Session TS6.4

[Programme]

TS6.4 | The Caledonian Orogen of the North Atlantic region: insights from geological and geophysical studies

The Caledonian Orogen of the North Atlantic region: insights from geological and geophysical studies

Co-organized by GD9/GMPV10

Convener: Jaroslaw Majka | Co-conveners: Deta Gasser, Johannes Jakob, Christopher Juhlin, Karolina Kośmińska

Orals

| Fri, 28 Apr, 16:15–18:00 (CEST)

Room K1

Posters on site

| Attendance Fri, 28 Apr, 10:45–12:30 (CEST)

Hall X2

Posters virtual

| Attendance Fri, 28 Apr, 10:45–12:30 (CEST)

vHall TS/EMRP

The Caledonian mountain belt represents a world-class example of a deeply denudated Himalayan-style orogen. The exposed crustal sections allow the study of all stages of the Wilson cycle and may contribute to our understanding of many fundamental processes in Earth Sciences, including (1) continental-rifting, break-up and ocean formation, (2) subduction plus arc-continent and continental collisions, (3) marginal basin formation, (4) deep crustal architecture of orogens, (5) (U)HP metamorphism, (6) orogenic wedge formation and dynamics, (7) the formation and evolution of crustal-scale shear zones, (8) fluid-rock interactions, (9) ductile and brittle deformation mechanisms, and (10) the dynamics of late- to post-orogenic extension and deep crustal exhumation.

This session aims to bring together scientists studying rocks and geological processes from all stages of the Caledonian Wilson cycle, i.e. from rifting to collision and post-orogenic extension, and welcomes sedimentological, petrological, geochemical, geochronological, geophysical, structural, and modelling contributions that help to improve our understanding of the Caledonides and mountain belts in general. Contributions related to the ICDP drilling project Collisional Orogeny in the Scandinavian Caledonides (COSC) are especially welcome.

Orals: Fri, 28 Apr | Room K1

Chairpersons: Jaroslaw Majka, Deta Gasser, Christopher Juhlin

16:15–16:20

5-minute convener introduction

Results from the Collisional Orogeny in the Scandinavian Caledonides (COSC) project

16:20–16:30

EGU23-7875

ECS

On-site presentation

Seismic site characterization around the COSC-2 drill hole (Järpen, Sweden)

Lena Bräunig, Stefan Buske, Rüdiger Giese, Katrin Jaksch, Jochem Kück, Sebastian Krastel, Henrik Grob, Christopher Juhlin, Henning Lorenz, and Bojan Brodic

Within the ICDP-funded project COSC (Collisional Orogeny in the Scandinavian Caledonides), mountain building processes are investigated with the help of two ~2.5 km deep fully cored boreholes in Central Sweden. Drilled in 2014, borehole COSC-1 near Åre studied the emplacement of the high-grade metamorphic allochthons and obtained a section through the Lower Seve Nappe as well as the underlying mylonite zone. The second borehole COSC-2, drilled in 2020 near Järpen/Mörsil, focuses on defining the character and age of deformation of the underlying greenschist facies thrust-sheets, the main Caledonian décollement and the Precambrian basement.

An extended walkaway VSP survey at the COSC-2 drill site was performed in September-October 2021. This study aims to support the geological interpretation with a high-resolution 3D image of the subsurface in the direct vicinity of the borehole. This allows the determination of the origin of the basement reflections and reveals the nature of the main décollement as well as the degree of basement thrusting. Two 2D surface seismic lines approximately perpendicular to each other (North to South, West to East) and centered around the COSC-2 drill site were acquired using single (1C) and three-component (3C) geophones at 5-30m intervals. Furthermore, the West-East line was extended by 30 geophones at 100m intervals on each line end to allow the registration of wide-angle shots. A 32 t Vibroseis source operated along both lines with source point distances of 100 m within the central part of the line and 500 m at the wide-angle stations, respectively. Ocean bottom seismometers (OBS) were deployed on the bottom of a lake north of the borehole along a ~1.5 km portion of the North-South line. An airgun source was activated on this part of the profile. Along the entire borehole down to a depth of 2.26 km a 3C geophone chain recorded the seismic wavefield from all source points with a geophone spacing of 10 m, complemented by the recording from one single zero-offset source point with a geophone spacing of 2 m.

The obtained surface seismic and VSP data set exhibits exceptionally good quality and shows many pronounced and clear reflections in the raw gathers. They are observed even at the largest source-receiver offsets (~11 km) and are visible at two-way-traveltimes up to 3-4 s, corresponding to structures at a depth of approximately 11 km. We present results of the ongoing surface seismic data processing and analysis, including a P-wave velocity model obtained from first arrival traveltime tomography, an analysis of seismic anisotropy related to the geological structures in the area and a first imaging result from the surface seismic data.

How to cite: Bräunig, L., Buske, S., Giese, R., Jaksch, K., Kück, J., Krastel, S., Grob, H., Juhlin, C., Lorenz, H., and Brodic, B.: Seismic site characterization around the COSC-2 drill hole (Järpen, Sweden), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7875, https://doi.org/10.5194/egusphere-egu23-7875, 2023.

16:30–16:40

EGU23-13822

On-site presentation

COSC-2 and the importance of scientific drilling: discovery of an unexpected Proterozoic igneous and Lower Palaeozoic sedimentary succession beneath the Caledonian nappes

Oliver Lehnert, Mark Anderson, and Simon Cuthbert and the COSC-2 logging team

The COSC (Collisional Orogeny in the Scandinavian Caledonides) project is an integral of the International Continental Scientific Drilling Program (ICDP), performed by a multidisciplinary and international team of geoscientists. It focuses on processes related to the Early Palaeozoic continent-continent collision between Baltica and Laurentia. The collision resulted in the final closure of the Iapetus Ocean in the Middle-Late Silurian when the Baltoscandian margin was partially subducted beneath Laurentia, forming a Himalayan-type orogen. In west-central Sweden this collisional mountain belt is deeply eroded and COSC-2 successfully recovered a continuously cored succession to a depth of 2276 m..

Based on seismic profiling, geophysical models and the resulting interpretations, COSC-2 predicted a continuous Lower Palaeozoic allochthonous sedimentary succession, the main Caledonian décollement in the Cambrian Alum Shale Formation, and a Fennoscandian basement. The unexpected core record therefore perfectly underlines the importance of deep continental drilling. Logging and early studies show that the succession intruded by dolerite dykes involves a thick porphyry sequence instead of Paleoproterozoic granitic basement. Drilling shows that an imbricate zone with Proterozoic and Cambrian sandstones, formed in different settings, covers the basement. The basal sandstones are overlain by deformed Alum Shale comprising the main décollement and by Lower Palaeozoic siliciclastics formed in more outboard and deeper environments. This differs significantly from interpretations based on the preliminary site investigations, which also suggested a main detachment hosted in Alum Shale, but close to the top of the basement, overlain by a zone of imbricates.

New detailed core descriptions show that there is a continuous sedimentary succession on top of a weathered basement (saprock and saprolith) covered by regolith (level of the Sub-Cambrian Peneplain?) which is overlain by basal conglomerates and a few meters of heterogeneous sediments (Lower Cambrian?), displaying the unusual development of a basin filled initially by mostly coarse-grained sediment gravity flows grading into finer-grained turbidites. This sedimentation was interrupted by a longer period of Alum Shale deposition (Middle Cambrian through Tremadocian), which transitioned into turbidite sedimentation again. This higher turbidite sequence (Tremadocian and younger) shows fining upward indicating a general deepening and was previously regarded as a much younger foreland basin fill (Föllinge greywackes). However, local sources of the turbiditic sediments below the Alum Shale and the extended time of deposition may rather point to a continuous sedimentation in a long-lived pull-apart basin preserved in a window beneath the Caledonian thrust sheet.

After many delays caused by Covid pandemic restrictions, the core was logged in fall 2021 and afterwards by the sampling party at the BGR Core Repository in Berlin/Spandau (summer 2022). Dating of the sedimentary units is the base of a stratigraphic framework for further correlations of geotectonic events, sea-level fluctuations, evolutionary pulses, climate changes, and the re-interpretation of seismic models. The continuous COSC-2 sequence provides various possibilities for interdisciplinary collaborations and studies performed by the COSC science team. The first scientific results are presented in session TS6.4 "The Caledonian Orogen of the North Atlantic region: insights from geological and geophysical studies".

How to cite: Lehnert, O., Anderson, M., and Cuthbert, S. and the COSC-2 logging team: COSC-2 and the importance of scientific drilling: discovery of an unexpected Proterozoic igneous and Lower Palaeozoic sedimentary succession beneath the Caledonian nappes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13822, https://doi.org/10.5194/egusphere-egu23-13822, 2023.

16:40–16:50

EGU23-15277

Highlight

On-site presentation

Heat flow in the COSC-1scientific borehole, implications for the Caledonian paleothermal state

Christophe Pascal and Niels Balling and the COSC geothermal team

The scientific drilling project “Collisional Orogeny in the Scandinavian Caledonides” (COSC), supported by ICDP and the Swedish Research Council, involved the drilling of two vertical boreholes through carefully selected sections of the Paleozoic Caledonian orogen in Central Sweden. The main objectives of the COSC geothermal team are: a) to determine the vertical variation of the geothermal gradient, heat flow and thermal properties, and to determine the required corrections for shallow (< 1 km) heat flow data; b) to advance basic knowledge about the thermal regime of Palaeozoic orogenic belts, ancient shield areas and high heat-producing plutons; c) to improve understanding of climate change at high latitudes (i.e. Scandinavia), including historical global changes and recent palaeoclimate development (since last ice age); d) to explore the geothermal potential of the Åre-Järpen area; e) to assess to what degree the conductive heat transfer is affected by groundwater flow in the uppermost crust, and f) to determine the heat generation input and impact from the basement and the alum shales.

