Late Carboniferous thin-skinned compressional deformation above the SW edge of the East European craton as revealed by seismic reflection and potential field data—Correlations with the Variscides and the Appalachians
Published:September 11, 2017
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Piotr Krzywiec, Stanisław Mazur, Łukasz Gągała, Mateusz Kufrasa, marek Lewandowski, michał malinowski, Vinton Buffenmyer, 2017. "Late Carboniferous thin-skinned compressional deformation above the SW edge of the East European craton as revealed by seismic reflection and potential field data—Correlations with the Variscides and the Appalachians", Linkages and Feedbacks in Orogenic Systems, Richard D. Law, J. Ryan Thigpen, Arthur J. Merschat, Harold H. Stowell
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For the first time, modern seismic reflection data along with gravity and magnetic data were used to image the structure of a fold-and-thrust belt overlying the SW margin of the East European craton in SE Poland. These data demonstrate that the Variscan orogen extends eastward much farther than previously believed and terminates against the East European craton basement slope. The structural setting of this newly documented eastern extension of the Variscan fold-and-thrust belt in SE Poland is comparable to that of the Alleghanian orogen emplaced on the margin of the North American craton. Variscan deformation documented in SE Poland is more intense than anywhere else beneath the Permian–Mesozoic German-Polish Basin east of the Harz Mountains, probably because of buttressing by the relatively shallow basement of the East European craton. Our study focused on two regional tectonic units: (1) the Radom-Kraśnik block, a NW-SE–elongated structural high where early Paleozoic to Devonian strata subcrop beneath the Permian–Mesozoic cover, and (2) the Lublin Basin, a major Paleozoic sedimentary basin developed above the SW slope of the East European craton. The seismic data image the Radom-Kraśnik block as a thin-skinned fold-and-thrust belt with a 10–12-km-thick pile of Ediacaran (?) to Devonian sediments tectonically emplaced on the margin of the East European craton. These sediments are involved in a NNE-vergent stack of thrust units striking oblique to the East European craton margin slope. Individual thrusts branch off from a basal detachment that is located in the basal part of Ediacaran sediments unconformably overlying the East European craton crystalline basement. The frontal part of the Radom-Kraśnik fold-and-thrust belt is a triangle zone related to the jump of the basal detachment from the intra-Ediacaran position to the base of the Silurian shales. The base-Silurian detachment continues under the gently folded Lublin Basin and emerges along the Kock fault zone, which is a thin-skinned ramp placed over a basement step at depth. The Kock fault zone could be considered an analogue to the so-called mushwad structures described within the frontal part of the Appalachians.
The eastern extremity of the Variscan orogen is deeply concealed beneath a thick succession of Permian and Mesozoic sediments, including an extensive horizon of Zechstein evaporites that attenuate seismic energy and restrict seismic penetration of pre-Permian basement. Therefore, the extent and magnitude of Variscan deformation have remained difficult to determine, and new data are required to constrain the eastern limits of the Variscan deformation front. Hence, the high-quality seismic reflection data recently acquired in SE Poland provide unique insight into the structure of the contact zone between the East European craton and the younger terranes adjoining the East European craton along the Teisseyre-Tornquist zone. Beneath Permian and younger sediments, for the first time, the new data image a zone of intense late Carboniferous deformation, with an elongate belt of folded and thrust-faulted Ediacaran (?) to Devonian rocks tectonically emplaced on the margin of the East European craton. This poses a question about the relationship between the observed fold-and-thrust belt and the Variscan orogen extending farther to the SW. In this paper, we consider whether the late Paleozoic fold-and-thrust belt stretching along the East European craton margin represents a localized zone of Variscan reactivation at the junction of basement blocks right- laterally displaced in the foreland along a Variscan strike-slip fault or a continuation of the Variscan deformation front emerging to shallower depths on the slope of the East European craton. This dilemma can be placed in the wider context of the Variscan orogen and its relationship to the adjacent craton that formed in the late Carboniferous along the margin of the Laurussia continent (Fig. 1). Although the continuity of the Variscan-Alleghanian belt was interrupted by the opening of the Atlantic Ocean, analogues for the deformation front encroaching on the East European craton are presently exposed and thus easily accessible in sections of the Appalachian orogen. The Appalachian fold-and-thrust belt was thrust over the margin of the Precambrian craton similar to the situation that we encountered in SE Poland. This correlation addresses the Variscan-Alleghanian belt features, which may include a sharp deformation front when emplaced on relatively shallow cratonic basement, becoming more diffuse and transitional toward the foreland if it adjoins terranes with an extensive sedimentary cover.
Our study focused on two regional tectonic units: (1) the Radom-Kraśnik block, a NW-SE–elongated structural high where early Paleozoic to Devonian strata subcrop beneath Permian–Mesozoic cover, and (2) the Lublin Basin, a major Paleozoic sedimentary basin developed above the SW slope of the East European craton and filled by Ediacaran–Carboniferous strata (Fig. 2). A unique collection of seismic data (Fig. 3) characterized by deep imaging (down to 8–9 s two-way traveltime [TWT], ~20+ km) and high quality for the entire sedimentary section provides unequivocal evidence for a complex system of pre-Permian thin-skinned compressional deformation within these two units. Some of the structures detected on the seismic data have been explained using an analogy with well-studied and partly exposed structures from the Appalachian fold-and-thrust belt in Alabama. Therefore, in line with some earlier concepts (Antonowicz et al., 2003; Antonowicz and Iwanowska, 2004), we challenge the classical view of the Variscan orogen, according to which the Variscan front is deeply concealed beneath Permian–Mesozoic strata covering the Polish Lowland far away from the East European craton margin (e.g., Pożaryski et al., 1992; Dadlez et al., 1994).
