Deep seismic reflection profile in Central Europe reveals complex pattern of Paleozoic and Alpine accretion at the East European Craton margin

Authors


Abstract

[1] The first deep reflection seismic profile was acquired across the Teisseyre-Tornquist Zone (TTZ), a major European suture zone, southeast of the Avalonia-Baltica collision zone, portraying different tectonic deformation styles (thick- versus thin-skinned) and Phanerozoic crustal accretion episodes over a relatively short distance (240 km). The overall crustal structure preserves image of the Neoproterozoic passive margin setting but with a significant overprint of the repetitive tectonic convergence (Cadomian to Alpine) acting at the Baltica margin. In contrast to the Avalonia-Baltica collision zone, here the polygenetic crustal accretion occurring at the cratonic margin was ultimately shaped by significant strike-slip faulting. TTZ is imaged as a narrow (10–15 km) transform zone separating the thinned East European cratonic crust from the Małopolska Block.

1 Introduction

[2] The lithosphere of western and central Europe is a natural laboratory for studying different crustal growth modes. It is dominated by various crustal accretion phases that formed the West European Paleozoic Platform (WEPP) encroaching eastward and northeastward onto the Precambrian East European Craton (EEC; a part of Baltica paleocontinent) (Figure 1). The tectonic contact between these major tectonic units is referred to as the Teisseyre-Tornquist Zone (TTZ) which represents the most prominent lithospheric boundary in Europe [Pharaoh et al., 2006]. The WEPP crust is heterogeneous, being a tectonic collage of various Paleozoic terranes (including Avalonia) [e.g., Winchester et al., 2002] accreted to the Baltica margin and affected by the Alpine orogeny in the south. Due to its complex history, the region is particularly suitable for investigating the impact of different constructive and destructive processes acting upon a cratonic margin.

Figure 1.

Location of the POLCRUST-01 profile against the Central European geology. Elevations of the East European Craton (pink shade). The extent of the Eastern Avalonia crust (brown shade). Thin lines show location of POLONAISE '97 and CELEBRATION 2000 deep refraction surveys [Guterch et al., 1999, 2001] and the DEKORP deep reflection transect [Krawczyk et al., 2008]. Abbreviations: CDF, Caledonian Deformations Front; HCM, Holy Cross Mountains; ŁB, Łysogóry Block; MB, Małopolska Block; PKB, Pieniny Klippen Belt; TTZ, Teisseyre-Tornquist Zone; USB, Upper Silesian Block. Abbreviations in the inset: EEC, East European Craton; IS, Iapetus suture; WEPP, West European Paleozoic Platform.

[3] Tectonic complexity of the Baltica margin has been studied by several refraction/wide-angle reflection profiles (see recent summary in Guterch et al. [2010]), including POLONAISE'97 and CELEBRATION 2000 experiments [Guterch et al., 1999, 2001] (see location in Figure 1), but with the limited ability to track fine-scale continuous and discontinuous tectonic deformations. Only a few deep reflection profiles supplemented crustal models obtained from refraction and provided detailed reflectivity images [Guterch et al., 2010], and only two of the transects crossed TTZ (DEKORP [Krawczyk et al., 2008] and BABEL A profiles [BABEL Working Group, 1993]). Those profiles were located in the inferred Avalonia-Baltica collision zone (Figure 1).

[4] Here we summarize results of the first deep reflection seismic profile (called POLCRUST-01) that crossed the TTZ ~ 800 km south of the DEKORP and BABEL profiles (Figure 1). The tectonostratigraphic position of this seismic profile is unique for several reasons. First of all, it portrays different tectonic deformation styles over a relatively short distance (240 km). Thick-skinned tectonics is represented by the Cadomian (pre-Ordovician) and Caledonian (pre-Devonian) accreted terranes (part of the WEPP) juxtaposed against the EEC margin. Thin-skinned deformations are represented by the young Alpine orogen (Carpathian Mts.) emplaced over the Paleozoic basement. Moreover, since the profile extends ~ 300 km east of the Variscan deformations front, it is outside the area of Late Paleozoic postorogenic extension and thermal-magmatic reworking of the crust [Artemieva and Meissner, 2012]. The Neoproterozoic and lower Paleozoic strata are covered by less than 3 km of younger sediments (except for the Carpathians) and are calibrated using tens of deep boreholes, which allows us to investigate the contact between the EEC margin and accreted Paleozoic crust in an unprecedented detail.

