Structural–Kinematic Evolution of the Central Belomorian ... · 212 GEOTECTONICS Vol. 41 No. 3...

210

ISSN 0016-8521, Geotectonics, 2007, Vol. 41, No. 3, pp. 210–230. © Pleiades Publishing, Inc., 2007.Original Russian Text © S.Yu. Kolodyazhny, 2007, published in Geotektonika, 2007, No. 3, pp. 46–68.

INTRODUCTION

The reconstruction of the tectonic evolution of thelithosphere in the Early Precambrian is one of the mostcomplicated problems of modern geology. The newgeophysical, isotopic, and geochemical data allowapplication of the actualistic approach to geodynamicinterpretation of the Archean and Paleoproterozoicevents. However, the same initial data are often inter-preted from different standpoints based on the theory ofplate tectonics, intraplate models of crustal evolution,or the ideas of mantle plume ascent. In this context, theapplication of the methods that provide additionalinformation for the solution of debatable issues wouldnot be out of place. In particular, the structural–kine-matic data on the character and direction of tectonicmovements supplement other geological evidence andpromote the development of adequate geodynamicmodels. An attempt to reconstruct the evolution of thecentral Belomorian–Lapland Belt in the Baltic Shield ismade on the basis of the original data obtained withmodern structural–kinematic methods.

GEOLOGICAL OUTLINE

The Belomorian–Lapland Belt—one of the mostimportant structural zones of the Baltic Shield—sepa-rates the Neoarchean Kola and Karelian massifs. Thistectonic province is known in the literature under dif-ferent names: the Belomorian–Lapland granulite–

gneiss belt, the Belomorian geoblock or microconti-nent, the Belomorian fold zone or rift system, a belt oftectonothermal reworking, and the Belomorian–Lap-land mobile or collision belt. Such a wide range ofterms is due to consideration of the origin of this zonefrom different viewpoints [3–5, 11, 12, 23–25, 27, 36,40, 44].

The Belomorian–Lapland Belt is composed of gran-ulite–gneiss and amphibolite–gneiss rock associationsreworked by granitization to a variable extent. Thesteady, polycyclic development of high-pressure meta-morphism under conditions reaching granulite andeclogite facies is one of the relevant attributes of thisbelt [11, 12, 17, 36]. In addition, the multiphase fold-ing, doming, migmatization, and ultrametamorphism;abundant minor basic intrusions emplaced at a highpressure (drusites dated at 2.45–2.35 Ga); and rare-metal and muscovite pegmatites (1.90–1.75 Ga) arespecific features of this province. As has been shown inseveral publications, the Belomorian–Lapland Belt is along-lived mobile structural unit that actively devel-oped in the Neoarchean and Paleoproterozoic and sub-sequently underwent multifold remobilization in theRiphean, in the Phanerozoic, and recently [7, 11, 12,23–25]. This region is heterogeneous and consists oftwo large tectonic units: the Belomorian Amphibolite–Gneiss Belt and the Lapland–Kolvitsa Granulite Belt(Fig. 1).

Structural–Kinematic Evolution of the Central Belomorian–Lapland Belt in the Paleoproterozoic

S. Yu. Kolodyazhny

Geological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017 Russiae-mail: [email protected]

Received July 5, 2006

Abstract

—A model of the evolution of the central Belomorian–Lapland Granulite–Gneiss Belt is proposed onthe basis of analysis of the Paleoproterozoic structural–kinematic assemblages. It is shown that this tectoniczone is a long-lived mobile structural unit that evolved through several stages of tectonic transformation andmetamorphism of rocks, including (1) the Reboly stage, which comprises subduction (2.88–2.82 Ga) and col-lision (2.74–2.53 Ga) substages; (2) the Selet stage of rifting and extension of the continental crust accordingto the model of simple shear (2.45–2.35 Ga); and (3) the Svecofennian stage, characterized by collision andgeneral transpression (1.94–1.75 Ga). The results of structural–kinematic study indicate that tectonic flow inthe Svecofennian time was nonuniform and related to the formation of the Kolvitsa–Umba near-horizontal pro-trusion. The propagation of this protrusion was caused by transpressional extrusion of plastic lower-crustalmasses to the surface as a gently ascending tectonic flow directed to the northwest (in present-day coordinates).Thereby, a thrust–normal-fault kinematic effect was expressed in pushing-out of deep-seated complexes con-temporaneously with tectonic erosion of the upper portions of the sequence owing to the development of low-angle normal faults.

DOI:

10.1134/S001685210703003X

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STRUCTURAL–KINEMATIC EVOLUTION 211

The Lapland–Kolvitsa Granulite Belt

This belt consists of the Lapland and the Kolvitsa–Umba segments (Fig. 1). According to the geologicaland geophysical data, the belt has a nappe–thrust struc-ture. The Lapland Nappe in the northwestern part of thebelt was displaced for no less than 100 km to the south-west [26, 27, 32]. The time of thrusting is estimated at1.95–1.91 Ga from the age of synkinematic metamor-phism [15, 27, 32, 51, 52]. The belt is composed ofmafic and felsic granulites, enderbite, tonalite, andcharnockite. As follows from Sm–Nd model ages andU–Pb zircon isochron ages, the protolith of Paleoprot-erozoic metamorphic rocks was formed 2.28–1.95 Gaago [8, 46, 51, 52]. The model Sm–Nd age and positive

ε

Nd

(

t

)

values testify to the predominance of the juvenilecomponent in their composition. This fact, togetherwith the geochemical signature of granulites, indicatesthat the origin of these rocks was related to the subduc-

tion that determined not only the formation of island-arc volcanic and plutonic series but also their erosionand deposition of clastic material in the interarc basins[16, 47, 48, 51, 52].

The large sheets of metamorphosed gabbroan-orthosite and the underlying complexes of the Tanaelv–Kandalaksha Zone lie at the base of granulite alloch-thons (Fig. 1). The isotopic data testify to the long-termevolution of these rocks from 2.5 to 1.90–1.85 Ga[28, 48]. The Neoarchean gneiss and amphibolite of theBelomorian Group occupy a lower structural position.

The retrograde reaction structures in granulitesmark (1) decompression and cooling, (2) nearly iso-baric cooling, and (3) retrograde hydration [52, 54, 55].The reactions of nearly isobaric cooling are character-istic only of marginal portions of granulite nappes. Therocks of the Tanaelv–Kandalaksha Zone that underliegranulite nappes demonstrate the inverse metamorphic

0 50 100 km

65°

30° 40°

65°

30° 40°

B A R E N T S S E A

W H I T E S E A

B

P

LK

LK

L

Kl

NK

IV

LKKU

KU

TK

TKKr

P

EK

EK

Fig. 2

Fig. 6

Fig. 9

Fig. 2

Caledonides

1 2 3 4 5 6 7 8

a b

Ko l a M

a s s i f

Ka r e l i a n M

a s s i f

Fig. 1.

The main tectonic units in the northeastern Baltic Shield. (

1

) Archean granite–greenstone complexes; (

2, 3

) Paleoproterozoic:(

2

) mainly epicontinental volcanic and sedimentary rift-related complexes, (

3

) sedimentary, volcanic, and intrusive complexes ofthe Svecofennian accretionary–collision belt; (

4, 5

) Archean–Proterozoic mobile belts: (

4

) Belomorian Amphibolite–Gneiss Belt,(

5

) Lapland–Kolvitsa Granulite Belt; (

6

) Devonian intrusions; (

7

) Riphean sedimentary cover; (

8

) faults: (

a

) steeply and (

b

) gentlydipping. Archean–Proterozoic mobile belts (letters in figure): B, Belomorian, LK, Lapland–Kolvitsa Belt and its segments: L, Lap-land and KU, Kolvitsa–Umba; Paleoproterozoic volcanic–sedimentary belts: EK, East Karelian; NK, North Karelian; Kl, Kuolo-jarvi; Kr, Karasjok; P, Pechenga; IV, Imandra–Varzuga; TK, Tanaelv–Kandalaksha.

212

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KOLODYAZHNY

zoning that follows a clockwise

P

–

T

–

t

path. It is sug-gested that the rocks of the Tanaelv–Kandalaksha Zonewere thrust under granulites, which, having come incontact with cold rocks, underwent isobaric cooling.These events proceeded at

600–700°ë

at a depth of~20 km [54, 55].

