ISSN:0016-786X    

DOI:10.1080/11035898009454490

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The mechanical behaviour, microstructure and substructure of the classic Tännäs augen gneiss of the Swedish Caledonides are presented. Progressive deformation of granodiorite to augen gneiss and mylonite is shown to take place in an isochemical environment with redistribution of megacrysts to surrounding matrix. Experimental deformation of natural rocks from the Tännäs Augen Gneiss Nappe has been carried out in the temperature range of 20–600°C and under confining pressure of 100–700 MPa. The highest strength found is for the fine-grained mylonite. The augen of the gneiss show brittle behaviour under all test conditions, and temperature has a limited effect on the ultimate strength of the matrix of the augen gneiss. Microstructures have been analysed by means of optical microscopy and transmission electron microscopy. The progressive deformation is brought about by dislocation glide and dislocation creep. Subgrain formation, recrystallization and straining of new grains are observed at all stages of the progressive deformation of the matrix while the augen behaviour is brittle.

ISSN:0016-786X    

DOI:10.1080/11035898009454491

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Metamorphism in the nappe terrains of the Central Scandinavian Caledonides is characterized by inverted gradients and other indications of disequilibrium caused by the superposition of nappes of various metamorphic grade and the translation of the nappe pile onto the margin of the Baltic Shield. In the lower nappes there is a gradual westward increase of grade, but some inverted gradients persist. In the upper allochthon, the variation of metamorphism is essentially vertical. It is possible to identify and evaluate parageneses formed prior to, during, and after nappe emplacement. Their interpretation in terms of separate Caledonian or Precambrian events is often ambiguous. Early Caledonian metamorphism apparently represented a range ofP-T-conditions. The build-up and translation of the nappe pile was characterized by transient high pressure conditions. A metamorphic pulse, manifest particularly in the west, succeeded nappe emplacement onto the Baltic Shield. Strain-induced metamorphism in the vicinity of thrusts contributes to the overall metamorphic pattern. A variety ofP/T-ratios characterized metamorphism in the Scandinavian Caledonides. This conclusion modifies current concepts about an essentially Barrovian metamorphic evolution of the orogen.

ISSN:0016-786X    

DOI:10.1080/11035898009454492

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Two Seve Nappes occur in the Åreskutan area. The lower nappe is composed of a layered sequence of tectonized metasediments and amphibolites, metamorphosed in the amphibolite facies. The upper nappe (Åreskutan Nappe) consists of high-grade migmatitic gneisses and metabasites. The two nappes are separated by a mylonite zone. A remnant of extensively mylonitized Särv Nappe is believed to occur between the Lower Seve Nappe and the Silurian rocks of the Lower Allochthon. The Särv Nappe is metamorphosed in greenschist facies. A phyllonitic zone separates the Särv Nappe from the Lower Seve Nappe. The mineral assemblage in the amphibolite facies Lower Seve Nappe indicates recrystallization temperatures around 550°C and pressures of 200–500 MPa. The distribution coefficients for the partitioning of Mg and Fe between biotite and garnet are aboutKD= 0.18. Strong retrogression occurred in the mylonitic basal parts of the Särv and Seve Nappes during Caledonian nappe translation. In the gneisses of the Åreskutan Nappe, a granulite facies metamorphism at >650°C and > 500–700 MPa was overprinted by amphibolite facies metamorphism. Biotite-garnetKD-vaIues from the Åreskutan Nappe cluster around 0.30, indicative of high-grade metamorphism. Polymetamorphism is possibly reflected by a considerable spreading of theKD-values from 0.26 to 0.58. Caledonian pegmatites cut the Åreskutan Nappe. They are mylonitized at the base of the nappe. No retrogression has taken place during the formation of the mylonitic foliation, which indicates that this process took place under relatively high-grade conditions. This is confirmed by theKD-values from recrystallized garnets and biotites in the mylonite. It is believed, therefore, that this mylonite formed during Caledonian nappe translation, when the Åreskutan Nappe was superposed on the Lower Seve Nappe. Subsequent displacement of the two Seve Nappes together on to the underlying tectonic units occurred under much lower metamorphic conditions. Recent Rb-Sr age determinations suggest that a high-grade metamorphism of the Åreskutan Nappe is of Caledonian age, although the rocks of this nappe may have had a pre-Caledonian history. The few age determinations (fission track) of minerals in the Lower Seve Nappe gave Proterozoic ages.

ISSN:0016-786X    

DOI:10.1080/11035898009454493

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The Middle and Upper Cambrian shaly sequence of the Östersund-Åsarna area along the Caledonian margin is reviewed, partly on the basis of new borings performed by the Geological Survey of Sweden. The uppermost Middle CambrianLejopyge laevigataZone is met with for the first time. An exceptional occurrence of fossils of Middle Cambrian character such asAcrothelecf.granulatain limy sediments laid down in oxidating environment in the Upper CambrianParabolina spinulosaZone indicates that the profound difference in faunal aspect between the Middle and the Upper Cambrian has an ecological background.

