Torgeir Andersen

Abstract Caledonian nappes of southern Norway comprise basement slices and sedimentary cover, and generally have close affinity with autochthonous terranes in the Fennoscandian foreland. The nappes can be distinguished in those having Gothian (1700–1600 Ma) character and those of Telemarkian (1550–1480 Ma) type. There were also several periods of variously intense magmatism between these early accretionary events and the end of the Sveconorwegian Orogeny. Most prominent features are the anorthositic-noritic massifs, which formed mainly during the Telemarkian period and in the late stages of the Sveconorwegian Orogeny between 980 and 920 Ma. In this paper, we review the main events affecting the crystalline parts of the nappes and report new results from the Eikefjord Nappe. An augen gneiss yields a crystallization age of 1191 ± 2 Ma, which is an event rarely seen in nappes or the autochthon. An amphibolitic gneiss formed at about 1500 Ma and underwent high-grade metamorphism at 986 ± 3 Ma, followed by lower grade overprints as reflected by titanite dates ranging to 915 Ma. The Sveconorwegian metamorphism was ubiquitous in the nappes, although its intensity was locally highly variable. The association of metamorphism with the second main anorthosite generation event points to intense subcrustal magmatism, heating and deformation of the lithosphere. The various crustal forming events have different geneses, some are clearly related to subduction processes, others more consistent with localized mantle melting. Those closer in character to classical Large Igneous Provinces are the events at 1470–1420, 1280–1240 and 1220–1180 Ma.

International audienceA mélange in southern Norway comprises a matrix of garnet, mica- and black carbonaceous schists and phyllites of abyssal origin, interlayered with originally coarser grained siliciclastic metasediments, serpentinite conglomerates and sandstones, solitary metaperidotites and thin slivers of gneisses. Several models for the formation of the mélange have been suggested, including formation as a) an ophiolitic mélange formed during ophiolite obduction, b) an unconformable post-obduction transgressive sequence or c) a mélange formed during hyperextension along the pre-Caledonian margin of Baltica. In the past, the mélange has therefore not been treated as one single tectonic unit, but has been assigned to various tectonic positions with both outboard Iapetus and inboard Baltican origins. In this study, we argue that the mélange occupies a tectonostratigraphic position below major Baltican basement nappe-complexes previously assigned to the Middle Allochthon. Furthermore, we present new consistent resultson the peak metamorphic temperatures (T ∼500 °C), based on RSCM, and a characteristic δ18Ocarb isotope composition (11-15.5 permille SMOW), both consistent for more than 250 km along strike of the mélange. δ13Ccarb values fall within three clusters around 1, −2, and −7 permille (PDB), respectively.The stable isotope investigation presented here was carried out in order to explore if pre-Caledonian isotope signatures in various generations of carbonate veins and the early Ordovician fossils at Otta, could have been preserved through a later Caledonian metamorphic overprint. The results presented here, however, suggest that reequilibration of the carbonates took place in the Silurian, most likely coeval with peak metamorphism of ~500 °C at ~420 Ma, and the main fabric development close to the base of the nappe stack. Reequilibration of the carbonates was assisted by the presence a pervasive static fluid, allowing for oxygen isotope exchange with the surrounding schists and resulting in a mélange-wide uniform δ18Ocarb signature. The carbon isotope composition was re-equilibrated only within each lithological body and notably not beyond lithological boundarie

Metabasalts of the Upper Ordovician Solund-Stavfjord Ophiolite Complex of the westernmost Norwegian Caledonides, show N-to E-MORB affinity, with high Th/Ta (or Nb) ratios giving evidence of subduction influence. The Solund–Stavfjord Ophiolite Complex is overlain by a heterogeneous assemblage of sedimentary and volcanic rocks, the Stavenes Group, of which the Heggøy Formation of metasandstones and phyllites conformably overlies the metabasalts of the Solund–Stavfjord Ophiolite Complex. The Heggøy Formation contains, in places, abundant metabasalt pillow lavas and minor intrusions, geochemically similar to those of the Solund–Stavfjord Ophiolite Complex, and basic metavolcaniclastites of island arc tholeiite (IAT) composition. This indicates that the Solund–Stavfjord Ophiolite Complex and Heggøy Formation developed in a marginal basin between a continental margin and an active subduction system, for which the present-day Andaman Sea may provide a realistic model. The other magmatic ro...