The European Variscides and the North American Ouachita-Appalachian belt are first-order branches of the same, late Paleozoic orogen (Keller and Hatcher, 1999) shaped by the collision between Gondwana and Laurussia (Fig. 1). In Europe, Variscan structures display a curvilinear trend, as a result of repeated indentation of several Gondwana-derived terranes (Matte, 2001; Martínez-Catalán, 2011; Schulmann et al., 2015) in a right- lateral transpressional regime (Arthaud and Matte, 1977; Badham, 1982; Lewandowski, 1993, 2003). This transpressional tectonic context resulted from sublatitudinal, right-lateral displacements between the Gondwana and Baltica-Avalonia margins of Laurussia during Late Devonian to early Carboniferous (Mississippian) time (Lewandowski, 2003; Eckelmann et al., 2014).
After cessation of the Variscan orogeny near the Carboniferous-Permian boundary (Franke, 2000; Kroner et al., 2008), Gondwana decoupled from the newly formed European Variscides along the right-lateral Gibraltar fault zone (Kroner and Romer, 2013) and proceeded westward, toward the southern edge of the Laurentian segment of Laurussia. Progressive development of the Appalachian subduction system continuously pulled Gondwana toward North America (Ziegler, 1990), causing right-lateral oblique convergence of both continents. The resultant right-lateral transpression lasted until the Early Permian time, generating strike-slip displacements of the Meguma terrane (e.g., Pe Piper and Jansa, 1999) and other terranes (see Ziegler, 1988), as well as dextral ramping of the Meguma terrane over the Avalon terrane (Murphy, 2000; Murphy and Hamilton, 2000). Since the leading edges of Gondwana and Laurussia collided orthogonally along the Laurentian margin, the Appalachians are mostly a linear belt, in contrast to the curvilinear trend of the European Variscides (Lewandowski, 2003). The peak of the Alleghanian orogeny can be dated as late Early Permian (ca. 290 Ma; e.g., Hatcher, 2002; Bartholomew and Whitaker, 2010). At the same time, the older Variscan belt experienced reactivation of the major tectonic zones along a set of right-lateral strike-slip faults (Arthaud and Matte, 1977). The Permian convergence of Gondwana and Laurussia led to the final collisional event along the Mauritanides-Alleghanides by the end of the Permian (Kroner and Romer, 2013).
The external fold-and-thrust belt of the Variscan orogen can be traced at the surface and beneath the Permian–Mesozoic cover from the Cornwall Massif in western England to the eastern outskirts of the Bohemian Massif in Czech Republic (Fig. 2). Where buttressed by foreland massifs, such as, for example, the Brunovistulian terrane or the London-Brabant Massif, the Variscan fold-and-thrust belt features a sharp deformation front that separates it from a weakly deformed foredeep. Elsewhere, however, delineation of the Variscan orogenic front is often interpretative due to sparse control on deep structure. This is the case in Poland, where the eastern termination of the European Variscides is accentuated by an ~90° bend of the external flysch belt convex toward the SW margin of the East European craton (Fig. 2). Several variants of the position of the orogenic front exist (e.g., Pożaryski et al., 1992; Dadlez et al., 1994). Although they differ in detail, all these interpretations assume that the deformation front of the Variscides does not reach the SW margin of the East European craton, as defined by the Teisseyre-Tornquist zone (Fig. 2), and dies out passing into the undisturbed sedimentary succession of a distal foreland basin (for a summary of published concepts, see, e.g., Narkiewicz and Dadlez, 2008; Mazur et al., 2010). However, Antonowicz et al. (2003) and Antonowicz and Iwanowska (2004) postulated the occurrence of Variscan thin-skinned, compressional deformation in the Radom–Kraśnik block and the Lublin Basin (Fig. 2), i.e., areas located far beyond the accepted eastern extent of the Variscan fold-and-thrust belt. They attempted to explain the present structure of the Lublin Basin and the Radom-Kraśnik block by fold-and-thrust deformation above a regional detachment developed along the basement-cover interface. The basement would not be involved in this deformation. The Radom-Kraśnik block was interpreted as a thin-skinned duplex stacked along low-angle thrusts that converge upward into an intermediate detachment within Silurian strata. The basal detachment continues beneath the entire Lublin Basin and emerges along its NE flank as a small imbricate fan. Hence, the Lublin Basin was interpreted to be a passively transported piggyback syncline. Such an interpretation of the present-day structure of the Lublin Basin prompted Antonowicz et al. (2003) to include this entire area in the external fold belt of the Variscides. Although the thin-skinned model has received significant criticism (e.g., Dadlez, 2003), it has not been rigorously tested due to the lack of conclusive seismic images from the Radom-Kraśnik block.