ISSN:0016-786X    

DOI:10.1080/11035898009454494

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Based on collections from 110 localities, the temperature-induced color change associated with low-grade metamorphism in conodonts is used for the first regional assessment of the degree of heating of Ordovician rocks in Scandinavia and the British Isles. Baltoscandian Platform rocks without intrusives in southern Sweden have apparently not been heated above 90°C. The autochthonous rocks in Jämtland appear to have been heated to a locally specific temperature between 110 and 400°C (perhaps even higher in some areas). The samples from the Oslo region, as well as those from the allochthon in the Trondheim region and the eastern nappes in Jämtland, suggest heating temperatures in excess of 300°C. Most collections studied from the British Isles indicate heating to more than 300°C, but those from the Girvan area and the Welsh Borderland show little, if any, thermal effects. In most cases, the conodont temperature indications fit well into, and clarify, the known geologic history of the study areas but at least two areal ‘anomalies’ are currently difficult to explain adequately.

ISSN:0016-786X    

DOI:10.1080/11035898009454495

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The dolerites of the main part of the Särv Nappe contain, after progressive alternating field and thermal demagnetization, a stable magnetic component of decl. 21.4°, incl. −9.1°, α25= 12.4°,k= 11.3. This gives a geomagnetic pole at lat. 21°N, long. 170°E which is in agreement with published Silurian-Devonian poles from Fennoscandia. This together with a simple ‘fold test’ implies that the Proterozoic dolerites became remagnetized during the Caledonian orogeny. The magnetic directions from the dolerites in the northern part of the nappe, the Ottfjället block, indicate that this area has rotated ca. 100–110° counterclockwise in relation to the rest of the nappe.

ISSN:0016-786X    

DOI:10.1080/11035898009454496

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The Tännäs Augen Gneiss Nappe and parts of the related, subjacent Veman Nappe are composed of K-feldspar megacryst bearing granodiorites, which have been subject to low-grade metamorphism and varying degrees of deformation. They form parts of the Middle Allochthon — a long-transported nappe unit in the central Scandinavian Caledonides. The least-deformed granodiorite has a Rb-Sr age of 1610 ± 85 Ma and Sr1= 0.7029, while zircons from the same rock give a discordia line with an upper intercept at 1685 ± 20 Ma. These dates reflect the age of intrusion of the rock. The granodiorite is comparable in composition and age with post-Svecokarelian granitoids and related volcanites east of the mountain belt. The more typically deformed augen gneiss does not yield isochrons, but shows disturbed Rb-Sr patterns both for specimens sampled regionally and on smaller scales. At the thrust contacts towards over- and underlying units, where the deformation was extreme, the gneisses have been transformed to dense mylonites. One specimen from the mylonite which separates the Tännäs Augen Gneiss Nappe from the overlying Särv Nappe has been divided into thin slices; these give a Rb-Sr isochron with an early Ordovician age of 485 ± 50 Ma, while mylonites collected on a larger scale scatter about this isochron. The age most probably reflects the time of development of the mylonite. The Sr isotopic equilibration during the Caledonian orogeny has approached completion only in the mylonite and even there only on a decimetre-scale. It has probably taken place mainly by grain boundary diffusion combined with extensive recrystallization.

ISSN:0016-786X    

DOI:10.1080/11035898009454497

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Total magnetic field data have been measured and interpreted along selected profiles in an area through the Jämtland-Trøndelag Caledonides. The object has been to model local and regional structures and investigate their possible relevance for interpretation of Caledonian tectonics. Two different methods of interpretation have been applied: analytic signal analysis, and fitting of polygonal models: they yield partly consistent, partly supplementary results. With few exceptions, the Caledonian cover units are weakly magnetized and are only responsible for very local anomalies. The main anomalies derive from the autochthonous-parautochthonous crystalline basement. The prominent regional anomaly in the area (the Jämtland Anomaly) is found to originate from the Proterozoic sequence of Dala granites and volcanics that crop out in the autochthon south of the area and obviously continue northwards as a basement below the Caledonian rocks. It forms a huge rock volume, 500 km long, 200 km wide and about 10 km thick. The assigned magnetic susceptibility, 0.075 SI-units, is supported by the existing rock magnetic data. The magnetic model of the basement surface below the Caledonian cover shows a pattern of synforms and antiforms in general agreement with the existing geological information. However, a previously unpredicted antiform is found at the eastern front of the Offerdal Nappe near Landön. Caledonian activity appears to have shaped the basement surface. It may also be responsible, at least partly, for some internal structure in the Precambrian basement, such as is seen in the alignment of the regular linear western flank of the Jämtland Anomaly with the western limb of the Sylarna Antiform.

ISSN:0016-786X    

DOI:10.1080/11035898009454498

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Density and magnetic properties were determined on some 1350 rock samples, taken from the different lithologies in the Caledonides of Jämtland, Sweden. The density determinations showed a strong trend of increasing density when moving from east to west in the investigated area. There was also a general increase in density upwards in the tectono-stratigraphy from the autochthon to the Seve of the Seve-Köli Nappe Complex. The determination of magnetic properties showed that east of the Caledonian Front the dominating high-susceptibility rock was the Rätan granite. In the Eastern Complex, west of the Caledonian Front, high-susceptibility rocks were found in the parautochthonous and allochthonous crystalline basement, whereas in the Western Complex the Ottfjället dolerite in the Särv Nappe was the dominating high-susceptibility rock.

ISSN:0016-786X    

DOI:10.1080/11035898009454499

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor