Together but separate: decoupled Variscan (late Carboniferous) and Alpine (Late Cretaceous-Paleogene) inversion tectonics in NW Poland

In Europe, formation of the Palaeozoic Variscan orogenic belt, and the Mesozoic-Cenozoic Alpine-Carpathian orogenic belt led to a widespread inversion events within forelands of both orogenic domains. We used legacy 2D seismic data together with the newly acquired 3D seismic data that, for the first time, precisely imaged sub-Zechstein (i.e. sub15 evaporitic) upper Palaeozoic successions in NW Poland in order to develop a quantitative, balanced 2D model of the late Palaeozoic – recent evolution of this area, characterised by a complex pattern of repeated extension and inversion. Four main tectonic phases have been determined: (1) Late Devonian – early Carboniferous extension and subsidence possibly related to extensional reactivation of Caledonian thrusts, (2) late Carboniferous inversion caused by the Variscan orogeny, (3) PermoMesozoic subsidence related to the development of the Polish Basin, and (4) its Late Cretaceous – Paleogene inversion. 20 Variscan and Alpine structures form a superimposed multilayer inversion system, mechanically decoupled by the Zechstein evaporites.


Introduction
Inversion tectonics has been intensely studied since the early 1980s', when a fully developed concept of sedimentary basin 25 inversion, exemplified by a tectonic graben bounded by deeply rooted normal faults subsequently reactivated as reverse faults or thrusts, was formulated (Glennie and Boegner, 1981;Bally, 1984). During this period, numerous papers and two dedicated volumes were published in which the geometry and evolution of inversion structures in different geodynamic settings were discussed Buchanan and Buchanan, 1995). Published case studies, documented by surface and / or subsurface data and supported by results of analogue and numerical modelling studies, dealt with the 30 geometry of inversion systems, the role of differential lithologies in the formation of inversion structures, the evolution of 1988; Del Ventisette et al., 2021;Krantz, 1991;Tari et al., 2021;Tortorici et al., 2019). In this case however "inversion" seems to be used as an equivalent of "extensional reactivation" which is much broader term that does not fully comply with the original concept of "inversion tectonics" coined by Bally (1984) (see also Holdsworth et al., 1997). Bally's original model was not focused on inversioni.e. reactivationof basin-bounding faults but on inversioni.e. partial or full destructionof a sedimentary basin. Destruction of a sedimentary basin (half-graben) in Bally's model is genetically linked 70 to reactivation (reverse in this case) of a basin-bounding fault(s) but fault reactivation is a secondary process here while the main emphasis is on demise of half-graben formed during extensional phase (Bally, 1984). On the other hand, negative inversion might be related to formation, not destruction, of a new basin that develops above extensionally reactivated thrust (e.g. Babaahmadi et al., 2018;Constenius, 1982Constenius, , 1996Deng et al., 2021;Powell and Williams, 1989;Tari et al., 2021;Velasco et al., 2010). Taking this into account it seems appropriate to delimit usage of the term "inversion tectonics" to 75 "positive inversion" as originally proposed by Bally (1984), and to abandon the term "negative inversion" that in fact is related to rather different tectonic scenario. End of the day, this is all a matter of terminology and definitions, and general consensus (or lack thereof) around them. Different opinions on that issue have been already expressedfor example, Cooper and Warren in their recent chapter (2020) provided short overview of various opinions on the term "negative inversion" and explicitly stated that it should be discarded. On the other hand, there are recently published important papers in which the 80 term "negative inversion" has been successfully used to describe regional tectonic evolution of particular regions and formation of various structures within the Earth's crust (e.g. Tari et al., 2021;Connors and Houseknecht, 2022). In this paper we follow our line of reasoning described above, describing inferred extensional reactivation (not negative inversion) of lithological variations of the sedimentary infill formed prior to onset of inversion i.e., prior, during and after extension. The most obvious example of influence exerted by lithology on inversion tectonics is the presence of ductile evaporites that leads to partial or full mechanical decoupling and formation of sub-and supra-salt / evaporitic structures, often of different geometries and kinematics (e.g., Brun and Nalpas, 1996;Jackson and Larsen, 2008;Jackson et al., 2013;Dooley & Hudec 2020;Hansen et al., 2021). A model of thin-skinned inversion system detached above evaporites is shown on Fig. 1C. One 90 important conclusion might be drawn from comparison of "classic" thick-skinned inversion system (Fig. 1A) and two thinskinned inversion systems, one without evaporites (Fig. 1B) and one with evaporites ( Fig. 1C)that despite their fundamentally different structural characteristics at depth they all have the same shallower tectono-sedimentary expression with identical key elements of the inverted extensional system such as inversion anticline and syn-inversion growth strata.
Taking this into account, it could be postulated that "classic" inversion scenario of extensional graben (sedimentary basin) 95 should not be restricted to basins with their bounding faults rooted within the crystalline basement and could equally well be also be applied to thin-skinned systems, either developed above ductile evaporites or simply located above thick preextensional sedimentary cover that prevented faulting deeply rooted within the crystalline basement during extension and subsequent inversion (see also Cooper and Warren, 2020).
In Europe, formation of the Palaeozoic Variscan orogenic belt, and then the Mesozoic-Cenozoic Alpine-Carpathian orogenic belt led to a widespread inversion events within forelands of both orogenic domains. Variscan (i.e., late Carboniferous) inversion is well documented in areas where either suitable outcrops of deformed Palaeozoic rocks are present or deeper seismic imaging is not hampered by a thick Upper Permian (Zechstein) evaporitic cover, such as for example Southern UK, Belgium or N Germany (e.g. Chadwick and Evans, 2005;Pharaoh et al 2020;Deckers and Rombaut, 2021;von Hartmann, 105 2003). On the other hand, Alpine (i.e., Late Cretaceous -Paleogene) inversion is well documented by seismic reflection data that image usually complex supra-salt / supra-evaporitic thin-skinned structures that are however characterized by not always clear relationship to the sub-salt / sub-evaporitic deformations (e.g. Chadwick and Evans, 2005;Mazur et al., 2005;Krzywiec, 2006).

