Crustal structure of the Fennoscandian Shield: A traveltime interpretation of the long-range FENNOLORA seismic refraction profile

Tectonophysrcs. 195 (1991) 105-137 Elsevier

Science Publishers

105

B.V.. Amsterdam

Crustal structure of the Fennoscandian Shield: A traveltime interpretation of the long-range FENNOLORA seismic refraction profile B. Guggisberg a-*, W. Kaminski b and C. Prodehl b a Instrtutfir Geophysrk, ETH-Ziirrch, CH-8093 Ziirrch, Swrtrerland ’ Geophysrkalrsches Instrtut. Unruersrtiit Karlsruhe, Hertzstraw (Received

August

16. D-W-7500

18, 1989; revised version accepted January December 10.1990)

european

geotraverse

Karlsruhe 21 FRG

10. 1990; received by publisher

ABSTRACT Guggtsberg, B., Kaminski, W. and Prodehl, C., 1991. Crustal structure of the Fennoscandian Shield: A traveltime interpretation of the long-range FENNOLORA seismic refraction profile. In: R. Freeman. M. Huch and St. Mueller (Editors), The European Geotraverse, Part 7. Tectonoph.vsrcs, 195: 105-137. The 1979 Fennoscandian Long-Range ProJect (FENNOLORA) was aimed at the determmation of the detailed structure m the earth’s mantle down to a depth of about 400 km. Observation distances reached almost 2000 km within Scandinavia between shotpoints off the North Cape and the southern coast of Sweden. To achieve an unbiased regarding the upper mantle structure, a careful crustal survey was carried out along the entire profile at the same time. Beneath the Fennoscandian Shield, i.e. the central section of the profile, the crust is characterized by quite a smooth increase in P-wave velocity down to the Moho which lies at a depth of about 50 km in the southern half of the shield and at about 45 km further north. The mean crustal velocity is 6.6-6.7 km/s. At the base of the crust the velocity mcreases gradually from about 7 km/s to 8.0-8.4 km/s m a 5-10 km thick crust-mantle transition zone. Both in the south and m the north, the relatively homogeneous crust of the Baltic Shield borders on areas with a more differentiated velocity structure. First-order discontmuities at shallower depth characterize the crust-mantle boundary near the southeastern tip of Sweden (38 km) and under the Caledonides m the north (43 km).

Introduction In August

1987). To control the data on lateral inhomogeneities in the crust and lower lithosphere, a series of 1979 a seismic refraction

survey was

intermediate shotpoints average spacing of about

carried out along a 1900 km line through Scandinavia in order to investigate the structure of the lithosphere and asthenosphere Baltic Shield. Large explosions at Karlskrona in the south (B) and Cape in the north (H and I, Figs. recorded along the whole line

G). Recording

beneath the shotpoints near near the North 1 and 2) were and served to

Meilen,

address:

SIMULTEC

AG,

Burgrain

37. CH-8706

Switzerland

0040-1951/91/$03.50

(‘1 1991 - Elsevier Science

Pubhshers

were positioned

at inter-

vals of about 3 km for distances up to 1000 km and at 12 km intervals for the large shots recorded at distances beyond 1000 km. In addition to the line through Scandinavia from shotpoints B to H, the shots at G were recorded along additional lines in Finland and the major shots at B and I were recorded along two lines south of the Baltic Sea, one running towards the southeast through Poland into the Ukraine and one running along the East German-Polish border into Czechoslovakia. The additional shotpoints W and BW in East Germany were also

penetrate the earth’s mantle to depths of approximately 400 km (Fuchs and Vinnik, 1982; Fuchs et al., 1987; Guggisberg and Berthelsen,

’ Present

stations

was arranged with an 300 km (C, D, E, F and

B.V.

Ilk ____.___

.~~____.

._____._.. __--In addition to P-wave data, high-quality S-wave data were also recorded. Kullinger and Lund (1986)

and

Stangl

record sectrons preliminary

interpretations

wave models. data

data km)

both and

(> 250 km,

show that

which

prepared

for upper pers.

P-

the S-wave

is comparable

for crustal

Stangl,

interpretation

have

based on published

The sections

are of a quality

P-wave O-250

et al. (1989)

of S-wave data and have presented

(distance

to the ranges

of

mantle

investigations

commun.,

1989).

of the S-waves,

however,

The

is beyond

the scope of this paper, which will this concentrate on the interpretation

of P-waves

only.

All record sections have been similarly plotted after application of a 1.3 Hz high-pass filter and a 33 Hz low-pass filter. In spite of the similar treatment some of the plotted data show predominantly

low frequencies

points

E and

(Figs.

I), others

lo-11

and 17, shot-

a predominantly

high-

frequency content (Figs. 12-15. shotpoints F and G). Shotpoints E and I were sea shots with charges

-3O

3”

9’

15’

21’

‘7’

33’

70’

-

FENNOLORA

Fig. 1. MaJor tectonic lmes = FENNOLORA

1979 features

of Northern

Europe.

Heavy

1979; lighter lmes = earher crustal and

upper mantle surveys m Scandinavia.

successfully recorded in the southern part of Sweden, starting at recording distances of 140 and 420 km respectively (Fig. 4). To extend the main line running

along the east

coast of Sweden into the Baltic Sea south of B, the southernmost part of the line was shifted to the west to the Schonen peninsula, thus (with two stations on the island of Bomholm) bridging the Baltic Sea as far as possible. By using this scheme, multifold coverage of the main line between W and I was obtained for the structure of the crust and lower lithosphere. Details of the experiment, data preparation and some highlights of first results have been described by Guggisberg (1986).

9” Fig. 2. Map of Scandinavia

15” showing posltion

21” of shotpoints

individual stations of the FENNOLORA

tine.

and

CRUSTAL

detonated

STRUCTURE

OF THE FENfVOSC,ANfXAN

at particularly

SHfELD

great depths

TR4VELTiME

(SO m at E

sea shotpoints

(B, C, D and H) was between the two shotpoints

20

The Fennoscandian

F and G

were Iocated on land in shallow lakes at about

the Fennosarmatian

350

warping,

Oslo graben and Khrbrny cntruscons

I t

Phanerozofc Rrphean Jotnion

platform 570

Caledonides

-

350

Ma

<

570

Ma

- 1600 ~1300

LOO -

600

Ma Ma MO

rock

1x1

Dalslondian

mj

Cothron

i’“i

Svecofenno -Karetioh folded region

pz--q

Archoeon Ftg. 3. Tk

or Baltic

folded

region

complex

folded

main geologrcai

(mc~niy

gfonitesl

region

units of Fennoscandra

Shield

palaeocontinent.

it rises from the Palaeozoic

2%

platform plctform

Coledonized frecambrtan

107

m above sea level (F at a water depth of 16-17 and C at a water depth af 4-5 m).

and 260 m at I}, while the water depth at the other and 45 m. In contrast,

INTERPRFTATION

800

- 1200

Ma

1200

- 1750

Mo

1750

- 2600

Mo

22600

(Kahma. 1978).

