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RESEARCH ARTICLE
Caledonian reactivation and reworking of Timanian
thrust systems and implications for latest Mesoproterozoic to
mid-Paleozoic tectonics and magmatism in northern Baltica
[version 2; peer review: 1 approved with reservations, 1 not
approved]
Jean-Baptiste P. Koehl 1,2, Eirik Stokmo3
1Earth and Planetary Sciences, McGill University, Montreal, Québec, H3A 0E8, Canada
2Geosciences, Universitetet i Oslo, Oslo, Oslo, 0371, Norway
3Brønnøy Kalk, Velfjord, Nordland, 8960, Norway
First published: 16 Feb 2024, 4:30
https://doi.org/10.12688/openreseurope.17033.1
Latest published: 15 Oct 2024, 4:30
https://doi.org/10.12688/openreseurope.17033.2
v2
Abstract
Background
The Trollfjorden–Komagelva Fault Zone is the southernmost thrust
fault of the Timanian Orogen and extends for thousands of kilometers
from northwestern Russia to northern Norway. Though there is little
about its location onshore northeastern Norway, where it is mapped
as a major fault system dominantly comprised of NNE-dipping thrust
faults, its continuation to the west below Caledonian nappes and
offshore post-Caledonian sedimentary basins remains a matter of
debate.
Methods
The present study provides a more definitive answer about the
continuation of Trollfjorden–Komagelva Fault Zone west of the
Varanger Peninsula by using seismic reflection, bathymetric,
topographic, and magnetic data onshore Finnmark and offshore on
the Finnmark Platform.
Results
Open Peer Review
Approval Status
1 2
version 2
(revision)
15 Oct 2024
version 1
16 Feb 2024 view view
Simon Stewart, Saudi Aramco, Dhahran,
Saudi Arabia
1.
Wenjiao Xiao, University of Chinese Academy
of Sciences, Beijing, China
2.
Any reports and responses or comments on the
article can be found at the end of the article.
Open Research Europe
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The present study demonstrates that the Sørøya–Ingøya shear zone
represents a portion of the Trollfjorden–Komagelva Fault Zone that
was folded into a NE–SW orientation and reactivated as a top-
southeast thrust during the Caledonian Orogeny, while other portions
of the Trollfjorden–Komagelva Fault Zone (e.g., on the Varanger
Peninsula) were reactivated as strike-slip faults. The study also
documents the presence of another major, NNE-dipping Timanian
shear zone with a similar geometry to the Trollfjorden–Komagelva
Fault Zone north of the Varanger Peninsula.
Conclusions
The Trollfjorden–Komagelva Fault Zone may continue offshore as a
NE–SW-striking folded structure. This has the following implications:
(1) the Seiland Igneous Province likely formed in a backarc setting, (2)
metasedimentary rocks of the Kalak Nappe Complex deposited along
the Baltican margin of the Iapetus Ocean, possibly in a late–post-
Grenvillian collapse basin, (3) the Iapetus Ocean was much narrower
than the several thousands of kilometers width commonly proposed,
and (4) early Neoproterozoic magmatism in northern Norway is
possibly related to the initial breakup of Rodinia.
Keywords
Baltica, Neoproterozoic, Paleozoic, Timanides, Timanian Orogeny,
Caledonides, Caledonian Orogeny, thrust, shear zone, seismic
reflection, magnetic, bathymetric, Porsanger Orogeny, Kalak Nappe
Complex, Iapetus Ocean, Grenvillian Orogeny, Seiland Igneous
Province, Trollfjorden–Komagelva Fault Zone
This article is included in the Horizon 2020
gateway.
Open Research Europe
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Open Research Europe 2024, 4:30 Last updated: 15 OCT 2024
Corresponding author: Jean-Baptiste P. Koehl (jeanbaptiste.koehl@gmail.com)
Author roles: Koehl JBP: Conceptualization, Data Curation, Formal Analysis, Funding Acquisition, Investigation, Methodology, Project
Administration, Resources, Validation, Visualization, Writing – Original Draft Preparation, Writing – Review & Editing; Stokmo E: Formal
Analysis, Investigation, Validation, Visualization, Writing – Original Draft Preparation, Writing – Review & Editing
Competing interests: No competing interests were disclosed.
Grant information: This work was funded by the Research Centre for Arctic Petroleum Exploration (ARCEx; grant number 228107), and
by the ArcTec project funded by the European Union’s Horizon 2020 research and innovation program under a Marie Sklodowska-Curie
Action grant (agreement number 101023439).
The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Copyright: © 2024 Koehl JBP and Stokmo E. This is an open access article distributed under the terms of the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
How to cite this article: Koehl JBP and Stokmo E. Caledonian reactivation and reworking of Timanian thrust systems and
implications for latest Mesoproterozoic to mid-Paleozoic tectonics and magmatism in northern Baltica [version 2; peer review: 1
approved with reservations, 1 not approved] Open Research Europe 2024, 4:30 https://doi.org/10.12688/openreseurope.17033.2
First published: 16 Feb 2024, 4:30 https://doi.org/10.12688/openreseurope.17033.1
Open Research Europe
Page 3 of 47
Open Research Europe 2024, 4:30 Last updated: 15 OCT 2024
Introduction
The Timanian Orogeny is an episode of NNE–SSW-oriented
contraction (all the structural directions are defined rela-
tive to present-day coordinates) that occurred in the latest
Neoproterozoic (ca. 650–550 Ma) during which oceanic crust
was possibly subducted under Baltica (Pease et al., 2004).
Though footprints of this tectonic episode were found all over
the Arctic (e.g., Estrada et al., 2018a; Estrada et al., 2018b;
Rekant et al., 2019), actual Timanian structures only crop
out in northwestern Russia (Olovyanishnikov et al., 2000),
northeasternmost Norway (Siedlecka & Siedlecki, 1967;
Siedlecka, 1975), and southwestern Spitsbergen (Faehnrich
et al., 2020; Mazur et al., 2009). Related structures also
occur offshore in the Barents Sea and are buried deeply in
Svalbard (Eldholm & Ewing, 1971, their Figure 4 profile
C–D; Koehl, 2020; Koehl et al., 2022a; Koehl et al., 2023;
Korago et al., 2004; Lopatin et al., 2001).
In northeasternmost Norway, the Timanian thrust front, the
Trollfjorden–Komagelva Fault Zone (Figure 1a), crops out on
the Varanger Peninsula. In the east, this major fault boundary
continues as the Sredni-Rybachi Fault Zone between the
Sredni and Rybachi peninsulas, and as the West Timan Fault
or Central Timan Fault farther east in the Timan Range and
Kanin Peninsula (Olovyanishnikov et al., 2000; Figure 1a). In
the west, the Trollfjorden–Komagelva Fault Zone continues
as a series of WNW–ESE-striking brittle faults that merge with
a NW–SE-striking segment of the Troms–Finnmark Fault Com-
plex (Gabrielsen, 1984; Gabrielsen & Færseth, 1989; Lea, 2016;
Roberts et al., 2011). However, recent studies suggest that track-
ing the western continuation of the Trollfjorden–Komagelva
Amendments from Version 1
Designed a tectonostratigraphic chart of the study area after
comments by Dr. Stewart. Updated the result chapter on
interpretation of seismic and bathymetric data following the
comments of Dr. Stewart, e.g., added further details about
specic structures and artifacts on seismic data, updated all
seismic gures, and added details about glacial features on
bathymetric data. Moved the uninterpreted version of each
seismic prole from the manuscript’s supplement to the
manuscript based on Dr. Stewart, Prof. Doré, and Prof. Xiao’s
suggestion. Deleted all acronyms from the text and rephrased
multiple sentences, following Dr. Stewart’s suggestion. Reworked
the section of the discussion about the “Western continuation
of the Trollfjorden–Komagelva Fault Zone” after Prof. Xiao’s
comments. In addition, added further information about
Large Low-Shear Velocity Provinces in the discussion and on
the Caledonian Orogeny in the Introduction after Prof. Doré’s
suggestion. Finally, implemented minor modications to most of
the gures after the reviewers’ comments (e.g., missing locations,
Timanian suture, and graticules in Figure 1).
Any further responses from the reviewers can be found at
the end of the article
REVISED
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Figure 1. (a) Elevation map of the Barents Sea, northern Norway and northwestern Russia showing major structural elements from the
Norwegian Oshore Directorate (thin white lines) and major fault trends in the Barents Sea (Timanian; yellow lines). The location of (a) is
shown as a red rectangle in the upper inset map and the location of (b) as a black rectangle. The basemap and the upper inset are from
Jakobsson et al. (2012). A Polar Stereographic projection was used (datum: WGS84). Abbreviations: AFC: Asterias Fault Complex; AKTW: Alta–
Kvænangen tectonic window; BaFZ: Baidaratsky Fault Zone; BB: Bjørnøya Basin; BFC: Bjørnøyrenna Fault Complex; BP: Bjarmeland Platform;
CTF: Central Timan Fault; ETF: East Timan Fault; FG: Forlandsundet Graben; FP: Finnmark Platform; FSB: Fingerdjupet Sub-Basin; FTB: Fold-
and-Thrust Belt; HB: Harstad Basin; HfB: Hammerfest Basin; HFC: Hoop Fault Complex; JFC: Jason Fault Complex; KCFZ: Kongsfjorden–
Cowanodden fault zone; KDFZ: Kinnhøgda–Daudbjørnpynten fault zone; LH: Loppa High; MB: Maud Basin; MFC: Måsøy Fault Complex;
MFZ: Molloy Fracture Zone; MH: Mercurius High; NB: Nordkapp Basin; ND: Norsel Dome; NH: Norsel High; OB: NP: Nordkinn Peninsula;
Olga Basin; PSP: Polhem Sub-Platform; RFC: Ringvassøya Fault Complex; RP: Rybachi Peninsula; SaD: Samson Dome; SeFZ: Senja Fracture
Zone; SD: Sredni Peninsula; SFZ: Spitsbergen Fracture Zone; SH: Stappen High; SIP: Seiland Igneous Province; SISZ: Sørøya–Ingøya shear
zone; SkB: Sørkapp Basin; SR: Senja Ridge; SRFZ: Sredni–Rybachi Fault Zone; SvB: Sørvestnaget Basin; SvD: Svalis Dome; TFFC: Troms–
Finnmark Fault Complex; TKFZ: Trollfjorden–Komagelva Fault Zone; TyB: Tiddlybanken Basin; TøB: Tromsø Basin; VH: Veslemøy High; VKSZ:
Vimsodden–Kosibapasset Shear Zone; VP: Varanger Peninsula; WTF: West Timan Fault. (b) Geological map of northern Norway and the
southern Barents Sea showing the main onshore and oshore structures and tectonostratigraphic units. The map is after Koehl et al.
(2019) with updates after Siedlecka and Siedlecki (1967), Siedlecki (1980), Townsend et al. (1986), Rice (1994), Kirkland et al. (2005; 2006a;
2007a; 2007b; 2008a; 2008b), Indrevær et al. (2013), Corfu et al. (2014), Koehl et al. (2018a), Faber (2018, manuscript 3), and Roberts and
Siedlecka (2022). Abbreviations: AFC – Asterias Fault Complex; AsW: Altenes tectonic window; AW: Alta–Kvænangen tectonic window; Bf:
Båtsfjorden; BFC: Bjørnøyrenna Fault Complex; Bj: Bjørnøya; Bn: Båsnæringsfjellet; Bv: Berlevåg; DP: Digermulen Peninsula; GL: Gjesvær
Low; Hj: Hjelmsøya; Ig: Ingøya;Kf: Kongsfjorden; Kv: Kvaløya; LG: Lillefjord Granite; Lk: Laksefjorden; Ln: Langfjorden; LVF: Langfjorden–
Vargsundet fault; Ma: Magerøya; MFC: Måsøy Fault Complex; NFC: Nysleppen Fault Complex; NP: Nordkinn Peninsula; Pf: Porsangerfjorden;
PP: Porsanger Peninsula; RA: Ragnarokk Anticline; Rf: Rolvsøya fault; Rk: Reinøykalven; RG: Revsneshamn Granite; RLFC: Ringvassøya–Loppa
Fault Complex; Rv: Rolvsøya; RW: Repparfjord–Komagfjord tectonic window; Sf: Syltefjorden; S: Syltefjordfjellet; SFZ: Senja Fracture Zone;
SISZ: Sørøya–Ingøya shear zone; Sk: Stikonjargga Peninsula; sNB – southwesternmost Nordkapp basin; SP: Sværholt Peninsula; Sø: Sørøya;
TFFC: Troms–Finnmark Fault Complex; TKFZ: Trollfjorden–Komagelva Fault Zone; Tn: Tanafjorden; TyB: Tiddlybanken Basin; VP: Varanger
Peninsula; VVFC: Vestfjorden–Vanna Fault Complex.