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In this paper, we analyse complex inversion tectonics in NW Poland, where the superimposed effects of Variscan and Alpine foreland compression led to the formation of a multilayer decoupled inversion system. Our analysis is partly based on a newly acquired high-quality 3D seismic data that provided unique, so far unavailable insight into the pre-Zechstein sedimentary cover. Geometry, kinematics and casual links between consecutive phases of extension / subsidence and compression / inversion are discussed in the context of the regional Palaeozoic and Mesozoic evolution of Central and 115 Eastern Europe.

Geological setting
The study area in NW  is located in region where crystalline basement is buried to a depths exceeding 11 km, as documented by deep seismic refraction data and gravity-magnetic modelling studies (Guterch et al., 1999;Guterch and Grad, 2006;Grad and Polkowski, 2016;Mazur et al., 2021). This is also compatible with the results of deep seismic 120 reflection surveying in the area adjacent to the east that documented the Caledonian orogenic front thrust over the undeformed lower Palaeozoic foreland and underlain by a lower plate that gradually descends towards the SW to a depth of at least 10 km (Krzywiec et al., 2014;cf. also Lazauskienė et al., 2002;Mazur et al., 2018;Poprawa, 2019Poprawa, , 2020Poprawa et al., 1999). Structures analysed in this paper evolved above the frontal part of this deeply buried Caledonian thin-skinned orogenic belt (Mazur et al., 2016), below which the Precambrian suture of the Teisseyre -Tornquist Zone is located (Mazur 125 et al., 2015).
The Caledonian orogenic belt in N Poland ceased to exist due to early Devonian uplift and widespread denudation (Dadlez, 1978;Poprawa, 2019). In the Variscan geological framework, the study area is located approximately 150 km NE from the front of the Variscan orogen that extends from south-western England across northern France, Belgium, northern Germany 130 to Czech Republic, Poland and Western Ukraine, and then on to Romania, Bulgaria and Turkey (Fig. 2;e.g., Catalan et al., and Topuz, 2017;Warr, 2012). The most external part of the Variscan orogen i.e., the Rhenohercynian zone forms a foreland fold-and-thrust belt built of the deformed Devonian -Carboniferous sedimentary succession deposited along the southern margin of Laurussia and subjected to progressive thrusting and folding (Oncken et al. 1999). The final emplacement of the 135 Variscan fold-and-thrust belt onto its foreland plate took place in the late Carboniferous (Kröner et al., 2008;Mazur et al., 2010Mazur et al., , 2020. It led to the regional flexure of the foreland plate and formation of extensive Carboniferous foreland basin filled with a thick synorogenic sedimentary succession (Burgess and Gayer, 2000;Deckers and Rombaut, 2021;Franke, 2014;Kombrink et al., 2010;Leveridge and Hartley, 2006;Maynard et al., 1997;McCann, 1999;McCann et al., 2008;Narkiewicz, 2007Narkiewicz, , 2020Opluštil and Cleal, 2007;Tomek et al., 2019). Final stages of evolution of the external Variscan 140 fold-and-thrust belt were associated with a widespread inversion of Palaeozoic basins located within its foreland (e.g., Corfield et al., 1996;Glen et al., 2005;Peace and Besly, 1997;Pharaoh et al 2020;Shail and Leveridge 2009;Smith, 1999;von Hartmann, 2003). Until recently, no Variscan inversion structures have been recognized within the NE part of the Variscan foreland, including NW Poland, mainly because of low quality of sub-salt seismic imaging of the sub-Zechstein interval. Relatively intense compressional deformations were documented in the area devoid of the Zechstein evaporitic 145 cover in SE Poland and western Ukraine within the Lublin -Lviv Basin (Krzywiec et al., 2017a,b;Kufrasa et al., 2020;Tomaszczyk and Jarosiński, 2017;Zayats, 2015) and adjacent areas including the Holy Cross Mountains (e.g., Czarnocki, 1957;Lamarche et al., 1999;Konon 2006Konon , 2007. However, they have been recently interpreted as belonging to the frontal Variscan fold-and-thrust belt rather than being foreland inversion structures (see Krzywiec et al., 2017a,b;Kufrasa et al., 2020;Mazur et al., 2020 and references therein). 