Bulletin of the Geological Society of Denmark • Volume 46/2

Festskrift i anledning af professor Asger Berthelsens 70 års fødselsdag den 30. april 1998

Skjernaa, L.: Festskrift i anledning af professor Asger Berthelsens 70 års fødselsdag den 30. april 1998. Bulletin of the Geological Society of Denmark, Vol. 46, pp. 121–123. Copenhagen.

This volume is in honour of Professor Asger Berthelsen on the occasion of his 70th birthday. Asger Berthelsen has made important contributions within the areas of basement geology, tectonics, structural geology, Quaternary geology, salt tectonics and deep seismic interpretations. He is an outstanding writer and illustrator.

L. Skjernaa [ ] Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. 7. January 2000.

Get pdf-file of paper (274 kB)

Superimposed deformation in glaciotectonics

Pedersen, S.A.S.: Superimposed deformation in glaciotectonics. Bulletin of the Geological Society of Denmark. Vol. 46, pp. 125–144, Copenhagen.

The identification of glaciotectonic structures is an exclusive field for the structural geologist. The structures comprise a series of different types and regimes. The sequential development of the glaciotectonic structures reflects superimposed subglacial and proglacial deformation processes. The glaciotectonic structures may involve earlier formed structures thus superimposed by the glaciotectonics, or the glaciotectonic structures may eventually be overprinted by neotectonic deformations. Four different superimposed settings may be distinguished: 1) glaciotectonic deformation superimposed on pre-Quaternary tectonics, 2) glaciotectonic deformation superimposed on earlier formed glaciotectonic structures (superimposed deformation involving two or more glaciodynamic events), 3) glaciotectonic deformations superimposed sequentially in the same glaciotectonic unit (two or more glaciotectonic phases in the same glaciodynamic event), and finally 4) neotectonic deformation superimposed on glaciotectonic structures. Examples of type 1 are taken from the deformed Palaeogene diatomites with ash layers at Hanklit, Mors, and Hestegården, Fur. Dokumentation of glaciotectonic deformation superimposed on halokinetic structures is demonstrated from Erslev, Mors, and further examplified by structures occurring at Junget on the north side of the Batum salt diapir in Salling. Type 2 is examplified by glaciotectonic structures in the Skarrehage mo-clay pit on Mors. An example of Elsterian glaciotectonics superimposed by Saalian glaciotectonics is recorded from the hilly island Møborg, central part of western Jylland. The classic glaciotectonic site Møns Klint is described as a combination of an imbricate fan and an antiformal stack formed by the Young Baltic ice advance in the Late Weichselian superimposed by a regressional re-advance from the east. Type 3 is exemplified by the glaciotectonic complex at Feggeklit. Type 4 is described from the island of Fur where glaciotectonic structures are cut by neotectonic faults roughly parallel to the main E-W trend of Limfjorden.

Key words: Glaciotectonics, superimposed deformation, structural geology, Quaternary geology.

S.A. Schack Pedersen [ . ], Geological Survey of Denmark and Greenland, Thoravej 8, DK 2400 København NV. 18 January 1999.

Get pdf-file of paper (2100 kB)

Crustal structure and tectonic evolution of the Tornquist Fan region as revealed by geophysical methods

Thybo, H.: Crustal structure and tectonic evolution of the Tornquist Fan region as revealed by geophysical methods. Bulletin of the Geological Society of Denmark, Vol. 46, pp. 145–160. Copenhagen.

Crustal structure derived primarily from geophysical investigations reveals features that may be related to the complex tectonic evolution of the Tornquist Fan region. This northwestwards widening splay of Late Carboniferous – Early Permian fault zones in the Danish region emanates from the Teisseyre-Tornquist Zone in northern Poland. Seismic reflections and velocity anomalies image collisional fault zones that formed during the Proterozoic and Palaeozoic amalgamation of the crust. Re-equilibration of Moho appears to have taken place before late Palaeozoic rifting and magmatism initiated the main phase of basin formation that continued into the Mesozoic. The resulting, strong Moho topography, with variation between depths of 26 and 48 km, has been practically “frozen in” since then, although the late Cretaceous – early Cenozoic inversion tectonics may have formed a crustal keel underneath part of the Sorgenfrei-Tornquist Zone which cuts across the Proterozoic crust of the Tornquist Fan region.

Key words: Crust, Mantle, Geophysics, Seismic velocity, Tornquist Fan

Hans Thybo, Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. 3 May 1999.

Get pdf-file of paper (745 kB)

Structural and geochronological evolution of the northeastern part of the Sveconorwegian orogen, south-central Sweden

Wahlgren, C.-H. & Stephens, M.B.: Structural and geochronological evolution of the northeastern part of the Sveconorwegian orogen, south-central Sweden. Extended Abstract, Bulletin of the Geological Society of Denmark Vol. 46, pp. 161–163. Copenhagen 2000–02–10.

An important tectonic model for the Sveconorwegian orogen north and northeast of Lake Vänern, south-central Sweden, was presented by Berthelsen (1980). Both the Mylonite Zone and the frontal area of the Sveconorwegian orogen were interpreted to be related to large-scale compressional tectonics. Earlier thrusting to the west was inferred to be followed by later thrust movement to the east.

