Abstract The study of complex orogenic belts commonly begins in the frontal regions with well-defined tectonostratigraphy, and relatively simple structure and metamorphism, and proceeds into the progressively more complex hinterland, which nevertheless may contain the best geochronological record of the most intense orogenic events. The northern part of the Western Gneiss Region in the hinterland of the Scandian orogen contains a robust U–Pb zircon geochronological framework on rocks subjected to high-pressure (HP) and ultra-high-pressure (UHP) metamorphism that has implications for tectonic development both there and towards the foreland. HP and UHP eclogite crystallization occurred at 415–410 Ma (Early Devonian, Lochkovian to Pragian), followed by pegmatite crystallization at c. 395 Ma (Late Emsian) during exhumation and return to amphibolite-facies conditions, thus limiting the process to 15–20 myr. The nature and sequence of events are much more complex than in the foreland, causing difficulty in correlation, yet the combined tectonics in the two regions provides the necessary context to explain, for example, how rocks were subjected to deep-seated, high-temperature metamorphism and then exhumed to shallower levels. Here, we suggest how a recently recognized extensional detachment fault and a recently recognized out-of-sequence thrust might be linked to the timing of HP metamorphism and later exhumation. The postulated Agdenes extensional detachment in its footwall has basement gneisses containing Mesoproterozoic igneous titanite fully reset at 395 Ma, as well as Devonian pegmatites, and in the hanging wall Ordovician to Early Silurian granitoids of the Støren Nappe containing igneous titanite barely influenced by Devonian recrystallization and no evidence of post-Ordovician melts. This implies removal of a significant crustal section on a large-scale detachment. Rocks both above and below are overprinted by the same late, subhorizontal, sinistral ductile extensional fabric, obscuring any fabrics produced during development of the detachment itself. Eastern Trollheimen escaped the late, strong, subhorizontal overprint, and shows: (1) early emplacement of thrust nappes of Lower and Middle Allochthons over Baltican basement and its Late Neoproterozoic quartzite cover; (2) major, SE-directed, recumbent folding of the entire thrust-imbricated sequence; and (3) major, out-of-sequence, SE-directed thrusting (Storli Thrust), for an 80 km minimum transport across-strike, of the recumbent-folded sequence over deeper, less deformed, lower basement gneisses and unconformable Neoproterozoic quartzite cover. The upper basement contains boudins of eclogite and garnet-corona gabbro lacking in the lower basement. Similar basement imbrications occurred in the Tømmerås window, the Skarddøra Antiform, the Mullfjället Antiform and the Grong–Olden Culmination, up to 240 km NE of Trollheimen, as well as in the Reksdalshesten antiform 100 km west, all within the postulated minimum 400×180 km area of the Agdenes detachment.

Abstract The Trondheim Region ophiolites in the Mid-Norwegian Caledonides represent variably tectonized ophiolite fragments. We present high-precision thermal-ionization mass spectrometry and secondary-ion mass spectrometry (SIMS) U–Pb zircon dates, whole-rock geochemical and Sm–Nd data and Lu–Hf zircon analyses that permit the timing and nature of various stages in the evolution of the ophiolite to be elucidated. Plagiogranite intrusions dated at 487 and 480 Ma have relatively juvenile Nd and Hf isotopic compositions (ɛ Nd( t ) =6.3, ɛ Hf( t ) =8.2–12.4). Geochemical data indicate a subduction-zone influence, suggesting formation in an oceanic back-arc setting. At 481 Ma, a granitoid body with a relatively strong unradiogenic Nd and Hf isotopic composition (ɛ Nd( t ) =−2.6 to −4.0, ɛ Hf( t ) =3.8–6.4) and subduction-zone geochemical signature intruded the ophiolite. We interpret this stage to reflect the formation or migration of an oceanic arc above a subduction zone influenced by continentally derived sediments. At c. 475–465 Ma, a greenstone-dominated conglomerate and volcaniclastic sequence was deposited on the eroded ophiolite, indicating obduction between about 480 and 475 Ma. At c. 468–467 Ma, the deformed ophiolite and its sedimentary cover was intruded by trondhjemite dykes and shoshonitic volcanic rocks with intermediate Nd and Hf isotopic compositions (ɛ Nd( t ) =3.0–3.9, ɛ Hf( t ) =4.4–10.2). We interpret this magmatism to reflect subduction-polarity reversal and establishment of a magmatic arc at the continental margin shortly after obduction. Supplementary material: Whole-rock geochemistry, Sm–Nd isotopic data, SHRIMP U–Pb zircon, TIMS U–Pb zircon and Lu–Hf isotopic data are available at http://www.geolsoc.org.uk/SUP18689 .