The Belomorian Belt

This belt is composed largely of Neoarchean rocksthat have features similar to granite–greenstone associ-ations of the Karelian Massif [11, 36]. The geologicaland structural interpretation of the Belomorian Beltremains equivocal. The belt is regarded as a domain oflong-term tectonic and metamorphic remobilizationand multiphase folding [4, 39], as a system of oval (inplan view) structural elements in the framework ofzones of ductile flow [11], as a zone of strike-slip fault-ing and transpression [30], or as a zone of long-termextension and formation of low-angle normal faults[40]. According to [2, 12, 21–25], the Belomorian Beltconsists of a series of intricately deformed tectonicsheets (Fig. 2): (1) the Kovdozero Sheet of biotite tonal-itic gneiss, gneissic granite, and volcanics of the Tik-shozero greenstone belt; (2) the Chupa Sheet of variousaluminous gneisses; (3) the Khetolambina Sheet ofamphibole tonalitic gneiss, large orthoamphibolite ski-aliths, and relict zones of mafic and ultramafic rocks;(4) the Keret Sheet of biotite tonalitic gneiss; (5) theOrijarvi Sheet of biotite–amphibole tonalitic gneiss;and (6) the Riikolatvi Sheet of biotite and amphiboletonalitic gneisses with stratal bodies of orthoamphibolite.

A subduction–collision model of the evolution ofBelomorides in the Neoarchean is based on structural,lithologic, geochemical, and isotopic evidence [24]. Atthe subduction stage, the oceanic plate (KhetolambinaComplex) and the overlying sedimentary rocks (ChupaGneiss) were subducted beneath the Karelian granite–greenstone domain (Kovdozero Complex). The time ofsubduction is estimated at 2.88–2.82 Ga from the age ofisland-arc complexes of the Tikshozero greenstone beltand early metamorphism [9, 45]. The subsequent colli-

sion resulted in overturning of systems of the oldernappes and their obduction upon the margin of theKarelian accretionary region, phenomena that wereaccompanied by high-pressure metamorphism and for-mation of the younger tonalite 2.74–2.69 Ga ago[12, 20, 21]. The collision was completed with develop-ment of variously oriented folds and migmatite–granitedomes. As a result, the collisional orogen that weldedthe Karelian and Kola cratons formed by the end of theArchean.

The early Paleoproterozoic igneous rocks

in theeastern part of the Baltic Shield make up a vertical col-umn as follows (from top to bottom): (1) Sumian volca-nic rocks, including mafic–ultramafic layered intru-sions; (2) drusite complex; and (3) gabbroanorthosite.The drusite complex (lherzolite, gabbronorite, diorite,gabbroanorthosite) and genetically related charnockiteare known in the Belomorian–Lapland Belt as minorintrusions dated at 2.45–2.35 Ga [11, 37, 38, 40, 50].The corona (drusite) textures resulting from melt crys-tallization at

P

= 6–12 kbar and the subsequent high-pressure metamorphism are typical of these rocks[11, 37, 38, 50, 53]. In terms of geochemistry, theserocks are similar to the coeval volcanics that fill rifttroughs and the layered intrusions from marginal partsof the Karelian and Kola massifs [37, 38, 42, 43]. Thedrusite complex includes gabbroanorthosites that occurin layered plutons and as isolated sheetlike intrusivebodies. According to [40], these rocks indicate theextensional setting and development of low-angle nor-mal faults.

In general, the association of the early Paleoprotero-zoic igneous rocks of the Belomorian–Lapland Belt isa bimodal series of mafic–ultramafic mantle-derivedrocks and lower-crustal granitoid rocks that character-ize the tectonic setting of intracontinental rifting. Thismagmatism was related to the ascent of a mantle diapirwhose axis was projected on the Belomorian Belt[35, 37, 40, 42, 53].

Metamorphism.

With allowance for the data pub-lished in [11, 12, 21, 31], the following sequence of tec-

Fig. 2.

Geological sketch map of the central Belomorian–Lapland Belt. Compiled after [4, 11, 12, 18, 21–23, 31, 40, 44]. (

1–7

) Neoarcheanrocks of the Belomorian Belt: (

1

) Kovdozero Nappe (mainly biotite and less abundant biotite–amphibole tonalitic gneisses andgneissic granite), (

2

) Orijarvi Nappe (biotite–amphibole tonalitic gneiss), (

3

) Chupa Nappe (aluminous garnet biotite, kyanite–gar-net–biotite, kyanite–garnet–biotite–muscovite, and biotite–muscovite gneisses), (

4

) Khetolambina Nappe (mainly amphibole andbiotite–amphibole tonalitic gneisses, trondhjemite, granodiorite, and skialiths of ortho- and paraamphibolites), (

5

) Keret Nappe(mainly biotite tonalitic gneiss), (

6

) mafic zones (granitized metamorphosed mafic and ultramafic rocks), (

7

) Riikolatvi Nappe(biotite and amphibole tonalitic gneisses with stratal orthoamphibolite bodies; (

8–15

) Paleoproterozoic rocks of the Kolvitsa–Umbasegment (belt) of the Lapland–Kolvitsa Belt: (

8

) Kandalaksha Complex of the Tanaelv–Kandalaksha Zone (garnet amphibolite,amphibole gneiss, and basal metaconglomerate), (

9

) Por’ya Guba Complex (basic granulite, two-pyroxene crystalline schist, andless abundant intermediate and felsic granulites), (

10

) zone of tectonic melange, (

11

) Umba Complex (mainly felsic garnet–silli-manite granulite), (

12

) Tersky Complex (biotite–amphibole gneiss), (

13

) gabbroanorthosite (2.46–2.45 Ga), (

14

) enderbite andcharnockite (1944–1912 Ma), (

15

) porphyritic granite; (

16–18

) intrusive rocks of the Belomorian Belt: (

16

) Neoarchean enderbiteand charnockite (2.4 Ga), (

17

) mafic and ultramafic drusites (2.45–2.35 Ga), (

18

) subalkali granite (2.3–1.9? Ga); (

19, 20

) Neoarcheanrocks in basement of the Karelian Massif: (

19

) granite gneiss, (

20

) greenstone complexes; (

21

) Paleoproterozoic volcanic–sedimen-tary rocks; (

22

) Riphean sedimentary cover; (

23

) faults: (

a

) steeply and (

b

) gently dipping. Structural units of the Lapland–KolvitsaBelt (letters in figure): KU, Kolvitsa Umba segment (belt); TK, Tanaelv–Kandalaksha Zone; segments of the Belomorian Belt:SK, Seryak–Kovdozero; CH, Chupa; EN, Engozero; YE, Yena; Kv, Kovdozero Strike-Slip Fault; Paleoproterozoic volcanic–sedi-mentary belts: EK, East Karelian; NK, North Karelian; IV, Imandra–Varzuga.

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67° N

32°

0 10 20 km

N

S

34° E

66°

65°

W H I T E S E A

Kandalaksha Bay

IV

KU

KU

TK

SKKv

SK

SK

CH

NK

NK

YE

CH

EN

EN

EN

EK

EK

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

‡ b

Karelian

Massif

L. Topozero

Kem’

KU

214

GEOTECTONICS

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2007

KOLODYAZHNY

tonic and metamorphic events may be outlined: (1) theReboly stage with subduction (2.88–2.82 Ga) and col-lision (2.74–2.53 Ga) substages, (2) the Selet riftingstage (2.45–2.35 Ga), and (3) the Svecofennian stage(1.94–1.75 Ga).

The Reboly stage

predetermined the main featuresof the Belomorian metamorphic complexes and theirzoning that is reflected in the northeastward increase inthe metamorphic grade with an increase in the distancefrom the margin of the Karelian Massif [12].

The Selet rifting

is characterized by syn- and post-magmatic mineral reactions in rocks of the drusite com-plex (2.45–2.35 Ga). The early metamorphism ismarked by ortho- and clinopyroxene rims at olivine–plagioclase boundaries as a result of the subsolidusreaction Ol + Pl = Opx + Cpx

±

Sp at

P

> 8 kbar and

T

=

700–800°ë

. Garnet-bearing and amphibole rimsappeared later [37, 38]. The mineral assemblagesformed at

P

= 7–9 and 11–12 kbar and at

T

= 570–620

°

C and 700–710

°

C were identified in drusites [50].

