Magnetic crustal structures in northern Fennoscandia

Tec~onaph~s~cs, 192 (1991) 57-79

57

Elsevier Science Publishers B.V., Amsterdam

Magnetic crustal structures in northern Fennoscandia Herbert

Henkel

Geological Suruey of Sweden. Box 670, S-751 28 Uppsalu, Sweden

(Revised version accepted September 15, 1989)

ABSTRACT Henkel, H., 1991. Magnetic crustal structures in northern Fennoscandia. Anomalies-Land and Sea. Tectonophysics, 192: 57-79.

In: P. Wasilewski and P. Hood (Editors). Magnetic

Cooperation between the Nordic countries has produced a set of geophysical and geological maps in the region north of the 66th parallel in northernmost Europe. The area covers the transition from oceanic crust to the Baltic Shield, a section of the Caledonides, and a part of the Arctic platform. An interpretation of the aeromagnetic and gravity maps reveals several large structural units with distinct boundaries and characteristic anomaly patterns. In combination with data on rock physical properties, some general magnetic cmstal features can be outlined: (1) typical occurrence of magnetically banded ocean crust anomalies, (2) the occurrence of magnetically banded patterns in the continental crust connected with metasupracrustal formations separated by culminations of high-density and high-magnetization basement domes and megashear zones (thrusts and transcurrent faults) with large displacements, (3) occurrence of magnetic granitoid batholiths, (4) general low remanence of shield rocks, allowing straightforward interpretation of magnetic total field anomalies, and (5) general low magnetization of Caledonian thrusted units allowing mapping of the depth to their magnetic basement. Merging magnetic, gravity and petrophysical data allows constraints to be applied when modelling crustal structures. Some of the large-scale structural features are illustrated and some examples of profiles through the upper crustal structures are shown.

Introduction

production of the two anomaly maps, aeromagnetic interpretation of detailed maps (at scales from

The large amount of magnetic and gravity data collected in northern Fennoscandia by the na-

1: 50,000 to 1 : 400,~O) was carried OUI., resulting in a 1 : l,OOO,OOO compilation (Fig. 6 I (Henkel.

tional together

geological

and

land

surveys

were brought

into a common database for the NordkaFrom these data, 1 : l,OOO,OOO maps

lott Project.

1986). In this paper,

some of the more

regional

aspects of the magnetic and gravity crustai structures will be discussed, and examples of some

have been produced (Figs. 4 and 5) (Korhonen, 1986; Korhonen and Kiviniemi, 1986). The database consists of a 1 x 1 km grid of magnetic total intensity anomalies and a 2.5 x 2.5 km grid of

particular structures will be considered. The maps produced during the Nordkalott Project are available from the Nordic geological survey>.

terrain-corrected Bouguer anomalies. The aeromagnetic dataset comprises twelve components with different line spacing and flight altitude

Aeromagnetic

specifications. The data covering the Precambrian terrains to the east of the Caledonides was reduced to a common apparent elevation of 200 m above ground. The other areas have been incorporated without any change with respect to the original (more than 200 m) elevations. Parallel with the 0040-1951/91/$03.50

0 1991 - Elsevier Science Publishers B.V.

interpretation

map (Fig. 6)

The interpretation method for low-altitude (30 or 40 m) aeromagnetic measurements in Precambrian shield areas as proposed by Henkel (in Witschard, 1975) was tested in regions where higher flight altitude (150 or 160 m) data had already been acquired. A test area in the Precambrian shield region overlapping the three par-

H. HENKEl.

58

ticipating countries was used for further refinement of the interpretation method, and for adaptation to the more regional requirements of the Nordkalott Project. The results from the test area are recorded in Henkel (1984). In the Caledonides and shelf areas, depth to (magnetic) basement computations were performed. The aeromagnetic interpretation used original aeromagnetic maps at scales ranging from 1 : 50.000 to 1 : 400.000 together with existing interpretations acquired in connection with other mapping projects and regional studies. The results were then brought to a common scale of 1 : 500,000, at which information was generalized to fit the final reproduction scale, 1 : l,OOO,OOO. Along the national borders, the structures were fitted together by appropriate revision of the original interpretations. Traditional approaches to aeromagnetic interpretation, such as defining magnetic sources and structures together with the recognition of a few basic patterns, were attempted. The high- to intermediate-resolution measurements used for the interpretation of the Nordkalott data permitted a quite good visual pattern identification and provided for a series of criteria allowing enhanced interpretation.

parallel

dlscordant

DYKES

very

CONCENTRIC

Fig. 1. Illustration

of different

cmtmuous

parallel subctrcular

no pattern

IRREGULAR

discontinuous types of magnetic

In the interpretation procedures, anomalies having similar amplitude, symmetry and position (within a structure) are connected b,y’lines representing the location of the source body at the surface. This location will change w.th respect to the anomaly location due to strike and dip and the direction of magnetization. The rock data are therefore extremely useful. Induced magnetization parallel to the local geomagnetic field is assumed to be the dominant type of magnetization. This assumption is correct for most of the Precambrian rocks of the region, as shown by their low Q-values (ratio of remanent to induced magnetization) in the range 0.1-0.5 (Henkel, this issue). In some mafic intrusions, however, significant remanent

Five types of patterns (Fig. 1) are considered to be of prime interest. These are listed in Table 1.

