Download Diapirism, poly deformation and amoeboidal tee tonic patterns in the

GEOLOGINEN TUTKIMUSLAITOS
GEOLOGICAL SURVEY OF FINLAND
TUTKIMUSRAPORTTI N:o 41
REPORT OF INVESTIGA TION No. 41
Villard S. Griffin, Jr.
Diapirism, poly deformation and amoeboidal
tee tonic patterns in the Svecofennidic
area of southwestern Finland
Espoo 1979
GEOLOGINEN TUTKIMUSLAITOS
Tutkimusraportti n:o 41
GEOLOGICAL SURVEY OF FINLAND
Report of Investigation no. 41
Villard S . Griffin, Jr.
DIAPIRISM, POLYDEFORMATION AND AMOEBOIDALTECTONIC PATTERNS
IN THE SVECOFENNIDIC AREA OF SOUTHWESTERN FINLAND
Espoo 1979
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Griflin, V. S., Jr., 1979. Diapirism, polydeformation and amoeboidal tectonic patterns inthe Svecofennidic area
of southwestem Finland. Geological Survey 0/ Finland, Report o/Investigation No 41, 16 pages and 8 figures.
The Proterozoie Svecofennidic bell of southwestern Finland possesses amoeboidal map patterns in the
supracrustal rocks. This irregular macrogeometry is very similar to map patterns of Archean greenstone belts. Granitoid diapirism is cited often as the "orogenie" agent of the latter, and is the probable deformational cause in the
former. Such an agent explains the wide local variation of foliation trends in the supracrustal rocks, conforming to
the curved borders of adjacent kinematic granitoids . The lack of extensive linear deformation patterns seems to
weaken arguments for true orogenie deformation in this part of Finland . The Karelidic bel! in northern and eastern
Finland has an amoeboidal pattern similar to the Svecofennides. Thus it also may have been deformed by a poorly
integrated local type of diapirism during the Svecokarelidic " orogeny" .
Key words : diapirism, tectonic style, deformation, orogeny, Precambrian, Finland .
Villard S. Griffin, Jr., Department o/Chemistry and Geology, Clemson University, Clemson, South Carolina
29631, U.S.A.
ISBN 951-690-107-7
ISSN 0430-5124
Helsinki 1979. Valtion painatuskeskus
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CONTENTS
lntroduetion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Geologie setting . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reeent teetonie studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Map patterns .... . ... . . . ................. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Diapiri sm as a eause of polydeformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Teetonie similarities with Arehean greenstone belts .. . . . . . ... .. .. . . .. . . . .... . ..
Conelusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Aeknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Referenee s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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INTRODUCTION
Geologie setting
The crystalline rocks of Finland first came under the close scrutiny of geologists in the
latter part of the nineteenth century. Many fundamental and original insights into Finnish
rocks and Precambrian terranes in general were contributed by J. J. Sederholm at the turn of
the century. Geologie thinking of the time was captivated by uniformitarianism, and few had
any concept of the true vastness of the Precambrian. Uniformitarianism, rooted firmly in clastic sedimentation and volcanic processes, seemed to answer many questions concerning these
ancient depositional environments. Only with regard to igneous plutonic processes did real
controversy arise.
The rocks of southwest Finland (Fig. I), the Svecofennidic belt, are believed to comprise a suite of eugeosynclinal rocks. This view is based on the large quantities of graywacke
and volcanogenic rocks present in the area. Swedish mappers demonstrated that this zone continues westward to the Caledonian thrusts . No basement rocks have been found in the Svecofennidic belt.
The Karelidie belt occupies large areas in northern Finland and extends to eastern and
southeastem Finland (Fig. I). This area contains known pre-Karelidic basement rocks. The
overlying Karelidie rocks are considered to be miogeosynclinal.
Fig. 1 Generalized tectonic map of the Fennoscandian shield in Finland and Sweden. Structural trends of
supracrustal rocks are indicated by dashed lines, Plutonic rocks are shown in white, and pre-Karelidic basement rocks in black (Modified after Welin 1970 and Simonen 1971) .
6
Much uncertainty has existed about the correlation of these two zones. At the present
time these rocks are believed to have formed contemporaneously, and the deformation and
metamorphism which affected them are collectively termed the " Svecokarelidic orogeny " (Simonen 1971) . Svecokarelidic replaces the older individualized concepts of the "Svecofennidic
orogeny" and the "Karelidic orogeny" (Simonen 1960).