The Svecofennian collisional stage

was accompa-nied by tectonic transformation and metamorphism thatlocally developed along zones of ductile deformationand under conditions of kyanite–muscovite subfaciesof almandine–amphibolite facies (

T

= 590–630

°

C and620–720

°

C;

P

= 5.8–7.5 kbar) [11, 12, 18, 19, 33, 34].The intensity of metamorphism increases toward theallochthonous Lapland–Kolvitsa granulites; inversemetamorphic zoning has locally developed in theirframework. The retrograde Svecofennian metamor-phism proceeded against the background of falling tem-perature and a fall in pressure from 7.5 to 3.6 kbar[11, 31]. Several areas with different U–Pb ages oftitanite are recognized within the Belomorian–LaplandBelt: (1) an area of uniform metamorphism thatembraces granulite allochthons and underlying rocks(1.94–1.87 Ga), (2) an area of discrete metamorphismin the central zone of the Belomorian Belt (1.87–1.815 Ga), and (3) an area of zonal metamorphismalong the boundary between the Karelian Massif andBelomorides (1780–1750 Ma) [49]. Taking intoaccount that the U–Pb age of titanite characterizes thetime of closure of its isotopic system at

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2007


67°

34°

0 10 20 km

N

S

W H I T E S E A

Ka

re

li

an

M

as

si

f

32°

N

E

66°

65°

2361/U-Pb(Zir)

PR

1

2400

AR2

Sh

2420–2450

To2435/U-Pb(Zir)

PR1

2400/U-Pb(Zir)

L. Topozero

L. Pyaozero

AR2

AR2

AR2

2395

L. ChupaNappe

2400/U-Pb(Zir)

AR22446/U-Pb(Zir)

2441/U-Pb(Zir)

2442/U-Pb(Bd)

Ts

Lk

AR2

Ni

Tl

Zh

PR1

2400–2500

2440/U-Pb(Zir)

2423/U-Pb(Zir)2451/U-Pb(Zir)

2405/U-Pb(Zir)Kl

2467/U-Pb(Zir)

Kl

2450–2462/U-Pb(Zir)

2350–2217–1948/Rb-Sr

2329/tDM

2452/tDM

2380/tDM

PR1 2352/tDM



2450/U-Pb(Zir)2440/U-Pb(Zir)2436/U-Pb(Zir)2423/Pb-Pb(Zir)

íp

Kv

2415/U-Pb(Zir)

AR2

1Vt

2 3

4 5 6

‡ b

2443/U-Pb(Zir)

La

Kh

216

GEOTECTONICS Vol. 41 No. 3 2007

KOLODYAZHNY

Fig

. 4. S

truc

tura

l–ki

nem

atic

ass

embl

ages

of t

he S

elet

sta

ge. (

A) I

nteg

ral d

iagr

am il

lust

ratin

g lo

caliz

atio

n of

syn

kine

mat

ic in

trus

ions

(2.4

5–2.

35 G

a), (

B–E

) occ

urre

nce

and

stru

ctur

eof

intr

usio

ns o

f th

e Se

let s

tage

in th

e ar

eas

of (

B)

Khe

tola

mbi

na, (

C)

Lyag

kom

ina,

(D

) N

il’m

ogub

a, a

nd (

E)

Cap

e To

lstik

. See

Fig

. 3 f

or a

rea

loca

tion.

(B

) G

eolo

gica

l str

uctu

ral

sect

ion

(pho

topa

nora

ma)

of

the

east

ern

wal

l of

an a

band

oned

qua

rry.

See

text

for

exp

lana

tion.

(1)

Ban

ding

in g

neis

s; (

2) lo

w-a

ngle

faul

t; (3

) zo

nes

of f

olia

tion

and

C–S

-str

uctu

res;

(4)

dire

ctio

n of

dis

plac

emen

t at t

he (

a) S

elet

and

(b,

c)

Svec

ofen

nian

sta

ges,

acc

ordi

ng to

firs

t- a

nd s

econ

d-ge

nera

tion

stru

ctur

es o

f dy

nam

odia

phto

rite

s ge

nera

tions

, res

pect

ivel

y;(5

) di

rect

ion

of r

otat

ion

of s

truc

tura

l uni

ts a

t the

Sel

et s

tage

. (C

) Ph

otog

raph

of

a le

ntic

ular

gab

bron

orite

bod

y in

the

hing

e of

the

Sele

t fol

d w

ith a

hor

izon

tal a

xial

pla

ne. (

D)

Blo

ckdi

agra

m il

lust

ratin

g th

e oc

curr

ence

of

gabb

rono

rite

in th

e de

lam

inat

ed h

inge

of

the

Sele

t ant

iclin

e (F

Sl)

dist

urbe

d by

Sve

cofe

nnia

n th

rust

fau

lts a

nd f

olds

(F S

v). (

1) B

iotit

e gn

eiss

,(2

) ga

bbro

nori

te, (

3a)

thru

st f

aults

and

(3b

) zo

nes

of f

olia

tion

and

C–S

str

uctu

res,

(4)

mus

covi

te p

egm

atite

, (5)

tra

ck o

f th

e ax

ial

surf

ace

of t

he S

elet

ant

iclin

e; (

6) d

irec

tion

ofdi

spla

cem

ents

at t

he (

a) S

elet

and

(b)

ear

ly S

veco

fenn

ian

stag

es, (

7) d

irec

tion

of r

otat

ion.

(E

) B

lock

dia

gram

illu

stra

ting

rela

tions

hips

of

stru

ctur

al a

ssem

blag

es o

f th

e Se

let a

ndSv

ecof

enni

an s

tage

s in

Pal

eopr

oter

ozoi

c m

igm

atite

-gra

nite

s, s

ee t

ext

for

expl

anat

ion.

Str

uctu

ral

elem

ents

(le

tters

in

figur

e):

F Sl a

nd F

Sv a

re f

olds

of

the

Sele

t an

d Sv

ecof

enni

anst

ages

, res

pect

ivel

y; L

Sl a

nd L

Sv a

re li

neat

ions

of

the

Sele

t and

Sve

cofe

nnia

n st

ages

, res

pect

ivel

y.

0.5

m

C

D

E

ÄB

125°

∠75

°

100°

∠45

°

340°

250

mN

S

12

34

56

7

12

34

5a

bc

10 m

10 c

m31

0°

L Sv

L Sl

ë–S

SlL

euco

som

e of

mig

mat

itean

d m

icro

clin

e ag

greg

ates

Gab

bron

orite

Gab

bron

orite

FSl

FSl

FSv

Pegm

atite

Ban

ding

100°

∠40

°

100°

∠30

°B

t–A

mph

gne

iss

Zon

esof

fol

iatio

nG

abbr

onor

iteB

t gne

iss

FSl

FSl

Bou

dina

ge st

ruct

ure

Low

-ang

le n

orm

al fa

ults

Mag

mat

ic

dupl

exes

Fol

d of

long

itudi

nal

flow

(F

Sl)

with

B-l

inea

tion

Dir

ectio

nof

tect

onic

tran

spor

tSt

ruct

ure

of h

inge

dela

min

atio

n

Dru

site

bodi

es

Lin

eatio

nof

tran

spor

tL S

l

Gra

nite

and

th

read

like

mig

mat

ite

FSl

ab

ba

Gabb

rono

rite



In many cases, the sill-like drusite intrusions arecontrolled by the low-angle Selet normal faults thatinherit the surfaces of the Reboly thrust faults (Fig. 4B).In the wall of a quarry shown in Fig. 4B, one can seetwo closely spaced gabbronorite sills that mark listricfaults. In the central portions of these intrusive bodies,the mafic rocks undergo only slight alteration and retainophitic and drusitic textures, whereas in contact zonesthey become strongly foliated. The stratal body ofbiotite gneiss sandwiched between the sills experienceda thermal effect and became partly recrystallized andamphibolized. The foliation of gneiss that developedsynchronously with the emplacement of drusites indi-cates the normal faulting (Fig. 4B).

The drusite intrusions are often located in hinges offolds with vertical and horizontal axial planes(Figs. 4A, 4C). A typical exposure was studied near thesettlement of Nil’moguba. A series of isolated lherzo-lite and gabbronorite bodies are clustered here asextended chains. Initially, this was a common linearbody related to the hinge of the Selet fold (Fig. 4D).The systems of low-angle normal faults developed atthe late magmatic stage and were marked by narrowzones of migmatization provided a certain fragmentationof this body. The Svecofennian deformations wereexpressed in thrusting and the associated overturnedfolds. The thrusting developed along the initial strike ofthe intrusive body and resulted in further fragmentationand formation of asymmetric transverse folds. In manycases, such structural transformation led to the frag-mentation of drusite intrusions into the chains thatextend for many kilometers.