1

Aeromagnetic

patterns

Pattern

Definition

Banded

Continuous

Dykes

Very continuous

Typical and parallel

anomalies

and/or

discordant

and parallel

anomalies

1itholo;ies

Supracrustal. anomalies

with typical

asymmetry

Dykes and sillzI.

and strike Concentric

Continuous

Irregular

Lack of distinct

No pattern

Lack of anomalies,

patterns,

irregular

magnetically

with subcircular dist~bution transparent

patterns.

Reference structures (connections, con !acts and dislocations)

Patterns

TABLE

continwus

BANDED

arrangement

of anomalies

Mafic intrusions. Igneous

rocks.

Low- to intermediategrade sediments.

MAGNETIC

CRUSTAL

magnetization nence

STRUCTURES

may

be found.

has a direction

field

magnetic

bands

minimum

width

flight altitude,

can

be

will

but

sources

than

the

flight

sources

with

will

to

a

of less

anomaly only (see Fig. 2); a flight altitude m was used in the Finnish and Norwegian

netic are

one of 150 Shield

source

displaced

along

faults

high-resolution mated

horizontal down

m). The

at depth has

is indicated,

vertical

sides

and

where

the change

of

pattern occurs at the position of the contact

assuming

the source

the magnetization

body is in-

duced. Side shifts due to inclined contacts can also be accounted for in the inte~retation as well as southward to location Changes

shifts of the source body of the magnetic anomaly. in the continuity

with respect

aeromagnetic

of the magnetic

ref-

erence patterns are used to map and characterize magnetic dislocations. The location of such dislocations at the surface is indicated with straight-line segments in the interpretation map (Fig. 3). Displacement

of

reference

Fig. 2. Resolution

of magnetic

structures

measurements

along

linear

100

faults

is

in fractured (Henkel

is more typically

measurements.

A very

dislocatiors

is their

orthogonal

is greatly

among

which at

set can be observed.

In

reduced,

the

more

general

lineaments has been used. area, in the southern central

Nordkalott area where the N-S shear zone and the NW-SE

Bothnian-Senja Nordkalott combining

esti-

the shield region, where the magnetic

region of the Bothnian-Seiland study

be

to haemati:e

along only a few trends,

notion of magnetic In the LansjHrv

tailed

In

(about

with

oxidation

of magnetic

areas outside

defining

can

low associated

1977). This feature

feature

resolution

types.

accumulated

displacement

in low-altitude

least one nearly

contacts

the

of magnetite

and Guzman,

location

and structures

of various

to the increased

rocks (oxidation

typical

bodies

linear

in the mag-

to half the line spacing

magnetic

exhibited

associated

arising

measurements,

Areas having different magnetic patterns and/or different magnetizations are separated by magnetic

and

effects

when

area studies.

magnetization and/or surface. The approximate

lows

patterns

attributed

can still be produce

magnetic are typical

apparent

increasing

separations

altitude

resolu-

down

10 m. With

well-separated

as

of 40 m), single

observed

of about

of

gradients

Proj-

In the highest

(at elevations

identified, half

anomalies

can be considered

the same direction.

tion measurements

zones

rema-

In the area of the Nordkalott

ect, the geomagnetic having

this

from the geomag-

asy~et~c

59

FENNOSCANDIA

When

different

netic field, conspicuous be produced.

IN NORTHERN

shear zone intersect, of fault

zones

inte~retations digital elevation data (Henkel,

In some areas, can be identified

a fairly

was carried

de-

out. The

were improved by data with digital 1988).

the effects of low-angle thrusts in the magnetic anomaly pat-

terns. Typical features associated with this phenomenon are (1) discordant structures on either side, lower

(2) strongly unit

where

asymmetric the upper

spaced 20 m apart at 40 and 160 m above differently

anomahes unit

in the

is magnetically

spaced 50 m wide sources.

H. HENKU

REFERENCE

STRUCTURES

MAGNETIC DISLOCATION

_-

magnetic

connectlons

=

disiocatlon

-

magnettc

contacts

*****

linear

low

-

lateral

displacement

vertical O”l

Fig. 3. Magnetic

strong, (3) deep source anomalies

reference

structures

in the lower unit

where the upper unit is magnetically weaker, and (4) a generally more curving course than is typical for magnetic

dislocations.