These supracrustal assemblages have been established as Proterozoic or Middle Precambrian, with ages ranging from 1850 m.y. to 1900 m.y . However, some rocks in the Karelidic area are as old as 2000 m.y . (Simonen 1971; Kouvo and Tilton 1966) . The deformation
of the Svecofennidic rocks , along with their metamorphism, are synchronous with a generation of granitoid plutons of granitic to quartz dioritic compositions. The ages range from 1850
to 1750 m.y. (Kouvo and Tilton 1966). Minor gabbro plutons accompany or slightly predate
these synkinematic granitoids.
A similar situation exists in the Karelidic belt. However, remobilized gneiss and granitoids of the pre-Karelidic basement have been involved in the famous 'm antled gneiss doming process (Eskola 1948).
A late kinematic granite formation event followed on the heels of this synkinematic or
"synorogenic" plutonic activity . This potash-rich granite is developed to a marked degree in
southwest Finland . Härme (1965) has cIearly defined the unusual chemical and physical nature
of this granite which produced the cIassical migmatites made famous by the works of Sederholm . This K20-rich granite locally had sufficient viscosity to cause an updoming of the
country rock (Härme 1954; Edelman 1960, Figs. 42 and 43) similar to that caused by the
slightly earlier synkinematic granitoids. Yet, this potassium-rich magma also possessed such
large quantities of volatiles that country rock - both supracrustal and plutonic - was granitized extensively without mechanical disruption (Härme, 1965) . The origin and nature of this
unusual granite have been the subjects of controversy since the famous arteritic-venitic debates
of 1. 1. Sederholm and P. J . Holmqvist (Sederholm 1967, p. 106,209,370-371,420).
Postkinematic intrusions of coarse to medium grained granite plutons of the Onas-Obbnäs types represent the last major plutonic event. These plutons are postkinematic with the Rapakivi being the terminal one.
The "orogeny" affecting both the Svecofennidic and Karelidic belts around 1800 m.y.
has been written about by workers only in vague terms . Often the implication has been that it
was a type of "alpine" orogeny . Kranck (\956) recognized , however, that there are some specific differences between alpine orogenic styles and that of the Precambrian deformation in the
Fennoscandian shield, as weil as in parts of the Canadian shield. Most outstanding in his view
is the lack of long linear tectonic patterns in these shields, which are so prominent in alpinetype deformation belts. Arecent analysis of the Outokumpu region in the Karelidic area interprets the deformation there in terms of an alpinetype orogenesis, however (Huhma 1976).
Recent tectonic studies
Stephansson (1975) has interpreted the corresponding deformation event in Sweden according to the diapirism concept, dramatically illustrated by the experiments of Ramberg
(I 966a; 1966b) . Accordingly, the usually steep isocIinal and irregularly oriented folds both in
Sweden and Finland can be accounted for by crustal disturbances associated with upwelling of
balloon-like plutons. This style of deformation was recognized earlier by Wegmann (1929a;
1929b) in southern finland .
Hopgood et al. (1976), after cIose and critical examinations of numerous fine exposures near Tammisaari , recognize a complex sequence of folding and intrusion . They concIude
that this polyphase sequence of structures and intrusions can be widely applied in the Svecofennidic area. Thus, they imply a more uniform and regional deformation plan than that proposed by Stephansson.
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MAP PATTERNS
Supracrustal volcanic, volcanoclastic and sedimentary belts throughout Finland are outlined in Figure 2. The main sulphide ore belt (Kahma 1973) trending northwestward across
Finland approximately separates the Svecofennidic belt in the southwest from the Karelidie
belt in the remainder of the country. In both areas the supracrustal map patterns are curved,
twisted and even ragged in places , and are amoeba-like in general. The intervening plutons and basement also in the Karelidic area - conform weil to the irregular supracrustal pattern.
Such an excellent conformity suggests a causative relationship .
N
I
150
,
FIG . 3
Fig. 2 Map of Finland showing supracrustal metavolcanic-metasedimentary
beils (stippled) . Plutonic and basement rocks are indicated in white . The
map is simplified from Härme (1965) and Simonen (1960; 1971; Figs . 1 and
2) . Map areas of Figures 3 and 5 are shown in rectangles.
- -- - - -- - - -- -
----------------
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Fig. 3 Geologie map of the Marttila-Somero area in southwestern Finland, mueh simplified from Simonen .
(1960, Map 11). Supracrustal metavolcanie-metasedimentary rocks are stippled. Plutonlc rocks, mcludmg synkl: .
nematic granitoids and gabbros and late kinematic potash migmatite granite, are shown m whlte . Map locatlon mdlcated in Figure 2.
Figure 3 provides a view of this pattern in more detail , showing how intricate it can be .
