figure>

**Fig. 12.** Simplified tectonic maps of the border zone between Laurussia and the Variscan belt showing the relationship between two successive convergence events. (a) Present-day extent of Baltica, Avalonia and the Variscan terranes in the basement of Central Europe. (b) Major magnetic and gravity anomalies indicating partitioning of the deformation pattern between the effects of the east–west shortening D₁, dominating SSW of the Elbe Fault, and the superimposed north–south shortening D₂, prevailing NNE of the fault. D₁ and D₂ shortening directions indicated by red and dark blue arrows, respectively. Orientation of the Variscan deformation front in Poland after Mazur *et al.* (2020). Dark blue cross-hatching indicates localized zones of compression or transpression associated with D₂. BH, Bielawy High; BV, Brunovistulian Block; CDF, Caledonian Deformation Front; CTF, Carpathian Thrust Front; DF, Dolsk Fault; EF, Elbe Fault; HCM, Holy Cross Mountains; ISF, Intra-Sudetic Fault; KLF, Kraków–Lubliniec Fault; MS, Moravian Suture; MSB, Moravo-Silesian Belt; NPZ, Northern Phyllite Zone; OF, Odra Fault; RhA, Rheno-Hercynian Arc; RhS, Rheno-Hercynian Suture; MS, Moravian (Staré Město) Suture; STS, Saxo-Thuringian Suture; SxA, Saxo-Thuringian Arc; TMB, Teplá–Barrandian–Moldanubian boundary; VDF, Variscan Deformation Front; WH, Wolsztyn High.

belt to the SW margin of Laurussia had to be partitioned into two successive events defined herein as D₁ and D₂ (Figs 12, 13).

Data collected since the beginning of this century show that the initial convergence in the eastern part of the Variscan belt was in the west–east to WNW–ESE direction (D₁). These observations come from the Karkonosze–Izera Massif (Mazur and Aleksandrowski 2001; Jeřábek et al. 2016), the Erzgebirge Mountains (Konopásek et al. 2001), the northern periphery of the Moldanubian Zone (Verner et al. 2009; Pertoldová et al. 2010), the Klodzko Massif in the Polish Sudetes Mountains (Mazur et al. 2004), the Moravo-Silesian Zone (Racek et al. 2017; Štípská et al. 2020) and the Vosges Mountains (Edel et al. 2013).

East–west shortening resulted in top-to-the-west (WNW) tectonic transport and the emplacement of nappes. Opposing top-to-the-east kinematics characterized deformation in the Moravian Suture Zone along the eastern margin of the Bohemian Massif (Schulmann and Gayer 2000; Racek et al. 2017; Štípská et al. 2020). This first phase of the Variscan collision lasted c. 350–340 Myr (Verner et al. 2009; Racek et al. 2017; Jouvent et al. 2023; Młynarska et al. 2024). At this stage, the Variscan belt in its most eastern part had not yet interacted with the Baltica margin outside

the Brunovistulian Block, which was by then docked at Laurussia (Fig. 13a). The eastern Variscan belt was strongly shortened in the north–south direction only in the next time interval of c. 340–330 Ma (D₂) (Konopásek et al. 2001; Jeřábek et al. 2016; Racek et al. 2017; Jouvent et al. 2023). This was the result of a frontal collision between the already connected Variscan terrane complex and the margin of Baltica (Fig. 13b). Magnetic and gravimetric data reveal a package of strong east–west to WNW–ESE anomalies in the Bohemian Massif north of the Elbe Fault (Figs 5–6, 8–10 and 12b). These anomalies are probably related to structures that were formed during the accretion of the Variscan belt to the Baltica margin. Magnetic anomalies mostly represent the Rheno-Hercynian Suture (Arc), which is rotated into parallelism with the WNW–ESE Baltica margin. At the same time, the Saxo-Thuringian Arc, which was initially perpendicular to the Baltica margin, was disrupted in parts along dextral strike-slip faults (Figs 12, 13).

The north–south shortening is an important component of the deformation of the European Variscan belt, as suggested by Edel et al. (2013, 2018) based on a combination of structural and palaeomagnetic observations. The latter dataset indicates an anticlockwise rotation of the belt by c. 40°. The belt was then
<figure><img src="image_15.png" /><figcaption>**Fig. 13.** Plate tectonic reconstructions of Baltica, Avalonia and Variscan terranes in the Plate Carrée projection for (a) 350 Ma and (b) 330 Ma. Dashed blue lines show small circles centred on the Euler pole. The first phase of the accretion of the Variscan terranes included east-west convergence (present-day coordinates) between the Brunowistulian Block and Avalonia. In the second phase, the north-south shortening between the Variscan belt and Baltica dominated. The Rheno-Hercynian and Moravian sutures were then reactivated as sinistral and dextral shear zones, respectively. The disruption of the Saxo-Thuringian Arc and rotation of the Rheno-Hercynian Arc are important. Transverse dextral faults were also formed at that time. Dark blue cross-hatching indicates localized zones of compression or transpression associated with north-south shortening. BV, Brunovistulian Block; DBB, Devonian back-arc basin; HCM, Holy Cross Mountains.; MD, Moldanubian; MS, Moravian Suture; PC, Pomeranian Caledonides; RhA, Rheno-Hercynian Arc; RhO, Rheno-Hercynian Ocean; RhS, Rheno-Hercynian Suture; SxO, Saxo-Thuringian Ocean; STS, Saxo-Thuringian Suture; Sx, Saxo-Thuringia; SxA, Saxo-Thuringian Arc; TB, Teplá-Barrandian, TS, Thor suture. Source: partly based on Cocks and Torsvik (2011), Domeier and Torsvik (2014) and Edel et al. (2018).</figcaption></figure>