150 The upper Palaeozoic sedimentary cover within the study area consists of the (upper Emsian-?) Middle Devonian to lower Carboniferous (Mississippian) sediments deposited within the Western Pomeranian Basin (Fig. 2, Fig. 3). This succession is composed of mainly clastic and carbonate sediments, with subordinate evaporites (Lipiec and Matyja, 1998;Narkiewicz, 2007Narkiewicz, , 2020Narkiewicz et al., 1998;Matyja, 1993Matyja, , 1998Matyja, , 2006Matyja, , 2008Muszyński et al., 1996). Late Devonianearly 155 Carboniferous formation of sedimentary cover of Western Pomerania could be associated with regional extension and subsidence along the Laurussia southern margin (Smit et al., 2018).
The Pennsylvanian in the Western Pomeranian Basin is developed only locally in the northern part of the basin (Żelichowski, 1995;Matyja, 2006;Kuberska et al., 2007). Also locally, uppermost Carboniferous to lowermost Permian 160 volcanic rocks have been encountered by wells (Maliszewska et al., 2016).
The Variscan foreland basin ceased to exist in the latest Carboniferous due to regional post-orogenic uplift and erosion (Edel et al., 2018;McCann et al., 2006;Ziegler, 1990). This was followed by renewed subsidence and sedimentation related to formation of an extensive Permo-Mesozoic basin that covered the large part of Europe (Doornenbal & Stevenson, 2010, 165 Basin, together with its axial most subsiding part, the Mid-Polish Trough, formed the easternmost segment of this vast sedimentary basin (Dadlez et al., 1995;Stephenson et al. 2003;Ziegler, 1990). The Polish Basin underwent long-term Mesozoic thermal subsidence, punctuated by three major pulses of extension-related accelerated tectonic subsidence: during late Permian to Early Triassic times, in the Late Jurassic (Oxfordian to Kimmeridgian), and in the early Cenomanian (Dadlez 170 et al. 1995. Evolution of the NW and central segments of the Polish Basin, where a thick Zechstein evaporitic cover developed, was characterized by regional mechanical decoupling between the sub-Zechstein Rotliegend and older substratum and the Mesozoic supra-Zechstein cover (cf. Krzywiec, 2006a,b;Krzywiec et al., 2006). This led to formation of a system of peripheral extensional structures located above the basin margins', including grabens or halfgrabens detached in evaporites, and salt structures (Krzywiec, 2002a(Krzywiec, , 2012Rowan and Krzywiec, 2014;Warsitzka et al., 175 2021).
Inversion was associated with substantial uplift and erosion of the axial part of the Polish Basin i.e., the Mid-Polish Trough, which presently forms a regional anticlinal structure referred to as the Mid-Polish Anticlinorium (Swell), outlined by the 190 Cenozoic subcrop of the Lower Cretaceous and older rocks (Fig. 4). Inversion commenced in the late Turonian and lasted until the Maastrichtian -Palaeocene (e.g. Krzywiec, 2002Krzywiec, , 2006bKrzywiec et al., 2018;Resak et al. 2008). Due to regional inversion-driven uplift of the Mid-Polish Anticlinorium, the Upper Cretaceous mostly syn-inversion succession is presently preserved only along its flanks. Increased Late Cretaceous subsidence, related to flexural bending of both flanks of the uplifted Mid-Polish Anticlinorium (cf. Hindle and Kley, 2021) and combined with globally high Cretaceous sea level, 195 created relatively large accommodation space filled by syn-kinematic Upper Cretaceous strata. On the other hand, progressive growth of particular inversion structures, including also compressionally-reactivated salt diapirs, led to localised reduction of accommodation space and erosion, associated with formation of growth strata characterised by thickness reductions, progressive unconformities and facies changes (Leszczyński, 2012(Leszczyński, , 2002Krzywiec, 2002aKrzywiec, , 2006bKrzywiec, , 2012 associated with compressional reactivation of peripheral thin-skinned structures formed above the basin's flanks (e.g. Burliga et al., 2012;Krzywiec, 2002). The Koszalin -Chojnice Zone (Structure), located within the NE flank of the NW segment of the Mid-Polish Anticlinorium (Fig. 5, Fig. 6) and analysed in this paper (see below), is one of these peripheral structures that underwent substantial inversion well documented by seismic data (cf. Krzywiec, 2006a,b).