Granites associated with drusites are characterizedby formation of aggregates of potassium feldspar thatreveal synkinematic B-lineation oriented along the axesof the Selet folds (Figs. 4A, 4E). Granitic rocks gradeinto the thread-like migmatites with a similar lineation.As follows from observations, the linear structure ofgranitic rocks is related to their synkinematic crystalli-zation and autometamorphism under conditions of duc-tile flow in the near horizontal plane, when the exten-sional lineation is oriented in the direction of tectonictransport.

The available data show that the igneous rocks of theSelet stage were formed at a depth of more than 20 kmunder conditions of the near-horizontal ductile flow.The paths of the rock transport are recorded in the ori-entation of B-lineation and linear zones of folding thatstrike in the northeastern and near-meridional direc-tions (Fig. 3).

Structural–kinematic assemblages of the Sve-cofennian stage were formed in the tectonically aniso-tropic medium created at the preceding stages andpartly inherited in the newly formed structural features.These circumstances make it quite difficult to identifythe high-rank folds and faults on the basis of their geo-metric characteristics. At the same time, the Svecofen-nian transformations on the meso- and microscales are

expressed contrastingly. Their main indicator is the for-mation of tectonites, i.e., the products of retrogradedynamometamorphism.

In a generalized form, the Svecofennian structuralelements as kinematic indicators are shown in Fig. 5A.They make up an assemblage consisting of small-scaleasymmetric and sheath folds, asymmetric boudins,duplexes of compression and extension, and asymmet-ric veins. C–S structures, mineral lineation, δ- andσ-shaped porphyroblasts, en echelon arranged orienta-tions of mineral aggregates, and various rotation struc-tures are elements of this assemblage. To reveal thedirection of movements, these structural elements werestudied in different sections of hand specimens and out-crops. The particular displacements established in thisway were used further for reconstruction of the result-ant vector of local tectonic transport. In the case of low-angle structural elements, the obtained vector indicatesthe direction of displacement of the hanging lithon(block) relative to the footwall.

The C–S structures are immediately related to theretrograde metamorphism of the Svecofennian stageand composed of mineral assemblages that are definedas Svecofennian dynamodiaphtorites (Fig. 5B). Thephyllosilicates (Bt, Mu, Chl) are localized alongmicroshears C, thereby marking sigmoid bent surfaces S,which show a shear component. The retrograde charac-ter of these assemblages is expressed in the consecutivereplacements: dark brown Bt green Bt Chl +Mu + Ru, Grt Bt + Qtz Chl + Bi + Qtz, andHbl Act + Chl. Blastocataclasites with C–S struc-tures that characterize brittle–ductile stage of deforma-tion are often formed at the late stages of retrogrademetamorphism. The crosscutting relations allow recog-nition of several generations of C–S structures. Thetiming of C–S structures is based on their relationshipswith muscovite pegmatites (1.90–1.85 Ga) that revealindications of multistage formation. The early genera-tions of veins (pegmatite 1) bear distinct C–S struc-tures, whereas the late generations (pegmatite 2) arealmost completely devoid of these structures and cross-cut the early veins (Fig. 5C). Thus, C–S-structures arenearly coeval with muscovite pegmatites. Furthermore,these structures are superimposed on igneous rocks ofthe Selet stage (2.45–2.35 Ga) (Fig. 4B, 4E). Pressureshadows and clastic rims of porphyroblasts likewise areoften composed of minerals related to the retrogradestage, so that the structural elements formed by theseminerals are clear kinematic indicators (Fig. 5D). Otherstructures of this assemblage are identified by their spa-tial relations to the Svecofennian dynamodiaphtorites.For example, the zones of the Svecofennian blastomy-lonites (dynamodiaphtorites) are often conjugated withasymmetric and sheath folds that reveal kinematic com-monness with C–S structures (Figs. 5A, 5E). The zonesof the Svecofennian blastomylonites often demonstratethe retrograde sequence of ductile to ductile–brittle andbrittle deformations and the respective sequence ofmineral assemblages that are indicative of different

218


KOLODYAZHNY

10 cm

CB

D

Ä

E

F

S N

S

N

10 cm

10 cm2 cm

1 cm

S N

Blastomylonite

Blastomylonite zone

Massivepegmatite 2

Pegmatoid 1(C-S-structures)

Pl

Grt

Bt–Chl

Pl

Bt–Chl Ep

Resultant vectorof tectonictransport

ëS

C–S structures:

5 4

32

1

Asymmetricfaults F

En echelonarranged mineralaggregates

Crescent veins and stages of theirdevelopment1–5

Asymmetricboudinagestructures withreverse rotation

Dominoesstructures (extensionduplexes)

Kink zones – Kz

Compression

Riedel shears R

Fish-shaped schistosityof blastomylonite

R'-shearsVeins

Hinges of sheath folds – Ç

Mineral lineation (elongation) – L

Thrust amplitudein projectionon vertical plane

σ- Ë δ-shapedstructures of porphyroblasts

Harpoon-shapedand en echelonarranged veins

Bridgestructures

Sheath andtelescoped folds – F(Oblique sections)

duplexes

Fig. 5. Structural–kinematic assemblages of the Svecofennian stage. (A) Integral scheme (view in oblique sections), (B–F) photo-graphs. (B) C–S structures in biotite–muscovite gneiss of the Chupa Sequence. (C) Svecofennian synkinematic pegmatoids (1) andpostkinematic pegmatites (2) make up a common vein. (D) δ- and σ-shaped structures of garnet porphyroblasts in amphibolite ofthe Kandalaksha Sequence. (E) Asymmetric recumbent fold in blastomylonite zone of the Kandalaksha Sequence. (F) Kinematicinversion in blastomylonite zone: older C–S structures of the stage of ductile deformation are changed by younger structures of brit-tle–ductile deformation in the course of formation of asymmetric boudins in gneiss of the Chupa Sequence. Black and white arrowsindicate directions of older and younger displacements, respectively. See text for explanation.

Qtz



stages of retrograde metamorphism. As is judged fromthe structures related to specific tectonites, the kine-matic inversion is not a rare phenomenon (Fig. 5F).

The considered structural and metamorphic assem-blage developed in all Archean and Paleoproterozoiccomplexes of the Belomorian–Lapland Belt, so that thekinematics of tectonic processes in this province maybe reconstructed on the basis of the observations thatcover wide areas.

STRUCTURE OF DIFFERENT SEGMENTS OF THE BELOMORIAN–LAPLAND BELT

The Belomorian–Lapland Belt reveals transverseand longitudinal structural and compositional zoningand consists of several segments with specific tectonicfeatures. The Lapland–Kolvitsa Granulite Belt consistsof the Lapland and Kolvitsa–Umba segments (particu-lar belts), while the Belomorian Belt comprises theYena, Seryak–Kovdozero, Chupa, and Engozero seg-ments (Fig. 2).

The Kolvitsa–Umba Belt is well studied withrespect to its geology [4, 10, 15, 32, 41, 46, 52]. Thebelt is composed largely of Paleoproterozoic com-plexes separated by a basal detachment from gneiss ofthe Belomorian Group (Fig. 6). These complexes forma system of tectonic sheets that form the following ver-tical succession (from bottom to top): (1) garnet andmonomineral amphibolites of the Tanaelv–Kandalak-sha Belt (2.50–2.46 Ga) that correspond to tholeiiticbasalt and basaltic andesite in chemical composition,(2) a sheetlike body of tectonized gabbroanorthosite ofthe Kolvitsa massif (2.45–2.46 Ga), (3) two-pyroxeneand garnet–pyroxene basic granulites of the Por’ya GubaGroup (metatholeiite and metaandesite (2.5–2.4 Ga)),(4) a zone of tectonic melange that consists of frag-ments belonging to the adjacent complexes, and (5) fel-sic garnet–sillimanite granulites of the Umba Complex(metaterrigenous rocks deposited no earlier than 2.1 Gaago) [52]. The felsic granulites of the Umba Complexare cut through by granitoid intrusions of the Umba plu-tonic complex that consists of three intrusive phases:enderbite (1944 Ma), charnockite (1912 Ma), and lateporphyritic granite [15, 52]. The base of the tectonicsheet of the Umba granulites (melange zone) bearsindications of isobaric cooling, while the central por-tion of this sheet underwent decompression and cool-ing. These settings coexisted at the early stage of Sve-cofennian collision (1.95–1.91 Ga) [1, 41, 52]. Anatec-tic melting and dynamometamorphism are somewhatyounger processes.

