In NE Poland, Eastern European Craton (EEC) crust of
Fennoscandian affinity is concealed under a Phanerozoic platform cover and
penetrated by sparse, deep research wells. Most of the inferences
regarding its structure rely on geophysical data. Until recently, this area
was covered only by the wide-angle reflection and refraction (WARR) profiles,
which show a relatively simple crustal structure with a typical three-layer
cratonic crust. ION Geophysical PolandSPAN™ regional seismic
programme data, acquired over the marginal part of the EEC in Poland, offered
a unique opportunity to derive a detailed image of the deeper crust. Here,
we apply extended correlation processing to a subset (∼950 km) of the PolandSPAN™ dataset located in NE Poland, which
enabled us to extend the nominal record length of the acquired data from 12
to 22 s (∼60 km of depth). Our new processing revealed
reflectivity patterns, which we primarily associate with the
Paleoproterozoic crust formed during the Svekofennian (Svekobaltic) orogeny,
that are similar to those observed along the BABEL and FIRE profiles in the
Baltic Sea and Finland, respectively. We propose a mid- to lower-crustal,
orogeny-normal lateral flow model to explain the occurrence of two sets of
structures that can be collectively interpreted as kilometre-scale S–C′
shear zones. The structures define a penetrative deformation fabric invoking
ductile extension of hot orogenic crust in a convergent setting. Localized
reactivation of these structures provided conduits for subsequent
emplacement of gabbroic magma that produced a Mesoproterozoic
anorthosite–mangerite–charnockite–granite (AMCG) suite in NE Poland.
Delamination of thickened orogenic lithosphere may have accounted for
magmatic underplating and fractionation into the AMCG plutons. We also found
sub-Moho dipping mantle reflectivity, which we tentatively explain as a
signature of the crustal accretion during the Svekofennian orogeny. Later
tectonic phases (e.g. Ediacaran rifting, Caledonian orogeny) did not leave a
clear signature in the deeper crust; however, some of the subhorizontal
reflectors below the basement, observed in the vicinity of the AMCG Mazury
complex, can be alternatively linked with lower Carboniferous magmatism.
Introduction
The Precambrian East European Craton (EEC) is composed of three major
crustal blocks: Fennoscandia, Sarmatia, and Volgo–Uralia
(Gorbatschev and Bogdanova, 1993).
Fennoscandia was formed in the Paleoproterozoic during the Svecofennian
orogeny (see e.g. Lahtinen et al.,
2009). Its crust was imaged by several deep reflection profiles, mostly
offshore (Baltic Sea)
(Abramovitz
et al., 1997; BABEL Working Group, 1993; Korja and Heikkinen, 2005; Meissner
and Krawczyk, 1999), with a notable exception of the FIRE project in onshore
Finland (Kukkonen and Lahtinen, 2006; Torvela et al., 2013). In NE
Poland, the Fennoscandian crust is concealed under a Phanerozoic platform
cover and is penetrated by sparse, deep research wells. See
Krzemińska et al. (2017) for a recent summary. Therefore, most of the
inferences regarding its structure rely on geophysical data. Until
recently, this area was covered only by wide-angle reflection and refraction
(WARR) profiles from the POLONAISE'97 project (P2, P3, P4, P5 profile;
Czuba
et al., 2002; Grad et al., 2003; Janik et al., 2002; Środa et al., 1999)
and legacy transects (LT-7 profile; Guterch et al.,
1994). They portray relatively simple crustal structure with typical
cratonic three-layer crust (Grad et al., 2010). Experimental deep
reflection seismic profile GB1 shot between 1987 and 1988 revealed complex
reflectivity patterns in the deeper crust of the Pomerania region
(Dziewinska and Tarkowski, 2016), but
the low quality of the seismic data precludes any definite interpretation.
Recently this area was covered by the deep reflection seismic profiles of
the ION Geophysical PolandSPAN™ project. In 2012,
10
PolandSPAN™ profiles (with a total length of 2200 km) were
acquired in Poland over the marginal part of the EEC, east of the
Teisseyre–Tornquist Zone (TTZ). This large regional seismic programme aimed
to provide a better understanding of the sedimentary history, tectonic
architecture, and basement structure of the lower Paleozoic shale basins
(Krzywiec et al., 2013). Because of their regional character
and unprecedented imaging quality, PolandSPAN™ data have already
revolutionized several aspects of the regional geology of Poland. These
seismic profiles have been used as constraints for potential field modelling
that led to a new interpretation of the TTZ
(Mazur et al., 2015, 2016b) and Polish
Caledonides (Mazur et al., 2016a). In SE
Poland, interpretation of the PolandSPAN™ profiles proved that
the Variscan deformation extends much further to the east than previously
assumed (Krzywiec et al., 2017a,
b). Malinowski (2016) showed
that these data can be effectively used to study deep crustal structure by
employing the extended correlation method of Okaya and Jarchow (1989),
showing, e.g. presence of the reflective lower crust underlying the EEC in
SE Poland, previously imaged by the POLCRUST-01 profile
(Malinowski
et al., 2013, 2015).
Here, we apply extended correlation processing to a subset of the
PolandSPAN™ data located in NE Poland: three dip (5400, 5500, 5600)
and two strike profiles (1100, 1200) with a total length of ∼950 km. Since Precambrian crust in Poland is concealed beneath a Phanerozoic
platform cover, previous inferences were based mostly on the sparse, deep
research wells available (Krzemińska et al., 2017), but with these new
seismic reflection data it is now possible to shed light on the
characteristics of the deeper EEC crust in NE Poland. The key questions we
would like to address using these new data are as follows: (i) is the image
of the Svecofennian orogen in NE Poland similar to that observed further
north in Fennoscandia, e.g. in the Bothnian Bay (Korja and Heikkinen, 2005)
and onshore Finland (Torvela et al., 2013)? (ii) Do we see a crustal
expression of the Mesoproterozoic magmatism? (iii) Are the later tectonic
events (like Ediacaran rifting of Rodinia or Caledonian tectonics) also
recognizable in the crustal reflectivity patterns? We start with the
geological background, then we summarize the processing steps focused on
enhancing deeper reflectivity, and finally we present the new results and
integrate them with the existing geological observations to provide some
preliminary interpretation of the crustal structure in NE Poland.
Geological background
The study area is located in NE Poland at the western margin of the
EEC and Fennoscandia (Fig. 1). Its core was formed during the Paleoproterozoic
Svecofennian orogeny, which involved accretion of several microcontinents
and island arcs (Lahtinen et al., 2009).
Lahtinen et al. (2009) distinguish a
separate phase of the Svecofennian accretion called the Svekobaltic orogeny
(1.83–1.8 Ga). In the cross-Baltic correlations by
Bogdanova et al. (2015), the area of
NE Poland belongs to a microcontinent called Amberland (Fig. 1) with a
1.83–1.84 Ga accretion age. Subsequently, the Paleoproterozoic crust was
influenced by Mesoproterozoic (1.54–1.45 Ga) anorogenic magmatic
activity, producing anorthosite–mangerite–charnockite–granite (AMCG)
complexes in a ∼600 km long zone stretching from Belarus,
through Lithuania, NE Poland, and the southern Baltic Sea
(Dörr
et al., 2002; Skridlaite et al., 2003; Krzemińska et al., 2017). No
signature of the Svekonorwegian orogeny (1.14–0.9 Ga) affecting the western
rim of Fennoscandia (Bogdanova et al., 2008)
was recognized in our study area. Ediacaran rifting during Rodinia break-up
(e.g. Johansson, 2009) eventually led to the formation of a passive margin
of Baltica in the early Cambrian. No magmatic activity related to this stage
of the EEC margin development has been recognized in NE Poland. The western
part of the study area was also affected by Caledonian tectonics. An
extensive flexural basin, named the Baltic Basin, was developed in the
Silurian in front of the Caledonian orogen. The basin focused deposition of
a fine-grained siliciclastic succession up to 4000 m thick that gradually
thins out to the east and constitutes most of the Phanerozoic platform cover
of the EEC. The western part of the Baltic Basin was intensely folded to
form the Pomeranian Caledonides. The concept of Pomeranian Caledonides was
initially based on the analysis of the deep research wells
(Dadlez et al., 1994), but it was recently confirmed by
PolandSPAN™ line 5600, which was interpreted to image the
frontal thrust of the deformed upper Ordovician and Silurian sedimentary
succession with the undeformed lower Paleozoic sediments of the Baltic Basin
(Mazur et al., 2016a). The youngest magmatic
episode affecting the EEC crust included lower Carboniferous (354–338 Ma)
alkali magmatism with several syenite intrusions (Fig. 2; e.g.