Seismic and well data
The seismic data used in this study included legacy 2D data and newly acquired 3D data. The regional seismo-geological 210 transect shown on Figure 6 was assembled from several 2D profiles and, then, depth converted using velocity data from deep wells. The studied NE segment of this transects imaged the NE edge of the Mid-Polish Anticlinorium and the Koszalin -Chojnice Structurea peripheral structure developed within the NE flank of the Mid-Polish Trough / Anticlinorium. The 2D seismic data in this part of the basin, characterised by a relatively thin Zechstein cover, did image some structures within the sub-Zechstein succession, although relatively poor quality of seismic imaging did not allow constructing a reliable 215 geological model for the deeper substratum ( Fig. 7A; cf. Antonowicz et al., 1994). A major breakthrough was related to acquisition of high-quality 3D seismic data that provided a clear sub-Zechstein image in the study area (Trela et al., 2011;Fig. 7B). The seismic data was available in the depth domain and calibrated by several wells that, however, drilled the entire Permo-Mesozoic cover but encountered only the topmost part of the sub-Permian (i.e., sub-Zechstein) substratum, providing rather limited stratigraphic information on deeper seismic horizons within the Drzewiany Graben, excellently imaged by the 220 new 3D seismic data. The D-2 well and some indirect evidences were used to estimate the age of the sedimentary infill of this graben as Upper Devonian (Frasnian -Famennian?) to lower Carboniferous (Tournaisian; see also below).

