Sarv Nappe Research Articles

The Neoproterozoic Ottfjallet dike swarm of the Middle Allochthon, traced geochemically into the Scandian Hinterland, Western Gneiss Region, Norway

The Ottfjallet swarm of Late Neoproterozoic mafic dikes, cutting Neoproterozoic sandstones, became a key, 30 years ago, to the tectonics of the Scandian Caledonides. The sandstones were deposited in basins related to opening of lapetus, and intruded by dikes in distal parts of the Baltoscandian margin close to the developing spreading axis. The sandstones and underlying basement rocks were transported, from west of the present Norwegian coast, to as far east as western Sweden during the Silurian-Devonian Scandian Orogeny. The sandstones with dikes make up the Sarv Nappe, up to 2 km thick in Sweden, and the quartzites and amphibolites of the Saetra and equivalent nappes in Norway. These form the upper part of the Middle Allochthon. The lower part of the Middle Allochthon includes Middle Proterozoic basement gneisses and rapakivi granites, containing mafic rocks in some places. The dike-bearing quartzite is a key unit due to contrast with similar rocks lacking dikes at lower tectonic levels derived from inboard parts of Baltica. The Saetra Nappe and equivalents are well constrained on lithotectonic grounds in Norway at Oppdal, Leksdal, and Orkanger. It was also suspected to occur in deep, narrow synclines in the Western Gneiss Region where interlayered feldspathic quartzite and amphibolite are in correct tectonostratigraphic sequence, locally with a total thickness of only 1 to 3 m, and locally with dikes converted to eclogite. To test correlations, 127 samples of mafic rocks were collected from 14 areas west and southwest of Trondheimsfjord into the Western Gneiss region toward Alesund. Samples include mafic rocks in quartzites and others, some clearly dikes, from the underlying 1190 Ma rapakivi granite/augen gneiss of the Risberget Nappe and adjacent basement gneisses. Typical Saetra dikes in the Oppdal quarries, and mafic rocks from other quartzites and related rocks, have La n /Sm n ratios of 1.0 to 1.8, and Nb/La ratios of 0.8 to 1.4 (Oppdal group). Most REE patterns are moderately LREE-enriched with no or very small, mostly negative, Eu anomalies. All have similar multi-element patterns typically with small positive P anomalies, negative Zr-Hf anomalies and an absence of Nb-Ta anomalies, showing that the dikes are unrelated to arcs and have no notable continental crust component. A subset of Saetra, Risberget, and basement dikes is distinguished by higher La n /Sm n ratios of 1.8 to 2.5, but otherwise has very similar characteristics (Ystland Group). These data support correlation of the Saetra Nappe quartzite and dikes into highly deformed parts of the Western Gneiss Region and correlation of nearby dike-rich parts of basement gneiss with the Middle Allochthon. One sample in quartzite at Ura, at an unusual tectonostratigraphic position, has La n /Sm n = 0.7, Nb/La = 0.6, and a multi-element pattern different from Saetra dikes, suggesting it is unrelated to the Ottfjallet dikes. Non-Saetra-like amphibolites also occur in the Risberget Nappe, and have La n /Sm n ratios of 1.4 to 3, all Nb/La ratios <0.6, and multi-element patterns with sharp negative anomalies for Nb-Ta, P, Zr-Hf and Ti. These are probably Mesoproterozoic magmas and cumulates emplaced into the rapakivi granite protolith of the augen gneiss.

Density and magnetic properties of rocks in the Caledonides of Jämtland, Sweden

Abstract Density and magnetic properties were determined on some 1350 rock samples, taken from the different lithologies in the Caledonides of Jamtland, 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-Koli Nappe Complex. The determination of magnetic properties showed that east of the Caledonian Front the dominating high-susceptibility rock was the Ratan 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 Ottfjallet dolerite in the Sarv Nappe was the dominating high-susceptibility rock.

Palaeomagnetism of the dolerites of the Särv Nappe, southern Swedish Caledonides

Abstract The dolerites of the main part of the Sarv 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 Ottfjallet block, indicate that this area has rotated ca. 100–110° counterclockwise in relation to the rest of the nappe.

A probable glaciogenic sediment in the Särv Nappe, central Swedish Caledonides

Abstract A diamictite has been located near the base of the Sarv Nappe. The locality is situated 3 km NNW. of the village of Tannas in the province of Harjedalen. The rock is thought to be of glacial origin and probably of Varangien age. On this assumption the Ottfjall dolerites are not older than Varangien.