Krzemińska et al., 2017), coeval with the dolerite sills intruding
Silurian sediments offshore of Lithuania
(Motuza et al., 2015). According to the
revised lithostratigraphy (Krzemińska et al., 2017), crystalline
basement units of the study area can be further subdivided into the
Dobrzyń domain (DD), Mazury complex (MC), and Pomerania–Blekinge Belt
(PBB) (Fig. 2). The DD (1.82–1.76 Ga) basement comprises synorogenic
granites and supracrustal paragneisses. The PBB (1.79–1.74 Ga) basement
includes synorogenic granodiorites, quartz monzonites, and granites, whereas
the MC (1.54–1.49 Ga) is composed of the anorogenic AMCG association: quartz
monzonites, charnockitoids, diorites, and monzogabbros. Their occurrences are
clearly visible in the magnetic anomaly map as magnetic highs (Fig. 3).
Major Paleoproterozoic tectonic domains of Fennoscandia across the
Baltic Sea area. The black rectangle shows the study area. The location of the
BABEL A–B (BABEL
Working Group, 1993) and DEKORP-PQ (Meissner
and Krawczyk, 1999) deep reflection profiles is also marked. The known
extent of the domain affected by the Svecofennian orogeny (coloured
terranes) is bounded by the Teisseyre–Tornquist Zone (TTZ) in the SW and the
Belarus–Podlasie granulite belt (BPG) in the SE. Modified from
Bogdanova et al. (2015).
Location of the PolandSPAN™ seismic profiles (yellow
lines) on the background of a simplified geological map of the East European
Craton crystalline basement units (after Krzemińska et al.,
2017). TTZ – Teisseyre–Tornquist Zone, FSB – Fennoscandia–Sarmatia
boundary, AMCG – anorthosite–mangerite–charnockite–granite complexes, MLSZ
– Mid-Lithuanian Suture Zone, Paleozoic massifs: 1 – Olsztynek, 2 – Mława, 3 – Pisz, 4 – Ełk. Locations of WARR profiles LT7
(Guterch et al., 1994), POLONAISE'97 P2
(Janik
et al., 2002), P3
(Środa
et al., 1999), P4 (Grad et al., 2003), and P5
(Czuba
et al., 2002) are marked as thin black lines.
Data and methodsAcquisition
The PolandSPAN™ project employed acquisition parameters that
were primarily optimized to provide a continuous image of the lower
Paleozoic shale basins. Data were acquired with a 25 m receiver–shot spacing
and 960-channel symmetric spread (max. offset of 12 km), providing a nominal
fold of 480 with a common depth point (CDP) spacing of 12.5 m. The source array consisted of four
INOVA AHV-IV Commander (62 000 lb. peak force) Vibroseis trucks. A custom
broadband (2–150 Hz) 16 s long (τsweep) upsweep was used. In the
field, uncorrelated data (28 s of listen time, τrecord) were
recorded with auxiliary data containing measurements of weighted-sum ground
force (FWS), an estimate of the vibrator ground force (Fg)
(Ziolkowski, 2010) for each vibrator in the array.
Processing
ION Geophysical original time and depth imaging were focused on the
sedimentary cover structure with a processing sequence optimized to preserve
the original sweep bandwidth in the sedimentary cover. Reflection tomography
was used to build the velocity model for pre-stack depth migration (PSDM) in
the sedimentary section, while below the basement, WARR-derived velocities
were used. The nominal record length of 12 s enabled imaging down to the
lower crust on average. Malinowski and
Brettwood (2013) and Malinowski (2016) provided a proof of concept: by
using the extended correlation method of Okaya and Jarchow (1989),
PolandSPAN™ data could be extended to greater times
(∼20 s). Malinowski (2016) also demonstrated that despite
relatively short (16 s vs. 45–60 s long sweeps used during the POLCRUST-01
acquisition; Malinowski et al., 2013) and broadband (2–150 Hz as opposed to
6–64 Hz) upsweep, reliable imaging of the deeper structures (including the
Moho) can be obtained.
Therefore, the first step in our reprocessing was the application of a
self-truncating extended correlation, which increased the nominal record
length tprofile from 12 to 22 s. “Self-truncating” means that the
reference signal we correlate with the recorded data was truncated during
the correlation process, preserving the full bandwidth for the original
record length but losing bandwidth at later times. Given the acquisition
parameters of the PolandSPAN™ survey, a maximum frequency
fmax was limited to 57.5 Hz at 22 s of extended time. It can be derived
using the following formulas of Okaya and Jarchow (1989), assuming linear
upsweep.