The Svecofennian mega- and mesoscopic struc-tural–kinematic assemblages are systems of large tec-tonic nappes and imbricate thrust sheets that have ahorseshoelike form and demonstrate a festooned gen-eral structural pattern in plan view (Fig. 6). The tectonicsheets make up the separate Kolvitsa and Umba syn-forms and gently plunge in the eastern bearings. Their

vertical succession is characterized by inverse meta-morphic zoning and blastomylonitic and melangezones at the centroclinal closures, which are accompa-nied by thrust faulting, whereas strike-slip faults arelocalized at synform limbs (Fig. 6). The structures oftectonic pumping (systems of fold–thrust hummock-ing) oriented transversely to the general trend of thebelt developed in frontal horseshoe-shaped thrust-faultzones. Several generations of folds are recognized; therecumbent and overturned folds grading into thrustfaults are the oldest (Fig. 5E). The younger fold sys-tems have various orientations and exhibit cylindricaland conical morphology (Fig. 6, II). The hinges ofsmall folds and mineral lineation disperse along the arcof a great circle, thus indicating their rotation around anear-vertical axis (Fig. 6, I, III).

The Svecofennian meso- and microscopic struc-tural–kinematic assemblages make up a successiveretrograde series: (1) structures of bedding-plane andpervasive ductile deformation that are synchronouswith amphibolite- and granulite-facies metamorphism,(2) structures of ductile deformation that are related tothe formation of the first-generation dynamodiaph-torites that took place under conditions of epidote-amphibolite facies, and (3) structures of brittle–ductiledeformation that contain second-generation dynamodi-aphtorites that formed under conditions of greenschistfacies. The first structures are little informative in kine-matic terms because they are devoid of symmetry andstrongly obscured by subsequent deformations. Thesecond and third structural elements commonly havedistinct monoclinic symmetry and may be regarded asthe respective generations of the structural–kinematicassemblages. The first-generation pervasive assem-blages comprise C–S structures, structures resultingfrom rotation of porphyroblasts, en echelon arrangedmineral aggregates, transport lineation, etc. (Fig. 5A).The relationships of these structures with granitoids ofthe Umba Complex indicate that these structuresformed at the late magmatic stage of charnockite crys-tallization (1912 Ma) before emplacement of porphy-ritic granite. The second-generation structures devel-oped locally and are superimposed on the youngergranites.

The Kolvitsa Synform strikes in the latitudinaldirection and closes near the town of Kandalaksha,while slightly widening owing to the gradual plungingof its hinge to the east. In the section, this is an uprightsymmetric fold consisting of tectonic sheets and slices(Fig. 7A). At the northern limb of the synform, the mostrepresentative section is exposed in the rocky valley ofLake Sredny Luven’ga (Fig. 7B), where the stronglytectonized gabbroanorthosite of the Kolvitsa massiftransformed into blastomylonite is thrust over amphib-olite of the Kandalaksha Group. The first-generationstructures crosscut and deform structural elements ofthe older tectonic and metamorphic delamination: thethin lenticular banding expressed in alternation of pla-gioclase and garnet–pyroxene–amphibole laminas,

220


KOLODYAZHNY

Fig

. 6. S

truc

tura

l–ki

nem

atic

ske

tch

map

of

the

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a–U

mba

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led

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r [4

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tes;

(17

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rect

ions

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thru

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g, (

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ip f

aulti

ng, a

nd (

c) r

otat

ion

at t

he S

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ong

zone

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sec

ond-

gene

ratio

n dy

nam

odia

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rite

s. S

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tura

l un

its (

lette

rs i

n fig

ure)

: K

U,

Kol

vits

a–U

mba

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t; K

l an

d U

m,

Kol

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orm

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, (II

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les

of t

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mea

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are

spa

ced

at 5

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%),

(II

I) a

ggre

gativ

em

iner

al li

neat

ion.

See

Fig

. 5A

for

the

lette

rs o

n pr

ojec

tions

.

A

B

C

D

0km

N S4

8SKK

U

Kl

KU

Um

Um

KU

Um

bapl

uton

K an d a

l a ks h a

B ay

SK

SK

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NN

N

Ç Ç

L

Ç

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F

Kol

vits

aSy

nfor

m

Um

baSy

nfor

m

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15

16 17a

bc

ab

c

ab

ab

ab

L. Ko

lvitsa



1 cm

B

Ä

C

1 cm2 cm

10 m

4 km~2

km

N S

N S

NE

30° 10 cm

1 cm

50 m1 2 3 4 5 6

1 2 3 4 5 6 7 8 9a ba ba b

C–S2

C–S1 C–S2C–S1

Asymmetricmicroboudins

σ-shaped garnet porphyroclasts

Dominoes

10°

15°10°

15°

15°

10°

10°

20°40°

A B

Kolvitsa SyformMt. Okat’ev Umba Synform

δ- and σ-shapedgarnet porphyroclasts

Leucosomeof migmatite

Amph

C–S1

C–S1

Biotite gneiss

Telescopedand sheath folds

structure

a b

Fig. 7. (A) Sketch geological section of the Kolvitsa Synform along line AB (see Fig. 6A for line location and legend) and geologicalstructural sections of (B) northern and (C) southern limbs of the Kolvitsa Synform. Panel B: (1) thin-banded garnet-bearing blasto-mylonites after gabbroanorthosite, (2) lenticular banded garnet blastomylonite after gabbroanorthosite, (3) zones of tectonic dislen-sing and melange, (4) metapyroxenite and metagabbroid rocks, (5) intermediate granulite affected by retrograde metamorphism,(6a) faults and (6b) foliation zones, (7) vectors of tectonic transport based on measurements of Svecofennian dynamodiaphtoritestructures of (a) first and (b) second generations, (8) directions of strike-slip based on measurements of Svecofennian dynamodi-aphtorite structures of (a) first and (b) second generations, (9) orientation of (a) fold hinges and (b) aggregative mineral lineation.Panel C: (1) biotite gneiss of the Belomorian Group, (2) garnet amphibolite of the Kandalaksha Sequence, (3) biotite–amphibolegneiss of the Kandalaksha Sequence, (4) zone of tectonic melange, (5) zone of intense migmatization, (6) faults and direction ofdisplacement.

222


KOLODYAZHNY

migmatite veinlets, and anatectic lenses. The recum-bent and asymmetric folds are overturned in the north-ern bearings (Fig. 7B). The planar first-generationstructures occasionally develop along the older bandingbut more frequently cross the banding at different angles,in particular, in hinges of folds. These thin (1–2 mm)C–S structures of blastomylonites (dynamodiaph-torites) pervaded throughout the rocks and accompa-nied by the deformation and replacement of older gar-net porphyroblasts and pyroxene–amphibole aggre-gates as well as by the recrystallization of plagioclase.The mineral assemblages of epidote-amphibolite faciesoriented in line with the morphology of C–S structuresdeveloped as a result of retrograde dynamometamor-phism. The deformed garnet grains are transformed inσ- and δ-shaped porphyroclasts rimmed by clastic tails(Fig. 7B, close-ups). The older porphyroblasts oftenpull apart and make up a pencil-type lineation, which insome sections looks like a dominoes microstructure(Fig. 7B, close-ups). The segregation laminas that makeup the early banding are often transformed into asym-metric microboudins. The pencil-type lineation oftransport develops as retrograde chlorite–amphiboleaggregates. The study of kinematic indicators in differ-ent sections of outcrops allowed reconstruction of dis-placement directions of the hanging lithons that corre-spond to the obliquely oriented paths of tectonic flow inthe northwestern direction (Figs. 6, 7B). Some zonesreveal a predominance of right-lateral displacementsalong with poorly developed thrust kinematics. Thesecond-generation structural forms are local, thin cata-clastic zones with relatively rough C–S structuresaccentuated by stringers of the youngest minerals ofretrograde metamorphism (Ep, Chl, Qtz, Ab, Cal),which cut the older C–S structures at various angles andpartly rearrange them (Fig. 7B, close-ups). The recon-struction of displacement vectors pertaining to this gen-eration of structures indicates the eastward tectonictransport; the left-lateral dislocations are noted as well.