The foreland of the Variscan orogen in Poland encompasses the East European craton and a series of ESE- to SE-striking pre-Permian tectonic units that partly overlie and partly adhere to its southwestern margin: the Lublin Basin and the Radom-Kraśnik block, covered by our seismic data, and the Łysogóry and Małopolska blocks farther to the southwest (Fig. 2). In contrast to the majority of the East European craton, which has remained tectonically stable since the late Proterozoic, all these structural units passed through a complex Ediacaran–Paleozoic tectonic evolution that included long periods of basin-forming subsidence punctuated by phases of tectonic shortening and arguably also wrenching (e.g., Lewandowski, 1987, 1993; Nawrocki, 2000; Belka et al., 2002; Lamarche et al., 2003; Narkiewicz et al., 2007, 2011a; Nawrocki and Poprawa, 2006; Poprawa, 2006a, 2006b; Nawrocki et al., 2007; Krzywiec, 2009). Although the extent of the East European craton to the southwest is disputable (e.g., a summary of proposed positions of the Teisseyre-Tornquist zone was provided by Narkiewicz et al., 2015), it is accepted that the East European craton crust floors the Lublin Basin and the Radom-Kraśnik block. This has been substantiated by deep seismic refraction surveys that image an East European craton–type crustal structure beneath the Lublin Basin and the Radom-Kraśnik block (Malinowski et al., 2005; Guterch and Grad, 2006; Narkiewicz et al., 2011a) and also by depositional similarities (in particular, the distribution of Ordovician and Silurian facies) between the Lublin Basin and the Radom-Kraśnik block on one side and the East European craton on the other (Tomczyk, 1988; Kozłowski, 2003; Poprawa, 2006b). Recently, a deep seismic reflection profile (POLCRUST-01) provided the first image of the continuous top of the East European craton basement that descends from ~5 km below sea level (bsl) beneath the Lublin Basin to ~20 km bsl beneath the SW part of the Radom-Kraśnik block (Malinowski et al., 2013, 2015; Narkiewicz et al., 2015). On the contrary, the Łysogóry and Małopolska blocks adjacent to the southwest are believed to represent terranes transported along the Teisseyre-Tornquist zone in the early–middle Paleozoic (e.g., Lewandowski, 1993; Belka et al., 2002; Nawrocki and Poprawa, 2006; Nawrocki et al., 2007; Narkiewicz et al., 2015).
The Lublin Basin and the Radom-Kraśnik block, which were the main targets of our study, are unconformably covered by mildly deformed Permian–Mesozoic sediments of the German-Polish Basin deposited above a distinct pre-Permian, Variscan unconformity (e.g., Dadlez et al., 1995; Kutek, 2001; Krzywiec, 2002, 2009; Krzywiec et al., 2009). The pre-Permian structure and stratigraphy for these regional structural units are known from boreholes and industrial seismic lines. The Lublin Basin is a SE-trending structural depression marked by the widespread occurrence of Carboniferous strata, with minor inliers of older rocks emerging from beneath the Carboniferous cover at the base-Mesozoic subcrop. The NE limit of the Lublin Basin is conventionally delineated by the erosional pinch-out of the Carboniferous strata at the sub-Mesozoic subcrop (Fig. 3). In the southwest, the boundary between the Lublin Basin and the Radom-Kraśnik block is defined by the emergence of pre- Carboniferous strata (Fig. 3). This boundary consists of two fairly straight segments. Owing to their linearity, these are often considered as steep regional fault zones: the Ursynów-Kazimierz fault zone to the northwest and the Izbica-Zamosc fault zone to the southeast (Fig. 3). The general NE-SW profile of the Lublin Basin is asymmetric. It features a gently inclined NE slope that results from the regional dip of the East European craton basement and a steep SW flank, which is tectonically controlled (Fig. 4). This simple geometry is disturbed by the SE-striking (parallel to the basin axis) Kock fault zone. This tectonic feature reveals a complex record of late Carboniferous inversion (cf. Krzywiec, 2009), and it divides the NW Lublin Basin into the uplifted part in the northeast and the subsided part in the southwest (Fig. 4).
The Radom-Kraśnik block is a structural uplift marked by the predominance of pre-Carboniferous strata at the sub-Mesozoic subcrop. In contrast to the Lublin Basin, the Carboniferous rocks are reported only as isolated inliers (Żelichowski, 1974). The structural relief exposed at the base-Mesozoic surface is of low amplitude, which is expressed by the large subcrops of Devonian rocks (Fig. 3). Along the SE rim of the Radom-Kraśnik block, pre-Devonian strata ascend to the sub-Mesozoic subcrop (Fig. 3; Drygant et al., 2006).
The Lublin Basin and the Radom-Kraśnik block consist of deformed Ediacaran–Carboniferous sediments (e.g., Tomczy-kowa and Tomczyk, 1979; Modliński et al., 1995; Modliński and Szymański, 2001; Jaworowski and Sikorska, 2006; Moczydłowska, 2008; Pacześna, 2010; Narkiewicz, 2011; Narkiewicz et al., 2011b). These attain ~10 km in thickness close to the SW margin of the Lublin Basin and thin out toward the interior of the East European craton. The original thickness of the sedimentary section in the Radom-Kraśnik block is not constrained; however, it could have been even higher than in the Lublin Basin owing to the more distal position of this tectonic unit with respect to the East European craton.
DATA AND METHODS
We analyzed two PolandSPAN™ seismic profiles, 5100 and 5000 (Fig. 3). Data were acquired down to 12 s TWT, with high nominal fold (480), long offsets (12 km), tight receiver/shot spacing (25 m), and broadband sweep (2–150 Hz) provided by four 62,000 lb (28,122 kg) Vibroseis trucks.
The gravity data were derived from gravity ground stations and gridded at a 2000 m interval. The coordinate system used was Poland 1992, which is based on the Geodetic Reference System 1980 (GRS80) datum and the Gauss-Krüger projection in a meridional zone, with 19°E as the axial meridian, on which the linear scale is equal to 0.9993 (linear distortion = 70 cm/1 km). The Bouguer correction reduction density was 2.67 g/cm3. The total magnetic intensity grid was compiled from ground and airborne surveys. The magnetic data were gridded at 500 m intervals and continued upward to 500 m mean terrain clearance for the whole area of interest.
The Moho can have a large influence on the gravity anomalies because the density contrast between the lower crust and upper mantle can be large (0.4 g/cm3 in this study). If the crust is assumed to be in isostatic equilibrium (Airy-type), then topography should be compensated by corresponding changes in the depth of the Moho, and this can be used to model the gravitational effect of the Moho. This isostatic gravitational effect is then subtracted from the Bouguer anomaly to produce an isostatic residual anomaly, which should highlight the remaining lateral density contrasts that exist within the crust (e.g., Simpson et al., 1986).
