Scandian Deformation Research Articles

Tectonic Transport Directions, Shear Senses and Deformation Temperatures Indicated by Quartz c-Axis Fabrics and Microstructures in a NW-SE Transect across the Moine and Sgurr Beag Thrust Sheets, Caledonian Orogen of Northern Scotland

Moine metasedimentary rocks of northern Scotland are characterized by arcuate map patterns of mineral lineations that swing progressively clockwise from orogen-perpendicular E-trending lineations in greenschist facies mylonites above the Moine thrust on the foreland edge of the Caledonian Orogen, to S-trending lineations at higher structural levels and metamorphic grades in the hinterland. Quartz c-axis fabrics measured on a west to east coast transect demonstrate that the lineations developed parallel to the maximum principal extension direction and therefore track the local tectonic transport direction. Microstructures and c-axis fabrics document a progressive change from top to the N shearing in the hinterland to top to the W shearing on the foreland edge. Field relationships indicate that the domain of top to the N shearing was at least 55 km wide before later horizontal shortening on km-scale W-vergent folds that detach on the underlying Moine thrust. Previously published data from the Moine thrust mylonites demonstrate that top to the W shearing had largely ceased by 430 Ma, while preliminary isotopic age data suggest top to the N shearing occurred at ~470–450 Ma. In addition, data from the east coast end of our transect indicate normal-sense top down-SE shearing at close to peak temperatures at ~420 Ma that may be related to the closing stages of Scandian deformation, metamorphism and cooling/exhumation.

Dealing with discordance: a novel approach for analysing U–Pb detrital zircon datasets

Detrital zircon U–Pb geochronology is a rapidly expanding and useful technique for addressing the sedimentary rock record. However, because of difficulties in evaluating discordant analyses, common in most datasets, a high proportion of results may be discarded. These analyses, if interpreted correctly, can provide valuable information regarding the deformation, alteration or metamorphism of the zircon population, as these events are all capable of producing U–Pb discordance. A novel modelling procedure permits analysis of probabilistic relationships within U–Pb datasets to deconvolve the age information present within discordant analyses by assessing the relative likelihood of potential discordia lines. Additionally, because we retrieve useful information from discordant data, a stricter filter can be used to assess concordance, increasing confidence in those distributions. The validity of this modelling method is demonstrated using two previously published cases from the Caledonide orogen where clearly discordant analyses exist. In the Southern Uplands of Scotland, these analyses indicate resetting of the U–Pb systematics in metasedimentary rocks in the Grampian orogen prior to Ordovician erosion and redeposition. In the second case, from the Greenland Caledonides, discordant data provide additional constraints on previously proposed in situ resetting during Scandian deformation and fluid flow events. Supplementary material: The code used for writing the model, along with an instruction guide, is available at http://doi.org/10.6084/m9.figshare.c.2182197 .

Constraints on the timing of Scandian deformation and the nature of a buried Grampian terrane under the Caledonides of northwestern Ireland

In the Caledonides of northwestern Ireland there is little evidence for the crystalline basement to the Dalradian Supergroup, though ascertaining its nature is a critical prerequisite for palaeogeographical reconstructions. In Donegal a lamprophyre intrusion has sampled material with isotopic composition, age and fabrics that permit the investigation of the hidden basement of this region. U–Pb and Lu–Hf isotopes in concert with structural analysis are used to elucidate the origin of three morphologically distinct zircon populations from the lamprophyre. The first of these comprises c . 828 to c . 1660 Ma rounded inherited grains. The second population, of 476 ± 3 Ma large xenocrystic grains, displays oscillatory zoned cores embayed by cross-cutting overgrowths, implying a multi-stage magmatic genesis. The third population, of 437 ± 5 Ma high aspect ratio euhedral grains, is interpreted to date the magmatic crystallization of the lamprophyre. Hf isotopic data from the Silurian and Neo- to Mesoproterozoic zircons yield crustal residence ages of 1.0–2.3 Ga. However, the Ordovician grains have ϵHf i values of c . −20 and model ages of 2.7–2.8 Ga. These data are consistent with the presence of either Archaean crust or an underthrust Ordovician magmatic rock with an Archaean source component, at depth in NW Ireland. Felsic xenoliths, interpreted to be derived from the country rock Dalradian metasediments, exhibit a pre-entrainment foliation and are randomly oriented relative to the foliation within the lamprophyre matrix, indicating deformation prior to c . 437 Ma, probably related to the Grampian orogeny. The host lamprophyre contains a magmatic foliation parallel to a foliation in the country rock, implying synkinematic emplacement during regional NW–SE compression at 437 Ma. Supplementary material: Details of the analytical methods are available online at www.geolsoc.org.uk/SUP18614 .

