DEEP SEISMIC-REFLECTION REFRACTION INTERPRETATION OF CRUSTAL STRUCTURE ALONG BABEL PROFILE-A AND PROFILE-B IN THE SOUTHERN BALTIC SEA

Abstract

In 1989 the BABEL Working Group collected 2268 km of near-vertical reflection data in the Baltic and Bothnian Seas. As an integrated part of the field survey, the marine airgun shots were recorded by 64 multicomponent land stations. In this paper results are presented from interpretation of profiles B and A in the Baltic Sea, extending from the Aland Archipelago (Finland) into the Bay of Lubeck (Germany). In the shield part of the profiles northeast of the Sorgenfrei-Tornquist Zone, crustal reflectivity is observed at all levels and its termination in depth coincides largely with the crust-mantle boundary. The wide-angle data indicate a three-layer crust with velocities of 6.1-6.4, circa 6.6, and 6.9-7.2 km s-1. The Moho is found between 40-48 km depth, corresponding to 12-15 s TWT. In the northeastern part of profile A and the southern part of profile B, steeply northeast-dipping reflections are found at all crustal levels. The tectonic inversion of the Sorgenfrei-Tornquist Zone is clearly imaged above a thickened, high-velocity lowermost crust (7.1-7.4 km s-1). At depth the Sorgenfrei-Tornquist Zone widens and displays some asymmetry that is believed to be indicative of crustal shortening across the zone. An undulating Moho is observed along profile A where the lateral variability in structure and velocity field primarily is in the lower crust. Beneath the Skurup Basin south of the Sorgenfrei-Tornquist Zone, no intracrustal discontinuities are seen in the wide-angle data and a highly reflecting lowermost crust between 8 and 10 s TWT corresponds to a zone between 25 and 31 km depth with high velocity gradient (6.7-7.1 km s-1). A bright upper mantle reflection at 12 s TWT below the Skurup Basin can be explained by a velocity increase from 7.8 to 8.2 km s-1. Southwest-dipping reflections in the basement of the Mon High, an eastward continuation of the Ringkobing-Fyn basement High, indicate that the Caledonian Deformation Front is located at least 50 km further north than previously believed. The crust below the Mon High is 38 km thick with high velocities (7.1-7.4 km s-1) in the lower crust. In the North German Lowlands, the crystalline crust below the 10 km thick post-Caledonian sedimentary sequence is only 20 km thick and has velocities between 6.0 and 6.9 km s-1. It is hypothesized that during the Caledonian evolution, Baltica's Pre-Cambrian crust protruded into the docking Avalonian terrain as a major crustal flake structure.

Full text

Geophys.
J.
Int.
(1993)
112,
325-343
Deep seismic reflection/refraction interpretation
of
crustal structure
along BABEL profiles
A
and
B
in the southern Baltic Sea
BABEL
Working
Group*
Accepted 1992 October 9. Received 1992 June 30;
in
original form 1992 February
11
SUMMARY
In 1989 the BABEL Working Group collected 2268 km of near-vertical reflection
data in the Baltic and Bothnian Seas. As an integrated part
of
the field survey, the
marine airgun shots were recorded by 64 multicomponent land stations. In this
paper results are presented from interpretation of profiles B and A in the Baltic Sea,
extending from the &and Archipelago (Finland) into the Bay
of
Liibeck (Ger-
many). In the shield part of the profiles northeast of the Sorgenfrei-Tornquist Zone,
crustal reflectivity is observed at all levels and its termination in depth coincides
largely with the crust-mantle boundary. The wide-angle data indicate a three-layer
crust with velocities
of
6.1-6.4, circa 6.6, and 6.9-7.2 km
s-'.
The Moho is found
between 40-48 km depth, corresponding to 12-15
s
TWT. In the northeastern part
of
profile A and the southern part of profile B, steeply northeast-dipping reflections
are found at all crustal levels. The tectonic inversion of the Sorgenfrei-Tornquist
Zone is clearly imaged above a thickened, high-velocity lowermost crust (7.1-
7.4 km
s-').
At depth the Sorgenfrei-Tornquist Zone widens and displays some
asymmetry that is believed to be indicative
of
crustal shortening across the zone. An
undulating Moho is observed along profile
A
where the lateral variability in
structure and velocity field primarily is in the lower crust. Beneath the Skurup Basin
south of the Sorgenfrei-Tornquist Zone, no intracrustal discontinuities are seen in
the wide-angle data and a highly reflecting lowermost crust between
8
and
10
s
TWT
corresponds to a zone between 25 and 31 km depth with high velocity gradient
(6.7-7.1 kms-'). A bright upper mantle reflection at 12s TWT below the Skurup
Basin can be explained by a velocity increase from 7.8 to 8.2kms-'. Southwest-
dipping reflections in the basement of the Men High, an eastward continuation of
the Ringkebing-Fyn basement High, indicate that the Caledonian Deformation
Front is located at least 50km further north than previously believed. The crust
below the Man High is
38
km thick with high velocities (7.1-7.4 km
s-')
in the lower
crust. In the North German Lowlands, the crystalline crust below the 10 km thick
post-Caledonian sedimentary sequence is only 20 km thick and has velocities
between 6.0 and 6.9 km
s-'.
It is hypothesized that during the Caledonian evolution,
Baltica's Pre-Cambrian crust protruded into the docking Avalonian terrain as a
major crustal flake structure.
*The BABEL Workin Group comprises: Sweden: C.-E. Lund, H. Palm, L. B. Pedersen, R. G. Roberts (Solid
University
of
Copenhagen, aster Voldgade
10,
DK-1350 Copen-
S.
A.
Elming (Tilltimpad Geofysik, Lulefi Technical University,
hagen K); N. Balling, E. Nprmark (Department
of
Earth Sciences, S-95187 Lulel).
Aarhus University, Finlandsgade
6,
DK-8200 Aarhus N);
T.
UK: R. W. Hobbs,
S.
L. Klemperef, D. H. Matthews, D. B. Snyder
Dahl-Jensen (Geological Survey
of
Greenland, aster Voldgade 10, (British Institutions' Reflection Profiling Syndicate (BIRPS),
DK-1350 Copenhagen K). Bullard Laboratories Cambridge University, Madingley Rise,
Finland: P. Heikkinen, H. Korhonen, U. Luosto (Institute of Madingley Road, Cambridge CB3 OEZ); R. Long,
T.
Matthews, D.
Seismology, Helsinki University, Et. Hesperiankatu 4, SF-00100 Graham (Department
of
Geological Sciences, Science Laboratories,
Helsinki);
S.
E. Hjelt, K. Komminaho, J. Yliniemi (Geophysics Durham University, South Road, Durham DH1 3LE); D. J.
Observatory,
Oulu
University, SF-90570 Oulu). Blundell, R. Scott-Robinson (Department
of
Geology, RHB New
Germany:
T.
Dickmann,
E.
R. Flueh' (GEOMAR, University
of
College, Egham
TW20
OEX).
Kiel, WischhofstraBe 1-3, D-2300 Kiel 14); R. Meissner,
P.
'Authors for correspondence
Sadowiak,
S.
A. Thomas, Th. Wever' (Institute for Geophysics, 'Now at: FdBW
f.
Wasserschall- u. Geophysik, Kiel.
University
of
Kiel, OlshausenstraBe
40,
D-2300 Kiel
1).
'Now at: Geophysics Department, Stanford University, USA.
Denmark:
H.
Thybo
Q
,
A. Berthelsen' (Institute
of
Geology, Earth Geophysics, Uppsala University, Box 556, S-75122 Uppsala);
325