divided into segments by NW-SE-trending dextral transpression zones. These zones are typically c. 30-50 km wide and are marked by arcuate lobes along the eastern boundary of the Teplá-Barrandian Zone (Fig. 12). These are: (1) the southern termination of the Orlica-Śnieżnik dome and adjacent Teplá-Barrandian rocks affected by the NW-SE-trending subvertical fabric (e.g. Chopin et al. 2012); (2) a wide belt south of the Elbe Fault characterized by a modification of the NE-SW structural trend to NW-SE (Schulmann et al. 2005); and (3) the c. 30 km wide and 100 km long Bavarian Zone limiting the Moldanubian Zone to the south. The age of this major deformation is estimated at 330-325 Ma based on the deformation of syntectonic granite intrusions distributed parallel to this zone (Megerssa et al. 2023).

Edel et al. (2013, 2018) proposed that the north-south shortening along the Bavarian Zone resulted from the indentation effect of the north-advancing Salzburgia Block (the former Vindelicia of Vollbrecht et al. 1989; Fig. 12). The ‘Salzburgia Block’ coincides with an important gravity high and a region of high velocity lower crust north of the Alps, indicating the presence of cratonic basement, possibly of northern Gondwana origin (Schulmann et al. 2022). Overall, our geophysical data indicate that the zone of maximum north-south shortening of the Variscan belt occurred along the southern margin of Baltica, whereas shortening of the other parts of the belt was caused by the northward movement of Gondwana-derived promontories (Figs 12, 13).

The north-south frontal collision of the Variscan belt with the Baltica margin east of the Harz Mountains would not have been possible without significant strike-slip displacements along the

western margin of the Brunovistulian Block. Since the beginning of the 1990s, highly oblique tectonic transport has been postulated in the Moravo-Silesian Zone at the western margin of the Brunovistulian Block (Schulmann 1990; Schulmann et al. 1991; Schulmann and Gayer 2000). Large-scale shearing towards the NE was parallel to the Moldanubian-Moravian boundary (i.e. the Moravian Suture) (Schulmann 1990; Schulmann et al. 1991). The right-lateral transpression, superimposed on the original nappe stack resulting from east-west shortening, was later confirmed by Racek et al. (2017), who provided time constraints in the range of 335-325 Ma for the north-south strike-slip displacement. This time interval probably represents the timing of the main collisional phase between the Variscides and the Baltica margin. A zone of important sinistral transpression affecting the NE-SW-trending Avalonia-Saxothuringia boundary and dominant dextral transpression operating along the SW boundary of Avalonia and Armorica, mapped by Stephan et al. (2016), may indicate the same indentation process. All this is consistent with the global north-south shortening of the Variscan belt, where various cratonic blocks (such as Avalonia in the west, Baltica in the NE and Salzburgia (Vindelicia) in the SE) acted as large indenters responsible for the massive deformation of the internal part of the European Variscan belt.

If the Euler pole for Laurussia and the Variscan terranes was located somewhere at the confluence of the Variscan sutures (Fig. 13), then the consequence was a decrease in the convergence rate down to zero towards the pole. In fact, if the MGCR extends along the Odra Fault, the intensity of the associated magnetic anomaly decreases. This means, first of all, a weaker source – that is,
fewer plutonic intrusions – and, indirectly, also a smaller amount of subducted material. Importantly, ophiolites, probably associated with the Saxo-Thuringian Suture, only occur in the Central Sudetes Mountains where the subduction must have been very slow. Nevertheless, due to plate kinematics, a frontal collision must have occurred in the Polish section of the Rheno-Hercynian subduction zone. This is consistent with the geophysical data, which indicate that the Baltica margin was underthrust beneath the Variscan accretionary wedge at a distance of c. 100 km (Fig. 4; Grad et al. 2002, 2003, 2008). It is therefore highly probable that both the Rheno-Hercynian Suture itself and the accompanying low-grade metamorphic slates drilled on the Wolsztyn and Bielawy highs occur in an allochthonous position. The Variscan metamorphic complexes at the level of the sub-Carboniferous surface therefore extend at least to the Dolsk Fault. At the same time, at the level of the lower crust, the Baltica margin, may extend up to the Odra Fault (Fig. 12).

