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A major global suture in the Precambrian basement of SW Sweden?
Tectonophysics, 31 (1976) T35-T40 o Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
T35
Letter Section A major global suture in the Precambrian basement of SW Sweden?
H.P. ZECK and S. MALLING* Institute of Petrology, Copenhagen
University, Copenhagen
(Denmark)
(Submitted October 28, 1975; accepted January 16, 1976)
ABSTRACT Zeck, H.P. and Malling, S., 1976. A major global suture in the Precambrian basement of SW Sweden? Tectonophysics, 31: T35-T40. The traditional interpretation of the Gillberga synform north of Lake Vanern in SW Sweden pictures it as a tectonically undisturbed rock sequence decreasing in age upwards. Our investigations suggest that a number of major sub-horizontal thrust planes are present in the sequence. Low-angle thrusting seems to be a major feature in the Precambrian basement of SW Sweden and adjacent parts of SE Norway. It is suggested that this may be indicative of a major global suture, be it interplate or intraplate.
The open synform in the Precambrian basement of South Varmland, SW Sweden, known as the Gillberga synform (Fig. l), has a N-S axis with dips of lo-20% in the northern part. The outcropping structure has a length of about 40 km and a breadth of about 20 km. The rocks involved are mainly gneisses, and their foliation, and the layering parallel thereto, outlines the structure. The traditional interpretation of the geology of the Gillberga area, as it appears from the regional geological model given by Magnusson and coworkers (Magnusson et al., 1958; and Magnusson, 1960) is based on earlier work by Swedish geologists (Tornebohm, 1883; Holmquist, 1906; Magnusson, 1929 a, b). In this interpretation, the rock pile in the Gillberga area is considered to consist of three main units (see Figs. 1 and 2): a basal unit (1) -“Pre-Gothian” - which would represent the oldest rocks in the area (and in fact the oldest rocks in Sweden, Magnusson, 1965), overlain by younger rocks - “Gothian” - which are subdivided into an older supracrustal unit (2) and a younger unit of metaplutonic rocks (3), the latter regionally overlying the supracrustals. An unconformity was indicated between the Pre*Present address: Precambrian Research Unit, Geology Department, sity, Rondebosch, South Africa.
Cape Town Univer-
T36
1
50 km ,
I
DALSLANDIAN
GOTHIAN
GROUP
GROUP
PRE- GOTHIAN GROUP (-and Stora - Le Marstrand Formation) / 0
0 ’
“myloniten
zone
T37
GOTHIAN
GROUP
PRE - GOTHIAN
GROUP
~
Fig.2. Schematic geological section A-B (Fig. l), showing the traditional interpretation, after Magnusson et al. (1958) and Magnusson (1960).
holoclostic r-
I
j
1
supracrurtal
i- ’ .-I f??&Y’ 5 other *,......:. ?:._d
thrust
rocks
rocks,
corresponding
lithortratigraphic
units
to
(see
those
Zeck
of
8
Fig.
Moiltng.
- plane
2
1974
I
geological section A-B (Fig. l), illustrating the model proposed in this Fig.3. Schematical _. paper. The rock complex has been subdivided into a number of rock-stratigraphic units (Zeck and Malling, 1974) shown by different shading in the section. Three major thrust planes are distinguished in the complex. The planes are delineated by zones of holoclastic rocks. Where thickest these zones are indicated by a proper signature.
Gothian and the overlying Gothian supracrustals; this was based on the supposedly consistently higher muscovite content at the top of the PreGothian, a phenomenon thought to represent a metapaleosol. Outside the Gillberga area, further to the south and west, a still younger rock unit occurs. This younger unit, the Dalslandian, consists of low-grade metamorphic volcanics and sediments and a suite of granitic rocks which, respectively, overly discordantly and show intrusion contacts towards both the Gothian and PreGothian rocks. The metamorphic and plutonic activity reflected in the Dalslandian rocks would have taken place in the period 1200-900 m.y. B.P. (Kratz et al., 1968). Our recent investigations (Zeck and Malling, 1974) suggest that the rock complex in the Gillberga area, including both the Pre-Gothian and Gothian of earlier authors, harbours three major thrust planes running parallel to the foliation and layering in the gneisses. The planes are delineated by strongly cataclastic rocks (“holoclastic rocks”, Zeck, 1974), see Fig. 3. The aboveFig.1. Geological sketch map of South Varmland and North Dalsland, SW Sweden, showing the traditional interpretation, after Magnusson et al. (1958) and Magnusson (1960).