326
BABEL
Working
Group
Key
words:
BABEL
Project, Baltic Sea, Caledonian Deformation Front,
seismic
reflection, seismic refraction, Tornquist
Zone.
INTRODUCTION
Project BABEL (Baltic and Bothnian Echoes from the
Lithosphere) investigated the seismic structure of the
Lithosphere under the Baltic Shield and its southwestern
margin. The two main objectives of the project were to
determine the reflection characteristics of the Proterozoic
shield to establish its structural geometries with depth, and
for the first time image the Tornquist Zone, the major
European tectonic lineament, by high-resolution seismic
data in the Baltic Sea. In addition the transition from the
Baltic Shield to the Caledonides of the North German
Lowlands was an important target
of
the BABEL survey.
In autumn 1989 the BABEL Working Group, a
consortium of 31 scientists from 12 British, Danish, Finnish,
German and Swedish research institutions, collected
2268 km marine near-vertical reflection data in the Baltic
and Bothnian Seas.
As
an integrated and important part of
data acquisition, the airgun shots from the ship were
recorded at
64
stations on land. Thus, coincident
high-density, wide-angle data and near-vertical reflection
data allow an integrated interpretation of seismic structure
and velocity distribution in the area
of
investigation.
In this paper we present the first results from seismic
interpretation of the wide-angle land recordings together
with a comparison to the marine near-vertical reflection data
along BABEL lines B and A, reaching from the Aaland
Archipelago (Finland) to the Bay of Liibeck (Germany).
This part
of
the BABEL lines extends for 860km and
observations were made at 16 on-shore locations (Fig. 1). In
particular the general velocity field along the profiles is
shown and compared to the near-vertical reflection data.
6PN
60°N 60°N
58ON
58'N
56"N
56'N
54'N 54'N
26'E
Figure
1.
Location map showing BABEL profiles A (consisting
of
parts
A1
and A) and
B
by thick lines and land stations by triangles.
Profiles from other surveys in the area are shown by thin lines: the
EUGEMI
profile, EUGENO-S profiles
(I
to
V),
Polish
international profile
LT7
and the FENNOLORA profile.
Results from the Gulf of Bothnia will be presented in
a
companion paper (BABEL Working Group 1992).
PREVIOUS SEISMIC INVESTIGATIONS
Many of the previous deep seismic investigations in the area
formed parts of the European GeoTraverse (EGT)
programme (Mueller
&
Banda 1983). Most studies applied
refractionlwide-angle explosion techniques and only a few
deep near-vertical seismic data sets are publicly available in
the vicinity of the BABEL profiles (e.g. Juhlin
et
al.
1989;
Trappe 1989;
Ro
et
nl.
1990; Balling 1990; Juhlin 1990;
Dahl-Jensen, Dyrelius
&
Palm 1991).
In 1979 wide-angle seismic data were acquired for the
FENNOLORA project along a north-south profile through
Sweden and northern Norway (Clowes
et
al.
1987;
Guggisberg
&
Berthelsen 1987; Guggisberg, Kaminski
&
Prodehl 1991). The experiment was designed primarily to
provide information from the subcrustal lithosphere. The
profile line is located on land, almost parallel to BABEL
line
B
at a distance of circa 100 km. Early interpretations of
the crustal structure showed strong Moho topography, with
a major step
of
circa
10km
where the profile crosses the
Aseda Shear Zone. Later interpretations of the seismic
(Hauser
&
Stangl 1990) and the gravity data (Henkel, Lee
&
Lund 1990) indicate a more smooth change of Moho depth.
Several hundred kilometres northeast of line B, results of
the SVEKA profile
in
southern Finland reveal a very thick
(>55
km) crust (Luosto 1990).
The Tornquist Zone has been studied by many refraction
profiles. In Poland all interpretations show a pronounced
thickening of the crust under the Teisseyre-Tornquist Zone
(TTZ), which generally is taken to mark the boundary of the
East European Platform in this area (Guterch
el
al.
1986).
During the EGT joint project EUGENO-S, crustal
refraction data were acquired in Denmark and southwestern
Sweden along five profiles of which three cross the
Sorgenfrei-Tornquist Zone (EUGENO-S Working Group
1988). The transition area
from
shield
to
basins shows strong
Moho topography with depths ranging from
48
km to 26 km
(EUGENO-S Working Group 1988; Thybo 1990).
The EGT joint project EUGEMI revealed a 26 to 32 km
thick crust under the North German Basin (Aichroth,
Prodehl
&
Thybo 1992). Two deep near-vertical reflection
profiles (Trappe 1989) show pronounced short wavelength
undulations
of
Moho depth in the northern part of the
North German Basin.
Sedimentary structure within most of the traversed basins
is well investigated by commercial seismic reflection
profiling and drilling.
GEOLOGY
ALONG
THE PROFILES
Sedimentary
cover
The following review of the shallow, pre-Pleistocene geology
is primarily based on Floden (1984), Ahlberg (1986) and

BABEL
profiles
A
and
B
327
Figure
2.
Simplified structural map
of
the area. Abbreviations:
SNF-Sveconorwegian Front, TSB-Trans-Scandinavian Igneous
Belt, BBB-Bornholm-Bleking Block,
STZ-Sorgenfrei-Tornquist
zone,
'ITZ-Teisseyre-Tornquist
Zone, RFH-Ringkobing-Fyn
High, TEF-Trans-European Fault Zone, CDF-Caledonian
Deformation Front (after Berthelsen
1992).
BABEL seismic profiles
A
and
B
are shown with distances in kilometres along them.
Jubitz, Znosko
&
Franke (1986). Fig. 2 gives an overview of
the geology around BABEL profiles A and B.
Along the northern part of profile B (km 431-315),
Pre-Cambrian metamorphics and igneous rocks subcrop at
the pre-Pleistocene surface. Between km 315 and 265, the
profile crosses a northwest-trending,
50
km wide graben with
a
fill
of
Jotnian to Vendian
(?
1300-700Ma) clastic
sediments. The southeastern part of the graben is hidden
under a cover of platform-type Early Palaeozoic sandstones,
shales and limestones. Early Palaeozoic sediments also form
the pre-Pleistocene surface along the southern part
of
line
B
(km265-0) and the northeastern part of line
A
(km435.5-
375). The Early Palaeozoic sediments dip gently to the east
or southeast with a strike direction parallel to profile B. At
the profile, thicknesses are less than
500
m but further to the
south and southeast, thicknesses
of
2-3 km are reached
below the southeasternmost part
of
the Baltic Sea in the
so-called Baltic Syneclise (Jubitz
et
al.
1986).
Near the middle part
of
Oland, shallow boreholes have
proven the existence of a local, circa
30
km wide, graben
structure with a fill of Jothnian sandstones beneath the Early
Palaeozoic sediments. Conceivably the graben extends into
-Gofile B (at about km
80-50).
South of Oland, at circa km
375
along line A, the Early Palaeozoic platform cover is
truncated by a sequence of Mesozoic and younger sediments
in the Hano Basin which shows increasing thickness towards
the Sorgenfrei-Tornquist Zone, reaching about
lo00
m close
to the Christians0 Fault. Mesozoic and Tertiary deposits also
form the pre-Pleistocene surface along line A from the
Sorgenfrei-Tornquist Zone to the Bay
of
Liibeck.
The Sorgenfrei-Tornquist Zone (km 300 to 255)
underwent strong tectonic inversion during the Late
Cretaceous and Early Palaeocene. Prior
to
this
it
was
affected by normal faulting and possibly rifting in Late
Carboniferous to Early Permian and Mosozoic times. Within
the zone, various Mesozoic and Lower Palaezoic sediments,
or even Pre-Cambrian crystallines subcrop close to, or at,
the pre-Pleistocene surface like on the nearby island
Bornholm. Where preserved
in
halfgrabens, Early Palaeo-
zoic sediments are found with thicknesses
of
up to circa
1
s
TWT
(BABEL
Working Group 1991b). Prior to the Late
Cretacous to Early Palaeocene inversion, the Hano and
Skurup basins were possibly interconnected and linked the
Mesozoic Norwegian-Danish and Polish basins.
Both within the Sorgenfrei-Tornquist Zone and further
southwest, depth to acoustic basement shows considerable
variation (Baartman
&
Christensen 1975; Vejblek 1984;
BABEL Working Group 1991b). Between km205 and 125,
line A crosses an acoustic basement high, the M0n High
which is the eastward continuation of the Ringkabing-Fyn
basement high. Pre-Cambrian rocks have been drilled in the
Glamsbjerg and Grindsted wells at least
150
km further west
(EUGENO-S Working Group 1988), but
no
boreholes have
penetrated the circa 700m thick sedimentary cover
of
the
Mon High.
Further southwest along the line, the crystalline basement,
presumably consisting of Caledonian low-grade metamor-
phics, is overlain by an up to 10km thick sequence of
Devonian, Lower Carboniferous, Permian, Mesozoic and
Cenozoic sediments of the North German Basin. The North
German Caledonides formed in relation to Avalonias
docking and collision with the Baltica craton during the
Middle-Late Silurian (Blundell, Mueller
&
Freeman 1992).
Prior to the BABEL experiment, the Caledonian Deforma-
tion Front, marking the northern thrust border
of
Caledonian low-grade metamorphics, was expected to lie
close to the southern border of the Ringkobing-Fyn and
Mon highs (Ziegler 1990; Liboriussen, Ashton
&
Thygesen
1987).
Basement
rocks
Based largely on the nearby on-shore geology, we
distinguish three major basement units along the BABEL
profile (Fig. 2). From north to south they are:
(1)
The
Suecofennian
Crustal
Unit.
This basement unit
belongs to the Early Proterozoic (2100-1750 Ma) Svecofen-
nian orogen of central Sweden and southernmost Finland.
The Svecofennian terrain is characterized by curving
supracrustal belts, migmatitic gneisses and plutonic rocks,
including massive granite batholiths.
Two tectono-magmatic episodes are distinguished, the
oldest about 1900-1860Ma, and the last one around
1820-1780 Ma. However,
SHRIMP
dating (Claesson
et
al.
1990) suggests that the Svecofennian evolution started
earlier with construction of 2200-2000 Ma old magmatic arcs
which prior to, and during, their collision with the shield
supplied detritus to the Svecofennian clastic sediments.
Pre-2000Ma arc terranes have
so
far not been identified
directly at surface, but are possibly present at deeper crustal
levels
(cf:
BABEL Working Group 1990). Illustrative
examples of the near-surface Svecofennian fold structures