Due to the low dips involved, strong minima will occur at wedges oriented

magnetic towards

north, and corresponding maxima will occur when the wedge is oriented south. A situation typical along the Granulite Belt has magnetic rocks thrusted on magnetic rocks (usually with differently oriented structures). Similar structures have been

identified

Nordkalott

in

Project

a

few

area.

thrusted units are generally sity, a different situation more magnetic tected beneath Culcuiation

other As

areas the

the

of contacts

of magnetic

ture and where netization

(km)

dislocations.

there is good control

parameters,

normally

estimates will be obtained. have a negative contact

Usually, anomaly

on the mag-

very

good

northern edge. This effect disappears at less than 60 O to the north. Simi arly, southern edges of magnetic structures a maximum is developed (inside the contact) disappears when the dip is smaller than 60 north. Regional

dip

all structures outside their dips

of

on the contact which o to the

crustal structures

Caledonian

of low magnetic intenis created where the

Precambrian structures the Caledonides.

of dips

in

and interpretation

displacement

displacement

can be de-

or sheet-like

bodies

The dips of a large number of locally significant magnetic structures have been analyzed. For this purpose, the magnetic anomalies have been digitized from original maps and analyzed with an interactive model calculation program. The total intensity magnetic anomaly is very sensitive to variations in the orientation of the magnetic struc-

Grauity crustal blocks (Fig-. 7) Paired regional anomaly belts Three sets of paired (positive and anomaly belts appear in the Bouguer

negative) anomaly

map. One follows the continental edge to the west and the shelf-continent transition to the north. The positive anomaly is located off the coast and the negative anomaly is centred on the high elevated areas of the Caledonian mountains up to Porsangen fjord. The amplitude variation is from - 125 to + 120 mGa1. The second paired anomaly set follows the northeastern coastal areas from Porsangen where the positive anomaly is near the

-‘

pp.61-62 i

pp. 63-64

AEROMAGNETIC ANOMALY MAP NORTHERN FENNOSCANDIA TOTAL INTENSITY

REFERRED CoMpILEDAi

TD ,OGFiF-65

c

A

Pp. 65-68

,.T1_^_.^.-

r_

,~^-_.

..r,/.

^_

.IW__

MAGNETIC

CRUSTAL

STRUCTURES

IN NORTHERN

FENNOSCANDIA

69 ,POR$ANGEN

Fig. 7. Crustal

gravity

2

Crustal

gravity

Block

blocks.

to be displaced across major fault zones. In particular, the section between the Bothnian-Senja and the Bothnian-Seiland fault zones shows an anomalous pattern. The anomaly pair along the shelf edge represents the combined effects of a higher level Moho and the layer of low-density sediment (i.e. the thinning of the crystalline crust). East of Porsangen, the negative part of the anomaly pair is subdued, a thinning of the crust

coast and paralleled by a negative anomaly which is not correlated with any topographic relief. The amplitude variation is from - 35 to + 30 mGa1. The third anomaly pair follows the Granulite Belt, which gives rise to a positive gravity anomaly and a slight rise in the terrain which is flanked by a gravity low to the west and south. The amplitude variation is from - 50 to + 15 mGa1. The anomaly pairs of the continental edge can be seen

TABLE

FJORD

blocks Characteristics

Average Bouguer anomaly

(mGa1)

I

+100

Oceanic crust.

Ia

+130

Thinned

II

+40 to +70 0 to -25

III

crust.

Continental

edge, thinned

Sedimentary

cover on thinned

IV

+15 too

Granulite

Belt, tbrusted

Va

-t70

Local mafic intrusion surrounding

Vb VI

- 25

Continental

VII

-50

Normal

VIII IX X

Local circular

-35 0

crust.

middle crust diameter

of 50 km, approx.

depth 11 km, and

area of deformation. crust with fairly high gravity,

continental

Continental

-100

crust.

thinner

than VII.

shield crust.

crust with low gravity, felsic intrusion

Local, larger basement

thicker

or impact

culmination.

than VII (maximum

structure,

diameter

elevated

terrain).

65 km, approx.

depth 7-X km.

70

by about 10 km explains the positive part of the anomaly pair. In the case of the Granulite Belt anomaly pair, the negative anomaly represents a down-warping of the crust caused by the accumulation of heavy middle crustal materials (granulite facies rocks) down to about the 17 km depth in the belt proper. A detailed model of the Granulite Belt may be found in Marker et al. (1990).