Quite another kinematic situation is noted at thesouthern limb of the Kolvitsa Synform. Along thesouthern slope of Mount Okat’ev, biotite gneiss of theBelomorian Group is overthrust by amphibolite of theKandalaksha Group (Fig. 7C). The planar and linearstructures of the first-generation kinematic assemblagehave the same orientation in both sequences. A slightdisharmony is revealed in the character of folding,which is more intense and complex in the BelomorianGneiss and less distinct in amphibolites. The foldedmigmatite veinlets are seen in biotite gneiss. As a rule,these are small asymmetric and recumbent folds–bou-dins; fragments of hinges; and sheath and telescopedfolds, whose hinges are parallel to the mineral aggrega-tive lineation (Fig. 7C, block diagram). The Belomo-rian Gneiss is pervaded by a conformable system offirst-generation C–S structures (Fig. 7C, block dia-gram). These structural elements consist of sigmoidaggregates of plagioclase and quartz and flakes of chlo-ritized biotite. The vectors of tectonic transport

obtained for these structural features coincide in orien-tation with lineation and axes of sheath folds. Theintensity of migmatization in gneiss gradually increasestoward its thrust-fault contact with the KandalakshaGroup. The contact is accompanied by a foliated zoneof tectonic melange up to 50 m thick. The lenses ofbiotite and biotite–amphibole gneisses are incorporatedinto the matrix of amphibole–biotite blastomylonites;boudinage structures, small asymmetric folds, and nar-row migmatite zones are observed. The structural–kinematic situation is similar to that in the underlyingcomplexes. The overlying garnet amphibolite andamphibole gneiss make up a gently dipping homocline.The metamorphic banding and schistosity are conform-ably pervaded by C–S structures of first-generationdynamodiaphtorites. The en echelon arranged lines ofrecrystallized hornblende grains replaced with low-temperature amphibole and chlorite (Fig. 7C, close-up)are widespread. Small garnet porphyroclasts affectedby retrograde metamorphism are accompanied by pres-sure shadows and δ- and σ-shaped clastic tails. In gen-eral, the first-generation kinematic assemblages indi-cate thrusting in the WSW direction; this situation issubstantially different from the kinematic situation atthe northern limb of the Kolvitsa Synform (Fig. 6).The poorly developed subsequent deformations haveremained unestimated. A similar situation wasobserved along almost the entire southern limb of theKolvitsa Synform.

Based on the consideration of structural–kinematicassemblages, the vectors of tectonic movements in dif-ferent parts of the Kolvitsa and Umba synforms havebeen obtained and plotted on the structural geologicalscheme (Fig. 6). At the limbs of synforms, the vectorsof tectonic transport are divergent and symmetric rela-tive to the synform axes and characteristic of obliquestrike-slip and thrust displacements partly resemblingthe displacements that develop during formation of thetranspressional palm-tree structures. The first-genera-tion structures indicate NW-directed general tectonictransport and the resultant formation of telescoped sys-tems of tectonic sheets. Thus, the indications oftranspressional setting (divergent squeezing-out ofrocks toward the limbs of synforms) are combined withstill more distinct kinematic attributes of longitudinaltectonic flow and thrusting. This circumstance makes itpossible to consider the Kolvitsa–Umba Belt as a wholeas a near horizontal protrusion that formed undertranspression and underwent transport to the uppercrust as a system of telescoped thrust sheets verginglargely to the northwest. At the same time, the structureof first-generation dynamodiaphtorites in the uppersheet of the Umba granulites in the east of the UmbaSynform is related to the relative displacement in theopposite direction relative to the underlying nappes(Fig. 6). Hence, the under- and overthrusting developednonuniformly and the sheet of the Umba granulitemoved toward the surface at a lower velocity relative tothat of the underlying complexes. A similar tendency



was recorded in the second-generation structural–kine-matic assemblages. The overall northwestward tectonictransport was in progress, but the belt was doubled. Theregion of pumping was separated in the frontal zone asthe Kolvitsa Synform with structural features of lateralsqueezing of rock masses under the effect of wedgingprovided by protrusion of the Umba Synform (Fig. 6).

The central Belomorian Belt consists of the Ser-yak–Kovdozero, Chupa, and Engozero segments.

The Seryak–Kovdozero segment is situated alongthe southwestern coast of Kandalaksha Bay andembraces the drainage basins of lakes Kovdozero andSeryak. In the northwest, this segment is bounded bythe Kovdozero Strike-Slip Fault, while in the southeast,it is built on by the Chupa segment, which is separatedfrom the Seryak–Kovdozero segment by a near-latitu-dinal fault zone (Fig. 2). The results of the previousdetailed geological and structural investigations in thisdistrict were published in [4, 12, 21–25]. The Belomo-rian complexes make up the Neoarchean Kovdozero,Chupa, Khetolambina, and Keret nappes (Fig. 6). In thecentral part of the Seryak–Kovdozero segment, thesenappes are complicated by the Seryak Antiform, whosehinge gently plunges to the northwest. In the cross sec-tion, the antiform looks like an asymmetric isoclinalfold overturned to the southwest (Figs. 6, 8A). The foldabruptly widens southeastward, and its axial surface islost within the conformable transpressional shear zonethat is traced along the entire axis of the antiform. Therocks that compose the antiform are severely deformed

as a result of numerous low-angle detachments thateither are folded conformably to the general structureof the antiform or discordantly cut this structure(Fig. 8B). As is judged from the development of tecto-nites, the discordant detachments are systems of theSvecofennian thrust faults. The surfaces of detach-ments deformed into folds are often healed by drusiteintrusions and most likely are of the Selet age. Severalgenerations of C–S structures are observed in zones ofthe Svecofennian dislocations providing evidence forthe kinematic inversion: the older thrusting giving wayto the younger normal faulting (Fig. 8B, close-up).These observations lead to the suggestion that the Ser-yak Antiform is Svecofennian in age; however, it can-not be ruled out that this structural element began toform during the Selet stage.

A system of higher order tight folds that complicatesthe limbs of a large gentle synform is observed to thenortheast of the Seryak Antiform. This region isseverely imbricated as a result of the development ofhorseshoelike and festooned (in plan view) thrustsheets oriented across the northwestern trend of folds(Fig. 6). The thrust faults cut off the folds and, at thesame time, participate in younger folding. The study ofthrust faults in outcrops has shown that their surfacesactually are complexly built imbricate zones accompa-nied by Svecofennian dynamodiaphtorites with clearlyexpressed C–S structures (Fig. 8C). Some fault surfacesof older generations are deformed into folds and discor-dantly cut only by slightly deformed younger thrust

B C

A

Thrust fault 1Thrust fault 2

C–S1

C–S1C–S2

C–S1

S0

Grt

–Bt–

Mu

gnei

ss

NWMuscovite pegmatite

C D6 km

~6 km

WSW ENE

2 m

SeryakAntiform

6 m50 °

S0: 320°∠8°B: 315°∠10°

1 2 3 4 5

6 7 8a b c

1 cm

Fig. 8. (A) Sketch geological section of the Seryak Antiform along line CD (see Fig. 6A for line location and legend); (B) structureof the hinge of the Seryak Anticline near the settlement of Lyagkomina (Fig. 3), in section view; and (C) block diagram illustratingrelationships of low-angle faults of different generations with folding. (1) Biotite–amphibole and biotite gneisses, (2) garnet–biotite–muscovite gneiss, (3) feldspathized gneiss, (4) Paleoproterozoic gabbronorite, (5) pyroxenite, (6) Riphean or Paleozoic (?)lamproite, (7) direction of displacements at the (a) Selet and (b, c) Svecofennian stages as determined from structures of dynamo-diaphtorites of the first (b) and second (c) generations, (8) fault. In B: thrust fault 1 is deformed into upright folds, whereas fault 2cuts off older structural elements and is transformed afterward into a low-angle normal fault.

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KOLODYAZHNY

zones as a result of multiple thrusting and folding oflongitudinal flow with the fold axes oriented in thedirection of tectonic transport. The same structuralassemblage of festooned thrust faults and folds is trace-able to the north within a wide tract that gradually turnsto the northeast and conjugates beneath KandalakshaBay with the Kolvitsa–Umba Belt, similar in structure inmany respects (Fig. 6).

These data show that this segment was affected bythe front of the Kolvitsa–Umba protrusion and may beregarded as a continuation of the system of the tele-scoped Svecofennian nappes of the Kolvitsa Synformthat are spread over the Archean complexes of the Belo-morides (Fig. 6). At the early stages, these dislocationsdeveloped as thrust faults and afterward became partlyreactivated as normal faults, as is recorded in the struc-ture of tectonites.