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Fault Zone is not straightforward. New models involve a
possible truncation of the Trollfjorden–Komagelva Fault Zone
by a kilometer-thick, top-southeast Caledonian shear zone
(Koehl et al., 2018a) or a splaying and dying out geometry
(Koehl et al., 2019). The present study presents a new, more
realistic interpretation of the Trollfjorden–Komagelva Fault
Zone off northwestern Norway.
Previous work on the Varanger Peninsula (Roberts, 1996;
Siedlecka & Siedlecki, 1971; Figure 1a–b) and ongoing work
in Svalbard and the northern Barents Sea (Klitzke et al.,
2019; Koehl, 2020; Koehl et al., 2022a; Koehl et al., 2023)
show that Timanian folds, thrusts and shear zones were
refolded by NNE–SSW-trending Caledonian folds. The aim
of this contribution is to explore the Finnmark Platform
(i.e., the nearshore portion of the southwestern Barents Sea;
Figure 1a–b) for traces of similarly folded Timanian structures
and track their potential continuation onshore and in nearshore
fjords.
The results of the present study challenge previous models pro-
posed for the western, offshore continuation of the Trollfjorden–
Komagelva Fault Zone onto the Finnmark Platform. These
models involve a continuation of the Trollfjorden–Komagelva
Fault Zone as a WNW–ESE-striking brittle fault (e.g.,
Gabrielsen, 1984; Gabrielsen & Færseth, 1989; Roberts
et al., 2011), a truncation of the Trollfjorden–Komagelva Fault
Zone by a major northwest-dipping Caledonian shear zone
(the Sørøya–Ingøya shear zone; Koehl et al., 2018a), and that
the Trollfjorden–Komagelva Fault Zone dies out westwards
in Magerøya (Koehl et al., 2019).
The present study has implications for the onshore–offshore
correlation of tectonic structures in integrated studies and for
the interpretation of major thrusts consisting of ductile shear
zones and brittle faults on seismic data. Other implications
include a reevaluation of the meaning of “affinities” of
specific nappe units with specific plates, the tectonic setting
of formation of magmatic intrusions in northern Norway, and
the relationship between the opening of the Iapetus Ocean
and the Timanian Orogeny in the late Neoproterozoic. Notably,
the present work discusses the origin of the Seiland Igneous
Province and of the Kalak Nappe Complex (Figure 1a–b),
which were formerly thought to represent intrusions related
to the rifting of the Iapetus Ocean and an exotic terrane with
Laurentian affinities respectively. The present work also
briefly discusses the Porsanger Orogeny and the Finnmarkian
event. The former is a poorly constrained Neoproterozoic
episode of contraction in northern Norway, whereas the lat-
ter was previously believed to be an early phase of early
Cambrian of Caledonian deformation. We summarize our
findings and their implications in a model detailing the
geological evolution of northern Norway from the latest
Mesoproterozoic to the mid-Paleozoic. The present work
may be used as a framework for new plate tectonic recon-
structions for the Neoproterozoic–Paleozoic and for tectonic
models in regions deformed by two or more orogenic events.
Geological setting
Grenvillian-Sveconorwegian and Porsanger orogenies
Tectonothermal and magmatic activity of latest Mesoproterozoic–
earliest Neoproterozoic age was recorded in all three of
Svalbard’s basement terranes through various geochronological
studies of granites, gneisses, and schists and suggests that
Svalbard was involved in the Grenvillian Orogeny (Hellmann
et al., 1998; Johansson & Gee, 1999; Johansson et al.,
2000; Johansson et al., 2001; Johansson et al., 2004; Johansson
et al., 2005; Lorenz et al., 2012; McClelland et al., 2018;
Ohta & Larionov, 1998; Ohta et al., 2003; Pettersson
et al., 2009a; Pettersson et al., 2009b; Sirotkin &
Evdokimov, 2022). A similar record in southern Norway
and southern Sweden indicate that southern Scandinavia
was involved in a comparable event, the Sveconorwegian
Orogeny (Andersen et al., 2007; Bingen et al., 2008;
Slagstad et al., 2013; Viola et al., 2011). In northern
Norway however, this tectonothermal and magmatic activity
is recorded exclusively in blocks and terranes thought to
be exotic to Baltica (Agyei-Dwarko et al., 2012; Augland
et al., 2013), e.g., Kalak Nappe Complex (Daly et al., 1991;
Kirkland et al., 2006a; Figure 1b), whose deposition dated
to > 980 Ma (lower part; Kirkland et al., 2006a) and ca.
920–910 ± 15–20 Ma (upper part; Kirkland et al., 2007a)
overlaps with the Grenvillian–Sveconorwegian event (Figure 2).
The Porsanger Orogeny is a contractional event in northern
Norway (Corfu et al., 2007; Daly et al., 1991; Kirkland et al.,
2006a) and is connected to the intrusion of granites (e.g.,
Lillefjord and Revsneshamn granites; Figure 1b) and pegma-
tites dated respectively to 840± 6 Ma and 828 ± 4 Ma through
U–Pb geochronological analysis of zircon (Kirkland et al.,
2006a; Figure 2). The granites intrude Mesoproterozoic–Tonian
metasedimentary rocks of the Kalak Nappe Complex, which
show Laurentian affinities (Corfu et al., 2007; Kirkland
et al., 2007a; Slagstad et al., 2006; Figure 2). Note that simi-
lar 870–840 Ma ages were recently obtained for granitic and
gabbroic magmatic suites in northern Sweden, but that these
were correlated with the early breakup of Rodinia (Callegari
et al., 2023).
Timanian Orogeny
The Timanian Orogeny is a major episode of SSW-dipping
subduction and continental accretion that occurred on the
northern rim of the Baltican craton in the late Neoproterozoic
(ca. 650–550 Ma; Dovzhikova et al., 2004; Gee et al., 2000;
Glodny et al., 2004; Larionov et al., 2004; Olovyanishnikov
et al., 2000; Pease et al., 2004; Remizov & Pease, 2004;
Remizov, 2006; Figure 1a). The orogeny led to a succession
of magmatic and tectonic events, which resulted in the forma-
tion of a major fold-and-thrust belt characterized dominantly
by NNE-dipping thrusts and SSW-verging folds (e.g., Central
and West Timan faults; Korago et al., 2004; Kostyuchenko
et al., 2006; Lopatin et al., 2001; Lorenz et al., 2004;
Olovyanishnikov et al., 2000; Figure 1a), deformation reach-
ing blueschist and eclogite facies metamorphism (Beckholmen
& Glodny, 2004; Glodny et al., 2004; Remizov & Pease, 2004;
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Remizov, 2006), and the intrusion of various magmatic suites,
including subduction-related island- to continental-arc suites
(Dovzhikova et al., 2004; Glodny et al., 2004; Remizov &
Pease, 2004; Remizov, 2006), and syn–late-orogenic/subduction
calc-alkaline suites (ca. 700–515 Ma; Dovzhikova et al., 2004;
Gee et al., 2000; Larionov et al., 2004; Pease et al., 2004)
and elongated, post-orogenic alkaline suites (ca. 565–500 Ma;
Kuznetsov et al., 2007; Larionov et al., 2004).
In northern Norway, remnants of the Timanian Orogeny are
found on the Varanger Peninsula in easternmost Finnmark
(Figure 1a–b). There, a major, WNW–ESE-striking,
NNE-dipping brittle–ductile fault, the Trollfjorden–Komagelva
Fault Zone, represents the southernmost Timanian thrust fault
(Herrevold et al., 2009; Siedlecka, 1975; Figure 1a–b). This fault
is thought to have accommodated top-SSW, reverse-sinistral
movements in the latest Neoproterozoic (transpression;
Herrevold et al., 2009). Later on, it was reactivated during
the Caledonian Orogeny (dextral strike-slip to dextral-reverse
oblique-slip movements; Herrevold et al., 2009; Roberts,
1972), during post-Caledonian Devonian–Mississippian collapse
(normal-dextral strike-slip movements; Koehl et al., 2018a;
Koehl et al., 2019; Roberts et al., 2011), and potentially dur-
ing further episodes of rifting (Herrevold et al., 2009). Dextral
displacement along the Trollfjorden–Komagelva Fault Zone
was estimated to 207 kilometers (Rice, 2014).
East of Finnmark, the Trollfjorden–Komagelva Fault Zone
merges with the Sredni–Rybachi Fault Zone on the Sredni
and Rybachi peninsulas in northwestern Russia, and con-
tinues farther east into the Timan Range as the Central
Timan Fault and/or West Timan Fault (Olovyanishnikov
et al., 2000; Figure 1a). West of the Varanger Peninsula, the
Trollfjorden–Komagelva Fault Zone is thought to proceed
between the mainland and the Nordkinn Peninsula and off
the northern tip of the Sværholt Peninsula.
Farther west, the continuation of the Trollfjorden–Komagelva
Fault Zone is more debated and several models were pro-
posed. Interpretation of seismic data along the northern coast
of Finnmark by Gabrielsen (1984) and Roberts et al. (2011)
suggests that the Trollfjorden–Komagelva Fault Zone continues
Figure 2. Tectonostratigraphic chart and regional correlations in the study area. Abbreviations: E: Eidvågeid migmatite; L: Lillefjord
Granite; R: Revsneshamn Granite; Ø: Øksfjord Gabbro.
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north of Magerøya and merges with the NW–SE-striking,
northeast-dipping segment of the Troms–Finnmark Fault Com-
plex, a major post-Caledonian, overall northwest-dipping fault
that bounds Paleozoic–Cenozoic sedimentary basins (Gabrielsen
et al., 1990; Figure 1b). However, recent analysis of 2D
and 3D seismic data suggest that the Trollfjorden–Komagelva
Fault Zone does not merge with the Troms–Finnmark Fault
Complex and is, instead, truncated by a major, several
kilometers thick, northwest-dipping Caledonian shear zone, the
Sørøya–Ingøya shear zone (Koehl et al., 2018a; Figure 1b).
Another model based on the interpretation of high-resolution
bathymetric and magnetic data in northern Finnmark sug-
gests that the Trollfjorden–Komagelva Fault Zone splays into
multiple minor fault segments westwards on the island of
Magerøya and dies out just west of the coastline (Koehl et al.,
2019).
Late Neoproterozoic–Cambrian magmatism in
Northern Norway
Mafic–ultramafic and alkaline rocks of the Seiland Igneous
Province (Krauskopf, 1954; Robins & Gardner, 1975;
Speedyman, 1983; Sturt & Ramsay, 1965; Figure 1a–b)
intruded rocks of the Kalak Nappe Complex at ca. 580–560 Ma
and 530–520 Ma (Krill & Zwaan, 1987; Roberts et al.,
2006; Roberts et al., 2010; Figure 2). Initially, folds crosscut
by these intrusions were thought to reflect early Caledonian
contraction (Finnmarkian Orogeny; e.g., Kirkland et al.,
2006a; Kirkland et al., 2007a; Kirkland et al., 2008a; Sturt
et al., 1978). The folds are now known to be of magmatic
origin and related to dyke intrusion (e.g., Krill & Zwaan,
1987). The proximity of the Seiland Igneous Province with
the Iapetus paleo-margin and reconstruction of intruded rocks
of the Kalak Nappe Complex within the Iapetus Ocean sug-
gest that the Seiland Igneous Province is related to rifting
of Iapetus (Bergström & Gee, 1985; Elvevold et al., 1994;
Kirkland et al., 2008b; Krill & Zwaan, 1987; Larsen et al.,
2018). However, geochemical analyzes indicate that (back)
arc and collisional settings are also possible, as also suggested
by earlier petrochemical analyses (Roberts, 1975; Roberts
et al., 2006; Roberts et al., 2010; Robins & Gardner, 1975;
Speedyman, 1983).