The reduction-to-pole (RTP) transform was applied to the magnetic anomaly data. This is done because the vector nature of magnetization and variations in the inclination and declination of Earth’s magnetic field between the equator and the magnetic pole can lead to complications in magnetic anomalies. The RTP transform attempts to simplify the magnetic field by rotating the magnetic vector to be vertical, thereby centering magnetic anomalies above their causative bodies (MacLeod et al., 1993).
Further filters and derivatives of the gravity and magnetic data were used to enhance specific signatures and to attenuate long-wavelength signals related to deeper sources. These were applied using a combination of Geosoft Oasis montaj™ and Getech’s noncommercial software (GETGrid). The gravity signals record density variations throughout the entire crust. To better understand discrete, regionally pervasive changes in crustal structure and density, the total horizontal gradient (THD) derivative of the isostatic gravity was applied. The THD shows distinct linear trends along the location of faults that juxtapose blocks of contrasting density (Grauch and Cordell, 1987). Consequently, it is a powerful tool for mapping fault trends that may juxtapose sedimentary rocks of varying density against crystalline basement. The first vertical derivative sharpens up anomalies over their sources and tends to reduce anomaly complexity, allowing a clearer imaging of the causative structures (e.g., Blakely, 1996). This enhancement also amplifies short-wavelength anomalies in shallow sources.
Wavelength filtering is a means of enhancing features within a specified wavelength range from the full spectrum (e.g., Blakely, 1996). Low-pass filters remove wavelengths shorter than a specified cutoff and can therefore be used to remove short-wavelength upper-crustal anomalies, such as those caused by lithology contrasts and thin-skinned deformation, and to isolate the effect of regional crystalline basement structure. High-pass filters remove wavelengths longer than a specified cutoff and therefore are a useful tool for removing regional trends from the data, thus enhancing features related to shallower density contrasts or thin-skinned structures.
STRUCTURE OF THE RADOM-KRAŚNIK BLOCK AND THE LUBLIN BASIN
A regional seismic survey, PolandSPAN, together with newly acquired seismic reflection industry data, and reprocessed archived seismic profiles (cf. Antonowicz and Iwanowska, 2004) were jointly employed in this study to develop a new tectono-stratigraphic model of the Radom-Kraśnik block and the Lublin Basin. Comprehensive characteristics of both structural units are provided in the following sections.
The thin-skinned structural style of the Radom-Kraśnik block and the Lublin Basin is presented on the interpreted regional PolandSPAN seismic profiles 5100 and 5000 shown in Figures 5A and 5B, respectively. The quality of the seismic data within the Radom-Kraśnik block is lower than within the Lublin Basin, but it still allows for identification of the main tectonic subunits within the block. Key evidence is provided by the geometry of top Cambrian and top Ordovician reflectors, the only seismic reflectors that could be securely identified within most of the Radom-Kraśnik block. These reflectors, tied to the Narol IG-2 and Zwierzyniec-1 wells, delineate hanging-wall anticlines of thrust sheets and revealed the general imbricate-fan structural style of the Radom-Kraśnik block.
Based on the seismic data presented (Figs. 5 and 6), we interpret the Radom-Kraśnik block to be a NNE-vergent stack of thin-skinned thrust sheets, consisting of a deformed Ediacaran?–Devonian succession that was emplaced over the SW-descending crystalline basement of the East European craton. Since the basal detachment is located within the Ediacaran strata, the term “basement” is referred to in the following sections of the paper as a crystalline unit together with its Ediacaran cover, if not explicitly stated otherwise. The frontal part of the Radom-Kraśnik block is a triangle zone related to the jump of the basal detachment from the basement-cover interface to the base of the Silurian shales. The passive roof of the triangle zone is crosscut and kinematically disabled by a NE-vergent forethrust, documenting the forward growth of the imbricate belt beyond the ephemeral triangle zone (Figs. 5 and 6). It is not certain whether the formation of a ramp and stepping of the basal detachment from the basal part of Ediacaran sediments up to the base Silurian were triggered by structural (inherited architecture of the top of the East European craton basement) or stratigraphic (pinch-out of a Precambrian detachment horizon) controls. Our seismic data do not allow us to discriminate between these two end-member possibilities. The internal structure of the Radom-Kraśnik block is characterized by a few major thrusts sealed by a base-Mesozoic unconformity and a number of alternating thrusts that converge to the upper detachment in the Silurian incompetent sequence. The overlying Upper Silurian and Devonian strata are folded disharmonically (Figs. 5 and 6). PolandSPAN profile 5100 shows an intra–Lower Devonian unconformity (Fig. 5A) that was penetrated by the Zakrzew IG-3 well. The unconformity provides evidence for pre-Variscan tectonic movements.
The SE part of the Radom-Kraśnik block (Fig. 3), illustrated by PolandSPAN profile 5000 (Fig. 5B) and the industry seismic profiles (Fig. 6), reveals an internal structure similar to that imaged by PolandSPAN profile 5100 (Fig. 5A). However, because of higher structural relief and deeper post-Variscan erosion, the Lower Devonian is only locally preserved in this part of the block.
The post-Variscan overburden of the Radom-Kraśnik block is mildly folded and faulted. This deformation affected the entire Mesozoic cover, including the Maastrichtian strata, so it must have been associated with the Late Cretaceous–Paleogene inversion of the Mid-Polish Trough (cf. Krzywiec, 2002, 2009; Krzywiec et al., 2009).
