David Roberts

The Scandinavian Caledonides: event chronology, palaeogeographic settings and likely modern analogues

Abstract Following an outline of Scandinavian Caledonide tectonostratigraphy, tracing the Neoproterozoic, Baltoscandian, passive margin of Baltica outboard into the suspect, Iapetus oceanic terranes and farther into the highest allochthons, which carry evidence of Laurentian ancestry, descriptions are presented of the principal tectonothermal events recorded in different parts of the mountain belt. Four major compressive/transpressive events are recognised—Finnmarkian (Late Cambrian), Trondheim (Early Arenig), Taconian (Mid–Late Ordovician) and Scandian (Mid Silurian–Early Devonian). The first two events involved subduction scenarios with contraction/accretion between Baltica and/or an adjacent microcontinent and Iapetan arcs. The Taconian is also an arc-accretion, tectonothermal event, but this occurred along the margin of the Laurentian plate, remote from Baltica. The later Scandian event involved rapid subduction of the Baltican margin beneath Laurentia to depths exceeding 120 km and equally rapid exhumation. This was followed by an episode of orogenic collapse, widespread extension and Devonian basinal sedimentation, with local upright folding (Solundian phase). Based on comparisons between geochemical signatures and settings of Caledonide ophiolite fragments and diverse magmatic arcs, and modern examples of such, various analogies have been proposed by several authors with comparable elements in the southwestern and western Pacific region. Taking into account a predictive model for future oblique collision between the Australian and Eurasian plates, this resultant sinistral, oblique-convergent system of semi-linear terrane slices bears many similarities to the Caledonide geology and tectonics of Scandinavia.

Trace element geochemistry of Norwegian Lower Palaeozoic basic volcanics and its tectonic implications

Abstract The study of nearly 100 new trace element analyses of late Cambrian/early Ordovician greenstones from the Grong, Lokken, Storen and Stavenes areas of central and southern Norway indicates the presence of ocean floor type basalts in all four areas, and the presence of low-potassium tholeiites of island arc affinity in at least two of the areas. From this result an ocean floor-based Ordovician/early Silurian island arc complex and back-arc eugeosynclinal pile is recognised in the Trondheim region and its principal features described. This complex is considered to have been obducted during an early stage of the Middle Silurian orogeny upon an Eocambrian/Ordovician miogeosynclinal sedimentary succession which had developed on continental crust. During a later stage of the major folding and metamorphism of the juxtaposed sequences, further eastward thrusting took place such that total displacement of some allochthonous sheets may be in the order of several hundred kilometres.

Resurgent strike-slip duplex development along the Hitra-Snåsa and Verran Faults, Møre-trøndelag fault zone, Central Norway

Abstract Lineament analysis of the ENE-WSW Verran and Hitra-Snasa Faults of the long-lived More-Trondelag Fault Zone (MTFZ), has revealed the presence of a complex array of anastomosing faults and fractures. This has the geometric configuration of a major, dextral, strike-slip fault zone and compares well with characteristic patterns of braided wrench-faulting, e.g. the San Andreas Fault System of California. The fault array recognized in conjunction with the Verran Fault is clearly post-Caledonian and is considered to define a dextral, strike-slip duplex system. Associated with the parallel Hitra-Snasa Fault are Riedel-like structures which tend to point to an earlier component of sinistral movement. Rock products present along the Hitra-Snasa Fault and its secondary faults comprise mylonites, hydrothermally altered rocks and small-scale recrystallized breccias. Along the main Verran Fault, evidence of late polyphasal deformation is seen in several discrete episodes of brecciation, hydrothermal alteration, and locally pervasive prehnite and stilbite veining. The fault structures occurring along the Hitra-Snasa and Verran faults are thought to have originated as a sinistral fault system of late Devonian age, especially for the Hitra-Snasa lineament. Subsequently, strike-slip movement reversed in sense and shifted locus towards the Verran Fault system during the late Jurassic or early Cretaceous. During this strike-slip reversal, some earlier fractures related to the sinistral system were rejuvenated within the stress field of the evolving dextral duplex system. Strike-slip displacements of a similar age are known from fault complexes on the Norwegian continental shelf and in the northern North Sea. The regional picture indicates that the MTFZ was almost certainly linked to the fault systems of northern Scotland, prior to the opening of the Viking Graben.