In Varangerhalvøya, the Berlevåg Formation, which is com-
monly thought to be of Baltican origin, was affected by a
555 ± 15 Ma hydrothermal event (Kirkland et al., 2008b;
Figure 1b and Figure 2). This event is potentially related to
the intrusion of 577 ± 14 Ma and 550 ± 7.3 Ma metadolerite
dykes in northern Varangerhalvøya (Andersen & Sundvoll,
1995; Rice et al., 2004). These dykes were ascribed to the
opening of the Iapetus Ocean, though a backarc setting is
equally as possible (Alexander Hugh Rice pers. comm., 2022;
Rice et al., 2004). They are depicted by recent aeromagnetic
data on the Varanger and Nordkinn peninsulas (Nasuti et al.,
2015a), and therefore intrude both rocks of the Barents Sea
Group and of the Kalak Nappe Complex (Figure 1b). Farther
west, on the Porsanger Peninsula, the Kalak Nappe Complex
is intruded by (boudinaged) dolerite sills, which were meta-
morphosed during the Caledonian Orogeny, thus suggesting
a Precambrian, possibly Ediacaran, age for the intrusions
(David Roberts pers. comm., 2022; Roberts, 1987; Figure 2).
Caledonian Orogeny
In the early Paleozoic, the Caledonian Orogeny led to the
closing of the Iapetus Ocean and the collision of Baltica
(including Svalbard and the Barents Sea; Koehl et al., 2022a;
Koehl et al., 2023) and Laurentia (Corfu et al., 2014). In
northern Norway (Troms and Finnmark), this orogeny
resulted in the formation of overall top-southeast thrusts and
southeast-verging folds (Gayer et al., 1985; Townsend et al.,
1986; Townsend, 1987), which thrust Caledonian nappes
onto Neoarchean–Paleoproterozoic basement rocks (Bergh &
Torske, 1988; Bergh et al., 2010; Pharaoh et al., 1982;
Pharaoh et al., 1983; Reitan, 1963; Zwaan, 1995; Figure 1b). In
places, Caledonian metamorphism reached eclogite facies (e.g.,
Tromsø Nappe; Corfu et al., 2003; Ravna & Roux, 2006).
In northern Norway, the Caledonian Orogeny was initially
thought to have initiated with a first event at ca. 540–480 Ma,
the Finnmarkian Orogeny (Roberts, 2003; Sturt et al., 1978;
Torsvik & Rehnström, 2001). This event was later revised
and downgraded to a short-lived accretion event because of
inconsistencies in structural relationships around the intrusion
of the Seiland Igneous Province (Krill & Zwaan, 1987), lack
of corresponding major structures, and partial resetting of
geochronometers (Kirkland et al., 2006a; Kirkland et al.,
2008b).
In the latest Ordovician to Silurian, the collision of
Greenland and Norway during the Scandian phase of the
Caledonian Orogeny resulted in intense deformation and up to
eclogite-facies metamorphism (Augland et al., 2013; Corfu
et al., 2003; Corfu et al., 2006; Faber et al., 2019; Gaidies
et al., 2021; Gasser et al., 2015; Kirkland et al., 2006b;
Kirkland et al., 2007b; Ravna & Roux, 2006; Slagstad et al.,
2020; Ziemniak et al., 2019). Coevally, Silurian sedimentary
rocks of the Magerøy Nappe (Henningsmoen, 1961) were
deposited, thrust, and metamorphosed mostly to greenschist
facies (Andersen, 1981; Andersen, 1984; Figure 1b; Figure 2).
Deformation culminated at ca. 425–420 Ma prior to the collapse
of the Caledonian Orogen (Kirkland et al., 2006b).
During Caledonian deformation, Timanian thrusts and folds
were reworked into dominantly NNE-plunging, 3D dome- and
trough-shaped structures, both in the Barents Sea and
Svalbard (Koehl et al., 2022a; Koehl et al., 2023), and in the
Varanger Peninsula (Gabrielsen et al., 2022; Siedlecka &
Siedlecki, 1971). In addition, the arrangement of fault seg-
ments in duplex structures in map view suggests that the seg-
ment of the Trollfjorden–Komagelva Fault Zone on the
Varanger Peninsula was also partly reworked as a strike-slip
fault by Caledonian deformation (Herrevold et al., 2009;
Siedlecka & Siedlecki, 1967; Siedlecka, 1975).
Post-Caledonian extension
In the Devonian–Carboniferous, the Caledonian Orogen started to
collapse, which resulted in the formation of brittle normal faults
along preexisting zones of weakness and reactivation of Cal-
edonian thrusts (e.g., Koehl et al., 2018b; Torgersen et al., 2014;
Figure 1b). For example, Neoproterozoic metasedimentary rocks
of the Barents Sea and Tanafjorden–Varangerbotn regions were
intruded respectively by NE–SW- to ENE–WSW- and N–S-striking
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Late Devonian dykes, which intruded along brittle faults in
the eastern part of the Varanger Peninsula (Guise & Roberts,
2002; Nasuti et al., 2015a; Roberts, 2011) and adjacent area in
Russia (Roberts & Onstott, 1995). Analogously, Neoproterozoic–
Silurian metasedimentary rocks of the Kalak Nappe Com-
plex and Magerøy Nappe and Neoarchean–Paleoproterozoic
basement rocks were truncated by WNW–ESE-striking brit-
tle faults along which Mississippian dolerite dykes were
intruded in Magerøya (Lippard & Prestvik, 1997; Roberts
et al., 1991) and adjacent areas in Finnmark (Braathen &
Davidsen, 2000; Koehl et al., 2019; Nasuti et al., 2015a).
In northwestern Finnmark, Devonian–Carboniferous post–late-
Caledonian extensional faulting is marked by fault complexes
consisting of alternating ENE–WSW- and NNE–SSW-striking
segments (e.g., Langfjorden–Vargsund fault complex; Koehl
et al., 2019; Lippard & Roberts, 1987; Roberts & Lippard,
2005; Figure 1b). These fault complexes also developed along
preexisting folds and thrusts in Paleoproterozoic basement
rocks (Henderson et al., 2015; Koehl et al., 2019).
Methods and data
The present study uses 2D and 3D seismic reflection data
from the Norwegian National Data Repository for Petroleum
Data (DISKOS database), 2D data from the Norwegian
Defense Research Establishment, and 3D data from TGS (FP13;
Figure 3) to map hundreds of meters to tens of kilometers
large structures offshore (Figure 4a–b, Figure 5–Figure 7;
Koehl, 2024). Notably, the study highlights major shear zones
and thrusts and related folds on the Finnmark Platform, off
the coast of northern Norway (Figure 1a–b). The acquisition
Figure 3. Overview of the (a) 2D and (b) 3D seismic reection
database uses in the present study. The maps also display the
depth surfaces (in milliseconds TWT) of the main two Timanian
thrusts mapped on the Finnmark Platform.
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Figure 4. Interpreted (up) and uninterpreted (down) seismic reection data on the western Finnmark Platform. The location of
the data is shown in Figure 7. (a–c) NE–SW-trending seismic lines illustrating the moderately NNE-dipping geometry of the Sørøya–Ingøya
shear zone. (d) NW–SE-trending seismic line showing major thickness variations of the Sørøya–Ingøya shear zone related to Devonian–
Carboniferous core complex exhumation (Koehl et al., 2018a) and to Caledonian reworking into NE–SW-striking folds.
and reprocessing reports document that the smaller (hundreds
of meters wide) structures reported by the present study
are well within the horizontal and vertical resolution of the
seismic data analyzed.
The vertical resolution of the survey is ¼ of the wavelength,
which is computed from the velocity of basement rocks in
the study area, i.e., 6300 m.s-1 (from Gernigon et al., 2018),
and the frequency of the data, i.e., c. 40 Hz (TGS, 2014 their
Figure 12). In the present case, the vertical resolution of the
data is c. 39 m (= 6300/40/4; Table 1). Note that this is a
minimum estimate since a bandwidth enhancement was
attempted with frequencies up to c. 60 Hz (TGS, 2014 their
Figure 13), i.e., an improved resolution of c. 26 m (Table 1).
The regularization stage of the reprocessing summary for 3D
survey FP13 reports a bin size of 12.5 m x 25 m, i.e., a Fresnel
Zone radius of c. 313 m (TGS, 2014), which corresponds
to the horizontal resolution of the survey at shallow depth.
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Figure 5. Zoom in interpreted (up) and uninterpreted (down) seismic data on the western Finnmark Platform. The location of the
data is shown in Figure 6. (a) Zoom in a NE–SW-trending seismic line showing numerous SSW-verging folds and associated brittle thrusts
with top-SSW osets within the western portion of the Sørøya–Ingøya shear zone. Triangular-shaped packages in the upper left corner
are interpreted as deformed foreland basin deposits. (b) Zoom in a NE–SW-trending seismic line showing sigmoidal packages of Z-shaped
reections respectively interpreted as antiformal thrust stacks and duplexes within the Sørøya–Ingøya shear zone. (c) Zoom in a NE–SW-
trending seismic line showing the eastern portion of the Sørøya–Ingøya shear zone, which bends back into a NNE-dipping orientation.
Most folds within the shear zone display a vergence to the south-southwest indicating top-SSW movements. (d) Zoom in a NW–SE-trending
seismic line showing symmetric folds in shallow basement rocks in a major NE–SW-striking syncline possibly consisting of rocks equivalent
to the Magerøy Nappe (MN; upper right corner), northwest-verging folds in the northwestern limb of the major syncline (left hand-side), and
southeast-verging fold structures in the southeastern limb of the major syncline (lower right corner).
Deeper in the crust, the horizontal resolution worsens and is
a function of depth and the wavelength (Geldart & Sheriff,
2004). For example, the horizontal resolution of the survey at
5000 m depth is c. 627 m (= (5000 x (6300/40)/2)1/2);
Table 1).
The seismic data of survey BSS01 at shallow (0–3500 m
depth) and high depth (3500–8000 m depth) respectively
contain frequencies up to 60–80 Hz and 40 Hz (TGS-NOPEC,
2001). A minimum estimate of the vertical resolutions at shal-
low (using 60 Hz frequency) and high depth (using 40 Hz
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frequency) are therefore c. 26 m and c. 39 m. The bin size of
2D survey BSS01 is 12.5 m x 25 m (respectively the cable
group length and the shot point interval; TGS-NOPEC,
2001), hence giving a horizontal resolution of c. 313 m
(Table 1). Similarly to survey FP13, the horizontal resolution
of the BSS01 survey at a depth of 5000 m using a frequency
of 40 Hz is c. 627 m (Table 1).
The structures studied, including notably > 500 m wide and
> 150 m thick asymmetric folds are well within the horizon-
tal and vertical resolution, both at shallow and high depth
(≥ 5000 m) of the seismic data used in the present work
(Table 1). Noteworthy, the vertical resolution of seismic data
may be down to 1/32 of the wavelength in places (Kallweit &
Wood, 1982; Li & Zhu, 2000), i.e., down to c. 5 m for a 40 Hz
frequency (= 6300/40/32), thus further supporting the inter-
pretation presented. The water depth in the study area, the
Barents Sea, is about 200 m with little variations (Jakobsson
et al., 2012) and, thus, has little influence on the structures
mapped. Furthermore, seismic velocities for basement rocks
in northern Norway from Gernigon et al. (2018) suggest
that there is little to no vertical exaggeration in the basement
rock interval (Figure 4a–d and Figure 6a–b). Thus, the geom-
etry of the basement-seated structures described in the present
manuscript are in most likelihood similar to their actual
geometry.