1τrecord=τsweep+τlisten2tprofile=τrecord-τsweep3fmaxt=f10≤t≤tprofile4fmaxt=f1-f1-f0τsweep(t-tprofile)tprofile≤t≤τrecord
In the case of Vibroseis acquisition, data are usually correlated with the
theoretical (pilot) sweep. As mentioned above, for the
PolandSPAN™ data, we have the ground-force estimates for every
Vibroseis point (VP) location. When Malinowski (2016) compared stacks of
data correlated with both pilot sweep and ground-force estimates averaged
over all VPs, he found that substituting one for another in the
correlation process did not contribute to a significant change in the final
stack quality. However, in this study, we prefer to correlate raw data with
a ground force averaged for every VP, since spatially varying ground-force
estimates (which should compensate for variable baseplate coupling) are more
realistic than a simple theoretical signal.
After the re-correlation process, we started the basic processing sequence,
which was focused on the middle to lower crust and the upper mantle depths.
For quality control purposes, several stacked sections for each line were
produced at various stages and thoroughly assessed in terms of how
processing methods and their parameters affected the seismic signal.
Following this routine, the most effective processing sequence and parameter
configuration were determined. The processing is summarized in Table 1.
Data processing scheme.
1.Read uncorrelated SEG-D records2.Extended correlation with ground force3.Resample to 4 ms4.Geometry set-up and quality control5.Trace editing6.Surface-consistent amplitude scaling(receivers and shots)7.Spherical divergence correction8.Refraction statics (final datum 400 m)9.Minimum phase conversion10.Surface-consistent deconvolution11.Predictive deconvolution12.Residual statics13.FX deconvolution14.Bandpass filtering (2–6–38–48 Hz)15.Residual statics16.AGC17.Kirchhoff DMO18.CDP stack19.Linear coherency filtering20.Post-stack Stolt migration21.Bandpass filtering (8–10–20–30 Hz)22.Trace equalization23.Time–depth conversion
Location of the PolandSPAN™ seismic profiles on the
background of a total magnetic field anomaly map of NE Poland (reduced to
pole) (data compilation of Stanisław Mazur). The location of the AMCG complexes from
Fig. 2 is also indicated by a white dashed line.
We put a lot of effort into estimating the refraction static corrections
because we decided not to use the contractor's solution. Towards this end,
we employed an in-house neural-network-based algorithm
(Mezyk and Malinowski, 2018) for picking first breaks. Both
elevation statics and refraction statics were applied here using a datum
elevation of 400 m and a replacement velocity of 2250 m s-1
(the same as for
original processing). Initially, we processed the data with a
relative amplitude preservation; however, it turned out that qualitatively
better results for the deeper crust were obtained with a pre-stack AGC
scaling (5 s window). We used ION Geophysical pre-stack time migration
(PSTM) root mean square (RMS) velocity models for the NMO–DMO corrections.
Mild coherency filtering was applied pre-stack (only FX deconvolution). Dip
moveout corrections (DMO) appeared to be an essential step. We used
a Kirchhoff-integral-based DMO algorithm on the common offset planes. It
brought improvements into the sections by strengthening the continuity of
reflectors and correcting for conflicting dips. In general, migrating the
DMO-corrected stacked sections provided a clearer image with increased
reflection consistency in both the vertical and horizontal direction. After
the DMO stack, signal coherency was substantially increased with a
post-stack linear dip filtering. Careful tuning of the parameters was
required not to create artificial events. Dip-filtered stacks were
subsequently migrated. We tested the line-segment migration code
(Calvert, 2004), but because of the generally
noisier appearance of such migrated sections, we prefer to use simple F-K
(Stolt) migration. Finally, depth conversion was carried out. Velocity
models for depth conversion were merged from the PSDM velocity models
provided by ION Geophysical for the section above the basement and the
compilation of the crustal velocity model for Poland derived from WARR data
(Grad et al.,
2016) for the deeper section below the basement.
Results
The final migrated depth-converted sections presented in Figs. 4 and 5
formed the basis for defining the structural relationships and reflector
orientations. The reprocessed profiles illustrate a variety of crustal
reflectivity patterns, reflection Moho, and dipping mantle reflections. In
order to facilitate interpretation, the amplitude envelope is computed from
the final stacks, smoothed, and displayed as a colour background.
Signal-penetration depth was estimated from the amplitude decay curves (Fig. 6), extracted from the final stacked sections, following
Barnes (1994). In order to detect amplitude
variability along the profiles, each seismic section is divided into two
parts within which the corresponding decay curves are calculated.
Amplitude decay curves represent the root mean square (RMS) amplitude
generated using a 200 ms long sliding window that yields curves not too
smooth or overly spiky.
