Crustal structure across the TESZ along POLONAISE’97
seismic profile P2 in NW Poland
T. Janik a,*, J. Yliniemi b, M. Grad c, H. Thybo d, T. Tiira e
POLONAISE P2 Working Group1
aInstitute of Geophysics, Polish Academy of Sciences, Ks. Janusza 64, 01-452 Warsaw, PolandbSondakyla Geophysical Observatory, University of Oulu, FIN-90571 Oulu, FinlandcInstitute of Geophysics, University of Warsaw, Pasteura 7, 02-093 Warsaw, Poland
dGeological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, DenmarkeInstitute of Seismology, Teollisuuskatu 23, P.O. Box 26, University of Helsinki, FIN-00014 Helsinki, Finland
Received 2 August 2000; accepted 21 November 2001
Abstract
The POLONAISE’97 (POlish Lithospheric ONset—An International Seismic Experiment, 1997) seismic experiment in
Poland targeted the deep structure of the Trans-European Suture Zone (TESZ) and the complex series of upper crustal
features around the Polish Basin. One of the seismic profiles was the 300-km-long profile P2 in northwestern Poland across
the TESZ. Results of 2D modelling show that the crustal thickness varies considerably along the profile: f 29 km below the
Palaeozoic Platform; 35–47 km at the crustal keel at the Teisseyre–Tornquist Zone (TTZ), slightly displaced to the northeast
of the geologic inversion zone; and f 42 km below the Precambrian Craton. In the Polish Basin and further to the south, the
depth down to the consolidated basement is 6–14 km, as characterised by a velocity of 5.8–5.9 km/s. The low basement
velocities, less than 6.0 km/s, extend to a depth of 16–22 km. In the middle crust, with a thickness of ca. 4–14 km, the
velocity changes from 6.2 km/s in the southwestern to 6.8 km/s in the northeastern parts of the profile. The lower crust also
differs between the southwestern and northeastern parts of the profile: from 8 km thickness, with a velocity of 6.8–7.0 km/s
at a depth of 22 km, to ca.12 km thickness with a velocity of 7.0–7.2 km/s at a depth of 30 km. In the lowermost crust, a
body with a velocity of 7.20–7.25 km/s was found above Moho at a depth of 33–45 km in the central part of the profile.
Sub-Moho velocities are 8.2–8.3 km/s beneath the Palaeozoic Platform and TTZ, and about 8.1 km/s beneath the
Precambrian Platform. Seismic reflectors in the upper mantle were interpreted at f 45-km depth beneath the Palaeozoic
Platform and f 55-km depth beneath the TTZ.
The Polish Basin is an up to 14-km-thick asymmetric graben feature. The basement beneath the Palaeozoic Platform in
the southwest is similar to other areas that were subject to Caledonian deformation (Avalonia) such that the Variscan
basement has only been imaged at a shallow depth along the profile. At northeastern end of the profile, the velocity structure
is comparable to the crustal structure found in other portions of the East European Craton (EEC). The crustal keel may be
0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.
PII: S0040 -1951 (02 )00353 -0
* Corresponding author. Fax: +48-22-6915-915.
E-mail addresses: [email protected] (T. Janik), [email protected] (J. Yliniemi), [email protected] (M. Grad), [email protected]
(H. Thybo), [email protected] (T. Tiira).1 POLONAISE P2 Working Group: E. Gaczynski, M. Grad, A. Guterch, T. Janik (Poland); H. Thybo (Denmark); G.R. Keller (USA);
K. Komminaho, T. Tiira, J. Yliniemi (Finland).
www.elsevier.com/locate/tecto
Tectonophysics 360 (2002) 129–152
related to the geologic inversion processes or to magmatic underplating during the Carboniferous–Permian extension and
volcanic activity.
D 2002 Elsevier Science B.V. All rights reserved.
Keywords: Crustal structure; Trans-European Suture Zone; Teisseyre–Tornquist Zone; Polish Basin; Seismic modelling
1. Introduction
The Polish Basin forms the easternmost part of the
Permian–Mesozoic Basin in Central Europe, which
lies along the Trans-European Suture Zone (TESZ).
The TESZ is the most pronounced geologic zone in
Europe north of the Alpine orogenic belt between the
Black Sea and the British Isles. It marks the border
zone between Proterozoic Europe and Phanerozoic
Europe with Caledonian, Variscan and Alpine oro-
genic elements (Berthelsen, 1998). The basin, with
maximum depth of >10 km, is located to the west of
the Precambrian East European Craton (EEC) and
extends from northwest Poland to the Carpathian
mountains while ranging in width from 50 km in the
north to 90 km in the south. There was substantial
rifting, basin development and associated volcanic
activity in North and Central Europe around the
Polish Basin during the Carboniferous–Permian time,
290 Ma ago, followed by several reactivation events
during the Mesozoic and the Cenozoic. The trough
experienced continuous sedimentation from the Upper
Permian to the Mesozoic during phases of extension
since the Variscan orogeny (Dadlez et al., 1995).