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Figure 6. Interpreted (up) and uninterpreted (down) seismic reection data on the eastern Finnmark Platform. The location of
the data is shown in Figure 7. (a) Intra-basement N–S- to NE–SW-striking Caledonian folds. The data show dominantly symmetric folds. Note
that the URU reection coincides with the Top-basement reection here. (b) NNE-dipping ductile shear zone consisting of planar mylonitic
surfaces and SSW-verging folds indicating transport direction towards the south-southwest. Notice the mild reactivation of the shallow
portion of the thrust by a late Paleozoic listric normal fault. A dierent color scheme was used to enhance the contrast and better highlight
the structures described.
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Figure 7. Map showing the depth surfaces (in milliseconds TWT) of the main two Timanian thrusts on the Finnmark Platform.
Notice the correlation of major synforms and antiforms observed in the eld onshore and on onshore–oshore magnetic data (plain
and dashed pink lines) with folding of the Timanian thrusts on the Finnmark Platform in map view identied via seismic mapping (depth
surfaces). The map includes major faults from Figure 1. For abbreviations, see Figure 1b.
Table 1. Summary of the horizontal (for depths of 0 and 5000
m and using a frequency of 40 Hz) and vertical resolution (for
frequencies of 40 and 60 Hz and at a depth of 5000 m) of the
main seismic datasets used and a comparison with the size of
investigated structures in the present study.
Survey Horizontal resolution
(0 m/5000 m depth)
Vertical resolution
(40 Hz/60Hz)
FP13 312.5 m c. 627.495 m 39.375 m 26.25 m
BSS01 312.5 m c. 627.495 m 39.375 m 26.25 m
Asymmetric folds > 500 m > 150 m
The data were interpreted using Petrel (version 2021.3), but
can also be interpreted using OpendTect (open source). All
the structural direction refers to present-day coordinates.
Bathymetric and topographic data from the Norwegian
Mapping Authority were used to map escarpments and
lineaments (Figure 8 and Figure 9a–e). We further use existing
geological maps (e.g., Roberts, 1998; Siedlecki, 1980), struc-
tural datasets (Koehl et al., 2019), glacial feature studies (e.g.,
Ottesen et al., 2008; Vorren et al., 1986), and Digital Eleva-
tion Models such as Norgei3d and GoogleEarth (version
7.3.6.9345) to differentiate glacial features from tectonic
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Figure 8. Bathymetric and topographic data onshore Finnmark and nearshore fjords. The map shows major brittle faults, mac
dykes and sills, ductile fabrics (notably folded bedding surfaces), and glacial features. The location of seismic line BSS01-205 (Figure 4c) is
shown as a thick black line. The location of Figure 9a and c–e is shown as black frames.
structures, including ductile foliation and shear zones, brittle
faults, and folds. See also further interpretation and discussion
of the bathymetric data in Koehl et al. (2019).
Offshore structures are tied to onshore–nearshore structures
using aeromagnetic data (including tilt-derivative) from the
Geological Survey of Norway from the MINN (Nasuti et al.,
2015b; Figure 10, Figure 11a–d, Figure 12, and Figure 13a–c;
Koehl, 2024) and BASAR datasets (Gernigon & Brönner,
2012; Gernigon et al., 2014; Figure 14 and Figure 15; Koehl,
2024). Magnetic anomalies are used (1) to infer the pres-
ence of magmatic intrusions onshore and nearshore and to
study their deformation character (e.g., undulating anomaly
reflecting folded, pre-Caledonian dykes and linear anomalies
representing post-Caledonian dykes intruded along brittle
faults), (2) to map fold structures affecting highly magnetic
metasedimentary beds, and (3) to identify highly magnetic
rock units extensively intruded by dykes offshore.
Bathymetric, topographic, and magnetic data were interpreted
in GlobalMapper (version 13.0), and the figures were designed
with CorelDraw (version 2017). Alternative open-source
software are QGIS and GIMP respectively.
The structures described and discussed in the present study
include post-Caledonian brittle faults and dolerite dykes, and
Timanian and Caledonian folds, ductile shear zones, and brittle
thrusts. Note that the present study builds on the interpretations
of 2D–3D seismic, aeromagnetic, and bathymetric–topographic
data published in Koehl et al. (2018a; 2019). The reader is
therefore referred to these works for more information on
specific structures not discussed in the present contribution.
Results
Seismic data
Western Finnmark Platform. The Sørøya–Ingøya shear zone
and its overall geometry are described in Koehl et al. (2018a).
In map view, the shear zone strikes overall NE–SW with a
northwestern dip and bends into a NNE-dipping geometry
just northwest of Magerøya (Figure 1a–b). The shear zone
consists of kilometer-thick, gently dipping, planar, moderate- to
high-amplitude seismic reflections interpreted as mylonitic
fabrics and thrusts (Koehl et al., 2018a). In the present
section, we further highlight its overall geometry and focus on
smaller structures within the shear zone.
We further constrained the overall geometry of the
Sørøya–Ingøya shear zone to the southwest and identified a
bend into a NNE-dipping geometry just southeast of the bend
in the Troms–Finnmark Fault Complex in addition to the
bend into a NNE-dipping geometry northwest of Magerøya
(Figure 1a–b and Figure 4a–c). In the northwest, southwest,
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Figure 9. Interpreted (left) and uninterpreted (right) zoom in bathymetric data in (a) the Nordkinn Peninsula, (b) west of the Nordkinn
Peninsula in 3D with view towards the northeast, (c) the Repparfjord–Komagfjord tectonic window, (d) southeast of Rolvsøya, and (e) Sørøya.
The map shows major brittle faults, mac dykes and sills, ductile fabrics (notably folded bedding surfaces), and glacial features. The location
of seismic line BSS01-205 (Figure 4c) is shown as a thick black line. The location of Figure9a and c–e is shown as black frames.
and southeast, the shear zone is truncated by post-Caledonian
faults such as the Troms–Finnmark and Måsøy fault complexes
(Figure 4a–d) indicating it is pre-Devonian. The shear zone
is up to 3.0 seconds (TWT; i.e., possibly tens of kilometers)
thick and dips moderately (c. 30–40°) to the north-northeast in
the southwest and more gently (c. 10–15°) in the northeast in
NE–SW-trending cross section (Figure 4a–c), whereas it dis-
plays a curving up and down geometry with greater dip
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Figure 10. (a) Interpreted and (b) uninterpreted magnetic data onshore–nearshore northern Norway. The location of Figure 11a–d is shown
as black frames in (b).
angle with portions displaying a dome-shaped geometries in
NW–SE-trending cross section (Figure 4d). Overall, the
shear zone shows considerable thickness variations ranging
from 3.0 seconds (TWT) thick in the footwall of major
post-Caledonian fault complexes and in the southwest to
0.5–1.0 second (TWT) thick below the potential spoon-shaped
Devonian basin on the western Finnmark Platform in the
northeast (Figure 4a–d; Koehl et al., 2018a). These amount
to c. 9 km and 1.5–3 km respectively using an average veloc-
ity of 6100 km.s-1 for basement rocks (Gernigon et al., 2018
their table 1).
At smaller scale, planar moderate-amplitude reflection
interpreted as mylonitic fabrics and thrusts alternate with
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Figure 11. Zoom in interpreted (left) and uninterpreted (right) tilt derivative data from Nasuti et al. (2015a) in (a) Magerøya,
(b) Porsangerfjorden and the Sværholt Peninsula, (c) the Sværholt Peninsula and Nordkinn Peninsula, and (d) in Syltefjorden and
Syltefjordfjellet.
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Figure 12. (a) Interpreted and (b) uninterpreted tilt-derivative data onshore–nearshore northern Norway. The location of Figure 13a–c is
shown as black frames in (b).
asymmetric, wiggly to oscillating, low- to moderate-amplitude
reflections both within the Sørøya–Ingøya shear zone and in
adjacent basement rocks (Figure 5a–d). These reflections can
be traced for hundreds of meters to a few kilometers later-
ally (i.e., discontinuous to semi-continuous; Figure 5a–d). In
NE–SW-trending cross section, these asymmetric reflections
lean consistently towards the south-southwest (Figure 5a–c),
whereas they lean to the southeast and to the northwest respec-
tively in the southeastern and northeastern part of the western
Finnmark in NW–SE-trending cross section (Figure 5d). These
reflections typically show elongated, gently NNE-dipping
edges and short, steeply NNE- or SSW-dipping edges result-
ing in the SSW-leaning geometry (Figure 5a–c). In places,
the long and short edges are parallel to each other with
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Figure 13. Zoom in interpreted (left in a and up in b and c) and uninterpreted (right in a and down in b and c) tilt derivative data from
Nasuti et al. (2015a) in (a) Sørøya, (b) Porsangerfjorden, the Sværholt Peninsula, and the Nordkinn Peninsula, and (c) the Digermulen
Peninsula and the Varanger Peninsula.
the long, gently dipping edge leaning over the recumbent
short edge (Figure 5a–c). These asymmetric reflections are
truncated and offset in a reverse fashion by moderately to
gently NNE-dipping, moderate-amplitude disruption surfaces
(Figure 5a–c). The asymmetric reflections are interpreted as
SSW-, southeast-, and northwest-verging, in places isoclinal,
overturned to recumbent folds in basement rocks and the
disruption surfaces as minor top-SSW, top-northwest and
top-southeast thrust faults. Other less common geometries
include double-verging, box-shaped folds (Figure 5a) and
symmetric folds upright folds (Figure 5c–d). Notably, upright
folds occur mostly in basement rocks underlying and over-
lying the Sørøya–Ingøya shear zone, particularly within the
hinge zone of folded portions of the shear zone (Figure 5c–d).
Other interesting structures include packages or series of
Z- and S-shaped, low- to moderate-amplitude reflections adja-
cent to and/or stacked onto each other (Figure 5a–c). These
packages are commonly bounded upwards and/or downwards
by thrust surfaces. In places, individual S- and Z-shaped
reflections are found at the abrupt termination of asymmetric
folds against minor thrusts. We therefore propose that S- and
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Figure 14. (a) Interpreted and (b) uninterpreted tilt-derivative data from Gernigon et al. (2014). The data show a correlation between
fold structures in the eld and on seismic data with magnetic anomalies both onshore northern Norway and on the Finnmark Platform
oshore.
Z-shaped reflections represent offset bedding surfaces, and,
hence, packages of such reflections to correspond to forward- and
backward-dipping duplexes (Koehl, 2021).
The layering (bedding) and folding of pre-Carboniferous rocks
on the western Finnmark Platform and the non-occurrence of
the Late Devonian Svalbardian Orogeny in Norway (Chauvet
& Séranne, 1994; Fossen et al., 2013; Osmundsen & Andersen,
1994) and Svalbard (Koehl et al., 2022b) suggests that they
consist of pre-Devonian metasedimentary rocks. Furthermore,
the high magnetic character of the rocks on the western
Finnmark Platform (Johansen et al., 1994) suggests that they
also partly consist of magmatic intrusions. Potential analogs are
Neoproterozoic–lowermost Cambrian metasedimentary rocks of
the Barents Sea and Tanafjorden–Varangerbotn regions
(Gorokhov et al., 2001; Siedlecki & Levell, 1978; Siedlecka
& Siedlecki, 1967) and of the latest Mesoproterozoic–earliest
Neoproterozoic Kalak Nappe Complex (Roberts, 1998), both
of which are intruded by abundant Ediacaran dolerites and
Precambrian pegmatites (Kirkland et al., 2006a; Nasuti et al.,
2015a; Ramsay et al., 1979; Rice & Reiz, 1994; Rice et al.,
2004; Roberts, 1972; Siedlecka & Roberts, 1992; Figure 1b).
Conversely, less magnetic pre-Devonian basement rocks in
the Gjesvær Low (Johansen et al., 1994), which are located
within a major Caledonian syncline (Figure 4d and Figure 5d),
likely correspond to post-Cambrian metasedimentary rocks, pos-
sibly time equivalents to Silurian metasedimentary rocks of
the Magerøy Nappe (Kirkland et al., 2005; Ramsay & Sturt,
1976), which are only intruded by a few sub-vertical
Carboniferous dykes on Magerøya (Lippard & Prestvik, 1997;
Roberts et al., 1991). The dominantly metasedimentary charac-
ter of the rock successions on the western Finnmark Platform
is supported by the partly linear geometry (i.e., layered) of
seismic reflections there. The main differences of these reflec-
tions with mylonitic thrust surfaces are that they are of
lower seismic amplitude (i.e., less acoustic impedance con-
trast because metasedimentary rocks of more or less the same
density, whereas mylonites are denser) and are in places
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Figure 15. Interpreted (upper inset) and uninterpreted (lower inset) magnetic data from Gernigon et al. (2014) showing
tens of kilometers wide anomalies interpreted as major folds.
deformed into asymmetric folds (i.e., relatively mild defor-
mation), which is not typical of mylonitic fabrics because it
reflects relatively intense deformation.