Final migrated depth-converted section along
PolandSPAN™ profile 1200: (a) plot of positive
amplitudes;
(b) plot of positive amplitudes with amplitude envelope attribute in the
background.
Final migrated depth-converted section along
PolandSPAN™ profiles 5600, 5500, and 5400 (envelope and
amplitude combined plot as in Fig. 4b). Profiles are centred at the
intersection with line 1100 (vertical red line).
Analysis of the reprocessed seismic sections shows that, in general,
the reflectivity of the crust is not stationary, and its intensity may vary from
high (e.g. L1200 at CDP 3000–6000) to low (e.g. L5400 at CDP 3500–5000) or
even be characterized as acoustically transparent (e.g. L5400 at
CDP 1500–3500), indicating a gradual transition from crustal to mantle rocks.
Observed intracrustal reflections are mostly discontinuous but not chaotic.
They form patterns that can be either subhorizontal (e.g. L5400 at
∼8 km and CDP 5000–10 000) or gently dipping at an angle not
exceeding 20 degrees (e.g. L5600 at 12–22 km and CDP 7000–11 000). The
presence of abnormally strong reflectivity zones can also be marked,
especially in a depth range of 20 to 36 km in the area where lines 5400 and
1200 cross (Figs. 4–5). The transition between the lower crust and the
uppermost mantle is often trackable (as a change in a generally reflective
crust vs. transparent mantle or as a band of stronger reflectivity at the
expected Moho depth), undulating slightly between 36 and 42 km, yet in some
parts of the stacked sections, the signal penetration is insufficient to
image Moho. It is clearly visible in the case of line 5400, for which the
amplitude decay curves calculated for the SW and NE part are substantially
different in terms of reflectivity strength. Without averaging over
thousands of CDPs, the decay amplitudes would flatten out at 20 km for the
transparent CDP interval between 1500 and 3500. In contrast to the poorly
defined Moho in this part of line 5400, a very sharp boundary is observed
along line 5600 and 1200, in a CDP range of 3000–6000 and 1–2500,
respectively. The stacked section for line 5400 shows evidence of a
small symmetrical Moho uplift that emerges around CDP 6000 and extends for
∼90 km in the NE direction. The Moho can also be inferred
from the amplitude decay curves (which we present in the time domain as
originally calculated) as a change in decay rate at 13±1 s of
two-way time whereby the curves do not decay further. This time corresponds
roughly to a depth of 40 km, a level characterized by a sudden reflectivity
drop on the seismic sections presented in the depth domain. Some reflections
might continue into the upper mantle, such as the events visible on line 5600
and 1200 between CDP 1 and 2000 and between 13 500 and 14 500, respectively. Some of the
weaker sub-Moho reflectivity might be related to migration artefacts or
other processing footprints; however, the stronger ones (e.g. the one at
line 5600) seem to be real.
Discussion and preliminary interpretation
The reprocessed PolandSPAN™ profiles from NE Poland show a much
more complex architecture of the EEC crust compared with the WARR data
(Grad et al., 2010), which is a result of the different
methodologies employed. However, as discussed below, it is not only an issue
of more complex reflectivity observed in the reflection profiles but also a
redefinition of the middle–lower crust and Moho depths.
Amplitude decay curves extracted from the sections shown in Figs. 4 and 5. Red dashed line indicates average Moho depth inferred from WARR data.
Final migrated depth-converted section along
PolandSPAN™ profile 1200 with preliminary interpretation. Bars
atop the section are colour-coded according to the crystalline basement
lithologies following Krzemińska et al. (2017) (see Fig. 9 for a
legend). The magnetic profile at the top is extracted from the magnetic anomaly
map (Fig. 3). The dashed blue and red lines represent the top of the lower crust and
Moho boundary, respectively, taken from the WARR compilation of
Majdański (2012). The black dotted line is the interpreted Moho
boundary from reflection data. Arrows point to the upper mantle reflectors
(RUM). S – subhorizontal structural layering (Svekofennian orogenic
fabric); T – ductile thrust shear zones; C′ – extensional shear zones.
