Crustal structure across the TESZ along POLONAISE'97 seismic profile P2 in NW Poland

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


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.


Fig. 2 (continued).


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).


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).


(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.


Fig. 4 (continued).


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.


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.


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.

References

Abramovitz, T., Thybo, H., 2000. Seismic images of Caledonian

Collisional structures along MONA LISA profile 2 in the south-

eastern North Sea. Tectonophysics 317, 27–54.

Abramovitz, T., Thybo, H., MONA LISA Working Group, 1998.

Seismic structure across the Caledonian Deformation Front

along MONA LISA profile 1 in the southeastern North Sea.

Tectonophysics 288, 153–176.

BABEL Working Group, 1993. Deep seismic reflection/refraction

interpretation of BABEL profiles A and B in the southern Baltic

Sea. Geophys. J. Int. 112, 243–325.

Berthelsen, A., 1998. The Tornquist Zone northwest of the Carpa-


thians: an intraplate pseudosuture. Geol. Foren. Stockh. Forh.

120, 223–230.

Bogdanov, A.A., Khain, V.E. (Eds.), 1981. International Tectonic

Map of Europe. IGC Sub-commission of the Tectonic Map of

the World, Moscow.

Cerveny, V., Psencık, I., 1984. SEIS83—numerical modelling of

seismic wave fields in 2-D laterally varying layered structure

by the ray method. In: Engdahl, E.R. (Ed.), Documentation of

Earthquake Algorithm, World Data Centre A for Solid Earth

Geophysics, Boulder, Report SE-35, 36–40.

Czuba, W., Grad, M., Luosto, U., Motuza, G., Nasedkin, V., 2002.

POLONAISE P5 Working Group, Upper crustal seismic struc-

ture of the Mazury complex and Mazowsze massif within East

European Craton in NE Poland. Tectonophysics, 360, 115–128

(this volume).

Dadlez, R., Narkiewicz, M., Stephenson, R.A., Visser, M.T., van

Wees, J.-D., 1995. Tectonic evolution of the Mid-Polish Trough:

modelling implications and significance for central European

geology. Tectonophysics 252, 179–195.

DEKORP-BASIN Research Group, 1999. Deep crustal structure of

the Northeast German basin: new DEKORP_BASIN’96 deep-

profiling results. Geology 27, 55–58.

Doan, T.T., 1989. Zastosowanie modelowania sejsmicznego w ba-

daniach struktury skorupy i gornego p»aszcza Ziemi. PhD thesis,

Archives of the Institute of Geophysics, University of Warsaw

(in Polish, unpublished).

Grad, M., Trung Doan, T., Klimkowski, W., 1991. Seismic models

of sedimentary cover of the Precambrian and Paleozoic Plat-

forms in Poland. Publ. Inst. Geophys. Pol. Acad. Sci. A-19

(236), 125–145.

Grad, M., Janik, T., Yliniemi, J., Guterch, A., Luosto, T., Tiira, U.,

Komminaho, K., Sroda, P., Hoing, K., Makris, J., Lund, C.E.,

1999. Crustal structure of the Mid Polish Trough beneath TTZ

seismic profile. Tectonophysics 314 (1–3), 145–160.

Guterch, A., Grad, M., 1996. Seismic structure of the Earth’s crust

between Precambrian and Variscan Europe in Poland. Publ. Inst.

Geophys. Pol. Acad. Sci. M-18 (273), 67–73.

Guterch, A., Grad, M., Materzok, R., 1983. Structure of the Earth’s

crust of the Permian Basin in Poland. Acta Geophys. Pol. XXXI

(2), 121–138.

Guterch, A., Grad, M., Materzok, R., Perchuc, E., 1986. Deep

structure of the Earth’s crust in the contact zone of the Palae-

ozoic and Precambrian Platforms in Poland (TeisseyreTornquist

Zone). Tectonophysics 128, 251–279.