Above moderate-amplitude reflections of the Sørøya–Ingøya
shear zone in the southwest, seismic data show (inverted)
triangular packages consisting of poorly continuous to cha-
otic moderate- to low-amplitude reflections (Figure 4a and
Figure 5a). The poorly continuous reflections still show the
same dip and geometries as reflections within the Sørøya–Ingøya
shear zone. However, they are separated from underly-
ing asymmetrically folded reflections by southwest-dipping,
high-amplitude reflections with subtly undulating to rugose
geometries, which are in places truncated by northeast-dipping
thrust surfaces of the Sørøya–Ingøya shear zone (Figure 4a
and Figure 5c). The undulating to rugose reflections are inter-
preted as erosional unconformities later truncated by the
Sørøya–Ingøya shear zone. Thus, the less-continuous to cha-
otic character of the inverted triangular packages indicates
that they may represent broken up versions of folded and
sheared nappes composing the Sørøya–Ingøya shear zone.
The inverted triangular packages are therefore interpreted as
syn-orogenic foreland/piggyback deposits ahead of the major
thrust (Figure 4a–c).
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Eastern Finnmark Platform. On the eastern Finnmark Platform
near the coast of northern Finnmark, seismic data show the
occurrence of a gently curving up and down, high-amplitude,
intra-basement reflection defining major, 20–25 km wide,
NNE–SSW-trending troughs and domes in E–W-trending cross
section (Figure 6a). In places, this high-amplitude reflection is
truncated by the Upper Regional Unconformity (which cor-
responds with the Top-basement reflection in the area) and by
post-Caledonian normal faults (Figure 6a). We therefore
interpreted the 20–25 km wide troughs and domes as major
Caledonian synclines and anticlines. In E–W-trending cross
section, upright fold geometries are as common as asym-
metric folds and are typically found within the core of the
20–25 km wide folds. Note that previous works (e.g., Bugge
et al., 1995) interpreted the western flank of the eastern
major anticline in Figure 6a as a Carboniferous normal fault.
However, the apparent disruption of seismic reflections
there is clearly related to seismic artifacts, including multi-
ples and diffraction rays (see Figure 6a and Koehl et al., 2018a
their figure 6d).
The packages of seismic reflections above the folded
high-amplitude seismic reflection defining the main
Caledonian folds show more continuous and more mildly
folded, moderate-amplitude seismic reflections that are truncated
by the Upper Regional Unconformity upwards (Figure 6a).
Despite the less disrupted and more continuous character of these
reflections, their folded character, locally including east- and
west-verging, partly overturned folds (Figure 6a), suggests
that they are pre-Devonian in age. A possibility is that they
belong to the well-layered, low-grade metasedimentary rocks
of the Magerøy Nappe, which crop out to the south onshore
Magerøya (Figure 1b).
Other features of interest include asymmetric, U-shaped,
high-amplitude reflections disrupting intra-basement bedding
surfaces (Figure 6a). The disruptive character of these reflec-
tions together with the asymmetric U-shaped geometry sug-
gests that they represent saucer-shaped sills. The truncation
of folded basement bedding surfaces by the sills suggests
that they are post-Caledonian, and, therefore, may represent
offshore equivalents to the Mississippian dolerite dykes onshore
Magerøya (Lippard & Prestvik, 1997; Roberts et al., 1991).
Farther northeast in NNE–SSW-trending cross section, seismic
data show a major, NNE-dipping, kilometer-thick package
of moderate- to low-amplitude seismic reflections including
asymmetric, SSW-leaning, low-amplitude reflections with poor
lateral continuity separated by moderate-amplitude, planar, con-
tinuous reflections (Figure 6b) similar to those on the western
Finnmark Platform (Figure 5a–c). Upwards, the NNE-dipping
reflection package is truncated by the mid-Carboniferous
unconformity, which corresponds with the Top-basement reflec-
tion in the area. In addition, this reflection package is off-
set by a Devonian–Carboniferous normal fault that dies out
upwards below the Base Triassic unconformity and merges
downwards with planar, continuous reflections within the
NNE-dipping reflection package (Figure 6b). The NNE-dipping
reflection package is therefore interpreted as a major top-SSW
ductile thrust consisting of SSW-verging folds and top-SSW
mylonitic thrust surfaces. In E–W-trending cross section,
this kilometer-thick thrust is folded into 15–35 km wide,
NNE–SSW- to N–S-striking folds. The interaction of these
NNE–SSW- to N–S-striking folds and the north-northeastern
dip of the thrust results into NNE- to north-plunging fold axes
in map view (Figure 7). In the south and southwest, these
folds correlate with onshore folds mapped during field studies
on the Varanger Peninsula (Roberts, 1996; Siedlecki, 1980),
on the Nordkinn Peninsula (Roberts, 1998; Roberts, 2008a;
Roberts, 2008b; Roberts & Williams, 2013), and on Magerøy
(Andersen, 1981; Robins, 1990a; Robins, 1990b; Figure 7 and
Figure 8).
Bathymetric–topographic data
In the northeastern portion of the Kalak Nappe Complex,
bathymetric and topographic data reveal sets of high-frequency,
linear, smooth, gently dipping escarpments at rugose outcrops,
which trend dominantly NNE–SSW (Figure 9a–b), i.e.,
oblique to N–S-trending glacial lineations in the area (Koehl
et al., 2019; Ottesen et al., 2008). In the Nordkinn Peninsula,
Sværholt Peninsula, Magerøya, and the Porsanger Peninsula,
these escarpments can be tied with 5–15 kilometers wide,
NNE–SSW-trending, partly overturned to recumbent Caledonian
folds (Figure 8; Geul, 1958 in Andersen, 1981; Krill et al.,
1988; Ramsay et al., 1985; Roberts, 1987; Roberts, 1998;
Roberts & Williams, 2013), which are also supported by the
attitude of magnetic anomalies in the area (Figure 10 and
Figure 12). Typically, ductile fabrics on bathymetric and topo-
graphic data show rounded to oval geometries (e.g., at fold
hinges), and gently to steeply dipping attitudes matching
that the local folded foliation and bedding surfaces (e.g.,
Figure 9a–d). Note that the continuation of the western syn-
cline on Magerøya to the Stikonjargga Peninsula in the south
(Figure 1b) is based on the correlation of rocks of the
Hellefjord Group (Ramsay et al., 1985) to the Magerøy
Nappe (Kirkland et al., 2006b; Gasser et al., 2015; Figure 2).
This correlation is further supported by the presence of a
SSW-narrowing, NNE–SSW-trending, negative magnetic anom-
aly extending from Magerøya to the Stikonjargga Peninsula
(Figure 1b; Figure 11a).
Bathymetric data in northwestern Finnmark show major
bends in the strike of the ductile fabrics from NNE–SSW- to
WNW–ESE (Figure 8), which are supported by onshore field
mapping and magnetic anomalies. A concrete example is
the gradual bend from a NNE–SSW strike in Magerøya where
metasedimentary rocks of the Magerøy Nappe are deformed
into tight NNE–SSW-striking folds (Andersen, 1981; Robins,
1990a; Robins, 1990b) to ENE–WSW in rocks of the Kalak
Nappe Complex in western Magerøya and WNW–ESE in
Hjelmsøya and Rolvsøya (locations in Figure 1b) where
overturned fold structures in the field strike WNW–ESE,
and back to a NNE–SSW strike south and west of Rolvsøya
(Ramsay et al., 1979; Roberts, 1981). There, glacial lineations
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trend WNW–ESE and ENE–WSW and are largely oblique
to the ductile fabrics (Figure 8; Koehl et al., 2019; Ottesen
et al., 2008).
Comparable bends of dominantly NNE–SSW-striking ductile
fabrics are also observed onshore the northeastern part of the
Porsanger Peninsula (Ramsay et al., 1985; Roberts, 1998),
Sørøya (Appleyard, 1965; Krill et al., 1988; Roberts, 1973;
Speedyman, 1983; Figure 8), and the Repparfjord–Komagfjord
tectonic window (Pharaoh et al., 1982; Pharaoh et al., 1983;
Reitan, 1963; Torgersen et al., 2015; Figure 2). On the north-
eastern part of Porsanger Peninsula, gently northwest-dipping
escarpments show undulating geometries alternating between
NNE–SSW and ENE–WSW strikes. These correlate in the field
with the southeastern flank of a major, open, upright fold with
the hinge zone in the central part of the Porsanger Peninsula
(Gayer et al., 1987; Ramsay et al., 1985). These contrast
with glacial lineations, which trend NNE–SSW-trending and
WNW–ESE respectively in Porsangerfjorden and on the
Porsanger Peninsula (Figure 8; Koehl et al., 2019; Ottesen
et al., 2008).
Onshore and nearshore Sørøya notably, gently dipping escarp-
ments bend from a NNW–SSE strike in northeastern Sørøya
to an ENE–WSW strike in central Sørøya and a N–S to
NNE–SSW strike in the south (Figure 9e). These variations
coincide with the attitude of major, tens of kilometers wide,
steeply plunging fold structures in the field (Appleyard, 1965;
Krill & Zwaan, 1987; Roberts, 1973; Speedyman, 1983) and
are in places sub-orthogonal to glacial lineations (Figure 8).
Yet another example is the bending of gently northwest-dipping
escarpments in the Repparfjord–Komagfjord tectonic window
and between Rolvsøya and the Porsanger Peninsula (locations
in Figure 1b) from a NNE–SSW strike in the southwest to
a WNW–ESE strike in the northeast (Figure 9c–d). In the
Repparfjord–Komagfjord tectonic window notably, previous
studies mapped the occurrence of a several kilometers wide,
gently northeast-plunging fold within Paleoproterozoic base-
ment rocks showing a geometry comparable to that of the
described escarpments (Pharaoh et al., 1982; Pharaoh et al.,
1983; Reitan, 1963; Torgersen et al., 2015), which therefore
likely reflect the attitude of ductile fabrics.
Magnetic data
Regional aeromagnetic data in Finnmark and the Barents Sea
were described and interpreted in various works (Gernigon &
Brönner, 2012; Gernigon et al., 2014; Henderson et al.,
2015; Koehl et al., 2019; Nasuti et al., 2015a; Nasuti et al.,
2015b). The present section focuses on anomalies that delin-
eate structures (e.g., folds and faults) in basement rocks and
Caledonian nappes. Fold structures will be described from the
west to the east.
Folds. Overall, the tilt-derivative magnetic data show trian-
gular- (where partly imaged) to wedge-shaped (where com-
pletely imaged) magnetic anomalies interpreted as gently
NNE-plunging folds onshore northern Finnmark bending
into NNW-plunging geometries northward onto the eastern
Finnmark Platform (Figure 14). The three-dimensional geometry
of the folds likely results from top-SSW Timanian contraction
in the latest Neoproterozoic and superimposed top-southeast
Caledonian contraction in the Ordovician–Silurian, which is
partly shown in Gabrielsen et al. (2022).
Metasedimentary rocks of the Kalak Nappe Complex in
northern Sørøya consist of quartzite interbedded with marble,
pelite and graphite schist (Falkenes and Åfjord groups) and
garnet-rich micaschist (Storelv Group; Figure 2). In the
field, these units are deformed into subhorizontal, bending,
Caledonian, NNE–SSW- to ENE–WSW-striking folds. In
tilt-derivative magnetic data, the marble, pelite, graphite schist
and micaschist units correlate with high-positive anomalies
(Figure 13a). In map view, the bending of these Caledonian
folds defines steep folds with NW–SE-striking axial planes.
