Torgeir B. Andersen

Extensional tectonics in the Caledonides of southern Norway, an overview

The extensional collapse of the Scandinavian Caledonides resulted in rapid tectonic denudation of the orogen, exhumation of high- and ultra-high-pressure metamorphic rocks and provided a structural template for the formation of Devonian supra-detachment sedimentary basins. The geometry and intensity of the extensional deformation show considerable variation vertically in the crustal section as well as horizontally from east to west across the orogen. The most prominent structural feature related to the extension in central-south Norway is the change in the direction of tectonic transport, from the easterly directed nappe translation during the Silurian Scandian Orogeny, to top-westerly directed sense of shear during the extension. The Fennoscandian basement was little affected by extension in the eastern Caledonides. In the west, however, top-to-the-west shear zones are commonly observed in basement windows. Deformation affecting the Cambrian to Late Silurian rocks in the Caledonian foreland developed a typical of foreland fold and thrust belt geometry. Deformation in the foreland was apparently contemporaneous with the extension-related decompression of the high-pressure rocks in the hinterland. Thrusting in the foreland may thus have been driven by gravitational collapse, and as such have important similarities to the foreland‐hinterland relationships of the Himalayan‐Tibetan region. The basal contacts of the Jotun and other major nappes constitute prominent shear zones in which fabrics related to thrusting have been mostly destroyed by extensional shearing. The high structural levels of the Western Gneiss Region, adjacent to the western margin of the Jotun Nappe, were only moderately affected by the extensional deformation. Consequently, the Proterozoic orthogneiss complexes are generally well preserved in this area. The westernmost and structurally lowermost parts of the Western Gneiss Region have, however, been subjected to extreme overburden during the Caledonian continental collision. Initial, near-isothermal decompression of the high-pressure rocks occurred by non-rotational vertical shortening‐horizontal stretching at eclogite- to amphibolite-facies conditions; at a later stage, decompression and cooling from amphibolite to greenschist facies occurred by rotational deformation associated with the large-scale extensional detachments. The initial extensional deformation in the hanging wall of the detachments in western Norway commenced at greenschist-facies conditions, and became progressively more brittle and localised as the complexes were exhumed in the Late Silurian to Middle Devonian. Major syn-depositional normal faults in the hanging wall of the extensional detachments eventually controlled sedimentation in the Devonian supra-detachment basins.

The Sunnfjord Melange, evidence of Silurian ophiolite accretion in the West Norwegian Caledonides

A major composite terrane, the Sunnfjord Melange, has been identified in the West Norwegian Caledonides. The rocks of the melange provide a terrane-link between the allochthonous continental rocks of the Dalsfjord Suite with its cover of continental margin deposits and the oceanic terrane of the Solund-Stavfjord Ophiolite Complex. The melange was formed as the ophiolite was emplaced on the the fossiliferous Lower to Middle Silurian continental margin deposits of the Herland Group. This group unconformably overlies older metasedimentary rocks of the Høyvik Group and the crystalline basement of the Dalsfjord Suite. A structural style characteristic of thin-skinned thrust-foldbelts is locally preserved within the Herland Group on Atløy, and the thrust-foldbelt was developed in the foreland of the continental margin during the ophiolite accretion. A U-Pb zircon age of 443 ± 3 Ma from the ophiolite, the Silurian fossils in the Herland Group and the identification of the Sunnfjord Melange as an obduction melange provide the basis for a well constrained model of ophiolite accretion in the Scandinavian Caledonides.

Exhuming Norwegian ultrahigh-pressure rocks: Overprinting extensional structures and the role of the Nordfjord-Sogn Detachment Zone

[1] The Nordfjord-Sogn Detachment Zone (NSDZ) is widely cited as one of the primary structures responsible for the exhumation of Norwegian (ultra)high-pressure (UHP) rocks. Here we review data from the considerable volume of research describing this shear zone, and compile a strikeparallel cross section along the NSDZ from the Solund Basin in the south to the Soroyane UHP domain in the north. This cross section highlights several previously unrecognized patterns, revealing a shear zone with top-to-the-west asymmetric fabrics that (1) initiated at amphibolite facies, (2) overprints metamorphic breaks and tectonostratigraphic contacts, and (3) has a gradational continuum of muscovite cooling ages. These patterns constrain the kinematic evolution of the NSDZ and suggest a new three-step model for the exhumation of Norwegian (U)HP rocks. The initial stages of exhumation were characterized by the rise of crustal rocks from (U)HP depths to the base of the crust by buoyancydriven mechanisms not specified in this paper. Mantle exhumation was followed by top-to-thewest, normal-sense displacement within a broad noncoaxial ductile shear zone near the base of the crust that overprinted tectonostratigraphic contacts formed previously during mantle exhumation. In the final stages of crustal exhumation, top-W brittleductile detachments soled into and partially excised this ductile shear zone, dropping the Devonian basins into contact with rocks of varying tectonostratigraphic levels. This new interpretation of the NSDZ is significant as it accounts for the extreme crustal excision observed in western Norway using three sequentially overprinting structures active at different stages of UHP rock exhumation. Citation: Johnston, S. M., B. R. Hacker, and T. B. Andersen (2007), Exhuming Norwegian ultrahigh-pressure rocks: Overprinting extensional structures and the role of the Nordfjord-Sogn Detachment Zone, Tectonics, 26, TC5001,

Deep crustal fabrics and a model for the extensional collapse of the southwest Norwegian Caledonides

The exhumed deep crustal rocks in the Western Gneiss Region (WGR) of Norway experienced Caledonian high-pressure metamorphism during the Silurian, Scandian continental collision between Baltica and Laurentia. The record of coesite-bearing eclogites and pressure-temperature estimates from the WGR demonstrate extreme burial of these rocks at Pmax. Eclogite tectonite fabrics record coaxial deformation characterized by bulk horizontal shortening and vertical stretching. Many eclogites, particularly those with a high content of kyanite, quartz, phengite and clinozoisite have constrictional fabrics related to vertical stretching. Fabrics that developed during orogenic extensional collapse are of two main types. The deepest exposed sections are dominated by penetrative coaxial fabrics that are characterized by vertical flattening and horizontal, E-W, stretching. These fabrics developed during rapid decompression and were associated with, and locally enhanced by, partial melting of the deep crust. The collapse-related coaxial vertical shortening and horizontal stretching developed at granulite to amphibolite facies and is overprinted by non-coaxial deformation that formed thick mylonites along extensional detachments. The detachment zones are rooted in the coaxially deformed deep crust, and separate the exhumed deep-crustal rocks of the Lower Plate, from the rocks in the hanging-walls that are characterized by medium- to low-grade Caledonian metamorphism. Devonian basins were formed by extensional faulting in the upper crust, and the faults that controlled the sedimentation were rooted in the extensional detachments.

Primary sulphide melt inclusions in mantle-derived megacrysts and pyroxenites

Inclusions of sulphides are common in clinopyroxene megacrysts and Al-augite pyroxenite xenoliths in undersaturated continental basalts. The sulphides are typically FeS with 2–4 wt.% Ni and minor Co and Cu. The morphology of the inclusions and their relations to growth planes in the pyroxenes show that the sulphides were trapped as drops of immiscible sulphide melt. These nucleated on the surfaces of crystals growing from magmas, and are thus primary inclusions. Compound C02-sulphide inclusions are evidence for the coexistence of three immiscible fluids-silicate melt, sulphide melt and supercritical CO2. Hollow tubular to spherical sulphide inclusions result from the trapping of sulphide melts with up to 10 wt.% CO2 in solution. Primary CO2 inclusions have densities of 1.07–1.189 g cm-3; this corresponds to trapping pressures of 10–15 kbar at 1000–1200°C and confirms the origin of the megacrysts in the uppermost mantle. A wide variety of secondary inclusions formed by the decrepitation of the sulphide-CO2 inclusions during entrainment and transport by the host basalt. Clinopyroxene megacrysts containing sulphide inclusions show a very narrow compositional range, compared to the worldwide population of AI-augite megacrysts. This suggests that S-saturation was reached at a similar stage of mantle fractionation in similar magmas. Dumping of S (and Ni, Co, Cu) in the uppermost mantle is an important part of the overall process of mantle metasomatism.

The middle Devonian basins of western Norway: sedimentary response to large-scale transtensional tectonics?

Abstract The Devonian basins of western Norway were formed during late- to post-orogenic extension of overthickened Caledonian crust. The basins are situated in the hanging wall of the extensional Nordfjord–Sogn Detachment Zone (NSDZ) and display extensional half-graben geometries in sections parallel to the local direction of principal extension. Based on overall facies configurations, paleocurrent patterns and intrabasinal structures, we infer an anticlockwise rotation of the syndepositional extension direction from NW–SE in the south (Solund basin) to WSW–ENE in the north (Hornelen basin). The axes of folds that are roughly parallel to the local extension direction are rotated correspondingly. The Kvamshesten basin is located between the Solund and Hornelen basins. Sedimentological and structural data show evidence of an early, southeastwards tilt direction followed by a more eastwards tilt and associated E–W flowing paleodrainage. Correspondingly, NW–SE trending folds and reverse faults are superposed by E–W trending ones at low to intermediate stratigraphic levels. The variations in apparent tilt direction for the basins together with variations in intrabasinal structure is interpreted to reflect an anticlockwise rotation of the regional syndepositional strain field. The above observations and inferences indicate that the Devonian basins in western Norway formed in a strain field dominated by regional transtension, accommodated by extension along the NSDZ and sinistral strike–slip along orogen-parallel shear zones and faults to the north of the basins; alternatively, NW-directed extension preceded the introduction of a sinistral strike–slip component. The models are in accordance with recent work carried out in the footwall of the NSDZ and illustrates the tectono-sedimentary response to a complex interplay between extension and strike–slip that appears to have been fundamental in the late-stage disintegration of the Caledonian orogen.

Petrogenesis of spinel harzburgite and dunite suite xenoliths from Lanzarote, eastern Canary Islands: Implications for the upper mantle

Abstract We present data on petrography, mineral and whole rock major element relations and fluid inclusions on ultramafic xenoliths from Quaternary to Recent alkaline basalts in Lanzarote, eastern Canary Islands. The xenoliths have been divided into two main suites: the spinel harzburgite suite (harzburgites and rare lherzolites) and the spinel dunite suite (spinel dunites and rare spinel-plagioclase dunites) The spinel-harzburgite suite xenoliths from Lanzarote represent fragments of highly refractory, old suboceanic lithospheric mantle similar to that found beneath Hierro in the western part of the Canary Island chain. This mantle has been somewhat modified through a combination of melt extraction and metasomatism caused by infiltration of Fe-Ti-rich silicate melts and CO 2 fluids, probably in association with the formation of the Canary Islands. Also the spinel-dunite suite xenoliths show oceanic affinities, but are not directly related to the harzburgites through partial melting. Temperature estimates combined with isochores representing the densest CO 2 inclusions ( T h of − 12 ° C) in these nodules indicate a high geothermal gradient in the upper mantle under Lanzarote, 1100 ° C at ⩾ 26 km depth, and a correspondingly thinned lithosphere ( ⩾ 27 km). This implies much hotter conditions than those expected in “normal” suboceanic lithospheric mantle of an age corresponding to that off West Africa, and hotter conditions under Lanzarote than under the western Canary Islands. A possible explanation for this is the presence of a mantle plume under the Canary Islands, which causes thermal erosion at the base of the lithosphere, whereas ascending plume melts are responsible for heating, partial melting and metasomatism in the overlying mantle. Edge effects such as small-scale convection caused by interaction between hot plume material flowing eastwards underneath the lithosphere and the continental margin of West Africa, may account for enhanced thermal erosion under the easternmost Canary Islands and recurrent volcanism.

