Stratigraphic correlations between the Brabant Massif and the Stavelot, Rocroi and Givonne inliers (Belgium) and geological implications

The Caledonian basement crops out in the middle and southern part of Belgium in two major tectonic units: the Brabant Massif in the Brabant Parautochthon and the Stavelot-Venn, Rocroi, Givonne and Serpont inliers in the Ardenne Allochthon. The main aim of this work is to achieve a chronostratigraphic correlation between the Brabant Massif and the Ardenne inliers, from the lower Cambrian to the Middle Ordovician. Throughout his career, Michel Vanguestaine established an informal acritarch biozonation for this basement, which is only linked to the international stratigraphic scale in vigour at that time. Our first step was to correlate these informal biozones with the trilobite (Cambrian) and graptolite (Ordovician) biozonations which are currently well correlated with the chronostratigraphy. Then, compilation of the literature concerning each of these sedimentary units makes it possible to assign a chronostratigraphic position to their constituent formations. This work has permitted the establishment of a complete chart of the stratigraphic correlations between the Brabant Massif and the three main Ardenne inliers (Stavelot-Venn, Rocroi and Givonne). Geological implications are discussed: the Brabant Massif and the Ardenne inliers formed a single sedimentation basin with different and rheologically contrasting basements (rift and shoulder). New arguments confirm the presence of a Caledonian orogeny in the Ardenne.

The Billefjorden Fault Zone north of Spitsbergen: a major terrane boundary?

The Billefjorden Fault Zone is a major terrane boundary in the Norwegian Arctic. The fault separates basement rocks of Svalbard’s north-eastern and north-western terranes that recorded discrete Precambrian tectonothermal histories and were accreted, intensely deformed and metamorphosed during the Caledonian Orogeny. Although the fault represents a major, crustal-scale tectonic boundary, its northward extent is not well constrained. The present short contribution addresses this issue and presents new seismic mapping of structures and rock units north of Wijdefjorden, where the Billefjorden Fault Zone may continue. This study shows that there is no evidence for major faulting of the top-basement reflection, and therefore, that the Billefjorden Fault Zone may die out within Wijdefjorden–Austfjorden, step ≥ 20 km laterally, or be invisible on the presented seismic data. Seismic data also suggest that Caledonian basement rocks in Ny-Friesland (north-eastern terrane) are not significantly different from basement rocks below the Devonian Graben in Andrée Land (north-western terrane). Potential implications include the absence of a major terrane boundary in northern Spitsbergen.

Trøndelag Platform & Halten-Dønna terraces Composite Tectono-Sedimentary Element, Norwegian Rifted Margin, Norwegian Sea

The Trøndelag Platform and Halten-Dønna Terraces occupy a central part of the Norwegian Sea Rifted Continental Margin off the mid-Norway coast. The margin is expected to hold 4500 Mbboe of undiscovered oil equivalents and represents an economically important Arctic province. The geological evolution of the area is closely linked to processes involving the Caledonian orogeny and subsequent plate tectonic re-organizations, multiphase rifting, continental drift and glaciations across the northern hemisphere. In this chapter we review the geology of the Trøndelag Platform and Halten-Dønna terraces Composite Tectono-Sedimentary Element based on published data and results of Equinor ASA in-house studies. Three new structural elements are defined for the first time: the Leka fault complex, the Vikna high and the Sula basin.

Large-scale flat-lying mafic intrusions in the granitic Baltica crust of central Sweden and implications for basement deformation during Caledonian orogeny

<p>The role of inheritance in localizing basement deformation in the foreland has been demonstrated in orogens in different parts of the world. In the external domain of the central Scandinavian Caledonides, questions remain about the amount and the distribution of deformation accommodated by the Baltica basement during Caledonian orogeny. However, to answer these questions, it is necessary to understand the architecture of the Baltica crust underneath the Caledonian nappes and to determine the occurrence of potential detachment horizons or inherited structures that accommodated the shortening.</p><p>In this work, we study the lithological and structural architecture of the Baltica basement in central Sweden, east and west of the present-day Caledonian front. The aim is twofold: 1) identifying the main geological features of the Fennoscandian Shield and their regional extent underneath the Caledonian nappes to the west, and 2) to address their role in accommodating deformation during Caledonian orogeny.</p><p>The study area is characterized by mainly ~1.8 Ga granitic bodies intruded by various generations of mafic intrusions and locally bounded by major crustal shear zones. On the one hand, based on seismic interpretations, magnetic and gravimetry forward modeling and mapping, and results from the recently drilled COSC-2 borehole (as part of the Collisional Orogeny in the Scandinavian Caledonides (COSC) drilling project), we show that the basement underlying the Caledonian nappes is characterized by inclined to sub-horizontal mafic intrusions with large extent, emplaced at mid-crustal level. We propose that these intrusions are similar in size, geometry, and potentially age, to the 1.25 Ga Central Scandinavian Dolerite Group (CSDG) that are mapped as 100’s km long elliptic bodies or described as saucer-shaped intrusions further east. On the other hand, based on observations from COSC-2 drill cores and previous studies, analogue modelling and 2D seismic restoration, we propose that favorably oriented intrusions influenced, at least partly, crustal shortening in this area by localizing deformation along their margins. At a regional scale, we discuss the distribution of thick-skinned and thin-skinned deformation at the present-day orogenic front. On a broader scale, this study raises the question regarding the influence of pre-existing mafic intrusions in controlling the structural evolution and the segmentation of orogenic or rift systems in general.</p>