Arcuate and (right-) curving geometries of dominantly
NNE–SSW-striking magnetic anomalies between Rolvsøya and
the Porsanger Peninsula mimic the geometry of anomalies in
the Repparfjord–Komagfjord tectonic window associated with
a gently northeast-plunging fold (Koehl et al., 2019; Pharaoh
et al., 1982; Pharaoh et al., 1983; Torgersen et al., 2015 their
Figure 10e). The presence of a northeast-plunging fold is sup-
ported by bedding attitudes in southern Rolvsøya (north-dipping)
and in Reinøykalven and Bjørnøya (southeast-dipping; Roberts,
1998; locations in Figure 1b), and by correlation with near-
shore escarpments reflecting ductile fabrics (Figure 8 and
Figure 9d).
In the central part of the Porsanger Peninsula, a prominent
5–10 kilometers wide, NNE–SSW-trending, positive magnetic
anomaly aligns with outcrops of the Repparfjord–Komagfjord
tectonic window farther south, which are deformed into a
5–10 kilometers wide anticline that is also visible on magnetic
data (Koehl et al., 2019; Pharaoh et al., 1982; Pharaoh et al.,
1983). It is therefore possible that the two anomalies reflect
related, NNE–SSW-striking anticlines.
The syncline observed on seismic data just north of Magerøya
coincides with a magnetic low of comparable width (Gernigon
et al., 2014; Figure 14) that extends from offshore areas to
onshore areas of Magerøya where rocks of the Magerøy Nappe
crop out in a c. 20–25 kilometers wide syncline. Thus, the
synform on seismic data north of Magerøy is interpreted as the
northeastward continuation of this syncline with rocks of the
Magerøy Nappe in the core. It is also possible that the nega-
tive triangular magnetic anomaly represents Carboniferous col-
lapse basins on the Finnmark Platform (Koehl et al., 2018a).
However, the continuation of this negative anomaly below the
island of Magerøy, where post-Caledonian sedimentary depos-
its are absent, suggests that the anomaly is at least partly due
to the presence of the Magerøy Nappe in a wedge-shaped,
NNE–SSW-striking Caledonian syncline. This interpretation
is supported by the presence of NNE–SSW-striking folds in
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northeastern Magerøya in the field (Andersen, 1981, e.g., his
Figure 6), which also show as narrow magnetic anomalies on
the onshore–nearshore tilt-derivative magnetic map (Figure 11a).
The 5–8 km wide anticline at the mouth of Porsangerfjorden
was mapped as a right-bending, dominantly positive anomaly on
onshore–nearshore tilt-derivative magnetic data (Figure 11b).
In map view, the fold seems to affect 4–5 km wide, 10–20 km
long positive magnetic anomalies, which bend mildly to the
southwest and coincide with onshore occurrences of folded
Neoproterozoic intrusions in the northeastern part of the
Porsanger Peninsula (Nystuen, 2013; Waltham, 2003).
Farther east, 7–12 km wide, NE–SW-striking, gently
northeast-plunging syncline and anticline were mapped in the
field on the Sværholt Peninsula (Roberts, 1987; Roberts, 1998),
and a similar, 3–4 km wide syncline in Vindhamran in the
western part of the Nordkinn Peninsula (Roberts, 1998; Roberts
& Siedlecka, 2013; Roberts & Williams, 2013). The contours of
these folds are delineated by narrow, oval-shaped, high posi-
tive anomalies on onshore–nearshore tilt-derivative magnetic
data (Figure 11b), which are explained by the presence
of highly magnetic minerals (magnetite and titanite) in
metasedimentary beds of the Kalak Nappe Complex (Roberts,
2007; Roberts & Williams, 2013; Figure 11c).
Offshore magnetic and tilt-derivative data just north of the
Nordkinn Peninsula on the eastern Finnmark Platform shows
the presence of a major, 25–30 km wide, triangular, positive
anomaly (Figure 14 and Figure 15). Southwards, the triangular
shape of the anomaly widens onshore the central and eastern
parts of the Nordkinn Peninsula and continues as two posi-
tive magnetic anomalies (Figure 11c and Figure 13b), one
of which is correlated to metasedimentary beds enriched in
highly magnetic minerals in Vindhamran (Roberts, 2007). The
central and eastern parts of the Nordkinn Peninsula are therefore
believed to be deformed into a major, ≥ 30 km wide,
NNE–SSW-trending anticline. The metasedimentary successions
in the area include notably a c. 3 km thick succes-
sion of arkosic metasandstone overlain by a 3–5 km thick
phyllite-dominated metasedimentary succession (Roberts &
Siedlecka, 2013; Roberts & Williams, 2013), itself overlain by
the metasedimentary beds enriched in highly magnetic min-
erals of Roberts (2007). These successions are apparently
repeated symmetrically across a NNE–SSW-trending axis run-
ning near the center of the peninsula as supported by the
occurrence of a narrow, NNE–SSW-trending, positive magnetic
anomaly in the easternmost part of the peninsula similar to
that associated to the metasedimentary beds with highly mag-
netic minerals in the western part of the peninsula (Figure 11c).
Bedding surfaces in metasedimentary rocks both in the west-
ern and eastern limb of the major anticline dip moderately
to steeply towards the west-northwest (Roberts & Siedlecka,
2013; Roberts & Williams, 2013), thus suggesting that the east-
ern limb is overturned and giving the fold a south-southeast
vergence.
In the northwesternmost part of the Varanger Peninsula, the
outcrops of the Berlevåg Formation coincide with a 25 km
wide, wedge-shaped, curving, NNE–SSW-elongated, dominantly
positive anomaly on tilt-derivative magnetic data (Figure 14
and Figure 13c). This anomaly extends well into Tanafjorden
in the west (just east of the coastline of the Nordkinn Peninsula;
location in Figure 1b) and onto the eastern Finnmark
Platform to the north, where it broadens up to 50 km and
bends into a NNW–SSE trend, but pinches out rapidly to the
south/southwest. Since it is adjacent to the major anticline in
the central and eastern part of the Nordkinn Peninsula, this
magnetic anomaly is interpreted as a major wedge-shaped,
NNE–SSW-striking (trough-shaped) syncline. This synclinal
geometry of the Berlevåg Formation is confirmed by field stud-
ies and geological mapping in the area showing that the eastern
limb of the fold involves northwest-dipping bedding surfaces
(Siedlecka & Siedlecki, 1967; Siedlecki, 1980). Farther south-
west on the Digermulen Peninsula, field mapping by Siedlecki
(1980) and Siedlecka et al. (2006) reveals the presence of an
analogous, at least 15 km wide, wedge-shaped, NE–SW-striking
syncline extending from the western shore of Langfjorden to
Tanafjorden (Figure 1b and Figure 13c). Due to their occur-
rence in the footwall of the Caledonian thrust at the base of
the Kalak Nappe Complex and in a similar structure setting
(wedge-shaped syncline of comparable dimensions), we cor-
relate the Berlevåg Formation to the Ediacaran–Lower Ordo-
vician (Ebbestad et al., 2018; Högström et al., 2017; Jensen
et al., 2017; Siedlecki, 1980) Digermulen Group (Laksefjord
Nappe Complex), i.e., in agreement with previous correlations
(e.g., Corfu et al., 2014; Kirkland et al., 2008b; Figure 2). An
Ediacaran–Lower Ordovician age for the Berlevåg Formation
is in agreement with its position in a syncline, i.e., structurally
(although the Kalak Nappe Complex is partly thrust over the
Berlevåg Formation syncline) and stratigraphically above
Tonian to lower Cryogenian rocks of the Barents Sea Group
and of the upper Cryogenian Løkvikfjellet Group (Roberts &
Siedlecka, 2012; Siedlecki & Levell, 1978) and latest
Mesoproterozoic–Tonian metasedimentary rocks of the Kalak
Nappe Complex (Kirkland et al., 2006a; Kirkland et al., 2007a;
Figure 2). Nevertheless, since the Berlevåg Formation was
affected by a hydrothermal event at 555 ± 15 Ma (Kirkland
et al., 2008b), it is probable that it is mostly Ediacaran in age
(Figure 2).
In the northwestern part of the Varanger Peninsula, magnetic
data show 1–4 km wide, NE–SW-trending, spike-shaped posi-
tive anomalies in Kongsfjorden (Figure 13c). Since the bedrock
there consists of Tonian to lower Cryogenian metasedimentary
rocks of the Barents Sea Group (Båsnæring and Kongsfjord
formations; Siedlecka & Siedlecki, 1967; Siedlecki, 1980;
Figure 2) flanked in the northwest and southeast by younger
metasedimentary strata of the upper Cryogenian Løkvikfjellet
Group (Roberts & Siedlecka, 2012; Siedlecki & Levell, 1978;
Figure 1b) dipping respectively to the northwest between
Sandfjord and Berlevåg and to the southeast in Syltefjordfjellet
(Siedlecka & Siedlecki, 1967; Siedlecki, 1980), the area is
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therefore believed to be deformed into a 15–20 km wide,
NE–SW-striking anticline with the fold axis running within
inner Kongsfjorden. The northeastwards narrowing of the
spike-shaped magnetic anomaly suggests a northeast-plunging
geometry for the anticline (Figure 13c).
In Syltefjordfjellet, both onshore and offshore magnetic data
show the occurrence of a c. 15 km wide, rhomboidal to oval
positive anomaly (Figure 14 and Figure 11d and Figure 13c;
Koehl, 2024). In the southwest, the anomaly pinches out near
the bottom of Båtsfjorden, whereas in the northeast it pro-
ceeds into the Russian Barents Sea (Figure 14 and Figure 11d
and Figure 13c). Since the positive magnetic anomaly in
Syltefjordfjellet coincides with the occurrence of relatively
younger, gently dipping rocks of the Løkvikfjellet Group,
which are flanked to the west and southeast by relatively older
rocks of the Barents Sea Group (Siedlecka & Siedlecki, 1967;
Siedlecki, 1980; Figure 1b), we interpret the anomaly to reflect
the presence of a 20–25 km wide, NE–SW-striking syncline.
Noteworthy, Siedlecki (1980) mapped two synclines centered
(with fold axis running) along two NE–SW-elongated strips
of outcrops of the Båtsfjord Formation. However, relatively
younger rocks of the Løkvikfjellet Group crop out in between
these two strips and, therefore, suggest the presence of a
major, 20–25 km wide syncline (Figure 1b). This is further
supported by the presence of older rocks of the Båsnæring
Formation in Båsnæringsfjellet in the northwest and on the
southeastern shore of Syltefjorden in the southeast (Siedlecka
& Siedlecki, 1967; Siedlecki, 1980; location in Figure 1b).
Hence, the two smaller (5–10 km wide) synclines mapped by
Siedlecki (1980) may represent parasitic folds of the main
20–25 km wide syncline.
In the eastern part of the Varanger Peninsula, tilt-derivative
magnetic data show a 45–65 km wide, ENE–WSW-trending
negative anomaly extending from Syltefjorden to tens of
kilometers east of the Varanger Peninsula (Figure 14). Within this
broad negative anomaly, onshore tilt-derivative magnetic data
show narrow, hundreds of meters wide, curving positive mag-
netic anomalies with NNE-pointing spike shapes (Figure 13c).
These narrow anomalies coincide with and display similar
geometries to 3–5 km wide, NNE-plunging synclines in the
area (Sieldecki, 1980), thus suggesting that they reflect north- to
northeast-dipping bedding. Despite these 3–5 km wide syn-
clines, the dominance of rocks of the Barents Sea Group in the
area (older than rocks of the Løkvikfjellet Group) suggest that
the eastern part of the Varanger Peninsula consist of a tens
of kilometers wide anticline, which is supported by recent
field studies (e.g., Gabrielsen et al., 2022).
Faults. Magnetic data in northern Finnmark also show promi-
nent WNW–ESE-striking positive anomalies interpreted as
Carboniferous dolerite dykes intruded along brittle splays fol-
lowing the trace of the Trollfjorden–Komagelva Fault Zone
(Koehl et al., 2019; Nasuti et al., 2015a). In places, these dykes
show up to 2–3 km wide left bends (Figure 11a) mimicking
the local ductile fabrics attitude, e.g., in western Magerøya
where it bends from a northeasterly strike in the east to an E–W
strike in the west (Robins, 1990a). Other occurrences are seen
on the Sværholt Peninsula and in Porsangerfjorden (Figure 11b).