328
BABEL
Working Group
are supplied by Stglhos (1981). Basic dyke swarms and
rapakivi massifs were intruded
1650-1500
Ma ago, signaling
final establishment of cratonic conditions.
(2)
The Trans-Scandinavian Granite Porphyry Belt
is a
major magmatic province where undeformed batholith-type
granitoids dominate (Lindh
&
Gorbatchev 1984). The
eastern border of the Trans-Scandinavian Granite Porphyry
Belt cuts the regional Svecofennian deformation pattern.
U-Pb age determinations of zircons (Jar1
&
Johansson 1988)
have revealed that the granitoidal members of the
Trans-Scandinavian Granite Porphyry Belt either belong to
a 1840-1760 Ma or a 1700-1650 Ma suite. Except from close
to the eastern border, members of the older Trans-
Scandinavian Granite Porphyry Belt suite have not been
influenced by the Svecofennian deformation (Wikstrom
1984; Johansson 1988).
The Aseda Shear Zone is an eastwest-trending, steeply
north-dipping brittle to ductile shear zone at surface
(Skjernaa 1992). Large ‘inliers’
or
enclaves of Svecofennian
deformed gneisses and supracrustals from the Trans-
Scandinavian Granite Porphyry Belt have not been reported
south of the zone. Apparently, the Aseda Shear Zone forms
the southern limit of Svecofennian basement.
(3)
A
Relic
of
the Gothian Orogen-The Bleking-
Bornholm Block.
The basement of southeastern Skine,
southern Bleking and Bornholm forms a separate unit which
we call the Bleking-Bornholm block. In Bleking, an
east-west contact separates the deformed supracrustals,
gneisses, and associated granites of this block from the
undeforrned granites and porphyries of the Trans-
Scandinavian Granite Porphyry Belt (EUGENO-S Working
Group 1988). U-Pb, Sm-Nd, and Rb-Sr dating of the rocks
(Johansson
&
Larsen 1989) give comparable protolith ages
on both sides of the contact. Metamorphism and
deformation took place 1700-1600 Ma ago and the
Bleking-Bornholm Block is now considered to have evolved
during the Gothian orogeny (GaB1
&
Gorbatschev 1987).
Other Gothian rocks
of
southwestern Sweden were strongly
overprinted by the
SveconorwegianJGrenville
orogeny
1050-950Ma ago and the only portion
of
the former
Gothian orogen that escaped the overprinting is the
basement of the Bleking-Bornholm block. Presumably, line
B crosses the northern limit of the Bleking-Bornholm block
near the southern end of Oland.
How far to the southwest of Bornholm the Gothian
basement extends along the BABEL line is open for
speculation. However, if only limited (dextral) strike-slip
displacement has occurred along the Sorgenfrei-Tornquist
Zone (EUGENO-S Working Group 1988), the Gothian
basement could well underlie line A right up to the
Caledonian Deformation Front. At deeper crustal levels it
might even reach as far south as the Trans-European Fault
Zone, which is believed to mark the southernmost extension
of
the Pre-Cambrian crust of the Baltic Shield (Berthelsen
1984).
The southern off-shield region.
In the western Baltic Sea,
profile A crosses the Trans-European Fault Zone which,
probably, was active as an oblique collision zone/strike-slip
fault zone in Silurian time when the North German
Caledonides were thrust over the basement of Baltica. The
Tornquist Fan is a northwestward widening splay
o
Carboniferous-Permian fault zones in the Danish area
emanating from the Tornquist-Teisseyre Zone of northerr
Poland (Thybo
&
Berthelsen 1991). The fan is limited in thc
south by the Trans-European Fault Zone and in thc
northeast by the border zone of the Baltic Shield, also called
the Fennoscandian Border Zone (Liboriussen
et al.
1987).
The Tornquist Fan comprises west- to northnorthwest-
trending transtensional fault zones, linking northnortheast-
striking pull-apart structures (e.g. the Oslo-Skagerrak,
Brande and Ronne Grabens). The RingkGbing-Fyn High
formed between two of the faults (Cartwright 1990).
Transtensional movement along one of the faults caused
igneous activity at the northern edge of the Ringkobing-Fyn
High, circa
150
km northwest
of
the BABEL profile; with
probable implications for the development of both the
Brande Graben and the Zechstein basin north of the
Ringkobing-Fyn High (Thybo
&
Schonharting 1991). The
Sorgenfrei-Tornquist Zone coincides with the northeas-
ternmost of the fault zones of the Tornquist Fan at profile A
and in on-shore Sweden, whereas they trend apart in
Kattegat and further northwestward
(ct
Figs 2 and 13).
PROJECT
BABEL
The BABEL data were acquired during three weeks in
September-October 1989. The field layout facilitates
combined near-vertical and wide-angle interpretations with
additional 3-D control provided by supplementary fan-
observations. Favourable weather conditions during most
of
the period enabled collection of high quality near-vertical
and wide-angle reflection seismic data.
SV
Mintrop of Prakla-Seismos AG was chartered
for
the
marine work. Technical specifications
of
data acquisition
and processing are given by the BABEL Working Group
(1992). In short, shots from a 120.61, tuned airgun array
towed at 7.5m depth were recorded by a 3000111 long
streamer towed at 15m depth. Along line B, shot spacing
was 75m and recording time was 25s on 60 channels,
resulting in a 20-fold coverage per 25m CMP. For line A,
shot spacing and recording time were reduced to 50m and
18
s
respectively, and the number
of
channels was
120
which
increased the coverage to
60
fold. Onboard processing and
quality control produced brute stacks of good quality.
Around profiles A and B, the airgun shots from the ship
were recorded at 16 on-shore locations using a rather
heterogeneous pool
of
instruments operated by courageous
students from the participating institutions. Locations of
land recorders (Fig. 1, Table
1)
were chosen as close to the
line as accessible to allow better velocity control, avoiding
3-D effects as much as possible. At some locations, optimum
field layout could not be chosen because only land stations
were available for the wide-angle recordings. Three stations
(8,9 and 12) were located to provide images of expected
lower crustal or mantle inhomogeneities at critical distance
for Moho reflections. Station
12
was placed in the Tornquist
Zone and stations
8
and 9 were placed further northeast on
either side of the Aseda Shear Zone below which a Moho
offset
of
several km had been proposed on the
FENNOLORA profile (Clowes
et al.
1987; Guggisberg
&
Berthelsen 1987; Guggisberg
et
al.
1991). These stations

BABEL
profiles
A and
B
329
Station
No.
Coordinate
and location
Longitude Latitude
Equipment
No.
of
Seismometer
Operating
Distance range (km)
channels institute(a)
South Nearest
North
-
1
-
Uusikaupunki 21"1449" 60'5052"
PCM
(SF) 3 3-mmp (2 Hz) Oulu 355 165
2.
hand 19'55'41"
60024'01"
PCM
(SF) 3 3-mmp (2 Hz)
Oulu
280 95
3-
&and-Norlh 16'58'30" 571353" PC-based 3 3-mmp (2 Hz) Uppsala 280 35 305
Aarhus
4. bland-Middle 16038'27" 56"51'03" Mars-88FD 3 3-mmp (1 Hz) Copenhagen 235 35 352
5
-
&and-South (a) 1602640" 56°20'00" Lennartz 12 2 Smmp (2 Hz) Kiel 252 20 700
PCM
6
geoph. strings (4.5 Hz)
6
geoph. strings (4.5 Hz)
6
.
eland-South (b) 16028'37" 56'23'41" Lennartz 12 2 Smmp (2 Hz) Kiel 252 20
400
PCM
7-
Torhamn 15'50'1
5"
5697'02"
PCM
(SF) 3 3-mmp (2 Hz) Helsinki 262 23 285
8-
Oekna 15024'36" 57'28'58" PC-based 3 3-mmp (2 Hz) Uppsala 155 133 31 5
I
I
11
9
~
Lenhwda
[I
10
-
Bornholm
11
-
Simrishamn
12
-
Eslev
a
13-Mm
I
Aarhus
I
15025'1
7"
56'56'1
3"
Mars-88FD 3 3-mmp (1 Hz) Copenhagen 170
105
390
14'46'07" 55'1 5'31
"
Geometrics
26 20 geoph. strings (4.5 Hz) Aarhus 185 25 285
Pc-based 3-mmp (2 Hz) Uppsala
2
Smmp (2 Hz)
14'14'08" 5590'01"
PCM
(SF) 3 3-mmp (2 Hz) Helsinki 263
19
295
PC-based 3 3-mmp (2 Hz) Uppsala
Aarhus
130241 7. 55057'43n
MUS
MFD
3 3-mmp (1 Hz) Copenhagen 165 95 198
,
1291'02"
,
54'57'37"
,
MWS88FD
,
3
,
3-mmp(1 Hz)
,
Copenhagen
,
146
,
13
I
278
,
II
14
-
Gedser
15
-
Fehmarn
16
-
Lube&
I
11 '56'52" 5435'43"
Mars
88FD 3 3-mmp
(1
Hz) Copenhagen 91 16 224
1lo13'2O' 54027'20" Lennartz
8
2Smmp(2Hz) Kiel 41 213
PCM
2 geoph. strings (4.5 Hz)
MARS-88FD 3 3-mmp (1 Hz) Copenhagen 51 24 41
10°55'40" 54'1
0'50"
Lennartz
8
ZSmmp(2Hz)
Kid
95 247
PCM
2 geoph. sbings (4.5 Hz)
were positioned such that the reflection points roughly
coincide with the
FENNOLORA
line.
Further details on the BABEL, project, the field work and
some of the results obtained
so
far are given in other
contributions by the BABEL Working Group (1990,
1991a, b,
c
and 1992).
Near-vertical
reflection
seismic
data
Routine processing suitable for deep reflection data
(Klemperer 1989) was applied to produce stacked and
migrated sections, which are generally
of
excellent quality.
Sr-nial care was devoted to the velocity analysis where the
initial results of the wide-angle data modelling helped to
constrain the stacking and migration velocities that were
used in processing the normal incidence data. The full
processing sequence is described by the BABEL Working
Group (1992). Stacked and migrated sections for profiles B
anu
The near-vertical reflection section shows great diversity
in petterns of crustal reflectivity and structure along the
nr-f;l-s
(BABEL Working Group 1991~). Along profile B
ti--
;ust-mantle boundary
is
relatively non-reflective and
Moho depth can only be determined from wide-angle
01
%nations. At the northern end of profile B, the upper
are presented in Fig. 3.
part of the crust is non-reflective and the lower part of the
crust shows local subhorizontal reflections between
5
s
TWT
and Moho depth at 12-13s
TWT.
Further south, high
amplitude reflections from the crust are observed at all
crustal levels between km 315 and
265
where line B crosses
the fault limited Jotnian to Vendian
basin.
Also north
of
the
basin, strong reflections are observed at lower crust level.
East of Oland and further north, between km 300 and 100,
crustal reflectivity extends to
14s
TWT.
Between km 175
and
0,
individual reflections from the lower crust tend to dip
northward. A relatively well resolved, north-dipping
reflection extends from the top of the section at km
20
almost to 13s
TWT
at km 170. Other north-dipping
reflections from the crust are observed further north. East of
Oland, upper crust reflectivity shows remarkable variations.
In some parts the crust is reflective below
1.5s
TWT,
in
other parts the crust is almost transparent to
4
or
5
s
TWT.
Increased reflectivity at near surface (km
80-55)
could well
correspond to the eastward continuation of the down-faulted
Jotnian sandstones drilled below middle Oland.
The reflectivity is highly complex from the southern part
of profile B and 100 km into profile
A
northeast of the Hano
Basin. Strong reflection bands indicate a generally
northeast-dipping fabric
of
the whole crust and slight
northeast-dips are also found in a number of en echelon