Detrital zircon U–Pb ages of Silurian–Devonian sediments from NW Svalbard: a fragment of Avalonia and Laurentia?

Abstract: Detrital zircon populations from Silurian–Devonian clastic rocks of NW Svalbard were analysed by U–Pb laser ablation inductively coupled plasma mass spectrometry to investigate the pre-Caledonian provenance of Svalbard's Northwestern Terrane. Changes in the resulting age spectra suggest a major shift in sources from the Laurentian–Avalonian suture in the latest Silurian to the local metasedimentary basement of the Northwestern Terrane in the Late Silurian–Early Devonian, and in the Lochkovian to Grenvillian–Sveconorwegian sources. These data, together with structural, additional geochronological and metamorphic data from Svalbard, East Greenland and Avalonia, support the amalgamation of Svalbard as the result of long-distance transport along sinistral strike-slip faults. A unifying model for the final amalgamation of Svalbard, consistent with the stratigraphical and tectonothermal history of Svalbard, involves fragments from the Grampian orogen and Avalonian crust originally accreted to the Laurentian margin being subsequently transported northward along sinistral strike-slip faults during Scandian deformation. Supplementary material: The sample preparation procedures, laser ablation ICP-MS analytical methods, and data table are available at http://www.geolsoc.org.uk/SUP18421 .

High-temperature deformation during continental-margin subduction & exhumation: The ultrahigh-pressure Western Gneiss Region of Norway

A new dataset for the high-pressure to ultrahigh-pressure Western Gneiss Region allows the definition of distinct structural and petrological domains. Much of the study area is an E-dipping homocline with E-plunging lineations that exposes progressively deeper, more strongly deformed, more eclogite-rich structural levels westward. Although eclogites crop out across the WGR, Scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area. The Scandian reworking increases westward, culminating in strong Scandian fabrics with only isolated pockets of older structures; the dominant Scandian deformation was coaxial E–W stretching. The sinistrally sheared Møre–Trøndelag Fault Complex and Nordfjord Mylonitic Shear Zone bound these rocks to the north and south. There was moderate top-E, amphibolite-facies deformation associated with translation of the allochthons over the basement along its eastern edge, and the Nordfjord–Sogn Detachment Zone underwent strong lower amphibolite-facies to greenschist-facies top-W shearing. A northwestward increase in exhumation-related melting is indicated by leucosomes with hornblende, plagioclase, and Scandian sphene. In the western 2/3 of the study area, exhumation-related, amphibolite-facies symplectite formation in quartzofeldspathic gneiss postdated most Scandian deformation; further deformation was restricted to slip along biotite-rich foliation planes and minor local folding. That the Western Gneiss Region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation, implies that continental crust in general need not undergo pervasive deformation during subduction.

Granitic magmatism of Grenvillian and late Neoproterozoic age in Finnmark, Arctic Norway—Constraining pre-Scandian deformation in the Kalak Nappe Complex