The 45–65 km wide negative magnetic anomaly associated
with the major anticline in the easternmost part of the Varanger
Peninsula is transected by narrow, hundreds of meters wide, lin-
ear, ENE–WSW-trending positive anomalies correlated with
Late Devonian dykes (Nasuti et al., 2015a; Roberts, 2011).
Based on previous field mapping of ENE–WSW-striking
Caledonian thrusts in the northeastern part of the Varanger
Peninsula (Gabrielsen et al., 2022; Siedlecki, 1980), it is
probable that the Devonian dykes in the area intruded along
such structures as they seem to follow the Caledonian struc-
tural trend. This is supported by the reactivation of NNW- to
northwest-dipping Caledonian thrusts on the Varanger Penin-
sula as post-Caledonian brittle normal faults (Siedlecka &
Roberts, 2012). The geometry of these dykes contrasts mark-
edly with that of linear N–S-striking Devonian dykes intruded
along right-stepping brittle fractures in the southeastern part
of the Varanger Peninsula, which probably correspond to
post-Caledonian extensional faults (Guise & Roberts, 2002;
Herrevold et al., 2009; Nasuti et al., 2015a).
Discussion
The discussion focuses first on the timing of formation of
the identified two major NNE-dipping thrust systems on the
Finnmark Platform and on a possible correlation with onshore
structures. Then, we review the implications of the present
work for major tectono-magmatic events in northern Norway,
including the intrusion of the Seiland Igneous Province, the
deposition and thrusting of the Kalak Nappe Complex, the
rifting and width of the Iapetus Ocean, the Porsanger Orogeny,
and the Finnmarkian event. Finally, we summarize the findings
of the present study into a model for the geological evolu-
tion of northern Norway from the Grenvillian–Sveconorwegian
Orogeny in the latest Mesoproterozoic to the Caledonian
Orogeny in the early–mid Paleozoic (i.e., from ca. 1050 to
420 Ma).
Timing of formation of the top-SSW thrust systems on
the Finnmark Platform
Seismic data on the Finnmark Platform show the occurrence
of two major, overall NNE-dipping thrust systems, which are
folded into 15–35 km wide, NNE-plunging folds and show evi-
dence of top-SSW movement (Figure 4a–c, Figure 6a, and
Figure 7). The vergence of the major thrust systems and asso-
ciated numerous minor structures, such as asymmetric folds
and thrusts, strongly suggests that the two thrust systems
formed during the Timanian Orogeny (Herrevold et al., 2009;
Kostyuchenko et al., 2006; Olovyanishnikov et al., 2000;
Siedlecka, 1975).
This is supported by the apparent reworking of the southern
thrust system on the western Finnmark Platform previously
interpreted as the Sørøya–Ingøya shear zone (Koehl et al.,
2018a), where it is deformed into major, NE–SW-striking
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folds and shows indications for top-southeast movements
in the southeast and top-northwest movements in the north-
west (Figure 4d), i.e., most likely subsequently to the
episode top-SSW tectonism. A suitable event to have partially
reworked a preexisting NNE-dipping thrust system into
NE–SW-striking folds and to have generated top-southeast and
top-northwest movements on the folded portions of the thrust
is the Caledonian Orogeny (Gayer et al., 1985; Gayer et al.,
1987; Townsend et al., 1986; Townsend, 1987). Our inter-
pretation is in agreement with Gernigon et al. (2014)
who interpreted the basement ridge in the footwall of the
Troms–Finnmark Fault Complex on the Finnmark Spur as
Caledonian back-thrusts, thus further supporting a Caledonian
reworking of the top-SSW thrust system.
The (double-) folding patterns observed in metasedimen-
tary rocks affected by the Timanian Orogeny on the Varanger
Peninsula (e.g., dome-shaped Ragnarokk Anticline; Gabrielsen
et al., 2022; Roberts, 1996; Siedlecka & Siedlecki, 1971;
Figure 1a) is similar to what was inferred for NNE-dipping
Timanian thrust systems in Svalbard and the Barents Sea
(Koehl, 2020; Koehl et al., 2022a; Koehl et al., 2023). In
addition, the NNE-plunging folds identified on magnetic
data onshore–nearshore northern Finnmark (Figure 14 and
Figure 13c) correlate well with the NNE-plunging folding of
the northern thrust system (Figure 7 and Figure 8).
Notably, Koehl et al. (2018a) interpreted the significant thick-
ness variations of the southern thrust system to arise from
Devonian–Carboniferous core complex exhumation along
inverted portions of the shear zone. Though this interpreta-
tion may still be partly valid, it does not explain the opposite
vergence of contractional structures within the thrust system
(top-northwest in the northwest and top-southeast in the
southeast).
The western continuation of the Trollfjorden–
Komagelva Fault Zone oshore
A particularity of the southern thrust system is that it bends
into a WNW–ESE strike in the northeast and can be traced
up to c. 30 km off the Sværholt Peninsula and the Nordkinn
Peninsula, near the possible western continuation of the
Timanian thrust front, the Trollfjorden–Komagelva Fault Zone
(Gabrielsen, 1984; Gabrielsen & Færseth, 1989; Koehl et al.,
2018a; Koehl et al., 2019; Lea, 2016; Roberts et al., 2011).
Based on previous studies in northwestern Russia, the Timanian
Orogeny was associated with southwest-directed subduction
as suggested by eclogite and blueschist metamorphism and
related intrusions (Beckholmen & Glodny, 2004; Dovzhikova
et al., 2004; Gee et al., 2000; Glodny et al., 2004; Kuznetsov
et al., 2007; Pease et al., 2004; Remizov & Pease, 2004;
Remizov, 2006). The sigmoidal geometry of structures in the
southern thrust system (e.g., Figure 4a–c and Figure 5a–c) are
comparable in shape and size to subduction-related complexes
in Norway and the UK (Freeman et al., 1988; Péron-Pinvidic
& Osmundsen, 2020). In addition, the observed bend of the
southern thrust system on the western Finnmark Platform is
comparable in size to that of major plate-boundary faults such
as the San Andreas fault in southern California (e.g., Crowell,
1979; Janecke et al., 2018; Koehl et al., 2022c; Koehl et al.,
2022d). Considering the suitable location of the westernmost
trace of the southern thrust system near the continuation of the
Trollfjorden–Komagelva Fault Zone, we propose that it repre-
sents the western continuation of the Trollfjorden–Komagelva
Fault Zone offshore rather than the commonly proposed
WNW–ESE-striking brittle segment of the Troms–Finnmark
Fault Complex (Gabrielsen, 1984; Gabrielsen & Færseth, 1989;
Lea, 2016; Roberts et al., 2011). This correlation and partial
reworking of the offshore shear zone into a top-southeast thrust
during the Caledonian Orogeny on the western Finnmark
Platform explains the observed major bend of ductile fabrics
in the Kalak Nappe Complex from WNW–ESE in western
Magerøya and Hjelmsøya to NE–SW in Ingøya and
Rolvsøya in the field (Roberts, 1981; Robins, 1990a) and
on bathymetric data (Figure 8). In this case, truncation of
WNW–ESE-striking Timanian fabrics by a top-southeast
Caledonian shear zone in the western Finnmark Platform (e.g.,
Koehl et al., 2018a) is not required.
Regarding the fault-propagation tip model proposed by Koehl
et al. (2019), this model focused on shallow brittle overprints
of the Trollfjorden–Komagelva Fault Zone along which
(Devonian?–) Carboniferous dykes were intruded (Lippard
& Prestvik, 1997; Roberts et al., 1991). However, although
they may involve shallow brittle segments, orogenic structures
such as thrust fronts are generally dominantly ductile at depth.
Thus, even if the shallow and younger (Carboniferous) brittle
overprints of the Trollfjorden–Komagelva Fault Zone seem
to die out westwards around the island of Magerøya (Koehl
et al., 2019), these are distinct from the ductile portion of the
Trollfjorden–Komagelva Fault Zone at depth.
Alternatively, the folded southern Timanian thrust offshore
may represent the western continuation of another presumed
Timanian thrust just off the northern coast of Finnmark
(Figure 1b and Figure 7). This is supported by the large
proportion of graywacke in metasedimentary rocks of the
Barents Sea Group (Siedlecka, 1975; Siedlecki, 1980), which
are typical of active continental margins and accretion-
ary prisms (e.g., Dickinson, 1970), by the gradual transition
of the Barents Sea Group with more fluvial rocks of the
Tanafjorden–Varangerfjordbotn Group (Rice, 1994) on the
Varanger Peninsula, and by the interbedded character of the two
groups at the speculated location of the Trollfjorden–Komagelva
Fault Zone in the western part of the Varanger Peninsula
(Rice, 1994; Figure 2), which all suggest that the actual
Timanian thrust front lies just north of the coast of Finnmark
instead of onshore the Varanger Peninsula (Figure 1b and
Figure 7). This would imply that the Timanian subduction
trench and suture are located below or north of the Varanger
Peninsula depending on their vergence, which is possibly
dipping to the south (Pease et al., 2004).
Regardless of the correlation with the Trollfjorden–Komagelva
Fault Zone, it follows that other brittle–ductile structures
in Finnmark, which yielded late Neoproterozoic–earliest
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Cambrian ages but strike NE–SW at present, may represent
inherited Timanian thrusts, which were reworked during the
Caledonian Orogeny. Notably, Torgersen et al. (2014) obtained
531.1 ± 11.1 and 654.8 ± 13.4 Ma K–Ar ages for synkin-
ematic illite–muscovite along the Kvenklubben Fault, which
they interpreted as a tectono-thermal event. The present study
suggests that these ages may reflect early and late Timanian
thrusting respectively.
In western Troms, U–Pb geochronological analyses of
intensely deformed basement rocks in the West Troms Base-
ment Complex yielded four 650–550 Ma ages that corre-
spond with the time frame of the Timanian Orogeny (Bergh
et al., 2015). Notably, the 643 Ma intercept they obtained for
a pegmatite dyke in garnet–mica schist with marble lenses in
Baltsfjorden, Senja, was interpreted as secondary lead loss in
the Neoproterozoic because it did not show correspondence
with any geological events known to have affected rocks of
the West Troms Basement Complex (Figure 2). However,
considering the proximity of the Timanian front on the
western Finnmark Platform (< 100 kilometers instead of
> 250 kilometers; Figure 1b and Figure 4a–d) and related
magmatic complex (Seiland Igneous Province; < 50 kilometers),
it is conceivable that Timanian deformation and related
magmatism (at least mildly) affected rocks in western Troms,
thus leading to limited reactivation and/or overprinting and
intrusions in adjacent basement rocks.
Implications for folding in Finnmark
Bathymetric–topographic and magnetic data in Finnmark
confirm the presence of NNE–SSW-striking folds in the
northeastern portion of the Kalak Nappe Complex in the
Nordkinn Peninsula, Laksefjorden, Sværholt Peninsula,
Magerøya, Porsangerfjorden, and the Porsanger Peninsula, of
gently-NNE-plunging folds in the Repparfjord–Komagfjord
tectonic window and between the Porsanger Peninsula and
Rolvsøya (locations in Figure 1b), and of overturned,
WNW–ESE- to E–W-striking, moderately dipping to sub-vertical
folds in the west on Hjelmsøya, the northwesternmost Porsanger
Peninsula, Rolvsøya, and Sørøya (Figure 8–Figure 12).
More specifically, NNE–SSW-striking anticlines in the north-
eastern Kalak Nappe Complex broaden where adjacent syn-
clines become narrower or die out (Figure 8). These width
fluctuations notably occur along a WNW–ESE-trending axis
extending from the Stikonjargga Peninsula to the Sværholt
Peninsula (location in Figure 1b) and suggest the presence
of WNW–ESE-striking folds in the eastern part of the Kalak
Nappe Complex too, albeit with upright geometries (Figure 8).