Structural, mineralogical and petrophysical effects on deep crustal rocks of fluid‐limited polymetamorphism, Western Gneiss Region, Norway

The Proterozoic banded granulite facies complex of BÅrdsholmen, Western Gneiss Region, Norway (T=815–845°C) was locally transformed to eclogite (T=455–510°C, P>12 kbar) and amphibolite facies rocks (T=460°C) during the Caledonian continental collision. The granulite complex consists of mafic two‐pyroxene granulite and leucocratic orthopyroxene+garnet‐bearing layers alternating on a scale from 1 cm to 10 m. The granulite facies rocks change to eclogite facies rocks over centimetre‐scale distances along well defined fluid‐infiltration fronts. The mafic granulite was transformed to omphacite+garnet‐rich eclogites and the leucocratic rocks were converted to quartz+phengite‐rich assemblages with minor garnet and local omphacite. Melange‐like lithologies consisting of mafic lenses of eclogite surrounded by felsic material represent an advanced stage in the process of converting deep crust to eclogite facies. During amphibolitization this melange‐like lithology evolves to a rock where amphibolite lenses and layers are surrounded by granitoid gneiss, a lithology typical of the Western Gneiss Region. The BÅrdsholmen locality illustrates the profound control exerted by fluids on the timing of metamorphism, the structural make up and petrophysical properties such as density and rheology of crustal root zones. Fluid‐induced metamorphism will therefore exert control on the attributes of orogenic belts such as topography and Moho depth and influence the dynamics of collision zones by controlling the time of orogenic collapse and the buoyancy of the subducted crust. We suggest that orogens may develop differently depending on the fluid budget.

Devonian, orogen-parallel, opposed extension in the Central Norwegian Caledonides

Late orogenic, Early to Middle Devonian extension in the Scandinavian Caledonides was unidirectional in western Norway. New data from two detachment zones (Hoybakken and Kollstraumen) north of the More-Trondelag Fault Complex show that bidirectional, opposed, orogen-parallel extension dominated in this region. At this time, the fault complex acted as a transfer zone for the Hoybakken detachment. Extension and uplift in central Norway triggered significant magmatic activity, in contrast to the lack of granite intrusions during exhumation of western Norway.

Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region, Norway: indications of incomplete transition at high pressure

The Western Gneiss Region of Norway is a continental terrane that experienced Caledonian high-pressure and ultrahigh-pressure metamorphism. Most rocks in this terrane show either peak-Caledonian eclogite-facies assemblages or are highly strained and equilibrated under late-Caledonian amphibolite-facies conditions. However, three kilometre-size rock bodies (Flatraket, Ulvesund and Krakenes) in Outer Nordfjord preserve Pre-Caledonian igneous and granulite-facies assemblages and structures. Where these assemblages are preserved, the rocks are consistently unaffected by Caledonian deformation. The three bodies experienced high-pressure conditions (20–23 kbar) but show only very localized (about 5 %) eclogitization in felsic and mafic rocks, commonly related to shear zones. The preservation of Pre-Caledonian felsic and mafic igneous and granulite-facies assemblages in these bodies, therefore, indicates widespread (~ 95 %) metastability at pressures higher than other metastable domains in Norway. Late-Caledonian amphibolite-facies retrogression was limited. The degree of reaction is related to the protolith composition and the interaction of fluid and deformation during the orogenic cycle, whereby metastability is associated with a lack of deformation and lack of fluids, either as a catalyst or as a component in hydration reactions. The three bodies appear to have been far less reactive than the external gneisses in this region, even though they followed a similar pressure–temperature evolution. The extent of metastable behaviour has implications for the protolith of the Western Gneiss Region, for the density evolution of high-pressure terranes and hence for the geodynamic evolution of mountain belts.