Thrust sheets of the Uppermost Allochthon in the Caledonides of Scandinavia are distinguished by lithological assemblages and magmatic units that are, in many ways, quite different from those in subjacent nappe complexes. Supracrustal successions are derived mainly from platformal, shelf-edge and basinal-slope environments and are characterized in particular by extensive developments of carbonate rock units that range in age from Late Riphean to Early Silurian. Metasedimentary iron ore formations are also present. Another prominent feature is the Ordovician, arc-type, granitoid plutons and batholiths that dominate the geology in certain parts of the allochthon. In addition to these lithological elements, the Uppermost Allochthon carries an Ordovician tectonothermal record and early Caledonian, NW-vergent thrust polarity that is unique in Norway. Taken together, these features are indicative of a history of development and crustal growth along the eastern margin of Laurentia, involving an outboard magmatic arc, or arcs, and Taconian accretionary orogenesis. This was followed by recycling and deposition in Late Ordovician to Early Silurian successor basins prior to Laurentia-Baltica collision and the onset of the Scandian orogeny. The Taconian thrust sheets were then detached from their Laurentian roots and incorporated into the Siluro-Devonian, Scandian orogenic wedge on the Baltoscandian margin of Baltica. Taking into account the widely reported sinistral megashear arising from the Scandian, oblique collision and plate rotation, the rock units that constitute the Uppermost Allochthon are likely to have originally been located closer to the northern Appalachian segment of the margin of Laurentia, in view of the strikingly similar lithostratigraphic, magmatic, and tectonothermal histories of these two, now widely separated terranes.

Abstract Neoproterozoic, slope-to-basin, lithostratigraphic successions are discontinuously exposed within the Timan Range, in NW Russia, NE of a faulted basinal margin that marks the outer edge of a former, fluvial to shallow-marine, pericratonic domain. The Mid to Late Riphean, deep-water depositional systems of the Kanin Peninsula, and northern and central Timan attain considerable thicknesses, up to 10 000 m in the case of Kanin Peninsula. Basements to these successions are nowhere exposed. Although the successions accumulated along a comparatively stable, passive margin of Baltica, there are notable differences in sedimentary facies from area to area. Whereas the successions in northern and central Timan preserve a record of relative stability, with sedimentation keeping pace with subsidence, the nearby Kanin succession shows evidence of repeated faulting. This may reflect a non-contemporaneity of the diverse successions or a segmentation of the basin margin. Comparisons are also made with deep-water, turbidite-fan systems in northwestern parts of the Timan-Varanger Belt, on the Rybachi and Varanger Peninsulas. The lateral differences in sedimentary facies in these areas, seen in relation to the situation in Timan and Kanin, do, in fact, suggest that the 1800 km long Timanian Basin margin may have been segmented, and possibly into sub-basinal domains.

Abstract The northeastern margin of the East European Craton developed passively in an extensional regime from late Mesoproterozoic through to the later stages of Neoproterozoic time. Along the exposed parts of the Timan-Varanger Belt, a major fault zone separates pericratonic (platformal) and basinal domains. Successions of the basinal domain can be traced beneath the Pechora Basin, via drillcore and geophysical data, to where intra-oceanic subduction systems with island arcs are inferred to have existed in the later stages of the Late Riphean. In terminal Riphean to Vendian time, inferred subduction polarity reversal resulted in a progressive telescoping, dissection and accretion of these diverse magmatosedimentary assemblages against the northeastern margin of the craton, culminating in Mid to Late Vendian, Timanian orogenesis. The Timan Range exposes SW-verging upright folds with anchizone to lower greenschist-facies cleavages. Higher-grade rocks in the Kanin-North Timan area occur in anticlinal cores and thrust slices. Isotopic dating constraints suggest that peak Timanian metamorphism occurred during the time interval 600–550 Ma.