H. HENKEL

continent. The proposed suture is also partly visible on the aeromagnetic map. The +130 mGa1 Lofoten high and other anomalies over the thinned

TABLE Magnetic Discon-

Crustal gravity blocks Gravity blocks have been included here as background for the larger and deeper anomalous parts of the crust as compared to the shallow and smaller magnetic sources. These blocks are defined by their average relative Bouguer anomalies and boundary gradients and they represent large parts of the crust with different composition and/or thickness. A few more local blocks/structures have been included as they produce characteristic gravity anomalies. The blocks that may be seen are listed in Table 2. A change in the surface gravity of 40 mGal may correspond to a crustal thickness variation of 2-4 km, depending on the density contrast at the Moho. Apart from the described paired anomalies and blocks, numerous positive low-gradient anomalies appear to represent basement domes at different depths, as demonstrated in Lindroos and Henkel (1978) and Olesen and So& (1985). Remaining sharp gradient anomalies belong to various upper crustal sources such as granite intrusions (lows) and greenstone structures (highs).

discontinuities Characteristics

tinuity 1

Granulite

Belt southern

and western

Magnetic

thrust

are discordantly

slivers

magnetic

Precambrian

continuity

is large

dips about

20”

patterns. along

thrust

front.

piled on

The angular

the southern

dis-

edge and

to the north (Knorring

and Lund,

1988). 2

Southern

edge of discordant

Large angular 3

Eastern and

edge of N-S

striking

gravity

sinistral

Continuation

set of shear

gradier ts.

Large

offset of Precambrian

and

Marker,

fault zone(s) (Henkel,

zones locally

up to 40 km wide.

edge of approximately

of magnetic

unit.

batholiths

accu-

megashear Both-

1988). Sets of

1500 km long belt

extending

to 200 km wide (described

zones

structures.

1986) or of the

man-Seiland

Eastern

banded

of the Baltic-Bothnian

(Berthelsen

4

magnetic

discontinuity.

associated

mulated

to the south;

in Henkel

up

and Eriks-

son, 1986). 5

Northern

edge

(pm-Atlantic

of Precambrian

suture?}.

shield

Interrupted

structures

by discontinu-

ity 10. 6

7

Major crustal discontinuities The combination of the Bouguer anomaly with magnetic anomaly inte~retation allows a more complete delineation of major crustal discontinuities. Along the western flank of the negative Caledonian anomaly, several distinct gradients mark the termination towards the east of the western crustal structures. Similarly, anomalies belonging to eastern structural patterns are terminated. These discontinuities are tentatively interpreted as the suture zone created by the closure of Iapetus and the pre-Atlantic rifting tectonism. The slice of continental crust between this structure and the oceanic Atlantic crust is thus material belonging to the old Greenland and American

3

Western

edge of magnetic

trending

shear zones with large dextral?

Northern

edge

complex

greenstone

and

minor

of

E-W

trending

Appears

older magnetic

mafic intrusive

province.

9

Caledonian

eastern

front.

Precambrian 10

magnetically intrusives

structures.

Caledonian

thrust

offset.

:o be discordant

8

low magnetism

several NE-

belt with associated

branches.

with numerous

batholiths;

are discordantly

Thrust

slivers of

piled on magnetic

structures.

Bothnian-Senja

shear

zone

(Henkel,

1988).

Lo-

cally 80 km wide set of faults. 11

Edge of oceanic fracture netic tion

crust along continuation

zone to the north

oceanic

anomalies).

off the Norwegian

First identifiable

oceanic

age 58 Ma) (Kovacs

of Senja

(with discordant Parallel coast

mag-

with rift direc-

to the southwest.

magnetic

et al., 1986).

anomaly

(24B,

MAGNETIC

CRUSTAL

-6-

STRUCTURES

MAGNETIC

IN NORTHERN

FENNOSCANDIA

71

5lSCONTlNUlTY

L\

EOTHNIAN-SENJA

-:k

LOCAL STRUCTURAL

FAULT ZONE TRENDS

,,

,’

Fig. 8. Major magnetic

continental crust represent horsts of or thrusted lower to middle crustal slivers with, exposed at the surface in places, high-grade rocks (Olesen et al., this issue).

Fig. 9. Magnetic

upper crustal

discontinuities.

Kajar magnetic di~co~tinuitjes (Fig. 8) Several important discontinuities in the magnetic patterns can be inferred, along which re-

units. For explanation

of figures, see Table 4.

H. HENKEL

12

TABLE Magnetic

4 blocks

and units Characteristics

Block/unit Number

Name

1. Oceanic crust 11

Several parallel

Fallat

First positive

NE-striking anomaly

positive

and negative

at continental

magnetic

anomalies

edge is 24B (age 58 Ma) (Kovacs

et al., 1986).