The Lublin Basin is generally characterized by less-intense thin-skinned deformation, with the main detachment situated at the base of the Silurian shales, and secondary detachments within the Lower or Middle Devonian strata. Some basement-involved normal and reverse faults are also present (Figs. 5 and 6), the largest of which is associated with the Kock fault zone. PolandSPAN profile 5100 (Fig. 5A) illustrates the complex structure of the Kock fault zone, with significant across-strike variation in the Upper Devonian thickness. At the Precambrian–Ordovician level, the Kock fault zone is interpreted as a reverse fault (see also later herein) that created a significant basement step. The latter must have focused contractional strain during the Variscan orogeny owing to the buttressing effect. Consequently, strain localization produced a complex structure above the reverse fault with a small triangle zone in front of the foreland-verging thrust. A back thrust associated with this triangle zone is located in the Silurian shales.
The structure of the Lublin Basin differs between seismic profiles 5000 and 5100 (Figs. 5A and 5B). The main difference is the lack of the basement step associated with the deep Kock fault zone in the SE part of the basin. PolandSPAN profile 5000 and the industry seismic profiles show some rather minor faulting within the basement (Ordovician interval), but none of these faults accommodates substantial displacement (Figs. 5B and 6). Therefore, compressional deformation triggered by the emplacement of the Radom-Kraśnik block was conveyed into the Lublin Basin via a main basal detachment along Silurian shales, with additional detachments located within the Lower Devonian interval. All the compressional deformation within this part of the Lublin Basin involved the entire Carboniferous cover, and this testifies to its Variscan origin.
All the seismic profiles in Figures 5 and 6 show lateral thickness variations of the Upper Devonian within different parts of the Lublin Basin. This is an effect of widespread erosion that could be correlated with latest Devonian–earliest Carboniferous Bretonian tectonism (e.g., Ziegler, 1990; Kroner et al., 2008; Narkiewicz, 2007). The unconformity and overall structural style of Bretonian deformation are illustrated by the interpreted PolandSPAN regional seismic profile 1100 (Fig. 7), which is located on the NE side of the Kock fault zone (Fig. 3). On this seismic profile, the Bretonian unconformity is much better expressed than the Variscan (latest Carboniferous) unconformity (Fig. 7). The structural style of the eastern part of the Lublin Basin is dominated by deeply rooted, mostly reverse faults and associated basement uplifts. The main phase of activity on these faults was post-Famennian (Late Devonian) and pre-Visean (mid-Mississippian), as these are the youngest and the oldest rocks located beneath and above the Bretonian unconformity, respectively. Reverse faulting and formation of basement uplifts were followed by widespread erosion and succeeding deposition of the Carboniferous succession. Rather mild reactivation of some of these faults took place during the Variscan tectonic phase in the latest Carboniferous. Some of them were also reactivated during Late Cretaceous–Paleogene inversion of the Carpathian foreland. Reverse faulting during the Bretonian phase might have also been responsible for the formation of the basement step within the Kock fault zone.
Kock Fault Zone Compared to the Appalachians
The evolution of the Kock fault zone, probably the most spectacular thin-skinned structure in the Lublin Basin, can be better elucidated using a field analogue from the Appalachians, an area that is devoid of Mesozoic cover and thus accessible for direct field observations. A unique type of thin-skinned compressional structure is related in the external Appalachians in Alabama to the presence of thick Lower–Middle Cambrian shales and limestones within the Cambrian–Carboniferous sedimentary section (Thomas, 2001, 2007). These structures, termed mushwad (which stands for malleable, unctuous shale, weak-layer accretion in a ductile duplex; Thomas, 2001), nucleate in a thick, shale-dominated succession of rheologically weak rocks due to the buttressing effect exerted by a basement scarp or step associated with basement faults (Fig. 8). The stiff-layer roof is deformed by folds and faults and is elevated by accretion and duplexing of weak rocks in a mushwad. Mushwads can be located in a core of triangle zones associated with back thrusting within their frontal parts.
Important analogies between the structural setting of the mushwad structures and the Kock fault zone are defined by:
substantial basement steps,
a thick succession of incompetent rocks, and
intense late Carboniferous compressional deformation.
Indeed, seismic reflection images of the Kock fault zone (Figs. 9 and 10) from the NW part of the Lublin Basin (Fig. 3) document the presence of a basement step related to the steep reverse fault formed during the Bretonian tectonic phase (latest Devonian–earliest Carboniferous), which is similar to that imaged by profile 5100 (Fig. 5A). The fault offsets the crystalline basement and Ediacaran–Ordovician cover. The roof of this structure is built of the well-imaged Devonian–Carboniferous succession that was thrust and folded during Variscan tectonism. The Devonian is missing on the NE side of the Kock fault zone because of widespread post-Bretonian erosion, as explained earlier herein (cf. Fig. 7). Between the subdetachment Precambrian–Ordovician substrate and the competent-layer roof, there are thick Silurian shales that mechanically might have played the same role as Cambrian shales in the Appalachians. Due to the buttressing effect exerted by the basement step, they accommodated the bulk of the strain, and thus they were strongly shortened and deformed. The development of the Kock fault zone resulted in substantial uplift of the competent Devonian–Carboniferous sedimentary section. The frontal part of this zone might also have been associated with some back thrusting and the formation of a minuscule triangle zone.
The changing geometry of the underlying basement step resulted in a different structure in the Kock “mushwad” along strike that is flat and more extensive on the seismic section in Figure 9 and thicker and more localized in Figure 10. Such a direct relationship between the geometry of the stiff basement and strongly deformed weak layer also supports the proposed model in which the buttressing effect exerted by the basement step played a dominant role during the Variscan evolution of the Kock fault zone.