Structural restoration
Structural restoration was carried out along the cross-section that was located perpendicular to the strike of main structures 225 imaged by the seismic data (Fig. 6). It was constructed using the NE part of the regional seismo-geological transect and two inlines extracted from the 3D survey that was located in the central part of the cross-section.
Standard kinematic algorithms such as fault-bend folding and simple shear were used in order to obtain pre-deformational geometry of key seismic horizons. Within the Drzewiany Graben, a shear angle was set to 50° and corresponds to the plane 8 of maximum shear oriented parallel to minor normal fault surfaces (Dula, 1991;Xiao and Suppe, 1992). The relative timing of faults' activity was constrained based either on the presence of growth strata or cross-cutting relationships.
A maximum thickness of the syn-kinematic Upper Devonianlower Carboniferous sedimentary sequence is deduced from the Drzewiany Graben, where it attains up to 4 km. Given that the most complete stratigraphic profile of the syn-kinematic 235 strata is preserved within the SW part of the graben, it was used to approximate the currently missing portion of the Devonian-Carboniferous sedimentary rocks within the graben. Since little is known about a pre-deformation extent, geometry and thickness of the syn-tectonic strata at graben flanks, deposition of 50-m thick horizontal strata was assumed at each pre-inversion restoration step. It should be stressed though that, due to scarcity of data and widespread post-inversion erosion, reconstruction of the Devonian -Carboniferous cover outside the Drzewiany Graben was not the aim of this 240 balancing exercise. The cross-sectional shape of the unconformity at the base Permian was approximated by a regional trend line that connects the local depressions along this horizon. Flattening of the reference line resulted in preserving morphology of the unconformity. The initial thickness of Cretaceous sedimentary sequence eroded after the Alpine inversion was reconstructed using published paleothickness maps (Leszczyński, 2002(Leszczyński, , 2012.

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Modifications and improvements of the initial seismic interpretation were iteratively introduced until a satisfactory fit of the kinematic model to seismic and well data was achieved. The quality of the restored cross-section was verified via forward modelling by successively adding strain to the restored bed geometry, until the present-day shape of the horizons was obtained.

Seismic and well data
Well D-2 calibrated the Permo-Mesozoic succession and provided partial stratigraphic information about the upper Palaeozoic infill of the Drzewiany Graben (Fig. 7C). It drilled Tournaisian strata within the hangingwall of the master fault of the graben, went through the fault, and encountered Frasnian -Famennian within the footwall. Upper Devonian was interpreted within the axial part of the graben using similarity of seismic horizons from the footwall and the hangingwall. 255 Thickening of the large part of the sedimentary infill of the Drzewiany Graben towards the master fault suggests its syndepositional activity during extension, similarly to the model shown on Figure 1. On the other hand, the present day geometry of the infill, in particular an anticline above the master fault, suggests substantial inversion (cf. Fig. 1).
2D data provided information on the regional present-day geometry of the Permo-Mesozoic succession along the NE edge of 260 the Mid-Polish Anticlinorium (Fig. 6). The Koszalin -Chojnice Structure was interpreted as a thin-skinned anticlinal structure developed above the listric reverse fault rooted within the Zechstein evaporites, which is compatible with an overall geometry of this structure imaged by a large number of good-quality seismic data (cf. Krzywiec, 2006b(cf. Krzywiec, , 2012; see also below).

Cross-section balancing
Cross-section restoration permitted to distinguish four major deformation events in the tectonic evolution of the Drzewiany Graben (Fig. 8). According to the most plausible scenario, the Drzewiany Graben formed in response to Late Devonianearly Carboniferous NE-SW-oriented horizontal extension (Fig. 8a-i). Tectonic activity of the two conjugate, oppositelydipping graben-bounding normal faults created accommodation space successively filled with syn-tectonic deposits. A 270 heterogeneous displacement along the master faults resulted in vertical variation in the geometry of growth strata within the graben: antithetic rotation and thickening toward the northeast is the most pronounced at the basal section of the synkinematic sedimentary sequence, as opposed to the youngest sub-parallel layers maintaining almost constant thicknesses ( Fig. 8g-h). During the final stage of the extensional evolution of the Drzewiany Graben, the syn-tectonic strata were disrupted by secondary normal faults ( Fig. 8i; cf. Jagger and McClay, 2018;McClay, 1995;McClay and Scott, 1991). The 275 estimated total amount of the Late Devonianearly Carboniferous horizontal extension responsible for formation of the Drzewiany Graben was as high as 10 km (Fig. 8). However, this should be regarded as a minimum value as thickness of the post-Tournaisian strata that could have been deposited in this area is not known. In addition, the assumed thickness of the Tournaisian is a minimum estimate.