The present contribution focuses on themes “b” and “f” and evaluates the likely paleothermal state of the lithosphere of Baltica, in the region of the COSC boreholes, at the onset of the Caledonian orogeny. We concentrated on the results obtained from COSC-1, which was drilled, fully cored and repeatedly logged for temperature down to ~2.5 km depth. Average heat generation of the penetrated Caledonian metamorphic rocks was derived from the spectral gamma ray logs. The analysis yields a low average value of 0.8 µW/m³. Thermal conductivities were determined from 105 core samples. On average, thermal conductivity equals 2.8±0.4 W/(m K), down to ~2 km depth, and increases to 4.1±1 W/(m K) in the lowermost section of the borehole. The thermal gradient shows obvious paleoclimatic disturbances but seems largely unaffected below ~2 km depth and no advective signal is detected. The calculated heat flow for the deepest section of the well amounts to ~82 mW/m². This unusually high heat flow value for cratonic lithosphere reflects, most likely, dominant input from the underlying highly radioactive Transscandinavian Igneous Belt (TIB), which is Late Proterozoic in age. We therefore propose that the lithosphere of Baltica involving the TIB was relatively warm at the time of the Caledonian orogeny. We anticipate that the relatively high temperatures of the margin of Baltica strongly influenced deformation style.

How to cite: Pascal, C. and Balling, N. and the COSC geothermal team: Heat flow in the COSC-1scientific borehole, implications for the Caledonian paleothermal state, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15277, https://doi.org/10.5194/egusphere-egu23-15277, 2023.

16:50–17:00

EGU23-3271

Highlight

On-site presentation

Characterization of fluids in the Lower Allochthon and Baltican basement of the Scandinavian Caledonides (COSC-2 borehole, central Sweden)

Thomas Wiersberg, Katrin Jaksch, Jochem Kueck, Henning Lorenz, Samuel Niedermann, Simona Pierdominici, Jan-Erik Rosberg, Jessica A. Stammeier, and Franziska D. H. Wilke

The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project studies mountain building processes in a major mid-Paleozoic orogen in western Scandinavia by means of two boreholes (COSC-1 and COSC-2) in Åre municipality, Jämtland, central Sweden. The 2276 m deep COSC-2 borehole was completed in 2020. Subsequently, rising gas bubbles were observed in the borehole, rendering COSC-2 a target for downhole fluid sampling to better understand gas and fluid migration in the subsurface.

Seven downhole fluid samples were collected from the COSC-2 borehole with a Leutert Positive Displacement Sampler (PDS) at depths of potentially fluid-conducting fracture zones between 810 and 2081 m. Target depths for fluid sampling were determined by borehole seismic surveys and downhole acoustic logging conducted at COSC-2 from 2020 to 2022.

Downhole fluid samples were analyzed for their gas-to-water ratio, chemical gas composition (N₂, H₂, CH₄, CO₂, He, Ar, O₂), noble gas isotopes (He, Ne, Ar), and water composition (cations and anions). Gas analyses were also performed on two borehole headspace gas samples. The characterization of the fluids also includes determination of their age based on U/Th-He and K-Ar dating methods, as well as depth of phase separation (degassing) of fluids in the subsurface. These analyses provide valuable information for tracking fluid migration at different scales, i.e., from the microscale (core studies, mm-cm) and mesoscale (borehole studies, dm-m) to the macroscale (seismic, tens of metres-km). The fluid studies are accompanied by mineralogical studies on drill core samples from matching depths to constrain fluid-rock interaction by comparing solid and liquid (gas and aqueous) phases.

Our study of the chemical composition of fluids in the deep crust, as well as their age and interaction with rocks, will provide unique insights into fluid migration processes in a Paleozoic orogen and help understand similar processes in modern/current analogs such as the Himalaya.

How to cite: Wiersberg, T., Jaksch, K., Kueck, J., Lorenz, H., Niedermann, S., Pierdominici, S., Rosberg, J.-E., Stammeier, J. A., and Wilke, F. D. H.: Characterization of fluids in the Lower Allochthon and Baltican basement of the Scandinavian Caledonides (COSC-2 borehole, central Sweden), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3271, https://doi.org/10.5194/egusphere-egu23-3271, 2023.

Collisional and post-collisional tectonics in the Scandinavian Caledonides

17:00–17:10

EGU23-2089

On-site presentation

Palinspastic reconstructions constrained by sediment geochemistry; a new approach to correlating structurally dismembered lithostratigraphic units in the Caledonides of N. Scandinavia

A. Hugh N. Rice, Christa-C. Hofmann, Cornelius Tschegg, Mark Anderson, Gerhard Hobiger, and Thomas Griffiths

Lithostratigraphic units become fragmented during continental collisions and these may then undergo different strain and metamorphic histories. Correlating them subsequently can be difficult, especially where primary variations in thickness occur, and even more so if biostratigraphic constraints are poor or lacking. The resulting uncertainties impact attempts to reconstruct the palaeogeography and basin evolution.

As sediment composition is determined by source area composition, weathering before/during erosion, sorting, and biogenic, aeolian and diagenetic/metamorphic additions/alterations, shale sediments derived from the same source area at the same time should have similar chemical characteristics, differentiating them from other sediments.

Here, we outline part of a regional study of Neoproterozoic to Cambrian shale compositions in the mid- to lower structural levels of the Finnmark Caledonides and parts of the Norbotten Caledonides to test this hypothesis. The aim was to test the validity of presumed correlations between units separated by very large distances in palinspastic restorations. Do similarities in lithostratigraphic sections (crudely, sand vs. mud) reflect anything more than large-scale sea-level variations? Can different source areas be identified?

Major, trace and REE whole-rock data from 98 samples were compared using principal component analysis after the data had been recalculated to centred log-ratio values to mitigate problems associated with the constant-sum effect (Aitcheson 1982). Standard sediment discriminant methods (CIA, MFW and Zr/Sc-Th/Sc plots) support the interpretations given by the principal component analysis but in themselves generally do not show enough differences to yield reliable correlations on their own.

The results confirm some suggested correlations and indicate previously unsuspected ones: Although separated by ~350 km in branch-line/balanced section restorations, the data indicate that the Airoaivi Group in the west of the restored Gaissa Basin (Lower Allochthon) is a correlative of the Vadsø Group in the Autochthon of East Finnmark: The proposal that the Lille Molvik Formation is not part of the Vadsø Group is supported by its chemical similarities with the Tanafjord Group: Inclusion of the Veidnesbotn Formation within the Tanafjord Group, rather than being the basal unit of the Vadso Group, is confirmed by sediment geochemistry. Although these correlations are mostly small-scale and seem localized in importance, they change our overall understanding of the basin evolution, by making some areas that had different sedimentary histories more similar whilst in others they add to the complexity of the basin evolution.

Finally, geochemical differences between the late Precambrian to early Cambrian rocks in the Gaissa Basin of Finnmark and those ~300 km to the south in the Autochthon in Norbotten (Luo Pakte area) reflect deposition from different source areas, despite their detailed lithostratigraphic continuity.

Application of the approach proposed here could usefully be applied to the whole orogen to establish different sedimentary domains in space and time.

How to cite: Rice, A. H. N., Hofmann, C.-C., Tschegg, C., Anderson, M., Hobiger, G., and Griffiths, T.: Palinspastic reconstructions constrained by sediment geochemistry; a new approach to correlating structurally dismembered lithostratigraphic units in the Caledonides of N. Scandinavia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2089, https://doi.org/10.5194/egusphere-egu23-2089, 2023.

17:10–17:20

EGU23-8445

ECS

On-site presentation

Tectonic position and evolution of the Balsfjord Series in the North Norwegian Caledonides

Stephan Höpfl and Jiří Konopásek

The Balsfjord Series in Troms and Finnmark, N-Norway is part of a thrust-related nappe stack emplaced during the Ordovician–Silurian Caledonian orogeny. It overlies the Lyngen Magmatic Complex and Reisa Nappe Complex in the E and is overlain by the Nakkedal and Tromsø nappes in the W. Past research on the geological history of the Balsfjord Series was only undertaken locally and the tectonic meaning of this unit is still poorly understood. This is especially evident considering its role as a low–medium grade unit situated between two high grade complexes with diachronous evolution.

The structural evolution of the Balsfjord Series is characterized by three sets of deformation structures. In low-grade areas, the original bedding S0 was affected by boudinage with generally WSW-ENE-oriented stretching axes. In higher-grade regions, the S0 was folded by tight–isoclinal F1 folds showing flat axial surfaces parallel to the surrounding penetrative metamorphic foliation S1. The FA1 fold axes are parallel with the stretching lineation Ls1, and both show considerable rotation from a NW–SE orientation in the NW towards E–W and ENE–WSW in the SE of the area. The F1 folding was syn-metamorphic as it folded the bedding and simultaneously developed the peak metamorphic assemblage in the S1 fabric. A second deformation phase locally folds the metamorphic fabric S1 and Ls1. It is represented by open–tight F2 folds with flat–moderately dipping fold axial surfaces in higher-grade areas, or by development of deformation bands in low-grade rocks. The latest set of structures is represented by steep F3 folds and associated axial planar cleavage S3. The F3 folding and cleavage development becomes increasingly accentuated closer to the contact of the Balsfjord Series with the Lyngen Gabbro.

Mineral assemblages and P-T estimates show that the Balsfjord Series features an inverse metamorphic gradient with conditions increasing from the SE into higher tectonostratigraphic levels towards the W and NW. Thermodynamic modelling revealed maximum P-T conditions of ~450°C and 6.5 kbar in the garnet-zone of the unit, increasing up to 600 °C and 8 kbar in the staurolite-bearing uppermost levels. U–Pb dating of monazite associated with the peak mineral assemblage yielded ages between ca. 425–435 Ma, coeval with localized deformation of the basement rocks.

Our observations together with published data from the surrounding units suggest a tectonic scenario, which involves two suture/thrust zones. The uppermost Tromsø and Nakkedal nappes reached their metamorphic peak at ca. 450 Ma. Their exhumation to upper crustal levels likely occurred soon after that and there these units remained tectonically dormant. At ~440 Ma, the Nordmannvik Nappe of the Reisa Nappe Complex reached its peak metamorphism as a part of the eastern subduction channel. Final exhumation of the Nordmannvik Nappe and closure of the eastern suture took place at ~430 Ma. This was accompanied/followed by underthrusting of the Balsfjord+Lyngen nappe assembly in the west under the Tromsø+Nakkedal+nappe assembly causing the deeper burial and peak metamorphism of the Balsfjord Series at around the same time.