2 Data Acquisition and Processing

[5] The seismic data were acquired using the Vibroseis technique during August–October 2010 with parameters typical for exploration seismology (30 m receiver spacing and 60 m shot spacing) and high-CDP (common depth point reflection) fold (175), but with an unusually high source effort (40–45 s long sweeps) and long recording time (30 s). Table 1 lists the most important acquisition parameters. During the reflection data acquisition, refraction-type single-channel recorders were deployed every 1.2 km along the line in order to record Vibroseis points and produce common receiver gathers with an extended offset range. This piggyback experiment resulted in seismic sections with offsets up to 25–30 km, which were significantly above the nominal offset range of the reflection spread (10 km). This effort made it possible to build a first arrival-based tomography model used in subsequent imaging.

Table 1. Seismic Acquisition Parameters
ParameterValue
Spread typeSymmetric
No of channels700
Max. offset10,485 m
Source4 × IVI Hemi-50 Vibrators (24 t),
Sweep: frequency, duration, repeats6–64 Hz, 6–56 Hz, 6–48 Hz; 40–45 s; 4
Geophones10 Hz vertical, 12 geophones per group (in-line)
Receiver interval30 m
Source interval60 m
Nominal CDP fold175
Correlated record length30 s

[6] The reflection seismic data were processed by “Geofizyka Torun” Ltd. The basic processing flow in the time domain involved refraction statics calibrated by uphole times, F-K filtering, predictive deconvolution, iterations of velocity analysis, and residual statics. The long-spread and variable surface conditions resulted in reduced S/N ratios; therefore, various noise removal tools were applied prestack. The most effective tools were dipscan stacking and F-X deconvolution. Several sections were produced including conventional common midpoint stack, partially migrated stack (dip moveout and common reflection surface stacks), poststack migrations, and prestack time, and depth migrations. All deliverables were processed using an automatic gain control scaling.

[7] Additional in-house processing was implemented to enhance the signal from deeper parts of the crust and produce a depth section for interpretation. The migrated section was subjected to curvelet-based noise removal [Kumar et al., 2011] and envelope-based processing. It was very effective in delineating the reflection Moho, defined as the reflectivity contrast between the lower crust and the transparent upper mantle, especially in the southern part of the profile (Figure 2a). The velocities used for time-to-depth conversion (Figure 2b) were derived from the nearby deep refraction/wide-angle reflection CEL05 profile [Grad et al., 2006] and merged with the prestack depth migration model.

Figure 2.

(a) Migrated time section along the POLCRUST-01 profile and (b) depth-converted version of Figure 2a overlaid with the velocity model from the CELEBRATION 2000 CEL05 profile [Grad et al., 2006] (see Figure 1 for location). Symbols along refraction Moho in Figure 2b indicate quality of the wide-angle PmP arrivals along the POLCRUST-01 profile as determined from 3-D amplitude analysis: strong PmP reflection (open circles), average PmP quality (crosses), and PmP reflection very weak or not distinguishable from noise in record sections (dots).

3 Main Regional Crustal Features Recorded in the Seismic Data

[8] The NE end of the profile is situated in the marginal part of the East European Craton. The EEC crystalline basement was drilled in the vicinity of SP 1000. It is composed of various Archean and Paleoproterozoic rocks consolidated between 2.0 and 2.2 Ga. The top of the crystalline basement can be associated with an apparently continuous reflector (A-horizon in Figure 3) that can be traced over the distance of more than 100 km. However, under closer scrutiny, this horizon breaks into several segments, and west of SP 4000, it can be no longer considered as the top of the EEC basement (see the discussion in section 4). Up to SP 3000, the basement is overlain by generally flat-lying Lower-Middle Neoproterozoic to Mesozoic sedimentary and volcanic strata gradually thickening to the SW and terminated by the Izbica-Zamość Fault (IZF, Figure 3). The IZF corresponds to the SW boundary of large magnetic anomalies (Figure 3) [Królikowski, 2006; Petecki et al., 2003]. It can be inferred from seismic and borehole data that this nearly vertical fault is a long-lived structure, bearing a strong Carboniferous (Variscan) transpressive overprint [Narkiewicz, 2007; Krzywiec, 2009].

Figure 3.

Geological interpretation of the POLCRUST-01 profile. Bouguer gravity anomaly curve (∆g from measurements along the profile) and magnetic anomaly curve (∆T from Petecki et al. [2003]) are plotted atop the section. Abbreviations: IZF, Izbica-Zamość Fault; JF, Janów Fault; LF, Leżajsk Fault; MAB, magmatic bodies; HDB, high-density body.