330
BABEL
Working
Group
offset reflections at Moho level around 12s TWT. A
southwest-dipping reflection band cuts across the northeast-
dipping fabric between km
415
and 350 from
3
to
8
s
TWT
(profile A). Also the wide-angle data recorded at southern
Oland for shots to the south
of
the station (Fig. 8), indicate a
complex structure
of
reflectors in this part of the profile.
The inversion tectonics is dearly imaged in the
Sorgenfrei-Tornquist Zone where the basement block
shows an uplift
of
circa 1.5
s
TWT (BABEL Working Group
1991b). Below the block, there are some clear northeast-
dipping upper crustal reflections. Below the Hano Basin,
northeast of the Sorgenfrei-Tornquist Zone, the lowermost
reflection at 11s TWT is shallower than to the southwest
where it is found at 12-13s. Along line A, in the
Sorgenfrei-Tornquist Zone and further southwestward, a
reflective band is observed between
8
and 10
s
TWT. In the
inversion zone it is slightly higher which is only partly due to
velocity pull-up from the shallow basement. A reflection is
observed at 14s TWT (km280) below the reflective band,
suggesting a highly complex transition zone between crust
and mantle in the Sorgenfrei-Tornquist Zone.
The strong multiples from the top
of
the limestones in the
shallow Skurup Basin could not be totally removed during
processing. Nonetheless. reflections are observed from lower
crust and upper mantle level. The reflective band between
8
and
10s
TWT extends far into the Skurup Basin where it
thins from km
200
until it disappears at km 150. Between
km
230
and 200 at circa 12
s
TWT,
a bright, sharp reflection
is observed. Between km 180 and 140 a zone
of
primarily
northeast-dipping reflections are found between
11
and 14
s
TWT.
Southwest-dipping reflections are present at 1-5
s
TWT in
the basement
of
the M0n High between km175 and 145.
Further southwestward, slightly southwest-dipping, sub-
horizontal reflections at upper crust levels are present at
depths between
1
and
5
s
TWT from 145 km to somewhere
below the North German Lowlands.
Southward from the M@n High, the deep North German
Basin is imaged along profile Al. Below the thick,
post-Caledonian sedimentary succession (up to
5
s
TWT),
the crust appears almost void
of
reflections. Strong multiples
are created from the top Cretaceous limestones, the
Zechstein evaporites, and the Late Palaeozoic volcanics.
Special processing has eliminated these as much as possible
but the reflective energy these multiples represents, makes it
seem likely that, contrary to the Skurup Basin, little energy
penetrated below the sediments in this area where the
background noise level was high. Below the southern flank
of
the MBn High, northeast-dipping reflections at 10-12.5
s
TWT
are observed at the northeastern end
of
profile Al.
Wide-angle
seismic
data
Wide-angle recordings
of
the airgun shots along BABEL
lines A and B were acquired by operating landstations at 16
locations (Fig.
1).
In
general, data were acquired out to
distances
of
300 km, but at one location (station No.
5)
data
recorded to distances as large as 700km produced useful
signal (BABEL Working Group 1991a). The pool
of
equipment used was heterogeneous, ranging from instru-
ments with a single three-component seismometer to
complex
24
channel local geophone arrays of about
1000
m
aperture. At some locations two instruments were installed
to provide data redundancy.
All
stations were equipped with
a time signal receiver (DCF, 77.5 KH7) which was also used
onboard the seismic vessel to monitor shot-breaks
(BABEL
Working Group 1992). Ship-to-shore communication was
provided by a mobile telephone link. Recordings on bedrock
in Finland and Sweden were
of
excellent quality whereas
recordings on the sedimentary basins
in
Denmark and
Germany suffered to varying degrees from the high noise
level and strong absorption in sediments (low
Q).
Table
1
provides an overview of the locations, instruments and the
offset range for which good arrivals were detected.
Field data were reformatted at the participating
institutions, record sections were produced, and
thr
data
were made available in
SEG-Y
format for exchange. Record
sections of station
5
northward and station 10 have been
published elsewhere (BABEL Working Group 1991a, b),
and several vertical component P-wave sections are shown
here. With the exception of stations
13-16,
high quality
S-wave sections were also obtained. They will be analysed
later. All record sections presented here have been
processed according to the following scheme.
Frequency analysis of many seismograms showed that the
signals have a strong peak at about 9Hz and only slight
variation with offset is observed. Therefore an Ormsby
bandpass filter of 4-24Hz was employed for noise
reduction.
It was possible to stack adjacent shots for further
signal-to-noise improvement due to the close shot spacing,
which was 50m along line A and 75m along line B. At
stations where several vertical geophones were closely
distributed, all traces were sorted into equidistant offset bins
and subsequently stacked together. A bin width
of
100m
proved to be a good compromise between the desired
increase
of
fold and avoidance of destructive interference
and violating spatial aliazing.
The raw field records generally show considerable ringing
which may make the identification
of
secondary arrivals
difficult. In Fig. 4 a typical segment of a record section (a) is
shown together with its autocorrelation function (b).
Cascading spike deconvolution
(1500
ms operator length,
100ms gap to second zero crossing, and
3
per cent white
noise added) as described by Ferber
&
Koitka (1991)
effectively removed most of the ringing (Fig.4~). This
process was therefore applied to all data.
A representative set of example record sections are shown
in Fig. 6, 8-10 and 12 to give an impression
of
data quality
along the studied profile. Gaps in the sections are caused
mainly by necessary change
of
tape on the instruments, but
in some cases they are due to technical failure
or
communication problems. Very good signal-to-noise ratio is
seen throughout most
of
the sections, except for extreme
offsets (>200 km) at single-seismometer stations (13 and 14)
located in the basins. Stacking traces from neighbouring
receivers
of
the local geophone arrays made it possible even
to correlate signals beyond 200 km at the southwestern
stations
(15
and 16) in the North German Lowlands.
In the record sections, crustal phases are seen as first
arrivals to offsets
of
140 to 220 km. They are interpreted as
refractions from the upper and middle crust. Later phases
are interpreted as intra-crustal reflections which divide the
crust into two or three main layers. Other weak reflections