How to cite: Höpfl, S. and Konopásek, J.: Tectonic position and evolution of the Balsfjord Series in the North Norwegian Caledonides, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8445, https://doi.org/10.5194/egusphere-egu23-8445, 2023.

17:20–17:30

EGU23-16664

On-site presentation

On buoyancy and diapirism as drivers for exhumation of the basement infrastructure in the Western Gneiss Region, southern Scandinavian Caledonides

Simon Cuthbert

The Western Gneiss Region (WGR) is dominated by orthogneisses and bounded by normal-sense shear zones against overlying allochthons. This vast mass of granitoid rocks underwent subduction and re-emergence from the throat of the subduction channel, possibly rupturing the overlying orogenic wedge to open a tectonic window in the orogenic hinterland [2]. In this contribution I will explore available information regarding the role of buoyancy in driving tectonics during formation of this huge tectonic window (e.g. [5]) as an additional factor to permissive uprise within an externally-imposed kinematic system (e.g. [1], [8]).

The WGR is characterised by foliation domes (culminations) in which orthogneisses emerge from below the Scandian allochthons or UHP domains emerge from below HP rocks [4], [5] [8]. Some are metamorphic core complexes (MCC’s) with solid ductile cores [8] but others, cored by migmatite, resemble gneiss domes [7] such as the eastern part of the WGR, a classic area for the study of gravity tectonics [5]. The domes, ovoidal in plan form, are wrapped by the allochthons; the gneiss cores also over-ride the allochthons to form basement-cored fold-nappes. Ramberg’s analogue models of rising gneiss diapirs generated a similar architecture. A key factor is that the gneisses are initially overlain by a denser lid, which creates gravitational instability; this was possibly represented by the ophiolites and arc rocks of the Trondheim Nappe Complex. The density inversion is enhanced by partial melting in the gneisses. The Oppdal domes area have also been interpreted as giant sheath-folds in a simple-shear field [6]. This may be consistent with a scenario where lateral channel flow is combined with diapiric action [7] where breaching of the lid forms an “aneurism”. MCC’s and gneiss domes are important mechanisms for heat dissipation in orogens; in the eastern WGR metamorphic grade in the nappes flanking the domes increases towards the gneisses and with depth in infolded synformal “keels” [3], [4] suggesting transfer of heat advected by the gneiss into the cover. Inverted metamorphic gradients may be generated where domes over-ride the cover.

Understanding the relative roles of buoyancy as a direct driver of exhumation tectonics in the WGR versus permissive uprise controlled by the shear-zone framework will require more detailed mapping-out of Caledonian-age partial melting and metamorphic patterns in the orthogneisses, and new studies of kinematics of the eastern and northern dome systems of the WGR.

Financial support from the National Science Centre, Poland (grant 2014/14/E/ST10/00321) and from AGH UST, Krakow, Poland.

[1] Bottrill et al. (2014) Geochem. Geophys.Geosyst. doi:10.1002/2014GC005253

[2] Brueckner & Cuthbert (2013) Lithosphere doi:10.1130/L256.1

[3] Goldschmidt (1915) Skrift. Vid.-Selksk. Kristiana I. Mat.-Naturvid. Klasse, 6: 1-38

[4] Krill (1985) In: Gee & Sturt The Caledonide orogen: Scandinavia and Related Areas, pp. 475-483. J. Wiley & Sons Ltd., Chichester.

[5] Ramberg (1966) Bull. geol. Instn. Uppsala 43: 72pp.

[6] Vollmer (1988) Journal of Structural Geology 10, 735-743

[7] Whitney et al. (2004) Geol. Soc. America Special Paper 380: 1-19.

[8] Wiest et al. (2020) Journal of the Geological Society, London doi:10.1144/jgs2020-199

How to cite: Cuthbert, S.: On buoyancy and diapirism as drivers for exhumation of the basement infrastructure in the Western Gneiss Region, southern Scandinavian Caledonides, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16664, https://doi.org/10.5194/egusphere-egu23-16664, 2023.

Caledonian inheritance during post-Caledonian evolution

17:30–17:40

EGU23-13330

On-site presentation

3D-temporal structural and petrophysical characterisation of crystalline basement rocks on Smøla Island, Central Norway: Insights into onshore-offshore basement highs and post-Caledonian tectonic evolution

Matthew Hodge, Guri Venvik, Jochen Knies, Roelant van der Lelij, Jasmin Schönenberger, and Giulio Viola

The crystalline basement on Smøla Island, within the Mid-Norwegian Passive Margin of central Norway, exhibits intricate and polyphase brittle deformation feature arrays ideal for characterising fracture networks, tectonic evolution, and fluid flow and basement storage potential. As Smøla Island is considered an onshore analogue of offshore basement structural highs, which are currently poorly constrained in terms of unconventional georesource reservoir potential, this work may have important insights for the resource industry, and additionally for advancing basement-hosted greenhouse gas repository opportunities. In this ongoing study, we are integrating various datasets from four Smøla diamond drill holes and multiscalar surface/subsurface datasets, with K-Ar geochronology, providing a new 3D perspective of brittle deformation evolution through time and in space. We aim to outline a ‘toolbox’ methodology for producing robust deterministic 3D geological, and eventually, stochastic petrophysical models for deformed basement rock. Strike trends of pervasive cross-cutting lineaments over Smøla, identified from airborne magnetic and DTM data prior to their ground-truthing, high-resolution structural data and microscale petrographic analysis from the drill holes, and representative outcrops across Smøla Island provide geometric, kinematic, genetic, and cross-cutting relationships for a variety of multi-scalar deformation features (including brittle-ductile faults, fracture, and vein arrays). Field evidence and petrographic analysis suggest at least four major brittle deformation episodes (locally exploiting ductile precursors) linked to distinct mineral assemblages: I) epidote (3 types)-chlorite, II) chlorite-hematite-sericite, III) prehnite-calcite, and IV) hematite-calcite-zeolite. K-Ar dating results from seven selected oriented fault gouges indicate multiphase authigenic clay growth on faults oriented E-W, NW-SE, and NE-SW from the Late Carboniferous/Early Permian to the Late Triassic-Early Jurassic, and on N-S, NNE-SSW faults from the Late Carboniferous/Early Permian to the Mid-Cretaceous. Paleostress inversion from heterogeneous fault-slip data sorted according to the identified mineral assemblages indicates a polyphase tectonic evolution that broadly correlates with the known rifting and opening of the North Sea, and hyper-extension of the Mid-Norwegian margin. On-going 3D geological modelling of the oriented fault and fracture arrays coated by different mineral assemblages, through time, will provide a spatial and temporal evolution model for rock deformation on Smøla. These 3D deterministic geological models will subsequently be utilised to produce meaningful stochastic models, including discrete fracture network models (DFNs), to determine key petrophysical characteristics of the typical basement rocks and of their evolution through time.

How to cite: Hodge, M., Venvik, G., Knies, J., van der Lelij, R., Schönenberger, J., and Viola, G.: 3D-temporal structural and petrophysical characterisation of crystalline basement rocks on Smøla Island, Central Norway: Insights into onshore-offshore basement highs and post-Caledonian tectonic evolution, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13330, https://doi.org/10.5194/egusphere-egu23-13330, 2023.

17:40–17:50

EGU23-10396

On-site presentation

Ancient scars and rotating ribbons: how Appalachian-Caledonian orogenic inheritance seeded the rotations of the Flemish Cap and the Porcupine Bank during the Mesozoic rifting of the North Atlantic Ocean

J. Kim Welford, Michael T. King, and Pei Yang

The rifted continental margins of the modern Atlantic Ocean, spanning from pole to pole, encompass the full gamut of margin types and structural styles, with the Newfoundland-Iberia margins arguably having received the greatest amount of scientific scrutiny and attention. Still, the most interesting segment of the Atlantic appears to correspond to the Newfoundland-Galician conjugates and the Newfoundland-Irish Atlantic conjugates, where classic passive margin templates are suddenly replaced by failed rifts and numerous continental ribbons, still tethered to their continents (e.g., Flemish Cap and Porcupine Bank). This region of increased complexity corresponds exactly with the intersection of the Mesozoic rift with pre-existing, and obliquely-oriented, scars from the Paleozoic Appalachian-Caledonian Orogen, providing a world-class laboratory for investigating the influence of inheritance on rifting.

A recently published numerical modelling study, simulating the interaction of propagating rifts, revealed that such rifts, when laterally offset by approximately 400 km, can successfully generate and rotate continental ribbons away from their respective rifted continental margins. In particular, that study provided a compelling mechanism to explain the rotation of the Flemish Cap. In this work, we argue for the broader extrapolation of those modelling results to explain the rotations of both the Flemish Cap, offshore Newfoundland, and the Porcupine Bank, offshore Ireland, with the first rift corresponding to the northward propagating Atlantic rift and the second apparent rift corresponding to reactivated Appalachian-Caledonian scars. Consistent with the numerical modelling results, this conceptual rifting model results in failed rifts both within the Orphan Basin, offshore Newfoundland, and within the Porcupine Basin, offshore Ireland, with those failed rift features supported by numerous complementary geophysical studies. Future numerical modelling efforts will be dedicated to testing this relatively simple model of rift-inheritance interactions for the southern North Atlantic to confirm that they are sufficient to explain the observed complexity of margin structures between offshore Newfoundland and its conjugates.

How to cite: Welford, J. K., King, M. T., and Yang, P.: Ancient scars and rotating ribbons: how Appalachian-Caledonian orogenic inheritance seeded the rotations of the Flemish Cap and the Porcupine Bank during the Mesozoic rifting of the North Atlantic Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10396, https://doi.org/10.5194/egusphere-egu23-10396, 2023.