The Caledonian Orogen in Arctic Norway is characterized by a variety of nappes thrust from west to east onto the Baltic Shield. Traditionally, this has been regarded as the product of two orogenic events: an earlier Finnmarkian (540–490 Ma) and a later Scandian event (400–425 Ma). However, ion microprobe U–Pb zircon dating of discordant plutonic rocks within the lowermost nappes demonstrates that some of the deformation must have taken place in a Grenville (Sveconorwegian) event. This view is supported by the 981 ± 7 Ma, 978 ± 9 Ma and 973 ± 4 Ma ages of the Repvåg, Hårvika and Siedgoaivi adamellite bodies. These bodies cut the Sørøy Succession, apparently after an earlier deformation event. On these grounds a Grenville (Sveconorwegian) event is responsible for the D2 deformation within the Olderfjord and Kolvik nappes. Within the overlying Havvatnet Imbricate Stack, early deformation took place in the Neoproterozoic, “Porsanger Orogeny”. Evidence for this event is provided by the Litlefjord and Revsneshamn adamellite bodies dated at 841 ± 6 Ma and 839 ± 10 Ma and pegmatitic intrusions, dated at 826 ± 6 Ma and 833 ± 9 Ma, which show clear discordant structural relationships cutting F 2 fold structures that affect the Klubben Psammite, the oldest unit of the Sørøy Succession. Within the uppermost nappe (Sørøy-Seiland nappe) syn-deformational migmatitic leucosomes in the Eidvågeid Paragneiss yield crystallization ages of 709 ± 4 Ma. This age is indistinguishable from zircon overgrowths within the underlying Havvatnet Imbricate Stack. Hence, juxtaposition of these nappes predates Scandian tectonism and occurred during the Snøfjord event at c. 710 Ma. The component nappes of the KNC show decreasing ages of anatexis on moving up the nappe pile. Such temporal and spatial patterns are consistent with episodic terrane amalgamation from Grenvillian times. The KNC provides evidence for punctuated crustal anatexis and episodic orogenic deformation of c. 980 Ma (Grenvillian-Sveconorwegian), c. 840 Ma (Porsanger) and c. 710 Ma (Snøfjord) age, overprinted by intense Scandian deformation. These data support the notion of a Grenville segment extending between Greenland and Baltica and require a radical revision to the tectonometamorphic evolution of the KNC. The KNC represents a collage of exotic, diachronously accreted, terranes overthrust by Llandovery flysch of Laurentian affinity.

The early Caledonian (Finnmarkian) event reassessed in Finnmark: 40Ar/ 39Ar cleavage age data from NW Varangerhalvøya, N. Norway

The relative significance of early (Finnmarkian) and late (Scandian) Caledonian deformation in N. Norway is uncertain. Early studies suggested pervasive Finnmarkian deformation whilst later results indicated a restricted Finnmarkian domain. The present work suggests it was more widespread than accepted and that inter Finnmarkian–Scandian deformation occurred. 40Ar/ 39Ar dating of 2–6 and 6–11 μm pelitic fractions from the lower to mid-greenschist facies Tanahorn Nappe (five samples; base Middle Allochthon) and the epizone Løkvikfjellet and Barents Sea Groups (three samples; North Varanger Region) in the north Scandinavian Caledonides show slightly discordant spectra. Most spectra from the Tanahorn Nappe preserve possible evidence of an early Caledonian event in the high temperature steps, with recoil/excess Ar effects in the low temperature steps; no pre-Caledonian relict component has been recorded. The results indicate Finnmarkian deformation continued to ∼460 Ma, with Scandian reactivation at ∼425–415 Ma. From the North Varanger Region, a strongly crenulated sample yielded plateau ages (444–442 Ma); means of combined young steps from weakly to uncrenulated samples gave 470–450 Ma, suggesting penetrative strike-slip deformation occurred in the late Finnmarkian to inter-Finnmarkian–Scandian period. No Scandian ages were recorded in the North Varanger Region. Reassessment of published data from the Laksefjord Nappe and Gaissa Thrust Belt suggests they were affected by Finnmarkian deformation.

Evidence for Neoproterozoic orogenesis and early high temperature Scandian deformation events in the southern East Greenland Caledonides

Integrated field structural studies and SHRIMP U–Pb zircon and monazite dating have been undertaken in Renland, west of Scoresby Sund district in the southern part of the East Greenland Caledonides. Southwest Renland is dominated by metasedimentary rocks correlated with the Krummedal supracrustal succession of East Greenland and which on Renland were intruded by augen granites. Krummedal psammite from Renland yielded a spectrum of Mesoproterozoic to Palaeoproterozoic detrital U–Pb zircon dates, the youngest of which indicate deposition of the psammite occurred c. 1000 Ma ago, thus post-dating Grenvillian continent–continent collision in North American Laurentia. These Krummedal metasediments were deformed into regional nappe-scale folds prior to metamorphism, crustal anatexis and genesis of augen granites; an example of the latter has been dated at 915±18 Ma (U–Pb zircon). This demonstrates early Neoproterozoic high-temperature tectono-metamorphism affecting rocks within the southern East Greenland Caledonides, broadly contemporaneous with similar rocks farther north in East Greenland and with Sveconorwegian events on Baltica. Still in southwestern Renland, a later thermal event led to development of uppermost amphibolite to granulite facies metamorphic assemblages, veins and patches of in situ garnetiferous melt-bearing neosome in both metasediments (432±6 Ma, U–Pb zircon) and in the augen granites, and contemporaneous biotite-bearing granite sheets in top-down-to-the-E extensional shear zones (434±5 Ma, U–Pb zircon). Monazites from southwestern Renland record Caledonian thermal events as late as 410−400 Ma. In contrast, southeastern Renland is dominated by quartzofeldspathic migmatites with a strongly Caledonian signature but enclosing relicts of augen granite and retrogressed granulite facies psammitic and pelitic metasediment. There is also a sequence of Caledonian granitoid intrusions. Two samples from a hypersthene monzonite intrusion yielded U–Pb zircon dates of 424±8 Ma and 424±6 Ma. This pluton shows the marginal effects of the regional migmatization and was intruded early in the sequence of granitoid emplacement. An amphibolite facies migmatite, textural evidence from which suggests that it had never hosted granulite facies assemblages, records zircon growth at 423±6 Ma, and closure of monazite by 402±10 Ma. High grade metamorphism, and the protracted sequence of granitoid emplacement and still younger thermal events which together span the period between 430 and 400 Ma may, in part, reflect complicated lithospheric dynamics associated with subduction outboard of the Laurentian margin. Crustal segments carrying the relict evidence of Neoproterozoic and early Caledonian events must then quickly have been thrust northwestwards in foreland-propagating, northwesterly directed thrusts over Cambro-Ordovician platformal sequences on the Laurentian margin. This records the final closure of Iapetus, encroachment of Baltica and continent–continent collision from late Llandovery times (425–430 Ma).

The tectonic significance of pre-Scandian 40 Ar/ 39 Ar phengite cooling ages in the Caledonides of western Norway

Pre-Silurian continental-margin deposits in western Norway, non-conformably overlying allochthonous continental orthogneisses retain Ordovician 40 Ar/ 39 Ar cooling ages for phengites, implying either rapid cooling immediately after a Late Ordovician orogenic event, or less likely, a slow cooling following an Early Ordovician or older orogeny. The Dalsfjord Suite–Høyvik Group basement–cover pair are probably a lateral equivalent to Late Proterozoic sandstones (‘sparagmites’) covering the Jotun Nappe gneisses of the Middle Allochthon in central-south Norway. The Høyvik Group underwent polyphase deformation, greenschist-facies metamorphism ( T max <450°C) and exhumation prior to deposition of the unconformably overlying Wenlockian continental-margin deposits of the Herland Group. The Høyvik Group was only weakly metamorphosed during obduction of the Solund–Stavfjord Ophiolite and the Scandian continental collision between Baltica and Laurentia. Phengitic white micas from the Høyvik Group yield cooling ages of 446.1± 3.0, 449.1±2.2 and 447.5±4.0 Ma, respectively, identical within experimental error. One sample gives a plateau over 72% of the gas analysed, whereas the other samples were slightly disturbed after initial cooling, as indicated by systematically lower apparent ages at low experimental extraction temperatures. Minor 40 Ar loss probably occurred during subsequent Scandian deformation and late to post-orogenic extension.The Høyvik Group rocks were unroofed before the Wenlock time (423–428 Ma) and cooled through the temperature for argon retention in phengite at c . 447±4 Ma, indicating a maximum cooling rate between 14 and 22C/Ma- 1 through Ashgill and Llandovery times before being subjected to low-grade metamorphism during the Scandian orogeny. Rapid pre-Scandian cooling, combined with peak metamorphic conditions of 450C or less, may indicate that the Dalsfjord–Høyvik basement–cover pair were aVected by an orogenic event during the Late Ordovician (Caradoc) time. The data also suggest that the Caledonian margin of Baltica may have experienced a more protracted tectonism during the Caledonian cycle than previously models focusing on Early Caledonian and Tremadoc (or older) ophiolite obduction and the Scandian continental collision between Baltica and Laurentia.