2. Units west of Protogine Belt 21

Stuora Njuorjot

High-magnetic

22

Skoarra

Low magnetic

(high-grade) irregular

irregular

patterns.

patterns.

23

Lafol

Deep-seated

irregular

24

Cagan

Deep-seated

low-magnetic

magnetic

25

Unna Njuorjot

High-magnetic

pattern.

patterns.

(high-grade)

irregular

patterns.

3. Northern shelf area 31

SkilTi

Very deep seated low-magnetic

32

Davit Goaskin

Very deep seated high-magnetic

source.

33

Luosat

Very deep seated low and intermediate

34

Goalsi

Intermediate

deep-seated

high-magnetic

sources.

magnetic

magnetic

sources.

sources.

4. Protogine Belt 41

Lulit Goaskin

Deep-seated

42

Gironat

High-magnetic

banded

5. Between eastern edge of Protogine Belt and Bothnian-Seiland

irregular

zone

51

Albbas

High-magnetic

52

Geatki

Area dominated

by banded

53

Idjalotti

Area dominated

by high-magnetic

(high-grade?)

and several basement 54

Low-magnetic

Buoidda

6. Between Bothnian-Seiland

pattern.

pattern.

irregular pattern,

pattern,

basement

with several basement irregular

pattern,

regions

pattern.

zone and Granulite Belt

Njoammilat

Intermediate

62

Boazu

Large low-magnetic

63

Guorggat

Intermediate

64

Dovtta

Low-magnetic

65

Guovssahat

Irregular

magnetic

66

Vintanat

Banded

61

Skiret

Low-magnetic

banded

basement

magnetic irregular

pattern

(high grade?)

pattern. culmination.

banded

pattern

area with deep-seated

and several basement

pattern

and low-magnetic high-magnetic

pattern

72

Sarvva

Area dominated

-72

Gumppet

High-magnetic

irregular

by intermediate pattern.

anomalies.

and basement

culmination.

High-magnetic

irregular

pattern.

banded

banded

pattern.

8. Seiland intrusive province Giehka 2

91

Biebanat

High-magnetic

92

Oarre

Area dominated

pattern.

by low-magnetic

irregular

pattern.

IO. West-east belt of magnetic structures 101

Oarjit Vielpat

High (locally

very high) magnetic

banded

pattern

area.

102

Nuorttat

High (locally very high) magnetic

banded

pattern

area.

Vielpat

sources

culminations.

area with pyrrhotite-type irregular

irregular

pattern.

7. East and north of Granulite thrust front

9. Southeast of discontinuity

culminations

with banded

domes.

irregular

61

81

culmination

pattern

73

MAGNETIC CRUSTAL STRUtTURE.S IN NORTHERN FENNOSCANDIA

gional structures are terminated. Some of these magnetic ~scontinuities are also associated with gravity gradients indicating major changes in crustal structure, composition or thickness. These discontinuities are briefly described in Table 3. Together with a set of NW-trending shear zones, the shear zones were probably reactivated in connection with the Atlantic opening and the development of the Atlantic-Arctic shift of the rift zone. The NE-trending set of zones (1, Fig. lo) probably formed just prior to the Atlantic opening (pre-60 Ma) and the N-trending (discontinuity 3) and NW-trending set of zones (B, Fig. 10) when the north Greenland and SvaIbard foldbelts were formed by dextral strike-slip movements (60-30 Ma) (Harland, 1979, Schack et al., 1987). Coincidence with gravity discontinuities is observed along discontinuity 1, 2, 11, several parts of 3, and the northern part of 6. There is large uncertainty concerning the southwestern part of ~s~ntin~ty 5. Gravity discontinuities indicate a more western location of the pre-Atlantic suture. Discontinuity 2, which is only represented with a small section in the Nordkalott area, continues for about 200 km to the south. Major magnetic crustal blocks and units {Fig. 9) These blocks are bounded by the discontinuities described above and are defined by characteristic patterns and/or anomaly levels. These characteristic patterns and anomaly levels may represent uppermost crustal structural features or, in the case of larger anomalous areas, structures extending to considerable depths, but still remaining in the upper crust. The magnetic units are smaIler regions with more homog~eo~ anomaly patterns. Notice that the thrusted cover of Caledonian rocks is magnetically transparent, allowing continuation of the basement structures beneath the Caledonides. The magnetic blocks and units that have been identified are listed in Table 4. Around magnetic unit 81 (Giehka), a near-circular magnetic halo with disturbances due to adjacent structures is observed. This phenomenon has probably been caused by the deformation induced in the surrounding strata on the intrusion of the mafic rocks (Arkko, 1986).