17:50–18:00

EGU23-7360

Highlight

On-site presentation

Probing Into the Crust Through eastern Scotland: seismological contraints on the Highland Boundary Fault

Amy Gilligan, David Hawthorn, Robert Clark, Sophia Baker, Alice Blackwell, David Cornwell, Lukman Gani Inuwa, Heather Kennedy, Katrin Löer, Ahmed Madani, and Emma Watt

The Highland Boundary Fault (HBF) delineates a fundamental division in the topography and surface geology in Scotland, separating 1000-500Ma metamorphic rocks to the north from predominantly ~440-360Ma sedimentary rocks of the Midland Valley to the south. Despite detailed geological mapping of the HBF and surrounding areas, the role(s) of the HBF in the tectonic history of Scotland is contested. On one hand, the HBF may represent a major plate boundary that was active initially as a strike-slip, then reactivated as a high angle thrust fault. On the other hand, some argue that lateral movement on the HBF was limited, and the topographic break seen at the HBF is primarily due to differences in erosion rates. Seismicity on the HBF has been reported in both the instrumental and historical records, including a M4.8 earthquake in Comrie in 1839 and an earthquake swarm in Aberfoyle in 2003. Notably, no seismicity has been observed in northeast Scotland. It may be that there is no seismicity in this region, or that the distribution of seismic instrumentation has been insufficient to detect very small magnitude earthquakes (<M2).

To address these questions, in March-May 2022 we deployed a new network of 10 seismometers in north eastern Scotland as part of the PICTS (Probing Into the Crust Through eastern Scotland) project, which, together with a BGS Seismology permanent station, DRUM, form three transects across the HBF. These instruments form the first dense seismometer deployment in this region and data from them will allow us to place high-resolution constraints on the structure of the crust and uppermost mantle across the HBF, determine crustal thickness in this region, and to investigate if any seismicity is occurring on the eastern portion of the HBF.

Here we present preliminary results from the data recorded on seismometers from the PICTS project, including images of crustal structure from receiver function analysis that show differing crustal structure to the north and south of the HBF.

How to cite: Gilligan, A., Hawthorn, D., Clark, R., Baker, S., Blackwell, A., Cornwell, D., Gani Inuwa, L., Kennedy, H., Löer, K., Madani, A., and Watt, E.: Probing Into the Crust Through eastern Scotland: seismological contraints on the Highland Boundary Fault, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7360, https://doi.org/10.5194/egusphere-egu23-7360, 2023.

Posters on site: Fri, 28 Apr, 10:45–12:30 | Hall X2

Chairpersons: Karolina Kośmińska, Deta Gasser, Christopher Juhlin

X2.170

EGU23-2038

ECS

Pressure-temperature-time evolution of a blueschist and an eclogite from the Vestgötabreen Complex, Svalbard

Karolina Kośmińska and Jarosław Majka

The pressure-temperature-time (P-T-t) history of a blueschist and an eclogite from the high pressure-low temperature Vestgötabreen Complex, Svalbard, has been constrained using the conventional geothermobarometry, trace elements thermometry, and elastobarometry coupled with Lu-Hf garnet, U-Pb monazite, and U-Pb zircon dating. Three evolutionary stages for the eclogite have been distinguished thanks to the different textural positions and zoning of major minerals. The prograde growth (M1) happened at 15.9 kbar and 460°C, then the peak-P conditions (M2) 23.5 kbar at 507°C, followed by peak-T conditions (M3) of 21.4 kbar at 553°C. Only peak conditions of ca. 18 kbar at 520-550°C have been estimated for the blueschist. These P-T results indicate a low geothermal gradient of 7-8°C, as suggested by Agard et al. (2005). Secondary ion mass spectrometry (SIMS) analyses of zircon rims from the eclogite yielded the lower intercept of concordia at 478±17 Ma (n=11, MSWD=1.1), which is interpreted as a prograde growth. Monazite from the matrix and inclusions in garnet rim give a ²⁰⁶Pb/²³⁸U weighted mean age of 471±6 Ma (n=7, MSWD=1.4). Monazite could have formed due to florencite and/or lawsonite breakdown somewhere between M2 and M3 stages. Garnet in the eclogite is strongly zoned and Lu is concentrated mostly in the rims. Lu-Hf dating yields the age of ca. 471 Ma for the biggest fraction and ca. 466 Ma for smaller garnet separates. Monazite from the blueschist gives a ²⁰⁶Pb/²³⁸U weighted mean age of 486±6 Ma (n=4, MSWD=0.32) interpreted as a prograde growth. Lu-Hf dating of garnet from the blueschist provides an age of a peak metamorphism of 471.1±3.8 Ma (n=10, MSWD=2.8). in our opinion, the Vestgötabreen Complex represents the earliest Paleozoic subduction system, which could have developed proximally to the Baltican margin.

This work is supported by the National Science Centre of Poland project no. 2021/43/D/ST10/02305.

References:

Agard P, Labrousse L, Elvevold S, Lepvrier C (2005). Discovery of Palaeozoic Fe–Mg carpholite (Motalafjella, Svalbard Caledonides): a milestone for subduction zone gradients. Geology 33: 761–764.

How to cite: Kośmińska, K. and Majka, J.: Pressure-temperature-time evolution of a blueschist and an eclogite from the Vestgötabreen Complex, Svalbard, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2038, https://doi.org/10.5194/egusphere-egu23-2038, 2023.

X2.171

EGU23-6510

ECS

Metamorphic evolution of a garnet-bearing schist from the Bogegga Formation, Svalbard

Olga Turek and Karolina Kośmińska

The Bogegga Formation crops out on Oscar II Land in the western part of Svalbard archipelago. It is part of the Kongsvegen Group which belongs to the Southwestern Basement Province. This unit contains garnet-bearing mica schists and gneisses, pegmatites, and calc-schists which experienced up to a medium grade metamorphism (Hjelle et al., 1999). However, the petrological studies including estimation of the pressure-temperature (P-T) conditions have not been performed so far. Here we present the petrological characteristics of the highest grade garnet-bearing mica schist and the P-T estimates using a combined approach.

The studied schist consists of garnet porphyroblasts, white mica, biotite, quartz, and plagioclase. Tourmaline, epidote, allanite, zircon, and zoisite are accessory minerals. Garnet shows two distinctive compositions. Garnet-I forms cores and its composition is Alm_76-81Grs_6-9Prp_8-14Sps_2-4. It contains voluminous quartz inclusions. Garnet-II is generally calcium richer and forms rims or fills cracks within garnet-II. Its chemical composition can be characterized as Alm_71-72Grs_18-23Prp_4-7Sps_2-3. White mica is muscovite with Si content varying from 3.075 to 3.162 a.p.f.u. Biotite shows chemical zonation between the inclusions within garnet-I (X_Fe = 0.36 to 0.50) and matrix (X_Fe = 0.64 to 0.68). Plagioclase is dominated by albite endmember and its composition is Ab_77-97An_2-22Or_1-2. Rims of bigger porphyroclasts are albite rich, whereas cores are enriched in anorthitic component. Two metamorphic phases M1 and M2 were distinguished based on the petrological studies and P-T estimates. Preliminary P-T estimates suggest garnet-I growth at 4.3 – 8.5 kbar and 415 – 560 °C (M1), followed by garnet-II and matrix minerals formation at higher pressures and temperatures of 7.5 – 10.8 kbar and 590 – 675 °C (M2).

Amphibolite facies rocks that experienced similar P-T conditions are known from SW Svalbard (f.E. Müllerneset Formation, Berzeliuseggene unit, Isbjørnhamna Group, Pinkie unit). The correlations of the Boggega Formation with other amphibolite facies units cropping out along southwestern Svalbard require further studies including detailed geochronological analyses. This work was partly funded by the National Science Centre of Poland project no. 2021/43/D/ST10/02305.

References:

Hjelle A., Piepjohn K., Saalmann K., Ohta Y,. Salvigsen O., Thiedig W., Dallmann W.K. (1999). Geological Map, Svalbard 1:100 000, A7G Kongsfjorden, Norsk Polarinstitutt, Tromsø.

How to cite: Turek, O. and Kośmińska, K.: Metamorphic evolution of a garnet-bearing schist from the Bogegga Formation, Svalbard, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6510, https://doi.org/10.5194/egusphere-egu23-6510, 2023.

X2.172

EGU23-8869

Tonian to Mississippian magmatic pulses recorded within the Pearya Succession I in the vicinity of Yelverton Inlet, Ellesmere Island, Canada

Jarosław Majka, Karolina Kośmińska, and Jakub Bazarnik

The Pearya Terrane of northern Ellesmere Island is composed of a Tonian crystalline arc, Neoproterozoic to Paleozoic sedimentary successions, an Ordovician island arc complex and related volcaniclastics, and middle Ordovician to Silurian sedimentary rocks. Igneous rocks of the Pearya Succession I, dominated by Tonian gneiss, were targeted for ion microprobe U-Pb zircon dating. Two felsic gneisses yielded Tonian c. 960 Ma and 940 Ma ages, respectively. Another two felsic gneisses gave ages of c. 870 Ma and c. 750 Ma. The latter exhibited common inherited zircon cores dominated by a c. 870 Ma signature. Out of three dated mafic samples, a gabbro yielded an age of c. 470 Ma, while basaltic dykes gave c. 415 Ma and c. 340 Ma. The c. 415 Ma dyke is cutting the c. 940 Ma gneiss, whereas the c. 360 Ma dyke is emplaced within the c. 870 Ma gneiss. While the obtained ages in the range of c. 960-940 Ma are typically reported from the Pearya Succession I, felsic gneisses of c. 870 Ma and 750 Ma, to our knowledge, have not been reported so far. Tentatively, we interpret these two ages as a potential expression of post-Grenville extension, associated with an attempted, repeated, but unsuccessful rifting. The c. 470 Ma gabbro is interpreted to have formed in an active margin environment as a part of the Thores Arc during the main phase of the Caledonian (M’Clintock) subduction and amalgamation. The age of c. 415 of the older mafic dyke somewhat corresponds to other Early Devonian magmatic rocks known from Pearya. Interestingly, it slightly precedes the timing of prograde metamorphism within an adjacent Barrovian sequence of the Petersen Bay Assemblage. Thus, it may represent the earliest expression of a hypothesized igneous heat source for the Barovian sequence (Kośmińska et al. 2022, JPet). Lastly, the c. 340 Ma mafic dyke is coeval with metamorphism and granitic magmatism known from Pearya (Trettin 1998 GSC Bulletin, Estrada et al. 2016 JGeodyn, Powell & Schneider 2022 Tectonics). It is also coeval with regional extension and deposition of the Emma Fiord and Borup formations of the Sverdup Basin. Notably, the latter contains the Audchild basaltic lavas and pyroclastic sediments (Thorsteinsson 1974, GSC Bulletin). Thus, we postulate that the mafic dyke of c. 340 Ma age is closely related with extension and rifting responsible for the formation of the Sverdrup Basin. This discovery calls for much more careful interpretation of numerous undated mafic dykes occurring within the Pearya Succession I.

This research is funded by the National Science Centre (Poland) project no. 2019/33/B/ST10/01728.

How to cite: Majka, J., Kośmińska, K., and Bazarnik, J.: Tonian to Mississippian magmatic pulses recorded within the Pearya Succession I in the vicinity of Yelverton Inlet, Ellesmere Island, Canada, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8869, https://doi.org/10.5194/egusphere-egu23-8869, 2023.

X2.173

EGU23-5958

ECS

Tectonic reconstruction of the Lyngen Magmatic Complex

Marina Galindos Alfarache, Holger Stünitz, Mathieu Soret, Benoît Dubacq, and Guillaume Bonnet

The Lyngen Magmatic Complex (LMC) is the lowest unit of the Lyngsfjellet Nappe (Upper Allochthon, North Norwegian Caledonides). The fabrics of the LMC rocks range from undeformed to mylonitic. The undeformed rock is a gabbro-norite formed primarily by anorthite-rich (93%) plagioclase, enstatite, and augite. Two deformation events are distinguished in the LMC: (D1) an earlier shearing that has produced a N—S trending vertical foliation with sub-horizontal stretching lineation and dextral sense of shear, and (D2) a top-to-SE-directed thrust contact with the lower nappe series at the base of the meta-gabbro-norite. In the thrust contact region, the early vertical foliation is rotated into a flat-lying orientation and shows an ESE-trending stretching lineation. Deformed fabrics of D1 have developed successively from lower amphibolite, to epidote-amphibolite, and to greenschist metamorphic grades, i.e., on a retrograde temperature-path. The fabrics of the thrust contact have also developed from amphibolite to greenschist conditions.

Rock fabrics associated to D1 are dominantly located in the northern portion of the LMC (from Lyngstuva to the north side of the Kjosen fjord). The amphibole compositions of these rocks vary from core to rim, showing a trend from pargasitic to actinolitic composition, consistent with the transition from high- to low-temperature (amphibolite to greenschist facies). U-Pb dating of titanite associated with the greenschist grade in meta-gabbro-norite assemblages indicates a date of 485±9 Ma. This date is interpreted as a deformation/metamorphic age, because the analysed titanite forms from pargasite breakdown and is aligned parallel to the deformed fabric. As this deformation event is synchronous with the crystallization age of the LMC (481±6 Ma, Augland et al., 2014), the deformation associated to the N—S oriented stretching lineation and vertical foliation is linked to sea floor strike slip movements during back-arc spreading of the LMC. D2-rock-fabrics are dominantly located in the southern portion of the LMC and represent typical structures of nappe stacking during the Scandian collisional stage of the Caledonian orogeny. Close to the lower boundary of the LMC, garnet-bearing amphibolites, allow refining the P and T conditions for this unit. Thermobarometric estimates result in conditions of 650°C and 10kbar. This temperature is in contrast with the Raman spectroscopy values averaging around 530°C for the graphite bearing sediments below the lower contact of the LMC, i.e. sediments between the meta-gabbro-norite and the underlying Reisa nappe. The temperature difference between the two deformation events indicates re-heating of the meta-gabbro-norite during the Scandian thrusting.

The D1 structural relationships described in the LMC appears common for supra-subduction zone settings, and could potentially be observed at deeper mantle sections as reported in younger analogue tectonic settings as the Wadi al Wasit area of the Oman ophiolite. D2 appears linked to out-of-sequence thrusting at the base of the LMC with respect to the surrounding nappes, contributing to the north Norwegian Caledonides nappe transport sequence.

How to cite: Galindos Alfarache, M., Stünitz, H., Soret, M., Dubacq, B., and Bonnet, G.: Tectonic reconstruction of the Lyngen Magmatic Complex, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5958, https://doi.org/10.5194/egusphere-egu23-5958, 2023.

X2.174

EGU23-13021

ECS

Magmatism and metamorphism of the Mårma Terrane, Kebnekaise region, northern Swedish Caledonides

Riccardo Callegari, Karolina Kośmińska, Iwona Klonowska, Christopher J. Barnes, and Jarosław Majka

The Middle Allochthon of the Scandinavian Caledonides represents the Neoproterozoic distal continental passive margin intruded by a dyke swarm with minor Mesoproterozoic and Paleoproterozoic orthogneiss. Locally, it carries early Neoproterozoic plutonic rocks. For this work, we collected geochronological and geochemical data and carried out thermodynamic modelling on a variety of lithologies from the Vássačorru Igneous Complex (VIC) and surrounding rocks of the Mårma terrane of the Seve Nappe Complex (SNC) in the Kebnekaise area, northern Swedish Caledonides.

U-Pb zircon LA-ICP-MS geochronology yielded crystallization ages of c. 864±3 Ma (MSWD=0.92; n=9) and 856±3 Ma (MSWD=2.8; n=10) for the Vistas Granite and a gabbro from the VIC, respectively. A granodioritic intrusion yielded an age of 850±1 Ma (MSWD=1.5; n=38), whereas a granitic dyke and mylonitic orthogneiss yielded ages of 840±7 Ma (MSWD=4.3; n=50) and 835±8 Ma (MSWD=0.71; n=24), respectively. Younger populations of zircon at c. 626–610 Ma were dated in a banded amphibolite and the Aurek gabbro. Rare earth element (REE) geochemistry from felsic lithologies in the VIC indicate lower crustal contamination, while the REE pattern for the VIC gabbro suggests an N-MORB affinity for light REE and enrichment in the heavy REE due to crustal assimilation. The banded amphibolite records pressure-temperature (P–T) conditions in the melt stability field at 10.5–12.0 kbar and 600–680 °C. The Aurek gabbro records high-pressure metamorphism at 11.8–12.6 kbar and 480–565 °C. Phase equilibrium modelling of the peak metamorphic assemblage in the mylonitic orthogneiss yielded 11.2–11.7 kbar and 560–610 °C, while the retrograde assemblage yielded 7.4–8.1 kbar and 615–675 °C. Furthermore, P–T estimates of 6.5–7.5 kbar at 600–625 °C were obtained for the Vistas Granite.

The geochronological data indicate that the Kebnekaise region experienced several magmatic pulses during the Neoproterozoic. These geochronological and geochemical data suggest that the magmatic event responsible for the emplacement of the VIC is related to an attempted break-up of Rodinia between c. 864–835 Ma. The ages obtained from banded amphibolite and the Aurek gabbro represent the emplacement of mafic protoliths during the real break-up at c. 626–610 Ma.

Two metamorphic ages were obtained: one, c. 598 Ma, from the banded amphibolite, is interpreted as the age of the high temperature metamorphism in the melt stability field. The second, c. 443 Ma, from the mylonitic orthogneiss, is interpreted as the age of the amphibolite facies metamorphic condition reached during the collisional stage. The age of the metamorphic peak was not detected. However, the P–T estimates for the mylonitic orthogneiss and the Aurek gabbro are comparable with the results from other lithologies within the Kebnekaise region and in the northern Seve Nappe Complex. For this reason, we hypothesize that the age of the metamorphic peak is at c. 490–480 Ma.

This research is funded by the National Science Centre (Poland) project no. 2019/33/B/ST10/01728 to Majka.

How to cite: Callegari, R., Kośmińska, K., Klonowska, I., Barnes, C. J., and Majka, J.: Magmatism and metamorphism of the Mårma Terrane, Kebnekaise region, northern Swedish Caledonides, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13021, https://doi.org/10.5194/egusphere-egu23-13021, 2023.

X2.175

EGU23-9407

ECS

40Ar/39Ar geochronology of the Seve Nappe Complex in central Scandinavian Caledonides: Insights into exhumation processes

Iwona Klonowska and Christopher J. Barnes

The Seve Nappe Complex (SNC) is an exhumed high-to-ultra high pressure (HP-UHP) metamorphic unit exposed for >1000 km along the strike of the Scandinavian Caledonides. In the Åre region in Sweden, the SNC is subdivided into the Middle and Lower Seve nappes divided by a shear zone. The Middle Seve is dominated by migmatitic paragneisses metamorphosed in the UHP diamond stability field at c. 455 Ma, and overprinted in granulite facies conditions at c. 442-435 Ma (Gee et al. 2020, Geol. Soc. Lond. Mem. 50, 517-548 and references therein). The Lower Seve is dominated by metasedimentary rocks with minor orthogneisses and amphibolites. Garnet mica schists experienced peak-pressure metamorphism and a subsequent mylonitic overprint in amphibolite facies conditions (Jeanneret et al. 2022, JMG), dated to c. 460-430 Ma (Giuntoli et al. 2020; Tectonics 39, e2020TC006267). Lower Seve shearing is dated to c. 423-417 Ma, similar to the dividing shear zone at c. 424 Ma (e.g. Majka et al. 2012, J. Geosci. 57, 3-23; Giuntoli et al. 2020; Jeanneret et al. 2022).

In-situ laser ablation and step-heating ⁴⁰Ar/³⁹Ar geochronology was conducted on white mica and biotite in paragneisses and mylonites from Åreskutan Mt (Middle Seve), as well as orthogneisses and deformed metasediments from the Collisional Orogeny in the Scandinavian Caledonides (COSC-1) deep borehole in the Lower Seve to resolve the timing of exhumation and possible earlier metamorphic event(s).