Tectonic inversion during the late Cretaceous and
early Tertiary, probably due to compressive stresses
from the Alpine orogeny, produced shortening
between the two European tectonic domains and
thickened the lower crust below the TTZ.
The sedimentary rocks contain significant petro-
leum reserves and include the Zechstein salt deposits
that have hampered seismic imaging of the pre-Per-
mian strata structures. The trough is divided into a
(northern) Pomeranian and a (southern) Kujawy seg-
ment separated by a tectonic block, which was
uplifted in the lower Upper Rotliegendes and then
subsided in the Zechstein period (Pokorski, 1997),
Fig. 1b.
The coincidence between a tectonic inversion zone,
the Teisseyre–Tornquist Zone (TTZ), and an inter-
preted associated lower crustal thickening (e.g.,
Guterch et al., 1986) forms the primary background
for the POLONAISE’97 project (POlish Lithospheric
ONset—An International Seismic Experiment, 1997),
Fig. 1.
The tectonics and the varying density of the
sedimentary rocks significantly contribute to the gen-
eration of gravimetric anomalies of this area. The
basic feature of Bouguer anomalies along profile P2
(see Fig. 3) is the presence of an extensive depression
down to � 40 mGal; on this background, an increase
to 0 mGal is noted corresponding to the Mid-Polish
Anticlinorium (120–200 km along profile). The
Fore–Sudetic monocline in the southwest is charac-
terised by positive gravimetric anomalies of up to + 10
mGal and the East European Platform in the northeast
up to � 10 mGal (Krolikowski and Petecki, 1995;
Krolikowski and Wybraniec, 1996; Krysinski et al.,
2000). The Bouguer anomaly map over the TESZ in
northwest Poland, obtained after stripping off the
three-dimensional gravity effect of the sedimentary
cover down to the Zechstein formation, is character-
ized by a high gravity anomaly of about + 50 mGal.
The positive residual anomalies may be caused by
intrusive basic rocks of high density. The intrusive
masses occur mainly in the central part of Polish
Basin, within the TTZ (Krolikowski and Petecki,
1997).
The study area has been the object of intensive
seismic profiling (Fig. 1). Other investigations
include several profiles across the TTZ/TESZ: LT-
4, LT-5 (Guterch et al., 1983), LT-2, VII (Guterch et
al., 1986), BABEL-A (BABEL Working Group,
1993), LT-7 (Guterch et al., 1994). In order to obtain
the best possible depth sampling of the velocity
structure and to investigate the extent of along-strike
variability, an important part of the POLONAISE’97
project, and a previous project, included seismic
acquisition along five deep seismic refraction and
wide-angle reflection profiles that follow the strike
T. Janik et al. / Tectonophysics 360 (2002) 129–152130
Fig. 1. (a) Location of chosen deep seismic profiles on the background of major tectonic units of central Europe (compiled from Bogdanov and Khain, 1981; Guterch and Grad, 1996;
Guterch et al., 1996a,b). Abbreviations: GS=Sowie Mtns, GSW=Holy Cross Mtns, SB=Upper Silesian Coal Basin; dotted area: Sorgenfrei –Tornquist Zone (STZ) and Teisseyre–
Tornquist Zone (TTZ). (b) Location of the P2 profile and other POLONAISE’97 profiles (P1, P3, P4 and P5); asterisks—shot points, points—receiver positions; grey: area of main
Permo-Trias depocenters; dark grey: Upper Rotliegen main depocenters. Abbreviations: PS=Pomerania segment; KS=Kujawy segments (Pokorski, 1997).
T.Janik
etal./Tecto
nophysics
360(2002)129–152
131
direction of the TTZ/TESZ: TTZ (Grad et al., 1999),
P1 (Jensen et al., 1999), P3 (Sroda et al., 1999), and
P5 (Czuba et al., 2002), in addition to two cross-
profiles: P4 (Guterch et al., 1999) and P2 (this
paper).