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The following phase of the Variscan structural inversion affected only the SW-dipping master fault by inducing 2 km of reverse displacement (Fig. 8j). The remaining normal faults do not show any seismic-scale signs of compressional reactivation. The syn-extensional sedimentary infill of the Drzewiany Graben, occupying a proximal portion of the hanging wall, was, then, folded due to material translation over the underlying concave master fault. As a result, an asymmetric, open anticline with c. 1.3 km of structural relief was produced (Fig. 8j). Its topmost part, together with growth strata that might 285 have been deposited during inversion, was removed due to syn-to-post-inversion erosion.
The subsidence centre of the Polish Basin, illustrated on Figure 8l-o, developed SW from the analysed profile, within the Mid-Polish Trough, i.e., in the area, where currently the Mid-Polish Anticlinorium, formed by the second inversion event, is located (cf. Fig. 6). Subsidence within the Mid-Polish Trough led to tilt of the pre-Permian strata by c. 4° to the SW. It 290 generated accommodation space that was successively filled in with sedimentary rocks thickening to the SW (i.e., towards the basin depocenter). Normal faulting related to the basin formation only slightly affected the study area by growth of new SW-dipping faults beneath the Zechstein evaporitic cover. It should be noted that faults outlining the inverted Drzewiany Graben were not reactivated during this tectonic phase. This deformation event yielded up to 500 m of horizontal stretching. 295 major deformation stage discernible in the study area. It was caused by the NE-SW-oriented compression and formation of the thin-skinned anticlinal Koszalin -Chojnice Structure ( Fig. 8q; cf. Krzywiec, 2006aKrzywiec, , 2021. Its location might have been triggered by a buttressing effect of the sub-Zechstein basement steps related to small normal faults that were not reactivated during the inversion. Growth of the fault-related anticline was associated with c. 500 m of horizontal shortening. Post-300 inversion erosion removed the topmost part of the Koszalin -Chojnice Structure and part of the Upper Cretaceous syninversion cover. The Cenozoic post-inversion strata in this area are of negligible thickness and are not considered in this model.