Basins formed at magma-poor hyperextended margins that are partly floored by transitional crust exhibit a characteristic lithological assemblage including exhumed subcontinental mantle, basement slivers, syn-rift deep- marine basin sediments, coarse-grained sediments derived from pre-rift supracrustal rocks or basement, and minor mafic intrusives/volcanics. Such a lithological assemblage is preserved below the large crystalline nappes in South Norway, including the Jotun, Upper Bergsdalen, and Lindas nappe complexes and comprises metapelites, metasandstones, metaconglomerates, solitary serpentinite bodies, detrital serpentinite-soapstone conglomerates, minor late Scandian intrusives, and thin discontinuous sheets of Proterozoic and Ordovician gneisses. This rock association, non-genetically referred to as “melange”, can be traced almost seamlessly from Bergen to Otta and is interpreted to represent the vestiges of the pre-Caledonian magma-poor hyperextended margin of Baltica. This study focusses on the metamorphism and metasomatism of the metapelites and the serpentinites. The metamorphic mineral assemblage of the metapelites includes white mica, chlorite, biotite, plagioclase, and quartz as well as local garnet and amphibole. The ultramafics are to various degrees serpentinised, talcified, and carbonated. The metamorphic mineral assemblage of the serpentinites includes serpentine, olivine, talc, carbonates (magnesite, dolomite), magnetite, diopside. The majority of the olivine grains are markedly rich in Mg (Fo0.9-0.98) and the diopside grains are rich in Ca and poor in Al, which suggests that both olivine and diopside are secondary (Enger 2016, Master’s thesis, UiO). Locally, olivine, diopside, and carbonate are replaced by serpentine. Talc-carbonate, actinolite-chlorite, and talc-chlorite schists were locally developed during blackwall alteration of the ultramafics and the country rocks. Peak metamorphic temperatures for the metapelites were estimated at approximately 500 ±50 °C by the means of Raman spectroscopy of carbon-rich material (RSCM) and are remarkably consistent along strike of the melange zone. Comparisons of the metamorphic mineral assemblage and the RSCM results with thermodynamic calculations that are based on realistic whole rock compositions of the metapelites indicate pressures of 5.25-7.25 kbar during Scandian thrusting. The lateral consistency of the metamorphism is furthermore indicated by the isotope composition of carbonates in the melange. The δ18OCarb values of the partly carbonated serpentinites and the metasediments fall within a narrow range between +11 and +15.5 ‰ (SMOW) whereas the δ13CCarb values fall into three groups: (1) +1.6 to +0.3 ‰ (PDB), (2) −1.8 to −3.9 ‰, and (3) −6 to −8.6 ‰, irrespective of their textural setting, i.e. whether the carbonate is biogenic, vein material, or replacing serpentine minerals. The unit-wide homogenization of the δ18O values in a narrow range and the only locally occurring equilibration of the δ13C values suggest fluid-rock alteration with an externally derived, mostly aqueous fluid. During prograde metamorphism of metapelitic rocks aqueous metamorphic fluids are released by the formation of, e.g., biotite and garnet on the expense of chlorite and feldspar. Aqueous fluids are also released during the dehydration of serpentinite, e.g. by the breakdown of talc and serpentine minerals to form forsterite and orthopyroxene. Thermodynamic calculations indicate that the metapelites may have released up to 4 wt% and the metaperidotites up to 9 wt% of aqueous fluids during prograde metamorphism. Minor amounts of carbonic fluids may have been produced by the decarbonation of the metapelites and the partially carbonated serpentinites. We suggest that the ultramafics were already mostly or fully serpentinised prior to Scandian thrusting and underwent prograde metamorphism and dehydration together with the metapelites. Fluid-rock interactions with metamorphic aqueous fluids homogenized unit-wide the δ18OCarb values and locally the δ13CCarb values. The shared Scandian metamorphic history demonstrates that the rocks of the melange represent an almost seamless tectonic unit in the Caledonides of South Norway and were juxtaposed prior to the onset of Scandian thrusting most likely in the pre-Caledonian magma-poor hyperextended margin of Baltica.

The break-up of continents generates dyke swarms, basins and hyperextension assemblages. Once incorporated into new orogens, the latter can provide crucial information about origins and tectonic processes. The Nålfjell Complex in the Caledonian Skillefjord Nappe of northern Norway has many of the characteristics of a hyperextension assemblage, notably the presence of solitary serpentinite bodies exposed by the exhumation of serpentinized mantle and now embedded in schists, amphibolites, mylonites, marbles and felsic gneisses. The Skillefjord Nappe is a lithologically diverse, imbricated and discontinuous allochthon. It consists of felsic gneisses and dykes dated by zircon U–Pb to c. 3100, 2940, 2830, 2510 and 1800–1750 Ma. These rocks yield titanite ages of 2810–2700, 1750, 1660–1590 and 430–420 Ma. Metagabbro intruded at 1995 Ma. The ages and evolution of the Skillefjord Nappe are distinct from those of the structurally higher Svaertholt Terrane (>1030 Ma sediments deformed and ...

Devastating seismic events occur mainly in subduction zones, and a significant percentage of them are intraslab earthquakes. The geologic record of these events holds valuable information that needs to be investigated for a comprehensive seismic risk assessment. Here we investigate pseudotachylytes formed in oceanic peridotites and that are interpreted to result from intraslab seismic rupture. Each vein has recorded the seismic slip direction and slip sense of a single coseismic shear‐heating event. The well‐preserved exposures, showing individual veins up to 7 m in length and about 3 cm in width, of Cima di Gratera, in the Schistes Lustrés ophiolitic units of Corsica, offer unparalleled opportunities to investigate intraslab rupture kinematics in mantle rocks. The principal ferromagnetic phase in these rocks is a Ti‐poor magnetite. We use the anisotropy of magnetic susceptibility (AMS) recorded in pseudotachylyte generation veins (bulk susceptibilities range from 600 to 20,000 × 10...

The UHP province of Western Norway is the part of the baltic shield subducted to more than 100 km depth during the Caledonian orogeny. Field study in the Nordfjord area, north to the Hornelen Devonian basin reveas the heterogeneous behavior of the continental crust during its exhumation. Crustal-scale boudinage occurs during ductile constrictional stretching in amphibolitic conditions, preserving UHP and protolith in the core of boudins. The rims of the Nordfjord area boudin are characterized by a pervasive migmatization during exhumation, that lowers the average viscosity of the subducted crust. Top-to-the-west shearing associated to the Nordfjord-Sogn detachment overprints this constrictional fabric. Dating of a UHP eclogite in Stadlandet area by U-Pb method on rutile and omphacite fractions gives an age of 389.4+/-3.4 Ma for the crystallization of the UHP paragenesis. The same method used on titanite and Kfeldspars fractions from the surrounding migmatitic gneisses gives an age o...

The Caledonian orogeny in East Greenland and Scandinavia is typically portrayed as Ordovician to Silurian destruction of the Iapetus Ocean ending by continental collision of Baltica and Laurentia. The orogen is highly asymmetric, and typically modelled as subduction of Baltica, below the overriding Laurentian plate. The Scandinavian Caledonides are characterized by a deeply subducted rifted margin, metamorphosed to Ultra High

ABSTRACT The Solund basin of western Norway developed in the hanging-wall of a large extensional detachment zone in the lower Devonian. The basin consists of an approximately 10km thick succession of primarily conglomeratic sediments, but also includes an anomalous brecciated body composed of gabbro, rhyolite and metamorphosed quartzite of uncertain origin near the basal deposits of the basin. The body has been previously interpreted as an in-situ intrusive and volcanic rocks, as a thrust slice, and as a landslide. Previous U/Pb zircon dating of glassy felsic rocks within the body (439±3 Ma) suggest a lowermost Silurian age of the volcanic protolith. Detailed field mapping combined with microtexture analysis and laboratory modeling is currently used to determine the origin of this body and investigate the physical mechanisms that control the brecciation process. Based on the basal structure, the geometry of the body, and an unconformable upper contact with the conglomerate we present evidence to support the landslide interpretation. The unit is characterized by a large degree of brecciation, including "jigsaw breccias", particularly common within the foliated metamorphic rocks of the body. These breccias contain fragments ranging from microscopic grains to intact blocks up to 30m in length. The breccias are clast supported and the clasts have rotated around multiple axes, with little apparent space for these rotations to occur. The metamorphic rocks exhibit a higher degree of brecciation than the more massive gabbroic to granitoid rocks in contact with them. This apparent intrusive relationship, seen at many localities within the body, suggests that the majority of the jigsaw brecciation occurred prior to emplacement, with the body remaining quite coherent and preserving internal relationships through the emplacement event. A simple brecciation mechanism does not fully explain the observed morphology and arrangement of the breccia. We are currently addressing this problem and will present preliminary results from our analysis.