TABLE

5

Regional dislocation zones Line-

Characteristics

ament A

Homavan

shear zone: a sin&al

NW-stung

zone

which can be followed to zone J (a southern zone parallel to H). B

Boar-Senja

shear zone: a ~nt~nuati3n of Senja

fracture zone towards the south-southeast and southeast. Wide sinistral (approx.

45 km)

shear zone,

length approximately 4300 km. C

Botch-oiled

shear zone: a ~n~u;~tion

B~tic-Bother

of the

megashear (Berthelsen and Marker,

1986); N/NNW-trending

set of shear zones from the

Bay of Bothnia to the intrusive Seiland province or southern edge of zone H. Associated gravity anomaly (low to the east) and several major discordances. D

White Sea lineament: a topographic geological line with distinct magnetic trace through Granulite Belt; sin&ml displacement about 30 km possible, trends towards Seiland.

E

NW-trending

lineament with possible continuation

through Kola Peninsula, sin&al

displacement ap-

prox. 10 km. F

NNW-trending lineament: eastern branch could continue approx. 1500 km to the south, possible sinistral displacement approx. 10 km.

G

Porsangen invent:

approx. 800 km long, NNE-

trending, small movement of approx. 50 km? Reactivation in late glacial times (Olesen, 1?88). H

NE-trending Vestfjorden shear zone, approx. 200 km wide set of zones with very large dextral displacement (300 km with reference to the Protogine Belt structures). Approx. length at least 600 km beyond the Seiland intrusive province.

I

Lies north of and parallel to (H), displaces Protogine

J

Lies south of and parallel to (H), displaced Protogine

Belt reference structures approx. 135 km dextrally.

Belt reference strnctures approx. 260 km dextrally. K

NW-trending

lineaments

east of discontinuity

terminate at Bothnian-Seiiand

3,

shear zone (C).

Major magnetic dislocation systems {Fig. lo) Interpretation of the aeromagnetic map indicates the locations of major dislocations, and this information allows the definition of a number of regional dislocation zones (Table 5).

H. HENKEL

B

‘. . : BARENTS

SEA

0A’i OF BdTHNlA Fig.

10. Generalized

magnetic

dislocations

mark the western

edge of the Protogine

(2). For explanation

Along several shear zones, and especially along the Both&n-Senja and the Both&n-Seiland zones, typical shear lens patterns are visible, indicating intense transpressional strike-slip movements. By means of this process, lower stratigraphic units are thrust up on higher units while at the same time being displaced considerable distances. As a shear zone consists of a network of many individual faults, an overprinted pattern of mismatch tens of kilometres wide emerges (Henkel, 1988). The spatial frequency of such shear zones seems to be low: there are only two prominent examples in the Nordkalott study area (500 x 700 km). When overprinting is across older shear zones, a new shear lens pattern oriented parallel to the younger zone will dominate. This is apparently the

of letters,

Belt (1) and its possible

continus.tion

to the north

see Table 5.

case where zones H, I and J are traversed by the NNW-trending Bothnian-Senja shear zone. Low-angle faults are not readily identified in the aeromagnetic measurements except where large displacements have created characteristic discontinuities. Apart from the two large thrust fronts, the Caledonian and the Grant&e Belt, only minor thrusts have been identified. One is along parts of the northern edge of magnetic units 101 and 102 (Vielpat), the W-E

Generalized

trending greenst’one belt.

dyke occurrences (Fig. 1 I)

In the Nordkalott Project area, only minor magnetic dykes or dyke swarms have been found

MAGNETICCRUSTALSTRU~TURESINNORTHERN

..-‘-

STEEP

DYKES NORMAL NRM

,./

STEEP

DYKES REVERSED

. ...*+ .

75

FENNOSCANDIA

b

NRM

SILLS CONICAL DY

Fig. 11. Generalized dyke occurrence. For explanation of figures, see Table 6.

in the Precambrian shield region. They are indicated in the aeromagnetic interpretation map (Henkel, 1986) and in Fig. 11 a general compilation has been made. It should be noticed that only magnetic dykes wider than 10-20 m can be detected by the low to intermediate elevation aeromagnetic measurements. In addition, small (short) dykes are more readily identified in areas

with low magnetic variation (e.g. in basement culminations). The absence of large and dense dyke swarms shows that the entire area has remained distal to rifting events. A province of conical mafic dykes is associated with the basement culmination at Muorionalusta (crustal gravity block X) described by Lindroos and Henkel (1978). In the southwestern region, a

TABLE 6 Dyke systems Area

Dyke trend

Other characteristics

1

NNE

Small dykes in basement culmination.

2

NE

Individual, larger dykes with reversed NRM

3

N

Basement creation, large, slightly arcuate dykes do~nating northeast and northwest.