Unravelling the enigmatic Grampian Shear Zone: In-situ monazite and titanite U-Pb analysis of the juxtaposed Badenoch and Grampian Groups

<p>The Grampian Shear Zone (GSZ) represents a highly deformed tectonostratigraphic contact between the Proterozoic metamorphic rocks of the Dalradian Group from the underlying high grade metamorphic Neoproterozoic rocks of the Badenoch Group within the Grampian Highlands. The nature (tectonic suture or palaeo-unconformity), age and structure of the GSZ and indeed the underling Badenoch Group are poorly constrained. Previous studies of the GSZ and synkinematic (intruded during shearing) pegmatites found therein, yielded metamorphic/deformation (and magmatic) ages ranging from c.a. 808 to 440 M. This study reinvestigates this shearzone using in-situ (within section) petrochonological analysis on a range of U-Pb and Rb-Sr chronometers – Monazite, zircon, titanite, rutile and mica. Carrying out this analysis in-situ and using a variety of minerals allows us to directly date deformation fabrics over a wide range of deformation temperatures, giving us a far more detailed picture of the events recorded within these rocks. Large monazite grains (≥100μm) were mapped using in-situ LA-ICP-MS to show within grain variation of major elements and REEs. Monazite U-Pb spot analysis from the GSZ has yielded ages ranging from 784.11 ± 1.2Ma to 442.58 ± 0.58Ma. The same analysis was performed on a sample from the Grampian group which yielded an age of 441.34 ± 037Ma. In addition to this monazite data, in-situ U-Pb Titanite analysis from the Badenoch Group gave ages of 526.96 ± 1.33 Ma from a metabasite sample, with a metasedimentary sample giving a range of titanite U Pb ages from 540 to 460Ma. These age ranges show that the Badnoch Group and the GSZ have recorded a complex polyorogenic history relative to the “simple” overlying Dalradian metasediments. We propose that the Grampian Shear Zone represents a deep-seated Knoydartian (808 to 784Ma) age shear zone within the meso-Neoproterozoic Badenoch Group. This shear zone was then reactivated during the Grampian phase of the Caledonian Orogeny resulting in the tectonic emplacement of the Dalradian metasediments above the Badenoch group.</p>

Carbonatitic REE-Nb-Fe mineralisation in the Palaeoproterozoic Shin Group marble and calc-silicates, Loch Shin, Scotland

<p>Carbonatitic magmatism and metasomatism are rare in Great Britain & Ireland. However, REE-Nb minerals were identified in impure marbles and calc-silicates on the Aird of Shin and Arscaig near Lairg, Scotland. They belong to the Palaeoproterozoic Shin Group, a supercrustal succession of amphibolites, banded silicic gneisses, marbles and calc-silicates. The Shin Group is one of several Palaeoproterozoic to Neoproterozoic marble inliers in the North Highlands of Scotland that present evidence of Caledonian orogen carbonatitic metasomatism. The Proterozoic Bayan Obo ore complex in China was similarly deposited as a dolomite-limestone and later subject to alkali intrusions and carbonatitic metasomatism during the Caledonian orogeny. The Bayan Obo complex hosts REE-Nb-Fe carbonatitic fine-grained dolomites, REE-Nb deficient coarse dolomites, carbonatitic dykes, limestones and dolostones. The Aird of Shin marble mineralogy comprises niobium and tantalum Ca-bearing oxides, scheelite, strontian barite, ilmenite, REE-bearing monazite, REE-epidote and Fe-Mo sulphides in an impure Na-Fe-K calcite fabric. The Arscaig Qtz-Na-K calc-silicates are enriched in REE-bearing monazite, Mg-Fe chlorite and Fe-oxides, with minor REE-bearing xenotime, Mn-Fe garnet and strontian barite. The REE-Nb Aird of Shin marble and the REE-phosphate bearing Arscaig calc-silicates are comparable with carbonatitic mineral phases in the Bayan Obo complex. This study adds support to previous recognition of Caledonian carbonatitic magmatism and carbonatitic metasomatism of Proterozoic limestones and calc-silicates in Scotland. </p>

Pristine microstructures in pseudotachylytes formed in dry lower crust, Lofoten, Norway

Feldspar-rich pseudotachylytes from the island of Moskenesøya, Lofoten, formed in dry granulites under lower crustal conditions during the Caledonian orogeny. The central parts of the pseudotachylytes, where the cooling rates were slowest, are characterized by microlites and spherulites of plagioclase and K-feldspar. K-feldspar surrounding plagioclase is consistent with crystallization from a melt during cooling instead of devitrification as the origin of the spherulites. Very thin (a few micrometres wide) injection veins, which experienced very rapid quenching, contain amorphous or cryptocrystalline material. The preservation of this material and of the fine-grained microstructures shows that, under fluid-absent conditions, recrystallization and reactions are slow and the original microstructures of the pseudotachylytes can be preserved. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.