Ma

m

is part of

Slightly

up-

sediments

on

10x

the

East

European

platform

in

(SchBnenberg,

1971) and is flanked

gle overthrust

nappes

the northwest. exactly

Barents our

line. Its northern and

Sea (Calcagnile is

1980), its geological

all authors

the

by

is

in the

1978). Because of the

Fenno-

(Oftedahl,

has not been based

the study of tectonic

(Polkanow

to

by the

boundary

speculative

subdivision

and Gerling,

but on

The main geological units of the Fennnscandian Shield (Fig. 3) are the Belomorides in the northeast,

the Svecofennides

five units fall into four age classes (Kahma, the

Belomorides

Svecofennides

1961).

Using

continental

2600

range

complex

1978):

m.y.,

from

ranges

the

1750 to

from 1200

folded

rock type, the Svecofennides

little

size

in

region

from 800 to 1200 m.y.

towards

in

than

to 1750 m.y., and the Dalslandian ranges

has

of younger

older

2600 my. the Gothian

ther subdivided

enlarged

are

and Karelides

nucleus

accretion

and the Karelides

the central area, and the Gothian and the Dalslandian in the southwest (Kahma, 1978). These

agree that an Archaean

northeast

southwest

and Panza, very

through

age determinations in

orogen

lies somewhere

of the evolution

Shield

on reasoning Today

of the Caledonian

defined

knowledge

scandian

southeast

In the south it is terminated

Tornquist-Teisseyre not

the

by the low-an-

mica),

volcanites),

into Svionian Bottnian

and

the

can be fur-

(light gneisses

(greywackes younger

and

intrusions

with basic

of the

areas. Still uncertain is whether these events conform to a plate tectonic evolutionary scheme as

Rapakivi granites (Hietanen, 1975). The FENNOLORA seismic refraction

the theory is understood today (Magnusson et al., 1962; Hietanen, 1975; Oftedahl, 1980; Berthelsen, 1984; Wilson, 1984).

starts in the south at the Wolgast (W) shotpoint. crosses the Baltic Sea and the Tomquist-Teisseyre line, runs through the area of Gothian granites in

line

DISTANCE IN KM Fig. 4. Trace-normalized W = shotpoint; = 6 km/s.

record

section

of vertical-component

recordings

(Z)

of profile

W-N

N = profile direction towards north; reduced traveltime = traveltime-distance/reduction

The records are plotted using a band-pass two-dimensional

filter from 2.0 to 20 Hz. Superimposed

velocity-depth

(crustal

part).

fiplanattons:

velocity; reduction velocity

are the traveltime curves calcuhued

structures shown in Figs. 20-24.

from

CRUSTAL

STRUCTURE

OF THE

FENNOSCANDIAN

SHIELD

TRAVELTIME

INTERPRE?ATION

.

DISTANCE

IN

KM

Fig. 5. Record section of profile B-N (crustal part). For explanation, see Fig. 4.

southern traverses

Sweden between the Svecofennian

shotpoints B and C, part of the Fenno-

This paper describes an interpretation of the crustal profiles based on the crustal part of the record sections (Figs. 4-17). i.e. up to recording distances of 250-300 km. The final two-dimensional velocity-depth models shown in Figs. 19-24 fit the traveltime data superimposed on the record

scandian Shield between C and F, and passes into the Archaean province of the Belomorides near F. The northernmost halfway between

part of the line, G and H, is

from about located on

Caledonian rocks, which are considered as an overthrust of Caledonian nappes onto the underly-

sections includes

ing Fennoscandian

W (Fig 4) south of the Tornquist-Telsseyre

Shield.

DISTANCE

IN

in Figs. 4-17. The interpretation also the crustal data available from shotpoint

KM

Fig. 6. Record sectlon of profile C-S (crustal part). For explanation, see Fig. 4.

line.

/I /

Fig. 7. Record section of profile C-N

CorreIationand interpretation

(crustal part). For explanation,

of plkases

see Fig. 4.

range of less than 50 km. In profiles W-N, E-S and E-N, the corresponding velocities reach values of between 6.1 and 6.4 km/s at recording distances between 50 and 150 km. On profiles W-N, B-N, C-S, C-N, E-S, E-N G-S and G-N (Figs. 4-7, 10-11, and 14-15) the

With the exception of profiles W-N, E-S and E-N (Figs. 4, 10, ll), the data indicate the complete absence of sediments showing negative reduced traveltimes for the Pp phase in the distance

P 92

DISTANCE

IN

KM

Fil 3. 8. Record section of profile D-S (crustal part). For explanation,

see Fig. 4.

CRUSTAL

STRUCTURE

OF THE

FENNOSCANDIAN

SHIELD

TRAVELTIME

DISTANCE

IN

INTERPRETATION

KM

Fig. 9. Record section of profile D-N (crustal part). For explanation. see Fg. 4.

first-arrival data with continuously

(Fig. 7) and E-N (Fig. 11)). Regarding the other profiles, the energy of the first arrival data fades out at 60-70 km on profiles D-N (Fig. 9) and F-S (Fig. 12) and at 120-130 km on profiles D-S, D-N and F-N (Figs. 8, 9 and 13). First arrivals

align on the Pp traveltime curve increasing velocity up to dis-

tances beyond 150 km, possibly including a cusp at about 100 km indicating a more sudden increase

in velocity

at this range (see profiles

C-N

Y i

.

.

-

: DISTANCE

IN

KM

Fig. 10. Record sectlon of profile E-S (crustal part). For explanation, see Fig. 4

I

,,n

I5”

nlSrANCE

IN

KY

Fig. 11. Record sectton of profile E-N (crustal part). For explanation,

beyond this distance are delayed with respect to a hypothetical continuation of the Ps traveltime curve. Beyond 200 km, the clear first arrival data align on traveltime curves from which P, velocities of

DISTANCE

see Fig. 4.

between 8.0 and 8.2 km/s are derived. Only in a few cases are velocities as low as 7.9 or as high as 8.4 km/s observed. We interpret these as apparent velocities. On record sections plotted with a reduction velocity of 8 km/s (shown in Guggisberg,

IN

KM

Fig. 12. Record section of profile F-S (crustal part). For explanation,

see Fig. 4.

CRUSTAL

STRUCTURE

OF THE

FENNOSCANDIAN

SHIELD

TRAVELTIME

DISTANCE

INTERPRETATION

IN KM

Fg. 13. Record section of profile F-N (crustal part). For explanation, see Fig 4.

1986. but not in this paper),

these P,, arrivals

can

sphere as shown by Guggisberg et al. (1984) and Guggisberg (1986). Between 100 and 250 km on all profiles the bulk of the energy lies in secondary arrivals, which

generally be correlated up to distances of about 500 km. Further in the record section, phases with delayed indicating

traveltime the fine

but with higher energy structure

appear,

of the lower

litho-

DISTANCE

are on the whole

clear, correlated

IN KM

Fig. 14. Record section of profile G-S (crustal part). For explanation, see Fg. 4.

by two travel-

3;SIAIK’E

IN Kf?

Fig. 15. Record section of profile G-N (crustal part). For explanation, see Fig. 4

time curves and interpreted

as reflections

from

zones of increased velocity gradient at a midcrustal level and the crust-mantle boundary.

reflections are clearly expressed, they are weak on the profiles from C, and appear quite strongly again at D. Due to the low-frequency character of

The character of these phases changes on passing from south to north. At W-N and B-N both

the record sections from shotpoints E and I (Figs. 10, 11 and 17) the second and later reflection

DISTANCE

IN

KM

Fig. 16. Record section of profile H-S (crustal part). For explanation, see Rg. 4.

CRUSTAL

STRUCTURE

OF THE

FENNOSCANDIAN

SHIELD

TRAVELTIME

115

INTERPRETATION

150

DISTANCE

IN

KM

Fig. 17 Record section of profile I-S (crustal part). For explanation. see Fig 4

cannot record

easily be separated from the first. The sections of F-S show them to be clearly and

It should be noted that in both cases a major tectonic boundary is crossed. In the south, the crust-related part of profile B-N (Fig. 5) is mainly

G-S clearly show the first reflection, but less clearly the second one, while on G-N and H-N

located in the Gothian province and enters the Svecofennian at about 200 km. In the north, pro-

both reflections

file G-N (Fig. 15) enters the Caledonian 180 km north of shotpoint G.

differentiated;

the

record

sections

can again be traced

of F-N

separately.

The character of the phases just described suddenly changes on passing from B-N to C-S (Figs. 5 and 6). as well as from G-N to H-S (Figs. 15 and 16), in spite of the fact that the corresponding

A crustal

profiles plained

The correlation of the phases sections (Figs. 4-17) matches

reverse each other, This can only be exby assuming that here a major structural

about

model of FENNOLORA seen in the record the final derived

change occurs. Between B and C, the line crosses the boundary between Gothian granites and the

modkl in Figs. 19-24. The first step, however, towards the final model was to calculate one-di-

Svecofennian

mensional velocity-depth functions for each individual profile (Fig. 18, central part) assuming as a

province,

between

G and H the line

crosses from the Archaean province of the Fennoscandian Shield into the Caledonian area. This change

in character

of the record sections

is again

clearly established in the velocity-depth functions and the corresponding part of the crustal cross section low.

(see Fig. 18). which will be discussed

be-

On some record sections beyond 200 km, a high-frequency phase is visible that seems to be an apparent continuation of a phase refracted in the middle crust (Figs. 5 and 15), but which, because of its large amplitude cannot be explained as such.

first approximation that the crustal structure would not change in the horizontal direction along the line. Traveltime curves recalculated from these velocity-depth functions already fit the phases observed in the record sections quite well. From these, a first-approximation crustal CIOSS section has been constructed with lines of equal velocity (Fig. 18, upper part). Finally, in a second cross section (Fig. 18, lower part), the depth-distance ranges were plotted from which the corresponding reflections recognized in the observed data are

BALTIC SEA GOTHIAN

SSW

Fig. 18. Crustal cross section exaggerated

BALTIC

by 4: 1. Velocity

SHIELD

,CALEDO-,BARENTSINIDES * SEA

SVECOFENNIAN

through inversions

the Baltic Sea and the Fennoscandian are indicated by stipphng.

Shield of Scandinavra.

Intersection

Upper part: Cross section showing hnes of equal velocity at intervals of 0.2 km/s. for each individual

profile. Functions

for profiles observed

Mdile

are plotted from left to right for profiles observed

from north to south.

Lower pert: Cross section showing

dtstance range at which the respectrve phase is recorded.

indtcate the direction along the profile in which the corresponding

actually returned. This first-approximation

model

g&berg (1986) used this first-approbation model to calculate a two-dimensional model of the crust along the FENNOLORA line applying a ray-tracing method (Cerveny and Psencik, 1984) based on ray theory (Cerveny et al., 1977). As the shotpoints average a separation of 300 km information on the crustal structure for each section is restricted to the data obtained from adjacent shotpoints. This can be clearly seen on the lower section of Fig. 18, and in particular in

versus drstance

is

part: Velocity-depth

functions

caldated

from south to north, and from right to left

main reflectors half way between

Vertical shading = tr~sit~~

has previously been discussed in detail (Prodehl and Kaminski, 1984). With slight changes concerning some details in the correlation of phases and the subsequently derived velocity-depth functions of Fig. 18, Gug-

Depth

with the Blue Road profile of 1972 is also shown.

zones with strong velocity

velocity (in km/s)

shotpoint

and the

gradient. AKOWS

is observed.

Figs. 19-24 which show the detailed ray-tracing models from shotpoint to shotpoint. As can also be seen from these figures, the large separation of shotpoints restricted reversed observations to only those phases which penetrate to depths greater than 15-20 km. For the upper crust, the only data available are practically all unreversed. Therefore, for modelhng the upper crust additional information from tectonic and geologioal maps (e.g. Magnusson et al., 1962; Holtedahl. and Dans, 1960) was used to check lateral changes in the structures. While the first-approximation model (Fig. 18) of Prodeh.l and Kaminski (19&4) may give the impression that the general outline of the crustal structure under the FENNQLCRA line is reiatively homogeneous, the detailed ray-tracing mod-

CRUSTAL

STRUCTURE

OF THE FENNOSCANDIAN

SHtEtU:

TRAVELTIME

lNTERPRE,TATION

r---l----7---T’-

-I-----

STRUCTURE

:

CRUSTAL

STRUCTURE

OF THE FENNOSCANDIAN

SHIELD

TRAVELTIME

INTERPRETATION

121

I

CRUSTAL

STRUCTURE

OF THE FENNOSCANDIAN

SHIELD

1 ,

\

TRAVFLTIME

INTERPRETATION

173

ela for the individual that

significant

sections

lateral

line, and these cannot cross

section

(Fig.

(Figs.

changes

19-24)

occur

be ignored.

show

along

the

The final crustal

25) has been

compiled

from

differmg velocities and depths of Interlaces and layers may occur at distances of 60 km or more north and south of a shotpoint. Figure individual

Figs. 19-24 and reflects two important characteristics: the general characteristic of the crust is a

neous

clear separation

the North

into

an upper

and a lower part

along the 2000 km long line. whereas velocities

in the

near-surface transition From Prodehl main

crust

and

in detail the

in particular

in the

zone show substantial

features

Kaminski

model

(1984)

of the crustal

(Fig.

characterized

structure

beneath tance

from the Balttc Sea to

Cape. The dashed the shotpoints,

ranges

where

be resolved

mark

lines, especially the depth

uncertainties

those

and dis-

remain

which

with the data that are presently

18), the

Upper crust

as follows:

(1) Except under its southernmost crustal structure of the Fennoscandian

part, the Shield is

The more refined ray-tracing model shows that the upper crust along the FENNOLORA line is in general quite homogeneous. Its thickness averages 20 km and the velocity increases with increasing

relatively homogeneous. (2) Throughout the whole length of the FENNOLORA line a clear separation into upper and lower crust can be defined.

depth detail

(3) The average velocities crust are relatively high.

particular basin-like

in upper

and lower

(4) The upper crust is about 20 km thick. (5) The whole crust beneath the Fennoscandian Shield, except underneath most part, has an average thickness

cross section

to compile the into a homoge-

available.

vanations.

the first-approximation and

crustal

cannot

layers as well as at the crust-mantle

25 shows the attempt sections of Figs. 19-24

its southernof 45-50 km.