The horizons of the top of the basement and the base of the Permian superimposed on the velocity structure model along the POLONAISE'97 P-4 profile (Grad et al. 2003) show significant discrepancies (Fig. 4). Although both these horizons are the product of a 3D algorithm (Mazur et al. 2021) and are therefore excessively smoothed, they show that the model layer with a velocity of 4.75 km s⁻¹ jointly represents the lower part of the Permian-Mesozoic sediments and the upper part of the pre-Permian (Carboniferous?) rocks. Moreover, rocks with a velocity of c. 5.75 km s⁻¹ beneath the Polish Basin correspond to unmetamorphosed or low-grade metamorphic sediments, whereas rocks with a velocity of 5.9 km s⁻¹ within the Sudetes Mountains and Wolsztyn Block belong to the Variscan basement. In this situation, it is not surprising that our attempt to build a 2D gravity and magnetic model based on the P-4 profile velocity structure was unsuccessful.

## Conclusions

The Variscan belt of Europe terminates in western Poland and Moravia, reaching the SW margin of Baltica and the western edge of the Brunovistulian Block. Although elements of the Variscan basement occur much further east within the Carpathian belt, they cannot currently be correlated with the Variscan zones stretching between the Iberian Peninsula and western Poland. The presence of Variscides further SE on the eastern side of the Brunovistulian Block is indicated by the direction of the Variscan deformation front running WNW-ESE up to the Ukrainian border. Particularly in SE Poland, Variscan shortening resulted in thin-skinned deformation of the EEC sedimentary cover, indicating orogenic push from the south and SW.

A set of good quality magnetic and gravity data, integrated here for the first time, for the area covering Poland, the Czech Republic and eastern Germany allows for a better understanding of the location and geometry of the tectonic sutures separating the terranes and orogenic zones. These data do not confirm either the U-shaped oroclinal loop of the Variscan belt or large-scale strike-slip tectonics along the Baltica margin. Instead, they indicate a 90° bending of the Variscan belt from a NE-SW to a WNW-ESE direction (a semi-orocline). The tectonic sutures converge at a point located at the junction of the Baltica margin and the western edge of the Brunovistulian Block. This may have been the site of the Euler pole for a plate pair including Laurussia and the Variscan terrane complex. The wedging of the Variscan terranes towards the NE indicates the highly non-cylindrical nature of the Variscan belt near its eastern end.

Convergence of the Variscan terranes initially occurred along the east-west direction (present-day coordinates) due to the subduction of a few intervening oceanic domains. Only later did the entire

eastern section of the Variscan belt dock to the margin of Baltica due to the closure of the Rheno-Hercynian Ocean. This resulted in a strong north-south shortening in the northern part of the Bohemian Massif and its foreland, which is confirmed, in addition to local geological data, by clear magnetic and gravity anomalies with a WNW-ESE orientation.

The area located on the NE foreland of the Bohemian Massif between the Odra and Dolsk faults (the Wolsztyn Block) has low-grade metamorphic slates in the basement, probably representing deposits of the passive margin of Laurussia. However, at the level of the lower crust, the Baltica margin may be underthrust as far SW as the Odra Fault. The position of the SW boundary of Baltica therefore depends on the crustal level considered. The metamorphic age of the low-grade rocks in the basement of the Wolsztyn Block is dated at earliest Carboniferous (earliest Tournaisian), constraining the timing for the closure of the Rheno-Hercynian Ocean. Carboniferous clastic rocks occur in a transgressive position on top of this metamorphic basement, marking a back-stepping of the foreland basin onto the hinterland. Deep reflection seismic profiles would be necessary to understand the exact structure of the area in which the attenuated Baltica margin underthrust the Variscan orogenic wedge.

Scientific editing by Wenjiao Xiao

**Acknowledgements** The authors thank Wenjiao Xiao for editorial handling of this paper. Many thanks are due to Hao Wang and an anonymous reviewer for their helpful comments and corrections to the text. We gratefully acknowledge Getech Group plc for access to the gravity and magnetic data.

**Author contributions** SM: conceptualization (lead), data curation (lead), formal analysis (lead), investigation (lead), methodology (lead), software (lead), validation (equal), visualization (equal), writing – original draft (lead); KS: conceptualization (supporting), investigation (supporting), methodology (supporting), visualization (supporting), writing – review and editing (lead).

**Funding** The authors acknowledge financial support from the Polish National Science Centre, Grant No. UMO-2021/43/B/ST10/01392, awarded to S. Mazur and the Czech Science Foundation, Grant No. 23-08249Sand, awarded to P. Štípská.

**Competing interests** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

**Data availability** The potential field data that support the findings of this study are available from Getech Group plc (https://getech.com/getech-explore/), but restrictions apply to the availability of these data, which were used under licence for the current study and are not publicly available. Data are, however, available from the authors on reasonable request and with permission of the Getech Group.

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