[9] A characteristic feature that is rather atypical for the normal cratonic crust away from the midcontinental rift zones [Meissner et al., 2006] is the pronounced lower crustal reflectivity. It starts at ~ 10 s TWT (two-way time) between SP 1000 and 3000 (Figure 2a) and is in good agreement with the top of the lower crust derived from refraction (Figure 2b). The distinct lower crustal reflectivity ends abruptly against the downward extrapolation of the IZF. The reflective zone is thicker close to this tectonic line, while east of the IZF, the reflections are stratified into two zones—denser reflectivity occurs between 10 and 13 s TWT and then it fades out between 13 and 15 s TWT. The upper/middle crust is also reflective. A thick package of reflections in the interval 2.5–7.5 s TWT (Figure 2a), occurring between SP 1500 and 2500 likely represents mafic intrusions that can be linked with a Neoproterozoic trap basalt province extending over a large area of the EEC [Bogdanova et al., 1996]. It is also associated with a prominent magnetic high up to 300 nT (MAB in Figure 3) [Królikowski, 2006].

[10] In the SW, the POLCRUST-01 profile clearly images the outer part of the Carpathian fold-and-thrust belt with its major south-dipping thrusts, partly overriding the Miocene foredeep succession (Figure 3). The main Carpathian thrust is inferred to be a slightly south-dipping plane located at 9 km depth in the southern part [Gągała et al., 2012], although the increased reflectivity is observed down to ~ 15–18 km within a low velocity zone imaged in the refraction profile (Figure 2b). It could represent autochthonous sub-Carpathian sediments being highly tectonized during Alpine orogeny. The basement, as defined by the refraction data below the Carpathians and their foredeep, is relatively transparent. It represents thick Ediacaran-Cambrian folded and partly weakly metamorphosed clastic sediments with a discontinuous thin cover of flat-lying Ordovician to Jurassic strata [Bula and Habryn, 2008]. This regional unit, named the Małopolska Block (MB), is interpreted as a Gondwana-derived terrane accreted to the EEC in the Early Paleozoic [Belka et al., 2000; Winchester et al., 2002]. The MB lower crust south of SP 5000 shows very weak and short-wavelength reflectivity between 8–13 and 10–13 s TWT. Its top, identified based on the CEL05 refraction data (Figure 2b), coincides with two “bright spots” between SP 5500 and 6700, composed of south-dipping reflections packages.

[11] The reflection Moho can be clearly identified as the base of crustal reflectivity at ~ 15 s TWT between SP 1000 and 3000 with possible projection of the Moho reflection into the mantle between SP 2500 and 3000. Reflections at Moho depths (~ 12–13 s ) are evident although much weaker to the SW of SP 3000 and become elusive from SP 4000 southwards. Signal penetration limits estimated using the amplitude decay curves [Mayer and Brown, 1986] ranges from 20 s in the NE (EEC) to ~ 15 s in the SW (Carpathians). Thus, it can be assumed that the weak Moho reflection in the south is due to the physical properties of the crust/mantle boundary (transitional type of Moho). The latter has been validated through the analysis of the PmP wide-angle Moho reflection quality in SE Poland based on crossline recordings from the CELEBRATION 2000 experiment. As indicated in Figure 2b, wide-angle Moho reflections starting from SP 4500 towards SW have been classified as very weak or indistinguishable from the noise. Some more pronounced PmP reflections are observed close to SP 7000, where increased reflectivity is also observed in the migrated section (Figure 2a). The refraction and reflection Moho are colocated (within the error bounds of both methods) between SP 1000–2000 and 6500–9000. The biggest discrepancy between refraction and reflection Moho occurs between SP 3000 and 5750 (8–9 km maximum depth difference, Figures 2b and 3). This might be a result of different structures sampled by the two profiles, as supported by interpretation of other CELEBRATION 2000 profiles [Janik et al., 2009; Narkiewicz et al., 2011], where the Moho is shallowing southeast of CEL05 profile, reaching 38–40 km depth. Such depths are more compatible both with the reflection Moho and other deep refraction profiles crossing TTZ further north (e.g., profile LT7 [Guterch et al., 1994] and profile P4, [Grad et al., 2003]).