Guterch, A., Grad, M., Materzok, R., Perchuc, E., Janik, T., Gac-

zynski, E., Doan, T.T., Bia»ek, T., Gadomski, D., M»ynarski, S.,Toporkiewicz, S., 1991. Structure of the lower crust of the Pa-

leozoic Platform in Poland from seismic wide-angle and near-

vertical reflection surveys. Publ. Inst. Geophys. Pol. Acad. Sci.

A-19 (236), 41–62.

Guterch, A., Grad, M., Materzok, R., Perchuc, E., Janik, T., Ga-

czynski, E., Doan, T.T., Bia»ek, T., Gadomski, D., M»ynarski, S.,Toprkiewicz, S., 1992. Laminated structure of the lower crust in

the fore-Sudetic region in Poland, derived from seismic data.

Phys. Earth Planet. Inter. 69, 217–223.

Guterch, A., Grad, M., Janik, T., Materzok, R., Luosto, U., Ylinie-

mi, J., Luck, E., Schulze, A., Forste, K., 1994. Crustal structure

of the transition zone between Precambrian and Variscan Europe

from new seismic data along LT-7 profile (NW Poland and east-

ern Germany). C. R. Acad. Sci. Paris 319 (s.II), 1489–1496.

Guterch, A., Lewandowski, M., Dadlez, R., Pokorski, J., Wybra-

niec, S., Zytko, K., Grad, M., Kutek, J., Szulczewski, M., Ze-

lazniewicz, A., 1996a. Podstawowe problemy g»e�bokich badan

geofizycznych i geologicznych obszaru Polski. Publ. Inst. Geo-

phys. Pol. Acad. Sci. M-20 (294), 44 (in Polish with English

summary).

Guterch, A., Grad, M., Gruszynska, B., Bukowicki, J., Jakubowski,

A., Pietrzyk, K., Stawinoga, J., Bystron, M., 1996b. Crustal

structure from deep seismic reflection data in the Permian basin

in the region of TESZ (Poland). EUROPROBE TESZ Work-

shop, Ksia�z, Poland, 11–17 April, 1996, Abstracts.

Guterch, A., Grad, M., Antonowicz, L., 1998a. Crustal structure

from seismic near vertical reflection data in the Polish basin.

23rd General Assembly of the European Geophysical Society,

Nice, France, 20–24 April, 1998. European Geophysical Soci-

ety, Annales Geophysicae, Katlenburg-Lindau, Germany, Sup-

plement to Vol. 16, Part I, C106, Abstracts.

Guterch, A., Grad, M., Thybo, H., Keller, G.R., Miller, K., 1998b.

Seismic experiment spread across Poland. EOS, Transactions,

American Geophysical Union 79 (26), 302, 305.

Guterch, A., Grad, M., Thybo, H., Keller, R.G., POLONAISE

Working Group, 1999. POLONAISE’97—international seismic

experiment between Precambrian and Variscan Europe in Po-

land. Tectonophysics 314 (1–3), 101–122.

Hole, J.A., 1992. Non-linear high-resolution three-dimensional seis-

mic travel time tomography. J. Geophys. Res. 97, 6553–6562.

Janik, T., Materzok, R., 1991. The structure of the Earth’s crust and

its model representation in the Fore-Sudetic Monocline along

profile M-13 (2-X-77/78) (Gorzow Wlkp.-Kalisz). Pub. Inst.

Geophys. Pol. Acad. Sci. A-19 (236), 111–124.

Jensen, S.L., Janik, T., Thybo, H., POLONAISE Working Group,

1999. Seismic structure of the Palaeozoic Platform along PO-

LONAISE’97 profile P1 in NW Poland. Tectonophysics 314

(1–3), 123–144.

Komminaho, K., 1997. Software manual for programs MODEL and

XRAYS—a graphical interface for SEIS83 program package.

Univ. Oulu, Dep. Geophys. Rep. 20, 31.

Krolikowski, C., Petecki, Z., 1995. Gravimetric Atlas of Poland.

Panstwowy Instytut Geologiczny, Warsaw.