2. Data acquisition
Profile P2 (Rzepin–Gdansk) is located in north-
western Poland almost perpendicular to the TTZ
(Fig. 1). Measurements were carried out in May
1997 as part of the POLONAISE’97 experiment
(Guterch et al., 1998b, 1999). The 300-km-long
NW–SE profile crosses the main tectonic units in
the area: the Palaeozoic Platform, the TTZ and the
Precambrian Platform (Fig. 1a). At 11 shot point
locations (SP), indicated by stars in Fig. 1b, 200–
1000 kg of TNT (Table 1) were fired in specially
drilled boreholes, about 40–50 m deep and 10 m
apart, with each hole containing a maximum of 50
kg of explosives. About 100 mobile vertical compo-
nent geophones recorded the seismic waves at an
average spacing of 3 km. ‘‘Zero’’ distance along the
profile is defined as the southwestern end of the
profile at SP2010. The shot numbering increases to
the northeast up to SP2110 (two shot points SP9120
and SP9230 were situated at the crossings with
profiles P1 and P3). A total of 53 shots from four
other POLONAISE’97 profiles (P1, P3, P4 and P5)
were also recorded along profile P2.
3. Correlation of seismic phases
The seismic data is of high quality, with clear first
arrivals up to offsets of 150–250 km, although the
four record sections (especially SP2080 and SP2090)
from the northeastern end of the profile show lower S/
N ratios. The seismic sections are shown in Fig. 2a–d.
Seismic phases with apparent velocity from 3.0 to 4.8
km/s are observed as first arrivals to 40-km offsets
along the whole profile. At 40–110 km offset, the
sections clearly define three groups of waves, based
on distinct differences in apparent velocity of the first
arrivals, interpreted after modelling (Fig. 3) as
refracted waves from the upper crust, the Pg phase
(Fig. 4a–g). For shot points located at the southwest-
ern part of the profile [SP2010, SP2020, SP9120 (SW
branch) and SP2040 (SW)], the observed apparent
velocity is about 6 km/s out to ca. 100-km offset. In
the middle part of the profile, at distances between
100 and 200 km [SP9120 (NE), SP2040 (NE),
SP2050, SP2060 and SP2070 (SW)], the apparent
velocities are lower (5.0–5.6 km/s) out to 80-km
offset. In the northeastern part of the profile, from
200 km [SP2070 (NW) to SP2110], apparent veloc-
ities are again high, ca. 6 km/s. Particularly high
apparent velocities, Vp>6.2 km/s, are observed for
SP2070 (northeast) from 30- to 50-km offset. The
crossover offset between crustal and mantle refrac-
tions is observed at offsets of about 110–130 km. The
offset interval 70–110 km is characterised by weak
first arrivals of Pg phases in the trace normalised
Table 1
Details of used explosive sources
Shot point number Latitude N (/) Longitude E (k) Altitude, h [m] Date y:m:d. Time [GMT] h:m:s TNT charge [kg]
2010 52j24.171VN 15j05.503VE 94 1997:05:22 00:50:00.000163 200
2020 52j33.542VN 15j21.339VE 48 1997:05:20 23:35:00.000163 1000
9120 52j40.935VN 15j39.536VE 50 1997:05:20 22:35:00.000201 1000
2040 52j57.588VN 16j02.630VE 43 1997:05:21 23:35:00.000166 300
2050 53j09.000VN 16j19.694VE 136 1997:05:21 21:35:00.000225 1000
2060 53j17.753VN 16j35.882VE 107 1997:05:20 21:35:00.000225 300
2070 53j33.799VN 16j57.371VE 123 1997:05:28 00:05:00.000191 200
2080 53j43.876VN 17j18.999VE 160 1997:05:27 00:05:00.000191 200
2090 53j55.733VN 17j41.366VE 145 1997:05:27 00:35:00.000215 200
9230 54j01.738VN 17j58.446VE 156 1997:05:26 22:05:00.00 200
2110 54j15.477VN 18j18.840VE 237 1997:05:26 22:35:00.000203 1000
3030 54j03.981VN 17j36.880VE 176 1997:05:27 22:05:00.00 1000
3050 53j49.891VN 18j16.002VE 119 1997:05:27 22:35:00.000202 200
T. Janik et al. / Tectonophysics 360 (2002) 129–152132
sections, due to the high amplitudes of the reflected
waves from the lower crust. At greater offset, weak Pnarrivals refracted beneath the Moho are observed as
first arrivals, with apparent velocities of about 8.0 km/
s at offsets up to 150 km, but only for record sections
from a few shot points: SP2010, SP2020, SP9120 and
f 8.2 km/s for SP2070 (SW). Only sporadic reflec-
tions from the upper crust can be correlated. Strong
reflections from the top of the lower crust are observ-
able in all record sections (PCM). The lower crust is
highly reflective as indicated by strong reverberating
arrivals (ringing signal) in the seismic sections. PMP
reflections from the Moho are rather weak compared
to reflections from the lower crust (PLC, PCM). The
northeastern branches of record sections for SP2020,
SP9120 and SP2040 [at offsets: 50–100 and 200–250
km (SP2020); 160–230 and 110–180 km, respec-
tively] show strong late arrivals with high apparent
velocity of about 8.4 km/s, which we interpret as
reflections from the upper mantle (PI). Because of the
relatively poor quality of seismic record sections from
the northeastern part of the profile, we have addition-
ally made use of two high quality record sections,
acquired along P2 for shot points on profile P3
(SP3030 and SP3050). These two shot points were
about 30 km from the cross point between profiles P2
and P3. Together with SP2110, they confirm the
existence of two reflectors in the lower crust in the
middle of the profile. Record section for SP3030
(SW) shows also intramantle reflections at offset
130–180 km.