In the Middle Seve, in-situ laser ablation of biotite included in garnet, located between HP phases, replacing garnet, and within kyanite-sillimanite-biotite lenses produced c. 451 Ma in the UHP gneisses, and c. 453 Ma in both the migmatite and mylonite. Biotite defining the main foliations of these rocks provided c. 440, 437, and 438 Ma, respectively, with the youngest date of c. 428 Ma resulting from deformed biotite. Phengitic white mica defining the foliation in the migmatite provides a date of c. 443 Ma and a range of 430-422 Ma. Step-heating results are overall younger, with biotite plateau dates of c. 430, 420 and 413 Ma from the UHP gneiss, and a white mica date of c. 404 Ma from a migmatite.

In the Lower Seve rocks, the in-situ dates from deformed and undeformed white mica and biotite are more consistent, ranging from 434 to 424 Ma. Only biotite from one metasediment preserved older dates of 441-436 Ma. Similar to the Middle Seve, the step-heating results are younger with biotite yielding plateau ages of c. 414 Ma and 408 Ma, and white mica providing c. 418 Ma, and 407-404 Ma in all rocks.

Altogether, the oldest biotite dates likely inherited records of the Ordovician-Silurian UHP-HT subduction-exhumation events in the K-rich Middle Seve gneisses. In the other rocks from both Middle and Lower Seve nappes, both deformed and undeformed biotite and white mica resolve the timing of Silurian thrusting and exhumation of the nappes, followed by a second Devonian exhumation event, which is primarily recorded by white mica plateau dates.

This work is financially supported by the National Science Centre (Poland) research project no. 2018/29/B/ST10/02315.

How to cite: Klonowska, I. and Barnes, C. J.: 40Ar/39Ar geochronology of the Seve Nappe Complex in central Scandinavian Caledonides: Insights into exhumation processes , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9407, https://doi.org/10.5194/egusphere-egu23-9407, 2023.

X2.176

EGU23-8475

ECS

Monazite U-Th-total Pb dating of migmatites from the Krutfjellet Nappe, Upper Köli Nappes, Swedish Caledonides

Isabel S. M. Carter, Simon Cuthbert, and Katarzyna Walczak

The Köli Nappe Complex (KNC) in the Scandinavian Caledonides of Sweden originated as terranes within the Iapetus Ocean derived from subduction-related magmatic and basin systems. The Krutfjellet Nappe is part of the Upper Kӧli Nappes in Västerbotten, Sweden. Siliclastic, carbonate and volcanic protoliths^[3] underwent amphibolite facies metamorphism involving extensive migmatisation, which was of a distinctly higher grade than the other Koli Nappes. No modern P-T-t studies have been made in this nappe. Foliations and early folds in the metasediments (D1 and D2) are cut by latest Ordovician to earliest Silurian metagabbros and metagranites. Regional metamorphism and intrusion were syn-to-post D2. All these predate Scandian thrusting over the middle and lower KNC^[3]. A trondhjemitic pebble in a metaconglomerate was dated to c. 489 Ma^[4] so the main fabric-forming event is constrained to some time in the Ordovician. The mafic intrusions were partially converted to amphibolite and greenschist^[2] and the main greenschist-amphibolite metamorphism in the subjacent KNC was early Silurian, followed by early Devonian thrusting^[1], so a Scandian metamorphic imprint in the Krutfjellet Nappe is implied.

Four sillimanite and/or kyanite-bearing pelitic migmatite samples from the Norra Storfjället lens of the Krutfjellet Nappe were selected for U-Th-total Pb electron microprobe dating of monazite. Monazites from a variety of fabric elements including matrix, leucosome and inclusions within garnet yielded ages spanning the range 484-390 Ma. The monazites often have complex zoning patterns in Th and Y. However, discrimination of monazite populations based on trace element measurements was not resolvable so zoning appears to be decoupled from ages. There is also no discernable relationship between ages and location of the monazite within fabric elements. Weighted mean specimen ages were found to be 427 ±3.8 Ma, 442.5 ±4.0 Ma, 433.3 ±3.0 Ma and 438.3 ±2.7 Ma.

The large span of ages obtained suggests that more than one metamorphic event is recorded, however, some mixing and/or partial resetting of ages has occurred. The oldest ages (474-484 Ma), often outliers, are close to the early Ordovician conglomerate clast age^[4] and may have either been inherited from detrital monazite or formed during an early metamorphic event close to the clast age. The youngest ages (c. 430-400 Ma) are likely to be related to final thrusting of the Scandian nappe assemblage. The predominant age population falling around 445-435 Ma is similar to the ages of nearby early Silurian intrusions^[3], so monazite may have been generated or reset by the early Silurian intrusions, or by regionally-enhanced thermal regime associated with this magmatism.

Funded by the National Science Centre (Poland) grants no. 2021/41/N/ST10/04298 and 2021/41/N/ST10/04298.

[1] Bender, H., Glodny, J. and Ring, U. 2019. Lithos, 344–345, 339–359.

[2] Senior, A. and Otten, M.T. 1985. In: Gee, D.G. and Sturt, B.A., 953–978.

[3] Stephens, M.B. 2020. GSL Memoirs, 50, 549–575.

[4] Stephens, M.B., Kullerud, K. and Claesson, S. 1993. GSL, 150, 51–56.

How to cite: Carter, I. S. M., Cuthbert, S., and Walczak, K.: Monazite U-Th-total Pb dating of migmatites from the Krutfjellet Nappe, Upper Köli Nappes, Swedish Caledonides, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8475, https://doi.org/10.5194/egusphere-egu23-8475, 2023.

X2.177

EGU23-9416

The Grønfjellet unit – an alkaline volcanic complex of uncertain tectonic affiliation in the eastern Trondheim Nappe Complex, central Scandinavian Caledonides

Deta Gasser, Gurli Meyer, Anna K. Ksienzyk, Frode Ofstad, Lars Eivind Augland, Trond Slagstad, and Tor Grenne

The Trondheim Nappe Complex (TNC) of the central Scandinavian Caledonides is a key area for understanding the closure history of the Iapetus Ocean prior to the final collision between Laurentia and Baltica. In the western TNC, late Cambrian to early Ordovician oceanic arc formation, followed by arc–continent collision and ophiolite obduction onto a Laurentia-derived microcontinent, is well-documented. Following arc–continent collision, a mid-Ordovician phase of rifting has recently been identified, which produced a peculiar volcanic association of MORB-type basalts and a variety of alkaline, shoshonitic and ultrapotassic volcanic rocks. In the eastern TNC, the volcanic and tectonic evolution is less well constrained, but the Fundsjø Group is traditionally interpreted to represent an immature, ensimatic island arc of late Cambrian age.

Recent field mapping, geochemistry, and air-borne geophysical work in the eastern TNC has identified a distinctive volcanic complex in the Grønfjellet area, previously mapped as part of the Fundsjø Group. The complex covers at least 7 km² and comprises a variety of rock types: (1) pyroclastic volcanic deposits with up to 20x10 cm large, subrounded, flattened, fine-grained clasts with feldspar and amphibole crystals in a matrix of similar composition, (2) fine-grained greyish rocks with mm-sized white feldspar aggregates/crystals and/or mm- to cm-sized amphibole crystals, with and without subtle compositional layering, (3) homogeneous, fine- to medium-grained feldspar- and amphibole-rich rock (“micro-gabbro texture”), and (4) very fine-grained, flinty, light-grey-greenish rocks with a homogeneous texture. Along its northern and eastern borders, the complex is associated with abundant marble layers; the western border is associated with brownish-weathering biotite-muscovite schists, whereas the southern continuation of the complex is still unclear.

Preliminary geochemical data from ten fine-grained samples of volcanic origin reveal a peculiar composition: they plot as alkaline rocks in the Nb/Y vs. Zr/Ti diagram; they are enriched in LREE as well as Th, U, Nb and Ta; they plot close to the MORB–OIB array in the Nb/Yb vs Th/Yb diagram; and they do not show significant negative Nb-Ta anomalies typical for island-arc or back-arc settings. Ranging in composition from trachybasalt, through basaltic trachyandesite to trachyandesite, they are very different from the typical island arc tholeiites and back-arc basin basalts of the Fundsjø Group metavolcanic rocks elsewhere, and are more similar to rift-related alkaline rocks from the western TNC. Age dating of the Grønfjellet rocks is ongoing, as is a comparison with newly acquired geochemical data from adjacent areas of the Fundsjø Group, in order to shed light on the tectonic affiliation of this volcanic complex.

How to cite: Gasser, D., Meyer, G., Ksienzyk, A. K., Ofstad, F., Augland, L. E., Slagstad, T., and Grenne, T.: The Grønfjellet unit – an alkaline volcanic complex of uncertain tectonic affiliation in the eastern Trondheim Nappe Complex, central Scandinavian Caledonides, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9416, https://doi.org/10.5194/egusphere-egu23-9416, 2023.

X2.178

EGU23-13596

Pyroxene microstructures in eclogite from UHP domains and an interjacent area, Western Gneiss Region, Norway

Dirk Spengler, Adam Włodek, Xin Zhong, Anselm Loges, and Simon Cuthbert

The Western Gneiss Region (WGR) in W Norway exposes ultrahigh pressure (UHP) metamorphic eclogite of Scandian age in domains that are spatially separated from one another for unknown reasons. We studied five eclogites from the two northern UHP domains and the area in between (at the localities Årsetneset, Fjørtoftvika, Riksheim, Synes, Ulsteinvik) for petrography, mineral chemistry and by Raman spectroscopy. The peak metamorphic mineral assemblages contain garnet, Na-pyroxene (jadeite 0.13–0.46) and – depending on the sample – rutile, ilmenite, quartz, kyanite and/or orthopyroxene. Depending on strain accumulation, the eclogite facies fabric is poikiloblastic or has a foliation formed by elongated grains and grain aggregates of Na-pyroxene and garnet. Secondary processes formed amphibole, biotite and symplectite of plagioclase and diopside. Irrespectively, all samples contain Na-pyroxene with needle-shaped inclusions that are in parallel to the presumed c-axis of the host. These needles are either bi-mineralic (quartz + pargasite) or monomineralic (quartz). Chemically integrated compositions obtained at mineral surfaces with needle exposure using a scanning electron beam yielded lower Ca-Tschermak’s and higher Ca-Eskola components than the host. The molar ratios of these calculated endmembers are consistent with the needles being formed by the reaction: 2 Ca-Eskola = Ca-Tschermak’s + 3 quartz. If Ca-Eskola is regarded to be typical for UHP metamorphism, then the spatial distribution of eclogite with quartz needles does not support a separation of the two northern UHP domains by the interjacent area.