The thickness of the Phanerozoic platform cover varies from ∼7–8 km in the SW to less than 2 km in the NE (Figs. 7–9). With few exceptions
(e.g. SW part of line 5400; Fig. 8), reflection Moho is relatively well
defined as the base of bands of intermittent reflections dividing reflective
crust from the generally more transparent upper mantle. In the following
comparisons, we use the compilation of WARR data by
Majdański (2012), including the top lower crust and Moho
horizons. The depth to WARR Moho varies smoothly along the interpreted
PolandSPAN™ profiles between 38 and 43 km with a typical value
around 40 km, being close to the global average of the “normal”
continental crust (Christensen and
Mooney, 1995). The agreement between such defined WARR Moho and the assumed
crust–mantle boundary interpreted in the reflection data is good, with some
notable exceptions. Reflection Moho along line 1200 is ∼2–3 km shallower than the WARR Moho (Fig. 7). Reflection Moho in the NE part of
line 5600 is up to 4 km shallower. Reflection Moho along line 5500 is
∼2 km shallower. In the case of line 5400, a Moho uplift
(∼2–3 km) is observed. See the discussion on the AMCG complex
below. However, considering the fact that the velocities in the sedimentary
cover are poorly resolved in WARR models and we used reflection-derived
velocities at shallower depths, those changes can be attributed to the
differences between those two methods. The depth to lower crust was inferred
using WARR data and the change in the reflectivity patterns observed between
the mid- to lower-crustal depths. The lower crust generally has a
much-reduced thickness compared with the WARR model. We can note some
distinct lower-crustal reflectivity patterns, with a common observation that
the lower crust is reflective close to its top.
Final migrated depth-converted sections along
PolandSPAN™ profile 5400 with its tentative interpretation.
Bars atop the section are colour-coded according to the crystalline basement
lithologies following Krzemińska et al. (2017) (see Fig. 9 for a
legend). The magnetic profile at the top is extracted from the magnetic anomaly
map (Fig. 3). S – subhorizontal structural layering (Svekofennian orogenic
fabric); T – ductile thrust shear zones; C′ – extensional shear zones.
Final migrated depth-converted sections along
PolandSPAN™ profiles 5500 and 5600 with their tentative
interpretation. Bars atop the section are colour-coded according to the
crystalline basement lithologies following Krzemińska et al. (2017) (see
Fig. 9 for a legend). The magnetic profile at the top is extracted from the
magnetic anomaly map (Fig. 3). S – subhorizontal structural layering
(Svekofennian orogenic fabric); T – ductile thrust shear zones; C′ –
extensional shear zones.
The black lines in Figs. 7–9 delineate representative reflection fabrics and
shear zones, which we infer from the data. The main type of reflection
corresponds to the gently dipping to subhorizontal structural layering,
presumably representing Svekofennian orogenic fabric (labelled S in Figs. 7–9). A
number of low-angle discontinuities (15–20∘), inferred
from the seismic reflections, branch off from the subhorizontal fabric
and are followed by subparallel layering. These features, probably matching
ductile thrust shear zones, are dipping towards NE and SE in the NE–SW- and
NW–SE-oriented sections, respectively (labelled T in Figs. 7–9).
Collectively, their geometry is consistent with the previously postulated SW-to-W polarity
of the Svekofennian orogen (e.g. Park, 1985; Gorbatschev and
Bogdanova, 1993; Korja and Heikkinen, 1995, 2005; Nironen,
1997). The southwestward polarity of the orogen is also in accord with the NE-dipping upper mantle reflectors that may correspond to the preserved relics
of a Paleoproterozoic subduction zone. In several places, at a lower–middle
crust level, the subhorizontal reflectors or NE-dipping shear zones are
truncated by a package of reflectors with an opposite dip, i.e. NW or
SW directed, such as line 1200 between CDP 1000 and 5000 (Fig. 7) and line 5400
between CDP 8000 and 12 000 (Fig. 8). These SW-dipping events comprise straight
reflections flanked by reflections bent into parallelism with the
SW-inclined packages. Consequently, subhorizontal or NE-dipping sets of
reflectors often acquire a sigmoidal shape with terminations aligned into
the SW-dipping events. The latter presumably correspond to extensional or
transtensional shear zones of uniform geometry and kinematics throughout the
studied sections. Both sets of structures identified in the seismic images
jointly delineate a kilometre-scale S–C′ fabric (Fig. 10) related to the
SW-directed (in present-day coordinates) mid- and lower-crustal flow.