Krolikowski, C., Petecki, Z., 1997. Crustal structure at the Trans-

European Suture Zone in northwest Poland based on gravity

data. Geol. Mag. 134 (5), 661–667.

Krolikowski, C., Wybraniec, S., 1996. Gravity and magnetic maps

of Poland—historical background and modern presentation.

Publ. Inst. Geophys. Pol. Acad. Sci. M-18 (273), 87–92.

Krynicki, T., Materzok, W., Materzok, R., Miko»ajczak, A., 1995.Badania sejsmiczne refleksyjne wykonane w Polsce w celu

rozpoznania struktury dolnej skorupy ziemskiej. Geofiz., Bull.

Inf. Przedsie�biorstwa Badan Geofizycznych, Warszawa 1 (95),

27–44.

Krysinski, L., Grad, M., POLONAISE Working Group, 2000. PO-

LONAISE’97—seismic and gravimetric modelling of the crus-

tal structure in the Polish Basin. Phys. Chem. Earth, Part A Solid

Earth Geod. 25, 355–363.


Lassen, A., Berthelsen, A., Thybo, H., submitted. Reflection seis-

mic evidence for two orogens in the southern Baltic Sea.

Tectonics.

Miller, K.C., Keller, G.R., Gridley, M., Luetgert, J.H., Mooney,

W.D., Thybo, H., 1997. Crustal structure along the west flank of

the Cascades, western Washington. J. Geophys. Res. 102 (B8),

17857–17873.

MONA LISAWorking Group, 1997. MONA LISA—Deep seismic

investigations of the lithosphere in the southeastern North Sea.

Tectonophysics 269, 1–19.

M»ynarski, S., 1984. The structure of deep bedrock in Poland on the

basis of refraction results. Publ. Inst. Geophys. Pol. Acad. Sci.

A-13 (160), 87–100.

Novotny, O., Proskuryakova, T.A., Shilov, A.V., 1995. Dispersion

of Rayleigh waves along the Prague–Warsaw profile. Stud.

Geophys. Geod. 39, 138–147.

Novotny, O., Grad, M., Lund, C.-E., Urban, L., 1997. Verification

of the lithospheric structure along profile Uppsala–Prague using

surface wave dispersion. Stud. Geophys. Geod. 41, 15–28.

Pokorski, J., 1997. Paleotectonic map of the Upper Rotliegend. In:

Marek, S., Pajchlowa, M. (Eds.), The Epicontinental Permian

and Mesozoic in Poland. Prace Panstwowego Instytutu Geolo-

gicznego, Warszawa, p. 55.

Pyra, E., 1990. Dwuwymiarowe modelowanie sejsmicznej struktury

skorupy ziemskiej (profil M-7). MSc thesis, Archives of the

Institute of Geophysics, University of Warsaw (in Polish, un-

published).

Sroda, P., POLONAISE Working Group, 1999. P- and S-wave ve-

locity model of the southwestern margin of the Precambrian

East European Craton, POLONAISE’97, profile P3. Tectono-

physics 314 (1–3), 175–192.

Thybo, H., 2000. Crustal structure and tectonic evolution of the

Tornquist fan region as revealed by geophysical methods. Bull.

Dan. Geol. Soc. 46, 145–160.

Thybo, H., Abramovitz, T., Lassen, A., Schjøth, F., 1994. Deep

structure of the Sorgenfrei –Tornquist Zone interpreted from

BABEL seismic data. Z. Geol. Wissen. 22, 3–17.

Thybo, H., Perchuc, E., Gregersen, 5., 1998. Interpretation in statu

nascendi of seismic wide-angle reflections based on EUGENO-

S data. Tectonophysics 289, 281–294.

Zelt, C.A., 1992. Seismic program package ZPLOT.


Crustal structure across the TESZ along POLONAISE'97 seismic profile P2 in NW Poland

Documents

Transcript of Crustal structure across the TESZ along POLONAISE'97 seismic profile P2 in NW Poland