Age of monazite from metapelitic schists of Atomfjella and Mossel series, Western Ny Friesland, Spitsbergen according to the CHIME method

The article presents the results of CHIME (chemical Th–U-total Pb isochron method) dating of monazite from metamorphic rocks of Precambrian complexes located in the north part of the West Spitsbergen Island. It is shown that for rocks of Atomfjella Series and Mossel Series, monazite ages are coeval within error (Atomfjella Series: 381 ± 18 Ma, Mossel Series: 377 ± 23 Ma). These age estimates show that metamorphism of the crystalline basement possibly took place during the Late Caledonian orogeny.

Multi-chronometer dating of the Souter Head complex: rapid exhumation terminates the Grampian Event of the Caledonian Orogeny

ABSTRACTThe Souter Head sub-volcanic complex (Aberdeenshire, Scotland) intruded the high-grade metamorphic core of the Grampian Orogen at 469.1 ± 0.6 Ma (uranium-238–lead-206 (238U–206Pb) zircon). It follows closely peak metamorphism and deformation in the Grampian Terrane and tightly constrains the end of the Grampian Event of the Caledonian Orogeny. Temporally coincident U–Pb and argon/argon (40Ar/39Ar) data show the complex cooled quickly with temperatures decreasing from ca.800 °C to less than 200 °C within 1 Ma. Younger rhenium–osmium (Re–Os) ages are due to post-emplacement alteration of molybdenite to powellite. The U–Pb and Ar/Ar data combined with existing geochronological data show that D2/D3 deformation, peak metamorphism (Barrovian and Buchan style) and basic magmatism in NE Scotland were synchronous at ca.470 Ma and are associated with rapid uplift (5–10 km Ma−1) of the orogen, which, by ca.469 Ma, had removed the cover to the metamorphic pile. Rapid uplift resulted in decompressional melting and the generation of mafic and felsic magmatism. Shallow slab break-off (50–100 km) is invoked to explain the synchroneity of these events. This interpretation implies that peak metamorphism and D2/D3 ductile deformation were associated with extension. Similarities in the nature and timing of orogenic events in Connemara, western Ireland, with NE Scotland suggest that shallow slab break-off occurred in both localities.

Impact of Timanian thrusts on the Phanerozoic tectonic history of Svalbard

<p>Despite more than a century of investigation, the relationship between basement rocks throughout the Svalbard Archipelago is still a mystery. Though these rocks display similar geochronological ages, they show significantly different metamorphic grades and structures. Thus far, Svalbard was believed to be composed of three terranes of rocks formed hundreds–thousands of kilometers apart and accreted in the mid-Paleozoic during the Caledonian and Ellesmerian orogenies.</p><p>New evidence from seismic, gravimetric, aeromagnetic, seismological, bathymetric, and field data show that these terranes might have already been juxtaposed in the late Neoproterozoic. Notably, the data show that at least three–four, crustal-scale, WNW–ESE-striking fault systems segment Spitsbergen and merge with Timanian thrusts in the northern Barents Sea and northwestern Russia. These thrusts were reactivated as and/or overprinted by sinistral-reverse oblique-slip faults and partly folded during the Caledonian Orogeny and Eurekan tectonic event, and reactivated as and/or overprinted by sinistral-normal faults during Devonian–Mississippian extensional collapse of the Caledonides, thus offsetting N–S-trending Caledonian grain and post-Caledonian basins, and explaining the juxtaposition of basement rocks with seemingly different origin.</p><p>The presence of Timanian faults explains basement heterogeneities throughout the Svalbard Archipelago, strain partitioning during the Caledonian Orogeny and Eurekan tectonic event and, thus, the western vergence of early Cenozoic folds in Devonian rocks in central–northern Spitsbergen (previously ascribed to the Late Devonian Ellesmerian Orogeny) and the arch shape of the early Cenozoic West Spitsbergen Fold-and-Thrust Belt in Brøggerhalvøya, the distribution of Mississippian rocks and Early Cretaceous intrusions along a WNW–ESE-trending axis in central Spitsbergen, the transport of Svalbard in the Cenozoic from next to Greenland to its present position (c. 400 km southwards), the strike and location of transform faults and oceanic core complexes and gas leakage along the Vestnesa Ridge west of Spitsbergen, the continental nature and NW–SE strike of basement fabrics in the Hovgård Ridge between Greenland and Svalbard, and the occurrence of recent (< 100 years old) earthquakes in Storfjorden and Heer Land in eastern Svalbard.</p><p>Further implications of this work are that the tectonic plates constituting present-day Arctic regions (Laurentia and Baltica) have retained their current geometry for the past 600 Ma, that the Timanian Orogeny extended from northwestern Russia to Svalbard, Greenland and, potentially, Arctic Canada, that the De Geer Zone does not exist, that the Billefjorden Fault Zone (Svalbard) and the Great Glen Fault (Scotland) were not part of the same fault complex, and that the Harder Fjord Fault Zone (northern Greenland) possibly initiated (or was reactivated) as a Timanian thrust.</p>