The Chupa segment of the Belomorian Belt is sit-uated between the Seryak–Kovdozero and Engozerosegments at the flank of the Kolvitsa–Umba protrusion.This region differs from the adjacent territories largelyin the style and spatial orientation of the Paleoprotero-zoic structural assemblages that mainly strike in thenear-latitudinal direction (Fig. 9). The Chupa segmentis composed of the rocks belonging to the Kovdozero,Chupa, Khetolambina, and Keret complexes that makeup a system of overturned tectonic sheets in the hangingwall of the large recumbent synform [21] (Fig. 10A).

The substantial role of the Paleoproterozoic tectonicand metamorphic processes in the formation of thestructural assemblages of this territory has been pointedout in many publications [11, 13, 14, 33, 34, 39, 40].The Svecofennian transformation resulted in theappearance of wide fields of dynamodiaphtorites thatcontrol localization of muscovite pegmatites. The near-latitudinal right-lateral and more frequent left-lateralstrike-slip fault zones are marked by dynamodiaph-torites and metasomatic rocks of the Svecofennian age.The conjugate thrust and low-angle normal faults,which are located between these zones, often inherit thesurfaces of the Reboly nappes. These low-angle faultscommonly cut the folds and, in turn, participated in theyounger folding.

The Svecofennian thrust faults are often accompa-nied by tectonic melange zones up to a few meters inthickness (Fig. 10B) that consist of lenses and boudinsdiverse in lithology, fragments of detached fold hinges,sheets of discordantly folded gneisses, blastomylonites,dynamodiaphtorites, and metasomatic rocks of variouscompositions. The dynamodiaphtorites, greisen-likemetasomatic rocks, and related lenses of muscovitepegmatoids indicate that these structural elements areSvecofennian in age. The asymmetric shape, rotationstructures of garnet and feldspar porphyroblasts, andC–S structures are consistent with the thrust kinematicsof these zones. Relict fragments of the older metamor-phic banding deformed into the folds with horizontal

Fig. 9. Structural–kinematic model of the Chupa segment. Compiled after [13, 14, 21]. (1, 2) Neoarchean basement rocks of theKarelian Massif: (1) granite gneiss, (2) greenstone complex. See Fig. 6 for other symbols.

N

S

0 2 4 6 8 km

A B

1

2

WH

I TE

SE

A

Chupa Guba



axial planes are retained in the strongly tectonizedrocks (Fig. 10B, close-up). The Svecofennian disloca-tions closely pervade the Archean complexes of theBelomorides as lenticular–loop shear and blastomylo-nite zones (Fig. 4B). The dispersed C–S structures thatembrace considerable volumes of rocks develop in theareas adjacent to the zones of thrusting. In the structuralstyle and grade of metamorphism, two generations of

C–S structures are recognized: an older generation ofdynamodiaphtorites that formed under conditions ofepidote-amphibolite facies and younger blastocataclasitesof greenschist facies (Figs. 10C, 10D). Several kinematicpulses expressed in differently oriented C–S structures ofthe older generation are noted. The character of cross-cutting relations and superposition shows that the tec-tonic movements periodically resumed, while follow-

1 m

B C

D

Ä

60°

60°∠5°–10°

C–S1 Relict banding

5 cm NNW

AggregativemineralÇ-lineation

C–S1C–S2

C–S1

W E6 km

~6 k

m

BÄ

10°

0.2 m

Grt–Bt–Mu gneissRelict banding

C–S1

C–S2

C–S1

C

C

1 2 3

Sigmoid structures

1

2

3

4

5

6

Fig. 10. Structural–kinematic assemblages in the Svecofennian thrust fault zones: (A) sketch geological section along line AB (seeFig. 9 for line location and Fig. 6 for legend), (B) section of tectonic melange in zone of the Svecofennian thrust fault, (C) blockdiagram illustrating relationships of C–S structures of different generations, (D) relationships of Svecofennian C–S structures ofvarious kinematic pulses and generations in section view. Panel B: (1) serpentinite, (2) garnet orthoamphibolite affected by retro-grade metamorphism, (3) kyanite–garnet–muscovite gneiss, (4) greisenized porphyroclastic gneiss, (5) muscovite pegmatite,(6) tectonic surfaces and displacements along them. Panel D: (1) leucosome of migmatite, (2, 3) foliation planes of different gen-erations in tectonized gneiss.

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KOLODYAZHNY

ing the weakened surfaces oriented in either direction.The older generation of C–S structures commonlymark thrust and strike-slip displacements. As a rule, thesecond-generation structures are related to the normalfaulting (Figs. 10C, 10D).

In general, the Chupa segment is characterized by asystem of Svecofennian low-angle faulting that pro-gressively developed from thrusting to normal faulting.This system was combined with near-latitudinal strike-slip faults and variously oriented folds. The C–S struc-tures in near-latitudinal steeply dipping fault zonesindicate both right-lateral and left-lateral (more fre-quent) displacements. The eventual structural assem-blage may be defined as being the result of strike-slip–thrust and strike-slip–normal faulting. This assemblageconsists of large sliding domains bounded by strike-slipfaults (Fig. 9). The faulting most likely was controlledby the longitudinal transport of the Kolvitsa–Umbaprotrusion.

The Engozero segment may be regard as a flank ora back region with respect to the Kolvitsa–Umba pro-trusion (Fig. 2). A large vortex structure that formedhere in the Svecofennian time requires further investi-gation. Its evolution likely was caused by rotation ofrock masses around a vertical axis under the effect oflateral longitudinal tectonic flow.

DISCUSSION

The present-day structure of the Belomorian–Lap-land Belt exhibits the final result of the multistagedeformation that reached a peak of intensity at the earlystage of the Svecofennian cycle and gradually wanedafterward. Because of this circumstance, the obtainedkinematic data mainly pertain to the retrograde Sve-cofennian tectonic processes. The older structuralassemblages were obliterated to a great extent andretained only as relicts. The data presented above showthat the evolution of the Belomorian–Lapland Belt is aseries of three consecutive tectonic and metamorphicevents related to the Reboly stage (2.88–2.53 Ga), Seletstage (2.45–2.35 Ga), and Svecofennian stage (1.94–1.75 Ga).

The Reboly stage of the evolution of the Belo-morides in the Neoarchean was considered in [12, 21,24, 25, 36] in terms of the subduction–collision modelof the formation of the Belomorian Belt. As followsfrom the above-cited publications, a collisional orogenwas formed by the end of Archean and the Belomoriancomplexes proper were localized at its base.

The Selet stage was manifested in extensive occur-rence of Paleoproterozoic bimodal igneous complexes(2.50–2.35 Ma) that characterize the setting of epicon-tinental rifting. The Belomorian complexes thatoccurred at that time in the middle and lower crustunder a pressure of 8–12 kbar were pervaded by dis-persed drusite and granite intrusions emplaced undersynkinematic conditions throughout the crust. It is sug-

gested that these processes were related to the ascent ofa mantle diapir [35, 37, 40]. The structural assemblagesof the Selet stage comprise low-angle normal faults andzones of near-horizontal flow, folds of longitudinal flowwith vertical and low-angle axial planes, B-type aggre-gative mineral lineation, boudinage structures, andmagmatic duplexes (Fig. 4). Most these structures con-trolled localization of intrusions. The available dataindicate that the igneous rocks of the Selet stage wereformed deeper than 20 km under conditions of near-horizontal ductile flow. The paths of rock motion wererecorded in the orientation of B-lineation and foldzones oriented in the northeastern and near-meridionaldirections (in present-day coordinates) (Fig. 3). It maybe supposed that near-horizontal flow was accompa-nied by tectonic delamination and partial exhumationof deep-seated masses due to extension and slidingdown of the upper crustal sheets along gently dippingnormal faults (model of simple shear [56]). In manyrespects, this scenario is consonant with the ideas statedby Terekhov [40], who considers the simple shear to bethe pivotal mechanism of the evolution of the Belomo-rian–Lapland Belt in the Paleoproterozoic, includingthe Svecofennian stage. However, the data discussedabove constrain the period characterized by the tecton-ics of this style by the Selet time.