In many cases, crustal waves continue as strong
phases into in the over-critical distances to 180–220
km offset (e.g., SP2020, POV). Record sections along
profile P1 (Jensen et al., 1999) and TTZ (Grad et al.,
1999) show similar feature. They are important phases
to the modelling process because they provide infor-
mation about crustal velocities, including the maxi-
mum velocity within the crust. Also, characteristics of
the recorded wave field from the upper crust in the
southwestern and central part of profile are similar to
profile P1 (Jensen et al., 1999).
4. Seismic modelling
The travel times of refracted and reflected P waves,
as identified in the record sections along the profile,
provide the basis for the modelling of the velocity
distribution and depths to the seismic boundaries in
the seismic model of the crust and uppermost mantle
(Fig. 3).
For initial 2D modelling of the crustal and upper
mantle velocity structure, we applied the tomographic
inversion technique of Hole (1992), as presented by
Miller et al. (1997). For the subsequent, more detailed
modelling, we applied ray tracing calculations of
travel times and synthetic seismograms for which
we used the interactive version of the ray package
SEIS83 (Cerveny and Psencık, 1984) supported by
graphical interfaces MODEL (Komminaho, 1997) and
ZPLOT (Zelt, 1992).
Example results of the 2D modelling of the crust
for different parts of the profile are shown in Fig. 4a–
f. The basic geometry of the boundaries and near
surface velocity distribution (to 4-km depth) of the
sedimentary cover were obtained from an initial
model based on 14 boreholes located near the P2
profile together with results from industrial reflection
and refraction profiling in the area. Below a thin layer
of Cenozoic sediments, the Mesozoic sequence has
typical velocities of 2.1–4.5 km/s. The upper Palae-
ozoic strata of a few kilometres’ thickness has veloc-
ities of Vp = 4.5–5.1 km/s for the Zechstein complex
and Vp = 5.0–5.2 km/s for other sequences. There is
no clear boundary between the upper and lower
Palaeozoic sequences, and it is also difficult to dis-
tinguish a clear transition into consolidated metamor-
phic basement. A layer with Vp = 5.7–5.9 km/s has
been determined at ca. 6–8 km depth in the south-
western part of the profile and up to 15-km depth
between 100 and 190 km along the profile. However,
the basement is clearly defined by velocities larger
than 6 km/s from about 200 km along the profile, also
suggested by previous reflection data. Its depth
decreases northeastward from 8 to 4 km at the north-
eastern end of the profile. A high velocity body
(HVB), with Vp>6.1 km/s, explains an anomalous
early first arrival at offset 20–50 km on the north-
eastern branch of the SP2070 record section (PHVB).
The thickness of the basement (Vp>5.9 km/s—‘con-
solidated’) varies from 10 km in TESZ to 5 km below
Precambrian Platform (where Vp>6.0 km/s—crystal-
line). The middle crust, with a thickness of ca. 4–14
km, was modelled with velocity of 6.2 km/s in the
southwestern part of the profile and 6.8 km/s in the
T. Janik et al. / Tectonophysics 360 (2002) 129–152 133
Fig. 2. (a) Amplitude-normalised seismic record sections for SP2010, SP2020, SP9120; reduction velocity 8.0 km/s. (b) Amplitude-normalised
seismic record sections for SP2040, SP2050, SP2060; reduction velocity 8.0 km/s. (c) Amplitude-normalised seismic record sections for
SP2070, SP9230, SP2110; reduction velocity 8.0 km/s. (d) Amplitude-normalised seismic record sections for SP3030, SP3050; reduction
velocity 8.0 km/s.
T. Janik et al. / Tectonophysics 360 (2002) 129–152134
northeastern part of the profile. In the northeastern
part of the profile, from ca. 210 km, a layer with a
thickness of ca. 5–7 km and Vp>6.3 km/s has been
modelled below about 11-km depth, similar to a layer
along profile P3 (Sroda et al., 1999).