BABEL
profiles
A
and
B
331
-3
[SI
-2
-1
-0
I"
tg:
i7
170 175
180
185
X[kmI
Figure
4.
Example
of
cascading spike deconvolution applied
to
the
wide-angle data. (a) Section showing part
of
the original data from
station 6 (Oland-South).
(b)
Plot
of
autocorrelation
of
individual
traces. (c) Section after deconvolution.
from various levels in the crust are seen in the sections but
they appear relatively scattered and originate from rapidly
varying depth levels along the profile. The model we present
here is a smoothed picture
of
the general velocity field and
these phases have therefore been excluded from the
interpretation.
All sections show a strong wide-angle reflection phase
which can be interpreted to originate from the crust-mantle
boundary. The fine structure of these wide-angle
PMP
reflections is variable along the profiles, ranging from sharp
to more ringing signatures. The data from stations
5
and 6
on southern Oland show complicated reflections around the
PMP
for shots
to
the southwest (Fig.8a). The reflections
show high apparent velocities and probably originate from
the lowermost crust and possibly from the uppermost
mantle. A strong phase following the
P,P
is easily
correlated at
8
s
reduced time in the section from station
10
on Bornholm for shots to the northeast (Fig. 9a). At present
it remains unexplained and has not been interpreted in the
velocity model. In most sections, the
P,,
refraction from the
uppermost mantle can only be correlated for short distances
of
20-30 km, where it is the first arrival because strong
upper mantle reflections dominate the refraction and
become the first arrival at larger distances.
THE
VELOCITY MODEL
To determine the velocity field along the 860 km long profile
we have employed data from the 16 landstations that were
in operation during the southern part
of
the BABEL
experiment. Data were in general acquired out to
300
km
distance from each station
(cf:
Table 1). Profile B is located
in the shield area and only for profile A was it necessary to
assess the structure
of
the sedimentary succession and its
velocities. The resolution of the near-vertical reflection data
was sufficient for interpreting the structure of the sediment
layers and knowledge from boreholes helped to constrain
the velocities. Kinematic forward modelling using a
ray-tracing algorithm (Luetgert 1988; Thybo
&
Luetgert
1990) was employed to match observed traveltimes with the
calculated traveltimes from the model.
The final models
of
the compressional wave velocity
structure are shown in Figs
5
and
7
together with the
modelled wide-angle reflections and plots
of
the observed
traveltimes. The discrepancy between modelled and
observed traveltimes is everywhere less than 200ms and in
general the agreement is better.
Profile
B
The velocity model along profile B (Fig. 5b) appears
relatively simple and homogeneous, in general showing two
intra-crustal reflectors at 20 and
30
km depth. The Moho
varies between
40
and
48
km depth, with the larger Moho
depth occurring along the central part
of
profile B and
shallowing at both ends. We find no indications for any
major steps in crustal thickness over short distances near the
&eda Shear Zone
or
elsewhere. This conclusion is further
supported by wide-angle Moho reflections recorded at fan
stations
8
and
9
which indicate a smooth Moho reflector at
the reflection points between Oland and mainland Sweden.
Velocities vary from 6.1 to 6.4 km
s-'
in the upper part
of
the crust. A strong gradient was introduced to match the
curvature
of
the first arrivals. Detailed inspection
of
the
high-density wide-angle data shows that the first arrivals on
some
of
the seismic sections consist
of
several individual
phases (e.g. Fig. 8b arround 120km), indicating slight
velocity perturbations or layering. The effect is clearly
imaged along &and where, in a more detailed modelling, it
could be explained by a highly heterogeneous or layered
crust.
The middle crust, which is characterized by velocities
of
approximately 6.6 km
s-',
apparently merges with the upper
crust at the northern end
of
the profile where no upper
reflector can be identified (Fig.5b). The top
of
the middle
crust layer shallows at the southern end
of
the profile,
accompanied by gentle shallowing
of
the Moho and thinning
of the lower crust which shows slightly higher velocities to
the south.
Due to station distributon, the model along profile B is
best constrained by data along Oland and partly at the
northern end
(cf:
Fig.
5).
An example
of
the fit between an

332
BABEL
Working
Group
J
BB
I
BA
I
B
I
A
ssw
NNE
I
I
I
I
I
I
I I
12
I
I
12
0'
!o
Asz
JVB
516
4
3
BO
€10
20
30
k
40
50
60
S
I
w
.-
0
50
100
150
200
250 300
350
400 450
6.1
=
V,,
in
km/s
DISTANCE
in
km
V.E.
=
2
Figure
5.
Velocity model along profile B. (a) Traveltimes
of
seismic phases as correlated from the wide-angle/refraction record sections
(reduction velocity
8
km
s-'1.
Stations are identified by different symbols. (b) Model in scale 2:l showing the velocity field by first order
discontinuities and velocities at top and bottom
of
layers. Triangles show locations
of
land stations along the profile, except
for
the five stations
which were located outside the displayed range. Reflection points for modelled wide-angle reflections are shown by shaded boxes, where
shading density identify recording station. Abbreviations: ASZ-Aseda Shear Zone, JVB-Jothnian to Vendian basin.
observed seismic section and traveltimes calculated for the
velocity model is presented in Fig. 6(b).
The model corresponds with the near-vertical reflection
data as shown in Fig.
11.
The layers in the velocity model
correspond broadly to zones
of
different reflection
characteristics. Along Oland the upper layer
is
almost void
of near-vertical reflections; the middle crust dominated by
subhorizontal reflections, and the lower crust reflections
generally dip northward. The normal incidence, two-way
traveltime to Moho as calculated from the velocity model
coincides with the apparent termination
of
crustal
reflectivity along most
of
the section, with a possible
exception between km 250 and 175.
Between km 305 and 265 there is an apparent increase in
near-vertical reflectivity throughout the crust including the
upper crust. This
is
coincident with the Jothnian to Vendian
basin and the increased reflectivity may be caused by
different penetration characteristics. The shallowing
of
the
crust-mantle boundary north
of
km 325 is observed on both
the near-vertical reflection and the wide-angle data sets as is
the shallowing at the southern end of profile B.
Profile
A
The velocity structure along profile A (Fig.7b) is more
complex than along profile B and shows pronounced lateral
changes some
of
which are interpreted as indicators for
terrane boundaries. Data coverage along profile
A
is
relatively good, but it required extensive raytracing to
understand some
of
the complexity seen in the record
sections. During modelling, depths to main first order
discontinuities were constrained by the near-vertical
reflection section and the correlation between velocity
model and near-vertical reflection section is good
everywhere along the profile (cf. Fig. 11). Signal-to-noise
level was poor at some
of
the stations in the deep North
German Basin but was improved substantially by subse-
quent stacking and filtering. Even the southernmost station
16
(Liibeck), located
on
top
of
10
km sediments, recorded
signals out to distances
of
200 km (Fig. lob).
Overall, the crust-mantle boundary has a corrugated
shape. The crust is thick beneath the Sorgenfrei-Tornquist
Zone (and the nearby part
of
the Hano Basin) and the
transition from the North German Basin into the Men High,

Sou
I
h
weg
BABEL
profiles
A
and
B
333
Northeast
I!
li
-
0
$9
z
E"
F6
3
0
80
1
bo
Distance in
km
Oland-North
2b0
3b0
Southwest
Northeast
Oland-North
l!
12
-
0
$9
m
P
E
F6
3
0
15
12
9;
(D
0
m
62
-
-
3
0
-15
-1
2
-4
m
-9z
0
-cD
-
-6
-
-3
50
35
100
200 3b0
Distance in
km
Figure
6.
Section
of
seismic data acquired
for
shots
to
the north at station
3
on
northern Oland. Horizontal axes shows distance from station
in
km. (a) Raw section.
(b)
Section superimposed
by
traveltimes calculated for model in Fig.
5(b).
whereas it is thin beneath the northern part
of
the Hano
Basin, the Skurup Basin, and the deepest part
of
the North
German Basin.
The three-layer, shield-type crustal structure continues
from profile B into the northeastern part
of
profile A,
although the Moho rises to a depth
of
34 km beneath the
Hano Basin. Here Moho depth
is
constrained by clear
PMP
arrivals from stations
3,6,
7,
10 and
11.
In Fig.
8
parts
of
the
record sections from the split spread observation at station
6
(Oland south) show the
PMP
for comparison.
To
the
southwest,
PMP
arrives at
6.5
s
reduced time which is 1.5
s
earlier than
to
the northeast. Velocities in the middle crust
decrease by about
0.2
km
s-'
from profile B to below the
Hano Basin.
The Sorgenfrei-Tornquist Zone also shows a three-layer
crust, and the tectonic inversion is clearly indicated by the
thin sedimentary cover and a complex horst and graben
pattern near the surface (BABEL Working Group 1991b).
The upper crustal, wide-angle reflector vanishes to the
southwest within the Sorgenfrei-Tornquist Zone. However,
both land stations
(10
and
11)
used
to
constrain the upper
crustal structure were located at some distance perpendicu-
lar to the line and, therefore, possible
3-D
effects have to be
taken into account. Across the Sorgenfrei-Tornquist Zone,
velocities in the middle crust increase by 0.1-0.2 km
s-'
to
the southwest.
The Moho
is
found at circa 40km depth below the
Sorgenfrei-Tornquist Zone where velocities in the lower

334
BABEL
Working
Group
I
I
I
AB ,AA A,
AI
A1 AC
sw
NE
12
f
I
I
I
I
I I
I
I
I
1
I
I
I
I I
I
NGL
RFH
SB
STZ
HB
16
15
14
13
7
516
11
10
0
I I I I
I
;O
100
150 200
250
300
350
400
450
I
0
DISTANCE
in
krn
--
-
lsolines
V,
in
krn/s
V.E.
=
2
Figure
7.
Velocity model along profile A. (a) Traveltimes
of
seismic phases as correlated from the wide-angle/refraction record sections
(reduction velocity
8
km
s-').
Stations are identified by different symbols.
(b)
Model in scale
2:l
showing the velocity field by first order
discontinuities (full lines) and velocity contours (stippled lines). Triangles show locations
of
land stations along the profile and reflection points
for modelled wide-angle reflections are shown by shaded boxes. Abbreviations: NGL-North German Lowlands, RFH-Ringkobing-Fyn or
M@n High, SB-Skurup Basin,
STZ-Sorgenfrei-Tornquist
zone, HB-Hano Basin.
crust are as high as
7.1
to
7.5
kms-'. The northeastern part
of
this Moho deepening is constrained by
PMP
observations
from stations
10
and
11
whereas only
P,
arrivals from station
7
indicate the thick crust for the central part.
A
small
velocity anomaly within the lower crust at circa km 325 was
included to fit critical distances
of
reflections and apparent
velocities
of
diving waves.
On the record section from station
10
to the northeast, a
bright reflection is seen around
8
s
reduced traveltime, which
is about
2
s
later than the
PMP.
This phase has already been
,sw
Station Oland-South
,sw
Station Oland-South
160
iio
Distance in
km
60
10
08
0)
UI
C
.-
s
.-
;5
t-
2
-
60
10
110
Distance in
km
160
Figure
8.
Sections
of
seismic data acquired at station
5
on
southern Oland. Note the northeastward termination or delay
of
the
f'!
phase at
circa
120
km distance and the differences between
PUP
reflection times in the two directions. Horizontal axes show distance from station
in
km.