VARISCAN DEFORMATION OF THE RADOM-KRAŚNIK BLOCK AND LUBLIN BASIN—AN EXPRESSION IN GRAVITY AND MAGNETIC ANOMALY MAPS
Potential field methods were used here to provide information about the deep structure below the seismic penetration depth and extrapolate observations from two-dimensional (2-D) seismic profiles into a third dimension. We used the potential field data to understand the geometry of the top of the crystalline basement and constrain the strike of structures imaged on seismic 2-D cross sections. The seismic interpretation was supplemented using an integrated approach by interpreting seismic profiles jointly with the gravity and magnetic data. Various gravity and magnetic grids were used to delineate a set of structural elements, which were then compared and calibrated with the available seismic data to produce a consistent structural framework. The isostatic gravity and RTP magnetic data, along with their filters and derivatives, were used as the main data sets on which the position and kinematics of major structural elements were mapped and interpolated between seismic lines. The gravity and magnetic interpretation presented in this section is focused on understanding the thin-skinned structure of the Radom-Kraśnik block and the Lublin Basin.
The isostatic gravity map reveals two superimposed structural trends: NNW-SSE and WNW-ESE (Fig. 11A). The NNW-SSE trend is associated with a relatively long-wavelength density anomaly and most likely represents the response of the top of the metamorphic basement configuration. The WNW-ESE trend is related to short-wavelength anomalies in the range of a few kilometers. Consequently, density contrasts generating these WNW-ESE anomalies must be located at shallow depths not exceeding a few kilometers and almost certainly represent tectonic structures within the sedimentary cover.
The difference between the NNW-SSE and WNW-ESE structural trends is even better demonstrated by band-pass filtering (Fig. 12). Using a common assumption that the depth of a causative body is between 1/3 and 1/4 of the anomaly wavelength, the cutoff frequencies for the filters were adjusted to the depth of the expected sources using seismic data as guidance. Band-pass filtering (30–150 km) of the isostatic gravity anomalies suppresses gravity anomalies coming from the sources shallower than ~10 km (Fig. 12A). The additional 150-km-long wavelength cutoff eliminated anomalies generated by deep sources (>40 km) and/or Moho morphology. High-pass filtering of isostatic gravity (40 km) emphasized anomalies related to density contrasts not deeper than ~10–15 km, which is expected crystalline basement depth (Fig. 12B). Based on the relationship between specific structural trends and the frequency of anomalies emphasizing those trends, we draw a conclusion that the NW-SE structural trend must be produced by the crystalline basement–sediment interface, whereas the ENE-ESE trend is related to the internal structure of a sedimentary pile.
Structural mapping was mostly based on the first vertical derivative and the total horizontal derivative of isostatic gravity (supplementary Fig. DR11). These derivatives both enhance the anomalies generated by deformation within the sedimentary pile overlying the crystalline basement. The match between potential field anomalies and the seismic interpretation is not perfect due to the complex nature of gravity anomalies and incomplete seismic imaging, but some key features can be unambiguously correlated. First, the correlation is good for the frontal thrust defined in this study as an overthrust of the Lower Paleozoic to Devonian rocks of the Radom-Kraśnik block onto the Lublin Basin. There is also a good correlation with the thrust contact between the Łysogóry and Małopolska blocks on one side and the Radom-Kraśnik block on the other.
A characteristic feature of the study area is the lack of correlation between the top of the crystalline basement structure and magnetic anomalies. A major magnetic high located in the area of the Radom-Kraśnik block (Fig. 11B) is apparently not connected to a major basement high (see following). Instead, it must be associated with strong susceptibility contrasts within the crystalline basement. The increased susceptibility values are coupled with a generally strong gravity signal, since the study area overlaps the Małopolska gravity high (e.g., Grabowska and Bojdys, 2001). This may in turn suggest the presence of anomalous high-density and high-susceptibility bodies within the East European craton deep basement.
Depth to Basement
A depth-to-basement study was performed using a twofold approach. First, the top of the crystalline basement surface in the time domain was created by gridding (minimum curvature) the 2-D time horizons derived from a number of industrial seismic sections. The resultant seismic time horizon (Fig. 13A) does not provide definitive information about basement depth, but it illustrates possible crystalline basement highs and lows that can be used as reference for the subsequent quantitative depth calculations. Second, a three-dimensional (3-D) gravity inversion method was applied (Fig. 13B). The theoretical basis and details of the inversion procedure were described in Barnes and Barraud (2012). As a first step, the gravity response of the upper mantle was calculated from a simple two-layer forward model. A Moho surface grid based on seismic refraction data was used in this exercise (courtesy of the Institute of Geophysics, Polish Academy of Sciences). The density contrast across the Moho was assumed to be 0.4 g/cm3. Then, the calculated mantle signal was subtracted from the observed terrain-corrected (Bouguer) gravity. The gravity residual was then inverted for the top crystalline basement surface, assuming a low-density contrast across the top of the basement (0.2 g/cm3) and a lack of lateral density variation in the basement and sediments.
The depth-to-basement map expressed in two-way traveltime (TWT) covers the area that is limited by the top crystalline basement picks available (Fig. 13A). The isolines overlaid on top of the color-coded map provide depth-to-basement in meters for reference, as it is derived from the gravity inversion. Both the maps compiled in time and depth (Fig. 13) show a relatively smooth top of the East European craton crystalline basement rapidly sloping toward the SW beneath the Łysogóry and Małopolska blocks. The depth to basement increases from NE to SW by ~15,000 m from ~3000 to 18,000 m (Fig. 13B). The greatest crystalline basement depth is attained beneath the NE margin of the Łysogóry and Małopolska blocks, as the top of the East European craton seems to continuously plunge to the SW. The structural elements interpreted from the maps of isostatic gravity and its derivatives (Figs. 11A and 12B) contain no expression in the morphology of the top of the crystalline basement (Fig. 13). Only a long-wavelength gravity anomaly, emphasized by the 30–150 km band-pass filter (Fig. 12A), coincides with the general trend of basement flexure.