The lower crust also differs between the south-
western and northeastern parts of the profile. It is 8
km thick with a velocity of 6.8–7.0 km/s at a depth of
22 km in the southwestern part of the profile, whereas
it is ca.12 km thick with a velocity of 7.0–7.2 km/s at
a depth of 30 km in the northeastern part of the
profile. In the lowermost crust, a body with a velocity
of 7.20–7.25 km/s was modelled above Moho at a
depth of 33–45 km in the central part of the profile
(km 115–250). This velocity was determined by
refraction from record section for SP2040 with sup-
port of dynamic modelling of relationship between
reflections PCM and PMP identified in record sections
for SP2040, SP2110, SP3030 and SP3050. The depth
to Moho at the base of this body changes from 35 to
47 km. Modelling in the central part of the profile was
supported by modelling results along the crossing
TTZ profile (Grad et al., 1999). At the crossing point,
the depth to Moho is 34 km in the TTZ profile and 35
km in P2, determined on the basis of strong reflections
in the SP2040 and SP2060 record sections. The very
deep Moho (47 km), at a distance 200 km, is deter-
mined by strong reflections in the record sections for
SP2110 and SP3050. Sub-Moho velocities are f 8.25
km/s in the central and southeastern parts of the
Fig. 2 (continued).
T. Janik et al. / Tectonophysics 360 (2002) 129–152 137
profile, where the crust is 29 km thick with well-
documented Moho by reflections from record sections
for SP2010 to SP2050 (SW) up to a distance of 120
km. The sub-Moho velocity of 8.1 km/s and the Moho
depth (f 42 km) at the northeastern part of the profile
are in agreement with the model of profile P3 (Sroda
et al., 1999). Mantle reflections from below Moho (PI)
are modelled at a depth of 45 km between 50 and 65
km and at ca. 55-km depth at 130 and 180 km along
the profile (Fig. 4a–c,g; SP2020, SP9120, SP2040
and SP3030). The deepest velocity 8.6 km/s has only
been constrained from the critical offsets and ampli-
tudes of the mantle wide-angle reflections.
5. Discussion of errors
Uncertainties of the model parameters are esti-
mated from the uncertainties of subjectively picked
travel times, the misfits between calculated and
observed travel times, and the ray coverage in the
model. Uncertainties due to erroneous interpretation
of arrivals cannot be estimated, but the probability of
their accuracy increases with increasing quality and
amount of data.
In the kinematics modelling, the calculated travel
times were compared with the experimental travel
times. The model was successively altered by trial-
and-error and travel times were recalculated many
times until an agreement was obtained between the
observed and model-derived travel times.
The accuracy of our model is tested for Pg and PLCwaves for SP2020 record section (Fig. 5). The arrival
time of the Pg wave was calculated with the estimated
velocity of 5.9 km/s and for changes of + 0.2 and
� 0.2 km/s. The arrival times of PLC wave, reflection
from the top of lower crust, was calculated at the
estimated depth of f 22 km and for changes of
depth, + 2 and � 2 km. The grey belts cover the
uncertainty interval of F 0.1 s around the arrival
times calculated for the model. Black dots show our
picked travel times. It is clear from the figure that the
uncertainties of velocity determinations are much
lower than F 0.2 km/s. Similarly, uncertainties of
the depth to intracrustal reflections (and Moho) are
less than F 2 km, probably within F 1 km.
Generally, the velocity and depth uncertainties of
the forward 2D modelling are of the order of F 0.1
km/s and F 1 km. For complicated places of the
model as, e.g., the fault, etc., the accuracy could be
about F 0.2 s and F 2 km, respectively.
Synthetic seismograms were calculated to control
the velocity gradients within the layers and velocity
contrasts at the seismic boundaries. There is a qual-
itatively good agreement between the observed and
theoretically calculated amplitudes for the main
phases. Except for a small piece from SP2040, we
observe no refracted waves from the lowest crust.
Therefore, the velocity has only been modelled by use
of synthetic seismograms with bigger uncertainties, as
F 0.2 H 0.3 km/s.
The whole area of the profile cannot be determined
with the same accuracy in the process of modelling.
Only parts of the model’s boundaries are documented
by reflected waves and some parts of layers are
covered by refracted rays (see Fig. 6).
2D modelling does not take into account out-of-
plane refractions and reflections, which must have
occurred in such a complicated region. In the future,
the data from the all profiles in this area should be
interpreted using a 3D approach.
6. Crustal model
Profile P2 crosses the zone of contact between the
Palaeozoic and Precambrian Platforms in Poland.