BABEL
profiles
A
and
B
335
,sw
Station Bornholm
12
0
a
v)
.-
c9
I
E6
F
3
200
Distance
in
krn
,sw
Station Simrisham
NE,
100
12
0
a
v)
.-
c9
I
E6
F
3
100
Distance in
krn
200
12
9
6
3
12
9
6
3
Figure
9.
Sections
of
seismic data acquired at stations
10
(Bornholm) and
11
(Simrishamn). The two stations were separated
by
40
km
but there is
only
10
km
difference in projection points
on
the profile. R,, denotes a phase which has not been interpreted
by
the present velocity model. Horizontal axes show distance
from
stations in
km.
mentioned in an earlier paper (BABEL Working Group
1991b). In Fig. 9 parts
of
the record sections showing the
PMP
and this later reflection from station 10 (Bornholm) and
the
PMP
from station
11
(Simrishamn) are shown for
comparison. Such a phase is completely missing in the
record section from station
11
and also the reverse sections
from stations 6 and 7 show no indications
of
similar arrivals
(see also Fig. 8a). We therefore assume that this phase in
the station 10 record section is some kind of artefact and
have not included it in
our
modelling. Possible explanations
are side-swipe effects, reflections involving conversion
between shear and compressional waves,
or
reflections from
a northeast-dipping reflector in the upper mantle. The
record section from station
10
northeastward shows
exceptionally strong
S,
arrivals. Stations
10
and
11
show
pronounced differences in apparent velocities and arrival
times
of
first arrivals beyond 150km distance to the
northeast. This indicates rapid lateral changes perpendicular
to the profile.
In
the velocity model, the thick crust associated with the
Sorgenfrei-Tornquist Zone is much wider at depth than
outlined by near-surface fault structures. The deepest part
of
the Moho
is
found offset and extends to underneath the
Hano Basin
50
km northeast
of
the surface expression
of
the
Sorgenfrei-Tornquist Zone.
Under the Skurup Basin, between the Men High and the
Sorgenfrei-Tornquist Zone, a totally different velocity
structure is found. This part is mainly constrained by
observations from stations 10,
11
and 13, and partly by
P,
arrivals from more distant stations. Below a thin
sedimentary cover, including a hitherto unknown buried
sub-basin around km 220, the crustal velocities uniformly
increase from 6.1 to 6.7kms-' at
25
km depth. No
intracrustal reflections are observed, but in the lower crust,
between 25 and
31
km depth, a strong velocity gradient zone
(6.7 to 7.1 km
s-')
is included to match apparent velocities
of
the
PMP
observations. The velocity in the 'upper mantle'
of 7.8 kms-' is slightly less than normal, but well
constrained by the
PMP
critical distances and
P,
observations
from stations
7,
13,
15 and 16.
The high velocity gradient zone in the lower crust
corresponds to the highly reflective zone between
8.5
and
10.5
s
TWT in the near-vertical reflection section. In
addition, the reflection data contain a prominent reflection
at about 12
s
TWT,
but no straightforward evidence for such
a discontinuity occurs in the wide-angle data. Modelling
showed that a velocity increase from 7.8 to 8.2 km
s-'
at a
37 km deep interface does not conflict with the wide-angle
data. This may be a reasonable velocity contrast to generate
the strong reflection in the near-vertical reflection data.
Crustal structure in the area
of
the Men High resembles
that
of
the Sorgenfrei-Tornquist Zone. Below a thin
Mesozoic cover, a northeastward thinning sequence
of
Upper Palaezoic sediments, and the upper 10km
of
the
basement, a mid-crustal layer
of
velocity 6.6 km
s-'
overlays
a high velocity (7.0-7.4 km
s-')
lower crust and the Moho is
found at 38 km depth. Observations from stations 13, 14 and
15 provided this image.
Underneath the almost
10
km thick sedimentary sequence
in the North German Basin, the middle crustal layer is also
imaged, but the Moho depth is only 30 km and lower crustal
velocities remain just below 7.0 km
s-'.
These observations
are in agreement with Aichroth
et
al.
(1992) who found
26-30km depth to Moho and lower crust velocities
of
6.9s-' along the EGT profile, 50km west of the BABEL
profile. The crustal thickening from the North German
Basin to the M#n High is modelled by
PMP
and
P,
observations from stations 15 (Fehmarn) and 16 (Liibeck).
In Fig.
10
parts
of
the record sections from these two
stations are shown for comparison. On the section from
station 16 a complex wavefield with a clear
P,
arrival at 7.1
s
reduced traveltime and a crossover distance
of
110km is
seen, whereas on station
15
the
P,
arrival is seen at 7.5s
reduced traveltime and a crossover distance
of
170 km.
DISCUSSION
Major wide-angle reflectors and velocity discontinuities in
the velocity models (in Figs
5
and
7)
have been
superimposed on the near-vertical reflection sections in Fig.
11.
In the shield from the northern part
of
profile B to

336
BABEL
Working
Group
12-
-
v-
0
In
.E
9
-
2-
.-
;-
F
6-
-
4-
Station Fehmarn
__
NE4
reflection at
20s
TWT. However, at present we cannot
exclude the possibility that the variation in upper crustal
reflectivity may be caused by the internal structure and
composition
of
the seaward continuation
of
the Trans-
Scandinavian Granite Porphyry Belt. Future studies will
investigate further the cause
of
the variation.
The suggestion
of
a major Moho step beneath the Aseda
Shear Zone, based on interpretations
of
the FENNOLORA
data, is contradicted by the BABEL data set.
Our
new
model is consistent with all near-vertical and wide-angle
observations and is further supported by fan recordings at
stations
8
and
9.
The near-vertical reflection data clearly
show a smooth northward increase in crustal thickness along
Oland and fairly constant traveltime
to
the top
of
a zone
characterized by slightly north-dipping reflections. The
velocity model also shows a smooth increase in crustal
thickness and an almost constant depth to the top of the
lower crust.
If
there were any major Moho step below the
FENNOLORA line, this would indicate a major change in
deep crustal structure within the
80
km wide zone separating
the FENNOLORA and BABEL profiles. Moreover, the fan
observations at stations
8
and
9,
with reflecting points close
to
the FENNOLORA profile, also indicate a smooth
transition. Due to the 350 km distance between
FENNOLORA shot points B and C, no reversed coverage
was obtained for the lower crust and Moho reflectors and a
straightforward interpretation of the FENNOLORA wave-
field by a major step on Moho is apparently misleading.
However,
our
results regarding crustal thickness below the
Hano Basin and north
of
the Aseda Shear Zone are
consistent with previous interpretations
of
the
FENNOLORA data set (Clowes
et
al.
1987;
Guggisberg
&
Berthelsen
1987;
Guggisberg
et
al.
1991).
The crust
is
thin under the Hano Basin but the
near-vertical reflection section does not change character
across the Sorgenfrei-Tornquist Zone. There is, however, a
marked change in near-vertical reflection character further
northeast around km
350
(profile
A)
where a northeast-
dipping fabric is identified to the northeast. The wide-angle
data show increased reflectivity from most
of
the crust in
this area which may indicate
a
highly heterogeneous
or
layered crust with rapid lateral variations. Moho depth
varies from 34 km under the basin to 48 km further north
along &and. We also note that the very high velocity
of
7.4
to
7.5
kms-' identified below and south
of
the Sorgenfrei-
Tornquist Zone has not been identified further north where
lower crust velocities have been interpreted as less than
7.2kms-'. The crystalline crust under the Hano Basin,
characterized by shield-type velocity structure and numerous
northeast-dipping near-vertical reflections, may represent
the Bleking-Bornholm block with preserved Gothian
orogenic structures, possibly reworked by Jotnian to
Vendian extension and subsequently attenuated during Late
Palaeozoic and Mesozoic basin development.
The band
of
near-vertical reflections between
8
and
10s
TWT continues into the Sorgenfrei-Tornquist Zone where
it occurs between
7
and
9s
TWT, shallowest at the
northeastern edge of the zone. Deep near-vertical reflections
dip toward the Sorgenfrei-Tornquist Zone from both sides
and horizontal reflections can be seen as late as 14s TWT
within the zone. The velocity model shows a deepening
of
the Moho to 40km which corresponds to
12.5s
TWT,
the
12-
-
v-
8
v)
.E
9
-
:-
.-
E-
I-
6-
-
d-
100
sw
150
200
Distance in km
Station Lubeck
NE,
100
150
Distance in km
200
Figure
10.
Sections
of
seismic
data
acquired
at
stations
15
(Fehmarn)
and
16
(Liibeck).
Note
differences
in
arrival
time
and
cross-over
for
the
P,
phase. Horizontal
axes
show
distance
from
stations
in
km.
km350 along profile As the velocity profiles are formed by
three layers: an upper crust with velocities 6.0-6.3 km
s-l,
a
middle crust with velocities around
6.6
km
s-',
and a lower
crust where velocities are larger than 7.0 kms-'. Such
layering
of
the crust seems to be typical for the shields
(Meissner
1986;
Pavlenkova
1988)
whereas the high-velocity
lower crust has not previously been identified below the
Sorgenfrei-Tornquist Zone.
Distinct variation in upper crustal reflectivity is observed
in profile B east
of
Oland (Fig.3, km
160
to
0),
a feature
which is probably due to something other than near-surface
effects. A possible explanation is that old sedimentary
basins, possibly
of
Jothnian to Vendian age, cause the
observed, short-wavelength variation in depth
to
the
reflective crust. In this part of the profile, a north-dipping
reflection pattern is observed which includes a pronounced
reflection extending through most
of
the crust from km
20
to
170. In a model
of
northward extension this reflection
pattern could represent the major extensional fault in the
crystalline crust. Movement would then have been
transferred into the upper mantle along a ramp which may
be imaged at km 300 by the relatively strong, north-dipping