T38
mentioned unconformity between the “Pre-Gothian Group” and the “Gothian Group”, which is a crucial aspect in distinguishing both rock units could not be confirmed by our investigations. The rocks in the external parts of the synform were probably formed during the Svecofennian event (- 1700 m.y. B.P.), see Zeckand Malling (1974). The rocks in the central part of the synform the Gothian meta-plutonics of earlier authors - consist essentially of cataelastic equivalents of the rocks found in the external parts. The spatial extent of the Gothian supracrustals (mainly quartzitic rocks, often with well-preserved cross-bedding) is suggested from our investigations to be much more restricted than earlier maps show. Many of the holoclastic rocks seem to have been mapped as meta-sediments or meta-volcanics. Such confusion was described from elsewhere and seems not uncommon (Higgins, 1971). The age of the thrusting (and concomitant metamorphism and deformation, cataclastic and otherwise) seems to be Dalslandian (s Grenville). This is suggested by radiometric age determination on the consanguineous basement of South Norway (O’Nions and Baadsgaard, 1971), by our own unpublished dating results, and by analogy to large-scale thrust phenomena described further to the south which involve obvious Dalslandian metasediments (Heybroek and Zwart, 1949). Another indication of a Dalslandian (or younger) age is given by paleomagnetic measurements (Neuvonen, 1973) which indicate that the Precambrian in SW Sweden (and South Norway) does not plot on the polar wandering curve for the rest of the Fennoscandian shield in the period 1900-1100 m.y. B.P. This might be explained by proposing a lateral displacement of SW Sweden (and South Norway) relative to the rest of the shield (Neuvonen, 1973), implying large (sub)-horizontal movements which could fit in and be connected with the sub-horizontal thrust units proposed here. An indication of the place of origin of the overthrusted material is given by the E-W directions of the mineral lineation in the rocks. This lineation is interpreted as an a-lineation and thus as an indication that the thrust masses came from the east or the west. The three thrust masses were not necessarily independently emplaced. It is possible that all three or that two adjacent ones travelled together over some distance. The distance over which the thrust masses have travelled is unknown. There is the possibility that the thrust masses stem from the large, roughly N-S running cataclastic zone about 25 km of Gillberga (see Fig. l), or even from the much larger, also N-S running “mylonite” zone which passes 80-90 km east from Gillberga; such in similarity to a model proposed by Bryant and Reed (1970) for the Blue Ridge in the North American Appalachians. The large-scale, low-angle thrusting in the Gillberga area here described, in combination with similar structures more to the south (Heybroek and Zwart, 1949; Magnusson et al., 1958) and to the north and the east, in Sweden and in the adjacent part of SE Norway (Zeck and Malling, 1974; A. Berthelsen, personal communication, 1975, Copenhagen) suggest the existence of a regional N-S running zone - in its present scope 200-300 km in length and about 50 km in breadth - in which crustal shortening in E-W direction
T39