There is a clear change in crustal and uppermost
mantle velocity structure at 200 km between the
southwestern and northeastern parts of the profile
Fig. 3. Two-dimensional velocity model of the crustal structure along profile P2 developed by forward ray tracing (lower diagram) and
simplified sketch of the derived structure of upper crust: Cr—Cretaceous, J—Jurassic, T—Triassic, P—Permian, S +Or—Silurian and
Ordovician, OP—older Palaeozoic (middle diagram). Arrows on the top of box show location of shot points and thick lines location of crossing
places of the profiles P2 with profiles P1, TTZ and P3 (middle diagram). Those parts of the first order discontinuities that have been constrained
by reflections or/and refractions are marked by thick lines. The deepest velocity (8.6 km/s) has only been constrained from the critical offsets
and amplitudes of the mantle wide-angle reflections; white dashed line—contoured area covered by rays; (+) = Precambrian crystalline basement
(M»ynarski, 1984); M and 28-km deep boundaries at distance ca. km 265 are taken from P3 profile interpreted by Sroda et al. (1999). Bouger
anomaly curve (upper diagram).
T. Janik et al. / Tectonophysics 360 (2002) 129–152 139
(Fig. 5). The structure of the Earth’s crust between 0
and 180 km is characterised by about 3 km of
sedimentary sequences with velocities of 2.0–4.3
km/s that extend down to the top of Zechstein
deposits. The Zechstein and older sedimentary rocks
have velocities of 5.0–5.5 km/s and extend to depths
of 10–15 km. A 10-km-long and 2-km-thick high-
velocity body (HVB), with Vp>6.1 km/s, was mod-
elled at 200 km at a depth of 6 km. The total
sedimentary thickness ranges from 4 to 8 km in the
northeast (Precambrian Platform) to 8–15 km at the
central (TTZ) and southwestern (Palaeozoic Platform)
parts of the model (Fig. 3). The basement velocities
are very low in the Palaeozoic Platform and TTZ
areas; < 6.0 km/s down to ca. 17-km depth. This
difference is also evident in the middle crust where
the velocity is 6.1 km/s in the southwestern part and
6.8 km/s in the northeastern part of the profile. The
lower crustal velocity is 6.8–7.0 km/s and this unit is
8 km thick in the southwestern part of the model, and
7.0–7.25 km/s in the 12-km-thick northeastern part of
the model.
The crustal thickness increases from 29 km in the
southwest to 42 km in the northeast and shows
significant undulation in the central part of section.
The structure of the crust and the Moho depth in the
southwestern part of the profile are similar to findings
along profile P1 (Jensen et al., 1999) and LT-7
(Guterch et al., 1994).
In the central part of the profile, the Moho depth is
less than we expect from previous results of modelling
of seismic data along profiles LT-2, VII, LT-4 and LT-
5. Along these profiles, a crustal thickness over 50 km
has previously been interpreted. However, these
results were based solely on 1D techniques. The LT-
7 profile was the first profile to be modelled with 2D
techniques. The Moho depth along this profile is also
less than 50 km. Modelling in the central and north-
eastern parts of the profile was in agreement with the
models of profiles: TTZ (Grad et al., 1999) and P3
(Sroda et al., 1999).
7. Discussion
Crustal thickness varies strongly along profile P2:
f 29 km on the Palaeozoic Platform, f 47 km in the
Teisseyre–Tornquist Zone (TTZ) and up to f 42 km
on the Precambrian Platform. Beneath the Moho
boundary, rather high velocities of about 8.25 km/s
were found. Three different crustal entities are inter-
preted along profile P2. The first entity, the Palaeozoic
Platform at the southwestern end of the profile,
extends up to a distance of 100 km. The consolidated
basement is characterised by velocities ranging
between 5.7 and 5.95 km/s below the depths of 6–8
km. The velocities of the second entity (TTZ) in the
distance interval 100–200 km are similar to the
Palaeozoic Platform but the depths to the discontinu-
ities are larger. The low velocities ( < 6.0 km/s) of the
consolidated basement extend to depth of ca. 16–22
km. The third entity, the Precambrian Platform, which
ranges from 200 to 300 km, is different from the other
blocks: the consolidated basement with velocities
exceeding 6.0 km/s is reached at 4–8 km depth,
above a second layer with Vp = 6.2 km/s.
The crustal structure and depth of the Moho along
the P2 profile are consistent with previous results
obtained for nearby and crossing profiles (Fig. 1a).
The same velocity distribution and boundary depths
were found beneath the crossing-points with P1, TTZ
and P3 profiles. In general, the upper crust of TESZ
area is characterised by low seismic velocity, low
velocity gradients and significant velocity contrasts
at seismic boundaries (0.3–0.5 km/s). Low velocities
in the TESZ area were observed earlier on refraction,
near-vertical reflection and wide-angle reflection pro-
files. Interpretation of few short vertical reflection
Fig. 4. (a) Modelling result for SP2020: Synthetic seismic section (upper diagram) for the model in Fig. 3; Amplitude-normalised seismic record
sections and theoretical travel times of P waves calculated for the crustal model. Abbreviations used for panels (a)– (f): Pg—refracted arrivals
from the upper crust, PHVB—refracted arrivals HVB, Pn—sub-Moho refraction, POV—strong reflections within the crust in the over-critical
distances, PLM—reflection from the top of the lower crust, PCM—reflection from top of the lowermost crust, PMP—reflection from Moho
discontinuity, PI—reflection from mantle. Reduction velocity 8.0 km/s (middle diagram); Ray diagram with refracted and reflected waves in the
model (lower diagram). (b) Modelling result for SP9120: see panel (a) for further information. (c) Modelling result for SP2040: see panel (a) for
further information. (d) Modelling result for SP2060: see panel (a) for further information. (e) Modelling result for SP2070: see panel (a) for
further information. (f) Modelling result for SP2110: see panel (a) for further information. (g) Modelling result for SP3030: see panel (a) for
further information.