Garnet has minor compositional zoning with smooth gradients at grain rims. Mineral core compositions of garnet and needle-bearing Na-pyroxene suggest minimum metamorphic conditions after needle formation in the ranges of 700-790 °C and 1.0-1.6 GPa, when the calibrations of the Fe–Mg geothermometer of Krogh Ravna (2000) and the jadeite + quartz geobarometer of Carswell & Harley (1990) are applied. Subsequent retrogression partially transformed quartz needles into albite needles with irregular outline in two of the samples (Riksheim, Ulsteinvik) at the expense of jadeite in the proximal host. Rare associated needles of cristobalite and an unknown phase with albite chemistry in these two southernly samples, perhaps as a result of retrogression, were not observed in the three northernly samples. Hence, the evolution of the pyroxene microstructures after formation allows to investigate spatial differences in the retrogression history.

This work is financially supported by the Norwegian Financial Mechanism 2014-2021 and the Polish National Science Centre, project no. 2020/37/K/ST10/02784.

Carswell, D.A. & Harley, S.L. (1990): Mineral barometry and thermometry. In: Carswell, D.A. (ed.) Eclogite Facies Rocks. Glasgow and London: Blackie, 83-110.

Krogh Ravna, E. (2000): The garnet–clinopyroxene Fe²⁺–Mg geothermometer: an updated calibration. Journal of Metamorphic Geology 18:211-219.

How to cite: Spengler, D., Włodek, A., Zhong, X., Loges, A., and Cuthbert, S.: Pyroxene microstructures in eclogite from UHP domains and an interjacent area, Western Gneiss Region, Norway, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13596, https://doi.org/10.5194/egusphere-egu23-13596, 2023.

X2.179

EGU23-12123

Detrital zircon geochronology of Lower Paleozoic sedimentary rocks from COSC-2 borehole

Grzegorz Ziemniak, Iwona Klonowska, William McClelland, Oliver Lehnert, Simon Cuthbert, Isabel Carter, Ricardo Callegari, and Katarzyna Walczak

The Caledonian Orogeny in the Scandinavian Caledonides (COSC) project aims to investigate the orogenic processes involving Caledonian allochthons together with the underlying sedimentary cover and Proterozoic igneous basement. The basement comprises Transscandinavian Igneous Belt (TIB) rocks with Hallandian and Central Scandinavian Dolerite Group intrusions and is overlain by a regolith (sub-Cambrian peneplain?). A Lower Cambrian(?) sedimentary succession of conglomerate, carbonate and shale covers this immature soil, followed by coarse-grained gravity flows fining upwards and showing a transition into the Alum Shale Formation. The undisturbed middle part of the formation separates the lower sedimentary cover from its overlying turbiditic part and the Lower Ordovician(?) turbidite sequence fining up to the top of the COSC-2 core.

First results of detrital zircon geochronology from the Cambrian succession show that the basal section of the autochthonous cover is characterized by mainly late Paleoproterozoic (c. 45% of all grains) – early Mesoproterozoic (c. 52%) detrital grains with age signatures of c. 1.77 Ga, 1.66 Ga and 1.44 Ga and a subordinate 1.25 Ga age peak. The middle part of the succession is dominated by late Paleoproterozoic detritus (c. 62% of all grains) with minor Mesoproterozoic (c. 21%) and Archean (c. 11%) input. The main age signatures are c. 1.80 Ga and 1.90 Ga with subordinate age peaks at c. 2.72 Ga, 2.00 Ga, 1.16 Ga. The upper part of Lower Cambrian(?) succession is characterized by Archean to Cambrian detritus. Archean grains constitute 12% of grains with dominant age signature at c. 2.67 Ga. Paleoproterozoic grains (25%) are grouped in 2.15-1.65 Ga interval with peaks at c. 2.12 Ga, 1.80 Ga, 1.76 Ga and 1.67 Ga. The Mesoproterozoic population (41%) is characterized by major age peaks at c. 1.55 Ga and 1.20 Ga. Neoproterozoic – Cambrian group (17%) contains major populations at c. 0.60 Ga and 0.53 Ga and a significant peak at c. 0.72 Ga. The maximum depositional age calculated via the maximum likelihood age algorithm yielded 530.5±4 Ma for the upper part of the Lower Cambrian succession. Two samples from the Ordovician succession show Mesoproterozoic – Neoproterozoic sources (c. 75% of grains), with more than 38% of grains yielding late Mesoproterozoic – early Neoproterozoic (1.2-0.9 Ga) ages. The dominant population of c. 1.06-1.02 Ga is accompanied by c. 1.50-1.47 Ga, 1.15 Ga and 0.99-0.97 Ga age peaks.

The autochthonous Lower to Lower Middle Cambrian passive margin succession in the lower part is dominated by local detritus provided solely from the Eastern Segment of Sveconorwegian Orogen (including the basement investigated by the COSC-2). The provenance shifts up the profile towards TIB-1 and Svecofennian Orogen sources, with the youngest part of the succession characterized by an input of Timanian Orogen detritus, including the uplifted Karelian protocraton. The Ordovician succession is characterized by Meso-Neoproterozoic age populations most likely sourced from the Sveconorwegian Orogen with a minor cratonic contribution. The youngest detritus is early Neoproterozoic, suggesting a passive margin setting with no early Caledonian input present.

This work was funded by the National Science Centre (Poland) projects no. 2019/33/B/ST10/01728 and 2018/29/B/ST10/02315.

How to cite: Ziemniak, G., Klonowska, I., McClelland, W., Lehnert, O., Cuthbert, S., Carter, I., Callegari, R., and Walczak, K.: Detrital zircon geochronology of Lower Paleozoic sedimentary rocks from COSC-2 borehole, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12123, https://doi.org/10.5194/egusphere-egu23-12123, 2023.

X2.180

EGU23-2057

Overview of results from active source seismic reflection and refraction surveys acquired in the Swedish Caledonides and vicinity

Christopher Juhlin

The first larger scale seismic refraction survey over the Swedish Caledonides appears to date back to 1969 as part of the Trans-Scandinavian Deep Seismic Sounding project (Vogel and Lund, 1970). Forty-two receiver locations were occupied between Sundsvall and Trondheim with shot points off the coast of western Finland and in the water near Trondheim. Interpretation of P-wave arrivals and modeling showed a crust that is generally 40-45 km thick below the Baltic Shield, but that thickens some kilometers below the mountain belt, a result consistent with more modern interpretations (c.f. England and Ebbing (2012)). Since then a significant number of additional refraction surveys have been performed over the Swedish Caledonides, as well as larger scale reflection seismic surveying. The Collisional Orogeny in the Scandinavian Caledonides (COSC) reflection profile played a significant role in the siting of the two ICDP boreholes, COSC-1 (2.5 km deep) and COSC-2 (2.275 km deep) that were drilled in the mountain build in 2014 and 2020, respectively. Results from earlier active source seismic experiments will be reviewed in this presentation, as wells as more recent results from the COSC project.

Vogel A. and Lund C.-E., 1970. Combined Interpretation of the Trans-Scandinavian Seismic Profile, section 2-3. Internal Report No. 4, Dept. of Solid Earth Physics, Uppsala University, 25pp.

England R.W. and Ebbing J. 2012. Crustal structure of central Norway and Sweden from integrated modelling of teleseismic receiver functions and the gravity anomaly. Geophys. J. Int., 191, 1–11.

How to cite: Juhlin, C.: Overview of results from active source seismic reflection and refraction surveys acquired in the Swedish Caledonides and vicinity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2057, https://doi.org/10.5194/egusphere-egu23-2057, 2023.

X2.181

EGU23-3572

Present-day stress field analysis in the COSC-2 borehole, Sweden

Simona Pierdominici, Wenjing Wang, Douglas Schmitt, Jochem Kueck, Henning Lorenz, and Jan-Erik Rosberg

The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project studies mountain building processes in a major mid-Paleozoic orogen in western Scandinavia and its comparison with modern analogues (i.e. Alpine-Himalaya mountain belt) by two boreholes (COSC-1 and COSC-2) in Jämtland, central Sweden. The COSC-2 borehole was drilled from mid-April to early August 2020 with nearly 100% core recovery and reached a total depth of 2276m. COSC-2 drilling encountered, from top to bottom, 780m of turbiditic greywackes, about 50m of a sheared black shale unit followed by sandstones and conglomerates in a turbiditic background sedimentation to about 1250m. Ignimbrites and volcanic porphyries with sporadic intervals of doleritic intrusions dominate the deeper stratigraphic sequence (from 1250 m to the bottom depth). To acquire the petrophysical properties of the rocks, three downhole logging campaigns were carried out by Lund University and the ICDP Operational Support Group from 2020 to 2022. In this study, high-resolution acoustic images of the open borehole below 100m were analysed to identify and interpret past and present tectonic features. Two main categories were detected on the image log: geological structures (i.e. foliation, fractures) and stress-induced alteration of the borehole (i.e. breakout). The latter allows the orientation of the present-day stress field to be constrained. For breakout identification, both manual and automatic peak-detection was deployed. In the manual interpretation, the breakout azimuth is assumed to be the center of each breakout, whereas in the automatic selection, the breakout azimuth is set to the average location of the peak when the minimum location in the filtered amplitude and the maximum location in the filtered radius image logs are close (difference less than 25°), based on the assumption that the breakout shape is symmetric. In the COSC-2 borehole, the breakouts were mainly concentrated between 1600m and 1897m. Only a few and poorly-developed breakouts were manually identified outside of dolerite intrusions and gabbroid rocks. Based on the manual approach, about 104 borehole breakouts were identified for a total length of 93m with an average orientation of the maximum horizontal principal stress (SH) of 160°. Automatic peaking detected 216 breakouts for a total length of 43m with an average SH-orientation of 161°. A high correlation was found between these two methods, and the SH-orientation remains fairly constant among the borehole. We also compared the results of COSC-2 with those of the 2496m deep COSC-1 borehole, located about 20 km to the northwest of COSC-2: 1. the orientation in the two boreholes diverges by about 33° (SH orientation of COSC-1 is 127°), 2. in COSC-2 the breakouts are well developed in width and length, and 3. they show a much greater cumulative length (93m compared to 22m in COSC-1). The paucity of breakouts in the COSC-1 well has been attributed to the type of rocks (metamorphic and crystalline) that are generally elastically stiff and have high mechanical strength, which inhibits the formation of breakouts. In contrast, in COSC-2, the dolerite and gabbroid rocks seem more prone to stress-induced enlargements.