occurred. Part of the rocks in this zone are older gneisses that received a new metamorphictectonic imprint during this shortening event. Other parts consist of low-grade metamorphic, supracrustal rocks (Dalslandian Group), that only show this later metamorphic-tectonic imprint. It seems feasible to explain this large-scale feature in terms of plate tectonics. Plate-tectonic models for the Mesozoic and the Tertiary, and to a lesser extent the Paleozoic, are rather well defined and available in recent literature. If and how the plate-tectonic machinery functioned in the Precambrium is a subject of much discussion among Precambrian geologists. However, for the Proterozoic, to which the basement in SW Sweden belongs, many authors seem to favour a mechanism basically not unlike that in the Mesozoic-Tertiary (Dewey and Spall, 1975). If such a type of plate tectonism has been at work in the basement in SW Sweden, it would seem that the zone of low-angle thrusting was formed at a destructive lithospheric plate margin and represents a collision zone between two lithospheric plates. At some places within this suture zone complexes of oceanic or island-arc rock assemblages might be preserved in the form of exposures of the Dalslandian Group. At most places such indicators of the former presence of an oceanic environment would be absent and two continental fragments would be found in collision contact (cf. Dewey and Burke, 1973). In such a situation Oxburgh (1972) has shown the feasibility of a flake-tectonic model, in which sheet-like masses may be sheared from the top of one of the plates and driven over the other (abduction). Obviously, a precise characterization of the Dalslandian Group rocks is of crucial importance for the fate of this plate-tectonic model for SW Sweden. Detailed fieldwork and geochemical studies are in progress to define the environment in which these rocks were formed. In the meantime, as long as these investigations have not rendered proof for the presence of key petrogenetic assemblages for a destructive lithosperic plate margin, an alternative mega-tectonic solution might be considered, viz. a model that pictures the mobile zone developing within a continental fragment. However, even if this alternative is proven to be more feasible, it does not necessarily indicate that fundamental drift parameters like heat production and lithospheric plate thickness, structure and composition were drastically different in the Proterozoic. Also in the Phanerozoic, zones of crustal shortening and largescale low-angle thrusting seem to have been formed within continental fragments. A recent report of Forman and Shaw (1973) describes very extensive, intraplate, low-angle thrust zones formed 600-350 m.y. ago in the Precambrian basement of Central Australia. Major gravity anomalies are associated with these intraplate thrust zones, indicating that they reach down into the mantle. Armstrong and Dick (1974) claimed that large-scale, comparatively thin thrust sheets, be they interplate or intraplate, originated in regions of unusually steep geothermal gradient, where they were detached as rigid cover fragments from their thermally softened base.
T40
REFERENCES Armstrong, R.L. and Dick, H.J.B., 1974. A model for the development of thin overthrust sheets of crystalline rock. Geology, 2: 35-40. Bryant, B. and Reed, J.C., 1970. Geology of the Grandfather Mountain Window and vicinity, N. Carolina and Tennessee. U.S. Geol. Surv. Prof. Pap., 615, 190 pp. Dewey, J.F. and Burke, K.C.A., 1973. Tibetan, Variscan and Precambrian basement reactivation: products of continental collision. J. Geol., 81: 683-692. Dewey, J. and Spall, H., 1976. Pre-Mesozoic plate tectonics: How far back in earth history can the Wilson cycle be extended? Geology, 3: 422-424. Forman, D.J. and Shaw,-R.D., 1973. Deformation of the crust and mantle in Central Australia. Bull. Bur. Min. Res. Geol. Geophys., 144, 20 pp. Heybroek, P. and Zwart, H.J., 1949. The overthrusts between Te&kersjOn and Marsjon, Dalsland. Geol. Foren. Stockholm Fbrh., 71: 425-434. Higgins, M.W., 1971. Cataclastic rocks. U.S. Geol. Surv. Prof. Pap., 687,97 pp. Holmquist, P.J., 1906. Studien uber die Granite von Schweden. Bull. Geol. Inst. Univ. Uppsala, 7: 77-269. Kratz, K.O., Gerling, E.K. and Lobach-Zhuchenko, S.B., 1968. The isotope geology of the Precambrian of the Baltic Shield. Can. J. Earth Sci., 5: 657-660. Magnusson, N.H., 1929 a. Indledning and Berggrunden (+ hard rock map), pp. 5-38. In: N.H. Magnusson and L. von Post (1929),Beskrivning till Kartbladet SPffle. Sveriges Geol. Unders., Ser. Aa., nr. 167. Magnusson, N.H., 1929 b. Gillbergaskklens byggnad (inclusive 2 geological maps). Sveriges Geol. Unders., C 360, 84 pp. Magnusson, N.H., 1960. The Swedish Precambrian outside the Caledonian Mountain Chain, pp. 5-66. In: N.H. Magnusson, P. Thorslund, F. Brotzen, B. Asklund and 0. Kulling, Sveriges Geol. Unders. ser. Ba, nr. 16. Magnusson, N.H., 1965. The Pre-Cambrian history of Sweden. Q. J. Geol. Sot. London, 121: l-30. Magnusson, N.H., Asklund, B., Kulling, O., Kautsky, G., Eklund, J., Larsson, W., Lundeg&dh, P.H., Hjelmqvist, S., Gavelin, G. and Gdman, O., 1958. Karta over Sveriges berggrund (tre blad), &ala 1: l,OOO,OOO,sodra bladet. Sveriges Geol. Unders., ser. Ba, nr. 16. Neuvonen, K.J., 1973. Building up of the Proto-Fennoscandian Shield. XI Nordiska Geol. Vintermotet, Oulu, January 1974, Abstracts etc., p, 62. G’Nions, R.K. and Baadsgaard, H., 1971. A radiometric study of polymetamorphism in the Bamble region, Norway. Contrib. Mineral. Petrol., 34 : 1-21. Oxburgh, E.R., 1972. Flake tectonics and continental collision. Nature, 239: 202-204. Tornebohm, A.E., 1883. Gferblick bfver mellersta Sveriges Urformation. Geol. Foren. Stockholm Fbrh., 6: 682-607. Zeck, H.P., 1974. Cataclastites, hemidastites, holoclastites, blasto-ditto and myioblastites - cataclastic rocks. Am. J. Sci., 274: 1064-1073. Zeck, H.P. and Malling, S., 1974. The Gillberga synform (Precambrian Basement, SW VLmland, Sweden); literature synopsis and preliminary notes on its re-interpretation. Bull. Geol. Sot. Denmark, 23: 159-174.
T35
Letter Section A major global suture in the Precambrian basement of SW Sweden?
H.P. ZECK and S. MALLING* Institute of Petrology, Copenhagen
University, Copenhagen
(Denmark)
(Submitted October 28, 1975; accepted January 16, 1976)
ABSTRACT Zeck, H.P. and Malling, S., 1976. A major global suture in the Precambrian basement of SW Sweden? Tectonophysics, 31: T35-T40. The traditional interpretation of the Gillberga synform north of Lake Vanern in SW Sweden pictures it as a tectonically undisturbed rock sequence decreasing in age upwards. Our investigations suggest that a number of major sub-horizontal thrust planes are present in the sequence. Low-angle thrusting seems to be a major feature in the Precambrian basement of SW Sweden and adjacent parts of SE Norway. It is suggested that this may be indicative of a major global suture, be it interplate or intraplate.
The open synform in the Precambrian basement of South Varmland, SW Sweden, known as the Gillberga synform (Fig. l), has a N-S axis with dips of lo-20% in the northern part. The outcropping structure has a length of about 40 km and a breadth of about 20 km. The rocks involved are mainly gneisses, and their foliation, and the layering parallel thereto, outlines the structure. The traditional interpretation of the geology of the Gillberga area, as it appears from the regional geological model given by Magnusson and coworkers (Magnusson et al., 1958; and Magnusson, 1960) is based on earlier work by Swedish geologists (Tornebohm, 1883; Holmquist, 1906; Magnusson, 1929 a, b). In this interpretation, the rock pile in the Gillberga area is considered to consist of three main units (see Figs. 1 and 2): a basal unit (1) -“Pre-Gothian” - which would represent the oldest rocks in the area (and in fact the oldest rocks in Sweden, Magnusson, 1965), overlain by younger rocks - “Gothian” - which are subdivided into an older supracrustal unit (2) and a younger unit of metaplutonic rocks (3), the latter regionally overlying the supracrustals. An unconformity was indicated between the Pre*Present address: Precambrian Research Unit, Geology Department, sity, Rondebosch, South Africa.
Cape Town Univer-
T36
1
50 km ,
I
DALSLANDIAN
GOTHIAN
GROUP
GROUP
PRE- GOTHIAN GROUP (-and Stora - Le Marstrand Formation) / 0
0 ’
“myloniten
zone
T37
GOTHIAN
GROUP
PRE - GOTHIAN
GROUP
~
Fig.2. Schematic geological section A-B (Fig. l), showing the traditional interpretation, after Magnusson et al. (1958) and Magnusson (1960).
holoclostic r-
I
j
1
supracrurtal
i- ’ .-I f??&Y’ 5 other *,......:. ?:._d
thrust
rocks
rocks,
corresponding
lithortratigraphic
units
to
(see
those
Zeck
of
8
Fig.
Moiltng.
- plane
2
1974
I
geological section A-B (Fig. l), illustrating the model proposed in this Fig.3. Schematical _. paper. The rock complex has been subdivided into a number of rock-stratigraphic units (Zeck and Malling, 1974) shown by different shading in the section. Three major thrust planes are distinguished in the complex. The planes are delineated by zones of holoclastic rocks. Where thickest these zones are indicated by a proper signature.
Gothian and the overlying Gothian supracrustals; this was based on the supposedly consistently higher muscovite content at the top of the PreGothian, a phenomenon thought to represent a metapaleosol. Outside the Gillberga area, further to the south and west, a still younger rock unit occurs. This younger unit, the Dalslandian, consists of low-grade metamorphic volcanics and sediments and a suite of granitic rocks which, respectively, overly discordantly and show intrusion contacts towards both the Gothian and PreGothian rocks. The metamorphic and plutonic activity reflected in the Dalslandian rocks would have taken place in the period 1200-900 m.y. B.P. (Kratz et al., 1968). Our recent investigations (Zeck and Malling, 1974) suggest that the rock complex in the Gillberga area, including both the Pre-Gothian and Gothian of earlier authors, harbours three major thrust planes running parallel to the foliation and layering in the gneisses. The planes are delineated by strongly cataclastic rocks (“holoclastic rocks”, Zeck, 1974), see Fig. 3. The aboveFig.1. Geological sketch map of South Varmland and North Dalsland, SW Sweden, showing the traditional interpretation, after Magnusson et al. (1958) and Magnusson (1960).
T38
mentioned unconformity between the “Pre-Gothian Group” and the “Gothian Group”, which is a crucial aspect in distinguishing both rock units could not be confirmed by our investigations. The rocks in the external parts of the synform were probably formed during the Svecofennian event (- 1700 m.y. B.P.), see Zeckand Malling (1974). The rocks in the central part of the synform the Gothian meta-plutonics of earlier authors - consist essentially of cataelastic equivalents of the rocks found in the external parts. The spatial extent of the Gothian supracrustals (mainly quartzitic rocks, often with well-preserved cross-bedding) is suggested from our investigations to be much more restricted than earlier maps show. Many of the holoclastic rocks seem to have been mapped as meta-sediments or meta-volcanics. Such confusion was described from elsewhere and seems not uncommon (Higgins, 1971). The age of the thrusting (and concomitant metamorphism and deformation, cataclastic and otherwise) seems to be Dalslandian (s Grenville). This is suggested by radiometric age determination on the consanguineous basement of South Norway (O’Nions and Baadsgaard, 1971), by our own unpublished dating results, and by analogy to large-scale thrust phenomena described further to the south which involve obvious Dalslandian metasediments (Heybroek and Zwart, 1949). Another indication of a Dalslandian (or younger) age is given by paleomagnetic measurements (Neuvonen, 1973) which indicate that the Precambrian in SW Sweden (and South Norway) does not plot on the polar wandering curve for the rest of the Fennoscandian shield in the period 1900-1100 m.y. B.P. This might be explained by proposing a lateral displacement of SW Sweden (and South Norway) relative to the rest of the shield (Neuvonen, 1973), implying large (sub)-horizontal movements which could fit in and be connected with the sub-horizontal thrust units proposed here. An indication of the place of origin of the overthrusted material is given by the E-W directions of the mineral lineation in the rocks. This lineation is interpreted as an a-lineation and thus as an indication that the thrust masses came