T. Janik et al. / Tectonophysics 360 (2002) 129–152140
profiles near Poznan shows interval velocities of < 6.0
km/s down to 16–18 km depth (Krynicki et al., 1995;
Guterch et al., 1996b, 1998a). Beneath refraction
profiles M-9 and M-7, recorded in the 1970s and
reinterpreted using 2D forward modelling techniques,
velocities of 5.7–5.8 km/s at about 12 km depth and
5.9 km/s at 17 km, were found (Doan, 1989; Pyra,
1990; Grad et al., 1991; Guterch et al., 1991, 1992).
Also, from 1D models of profile M-13 (Janik and
Materzok, 1991), velocities of < 6.0 km/s up to 12 km
depth were found. Beneath the LT-7 profile (parallel to
the P2 profile, about 60 km to northwest) velocities of
5.75–5.9 km/s were found in a depth range of 6–20
km (Guterch et al., 1994). Low velocities in the TESZ
area were also confirmed from a study of surface
wave dispersion along profiles from Prague to War-
saw and from Prague to Uppsala (Novotny et al.,
1995, 1997).
The POLONAISE’97 project provided conclusive
evidence for the high velocity (>7.2 km/s) of the
crustal keel beneath and around the TTZ in Poland.
Determination of this velocity was one of the key
objectives of the project. The existence of the high-
velocity crustal keel may be ascribed to two different
processes (Thybo et al., 1994): either (1) it represents
underplating that developed during the late Carbon-
iferous to early Permian magmatic episode in the area,
or (2) it represents the lower crustal counterpart of the
upper crustal inversion features. The latter may be
related to late Cretaceous to early Tertiary compres-
sion of the area (BABEL Working Group, 1993).
Determination of shear wave velocity and Poisson’s
ratio appears to be the best way to possibly discrim-
inate between the two possibilities. Also, planned
reflection seismic profiles along profile P4 may show
whether the crustal keel is reflective or not.
Lower crustal and Moho reflectivity varies slightly
along the profile. The strongly ringing lower crust in
the southwestern part of the profile, together with
strong PMP reflections, appears to be characteristic for
crust of the Caledonian Avalonia terrain whereas the
moderate reflectivity of the lower crust and Moho is
typical of the Precambrian shield and platform litho-
sphere of Baltica (Thybo et al., 1998).
The upper part of the velocity model reveals a
sedimentary cover on the Precambrian East European
Platform of around 4 km thickness. A gradual thicken-
ing occurs toward the Polish Basin where the base of
the Palaeozoic (post-Caledonian?) sequence attains a
depth of up to 12 km. The asymmetric shape of this
Fig. 5. Test of the accuracy calculated for Pg and PLC arrivals for SP2020 record section. The arrival time of the Pg wave was calculated with the
estimated velocity of 5.9 km/s (solid line) and for changes of F 0.2 km/s (dashed lines). The arrival times of PLC wave, reflection from the top
of lower crust, was calculated at the estimated depth of f 22 km (solid line) and for changes of depth F 2 km (dashed lines). The grey belts
cover the uncertainty interval of F 0.1 s around the arrival times calculated for the model. Black dots show our picked travel times.
T. Janik et al. / Tectonophysics 360 (2002) 129–152148
basin may indicate a half-graben structure with an east-
dipping master fault, in agreement with a proposed
model for Late Palaeozoic basin formation in north-
eastern Europe (Berthelsen, 1998). Nevertheless, it
appears more likely that any principal basin forming
crust-cutting fault was southwest dipping, considering
the orientation of the predominantly south- to west-
dipping crustal structures in the area between Poland
and the North Sea (DEKORP-BASIN Research Group,
1999; BABEL Working Group, 1993; Lassen et al.,
submitted; MONA LISAWorking Group, 1997; Abra-
movitz et al., 1998; Abramovitz and Thybo, 2000).