BABEL
profiles
A
and
B
331
maximum depth at the northeastern edge of the zone.
Land-station 12 (Eslov) was located in Scania,
95
km from
fine A along strike
of
the Sorgenfrei-Tornquist Zone. By
wide-angle reflections, data from this station also shows the
deepening
of
the Moho toward the Sorgenfrei-Tornquist
Zone from both sides
(cf:
the record section in Fig.
12
which
has been normal-move-out corrected with velocity
6.73 km
s-').
The record section shows no wide-angle
reflections from Moho level in the central part
of
the
Sorgenfrei-Tornquist Zone, possibly due to a strong
velocity gradient in the lowermost crust or to the presence
of
a layer with velocity 7.8 km
s-'
at the base
of
the crust.
We suggest that the thickening
of
the lower crust and the
abnormal transition between crust and mantle was caused by
the Late Cretaceous to Palaeocene inversion in the
Sorgenfrei-Tornquist Zone. A possible explanation is that
compressive stresses not only caused the inversion near the
surface but also caused 'subversion'
of
lower crustal rocks
into the upper mantle, forming the abnormal thick crust
or
thick transition zone between crust and mantle. The
asymmetry
of
the deep Sorgenfrei-Tornquist Zone feature
could support an interpretation that the northeast-dipping
strong upper crustal, near-vertical reflections are from faults
which may have been active during previous crustal
shortening. The inversion tectonics, undoubtedly, were
accompanied by lateral movement along the Sorgenfrei-
Tornquist Zone. Blundell and the BABEL Working Group
(1992) suggest a model in which high-angle, strike-slip faults
in the upper crust detach into low-angle faults at the top
of
the reflective lower crust, with the Moho acting as the lower
bound. Displacement was then transferred into the lower
lithosphere along a dipping fault in the upper mantle.
Similar deep structure
of
the Tornquist-Teisseyre Zone
has been interpreted from refractionlwide-angle reflection
seismic data along several profiles in Poland (Guterch
et
al.
1986). In northern Poland, the zone
of
abnormal thickened
crust (Moho trough) is situated below the inversion zone
whereas in southern Poland, the trough is developed due
north
of
the inversion zone. Northwest
of
the BABEL
profile, there are refraction seismic indications from
P,,
delays along EUGENO-S profile
1
for a deepening from
basin to shield at the Sorgenfrei-Tornquist Zone near
Kattegat (Thybo 1990), whereas there is no sign of such
deepening along EUGENO-S profile 2 in the Kattegat area
(Flueh
&
Vieland 1989).
Beneath the Skurup Basin, between the Sorgenfrei-
Tornquist Zone and the M0n High, no wide-angle
reflections have been identified but a small pre-Mesozoic
basin has been identified around km 220 (Fig.7). In the
near-vertical reflection section, upper crustal reflections dip
in both directions below the small basin and the lower crust
reflective band (at
8-10
s
TWT)
continues from the
Sorgenfrei-Tornquist Zone below the Skurup Basin
although it thins southwestward and terminates at about km
160
(Figs 3 and 11). Below the termination, northeast-
dipping reflections are observed at 10 to 12.5
s
TWT
in the
upper mantle (Figs
11
and 3) and there is a Moho step in the
velocity model (Figs
11
and 7). These observations indicate
that the crust beneath the Skurup Basin
is
no longer
of
a
three-layer shield type. The different crustal structure may
be a result
of
Late Palaeozoic and Mesozoic extensional
tectonics and basin development. The pre-Mesozoic basin
around km220 might indicate the position
of
one
of
the
transtensional fault zones in the Carboniferous-Permian
Tornquist Fan north
of
the Ringkobing-Fyn and M@n highs.
Structure is varying along profile A at all crustal levels in
the near-vertical reflection section. In the velocity model,
the large lateral variability
of
the crystalline crust along
profile A
is
primarily in the lower crust whereas the upper
crust is more uniform. This suggests that tectonically
induced changes in crustal thickness primarily occurred at
lower crustal level in the southwestward transition zone
of
the Baltic Shield between km 350 and 80 along profile
A.
Future studies of the S-wave velocity field along the profile
will be valuable for understanding the importance
of
lower
crust composition.
The southwest-dipping and subhorizontal reflections
observed between
1
and
5s
TWT in the basement
of
the
M0n High and adjacent parts
of
the North German Basin,
may image the structure
of
overthrust Caledonian low-grade
metamorphics, in concurrence with the interpretation by
Vejbaek (1990)
of
a similar reflection seismic signature in
the pre-Permian basement
of
the Horn Graben. This
indicates that the Caledonian Deformation Front subcrops
on the M0n High at circa km 180 along profile A which is at
least
50
km further north than previously believed.
Information from deep boreholes in Schleswig-Holstein and
southern Jutland suggests that the Caledonian Deformation
Front
is
located south
of
the Ringkobing-Fyn High in this
area as also interpreted by Bialas, Flueh
&
Jokat (1990)
from EUGENO-S data along profile
1,
circa
100
km west
of
the BABEL profile. The deeper structure at the northern
edge of the M0n High also indicates a transition along
BABEL profile A. Hence, it is suggested that the
Caledonian Deformation Front trends along the southern
border
of
the Ringkobing-Fyn High eastward from the
North Sea, but then subcrops on the M0n High further east.
Although this may seem paradoxical, there is no
a
priori
reason that the Caledonian Deformation Front should not
intersect the Carboniferous to Permian basement highs
obliquely. At the southwest termination (km160)
of
the
band
of
subhorizontal reflections between
8
and 10
s
TWT
in
the near-vertical section, northeast-dipping reflections from
the upper mantle are observed at 10 to 12.5
s
TWT
(Figs
11
and 3) and there is a Moho step in the velocity model (Figs
11
and 7). This suggests that the Caledonian collision
between the Baltica craton and the Avalonian terranes led
to a flake-type suture in which the indenter has been imaged
by the mid-crustal band
of
reflections between
8
and
10s
TWT.
We thus hypothesize that Baltica has indented the
rheologically different crust
of
Avalonia, obducting its upper
crust and subducting its lower crust
(cf:
Fig.
13).
Lower crustal velocities
of
7.3-7.5 km
s-l
have been
interpreted below the M0n High and the North German
Basin. In EUGENO-S profile
2,
Flueh
&
Vieland (1989)
found a similar velocity structure in western Sweden and the
nearby part
of
Kattegat. They explained the observation by
crustal thickening due to stacking
of
Sveconorwegian
reworked crust over deep-lying Svecofennian crust. Based
on EUGENO-S and FENNOLORA data they limited this
velocity structure to the area north of the east-west
extension
of
the heda Shear Zone. Indications for similar
high velocities below the North German Lowlands have
previously been reported by Knothe
&
Walter (1971). Along

338
BABEL
Working
Group
DISlANCE,
UM
ssw
0
OUND
ASEDA
SHEAR
ZONE
100
COTUND
2M
MIGRA~ED
575014750
'
25
7825
sw
0
DISTANCE.
w
Cn
15
BABEL
LINEA~
UNMIGRATED
1650
SP
101
RFH
Mm
104
DISTANCE,
w
CDF?
200
BABEL
LINEA
"1
MIGRATED
I
240'
I
9680
MIM)
Figure-
11.
Near-vertical reflection sections along
BABEL
profiles
A
and
B
with main boundaries (velocity discontinuities) from velocity
models (Figs 5b and 7b). CDF-Caledonian Deformation Front, RFH-Ringkcibing-Fyn
or
Men High.
EUGENO-S profile
1,
thick three-layer crust and high
velocity lower crust have been interpreted below the
Ringkebing-Fyn High circa 100km west of the BABEL
profile (Thybo 1990) whereas further west, interpretations
of
EUGENO-S profile
5
have found velocities in the lower
crust no larger than
7.0
km
s-'
below the Ringkebing-Fyn
High circa 150-250 km west
of
the BABEL line (Thybo
et
al.
1990). The previous and present findings suggest different
composition of the basement high sampled by EUGENO-S
profile
5
from the basement highs sampled by EUGENO-S
profile
1
and BABEL profile A. Following the arguments of
Flueh
&
Vieland (1989) it could be speculated that the
western part, sampled by EUGENO-S profile
5,
may be of
Sveconorwegian origin and the eastern part, sampled by
EUGENO-S profile
1
and BABEL, may be characterized by
Sveconorwegian crust thickened by its thrusting over
Svecofennian or Gothian crust. Provided that the different
high-velocity lower crusts are all of Pre-Cambrian age and
origin, there is indication for up to
50
km right-lateral,
strike-slip displacement of the deep crust somewhere
between the Sorgenfrei-Tornquist Zone and the
Ringkobing-Fyn and MOn highs
(c$
Fig. 13).
The velocity model shows considerable crustal thinning
(from 38 to 30 km) south of the M0n High. This is in good
agreement with findings along the EUGEMI profile,
50
km
westward, where also strong topography on the Moho has
been interpreted (Trappe 1989; Thybo 1990). In the
near-vertical reflection section, the lower crust seems almost
void of reflections around the M0n High with no sign of a
shield-type crust as it appears along most of the BABEL