from the east or the west. The three thrust masses were not necessarily independently emplaced. It is possible that all three or that two adjacent ones travelled together over some distance. The distance over which the thrust masses have travelled is unknown. There is the possibility that the thrust masses stem from the large, roughly N-S running cataclastic zone about 25 km of Gillberga (see Fig. l), or even from the much larger, also N-S running “mylonite” zone which passes 80-90 km east from Gillberga; such in similarity to a model proposed by Bryant and Reed (1970) for the Blue Ridge in the North American Appalachians. The large-scale, low-angle thrusting in the Gillberga area here described, in combination with similar structures more to the south (Heybroek and Zwart, 1949; Magnusson et al., 1958) and to the north and the east, in Sweden and in the adjacent part of SE Norway (Zeck and Malling, 1974; A. Berthelsen, personal communication, 1975, Copenhagen) suggest the existence of a regional N-S running zone - in its present scope 200-300 km in length and about 50 km in breadth - in which crustal shortening in E-W direction
T39
occurred. Part of the rocks in this zone are older gneisses that received a new metamorphictectonic imprint during this shortening event. Other parts consist of low-grade metamorphic, supracrustal rocks (Dalslandian Group), that only show this later metamorphic-tectonic imprint. It seems feasible to explain this large-scale feature in terms of plate tectonics. Plate-tectonic models for the Mesozoic and the Tertiary, and to a lesser extent the Paleozoic, are rather well defined and available in recent literature. If and how the plate-tectonic machinery functioned in the Precambrium is a subject of much discussion among Precambrian geologists. However, for the Proterozoic, to which the basement in SW Sweden belongs, many authors seem to favour a mechanism basically not unlike that in the Mesozoic-Tertiary (Dewey and Spall, 1975). If such a type of plate tectonism has been at work in the basement in SW Sweden, it would seem that the zone of low-angle thrusting was formed at a destructive lithospheric plate margin and represents a collision zone between two lithospheric plates. At some places within this suture zone complexes of oceanic or island-arc rock assemblages might be preserved in the form of exposures of the Dalslandian Group. At most places such indicators of the former presence of an oceanic environment would be absent and two continental fragments would be found in collision contact (cf. Dewey and Burke, 1973). In such a situation Oxburgh (1972) has shown the feasibility of a flake-tectonic model, in which sheet-like masses may be sheared from the top of one of the plates and driven over the other (abduction). Obviously, a precise characterization of the Dalslandian Group rocks is of crucial importance for the fate of this plate-tectonic model for SW Sweden. Detailed fieldwork and geochemical studies are in progress to define the environment in which these rocks were formed. In the meantime, as long as these investigations have not rendered proof for the presence of key petrogenetic assemblages for a destructive lithosperic plate margin, an alternative mega-tectonic solution might be considered, viz. a model that pictures the mobile zone developing within a continental fragment. However, even if this alternative is proven to be more feasible, it does not necessarily indicate that fundamental drift parameters like heat production and lithospheric plate thickness, structure and composition were drastically different in the Proterozoic. Also in the Phanerozoic, zones of crustal shortening and largescale low-angle thrusting seem to have been formed within continental fragments. A recent report of Forman and Shaw (1973) describes very extensive, intraplate, low-angle thrust zones formed 600-350 m.y. ago in the Precambrian basement of Central Australia. Major gravity anomalies are associated with these intraplate thrust zones, indicating that they reach down into the mantle. Armstrong and Dick (1974) claimed that large-scale, comparatively thin thrust sheets, be they interplate or intraplate, originated in regions of unusually steep geothermal gradient, where they were detached as rigid cover fragments from their thermally softened base.