Contact relationships in the southeastern Trondheim Nappe complex, central-southern Norway: Implications for early Paleozoic tectonism in the Scandinavian Caledonides

The Upper Allochthon, a composite thrust complex containing elements of an accreted terrane, has long been a controversial feature in the Scandinavian Caledonides. Although final emplacement of the Upper Allochthon occurred during the Siluro-Devonian Scandian orogeny, the complex experienced an intricate and still debatable early Paleozoic history. In the central Norwegian Caledonides, the Upper Allochthon comprises the Trondheim Nappe Complex, which contains an enigmatic axial unit of metasedimentary and meta-igneous rocks flanked by oceanic crustal sequences to the east and west. Determination of the nature of internal contacts and timing of assembly of the Trondheim Nappe Complex has been hampered by wide variations in lithology, strain and metamorphic grade, and scanty age constraints in some rock units. In the Lake Savalen area, southeastern Trondheim Nappe Complex, a polydeformed continental margin quartzite/schist sequence occurs in the core of the Einunnfjellet dome. These rocks, which can be linked to the Baltoscandian miogeocline, were overthrust by oceanic crust and pelagic sediment, then eroded and unconformably succeeded by polymict conglomerates and fine-grained clastic sediments. Early thrust and unconformable contacts were later overprinted by Scandian deformation and metamorphism. The garnet isograd cuts across lithologie contacts, and does not appear to be folded or interrupted by faulting within the 250-km 2 map area. Regional correlations indicate the widespread occurrence of tectonostratigraphic relationships similar to the Einunnfjellet-Savalen area, and imply that prior to the Scandian orogeny, portions of the Upper Allochthon were assembled into a composite mass consisting of: 1. (1) a lower Baltoscandian miogeoclinal sequence 2. (2) a thrust complex of fragmented oceanic terranes 3. (3) an unconformable succession that overstepped both (1) and (2). This mass was subsequently thrust eastward, imbricated, and metamorphosed during Scandian orogenesis. As a result of the pervasive Scandian overprint, pre-Scandian faults and unconformable contacts in the Upper Allochthon are commonly cryptic.

A model for stratigraphic and metamorphic inversions at Sulitjelma, central Scandes

AbstractThe relationship between disposition of metamorphic isograds and early strain history is examined for the Sulitjelma Fold Nappe of the central Scandes. Evidence for stratigraphic inversion and the widespread inversion of the garnet isograd is reviewed, and new evidence for the nature of early Scandian deformation is presented. A two-stage model is presented which explains the inversions by (i) an initial phase of horizontal compression resulting in crustal thickening by vertical stretching during closure of a marginal basin, followed by (ii) continued horizontal compression and crustal thickening by eastward-directed nappe emplacement. The emplacement of the Sulitjelma Fold Nappe during the latter phase of progressive simple-shear-dominated deformation was accompanied by the development of sheath fold geometries and the overturning of isotherms to produce the widespread inversion of the garnet isograd.

Mega-lenses and Scandian deformation in the basement in the northern part of Western Gneiss Region, Vestranden, central Norwegian Caledonides

(1986). Mega-lenses and Scandian deformation in the basement in the northern part of Western Gneiss Region, Vestranden, central Norwegian Caledonides. Geologiska Foreningen i Stockholm Forhandlingar: Vol. 108, No. 3, pp. 287-289.