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.

Age and structure of the southern Rockall Trough new evidence

Abstract The Rockall Trough separates the Rockall Plateau microcontinent from the shelf and slope west of the British Isles. The structure and age of the trough has been the source of considerable discussion. Although widely considered to be of oceanic origin, postulated ages for the spreading range from Permian to Cretaceous. New seismic profiles linked to the IPOD sites in the Bay of Biscay and to oceanic anomalies of known age are used to present a new assessment of the age and structure of the southern Rockall Trough. It is concluded that about 120 km of ocean crust is present in the trough and that spreading took place in the Albian-Maastrichtian interval.

Carbonate formations and early NW-directed thrusting in the highest allochthons of the Norwegian Caledonides: evidence of a Laurentian ancestry

tructural, sedimentological and isotope-geochemical investigations on thick, carbonate formations in the Uppermost Allochthon of the Norwegian Caledonides have provided the first direct indications of the Laurentian origins of these shelf and slope/rise successions. Chemostratigraphical ages for marbles mostly range from Vendian to Cambrian. The earliest tectonic deformation, involving NW-vergent thrusts and folds in one of the nappes, is considered to relate to Taconian orogenic development along the margin of Laurentia. Subsequently, during Siluro-Devonian, Baltica–Laurentia collision, these thick successions, and plutonic complexes farther south, were detached from their Laurentian roots and retransported into the higher levels of the Scandian orogenic wedge.

Geochemistry of Pan-African volcanic arc sequences in southeastern Sinai Peninsula and plate tectonic implications

Abstract Major and trace elements of lavas, dykes and plutonic rocks of the late Proterozoic orogenic sequence in SE Sinai, the Kid Group, have been analysed. The dykes and lavas of the southermost sequences, the Tarr Complex and Heib Formation, are calc-alkaline, whereas the lavas of the Malhak Formation and the Sharira Gabbro (to the north of the Heib Formation) show both calc-alkaline and tholeiitic trends. The trace element characteristics of the Tarr Complex and the Heib and Malhak Formations, despite between-sequence variations, are all comparable with ensialic island arc magmatism, whereas the Sharira Gabbro shows some MORB characteristics in addition to the island arc imprints. The Sharira Gabbro and the lavas of the Malhak Formation possibly formed in a developing back-arc basin behind a continental-marginal ensialic island arc (the Tarr Complex and Heib Formation). During the Pan-African orogeny, the constituent units of the Kid Group were mutually juxtaposed along major ductile shear zones of thrust-fault character. This plate-convergence regime involved initial magmatic arc development following northward subduction, and subsequent collision between the arc complex and the Proterozoic continental margin.

The Devonian Nesna shear zone and adjacent gneiss-cored culminations, North–Central Norwegian Caledonides