VARISCAN DEFORMATION FRONT IN SE POLAND REVISITED—DISCUSSION
The seismic profiles from the Radom-Kraśnik block and the Lublin Basin image Variscan deformation that is more intense than anywhere else beneath the Permian–Mesozoic German-Polish Basin to the east of the Harz Mountains (Fig. 2). Furthermore, Variscan deformation in SE Poland seems to be more localized than in many other sections of the Variscan external belt of Europe. As already pointed out, this might be a consequence of buttressing by the relatively shallow basement of the East European craton, which rises steeply toward the east (Fig. 13). Similar to the margins of the Brunovistulian terrane (Moravia) or the London-Brabant Massif (Cornwall, Ardennes), the Variscan fold-and-thrust belt features a sharp deformation front in SE Poland (Figs. 5 and 6). This is also the situation where the Appalachian fold-and-thrust belt is emplaced over the margin of Precambrian craton (Fig. 8) in North America.
To the southwest, the Radom-Kraśnik block is adjacent to the Łysogóry and Małopolska blocks (Fig. 2), which have less intense Variscan deformation (e.g., Lamarche et al., 1999, 2003; Konon, 2006, 2007; Gągała, 2014, 2015). Consequently, a key question concerns the possible mechanism for the eastward transfer of Variscan shortening from the hinterland in the Bohemian Massif to the foreland in the Radom-Kraśnik block and the Lublin Basin. One explanation employs the Odra and Kraków- Lubliniec faults (Fig. 2), which have traditionally been interpreted as a zone of large-scale, right-lateral, strike-slip displacements (Arthaud and Matte, 1977; Aleksandrowski et al., 1997; Mattern, 2001). Since the Odra and Kraków-Lubliniec faults are oblique at a low angle to the Teisseyre-Tornquist zone, right-lateral motion of these faults might have generated localized shortening against the rigid buttress of the East European craton. In this interpretation, the Radom-Kraśnik fold-and-thrust belt would represent a local zone of Variscan reactivation at the termination of a regional-scale strike-slip fault.
A further-reaching but not conflicting interpretation assumes a southwestward continuation of the regional detachment beneath the Łysogóry and Małopolska blocks. Indeed, the apparent continuation of the East European craton basement beneath the Małopolska block has been recently imaged by the POLCRUST-01 seismic profile for a distance of at least 40 km (Malinowski et al., 2013, 2015), although it was interpreted to be genetically linked to the internal structure of the Małopolska block rather than an extension of the East European craton (Narkiewicz et al., 2015). If this is the case, the Radom-Kraśnik block would represent the leading edge of a thin- to thick-skinned Variscan orogenic wedge, similar to the structural setting of the Ardennes, which are emplaced onto the margin of the Brabant Massif along the Midi-Aachen detachment (e.g., Oncken et al., 1999; Sintubin et al., 2009). Consequently, Variscan horizontal compression would have been conveyed eastward along the basal detachment, causing shortening and localized deformation in front of a ramp formed by the slope of the East European craton crystalline basement. In this situation, the Radom-Kraśnik block would represent a proper orogenic front, where a deep-seated detachment emerges to the base-Mesozoic surface in front of a rigid buttress. The obliquity of the two structural trends in the study area (NW-SE and WNW-ESE; Fig. 12) proves that significant basement deepening was related to an independent stretching event rather than solely tectonic loading of the East European craton margin by the Radom-Kraśnik, Małopolska, and Łysogóry blocks.
The NE-SW–trending seismic profiles presented (Figs. 5 and 6) do not image any major subvertical faults that may correspond with the location of the Teisseyre-Tornquist zone as defined by Narkiewicz et al. (2015) along the Tomaszów fault, although their interpretation utilized an image of the whole crust provided by the POLCRUST-01 profile. Some basement-involved normal or reverse faults are distributed beneath the Lublin Basin, but they usually terminate within the rheologically weak Silurian shales. The most significant of them are associated with the Kock fault zone (Figs. 5, 9, and 10). However, the NW-SE profile (Fig. 7) shows that most, if not all, of these faults are related to the Bretonian (latest Devonian–earliest Carboniferous) tectonic phase and do not parallel the Teisseyre-Tornquist zone. Therefore, the data available for this study suggest that the Łysogóry and Małopolska blocks were thrust onto the East European craton margin rather than adjoining a major subvertical discontinuity corresponding to the Teisseyre-Tornquist zone.
The structural interpretation of the seismic profiles presented here is generally in line with the earlier concepts published by Hooper et al. (2002), Antonowicz et al. (2003), and Antonowicz and Iwanowska (2004). Although their model (Fig. 14) was partly conceptual, being only partially supported by seismic evidence, these authors had good intuition in recognizing the structural style of the study area. The present results, based on much better-quality seismic data, differ in the details of structural interpretation across the Radom-Kraśnik block and the Kock fault zone. In the previous model (Antonowicz et al., 2003; Fig. 14), the NE boundary of the Radom-Kraśnik block (Wilczopole/Ursynów-Kazimierz fault zone) and the Kock fault zone were defined as triangle zones, although their geometries as depicted on the diagram (Fig. 14) resemble more those of active duplexes. Both these zones are portrayed as antiformal stacks with presumably significant forethrusting. The present interpretation better shows the geometry of the triangle zones (passive-roof duplexes) modified by later imbricate thrusts (Figs. 5 and 6). Finally, the previous model postulates the continuation of the basal detachment beneath the entire Lublin Basin along the cover-basement interface (Fig. 14). The present interpretation locates the Lublin Basin detachment within the Lower Silurian shales and does not necessarily imply its continuity over the whole basin (Figs. 5 and 6). Also, the model proposed by Antonowicz et al. (2003) assumed a thin-skinned nature for all deformation associated with the Kock fault zone and the lack of any deformation at the basement level, while available data clearly show thick-skinned faulting in this area that resulted in the formation of a substantial basement step.