T40
REFERENCES Armstrong, R.L. and Dick, H.J.B., 1974. A model for the development of thin overthrust sheets of crystalline rock. Geology, 2: 35-40. Bryant, B. and Reed, J.C., 1970. Geology of the Grandfather Mountain Window and vicinity, N. Carolina and Tennessee. U.S. Geol. Surv. Prof. Pap., 615, 190 pp. Dewey, J.F. and Burke, K.C.A., 1973. Tibetan, Variscan and Precambrian basement reactivation: products of continental collision. J. Geol., 81: 683-692. Dewey, J. and Spall, H., 1976. Pre-Mesozoic plate tectonics: How far back in earth history can the Wilson cycle be extended? Geology, 3: 422-424. Forman, D.J. and Shaw,-R.D., 1973. Deformation of the crust and mantle in Central Australia. Bull. Bur. Min. Res. Geol. Geophys., 144, 20 pp. Heybroek, P. and Zwart, H.J., 1949. The overthrusts between Te&kersjOn and Marsjon, Dalsland. Geol. Foren. Stockholm Fbrh., 71: 425-434. Higgins, M.W., 1971. Cataclastic rocks. U.S. Geol. Surv. Prof. Pap., 687,97 pp. Holmquist, P.J., 1906. Studien uber die Granite von Schweden. Bull. Geol. Inst. Univ. Uppsala, 7: 77-269. Kratz, K.O., Gerling, E.K. and Lobach-Zhuchenko, S.B., 1968. The isotope geology of the Precambrian of the Baltic Shield. Can. J. Earth Sci., 5: 657-660. Magnusson, N.H., 1929 a. Indledning and Berggrunden (+ hard rock map), pp. 5-38. In: N.H. Magnusson and L. von Post (1929),Beskrivning till Kartbladet SPffle. Sveriges Geol. Unders., Ser. Aa., nr. 167. Magnusson, N.H., 1929 b. Gillbergaskklens byggnad (inclusive 2 geological maps). Sveriges Geol. Unders., C 360, 84 pp. Magnusson, N.H., 1960. The Swedish Precambrian outside the Caledonian Mountain Chain, pp. 5-66. In: N.H. Magnusson, P. Thorslund, F. Brotzen, B. Asklund and 0. Kulling, Sveriges Geol. Unders. ser. Ba, nr. 16. Magnusson, N.H., 1965. The Pre-Cambrian history of Sweden. Q. J. Geol. Sot. London, 121: l-30. Magnusson, N.H., Asklund, B., Kulling, O., Kautsky, G., Eklund, J., Larsson, W., Lundeg&dh, P.H., Hjelmqvist, S., Gavelin, G. and Gdman, O., 1958. Karta over Sveriges berggrund (tre blad), &ala 1: l,OOO,OOO,sodra bladet. Sveriges Geol. Unders., ser. Ba, nr. 16. Neuvonen, K.J., 1973. Building up of the Proto-Fennoscandian Shield. XI Nordiska Geol. Vintermotet, Oulu, January 1974, Abstracts etc., p, 62. G’Nions, R.K. and Baadsgaard, H., 1971. A radiometric study of polymetamorphism in the Bamble region, Norway. Contrib. Mineral. Petrol., 34 : 1-21. Oxburgh, E.R., 1972. Flake tectonics and continental collision. Nature, 239: 202-204. Tornebohm, A.E., 1883. Gferblick bfver mellersta Sveriges Urformation. Geol. Foren. Stockholm Fbrh., 6: 682-607. Zeck, H.P., 1974. Cataclastites, hemidastites, holoclastites, blasto-ditto and myioblastites - cataclastic rocks. Am. J. Sci., 274: 1064-1073. Zeck, H.P. and Malling, S., 1974. The Gillberga synform (Precambrian Basement, SW VLmland, Sweden); literature synopsis and preliminary notes on its re-interpretation. Bull. Geol. Sot. Denmark, 23: 159-174.