The subhorizontal-to-NE-dipping, often sigmoidal reflectors represent
first-order orogen-scale shear planes (S), whereas the SW-dipping events
correspond to extensional shear zones (C′) produced during orogen-scale
non-coaxial flow (Fig. 10). A similar fabric was described by Torvela et al. (2013) for the FIRE profiles of onshore Finland. These authors link the
structural pattern observed to the overall convergent tectonic setting of
the accretionary Svecofennian orogeny (1.96–1.76 Ga; Korja and Heikkinen,
1995, 2005; Torvela et al., 2013). Following classical studies
by Beaumont et al. (2001) and Vanderhaege and Teyssier (2001), Torvela et al. (2013) postulate syn-convergent flow of hot lower and middle crust
comparable to that presently connected with the Tibetan Plateau (e.g.
Beaumont et al., 2001, 2006; Lee and Whitehouse, 2007). According to these
models, partial melting of thermally mature thickened orogenic crust and
associated widespread migmatization results in the generation of
low-viscosity crustal layer that may undergo extension in an overall
convergent setting (e.g. Beaumont et al., 2001; Vanderhaege and Teyssier,
2001). Drill core data from the Paleoproterozoic basement of NE Poland
actually confirm widespread migmatization and synorogenic magmatism at the
time of the Svekofennian orogeny (Krzemińska et al., 2017).
Schematic illustration of S–C′ fabric. A set of extensional shear
bands C′ forms oblique to the shear zone boundaries, dipping towards the
shear direction in synthetic orientation. See also Fig. 16.22 in Fossen (2016).
We favour the syn-convergent crustal flow explanation over late orogenic to
post-orogenic extensional collapse (Korja and Heikkinen, 1995,
2005) due to the structural record from the AMCG igneous suite (Cymerman,
2004, 2014). Structural analysis of drill cores suggests localized
compressive deformation of the Mesoproterozoic (1.54–1.45 Ga) AMCG
intrusions (Cymerman, 2004, 2014), implying the cessation of orogenic-scale
extension by the time of their emplacement. The formation of the S–C′ fabric,
revealed by the seismic data, must have already been accomplished before the
AMCG magmatism. Furthermore, seismic-scale deformational features are not
imaged within the plutons (Figs. 7–9). However, some possible contacts of
the AMCG bodies coincide with zones of increased crustal reflectivity,
suggesting that reactivation of inherited shear zones may have provided
conduits for the emplacement of magma. Consequently, we propose that
delamination of over-thickened Svekofennian lithosphere may have accounted
for underplating of gabbroic magma that fractionated into the AMCG plutons
in NE Poland, following classical models of AMCG magmatism. See McLelland et al. (2010) for a review. The gabbroic parental magma yielded anorthositic
derivatives subsequently ascending into the middle to upper crust together
with granitoids derived by crustal anatexis (e.g. McLelland et al., 2010).
Increased mantle reflectivity in the vicinity of the AMCG bodies may signify
fragments of delaminated lower-crustal material. Sub-Moho reflectivity was
also observed along the POLONAISE'97 P4 profile between the P3 and P5 profiles
(Grad et al., 2002). The
exact shape of a lower- to mid-crustal gabbroic body and its position with
respect to the inferred subcrop of the MC AMCG rocks is likely controlled by
the interplay between the magmatism and the structure developed during the
Paleoproterozoic collisional and post-collisional deformations – a
mechanism suggested for the Korosten Pluton by Bogdanova et al. (2004).
Bright lower-crustal reflectors and their complex shape (with some
truncations) observed in the vicinity of the AMCG suite along lines 1200 and
5400 seem to support such an idea. We have to point out that the
interpreted shapes of the AMCG bodies in Figs. 7–9 are only tentative and
rely on three pieces of evidence: (1) the presence of mostly transparent
crust in seismic sections, (2) the occurrence of AMCG suite outcrops at the
top of the basement (after Krzemińska et al., 2017), and (3) the concurrence
of magnetic highs (Figs. 3, 7–9) due to the elevated magnetic susceptibility of
AMCG rocks.
An interesting reflector (marked SI in Fig. 8) is observed along line 5400
for more than 60 km between CDPs 5000 and 10 000 at a depth of ∼7–9 km. It was also visible in the original ION Geophysical time–depth
imaging, as well as in the industry seismic data from this area (Piotr Krzywiec, personal communication, 2019).
It is offset with respect to the magnetic high
crossed by line 5400 (Fig. 3). The S reflector can be tentatively linked
with the AMCG intrusion, representing a sill (or top of the layered
intrusion) fed by the mafic dykes as in the
Shumlyanskyy et al. (2017)
model for the Korosten Pluton in Ukraine. An alternative explanation invokes
a much younger magmatic event. Since the lower Carboniferous syenite
intrusion of the Olsztyn Massif (Fig. 2) is less than 100 km to the SE of
line 5400, the SI reflector (and associated deeper subhorizontal reflectors)
can be alternatively interpreted as intrusions of this age. Such an
explanation for the SI reflector origin is also supported by the fact that
the lower Carboniferous sills were drilled offshore of Lithuania (Motuza et
al., 2015).