Carl-Henric Wahlgren [ ] & Michael B. Stephens, Geological Survey of Sweden, Box 670, SE-751 28 Uppsala, Sweden. 24 February 1999.

Get pdf-file of paper (35kB)

Adakitic high-Al trondhjemites in the Proterozoic Østfold-Marstrand Belt, W Sweden

Hageskov, B & Mørch, B.: Adakitic high-Al trondhjemites in the Proterozoic Østfold-Marstrand Belt, W Sweden. Bulletin of the Geological Society of Denmark, Vol. 46, pp. 165–179. Copenhagen.

This paper investigates the first identified intrusives in SE Norway–W Sweden with the specific signature of adakitic arc magmas, which in recent settings are preferably explained as partial melts extracted from subducted oceanic crust. The studied adakitic high–Al trondhjemites occur as sheets in the Koster archipelago, W Sweden, where they form the oldest recognized granitoids in the metasupracrustals of the Stora Le–Marstrand formation. The trondhjemites were intruded during a short ca. 1.59–1.58 Ga interlude between the early and the main orogenic events of the Gothian orogeny (1.6–1.56 Ga, Åhäll et al. 1998). This interlude is otherwise characterized by ‘ordinary’ calcalkaline magmatism which on Koster is predated by the trondhjemites.
The typical adakitic signature suggests that the trondhjemitic magma was extracted from a MORB (Mid Ocean Ridge Basalt) like source, and that a hornblende eclogite restite was left in the region of melting. The restite composition indicates melt extraction at PT conditions in the range of 18–25 kb/800°C to 13-15 kb/950–1050°C. These requirement can only be met by subduction of warm (young or shear heated) oceanic crust beneath a crust including early Gothian metamorphosed and deformed Stora Le–Marstrand formation or by melting of metabasaltic material at a deep crustal level. The latter is a less likely possibility and demands that the Stora Le–Marstrand formation at the time of melt extraction was part of a > 45 km thick crust.

Key words: Adakites, high–Al trondhjemites, Mesoproterozoic, Gothian Complex, tectonics.

Bjørn Hageskov [ ] and Bente Mørch, Geologisk Institut, Københavns Universitet, Østervoldgade 10, 1350 Copenhagen K, Denmark. 25 June 1999.

Get pdf-file of paper (534kB)

Geology of the Setesdalen area, South Norway: Implications for the Sveconorwegian evolution of South Norway

Pedersen, S. & Konnerup-Madsen, J.: Geology of the Setesdalen area, South Norway: Implications for the Sveconorwegian evolution of South Norway. Bulletin of the Geological Society of Denmark, Vol. 46, pp. 181–201, Copenhagen.

Crust forming processes in the Setesdalen area in the central part of southern Norway are dominated by the development of pre-Sveconorwegian supracrustal rocks (immature clastic sediments and bimodal volcanics) with an assumed depositional age of 1150–1100 Ma and by the emplacement of scattered infracrustal granitoids. The supracrustal rocks were deposited on a partly older than 1300 Ma gneissic basement including rocks which may have suffered one or more pre-Sveconorwegian orogeneses. During the Sveconorwegian orogeny, with the main upper greenschist to middle amphibolite facies high-temperature metamorphism and deformational phases in the period 1060–970 Ma, igneous activity comprising K-rich rocks high in elements such as P, Ti, La, Sr, Zr, and LREE and Ba was dominant. Significantly younger than this activity is the development of many REE pegmatites which are so characteristic for the region. The Precambrian geological activity terminated (at about 830 Ma?) with the development of E-W trending tholeiitic dolerites.

Key words: Geochronology, granites, metagabbros, Norway, Precambrian, REE pegmatites, Sveconorwegian orogeny

Svend Pedersen & Jens Konnerup-Madsen, Geologisk Institut, Copenhagen University, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. 22 June 1999.

Get pdf-file of paper (490kB)

Late Sveconorwegian monzonitic dykes in the Setesdalen area of Central South Norway: Examples of structures in dykes and their indications of accumulated strain

Skjernaa, L. & Pedersen, S.: Late Sveconorwegian monzonitic dykes in the Setesdalen area of Central South Norway: Examples of structures in dykes and their indications of accumulated strain. Bulletin of the Geological Society of Denmark. Vol. 46, pp. 203–223. Copenhagen.

In the Setesdalen area, Central South Norway, a large number of monzonitic dykes and other minor bodies were intruded during the late stages of emplacement of Sveconorwegian magmatic complexes. Some of the dykes define cone sheet and bell-jar sheet systems. The dykes possess internal linear and planar fabrics, which are not seen in any of the host rocks. Also, cross-cutting granitic veins within the dykes are deformed and may show ptygmatic folds, whereas in the host rocks these same veins appear undeformed. The internal strain in the dykes was determined from vein deformation to be mainly simple shear with or without a volume reduction. The strain preferentially accumulated in the ductile monzonite dykes while the more rigid host rock blocks were passively displaced. In some cases, an additional minor component of flattening affected both the dykes and their host rocks.

Key words: Foliated dykes, monzonite dykes, strain analysis, ptygmatic folds, cone sheets, bell-jar sheets, ring intrusion, central intrusion, Setesdalen, Sveconorwegian.

L. Skjernaa [ ] & S. Pedersen [svendp@geo.geol.ku.dk] Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. 8 March 1999.

Get pdf-file of paper (1200 kB)