(6) The crust-mantle boundary beneath the Fennoscandian Shield is a 5-10 km thick transition zone.

from about 6.0 to 6.4 km/s. there are considerable lateral in the uppermost areas with decreased

However, variations,

in in

5-10 km, where velocity (5.7 km/s)

are found; in addition, there are also areas with relatively high (2 6.0 km/s) velocities near the surface. Due to the fact that no data are available between shotpoint W and the southernmost recording stations on the Schonen peninsula, i.e. between

0 and 140 km from W, details

of upper

(7) The crustal structure changes on crossing from the Fennoscandian Shield into its southern-

crustal structure and the Baltic Sea cannot be revealed, and it was necessary to assume an aver-

most province the Caledonian

age velocity for this part of the crust (Fig. 19). To the north of shotpoint B, be the Gothian complex a velocity structure similar to that under the Baltic Sea is derived from the data, the upper crust gradually increasing in thickness between B and C from about 21 to 29 km (Fig. 19).

and the Baltic orogen.

Sea as well as into

The detailed inversion of the seismic-refraction data into a velocity-depth model using the raytracing method of the earth’s

has revealed quite a detailed model crust beneath the Fennoscandian

As mentioned above and illustrated in the lower part of Fig. 18, information on crustal structure

Between C and D a sequence of alternating low- and high-velocity layers characterizes the upper part of the upper crust (Fig. 20). Beneath a thin (2 km) layer of 6.0-6.05 km/s a velocity inversion gradually develops, with velocity decreasing northward from 6.0 to 5.9 km/s at the

for each section is restricted to the data from the two nearest shotpoints only. Consequently, in the ray-tracing models of Figs. 29-24 information from the area around and below the shotpoints is not available. It is therefore not surprising that

surface south of D. Beneath a high-velocity layer occurs, with velocity varying laterally from 6.2 km/s in the south to 6.1 km/s near D; this layer cuts off the overlying low-velocity layer and occurs at the surface slightly north of D (Fig. 21).

Shield of Sweden and its adjacent regions to the south (Baltic Sea) and to the north (Norwegian coastal area). which will be discussed in the following sections.

CRUSTAL

STRUCTURE

OF THE

This layer is underlain with a velocity

SHIELD

TRAVELTIME

by a second inversion

near 6.0 km/s,

ally rises towards surface

FENNOSCANDIAN

the north

half way between

zone

and

appears

at the under-

neath

this layer can a more or less uniform

upper

crust

be traced

with

creasing velocity from near 6.2 km/s km/s (Figs. 20 and 21). Further

north,

from about

22), an 8 km thick low-velocity with an average the normal

upper

velocity crust,

gradually to about

upper crustal km/s

with increasing

km/s the

under

the Gothian

complex

layer

recognized in the one-dimensional model of Prodehl and Kaminski (1984) as is seen in the upper and lower part of Fig. 18. The upper crust under the Archaean province from south of F to the north (Fig. 23) quite uniform. with a uniform velocity increase from 6.0 km/s near the surface to 6.3-6.4 km/s at the top of the lower crust at a depth of 17-19 km. About half way between F and G another low-velocity near-surface layer occurs (mean in the lower

along

velocities

the

increasing

depth from 6.5 -6.8 km/s

are

overlies

uniform

with

Exceptions

which in this area has a

In addition,

gradually

is quite

line,

near 20 km to 6.9-7.3

This complexity in uppermost crustal structure between shotpoints C and F has already been

km/s).

crust

crust.

18-20 km. The upper layer with a velocity of less than 6.0 km/s terminates abruptly south of F.

5.7-5.8

lower

in-

E to south of F (Fig.

of 5.8-5.9

The

FENNOLORA

6.4

slightly reduced mean velocity, gradually rising from 6.1-6.2 km/s at 8 km to 6.3-6.4 km/s at

velocity

Lower crust

which also gradu-

D and E. Only

continuously

125

INTERPRETATION

near the base of the southernmost

section

and the adjacent

Bal-

tic Sea as well as some areas where the crust-mantle boundary

deepens

considerably

by a thick transition 7.5 km/s

and is overlain

zone where velocities

of 7.4-

are reached.

In general, continuous

the lower crust

velocity

At its upper from 6.3-6.4 Fennoscandian

jumps

boundary

is confined

by dis-

at its top and bottom.

minor

velocity

km/s to 6.6-6.7 km/s Shield and the Baltic

steps occur within the Sea to the

south. Only under the Caledonian orogen and the adjacent part of the shield area is there a considerable velocity jump from lower low-velocity layer 6.6-6.7 km/s at a depth The velocity jump at stantial. In areas with a

about 6.1-6.2 km/s (the as discussed above) to of 19-21 km. the Moho is more submore flat or only slightly

dipping crust-mantle boundary, north of shotpoint E the velocity jumps from about 6.9 km/s to 8.0-8.1 km/ south of E the velocity at the bottom of the lower crust is higher and the velocity step is from 7.2-7.3 km/s to 8.0-8.1 km/s.

part of the upper crust, an additional low-velocity zone is seen which evidently marks the beginning

Exceptional are the areas with thickened crust. Here the lowermost part of the lower crust be-

of a very complicated north of G (Fig. 24).

haves as a wide crust-mantle transition zone: the velocity increases gradually with depth to as much

As already

indicated

upper crust structure

to the

in the first-approximation

model of Prodehl and Kaminski (1984) the upper crust south of H (Fig. 24) is characterized by a crust of mainly low velocity (6.1-6.2 km/s) reaching to the top of the lower crust at a depth of 20-22 km. The upper crust in this area bears thin, intercalated high-velocity layers with velocities of 6.25 km/s at depths of about 4-8 km and 6.4 km/s at depths of lo-12 km. The geological boundary between the Archaean province of the Fennoscandian Shield and the Caledonian orogen is located about half way between G and H, and it seems that the Caledonian orogeny has influenced the upper crustal structure of the northernmost area of the Fennoscandian Shield.

as 7.4-7.5

km/s

and only then, on average 10 km mean Moho level, a veloc-

below the neighbouring

ity jump to 8.1-8.2 km/s occurs. Under the Gothian complex and the Baltic Sea at the bottom of the lower crust a low-velocity layer is encountered, resulting in a major velocity step from 6.56.6 km/s to 8.0 km/s. In contrast to the shield area where the lower crust has a thickness of 20 km or more, under the Caledonian area of the Baltic Sea between Sweden and Germany it is only 11 km thick and seems to be more or less uniform with a velocity of about 6.6 km/s (Fig. 19). This is the area where the the Tornquist-Teisseyre line traverses the Baltic Sea and forms the border zone between the Fenno-

scandian Shield and the Caledonian area underly-

Discussion

ing Denmark and northern Germany. To the north, under the adjacent Gothian complex in southern

Other geophysical

results

Sweden, the lower crust thickens gradually to as much as 16 km and is double-layered:

a 6.8-7.0

The

following

geophysical

parameters,

have

km/s layer overlies a layer with a decreased veloc-

been obtained for the area of the Fennoscandian

ity of about 6.5 km/s, which may terminate at the

Shield:

northern end of the Gothian complex (Fig. 19) as

regions

has already been seen in the first-approximation model (Fig. 18). Here, a sudden change in crustal structure oc-

in general

which are typical

for shield

(1) The traveltime residuals for teleseismic

P-

waves are negative (Herrin and Taggart, 1968). (2) Crustal thickness is more than 40 km (Vogel,

curs. The crust deepens abruptly from 40 to more

1971; Lund, 1979).

than 50 km, with velocities as high as 7.45 km/s at its bottom. The maximum depth of the Moho is

(3) The Bouguer gravity anomalies are negative (Balling, 1980). (4) Heat flow is lower than 0.04 J/m2 s

reached south of C, at 55 km. Such a sudden change was also a major feature in the first model

(Cermak and Rybach, 1979).

(Fig. 18). To the north, crustal thickness first decreases rapidly, but then gradually, to less than

(5) Seismicity al., 1984).

50 km at C, and reaches a minimum thickness of about 42 km south of shotpoint D (Fig. 20).

(6) There tions.

Another sudden increase in lower crust thickness is seen 80 km north of D (Fig. 21). Here the

In addition to the FENNOLORA project of 1979, other major explosion seismic projects in the

upper crust thins to less than 20 km and the crust-mantle boundary deepens abruptly from 47

area of the Fennoscandian Shield (Fig. 1) include the Trans-Scandinavian Seismic Profile (TSSP)

to 56 km, the velocity at the bottom of the lower crust reaching 7.5 km/s. At shotpoint E (Fig. 21) the crust returns to its average thickness of less than 50 km and to the north its thickness gradually decreases to 41 km north of F (Figs. 22 and 23).

shot in 1969 (Vogel, 1971), the 1972 BIue Road Project (Hirschleber et al., 1975), SVEKA 1981 (Luosto et al., 1984), BALTIC 1982 (Luosto, 198), the EUGENO-South Project of 1984 (Fhih and Berthelsen, 1986) and the POLAR Profile of 1985 (Luosto et al., 1989). Cassell and Fuchs (1979), as preparation for

About 150 km north of F the crust thickens a third time, from 41 to 49 km, the velocity at the Moho reaching 7.3 km/s. At shotpoint G the crustal thickness returns to slightly more than 40 km (Fig. 23). In contrast to the upper crust, the lower crust presents almost no change on passing from the Fennoscandian Shield into the Caledonian orogen. A minor increase in depth to the Moho of l-2 km is seen south of shotpoint H, from where the crust thins to 40 km to the north under the adjacent Barents Sea (Fig. 24). The internal velocity structure does not show any change when the line crosses this major geological boundary.

is low (Ahjos,

1984;

Slunga et

are only minor topographic

the planning of FENNOLORA,

varia-

reviewed the ex-

isting crustal models in detail and calculated

a

theoretical model of crust and upper mantle to a depth of about 600 km in order to predict the possible range of observation distances for various phases reflected from the interfaces within this depth range. The FENNOLORA line is crossed by three lines of the previous surveys (see Fig. 1 and 8). In the south it is crossed by Line 4-5 of the TSSP Profile south of Oskarshamn which was interpreted by Gregersen (1971) and revealed the same total crustal thickness of 37 km as was obtained

CRUSTAL

STRUCTURE

for B-N. profile

OF THE

Shotpoint

FENNOSCANDIAN

Berthelsen.

1986;

west, and the resulting similar to that along through

the Gothian

Further

north,

(1971)

Line

near

(Fhih

the FENNOLORA line, crustal thickness does not exceed 50 km. Beneath the Finnish part of the

and

Fennoscandian

the north-

reported

It is remarkable noscandian

Shield

granites

generally

area

2-3 of TSSP

north crosses

E. Vogel

a crustal

thickness

line is crossed

the south

major

velocity

Lund

neath

the FENNOLORA

of 45 km,

profiles

the

and

by the Blue Road

for the crust-mantle the existence

east

of

shotpoint

B (Luosto

with the exception

FENNOLORA

et al.,

1980).

FENNOSCANDIA

ally with depth

area,

Vp

In

Fig. 26. Representative shotpomt

C-North;

72, shotpoint

velocity-depth -

5-West (Lund,

1985). (c) -

Russian

Vp

functions

FENNOLORA 1979); platform

provmce,

shotpoint

Churchill

province

3-East (Berry and Fuchs,

(Mooney

et al., 1985). (f) - .-

(Green

North

E-South;

Australian

-t

the Archaean

:

craton

6.4 km/s

differences

are

of the upper

crust

and the Svecofennian

part

cration

TCMI-2

AFRICAlARABlA

1982).

Vp

shield areas of the earth.

platform

province, -

(Puzyrev

shopoint

G-South.

2-West (Berry

Australian

craton

(b) -.

1971) (d) -.

and Fuchs, TCMI-3

79,

- Blue Road

82, block III (Luosto

and Krylov.

III (Baier, et al., 1983); - .- Arabian North

IKn/sl

(a) - - - - - - FENUOLORA

79, shotpoint

et al., 1984); - - - - - - BALTIC

Profile

East;

AUSTRALIA

a

- FENNOLORA

- West Sibnian Grenville

Kalahari

similar

structure

[Km/s1

81, block III (Luosto 1979); -.

1973); -

et al., 1980). (e) -

6

Vp

of the crust of different

SVEKA (Pavlenkova,

in Finland

CANADA

[Km/s1

79, shotpoint

of around

of the SVEKA Profile (Luosto et al., 1’984, fig. 5) and on the BALTIC Profile (Luosto el al., 1985,

along

SOVIET UNION

(Km/s)

and velocities

because

between

5

(Km/s1

et al., 1984, 1985: 26). The Archaean

reached at depths of 15-20 km. This seems to be typical of the whole shield

seen in the velocity

EL-Vp

beother

et al., 1980; Prodehl

Luosto

et al., 1985) (Fig.

are only behaviour

et al. south-

of some points

neither

line nor beneath

upper crust north of shotpoint F the velocity increases quite abruptly to values around 6.4 km/s at shallow depths of between 4 and 6 km. Under the Svecofennian part the velocity increases gradu-

C. The mean crustal thickness of 45 km north of shotpoint E as found by the seismic refraction

Sweden,

zones. Also.

are found

1979; Luosto 1984;

found.

and the Svecofennian parts of the shield typically differ in terms of crustal velocities. In the Archaean

beneath

surveys have been confirmed by Bungum (1980) and is also seen in Finnish Lapland

(Lund,

of the Fen-

are not

thick transition

inversions

Kaminski,

Davydova

boundary,

of a trough

that in the crust

but instead

and

et al. (1984, 1985)

discontinuities

of B.