In this particular area the synkinematic granodiorites and associated plutons are undifferentiated from late kinematic potash-rich migmatite granite (all shown in white) . The location of this
detaiIed map is iIIustrated in Figure 2.
The foliation of the country rock and synkinematic plutons generally is steep (Figs. 4a
and 4b) . Pi-pole patterns for both the country rock and plutons are very similar. However, the
pluton pi-pole pattern suggests an overturning toward the west which has also been pointed out
earlier by Härme (1960). Edelman (1960; 1973) recognized a similar westward overturning in
the potash-rich granites of the Gullkrana area, farther westward. The lineation patterns also
are similar for the supracrustals and plutons (Figs. 4c and 4d), but the pluton pattern also
points to a westward overturning .
The eastward preferred orientation of the foliation in the supracrustal assemblage (Fig.
4b) is very strang. However, the partial girdle in Figure 4b reveals the importance of the wideIy deflected foliations which give rise to amoeba-like supracrustal patterns in the area . The 10cation of the area at the southern margin of the central Finnish plutonic complex (Fig. 2) explains the east-west bias in the structural grain . A north-south foliation bias should exist along
the eastern margin and along most of the western margin of this great igneous complex (Figs.
land 2) .
The nearly complete foliation pole girdle for the synkinematic plutons indicates a type
of tubular flowage in the direction of the tectonic a axis, Cloos (1946, p . 25-29) discusses
this type of flowage. Also, some flattening into the east-west direction is indicated by the concentration maxima.
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N
N
a
N
b
N
c
d
Fig. 4 Fabric diagrams of foliation and lineations in the Marttila-Somero
area (See Fig. 3), modified from Simonen (1969 , Fig. 1). a) 430 synkinematic pluton foliation poles with contour intervals at 0, I, 4 and 6 (black) percent concentrations per one percent area. b) 625 foliation poles of supracrustal metavolcanic-metasedimentary rocks. Contour intervals are at 0, 1,4 and
6 (black) per one percent area. c) 173 lineations from synkinematic plutons.
Contour intervals are at 0, I, 2,4 and 6 (black) percent concentrations per
one percent area.
DIAPIRISM AS A CAUSE OF POLYDEFORMATION
Stephansson (1975 p. 20- 23, Fig . 9) proposes a sequence of multiple intrusions of
diapirs in the Fennoscandian shield of central Sweden, corresponding to the granite series of
Sederholm. This scheme of tectonic movements conforms c10sely and is inspired by the diapirism experiments of Ramberg (I 969a; 1969b) . Synkinematic granodiorites and quartz diorites
began forming from a gravitationally unstable layer beneath the Svecofennidic supracrustal pile. These bulb-like diapirs forced a passage up through the overlying strata, according to the
same deformation mechanism of salt domes. Through isostatic compensation infolds of the
supracrustal rocks were then tightly squeezed down into the depressions or saddles left by the
upward rising diapirs of the underlying granitoid layer. This infolded pattern in the deformed
supracrustal pile was thus caused and controlled by the rising diapirs. A curved and ragged
pattern in plan view can be inferred from such a process.
Later granitoid intrusives, mostly granitic in composition, began rising diapirically
from a second layer more recently formed than the initial granitoid diapir layer. Many of these
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:~~:
SUPRAC R USTALS
I~,:;/d
GRANODIORITE
Iv/'.(, ~ 1
GABBRO
1+++1
::1:::1:::1:
POTASH
~'<~<i
.
0
~KM
GRANITE
Fig. 5 Tectonogram of the eastem fourth of the Riihimäki Quadrangle north of Helsinki in southem Finland .
Tectonic extrapolations from the map data reveal the importance of plutonic diapirs in the deformation of Finnish
Proterozoic rocks . Geology by Kaitaro (1956); tectonogram by V. S . Griffin . Location is indicated in Figure 2.
later granite-rich diapirs penetrated and otherwise deformed the older diapirs . This further
complicated the early structural fabric of the supracrustal belts. Magmatic segregation is the
implied source for these infracrustal layers .
Such a sequential intrusion-deformation scheme for Sweden can be logically applied to
similar rocks in Finland. Figure 5 is a tectonogram constructed according to methods applied
earlier to this region by Wegmann (l929a; 1929b) . This map area , a short distance north of
Helsinki (Fig . 2), is intruded by synkinematic granodiorites and minor gabbro. Late kinematic
K20-rich intrusives of migmatite granite penetrate some of the earlier granodiorites and at a finer scale form migmatites with all older country rock through metasomatic replacement and
lit-par-lit intrusion .