The BABEL seismic profiles in the Baltic Sea imaged several dipping sub-Moho
reflectors projecting into the Fennoscandian mantle
(Abramovitz
et al., 1997; BABEL Working Group, 1993; Balling, 2000; Korja and Heikkinen,
2005). The pronounced dipping mantle reflector observed NE of the Bornholm
area along the BABEL A line (from 40 to 65 km of depth) was interpreted by
Balling (2000) as a relic of
paleosubduction occurring at ∼1.8–1.7 Ga. The same
reflections projecting into the mantle were also imaged by the DEKORP-PQ
profiles parallel to the BABEL A profile close to Bornholm, and they were
also attributed to the Proterozoic terrane accretion
(Krawczyk et al., 2002; Meissner and
Krawczyk, 1999). Projecting the BABEL A and PQ mantle reflectors onto line
5600 suggests that we may observe the same feature at the SW end of this
profile. Krawczyk et al. (2002) concluded that the Baltica crust was not
mechanically involved in the Caledonian collision. This view can be
supported by the recent study by Mazur et al. (2016b), who suggested that the Caledonian Deformation Front (CDF) is a thin-skinned feature.
Therefore, we do not link the observed reflectivity patterns (including
mantle reflectors) with Caledonian deformation but consider them to
represent a Proterozoic accretion signature.
Conclusions
Reprocessing of ∼950 km of the regional seismic profiles from
the PolandSPAN™ project provided for the first time a detailed
picture of the EEC (Fennoscandian) crust in NE Poland. It revealed
reflectivity patterns that we primarily associate with Paleoproterozoic
crustal formation during the Svekofennian (Svekobaltic) orogeny and that
are similar to those observed along the BABEL and FIRE profiles in the
Baltic Sea and onshore Finland, respectively (Korja and Heikkinen, 2005;
Torvela et al., 2013). We suggest that a seismic-scale S–C′ fabric of the
Paleoproterozoic crust was shaped by mid- to lower-crustal flow in a
convergent setting during the Svekofennian orogeny. We propose that
delamination of the thickened Svecofennian lithosphere and resulting
asthenospheric ascent, partial melting of the lithospheric mantle, and ponding of
gabbroic melt at the crust–mantle interface (McLelland et al., 2010) can be
reconciled with the observed crustal fabric and explain the emplacement of
the Mesoproterozoic AMCG suites in NE Poland. We also found sub-Moho dipping
mantle reflectivity, which we tentatively explain as a signature of
paleosubduction occurring prior to the Svekofennian orogeny. Later tectonic
phases (e.g. Ediacaran rifting, Caledonian orogeny) did not leave a clear
signature in the deeper crust; however, some of the subhorizontal reflectors
below the basement may be linked to a lower Carboniferous magmatism.
Data availability
Data used in this research are not publicly
available and were provided based on the agreement between
Institute of Geophysics, Polish Academy of Sciences and ION Geophysical.
Author contributions
Conceptualization was carried out by MMal. MM and MMal were responsible for methodology, MM
was responsible for software, and MM and MMal were responsible for
validation. MM, MMal, and SM carried out the normal analysis; MM and MMal conducted the investigation,
and MMal secured resources. MM and MMal were responsible for data curation. MM, MMal, and SM participated in writing,
original draft preparation, review and editing, and visualization.
MMal was responsible for supervision, project administration, and funding acquisition.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “Advances in seismic imaging across the scales”. It is a result of the 14th International Symposium on Deep Seismic Profiling of the Continents and their Margins, Cracow, Poland, 17–22 June 2018.
Acknowledgements
We are grateful to Andrew Calvert for sharing his
coherency filtration and segment migration codes with us. We are indebted to
ION Geophysical for permission to use and show PolandSPAN™
data. Processing was done using GlobeClaritas™ software under
the licence from GNS Science, New Zealand. Comments from the two anonymous
reviewers are greatly appreciated. Comments by Svetlana Bogdanova on an earlier
version of the paper helped us to clarify some aspects of the
geological interpretation.
Financial support
This research has been supported by the National
Science Centre (grant no. UMO-2015/19/B/ST10/01612).
Review statement
This paper was edited by Ramon Carbonell and reviewed by two anonymous referees.
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