Bungum et al. (1980) and Brown et al. (1971) found a thin crust near B and between C and D. Brown et al. (1971) calculate a dip of 7.2’ towards

Luosto

of near 55 km.

FEN-

Profile near Lycksele (Hirschleber et al., 1975; Lund, 1979) half way between shotpoints E and F. Lund’s result of 46 km agrees well with our model.

which indicates

Shield,

Moho depths

crustal structure was quite the FENNOLORA Line

which is seen in our model north of E. Finally FENNOLORA

127

INTERPRETATION

Working

from B towards

shotpoint

interpreted

Project

EUGENO-South

1988) leading

NOLORA

TRAVELTIME

B was again used in 1984 for

IV of the EUGENO-S

Group,

SHIELD

et al.,

- Superior

1973); - - - - - -

Shield shotpoint West (Finlayson,

3

17x

fig. 9). Similar behaviour of velocity is also seen at

I-ABLE

greater depths. At the boundary

Characteristic

from upper to

1 velocities

and

velocities

gradients

for

lower crust at about 20 km in the older province, velocities higher than 6.6 km/s are seen. In the

VP-velocity

Gradient

younger province, however, velocities never reach

Upper crust

5.8-6.4

km/s

0.04-0.05

6.6 km/s.

Middle crust

6.2-6.6

km/s

Around 0.00 i _ ’

Lower crust

6.8-7.2

km/s

0.05-0.1

Such differences

are not seen in the

underlying uppermost mantle. General

:

‘ 5

’

structure of shield areas and the resulting differing crustal structures can

Detailed crustal structure studies have also been performed U.S.S.R.,

the

U.S.S.R.

in

other

shield

areas

(e.g.

in

be correlated with different geotectonic

the

A crustal investigation

in North America, in Southwest Africa

structures.

carried out in 1968 in

and in Australia) (Fig. 26). Pavlenkova (1979) has summarized the results

the Superior and Grenville provinces of Canada has been interpreted in detail by Berry and Fuchs

of such investigations

(1973). Their one-dimensional velocity-depth functions (Fig. 26d) are typically characterized by

in the U.S.S.R.

and has

defined a representative three-layer crust. The velocities and velocity-depth gradients in Table 1

a gradual velocity increase with depth. Only for areas of reduced velocity in the upper and middle crust are discontinuous velocity changes shown. In

seem to be characteristic for the Russian, Turanian and West Siberian platform areas. Regions with reduced velocity (Fig. 26~) are only observed in the middle crust (Pavlenkova, 1979). Following

the lower crust a low-velocity zone is observed. It cannot be ruled out that these zones might be falsely caused by lateral velocity changes at the

Landisman and Mueller (1966), Pavlenkova sees a connection between areas of reduced velocity and

border of the Grenville and the Superior provinces. However, from the analysis of P to S conversions of teleseismic events Jordan and

areas of increased seismicity. Alekseev et al. (1973) suggest that areas of reduced or increased velocity

#NW

Nor w:ir 1

NORTHERN

j

Cahdonides

2

ENE

SCANDINAVIA

j

Svecofennides

3

4

3

%!ic

Fig. 27. Crustal cross section along the Blue Road Profile (redrawn after Lund, 1979). Upper part: Cross section showing equal velocity. Lowerpurr: Velocity-depth functions from Lund (1979). For further explanation, see Fig. 18.

lines of

CRUSTAL

Frazer

STRUCTURE

OF THE FENNOSCANDIAN

(1975) confirm

inversion

within

SHIELD

the existence

TRAVELTIME

of a velocity

into

the

also reported

by Luosto

Damara

orogen

in

In

the

North

Southwest

Africa (Baier et al., 1983) also included

(1982) obtained

a profile

through

km/s)

the adjacent

The two-dimensional and Prodehl (1984) division upper

Kalahari

from

craton.

interpretation of Gajewski (Fig. 28) resulted in a clear

of the crust into three depth part,

the surface

areas. In the

to about

16 km,

within

Australian

shows

a general

velocity

inversion

velocities

crustal

east

with the velocity

gradually

and in the

increasing

from

Summarizing,

above

velocity

Hirschleber

of 7.9 km/s

km does the velocity

is observed

and only at 60

rise to values above 8.2 km/s

(Gajewski and Prodehl, 1984). A layer with a very high velocity of 7.4 km/s in the lowermost crust is

SOUTHWEST

WSW

2D-hK’LEl

20

I

Dwlhlkm!

recent

lower

results

of

(Fig. 26).

velocities

near

26 and

the surface

27;

Vogel,

et al., 1975; Pavlenkova,

reaches

The

(Fig. 26e).

in the uppermost

6.0 km/s

are 1971;

1379; Bath. 1 km often

(Fig. 26). When

this is

ENE

KALAHARI CRATON

6-O

.6.1 20

66

Deplhlkml

:a

Fig. 28. Two-dlmenslonal crustal velocity-depth Interpretation

AFRICA

km.

of shield areas, the following

(Figs.

1985). (2) The velocity quickly

13-15

can be drawn

5 km/s

from 6.3 km/s

from the more

(1) The P-wave

6.6 to 7.0 km/s. The crust-mantle boundary is difficult to define. At 45-60 km, a layer with a

crust

at 27 km with a slight

above 7.0 km/s

investigations

conclusions

The middle

increase

below

velocities of 5.6-6.2 km/s are observed. The middle part is split into two areas, in the west with 6.6 and 7.0 km/s,

of 6.2 km/s.

12 km to 6.85 km/s

velocity

Finlayson zones (5.85

the upper crust which in other areas

has a mean velocity near

craton,

two thin low-velocity

crust has velocities

of around

et al. (1984) for Finland

(Fig. 26b).

the lower crust.

Investigations

129

INTERPRETATION

structure through the Kalahan craton of Southwest Africa based on a ray-tracmg

(from GaJewskl and Prodehl, 1984).

Upper part:

Cross section showing lines of equal velocity.

Velocity-depth functions.

Lower part:

130

not the case, velocities depths

not greater

tulates

a first-order

of 6.0 km/s

increase,

FENNOLORA

discontinuity

have a limited

(6) Characteristic middie crust 6.5-6.7 km/s (Figs. 26 and 28).

at these depths,

(Fig. 26) derive a gradual

as is also observed

along

the

(7) In several gradual

extent

et al.,

areas,

the lower

increase

from

(8) The transition velocity

in the upper crust; lateral

velocity

(Figs. 26 and 27; Pavlenkova,

line (Fig. 18).