The Early to Mid-Devonian strain distribution in the Norwegian Caledonides was characterized by large-magnitude extension, extension-normal shortening and sinistral strike-slip. In SW Norway, previous work has shown that the angle between the orogen and the Devonian maximum elongation trend decreases northward towards a zone of sinistral strike-slip. We provide evidence that: (1) late- to post-orogenic extension was much more important in North–Central Norway than recognized previously; (2) the Early to Mid-Devonian, roughly orogen-parallel, maximum elongation trend persisted over more than 200 km farther north than hitherto recognized; (3) a number of gneiss-cored culminations in North–Central Norway were affected by, and probably owe their present geometry to, extensional shearing and faulting. The Nesna shear zone is identified here as a major, late Early to Mid-Devonian, low-angle extensional shear zone in the North–Central Norwegian Caledonides. In the study area, the Nesna shear zone now constitutes the lower boundary of the Helgeland Nappe Complex, a terrane exotic to Baltica. The present situation of the Helgeland Nappe Complex is thus that of an extensional allochthon. The transport direction along the medium- to low-grade Nesna shear zone was top-to-the-WSW, and thus roughly orogen-parallel. Extensional shearing in the Nesna shear zone may provide an explanation for a westward-younging, 40Ar/39Ar cooling pattern previously reported from the Ofoten–Lofoten area. Folding of the Nesna shear zone along axes parallel to the extensional transport direction indicates that extension-normal shortening continued at least into late Early and Mid-Devonian times. The WSW–ENE maximum elongation trend probably persisted in North–Central Norway through most of the Devonian period, and accompanied the transition from ductile to brittle deformation in the rocks of the Caledonian nappe-stack. Deformation in low-angle, initially medium-grade, extensional shear zones such as the Nesna shear zone was succeeded by shearing in steeper, low-grade, ductile-to-brittle shear zones that developed along the western margins of gneiss-cored culminations. Temporal overlap between activity in the Nesna shear zone and in the lower-grade shear zones that bound the culminations can be demonstrated. To the east, however, medium-grade extensional shear zones such as the present boundary between the Seve and Köli nappes were probably back-rotated as the culminations developed in the footwall of the low-grade shear zones. From the Mid–Late Devonian to Early Carboniferous, the low-grade, culmination-bounding shear zones constituted the eastern margin of a regional, transtensional system bounded in the south by the sinistral Møre–Trøndelag Fault Complex.

Isotopic stratigraphy suggests Neoproterozoic ages and Laurentian ancestry for high-grade marbles from the North-Central Norwegian Caledonides

Carbon and strontium isotope stratigraphy has been applied to constrain the depositional ages of high-grade marble sequences in the Ofoten district of the North-Central Norwegian Caledonides. Two marble formations hosted by diverse schists from the Bogen Group, all previously correlated over long distances with a Late Ordovician–Early Silurian, low-grade, fossiliferous succession, have been studied for carbon, oxygen and strontium isotopes. The least altered 87 Sr/ 86 Sr ratios ranging between 0.7062 and 0.7068, and the best preserved δ 13 C values falling between +5.0 and +6.5‰ obtained from two marble formations, are consistent with a seawater composition in the time interval 700–600 Ma. The results obtained do not support the previously proposed correlation of the Bogen Group with an Ordovician–Silurian lithostratigraphic succession further north. The apparent depositional ages suggest that the tectonostratigraphic succession studied is inverted and that the tectonostratigraphy of the region requires revision. The Neoproterozoic depositional ages combined with the palaeogeographic position of Baltica imply that carbonates were initially accumulated in seas on a continental shelf, probably Laurentia, and were tectonically transported onto Baltica during Early Silurian, Scandian collision, at c. 425 Ma. Prospecting for new dolomite marble deposits of the Hekkelstrand type and carbonate-hosted manganese–iron ores should be restricted to 700–600 Ma sequences in the Uppermost Allochthon of the Norwegian Caledonides.

Tectonostratigraphic development of the Trondheim region Caledonides, Central Norway

Abstract The metamorphic allochthon of the central Norwegian Caledonides comprises a complex of discrete nappes of metasediments and igneous rocks ranging in age from probable Svecofennian through Vendian to Silurian. This southeastward-translated allochthon overlies a thin cover of autochthonous Vendian to Cambrian sediments deposited upon a crystalline Precambrian basement, and is superseded by late-orogenic, intermontanebasinal sediments of latest Silurian to Middle Devonian age. Stratigraphical sequences in higher allochthonous units are floored by oceanic tholeiitic basalts with rare, subjacent sheeted-dyke and gabbro units, considered as fragments of an ophiolite assemblage which suffered initial eastward transport in pre-Middle Arenig times, an important orogenic event which is well represented in northern and southwestern Norway. The overlying Ordovician—Silurian sequences, disturbed by episodic parorogenic events, embrace a variety of sedimentary facies from shallow-water carbonates to deep-marine terrigenous turbidites and include both island arc and marginal basin lavas and intrusives. Polyphase Middle Silurian metamorphism and deformation resulted in a complex telescoping and dissection of the Lower Palaeozoic rocks and their Precambrian substrate, with nappe translation in the order of several hundred kilometres. Folding and thrusting of Old Red Sandstone molasse sediments attests to continuing tectonism well into Devonian times.