Discussion and conclusions
The quantitative, balanced model that was prepared using seismic and well data from NW Poland depicted four main 305 tectonic phases: (1) Late Devonianearly Carboniferous extension, (2) late Carboniferous inversion, (3) Permo-Mesozoic subsidence, and (4) Late Cretaceous -Paleogene inversion. Decoupling between two inversion events was caused by Zechstein evaporites. All these four phases will be discussed below in a regional context of geological evolution. Regional conceptual model depicting pre-Permian Palaeozoic evolution of the study area is shown on Figure 9. Its eastern part is based on regional seismic profile Pl-5400 of the PolandSPAN ® survey acquired above the Caledonian foredeep i.e. the Baltic 310 Basin (Krzywiec et al., 2014;Mazur et al., 2016), its western part on seismic data presented in this paper.
Often, such reactivation is associated with formation of half-grabens with syn-kinematic deposition focused above hangingwall of such asymmetric extensional system, and formation of growth strata characterised by divergent stratal pattern 320 and thickening towards the master fault, as shown on middle panels of Figure 1A, 1B and 1C (cf. Babaahmadi et al., 2018;Constenius, 1982Constenius, , 1996Powell and Williams, 1989;Tari et al., 2021;Velasco et al., 2010). As described by e.g. Ivins et al. (1990), dips of many normal faults shallowing with depth are caused by the reactivation of pre-existing thrust faults of underlying thrust belts. All these features are compatible with characteristics of extensional Late Devonianearly Carboniferous half-grabens illustrated on Fig. 8b-i, including both their syn-kinematic sedimentary infill as well as geometry 325 of the master fault. This fault could have been inherited from the Caledonian thrust belt although unequivocal seismic evidence of that is currently lacking. Late Palaeozoic extensional grabens have been widely documented in different parts of the extensionally reactivated Caledonides (e.g. Coward et al., 1987;Fossen, 2010;Koehl et al., 2018;Norton et al., 1987;Osmundsen and Andersen, 2001;Rowan and Jarvie, 2020;Scisciani et al., 2021;Séranne et al., 1989;Stemmerik, 2000).
Such grabens, very similar to the Drzewiany Graben, located above the Caledonian thin-skinned fold-and-thrust belt, have 330 been also documented using deep seismic data in the south-western Baltic Sea, NW from the Western Pomerania Basin (Lassen et al., 2001). In NW Poland, extensional reactivation of low-angle thrusts of the Caledonian thin-skinned orogenic wedge, characterised by rather large thickness (cf. Krzywiec et al., 2014;Mazur et al., 2016), explains both a listric geometry of the master fault that governed the evolution of the Drzewiany Graben as well as its thin-skinned character ( Fig. 9; see also coincided with a regional extensional phase that affected the southern margin of Laurussia (cf. Smit et al., 2018).
Inversion tectonics and reactivation of basement faults and fracture zones within the forelands of orogenic belts is a wellknown process (cf. Ziegler et al., 2002). Well documented examples include the Apennines (Costa et al., 2021;Scisciani, 2009), Andes (Delgado et al., 2012;Bilmes et al., 2013;Iaffa et al., 2011) and the Alps (Schori et al., 2021). Deformation 340 within the Variscan orogenic belt also have expressions within the Variscan forelanda whole array of inverted faults and basins in front of the Variscan belt have been documented in the S UK (Chadwick and Evans, 2005;Corfield et al., 1996;Glen et al., 2005;Peace and Besly, 1997;Shail and Leveridge, 2009;Smith, 1999). Figure 10 shows an interpreted seismic profile across the Eakring anticline that documents substantial Dinantian (Tournaisian -Visean) extension and subsidence, followed by late Carboniferous Variscan inversion, post-inversion erosion, and Permo-Triassic sedimentation. In this case, 345 there was no Late Devonian extension although lack of deep wells and inferior seismic imaging at deeper level probably does not preclude this. The Eakring anticline could be directly compared to the inverted Drzewiany Graben, both in terms of an overall geometry and main stages of development of Variscan inversion structures. There are also similar sub-Zechstein Variscan structures imaged on seismic data in N Germany (von Hartmann, 2003).

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Late Cretaceous inversion in the vicinity of NW Poland has been documented by numerous authors (Deeks and Thomas, 1995;Krzywiec et al 2003;Mazur et al., 2005;Meissner et al., 2002;Sopher et al., 2016;Seidel et al., 2018;Deutschmann et al., 2018). Seismic examples from areas without Zechstein evaporites show deeply rooted reverse faults, along which basement blocks have been uplifted. However, different geometries are observed in the areas where Zechstein evaporites were deposited. The evaporites led to regional mechanical decoupling between the sub-evaporitic basement and 355 supra-evaporitic cover, both during extension as well as inversion (e.g. Ahlrichs et al., 2020;Betz et al., 1987;Burliga et al., 2012;Dooley and Hudec, 2020;Lohr et al., 2007;Marsh et al., 2010;Soto et al., 2007;Stewart, 1999;Withjack and Callaway, 2000). In our study area, a thin-skinned inversion structurethe Koszalin -Chojnice Structurehas been documented by seismic data. Due to deep post-inversion erosion, the top of this inversion anticline and associated inversionrelated growth strata were removed. Better examples of the same structure are provided by seismic data from its more south-360 eastern segment, where relatively thick Upper Cretaceous succession is still preserved (Fig. 11A; cf. also Krzywiec, 2006bKrzywiec, , 2012. Similar structures have been documented in S England (e.g. Chadwick and Evans, 2005;Cosgrove et al., 2021). One of them is the Weymouth anticline, shown on Figure 11B. It evolved from Early Jurassic to Early Cretaceous due to detached thin-skinned listric normal faulting above the morphologically varied sub-salt basement. For this structure, Cenozoic (Miocene) inversion was postulated (Chadwick and Evans, 2005). 365 As it was described and illustrated above, the area analysed in this paper underwent two phases of inversion. Repeated extension and compression occuring along the same faults have been described by many authors (e.g., Bosworth and Tari, 2021;Dichiarante et al., 2020;Minguely et al., 2010). In our case, the presence of Zechstein evaporites resulted in mechanical decoupling between the Devonian-Carboniferous and Triassic-Cretaceous levels, and, despite close location, 370 both inversion structures evolved independently. Variscan inversion was associated with compressional reactivation of a listric normal fault that might have originally originated as a Caledonian thrust. Alpine (Late Cretaceous -Palaeocene) inversion was associated with listric thin-skinned reverse faulting detached within the Zechstein evaporites. Both inversion structures are compatible with certain elements of the classic inversion model by Bally (1984), but collectively form a complex, superimposed multilayer decoupled inversion system. 375