4

COniI?d

Four conical mafic dykes in large basement culmination

5

Ffat

Set of many small and several large sills; smaller, N-trending dykes also occur.

6

NW

Several large dykes, smaller NE-trending dykes also occur.

7

WNW and E-W

The only area with WNW- and E-W trending dykes, NE-trending dykes occur also.

smaller dykes striking

H. HENKEL

16

set of mafic (Witschard,

sills

forms

a prominent

1975). Table

erties of the different Depth to magnetic

structure

6 lists some of the prop-

a reasonably

study,

the automatically

calibrated

dyke systems.

basement

attain

(Fig. 12)

front,

the Precambrian

generally sheets

west

low-magnetic

of the Caledonian structures sediments

of low-magnetic

autocorrelation

are overlain and/or

metamorphic

method

(1982) has been applied

described for mapping

the top of the magnetic calculations along profiles

thrust tbrusted

rocks. by

by The to

structures. Automatic were made at intervals

of 6-8 km, and in the interpretation contours of the depth to the magnetic basement were drawn at 1 km intervals

(Lind,

1986a).

Low-magnetic

calculations

properties

Caledonides.

A few areas

In

show indications

the interpretation

map.

using

observed

the re-

of deep magnetic One

small high-magnetic cover

resents

the deeper

Profiles

type

of low-magnetic is connected of high-magnetic parts

known

in the Precambrian

by a thin

volumes

were

outside

comprises

surface

a pilot

depths

and these areas have been indicated

type of indication

Thorning

the depth

model

thickness.

determined

magnetization gions In the region

by

great

sources

separately

in

of indication

structures

overlain

rocks.

Another

with large, bodies

near-

and rep-

of these bodies.

across some basement

culminations

in the

Precambrian

au-

tochthonous Precambrian sediments cannot be separated from thrusted low-magnetic units and are included in the low-magnetic cover thickness.

Two

basement

culminations

elled along profiles using anomalies and petrophysical

Tests were run to illustrate the magnetic effects of thrusted Precambrian units (which have magnetization parameters similar to the outcropping Pre-

alusta

structure

cambrian) and show that such units will not generate significant magnetic anomalies until they

stones) deformed in turn overlie

(crustal

have

gravity

southern part of magnetic surrounded by supracrustal

been

mod-

gravity and magnetic data. The Muonionblock

X) in the

unit 52 (Geatki) is rocks (mainly green-

by underlying granitoids, which acid-intermediate high-grade

Fig. 12. Depth to magnetic basement. Contours at intervals of 2.5 km below sea level. Dotted line denotes larger deep magnetic sources in the shield region.

MAGNETIC

CRUSTAL

gneisses.

The structure

km (Lindroos

STRUCTURES

IN NORTHERN

has an amplitude

and Henkel,

The Gallivare

in the northern

unit 53 (Idjalottit)

magnetic

low with a diameter

structure

is encircled

consists

of low-density

nying

low-gradient

seated,

denser,

Conclusions

is a rounded

granitoids.

gravity

high

rocks

and

The accompaindicates

intermediate

deep-

composition

rocks rising 6 km to a depth of about

Some

central

of about 50 km. The

by supracrustal

more

of 8-10

3 km (Lind,

cerning

general

1986b). Because

there

are

numerous

similar

gravity

in magnetic blocks 5 and 6 with various magnetic anomalies, it is proposed that

these structures

represent

lying among occurrences

basement

by

volumes

rocks

while

of

(mainly greenstones). High-magnetic deep-seated sources are interpreted

rocks

anomalies of as high-grade

(granulite facies) rocks, in accordance with the findings in Lofoten (Olesen et al., this issue). In a few cases, the cause of the basement culminations is plate tectonic (thrusting and shearing), but they are mainly attributed to gravitational compensation of crustal

loading

with dense volcanic

the

restricted

con-

in a large

crust of the Baltic Shield. fact

have

that

measurements only

significant

of rocks

induced

very

small

remanence

to mafic intrusions

the majority to

patterns

of aeromagnetic

is simplified (mainly

can be drawn

structural

section of the continental The interpretation

and dykes),

have a 10~ ratio

magnetization

1988). This allows the interpretation

to be focussed

The

determination

(banded,

dyke-like

of

magnetic

and irregular)

patterns

is an instrument

for distinguishing between supracrustal, hypabyssal and plutonic lithologies; such an interpretation

must, however,

be based on original

data

+

4 M

++*++: ++++

rocks,

/

which causes the regions between the greenstone volcanic belts to rise. As the greenstones were mostly deposited on an acid (low-density) crust, the induced compensated usually

have

gravitational by granite diameters

instability diapirism of about

is also locally Such diapirs 5 km (Henkel,

1978). Figure 13 summarizes the general cal aspects of the basement culminations.