Using again the analogy to the Ardennes, the Radom-Kraśnik block represents a fold-and-thrust belt emplaced structurally above a sedimentary succession of foreland basin (cf. Lacquement et al., 2005; Sintubin et al., 2009). Although the separation of the orogenic wedge from the strongly tectonized foredeep is not straightforward, the boundary between the Radom-Kraśnik block and the Lublin Basin seems to define the position of the Variscan front in SE Poland. Reinterpretation of published tectonic models for western Ukraine demonstrates the continuity of structural style between the Radom-Kraśnik block and the Biłgoraj-Rawa Ruska block (Chiżniakow and Żelichowski, 1974). This implies along-strike extension of the Variscan front toward the southwest into the territory of Ukraine. On the other hand, the NW continuation of the Radom-Kraśnik block and the associated Variscan front does not seem to reach beyond the Grójec fault zone (Fig. 3).
The high-quality seismic reflection profiles recently acquired in SE Poland provide very good images beneath Mesozoic and younger sediments that delineate a thin-skinned fold-and- thrust belt with a 10–12-km-thick pile of Ediacaran (?) to Devonian sediments that was tectonically emplaced onto the margin of the East European craton. These sediments are involved in a NNE-vergent stack of thrust units, with the Radom-Kraśnik block, according to regional terminology, striking oblique at a low angle to the East European craton basement flexure. Individual thrusts branch off from a basal detachment that is located close to the base of the Ediacaran sediments. The frontal part of the Radom-Kraśnik fold-and-thrust belt is a triangle zone related to the jump of the basal detachment from the basement-cover interface to the base of the Silurian shales. The triangle zone is crosscut by a younger NE-vergent imbricate thrust that emplaced the Radom-Kraśnik fold-and-thrust belt structurally above the Lublin foreland basin. Since the Lublin Basin is filled with Upper Carboniferous (Pennsylvanian) and older sediments, the timing of thin-skinned thrusting is restricted to the latest Carboniferous.
Complex but generally less-intense thin-skinned compressional deformations were recognized in the Lublin Basin, where a main detachment is situated at the base of the Silurian shales, and secondary detachments are located within the Lower or Middle Devonian strata. The most distal deformation zone is the Kock fault zone, which developed above a step within the Precambrian basement due to the buttressing effect associated with Variscan thrusting. Due to localized strain, a complex structure was formed above the reverse fault, with a triangle zone developed in front of the foreland-verging thrust. The Kock fault zone could be regarded as an analogue to the so-called mushwad structures described within the frontal part of the Appalachians.
Deeply rooted, mostly reverse faults and associated uplifts are the dominant structural features in the eastern part of the Lublin Basin to the east of the Kock fault zone. Their activity led to widespread erosion during the latest Devonian–earliest Carboniferous that produced a Bretonian unconformity of regional extent. This important unconformity is responsible for the lateral thickness variations of the Upper Devonian across different parts of the Lublin Basin. The main phase of Bretonian tectonic activity was post-Famennian and pre-Visean, as these are the youngest and the oldest rocks located beneath and above the unconformity, respectively. Some of the Bretonian faults experienced rather mild reactivation during the Variscan tectonic phase in the latest Carboniferous.
The seismic lines available for this study do not image any subvertical faults penetrating from basement into the sedimentary succession that can be interpreted as the boundary between the East European craton and the adjacent Paleozoic terranes to the southwest. The lack of subvertical discontinuities representing potential candidates for a terrane boundary combined with the structural style of the thin-skinned fold-and-thrust belt suggest a compressional regime during Variscan deformation and the possible southwestward continuation of the basal detachment beneath the Łysogóry and Małopolska blocks. The Variscan deformation documented is more intense than anywhere else beneath the Permian–Mesozoic German-Polish Basin to the east of the Harz Mountain. This is probably because of buttressing by the relatively shallow basement of the East European craton. In the Appalachians, at the opposite end of the Variscan-Alleghanian belt, similar structures were also formed owing to interaction with the shallow basement of the North American craton.
We are indebted to ION Geophysical and Chevron Polska Energy Resources for allowing us to present the results of seismic and potential field data interpretation. PGNiG (Polskie Górnictwo Naftowe i Gazownictwo) S.A. is thanked for providing seismic data used to construct a new model for the Kock fault zone within the BlueGas project no. BG1/GAZGEOL-MOD/13. S. Mazur acknowledges financial support from the Polish National Science Centre (NCN) grant UMO-2011/01/B/ST10/04713. M. Mikołajczak is acknowledged for making some results of his Ph.D. potential field research available to us. R. Keller and R. Thigpen are thanked for helpful reviews.
Figures & Tables
Linkages and Feedbacks in Orogenic Systems
CONTAINS OPEN ACCESS
- Central Europe
- compression tectonics
- fault zones
- fold and thrust belts
- geophysical methods
- geophysical profiles
- geophysical surveys
- gravity anomalies
- gravity methods
- Lublin Basin
- magnetic anomalies
- magnetic methods
- Russian Platform
- sedimentary rocks
- seismic methods
- seismic profiles
- tectonic units
- thin-skinned tectonics
- Upper Carboniferous
- Variscan Orogeny
- southeastern Poland
- Kock fault zone
- Radom-Ktasnik Block