(3) The few zones of reduced ally observed

at

than 4-5 km. Bath (1985) pos-

while most other authors velocity

are reached

they are thin and

(100-200

1973;

are usu-

19-25;

Alekseev

Luosto

et al., 1984, 1985; Prodehl

km) (Figs.

Finlayson,

upper increase

(4) The velocity increase with depth is not discontinuous, but is characterized by more-or-less

26; Prodehl,

distinct zones of increased velocity gradient (Fig. 26; Luosto et al., 1980; Prodehi and Kaminski,

Tectonic inception

1984).

The evolution

between nyshov.

6.1 and 6.3 km/s

(Yegorkin

a

1979). a continuous

to the velocity

of the lower crust (Fig. 26; of the crust

varies consider-

ably from less than 40 to as much average

velocities

shows

1973).

(9) The thickness

1984).

(5) In the upper crust, the typical

with

at the bottom

depth

crust

are

7.0 to 7.6 km/s

from the lower crust

starts

Berry and Fuchs,

1982:

and Kaminski,

mantle

velocities

of the Moho

as 60 km. The

is about

45 km (Fig.

1984).

of the Fennoscandtan

Shield

are

and Cher-

1983).

A tectonic model

of

interpretation

the

of the velocity-depth

Fennoscandian

crust

requires

a

68

\

\

Kola nucleus 3.6 b.y.

64

62

60

Fig. 29. Fennoscandian (2800 Ma) continentat

Shield geoteztonic

elements

and their ra~ome~c

Karelian block from the younger Svecofennian

ages. The Sveco-Karelian

fault zone separates

part, which, during the Svecofennian

was reworked from oceanic to continental

material (Hietanen,

1975).

the older

orogeny (1900-1800

Ma),

(‘RUSTAL

STRUCTURE

tectonic tural,

model

which

petrological,

chronological

FENNOSCANDIAN

satisfies

geochemical

by Hietanen

of the shield

geological,

(Fig.

struc-

29). This model

mation

of oceanic

material

lian block (Fig. 30) (Berthelsen,

model shows

the

the shield might have taken place as follows:

of island

rifting,

which is separated marginal

crust.

the evolution

development

continental

a

by transfor-

into continental (1985)

131

INTERPRETATION

Ma ago, oceanic lithosphere was subducted northern edge of the Archaean continental

geo-

(1975) for the central

of the shield to the southwest to Berthelsen

TRAVELTIME

and isotopic

growth

According

SHIELD

data. The first plate tectonic

was introduced part

OF THE

of

From

2000

basin

the north

30) gradually

of the

1985). This caused arcs

Inari

and,

and

towards

intra-

proper

Marker,

the very old Kola

drifted

by

microcontinent.

from the continent (Berthelsen

at the Kare-

nucleus

this subduction

by a

1986a). (Fig. zone,

i4’N

Fug. 30. Tectomc map of the Fennoscandian Shreld and adjacent shteld regtons. MaJor tectonic boundanes are numbered from I to 9:

1 = Kola suture belt;

2 = southern margin of the Archaean

proto-shield;

3 = late Svecofennian thrust;

4 =zAsene fault;

5 = deformatton front along the eastern border of the Sveco-Norwegian orogen. Also shown are the 1900-1800 Ma old BaltrcBothnian (BB) and north Karelian (NK ) strike-slip megashears. A, to A, = Archaean crustal provmces. P, to P3 = Early to Mlddle Proterozotc crustal provmces (1900, 1700, and 1500 Ma). TEF= the North German-Pohsh

Trans-European

Caledomdes; TTZ = Tetsseyre-Tomquist

Fault: CDF = Caledoman Deformation Front of

Zone (southeast of the Baltic Sea); STZ = Sorgenfret-Tornqutst

Zone (northwest of the Balttc Sea); TIE = Trans-Scandinavtan Igneous Belt. Also shown IS the FENNOLORA

lure wtth shotpomts

W and B to H (from Guggisberg and Berthelsen, 1987, fig. 1). Dashed contours and spot values are sedtment thicknesses on the East European Platform.

until,

at I900 Ma ago, the two continental

collided by the pressed

and the marginal

basin were com-

into each other and shifted

tinental

Karelian the same

block

(Berthelsen,

inactive

the con1985).

At

zone to the

field” (Gaal,

19851,

of the isotopic

Fahick,

1984) and inves~gations (Claesson,

composition L.A..

(Wilson

and

1985;

Wilson

et al..

seyre line. From

between

transformed

1900-1850 Ma the oceanic lithothe two subduction zones was

into continental

tinental accretion. Between 1900 and

lithosphere

by con-

1800 Ma ago several

rigid

blocks of the Karelian continent moved past each other along large shear zones, first dextrally and later sinistrally, by several These shear zones include (see also Figs.

26b and

megashears, and shear (Bcrthelsen

hundred kiiometres. the Baltic-Bothnian

31), the north

Karelian

later the Raahe-Ladoga megaand Marker, 1986b). During the

sinistral phase of movement (1840-1800 Ma) the Svecofennian lithosphere must have already been rigid in order

to enable

Baltic-Bothnian

such movement

megashear

commun., 1985), Consequently, the candian Shield at 1840 Ma consisted Archaean block Belmorides, and

along

(A. Berthelsen,

the oral

Fennos-

of the of the Kola suture, the

north the Karelian

and

Svecofennian

orogens (Fig. 30). As time passed the subduction zone moved from a southern into a western position. Between 1750 and 1000 Ma the Fennoscandian Shield was first

enlarged towards the west through various (Fig. 30), and was then compressed (Berthelsen, 1980). The formation of the shield was then complete. The Caledonian orogeny at 4.50 Ma involved only the margins of the Fennoscandian continent (Berthelsen, 1984) and did not considerably alter the appearance of this old rigid block. stages

crust

5.6 and 6.4 km/s reduced

velocity

structure

of the crust

and

ual velocity

has velocities contains

m-

of between

several

which have limited

(2) The middle

of

extent.

by a gradfrom 6.6 to 7.0 km/s.

crust is characterized

increase

7.1 and 7.5 km/s, The strongly

zones

lateral

(3) The lower crust, with velocities

into the granitic

1985 j. Thereafter, the Svecofennian plate margin was far to the south, perhaps near the Tornquist-Teis-

velocity

(1) The upper

and was then sealed, as shown by

analysis intrusions

The general

rjf the crustal slrucm-t

volves three units (Fig. 31).

block, which was located

in the area of the “Skellefte became

onto

time the subduction

south of the continental

sphere

Tectonic interpretatron

(Berthelsen and Marker. 1986a). Caused collision with the island arc, the Inari

microcontinent

about

blocks

of between

is seen only in certain

undulating

areas.