BABEL
profiles
A
and
B
339
JOTNIAN-VENDIAN BASIN
3M
NNE
400
431
Tornquisf
Zone
NE
BLEKINC-BORNHOLM
BLOCK
BOUNHOLU
SKURUP
BASIN
I
I
238-
t
"
249D
I
225-
I
I
SP
102
3~~9012890
26501
1650
1000
6650
h650
5L
profiles. However as discussed in an earlier paragraph, the
lack
of
reflectivity here is likely to be due to lack
of
signal
penetration.
CONCLUSIONS
Deep seismic reflection profiles collected
.
and analysed
simultaneously with wide-angle/refraction data has provided
much new information about the southwestern margin
of
the
Baltic Shield and its transition into the Caledonides
of
Germany and southern Denmark (Fig.
13).
At this very
early stage
of
interpretation we can already note several
significant seismological and geological/tectonic observa-
tions.
(1)
The crust
of
the Pre-Cambrian Baltic Shield is
reflective at all levels as also seen further north in the Gulf
of
Bothnia. The reflectivity shows much lateral variability
that can be correlated with surface structures in some places
but not in others.
(2) Correlations between near-vertical reflections, model
velocities and outcrops are difficult on the shield, but some
are tentatively attributed to deformations during the
Gothian and Sveconorwegian orogenies and to Jothnian-
Vendian rifting.
(3)
The Baltic Shield along profile B can be described by
three layers
of
velocity 6.1-6.4, circa 6.6, and 6.9-7.2 km
s-',
as it seems typical
of
shield areas globally.
(4) The crust-mantle boundary coincides with the
apparent termination
of
crustal reflectivity in the shield. The
near-vertical reflection seismic section imposes valuable
constraints to the complex velocity structure at middle to
lower crust and upper mantle levels along profile A where
the relatively large distances between land stations make

340
BABEL
Working
Group
Southwest
ESLBV
Northeast,
0
3
6
5
2
-
rn
c)
I
9
12
14
1
ion
160.
150.
I400
1300
1200
Geographical direction from station in degree
1
ioo
1010
Figure
12.
Record section from fan station
12
(Eslov), showing wide-angle reflections across the Sorgenfrei-Tornquist Zone. Data have been
NMO-corrected with correction velocity
6.73
km
s-’.
4.P
is wide-angle reflection from the top
of
the lower crust. Arrows show unconformity
on
P,P
wide-angle reflections
from
Moho. Horizontal axis is geographical direction as viewed from the station.
interpretation difficult even though high-density record
sections are available.
(5)
In the shield, depth to Moho varies gradually and
smoothly, reaching
48
km in the central part of profile B and
decreasing to 34km below the Hano Basin. There is no
indication for any major Moho step below the Aseda Shear
Zone. Moho undulates between
30
and 41 km depth along
profile A where thick crust underlies the Sorgenfrei-
Tornquist Zone with its surficial inversion structures and the
M@n basement high.
(6) The thin crust beneath the Hano and Skurup basins
can be explained by Carboniferous to Permian and
Mesozoic crustal attenuation and basin formation.
(7)
The large lateral variability in the velocity model
suggests that tectonically induced changes in crustal
thickness are primarily in the lower crust of the transition
zone along profile A.
(8)
Significantly, the Sorgenfrei-Tornquist Zone is a
tectonic zone within Baltica. Several periods
of
activity are
indicated, including Late Palaeozoic to Mesozoic transten-
sion and Late Cretaceous to Palaeocene inversion.
Interpreted from seismic reflections, the tectonic zone
widens with depth and the associated crustal thickening
displays some asymmetry which we interpret by ‘subversion’
in relation to the tectonic inversion and crustal shortening.
(9)
The northeastward termination
of
southwest-dipping
crustal reflections has been tentatively interpreted as the
Caledonian Deformation Front between the two areas of
thickened crust. Based on the seismic data, we hypothesize
that Baltica indented Avalonia, obducting its upper crust
and subducting its lower crust.
Less than three years after acquisition, the BABEL data
have yielded only a small part
of
their wealth
of
information. Here we hope to document their acquisition
and introduce their high quality and potential in order to
encourage their use by more geoscientists. More detailed
geotectonic interpretations and more comprehensive velo-
city models are our most immediate goals.
ACKNOWLEDGMENTS
The BABEL Working Group acknowledge the participation
in acquisition of the wide-angle data by: T. Albrechtsen, L.
D.
Christensen, L. Clausen, L. Dynesius, L. Dyrelius, A.
El-Badrashini, K. Gyldenholm,
E.
R. Hansen,
S.
Hansen, K.
Hedrich,
J.
Hepper, M. Ibs von Seth, D. Klaeschen, R.
Larsen, T. Larsen, P. Lindblom,
J.
P. Nielsen, N.
K.
Nielsen,
N. Ochmann,
2.
Ping, M. Spranger,
P.
Trinhammer, A.
Tuppurainen, T. Vangkilde,
J.
Vendelbo,
Q.
Wei and
J.
Wiister.
Much
of
the interpretation presented in this article was
derived during a one-week workshop held in October 1990.
We thank GEOMAR, Kiel for organizing the workshop. We
also thank A. Lassen and L. Hansen (Copenhagen),
P.
H.
Nielsen (Aarhus) and
T
Reston (GEOMAR) for participat-
ing and for their contributions to the interpretation. Many
valuable comments and suggestions from an anonymous
reviewer are acknowledged.
BABEL seismic reflection profiles were acquired and
processed by Prakla-Seismos AG, and are available at the
cost of reproduction from the British Geological Survey
(Marine Geophysics Programme Manager), West Mains
Road, Edinburgh EH9 3LA, UK. BABEL was funded by
the British Natural Environmental Research Council,
BIRPS’ Industrial Associates Programme, the Commission
of the European Communities, the Danish Natural Science
Research Council, the Deutsche Forschungsgemeinschaft,
the Finnish Academy and the Swedish Natural Science
Research Council.

BABEL
profiles
A
and
B
341
lkm
0
Figure
13.
Schematic block diagram
of
the crustal structures around the southwestern border
of
the Baltic Shield. The vertical sections in the
front are 50km deep and the vertical exaggeration is approximately
2:l.
The section to the left (km
0-150)
shows the seismic velocity model for
EUGENO-S profile
5,
which trends along the Ringkobing-Fyn High and crosses the Brande Graben (BG). The section to the right (km
0-435.5)
shows the structure along BABEL profile A from the integrated interpretation
of
the coincident seismic near-vertical reflection and
wide-angle data. From southwest to northeast, this profile intersects the North German Basin (NGB), the Trans-European Fault Zone (TEF),
the Caledonian Deformation Front (CDF) on the Men High, and the Sorgenfrei-Tornquist Zone
(STZ)
which is limited by the Skurup and
Hano basins. From km
350
to
435.5,
the profile cuts relict Gothian crust of the Bornholm-Bleking Block. In EUGENO-S profile
5,
crosses
denote Pre-Cambrian crust belonging to Baltica (closely spaced in the lowermost crust). The broken signature indicates a presumed
Pre-Cambrian shear zone below the Brande Graben. In BABEL profile A (km
0-150)
the ‘v’-shaped signature under the NGB indicates crust
of
presumed east-Avalonian origin. Caledonian low-grade metamorphics are shown by broken lines between
TEF
and CDF. Further to the
northeast, crosses denote Pre-Cambrian crust belonging to Baltica and the closely spaced crosses show the high-velocity body below the STZ.
Full lines show prominent reflections, and broken lines show reflective bands, subhorizontal in the lower crust and steeply northeast-dipping
throughout the crust in the Bornholm-Bleking Block. In both profiles, the upper mantle is vertically ruled. Broken lines in BABEL profile A
(km
150-250)
indicate anomalous upper mantle
of
velocity
7.8
km
s-’.
On map surface, SG and OG denote the Skagerrak and
Oslo
grabens,
both
of
Carboniferous to Permian origin, and SNF denotes the Sveconorwegian Front.

342
BABEL
Working
Group
REFERENCES
Ahlberg, P., 1986. Den svenska kontinentalsocklens berggrund,
Suer. Geol. Unders. Rap., Medd.,
47, 1-101.
Aichroth, B., Prodehl, C.
&
Thybo, H., 1992. Crustal structure
along the central segment of the EGT,
Tectonophysics
207,
Baartman, J. C.
&
Christensen,
0.
B., 1975. Contributions to the
interpretation
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