The shape of the plutons, inferred or extrapolated from map information, is diapir-like.
Probable the roofs of these intrusions are more irregular than shown in Figure 5 (A . Simonen,
1975, oral communication). Nevertheless, the geometry is analogous to salt dome shapes and
to the shapes of the Ramberg diapirs. The K20-rich late kinematic diapirs partially intrude,
and perhaps replace in part , the earlier granodiorite plutons .
The intervening supracrustal assemblage, comprised mostly of volcanogenic and metasedimentary rocks raised to greenschist or amphibolite facies grade, has been folded into
tight accordian-like upright isoclines . The steep foliation, however, marks the low amplitude
undulations of the anticlinoria and synclinoria which the tight isoclines comprise (M . Härme
and A. Simonen, 1975, oral communication) . These tightly folded supracrustal rocks have
been crowded into troughs between the large diapiric plutons as they rose . Consequently, the
deformation of these cover rocks can be attributed directly to the local diapirism.
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Figure 6 is a hypothetieal and sehematie depietion of deformation events and effeets in
supraerustal eover roeks eaused by three peneeontemporaneous diapirie plutons developed in
sequenee. As eaeh sueeeeding pluton forms , the loeal struetures beeome more eomplex both in
the supraerustal assemblage, as weIl as in earlier plutons . Superimposed folds, multiple foliations and several episodes of small seale dike and sill development should be expeeted . However, the polydeformational paragenesis is entirely loealized, that is eonfined to the affeeted
areas around the deforming plutons. Different teetonie deformation regimes will prevail elsewhere as a result of different diapir sequenees and patterns .
"Orogeny" due to this style of diapirie deformation fundamentally is different from
that of the long, extensive folds , thrusts and nappes in Proterozoie mobile belts and in Phanerozoie fold belts . Amoeboidal patterns thus result from a poorly integrated teetonie framework in Fennoseandia, as opposed to weIl-integrated linear orogenie belts . Consequently, a
eareful and detailed teetonie sequential analysis based on outerop studies in a loeal region,
sueh as the reeent study by Hopgood et aL . (1976), ean be very useful for regional extrapolations in linear orogenie belts . However, the effeetiveness of this approaeh in Finland may be
limited only to restrieted areas reeording deformation eaused by loeal pluton diapirs.
I.
Fig. 6. Hypothetical scenario of sequential penecontemporaneous pluton diapirs deforming a layered supracrustal pile (plan view). In scene I Pluton A deforms the supracrustal pile into steep sided upright isoclines .
A second generation pluton B next in II overprints a "second generation" of structures , by its forceful intrusion, into the supracrustals and into pluton A. Finally, in III a third intrusion deforms pluton Band adds another generation of structures to the twice deformed supracrustal assemblage and the once deformed pluton
A. Pluton C penetrates forcefully the western margin of pluton B during diapirism.
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TECTONIC SIMILARITIES WITH ARCHEAN GREENSTONE BELTS
lnterest in Archean (early Precambrian) areas was enlivened by reports of work in
southern Africa (Anhaeusser et al. 1968; Kröner et al. 1973) following from earlier studies by
MacGregor (1951) and Hunter (1965). About the same time similar discussions of these very
old terranes appeared in the Soviet literature (Sheynman 1970; Pavlovkiy 1971). Prominently
mentioned were the studies in the Aldan shield of southern Siberia by N. V. Frolova, first published in the early 1950s (Pavlovskiy, 1971). lnvestigations from other shield areas, such as
in Australia (Glikson, 1972) were added so on to the discussion. Wilson (1972) and more recently McCall (1977) have summarized the popular thinking concerning these world-wide
rocks extending back toward the 4000 m .y. point in the earth's history.
Common to these accounts is mention of the unusual tectonic pattern possessed by the
ancient greenstone belts containing primitive basic and ultrabasic lavas with poorly sorted
graywacke-type sediments and minor siderite- and chert-rich sediments. Figure 7 presents a
c1assical assortment of such patterns, called "amoeboids" by Sheynman (1970, p. 217). The
ragged and curved outlines mark the effects of a granodioritic diapirism following on the heels
of supracrustal deposition (Anhaeusser et al. 1968, p. 2 I 86, 2 I 89). This tectonic style is considered to have continued until about 2500 to 2000 m.y. (Anhaeusser et al. 1968; Sheynman
1979; Pavlovskiy 1971; Wilson 1972). Then long, linear "mobile belts" appeared between
N
o
/
~
.'.