geophysi-

Caledonian

thrusted units

Major magnetic discontinulties banded MAGNETIC

pattern

Generalized

magnetic dislocations

high gmdwnts Short wwdengths

Generaked trends in magnetic banded pattern areas Generalized

high-mognetr

Basement

trends of oceanic magnetic bands culminations

Magnetic batholiths

I

lnternwdiote gramto~ds d

Fig. 13. Schematic W-E

grontte

-_= dlaplrs

profile through basement culmination

structures. The magnetic edge minima will have variable amplitudes due to the strike direction, the dip and the depth extent of the high magnetic rocks.

of

(Henkel,

on structural aspects, including dip determinations of contacts and magnetic bands.

culminations

of metasupracrustal

conclusions

the magnetic

remanent

structures associated

(Fig. 14)

1978).

structure

part of magnetic

71

FENNOSCANDIA

Large mafic intrusives Carbonatites Kiruna - Kolari greenstone 0

I

belt

100 km

t

Fig. 14. Magnetic crustal structures.

78

H. HENKEL

with high spatial magnetic ology, ring

resolution.

patterns

bimodal

may

observable

basic

tool for determination

dislocations

fault zones. Again,

various

of magneti-

of structures of steep

for the mapping

forming

is a

faults

Steep faults make up networks extent,

lithologies vertically,

and

resoluof these of con-

fault zones in which rock

are strongly dispersed horizontally and mainly as lens-shaped fragments of

dimensions.

the Nordkalott

Two such zones are noted

area,

the Bothnian-Senja

the Both&r-Seiland fault

zones.

in and

It is suspected

that these zones define areas in the upper continental crust where plate tectonic stresses are being continually (at variable rates over time) released. Discontinuities in the orientation of magnetic structures indicate the occurrence of low-angle fault zones. In the study area, several prominent examples are noticed. As there is a marked magnetic difference between Precambrian and younger lithologies

greenstone

structures

compensating

the

belts.

Acknowledgements

unimodal

data with high spatial

are a prerequisite

siderable

gravitational

occur-

in contrast,

having

and interpretation

cally

tion

and,

usually

lith-

as

distributions.

The occurrence

structures.

with

distributions

rocks,

rock lithologies

susceptibility

the banded

be correlated

susceptibility

in metasupracrnstal

plutonic

Further

(the

latter

being

generally

low-mag-

netic), it is possible to trace the continuation of the overthrusted structures beneath the thrusted lithologies. The mapped magnetic bound the magnetic crustal blocks Some of the discontinuities crustal sutures (or rather There may be significant

mark

discontinuities seen in Fig. 9. the location

the preceding discrepancies

of

rifting). between

the magnetic and gravity features associated with larger discontinuities as gravity anomalies arise from the entire lithospheric structure, while magnetic anomalies have their dominant sources in the upper crust. In addition, large gravity distortions tend to become compensated by processes and tectonic movements.

metamorphic

In two of the magnetic crustal blocks (5 and 6) significant amounts of greenstones occur, and a particular pattern of basement culminations appears to be related to these occurrences. These culminations represent upheavals of deeper, more dense parts of the upper crust and are interpreted

The 1 : l,OOO,OOOscale aeromagnetic tion map of the Nordkalott

project

cooperation

between

scientists

surveys

Finland

(Heikki

of

Tahvanainen),

Norway

and Atle Sindre) Hult,

Juhani

of the geological Saavuori

(Ola Kihle,

and Sweden

Korkealaakso

interpreta-

is the result of and

Odleiv

(Vesa Arkko,

and Jonas

Iris

Olesen Karin

Lind).

Dan

Nisca at the Swedish Geological Co. also contributed to the interpretation work. The depth to the magnetic

basement

performed

at the Geological

in Copenhagen Thoming.

interpretations

with

The naming

suggested by Lars checked the North

Survey

the kind

were

of Greenland

assistance

of magnetic

partly of Leif

provinces

was

Granar, and Hakan Rydving Lapp names. I would like to

express my gratitude to all these colleagues for lively discussion, good cooperation, and their interest in making the interpretation map. The project leader, Gunnar Kautsky, constantly encouraged all the participants.

References Arkko, V., 1986. The Nordkalott project-geophysical

aspects

of large mafic intrusions in northern Sweden. Geol. Surv. Swed. BRAP 86 401. Berthelsen, A. and Marker, M., 1986. 1.9-1.8

Ga old strike-slip

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of Actual Fault Zones

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in

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MAGNETIC

CRUSTAL

STRUCI’lJRES

IN NORTHERN

FENNOSCANDIA

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