Seismic velocities are very low ( < 6.0 km/s to
around 20 km depth) in the Palaeozoic Platform and
the Polish Basin. These low velocities are indicative
of old sedimentary sequences, which have been meta-
morphosed, partly because of deep burial during the
Caledonian orogeny, and possibly also because of
heating during Carboniferous–Permian basin devel-
opment and volcanic/magmatic activity (Jensen et al.,
1999). The high velocity body (f 6.1 km/s) around
200 km within the Palaeozoic sedimentary sequence is
interpreted as an igneous intrusion. It may be of
Devonian age (Dadlez et al., 1995; Krolikowski and
Petecki, 1997). However, it appears most likely that it
developed during the Carboniferous–Permian mag-
matic episode. This body lies above high-velocity
bodies at the base of the crust that may, in this context,
be interpreted as magmatic, underplated rocks from
the stretching event (Thybo, 2000).
Fig. 6. Ray coverage documentation. Those parts of the first-order discontinuities that have been constrained by reflections are marked by thick
lines. Thin lines represent the discontinuities in the calculated model constrained, at parts with contiguous grey areas, by refractions. Grey areas
below discontinuities show penetration of modelled layers by rays of refractions. Arrows on the top of box show the location of shot points and
thick lines location of crossing places of profiles P2 with profiles P1, TTZ and P3.
T. Janik et al. / Tectonophysics 360 (2002) 129–152 149
All profiles in the area, located to the southwest of
the Teisseyre–Tornquist Zone, show relatively high
sub-Moho velocities: 8.25 km/s on profile TTZ and
along profile P2. Profile P1 shows velocities of as
high as 8.4 km/s—significantly higher than the rates
found along profile P2. This difference may indicate
anisotropy in the uppermost mantle with the fast axis
trending parallel to the TTZ. Such possible anisotropy
may have a tectonic origin, indicative of alignment of
olivine minerals along the strike-direction of the TTZ
and the main basin axis in the area.
The nature and origin of the 45–55 km reflecting
boundaries in the upper mantle are unclear. They may
represent a crust–mantle transition layer composed of
high- and low-velocity materials (BABEL Working
Group, 1993).
8. Conclusions
. The seismic structure of the Earth’s crust
changes along profile P2. The crustal thickness varies
strongly: f 29 km on the Palaeozoic Platform, f 47
km in the Teisseyre–Tornquist Zone (TTZ) and up to
f 42 km on the Precambrian Platform. Beneath the
Moho boundary, rather high velocities of about 8.25
km/s were found in TESZ. These results are consistent
with other profiles, which cross profile P2.. The POLONAISE’97 project provided conclu-
sive evidence for the high velocity (>7.2 km/s) of the
crustal keel beneath and around the TTZ in Poland.
The existence of the high-velocity crustal body may
be explained by underplating or ‘‘subversion’’ as a
deep counterpart to tectonic inversion.. Lower crustal and Moho reflectivity varies
slightly along the profile. The Caledonian and Baltica
lithosphere may be distinguished based on their
reflectivity character.. The nature and origin of the 45–55 km reflect-
ing boundaries in the upper mantle are unclear. They
may represent a crust–mantle transition layer com-
posed of high- and low-velocity materials.. Seismic velocities are very low ( < 6.0 km/s to
around 20 km depth) in the Palaeozoic Platform and
the Polish Basin. These low velocities are indicative
of old sedimentary sequences—which have been
metamorphosed, partly because of deep burial during
the Caledonian orogeny, and possibly also because of
heating during Carboniferous–Permian basin devel-
opment and volcanic/magmatic activity. The high-
velocity body (f 6.1 km/s) around 200 km within
the Palaeozoic sedimentary sequence is interpreted as
an igneous intrusion of (?)Devonian age in the Palae-
ozoic sedimentary sequence.. Generally, the upper crust of TESZ area is
characterised by low seismic velocity, low velocity
gradients and velocity contrasts at seismic boundaries
(0.3–0.5 km/s).
Acknowledgements
Funding for ADGIP has primarily come from
Polish Oil and Gas, the National Fund for Environ-
mental Protection and Water Management and
Ministry of Environment. Data acquisition for the
POLONAISE’97 project was carried out under
international collaboration. We are grateful to the
many participants from: Institutes of Geophysics of
the Polish Academy of Sciences and University of
Warsaw, Poland; University of Texas at El Paso, USA;
University of Copenhagen and National Survey and
Cadastre, Denmark; GFZ-Potsdam, Germany; Uni-
versities of Helsinki and Oulu, Finland; Uppsala
University, Sweden; Geological Survey of Lithuania;
Geological Survey of Canada, and the shooting crews
from Geophysical Enterprises in Torun and Krakow,
Poland. Financial support from Academy of Finland,
the Carlsberg Foundation and the Danish Natural
Science Research Council and the National Science
Foundation (USA) is appreciated.
This is a EUROPROBE publication.
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