~
Fig. 7 Archean "greenstone" belt patterns in the Rhodesian shield. The supracrustal belts contain
metavolcanic and metasedimentary rocks. The white represents granitoid plutons (From MacGregor
1951, modified after Kröner 1973 . Fig. 4).
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N
~
LI
:.
o
J50
L
Fig. 8 Archean "greenstone" belts (supracrustals) of the Lake Superior area transected by east-west
gneiss belts. White represents plutonic rocks (modified from Goodwin and West 1974) .
N
~
Rhodesion
Croton
~ . ~
~
o
50
C roton
Km
Fig. 9 Close-up composite view of two individual " greenstone" belts of the Archean Rhodesian and Kaapvaal cratons, southern Africa (modified from Anhaeusser et ai. 1968 , Fig . 5) .
The stippled areas are supracrustals and the white represents diapiric plutons and migmatite
gneiss.
-_.~---~
14
the individualized Archean cratons containing the greenstone belts, possibly similar to the situation illustrated in the Lake Superior area of the Canadian shield (Fig. 8). Many workers
consider that the earth 's fundamental tectonic-metamorphic style irreversibly changed throughout the earth during this interval of time.
Diapirism has been cited by many as the mechanism of Archean' 'orogeny". Characteristic amoeboidal patterns found widely in the world's ancient shields present mute testimony to this. Highly irregular supracrustal map patterns reveal a type of unintegrated deformation, strikingly similar to the pattern characterizing the Fennoscandian shield. Figure 2 and
Figure 7 have much in common geometrically, as do Figures 3 and 9 also.
If the Archean greenstone belt tectonic pattern, and presumably deformation style, persisted into this well-documented Proterozoic terrane it would mark Fennoscandia as an unusual
area during the Middle Precambrian, but perhaps not unique. Cloud (1971, p. 15) briefly cites
a young "Archean" basin in the Prescott-Jerome area of Arizona where investigators have
established supracrustal ages of 1780 to 1820 m.y., and intruding granitic rocks of 1770 m.y.,
all very cIose to the Fennoscandian ages.
In the Svecofennidic area the analogy with the Archean greenstone belts probably extends only to the tectonic patterns and process inferred therefrom. The stratigraphic sequence
in the Svecofennidic belt does not contain greenstones at the base as do the Archean belts. Instead, the greenstone compositions are more similar to those of modern basaltic rocks (Simonen 1960, p. 55-59), however, the petrologic analogy might be better in the Karelidic
belt. Wilkman (1936) and Mikkola (1941) have described some very basic basalts in Karelia
and in Lapland which might belong to the Karelidic covering rocks and not to the pre-Karelidic basement. Recently, Mutanen (1976) reported komatiite compositions for some greenstones in the central Lapland, eastern Kainuu and Tipasjärvi provinces. However, age determinations seem to indicate that they belong to the 2400+m.y. Archean pre-Karelidic basement.
CONCLUSIONS
Based on the facts and interpretations presented here, the following concIusions can be
made about the Proterozoic terrane in Finland:
I . Neither the Svecofennidic nor Karelidic zones were deformed by the kind of integrated linear orogeny marking Proterozoic mobile belts and Phanerozoic fold belts.
2. Both of these geologic zones in Finland contain supracrustal rocks strongly resembling in map pattern the amoeboidal geometry of Archean greenstone belts.
3. The Proterozoic tectonic deformation in this part of the Fennoscandian shield was
controlled primarily by granodiorite and granite pluton diapirism, the same mechanism postulated by many to be responsible for Archean fold patterns.
4. Detailed outcrop structural analysis , employing methods developed in the Scottish
Highlands, is very useful in weil integrated mobile and fold belts. However, in Finland this approach probably will be less useful because of the non-integrated provincial style of diapir-induced deformation.
5. This amoeboid-diapir structural style may record an Archean crustal condition persisting weil into the Middle Precambrian in Fennoscandia.
ACKNOWLEDGEMENTS
Many of the ideas and views expressed in this paper were developed during my sabbatical study at the Geological Survey of Finland while working under Dr. Maunu Härme . I am endebted to Professors Härme, A. Simonen and H . Stigzelius for their kind assistance during my stay at Otaniemi as "Visiting Research Investigator"
during the winter and spring of 1975 . I thank also the staff of the Petrology Department for their generous hospitality during this visit and in the fjeld during the following summer. Appreciation is also expressed to C1emson
University for granting this sabbatical leave. Professor Härme is thanked for critically reviewing an earlier draft of
the manuscript . and for many stimulating and fruitful discussions.
15
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ISBN 951-690-107-7
ISS 0430-5124