4 Complex Structure of the the Cratonic Margin

[12] The area between the indisputable EEC crust on one side and the typical MB crust on the other (SP 3000–5000) poses an interpretative challenge given its complex seismic expression and upper crustal geology. The determination of the southwestern EEC extent largely depends on the interpretation of the continuity of the reflection package defined here as the A-horizon dipping from 2 to 23 km depth between SP 1000 and 5000 (Figure 3). To the NE, the A-horizon clearly corresponds to the top of the crystalline EEC crust, as supported by the CEL05 refraction model, where it coincides with a velocity contrast of 4.9–5.5 km/s to 6.0 km/s up to SP 3700 (Figure 2b). The A-horizon markedly steepens southwest of SP 3350. This change in dip correlates with a major nearly vertical unnamed fault apparent in the sub-Jurassic strata, separating strongly tectonized Devonian-Lower Paleozoic sediments from the gently dipping homoclinal Lower Paleozoic strata to the SW. There is a characteristic reflection-free zone close to SP 4000 separating the Mesozoic cover of the EEC from the Miocene Carpathian foredeep sediments. In surface geology, this boundary corresponds to another major subvertical fault called Janów Fault (JF). Its downward extent corresponds to the southwestern limit of the continuous A-horizon, suggesting that this fault is indeed a deeply rooted structure. Southwest of SP 4000, the A-horizon becomes diffuse, and from ~ SP 4400 southwards, its reflection character appears more compatible with the events observed still further south, between SP 5500 and 6700, close to the top of the lower crust. Notably, between SP 4000 and 4700, it is associated with the velocity contrasts from 6.0 km/s to 6.4 km/s inferred in the CEL05 refraction model (Figure 2b). A-horizon projects into the lower crust southwest of SP 4400 as supported by the CEL05 model. This place corresponds to a major tectonic discontinuity, Cieszanów Fault Zone (CFZ), traceable in sub-Miocene geology owing to a dense network of exploration seismic reflection data and numerous wells [Kowalska et al., 2000; Bula and Habryn, 2008]. It is worthwhile to stress that both the JF and CFZ coincide with two deep subvertical conductive zones imaged by the magnetotelluric soundings along a parallel profile located southeast of the POLCRUST-01 line [Ernst et al., 2002]. It provides independent support for the deeply rooted nature of those fault zones.

[13] Another characteristic feature of the EEC margin is the elevated lower crust observed between SP 3000 and 4000. The uplift is on the order of 10 km (from 30 to 20 km depth) and is associated with the thinning of the middle crust to only 7.5 km. In the same area, the sedimentary layer thickens from ~ 8 to 15 km, while the lower crust shows merely a small thinning. Reflections occurring below this uplifted segment of the lower crust can be tentatively interpreted as the mafic intrusions connected with the Late Precambrian rifting event, modeled as high-density bodies (HDB) along the EEC margin [e.g., Grabowska and Bojdys, 2001]. HDB in Figure 3 corresponds with the strong gradient of the Bouguer gravity anomalies measured along the profile.

[14] In view of the above considerations, the most likely explanation of the A-horizon is that its NE part (from SP 1000 to at least SP 4000) represents the top of the crystalline EEC basement, while southwest of the SP 4400, it corresponds to a different reflection zone in the uppermost part of the lower MB crust. The EEC crust reaches as far west as the vertical trace of the nearly vertical, crustal-scale Cieszanów Fault Zone (Figure 3). It seems to contradict the earlier inferences made based on the analysis of the CELEBRATION 2000 data [e.g., Malinowski et al., 2005; Grad et al., 2006; Janik et al., 2009], where the EEC lower crust was interpreted to extend as far west as 100–150 km from the TTZ.

5 Evolution From a Passive Margin Stage to Polygenetic Crustal Accretion and Strike-Slip Faulting

[15] At first sight, the overall crustal structure along the POLCRUST-01 profile is strikingly similar to the Atlantic-type passive margin setting (compare, e.g., with Austin et al. [1990]), but with the significant overprint of the repetitive tectonic convergence (Cadomian to Alpine) acting at the craton margin and expressed by the generally south-dipping reflections pattern southwest of the EEC.

[16] The geometry of the thinned EEC margin most probably preserves the passive margin stage after the Rodinia breakup in the Late Neoproterozoic [Nawrocki and Poprawa, 2006]. Terminal phases of the continental separation were accompanied by widespread Ediacaran mafic volcanism along the EEC margin, from SE Poland to Ukraine [Bogdanova et al., 1996]. Both the irregular reflective zone in the middle crust (MAB; Figure 3) and the pronounced lower crustal reflectivity at the EEC margin can be related to the Ediacaran extension and associated mafic magmatism. A reflective lower crust is an uncommon feature in the stable cratonic areas and is typically related to crustal extension [Meissner et al., 2006]. However, a similar reflection fabric was detected at the western margin of the EEC in the Baltic Sea [BABEL Working Group, 1993; Meissner and Krawczyk, 1999] and at the eastern margin of the EEC close to Urals [Kashubin et al., 2006]. Both cases are the result of crustal compression. Therefore, the observed lower crustal reflectivity may be polygenetic, related to both compressional and extensional regime, the latter commonly involving mafic sheet-like intrusives.