Moho relief (Fig. 31) is

essentially caused by the lowermost crust. If one ignores the anomalously thick crustal areas in and central Sweden, the crustal thickness increases from about 35--B km in southern Sweden to more than 42 km in central Sweden,

southern

and further to 44-47 km under northern These abrupt and well-defined changes

Sweden. in Moho

depth of the order of 10 km can be compared with similar observations in the Tethys area (Egloff, 1979; McCaig and Wickham, 1984; Allegre et al., 1984; Him et al., 1980, 1984). In recent Alpine continent-continent

collisions compressions an interfingering blocks involved is also observed Depending extent tions

on the power

of subsequent can

and of

subsequent the crustal

{Mueller,

of the collison

compression

1984). and the

such superposi-

be found at large distances from the

collision zone proper. During the collision of lndia and Asia, Archaean rocks were pushed onto Jurassic rocks even at distances of more than 1500 km

(e.g. south

of Lake

Baikai; Tappomer

and

Molnar, 1976) and the superpositions south of this suture are still active. Such an interfingering at large pressures in a continental block is called “crocodile tectonics” (Meissner, 1989; Sadowiak, 1989). In two areas along the FENNOLORA line the anomalously thick crust > 50 km may have been formed by this tectonic process. The step in the Moho between shotpoints B and C (Fig. 31), however, can be correlated with a steep mylonite zone between Aseda and Hiigsby (L. Skjernaa, pers. commun., 1985). The existence of this mylonite zone suggests relatively abrupt vertical movements of the southern block against the northern crustal block by about 10 km (No. 4, Fig. 30). This suggestion is supported by the depth

200

KM

by 5 : 1. shading

4

c

heavy line = crust-mantle

fess than 6.0 km/s; boundary;

cross hatching

model of the crust along the FENNOLORA

8

= layer velocity

Fig. 31. Velocity--depth

I

W

South

values are velocities

= zones of reduced in km/s;

velocity;

line of 1979. The tectonic

E

elements

megashear.

between

upper

in the text. Depth versus distance thin line = boundary = Baltic-Bothnian

&line;

G B-B Megashear

are explained B-B

line = 7.0 km/s W to I are shotpoints;

broken

and lithnlogical

Fi(eM

Skelldts

F

I

and lower crust;

is exaggerated

w

North

of the 7.0 km/s north

isoline,

which

of the mylonite

zone.

mylonite

zone

However,

we can not decided

isoline

change

in

whether

is abrupt

this

low-velocity point

isoline

south

of the

to 28 km (Fig. whether

occurs

31).

the depth

contmuously

zone in the lower crust beneath be explained

might be connected The

while

or

near the fault zone. Thts thick

B cannot

of the Rapakivi crust

rises

is seen at 35 km

shot-

by this feature.

with a later granitic

It

intrusion

type, but this is debatable.

zones

of reduced

velocity

in the upper

shotpoints

B and

E maybe

by batholiths

tle (Claesson,

which originated

S., 1985) and

which

metamorphosed by anatectic granitoids within the crust. granitoids

which

are embedded

ex-

in the manwere

further

melting of other The granites and in gneisses

and

metasediments

enriched

be recognized model (Fig.

as thin low-velocity zones in the 31). Around shotpoint E such

granitoids,

which

by mica (Gail,

originated

from

1985) can

arkoses

and

other sediments rich in quartz and feldspar, are found at the surface (Lundqvist. 1980). They are characterized by low seismic velocities (Fig. 31). The large crustal thickness between shotpoints D and E (Figs. 21 and 31) coincides with a typical thrust zone (No. 3 in Fig. 30). This means that the velocity structure in the lower crust here may be related to a process similar to “crocodile tectonics”. The zone of reduced velocity (Fig. 31) is possibly shifted along this thrust zone, as tt is located at a depth of 10 km south of it and at only 5 km north

of it (Fig.

21). The upper

crust

has

been pushed onto the southern rigid crustal block along the thrust zone (No. 3 in Fig. 30), while the lower crust was pushed down into the uppermost mantle. middle

the presence difficult

The shearing may have happened in the crust so that this depth range did not

suffer major depth changes. The increasing crustal thickness south of shotpoint G (Fig. 31) may be explained by the ‘Baltic-Bothnian megashear zone’, which has only recently been recognized (Berthelsen and Marker, 1986b). This wide shear zone, over which large dextral (1900-1850 Ma) and sinistral (1840-1800 Ma) movements have taken place (Berthelsen and Marker, 1986b) seems to widen with increasing depth.

of the Caledonides.

This, however,

The

Archaean F and

Bothnian

crustal

H/I

structure

section

is subdivided

megashear

The southern

between

zone into two units (Fig. 31).

unit shows a homogeneous

and

the low-velocity

effect caused

shot-

by the Baltic

zone

velocity

might

by the thick, heavily

mylonite zone of the Baltic-Bothnian In its northern part the FENNOLORA parallel tectonic

is

to prove.

points

lateral

between

plained

The decrease in crustal thickness towards the northern end of the line may be connected with

be a

sheared

megashear. line runs

to the most important Sveco-Karelian lineaments (Fig. 30). The complex se-

quence of two to three depth ranges and increased velocity may be caused intercalation

of Archaean

material

of reduced by tectonic and

Protero-

zoic metasediments with folding parallel to the seismic line (see fig. 5 in Berthelsen, 1985). As mentioned

in the introduction,

this crustal

study was part of a seismic study of the whole lithosphere under the Fennoscandian Shield involving long-range recording distance

seismic observations up to a of 2000 km. These data, their

conversion into a lithospheric seismic model with a depth penetration of 200 km and their tectonic interpretation have also been described in earlier contributions which the reader may also like to consult. e.g. Fuchs et al. (1987). Guggisberg (1986), Guggisberg et al. (1984) and Guggisberg and Berthelsen (1987). Of particular significance is Guggisberg and Berthelsen’s (1987) presentation of a tectonic model of the whole Fennoscandian lithosphere. Acknowledgements The FENNOLORA experiment along the main line between shotpoints W and I was made possible by funding by the Swedish Natural Science Research Council, the Norwegian Research Council, the Foundation for Research into National Resources in Finland the German Research Society, the Swiss National Science Foundation, the Danish Natural Science Research Council and the Royal Astronomical Society. Shots were organized by the Swedish Navy and Army in cooperation with the Research Institute for

CRUSTAL

STRUCTURE

OF THE

Swedish

National

Finnish

Defence

mological

Defence. Forces

Observatory

(shotpoints

SHIELD

(shotpoints

(shotpoint

B-F),

the

G), the Seisof Bergen

H and I), and the Academy

of Science

GDR

BW). Personnel

in Berlin

(shotpoints

and equipment

W and

for the recording

along the main line were provided

geophysical

research

Federal

Republic

Norway.

Spain,

United

TRAVELTIME

of the University

of the former stations

FENNOSCANDIAN

institutions

of Germany, Sweden.

by the

of Denmark, Finland,

the

France,

Switzerland

and

the

Kingdom.

The authors

135

INTERPRETATION

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