How to cite: Pierdominici, S., Wang, W., Schmitt, D., Kueck, J., Lorenz, H., and Rosberg, J.-E.: Present-day stress field analysis in the COSC-2 borehole, Sweden, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3572, https://doi.org/10.5194/egusphere-egu23-3572, 2023.

X2.182

EGU23-8838

Combined surface and borehole seismic investigations at the ICDP COSC-1 and COSC-2 drillholes (Sweden)

Stefan Buske, Helge Simon, Lena Bräunig, Christopher Juhlin, and Rüdiger Giese

The ICDP funded project COSC (Collisional Orogeny in the Scandinavian Caledonides) is investigating mountain building processes with the help of two ~2.5 km deep fully cored boreholes in Central Sweden. While borehole COSC-1, drilled in 2014, studied the emplacement of the high-grade metamorphic allochthons, borehole COSC-2, drilled in 2020, focuses on defining the character and age of deformation of the underlying greenschist facies thrust-sheets, the main Caledonian décollement and the Precambrian basement.

We have performed combined surface and borehole seismic investigations at both drill sites in order to characterize the Earth’s upper crust in the direct vicinity of the boreholes. Both surveys were designed as multi-azimuthal walkaway VSP surveys that have the potential to yield not only a 3D seismic image around the borehole both also to derive information about seismic anisotropy related to the drilled rock units.

During the COSC-1 survey in 2014, three surface lines were acquired centered radially around the COSC-1 drillsite. In the central part up to 2.5 km away from the borehole a hydraulic hammer was used as the seismic source, while for larger offsets up to 5 km explosives were employed. The wavefield of both source types was recorded using an array of 15 three-component receivers with a spacing of 10 m deployed at 7 different depth levels in the borehole. Simultaneously, the wavefield was recorded at the surface by 180 standalone three-component receivers along each of the three up to 10 km long lines, as well as by a 3D array of single-component receivers in the central part of the survey area around the borehole.

The COSC-2 survey in 2021 comprised two surface lines across the COSC-2 drillsite with densely spaced single- and three-component receivers and maximum source-receiver offsets of ~11 km. The location of the COSC-2 borehole right next to lake Liten made it necessary to design the survey as an amphibious seismic experiment using a 32 t Vibroseis truck and wireless geophones on land along the lake as well as an airgun and three-component OBS along the profile part across the lake. An array of 17 three-component receivers with a spacing of 10 m recorded the seismic wavefields of both sources along the entire borehole length.

In both cases, a 3D velocity model including anisotropy information was obtained from the seismic data by first-arrival traveltime tomography. In the case of COSC-1, the anisotropic velocity model was used to perform an anisotropic prestack depth migration of the surface data, while for COSC-2 this part of the data processing and imaging is still ongoing. We show a comparison of the characteristics of both data sets, compare the obtained results and present lessons learnt for the planning of similar projects in the future.

How to cite: Buske, S., Simon, H., Bräunig, L., Juhlin, C., and Giese, R.: Combined surface and borehole seismic investigations at the ICDP COSC-1 and COSC-2 drillholes (Sweden), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8838, https://doi.org/10.5194/egusphere-egu23-8838, 2023.

X2.183

EGU23-11329

ECS

Linking laboratory seismic velocity measurements with the minerlogical content and (micro)structures of the COSC-2 drill core, central Scandinavian Caledonides

Nora Schweizer, Markus Rast, Claudio Madonna, Bjarne Almqvist, and Quinn Wenning

The deep erosion of the Scandinavian Caledonides provides a unique opportunity to study the interior of an orogen. The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project aims to better understand orogenic processes and to verify interpretations of the Scandinavian Caledonides based on subsurface geophysical investigations. The second drill hole of the project (COSC-2) is located near Järpen in central Jämtland, Sweden (central Scandinavian Caledonides). Based on seismic images, the ∼2.3 km deep drill hole was assumed to transect the Lower Allochthon, the main décollement located in the Alum shale formation, the footwall sedimentary succession, and the underlying basement. Although a deformation zone in the Alum shale formation is found between ∼775 and ∼820 m depth, its related structures dip moderately towards ESE to E, which does not fit a décollement that is expected to dip gently to the west. The recent detailed description of the COSC-2 core also revealed a mostly continuous sedimentary succession deposited on top of a porphyry sequence, with no abrupt transition from autochthonous to allochthonous units.

The discrepancy between the interpretation of the seismic image and the drilled lithologies highlights the need to determine seismic properties of the drill core. The P-wave and S-wave sonic downhole logging performed after drilling may provide a first indication in high spatial resolution. However, laboratory seismic velocity measurements are required to link seismic velocities with mineralogical composition, (micro)structures, and associated anisotropy. We determine the P- and S-wave velocities of six samples covering main lithologies of the drill core: (1) a sand-to claystone (turbidite) from ∼380 m depth, (2) a sandstone from ∼690 m depth, (3) a phyllitic shale (Alum shale) from ∼815 m depth, (4) a fine grained conglomerate from ∼1175 m depth, (5) a porphyry from ∼1255 m depth, and (6) a dolerite from ∼1655 m depth. The seismic velocities are measured in three mutually perpendicular orientations, at different confining pressures up to 250 MPa. Measurements at pressurized conditions are used to simulate in-situ conditions and to estimate the intrinsic (crack-free) velocities. For all samples, we determine the density and describe the mineralogical composition as well as textures that may lead to seismic anisotropy. With the resulting data, we will be able to constrain the origin of the seismic velocity changes and associated reflections found in the seismic image. Furthermore, we can derive basic petrophysical properties such as seismic anisotropy and dynamic elastic moduli, which may serve as a basis for future studies related to similar tectonic settings.

How to cite: Schweizer, N., Rast, M., Madonna, C., Almqvist, B., and Wenning, Q.: Linking laboratory seismic velocity measurements with the minerlogical content and (micro)structures of the COSC-2 drill core, central Scandinavian Caledonides, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11329, https://doi.org/10.5194/egusphere-egu23-11329, 2023.

Posters virtual: Fri, 28 Apr, 10:45–12:30 | vHall TS/EMRP

Chairpersons: Johannes Jakob, Jaroslaw Majka, Karolina Kośmińska

vTE.7

EGU23-16708

ECS

Scottish Highlands Caledonian Granites: a fresh look at hot zone origins, emplacement and their relationship to Pb-Zn-carbonate mineralisation

Careen MacRae, Iain Neill, Joshua Einsle, Edward Dempsey, Anna Bird, Eilidh Milne, David Currie, and Chloe Gemmell

Plutons formed during the latter stages of the Caledonian Orogeny are a prominent feature of the landscape of the Northern Highlands of Scotland. Despite their prominence, and in rare cases mineralisation (Strontian) or high heat producing properties (Helmsdale), various intrusions lack critical analysis of their timing, emplacement mechanisms and geodynamic significance. For example, published emplacement ages are typically from small air abrasion isotope dilution studies of the 1970’s-1990’s¹. These have recently been argued to risk bias towards high quality grains which potentially grew during lower crustal processing of parental magmas². Here, we are conducting U-Pb zircon re-dating of six intrusions associated with the Great Glen Fault system: Glen Loy, Linnhe, Abriachan, Cluanie, Strontian and Helmsdale. Through a combination of extensive zircon picking, cathodoluminescence imaging and laser ablation mass spectrometry on multiple points per zircon we aim to reduce this selection bias.

Initial results, with titanite geochronology to follow, indicate that Glen Loy and Cluanie pre-date Iapetus slab breakoff and are therefore related to subduction beneath the Laurentian margin. All plutons studied so far demonstrate evidence of zircon growth which pre-dates final emplacement. We argue that, Iapetus subduction and Baltica-Laurentia collision were responsible for the generation of a lower crustal hot zone beneath the Northern Highlands. This hot zone lasted from ~450-430 Ma, prior to the upsurge in magmatism which followed slab breakoff. Re-dating of the ‘outer’ granodiorite facies of the Strontian pluton has produced a probable emplacement age at least 10 Myr younger than the previous accepted age of ~425 Ma. This finding raises questions about a) whether previous results reflected antecrystic zircon and titanite and b) the association of pluton emplacement with the timing of left-lateral motion on the Great Glen Fault system.

In addition, few Northern Highlands plutons are significantly mineralised, except for the Pb-Zn-hosting carbonate veins at the Strontian pluton. However, we do not know the age of mineralisation or its metal distributions, particularly any metals which have been designated as critical to society since surveys in the 1980's. In this study, we have also developed a workflow in collaboration with the Critical Minerals Intelligence Centre of the British Geological Survey to date mineralisation using U-Pb methods on calcite, and to compare results with U-Pb apatite dating of a mafic sub-volcanic dyke at the Strontian pluton, suspected to be Permian-Carboniferous in age. We will further address the distribution of metals using a combination of optical petrology, electron microscopy, laser rastering and focused ion beam nano-tomography. This further addresses the above knowledge gaps with correlative cm- to nano-scale and three-dimensional insights into the mineralisation process, a strategy that can be replicated for other potential critical element bearing deposits.

How to cite: MacRae, C., Neill, I., Einsle, J., Dempsey, E., Bird, A., Milne, E., Currie, D., and Gemmell, C.: Scottish Highlands Caledonian Granites: a fresh look at hot zone origins, emplacement and their relationship to Pb-Zn-carbonate mineralisation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16708, https://doi.org/10.5194/egusphere-egu23-16708, 2023.