[17] The transparent lower crust in the Małopolska Block is a feature typical for the Avalonian terranes of the WEPP that were not affected by the Variscan tectonothermal events [e.g., Meissner et al., 1994; Chadwick and Pharaoh, 1998]. The inclined midcrustal reflection fabrics, particularly developed in the extra-Carpathian part of the MB, are suggestive of N-verging thrusts and shear zones (similar to compressional structures commonly observed in the LITHOPROBE data [e.g., White et al., 1994]). The geological data support their relationship to Cadomian orogen constituting basement of the MB. The reactivation by similarly verging Carpathian compression cannot be excluded although it could have been relatively minor, given the generally thin-skinned tectonics of the outer Carpathian belt. However, in situ Cadomian convergence has been excluded by the geological data pointing to opposite processes of crustal extension, rifting and continental separation during late Neoproterozoic [cf. Nawrocki and Poprawa, 2006]. The zone of the EEC-MB contact may be alternatively considered as a Caledonian suture, comparable to the Avalonia-Baltica suture in the southern Baltic area [Krawczyk et al., 2002, 2008], where a stack of Lower Paleozoic sediments is thrusted ~ 150 km northward onto the EEC margin. Although the crustal geometry observed along the POLCRUST-01 profile (especially the A-horizon) suggests such a possibility, the imaged system of the deep subvertical faults parallel to the cratonic margin implies a strong strike-slip overprint. Those faults are displacing laterally thinned outer EEC crust (NE part) and finally separating the MB terrane (CFZ—SW part). The strike-slip faulting started during the Early Paleozoic when it acted as the major transform system, while later the faults were partly reactivated particularly during the Variscan and Alpide compression [Narkiewicz, 2007; Narkiewicz et al., 2011].

6 Conclusions

[18] The deep reflection profile POLCRUST-01 revealed unknown details of a contact zone between the East European Craton and West European Paleozoic Platform crust, constituting a unique example of a polygenetic crustal accretion occurring at the cratonic margin with the strong overprint of strike-slip faulting.

  1. [19] Lower EEC crust shows a pronounced reflectivity most probably related to the Late Neoproterozoic passive margin formation.

  2. [20] The Małopolska Block with a superimposed Carpathian thrust belt and its foreland basin shows a generally nonreflective lower crust, similar to the Avalonia-type crust of North-Central Europe unaffected by the Variscan events. The dipping midcrustal reflectivity is interpreted as thrust/shear zones of the Cadomian collisional orogen.

  3. [21] The intermediate zone between the EEC and MB (~ 50 km wide) shows a less reflective lower crust of the same thickness as in the EEC thinned middle crust, sediments thickening up to 15 km and a net reduction of a crustal thickness to ~ 35 km (versus 45 km in the EEC). It is bounded by subvertical faults penetrating deeply into the crust and constituting a major transform system that was repetitively and decreasingly active during the Early Paleozoic to Cenozoic.

  4. [22] The Teisseyre-Tornquist Zone is imaged as a narrow (10–15 km ) transform zone separating the thinned East European cratonic crust from the Małopolska Block.

[23] Overall, the present results demonstrate that Neoproterozoic to Early Paleozoic continental crustal accretion can be traced in deep seismic record. Although apparent reflectivity patterns may point to typical collisional-type accretion, major strike-slip displacements along the cratonic margins could play a significant if not a decisive role in ultimate development of the accreted belt.

Acknowledgments

[24] POLCRUST-01 profile was acquired by the Industry-Academia Consortium led by the Institute of Geophysics PAS (together with “Geofizyka Torun” Ltd. and PGNiG SA—Polish Oil and Gas Company) and was funded by the Polish Ministry of Environment through the National Fund for Environmental Protection and Water Management (contract 705/2009/Wn - 07/FG - BP - TX/D) and Polish Oil and Gas Company PGNiG SA. Z. Petecki (Polish Geological Institute, Warsaw) is acknowledged for providing the magnetic anomaly curve. Comments by R.G. Keller and an anonymous reviewer helped to improve earlier version of the manuscript.

[25] The Editor thanks David B. Snyder and Randell Stephenson for their assistance in evaluating this paper.

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