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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.