Data availability
Seismic and well data (excluding Jamno IG-2 well) used in this study are confidential and not available publicly. Data from Jamno IG-2 well could be accessed via Central Geological Database and National Geological Archive maintained by the Polish Geological Institute. 380

Author contribution
PK wrote most of the text, prepared most of the figures and compiled the paper, MK prepared balanced model, prepared relevant part of the text and Fig. 8, PP and SM provided expertize on regional geology, MK and PŚ were instrumental in providing the data used in this study and were involved in discussions on interpretation of seismic data.

Competing interests 385
The authors declare that they have no conflict of interest.

Special issue statement
This article is part of the special issue "Inversion tectonics -30 years later" that was organized as a follow-up to the similarly entitled technical session at the EGU General Assembly, Vienna, Austria, 7-12 April 2019.
Acknowledgements 390 PGNiG S.A. kindly provided all the seismic and well data used in this study. Seismic data was acquired and processed by Geofizyka Toruń S.A. IHS Markit and Petroleum Experts are acknowledged for providing software for seismic data interpretation and cross-section balancing, respectively. PK would like to thank colleagues from Geofizyka Toruń S.A., especially Paweł Pomianowski and Mariusz Łukaszewski, for stimulating discussions regarding various aspects of acquisition and processing of seismic data from the Pomerania region. This study was supported by PGNiG S.A., regional 395 analysis was completed within the NCN grant UMO-2015/17/B/ST10/03411. Gabor Tari and Mark Cooper are thanked for their reviews that helped to finally shape this paper.  Geol., 38, 289-305, 1994. Babaahmadi, A., Sliwa, R., Esterle, J. and Rosenbaum, G.: The evolution of a Late Cretaceous-Cenozoic intraplate basin (Duaringa Basin), eastern Australia: evidence for the negative inversion of a pre-existing fold-thrust belt, International 405

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Figure 2: Regional map showing main elements of the Variscan orogenic belt and its foreland between the UK / France and Poland / Ukraine (adopted from Krzywiec and Kufrasa, submitted, compiled after Mazur et al. 2020, Narkiewicz 2007, Opluštil and Cleal 910 2007Ziegler 1990, Catalan et al. 2020. Grey area: present-day post-erosional extent of the Carboniferous basin. Variscan orogenic front (thick dark violet line) is shown as a foreland-verging thrust but it should be kept in mind that this is regional generalization meant to illustrate general vergence of the entire thrust belt and that along that front also backthrusting and wedging could be observed. HCM: Holy Cross Mountains.    : A: Regional seismo-geological transect illustrating structure of the Mid-Polish Anticlinorium (Swell) in NW Poland (cf. Krzywiec, 2006a, Krzywiec et al. 2006). Red rectangle: part of the transect that was used to construct balanced model shown on Fig. 8. B: schematic model of a decoupled sedimentary basin developed above thick salt layer during thick-skinned sub-salt extension (modified after Withjack and Callaway, 2000; see also Krzywiec, 2006bKrzywiec, , 2012. Peripheral structures developed within the supra-salt sedimentary cover during decoupled extension often focus thin-skinned compressional deformation during ensuing basin inversion (cf. Burliga et al.,    Both seismic lines are displayed with the same horizontal and vertical scale so certain structural features could be directly compared.