The Svecofennian stage of the evolution of thestudy region is characterized most completely in struc-tural and kinematic terms. The results obtained aresummarized in the scheme that demonstrates the direc-tions of tectonic movements of this stage and the char-acter of the dynamic segmentation of the central Belo-morian–Lapland Belt (Fig. 11). The consideration ofthis scheme leads to the following conclusions. TheSvecofennian structural–kinematic assemblages of thegiven region have a complex spatial configuration thatreflects nonuniform, at first glance, chaotic tectonicflows. The analysis of the vector field and general struc-tural pattern makes it possible to explain these flowsand the character of segmentation of the studied area interms of the formation of the near-horizontal Kolvitsa–Umba protrusion. In plan view, this protrusion is mush-room shaped. The back decompression region that con-trols the Umba granitoid pluton, the zone of the mainprotrusion with telescoped thrust systems, the frontalzone of tectonic pumping and indentor action of theprotrusion, and its flank segments with thrust andstrike-slip or rotation–vortex displacements are recog-nized (Fig. 11).

The protrusion was formed as a result of tectonicpushing-out of granulite complexes from the lowercrustal levels to the upper levels in the form of a low-angle ascending tectonic flow accompanied by tectonicand retrograde metamorphic processes in the course ofprogressive exhumation of high-grade metamorphicrocks. The relatively narrow subthrust zones of granu-lite allochthons are the only exceptions. In particular,the clockwise P–T–t paths of the rocks of the Tanaelv–Kandalaksha Belt probably illustrate their underthrust-



0 10 20 km

N

S

C

B

A

Yena segment

Frontal zoneof protrusion

Kolvitsa–Umbaprotrusion

Back zone

Umbapluton

Flank regionof protrusion

Seryak–Kovdozero

segment

North

Karel

ian

Strike

-Slip F

ault Z

one

K a r e l i a n M a s s i f

E n g o z e r oV o r t e xS t r u c t u r e

East K

arelian Strike-Slip

Fault Zone

1

2

3

4

5

6

7

8

9

10

11

a b c

cba

a b

Chupa segment

WH

I TE

S EA

Zone of gneiss dom

es

1 3 42 Protrusion

Flank region

Flank

region

321 Active sheet Active sheet

(subthrust zone of Lapland nappes)

Fig. 11. (A) Structural–kinematic scheme of the central Belomorian–Lapland Belt and Kolvitsa–Umba near-horizontal protrusion(Svecofennian stage); (B) consecutive stages of the formation of the Kolvitsa–Umba near-horizontal protrusion in the process oftectonic telescoping (1–4) in plan view and (C) formation of a packet of tectonic slices in the process of pushing-out of active sheetand normal faulting in its back part (1–3), according to the model developed in [29], in section view. (1–3) Central portion of pro-trusion; (4) frontal zone of protrusion; (5, 6) flank regions of protrusion; (7) Engozero vortex structure; (8) zone of gneiss domes;(9) direction of (a) thrusting, (b) strike-slip faulting, and (c) rotation at the Svecofennian stage based on structures of first-generationdynamodiaphtorites; (10) direction of (a) thrusting, (b) strike-slip faulting, and (c) rotation at the Svecofennian stage based on struc-tures of first-generation dynamodiaphtorites; (11) faults: (a) steeply and (b) gently dipping.

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KOLODYAZHNY

ing and the subsequent emergence. In marginal parts ofgranulite massifs, the underthrusting is confirmed bytheir isobaric cooling.

The central portion of the protrusion has a synformstructure and indications of the divergent (outward)squeezing-out of rock masses, as occurs during the for-mation of the transpressional palm-tree structural ele-ments. Thus, the protrusion likely was formed undertranspressional conditions. At the same time, accordingto the reconstruction, the predominant longitudinal tec-tonic flow was oriented in the northwestern direction andresulted in multifold tectonic telescoping (Fig. 11B). Atthe early stages, the protrusion developed in the form ofa unilateral northwestward flow that gave rise to the for-mation of tectonic slices and sheets that were festoonedand enclosed into one another (Fig. 11B, stages 1–2).As follows from the tectonic doubling of the Kolvitsa–Umba Belt, a region of pumping and tectonic telescop-ing of a higher rank arose at the front of the protrusionas a result of the progressive displacement (Fig. 11B,stages 3–4). While the protrusion continued to wedge-in to the northwest, the rock masses tectonically spreadfrom the frontal zone of pumping (Fig. 11B, stage 4).The uppermost tectonic sheet of the Umba granulites inthe back zone of protrusion moved with a lower veloc-ity relative to that of the underlying complexes. As aresult, a system of gentle dipping normal faults devel-oped in the back zone under the conditions of decom-pression that promoted emplacement of the Umba gra-nitic pluton.

As was mentioned above, the variation of U–Pbtitanite ages corresponds to the following succession oftransportation of high-grade metamorphic complexesinto the upper crust: (1) Lapland–Kolvitsa granulitesand complexes of the (2) central and (3) marginal partsof the Belomorian Complex. Thus, an anomalous suc-cession of pushed-out tectonic sheets (from upper tolower) is suggested for the formation of protrusion.This succession is consistent with the structural–kine-matic data that show the progressive superposition ofnormal faults upon the older thrust faults. In line withthis evidence, the development of the low-angle faultsof the Belomorian–Lapland Belt may be interpreted asfollows. The thrusting started in the axial zone of pro-trusion and progressively spread over its frontal zoneand flanks; as a result, the activated fold–thrust regiongradually expanded. In the course of pushing-out, themetamorphic rocks reached the upper crust and under-went decompression and cooling (Fig. 11C, stage 1).The next generation of thrust sheets was squeezed outfrom under the older sheets and also reached the condi-tions of retrograde metamorphism (Fig. 11C, stage 2).A system of low-angle normal faults conjugated withthrust faults developed in the back zone of the pushed-out active sheet; thereby, the normal faults partly inher-ited the surfaces of the older thrust faults. The develop-ment of normal faults predetermined the tectonic exhu-mation of deep-seated rocks that was due to slidingaway of the upper tectonic sheets. The progressive

development of the thrusting and normal faulting indiametrically opposite directions gave rise to the for-mation of the general dominoes-type structure. As aresult, the tectonic sheets rotated around the horizontalaxis and flattened; the vertical thickness of the tectonicpacket diminished (Fig 11C, stage 3). Such a mecha-nism of pushing-out of tectonic sheets from under thepreviously formed nappe assembly was proposed byMorozov [29] on the basis of experimental data andfield observations.

Many questions concerning the genesis of granulitecomplexes and the geodynamic settings that precededtheir exhumation, as well as the causes of tectonicsqueezing of these metamorphic complexes toward thesurface, remain beyond the scope of this paper. Theavailable data indicate that the formation of theKolvitsa–Umba protrusion proceeded under conditionsof general transpression that served as a background forthe local longitudinal lateral flow, pumping, compres-sion, and decompression. However, this general state-ment requires further development in terms of the spe-cific geodynamic model.

CONCLUSIONS

(1) The Belomorian–Lapland Belt is a long-livedmobile zone that developed in different geodynamicsettings. Its evolution was related to a series of tectonicand metamorphic events: (1) the Reboly stage that com-prises the subduction (2.88–2.82 Ga) and collision(2.74–2.53 Ga) substages, (2) the Selet stage of the epi-continental rifting (2.45–2.35 Ga), and (3) the Sve-cofennian stage characterized by collision and generaltranspression (1.94–1.75 Ga).

(2) At the Selet stage, the early Paleoproterozoicstructural and metamorphic complexes of the Belomo-rian Belt occurred at the lower and middle levels of thecontinental crust that experienced extension related tosimple shearing. The rifting in the upper crust was com-bined with tectonic delamination and pervasive tectonicflow, structural lineation, folding of longitudinal flow,and low-angle normal faulting that controlled localiza-tion of synkinematic intrusions deeper in the crust.

(3) At the Svecofennian time, the deep-seated meta-morphic complexes of the central Belomorian–LaplandBelt underwent tectonic exhumation under transpres-sional conditions as a result of emplacement of thenear-horizontal Kolvitsa–Umba protrusion. The propa-gation of this protrusion was accompanied by multipletelescoping of tectonic slices and related to thetranspressional squeezing-out of high-grade metamor-phic rocks into the upper crust in the form of a low-angle ascending tectonic flow moving in the northwest-ern direction (in present-day coordinates).

ACKNOWLEDGMENTS

I thank M.G. Leonov, M.V. Mints, Yu.A. Morozov,M.L. Somin, Yu.G. Leonov, and V.V. Travin for theirhelpful consultations. This study was supported by the



Russian Foundation for Basic Research (projectnos. 06-05-64848, 07-05-01158); the Division of EarthSciences, Russian Academy of Sciences (program no. 6);and the Foundation for Support of Russian Science.

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