Similarly, the gently-NNE-plunging anticline in the Repparf-
jord–Komagfjord tectonic window and in the central part of
the Porsanger Peninsula appear to narrow and die out respec-
tively to the northeast and the southwest (Figure 8–Figure 12).
The alignment of these two anticlines, their similar strike and
width (5–10 km) suggest that they may correspond to the
same structure, which appears mildly deformed into broad,
WNW–ESE-striking folds (Figure 8– Figure 12).
The change in attitude of WNW–ESE-striking folds from
sub-horizontal in the east (eastern Porsanger Peninsula,
Magerøya, Nordkinn Peninsula, Sværholt Peninsula, Stikonjargga
Peninsula in Rykkelid, 1992; locations in Figure 1b) to
moderately (western Magerøya) and steeply east-plunging
(Hjelmsøya, western Porsanger), and to sub-vertical in the
west (Rolvsøya–Ingøya, Sørøya) is proposed to be related
to top-southeast Caledonian thrusting (and related fold-
ing) along the Sørøya–Ingøya shear zone segment of the
Trollfjorden–Komagelva Fault Zone, which is located ca.
15–20 kilometers northwest of the coast of Rolvsøya, Ingøya,
and Sørøya.
Implications for the Seiland Igneous Province
Recent studies show that the Seiland Igneous Province
likely reaches at least 9 km deep into the crust (Larsen et al.,
2018; Pastore et al., 2016). By contrast, thickness estimates
for the Kalak Nappe Complex in the area are in the range
of ≤ 7.5 km based on structural cross sections (Gayer et al.,
1985; Ramsay et al., 1985) and ≤ 1.7 km based on new
high-resolution magnetic data inversion (Nasuti & Roberts,
2023). This indicates that the Seiland Igneous Province reaches
well into Baltican Archean–Proterozoic basement rocks at
depth, onto which rocks of the Kalak Nappe Complex were
thrust (Figure 2). This is supported by the interpretation of
magnetic data showing that Baltican basement rocks extend
into western Troms (West Troms Basement Complex; Bergh
et al., 2010; Zwaan, 1995) and offshore onto the western
Finnmark Platform as major NNW–SSE-striking folds (Koehl
et al., 2019; their Figure 7a and paragraph 2 pp. 15). Together
with the proximity (< 30 km) of the Timanian front thrust
(Trollfjorden–Komagelva Fault Zone; Figure 1a–b and
Figure 4a–d), this suggests that the Seiland Igneous Province
was intruded on the Baltican margin of the Iapetus Ocean.
Most previous studies relate the 580–560 Ma and 530–520 Ma
magmatism of the Seiland Igneous Province to rifting
(Bergström & Gee, 1985; Elvevold et al., 1994; Krill &
Zwaan, 1987; Kirkland et al., 2008b; Larsen et al., 2018;
Roberts et al., 2006; Roberts et al., 2010), although backarc
and collisional settings are equally possible based on current
evidence (Roberts et al., 2006; Roberts et al., 2010; Robins &
Gardner, 1975). A continental backarc setting (e.g., Draut &
Clift, 2001; Vasey et al., 2021) is supported by calc-alkaline
rocks in the Seiland Igneous Province (Roberts et al., 2006;
Robins & Gardner, 1975; Speedyman, 1983; Stumpfl & Sturt,
1964; Sturt & Ramsay, 1965) and by the proximity of the
Seiland Igneous Province with the Timanian thrust front
in northern Norway (Trollfjorden–Komagelva Fault Zone;
Siedlecka & Siedlecki, 1967; Siedlecka, 1975) and on the
western Finnmark Platform (≤ 30 km; Figure 1a–b and
Figure 4a–d). In addition, contemporaneous ages were
obtained for collision- to subduction-related calc-alkaline and
late–post-orogenic alkaline magmatic suites in Russia respec-
tively dated to 700–515 Ma and 565–500 Ma (Kuznetsov
et al., 2007). The former intruded in a continental-arc (to
syn-collisional) setting with a southwest-dipping subduction
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zone (i.e., beneath Baltica) just north of the Timanian thrust
front in Russia (Timan Range, Urals, and under the Pechora
Basin; Dovzhikova et al., 2004; Gee et al., 2000; Kuznetsov
et al., 2007; Pease et al., 2004), whereas the elongate
shapes of the latter were interpreted to reflect the onset of
late–post-orogenic extension (Kuznetsov et al., 2007; Larionov
et al., 2004). The composition of felsic to ultramafic rocks of
the Seiland Igneous Province (e.g., Larsen et al., 2018) does
compare well with that of late–post-Timanian alkaline suites
in northwestern Russia (Larionov et al., 2004). It is therefore
proposed that the Seiland Igneous Province is part of a regional,
extensional–transtensional igneous suite extending from the
Russian Timanides to northern Norway and the Norwegian
Barents Sea (foliated gabbro/basalt in well 7120/1-1). This
implies that the 555 ± 15 Ma hydrothermal event in the
Berlevåg Formation (Kirkland et al., 2008b) and the 577 ± 14 Ma
metadolerite dykes on Varangerhalvøya (Rice et al., 2004;
Figure 2) are also related to the Timanian Orogeny. A backarc
setting was previously suggested for the Seiland Igneous
Province and the 577 ± 14 Ma metadolerite dykes based on
petrochemical analyses (Alexander Hugh Rice pers. comm.,
2022; Rice et al., 2004; Roberts, 1975; Robins & Gardner,
1975; Speedyman, 1983). Furthermore, thermochemical and
dynamic modelling by Griffin et al. (2013) suggests that the
Seiland Igneous Province is part of the last phase of melt-
ing of a rapidly ascending diapir of subducted oceanic litho-
sphere. Interestingly, the first calc-alkaline intrusions of the
Seiland Igneous Province are coeval with the onset of
Timanian contraction at ca. 650 Ma (653.2 ± 2.2 Ma grano-
diorite) and some intrusions show ages coeval with Timanian
tectonism at ca. 650–550 Ma (e.g., 596.4 ± 5.1 Ma Øksfjord
Gabbro; Roberts et al., 2006; Figure 2).
Alternative interpretations include a link of the Seiland
Igneous Province with the Central Iapetus Magmatic Province
(Fichler et al., 2020; Grant et al., 2016; Gumsley et al.,
2020; Larsen et al., 2018; Tegner et al., 2019). It is possible
that the Seiland Igneous Province is the product of both the
Central Iapetus Magmatic Province and subduction of oceanic
crust below Baltica during the Timanian Orogeny, the latter
potentially strengthening the former. In such model, the
Central Iapetus Magmatic Province (Bingen et al., 1998;
Gumsley et al., 2020; Higgins & van Breemen, 1998; Kjøll
et al., 2019; Meert et al., 2007; Pease et al., 2008; Tegner
et al., 2019), which is oriented perpendicular to the Timanian
Orogen and related subduction, may have partly formed as
a back-arc basin, i.e., in a similar setting as the South China
Sea with the Manila trench (Ding et al., 2018; Sun, 2016; Qin
et al., 2019), or as an impactogenic basin, i.e., extension
enhanced by subperpendicular contraction (Barberi et al., 1982).
However, Timanian-related southwest-dipping subduction under
Baltica (Pease et al., 2004) and Laurentia (Estrada et al.,
2018a; Rosa et al., 2016) might be problematic if these
blocks were indeed located above the edge of the Jason Large
Low-Shear Velocity Province (e.g., Tegner et al., 2019) because
it would imply overlapping of major mantle downwelling
(subduction) and upwelling processes (related to the Plume
Generation Zone at the edges of Large Low-Shear Velocity
Provinces; Torsvik et al., 2006). If a southwest-dipping sub-
duction zone indeed existed there in the late Neoproterozoic,
then it is inconsistent with the presence of a Large Low
Shear Velocity Province below Baltica at that time.
Implications for the Kalak Nappe Complex and Iapetus
Ocean
The rocks of the Kalak Nappe Complex are intruded by the
Seiland Igneous Province, which was most likely intruded
on the Baltican margin of Iapetus in the late Neoproterozoic.
This indicates that the latest Mesoproterozoic–early Neoprot-
erozoic sedimentary rocks of the Kalak Nappe Complex were
deposited on the Baltican margin or nearby. This is consistent
with the occurrence of late Neoproterozoic dolerite dykes
in northeastern Norway (Nasuti et al., 2015a) both in rock
units with presumed Baltican (e.g., Berlevåg Formation in
Varangerhalvøya; Kirkland et al., 2008a; Rice et al., 2004) and
Laurentian affinities (Kalak Nappe Complex in Nordkinnhalvøya;
see red boxes in Nasuti et al., 2015a their Figure 5b). It is also
supported by deformation structures in a paragneiss unit in the
Kalak Nappe Complex on Hjelmsøya (location in Figure 1b),
which suggest three discrete phases of deformation, includ-
ing (1) development of an intense gneissic foliation in Pre-
cambrian gneisses prior to the intrusion of pegmatite dykes,
(2) further ductile deformation and shearing of the gneisses
and pegmatite dykes and incorporation of the dykes into the
foliation, and (3) top-south recumbent folding during
Caledonian overprinting (Ramsay et al., 1979). Based on the
1025 ± 32 Ma age for the youngest detrital grain in the parag-
neiss on Hjelmsøya (Kirkland et al., 2008a), and since there are
only three major episodes of contractional deformation recorded
in Finnmark in the Neoproterozoic–early Paleozoic (possi-
bly Grenvillian–Sveconorwegian, Timanian and Caledonian),
the earlier two events recognized on Hjelmsøya may
correspond to the Grenvillian–Sveconorwegian and/or Timanian
events. Notably, the first event with intrusion of pegmatite
dykes is suggested to reflect the Grenvillian Orogeny based
on various ca. 980 Ma ages for intrusions and migmatites
(Kirkland et al., 2006b; Kirkland et al., 2008a; Figure 2),
and the second event involving shearing of pegmatite dykes
and their incorporation into the main foliation is proposed to
reflect top-SSW Timanian thrusting along a ductile thrust/shear
zone that was later folded into an E–W strike onshore
Hjelmsøya during subsequent Caledonian contraction. The
involvement of the Kalak Nappe Complex in the Timanian
Orogeny is also suggested by the ca. 500 Ma ages obtained
for greenschist metamorphism in this unit (Kirkland et al.,
2008b), which imply that it was thrust over Ediacaran meta-
sedimentary rocks of the Lomvatna Group (Pharaoh et al.,
1983) sometime between 650 and 500 Ma. Because of the
proximity of the Kalak Nappe Complex both to Laurentia and
Baltica (e.g., Corfu et al., 2007; Kirkland et al., 2007a;
Slagstad et al., 2006; present study), the Iapetus Ocean was
probably narrower than the several thousands of kilometers
width typically proposed by paleogeographic reconstructions
(e.g., Torsvik & Trench, 1991).
Kirkland et al. (2007a) and others argue for an exotic
provenance of the Kalak Nappe Complex that would require
hundreds of kilometers displacement of various sub-units of the
Kalak Nappe Complex in different directions. The similarities
of detrital zircons in the Sørøy Succession (upper Kalak
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Nappe Complex) and the Moine Supergroup (Scotland; prov-
enance from the Grenvillian–Sveconorwegian Orogeny; Bonsor
et al., 2012; Glendinning, 1988; Spencer et al., 2015) and in
the Sværholt Succession (lower Kalak Nappe Complex) and
the Krummedal supracrustal sequence (Greenland) are logical
because (1) both Scotland and Greenland were parts of
Laurentia in the Neoproterozoic (Cawood et al., 2004), (2) Baltica
and Laurentia were adjacent to each other from the latest
Mesoproterozoic to the break-up of Rodinia in the early–mid
Neoproterozoic, which initiated at ca. 825 Ma in northern
Norway (Koehl et al., 2018b), and (3) paleomagnetic data for
Baltica and Laurentia suggest that these two plates moved
together from 750 Ma to 550 Ma before they rifted apart dur-
ing the opening of Iapetus (Pease et al., 2008). Hence, the
similar detrital signatures may very well be explained by
erosion of uplifted portions of the southern Baltican margin
of the Grenvillian–Sveconorwegian Orogeny and transport
northwards towards Scotland and north-northeastwards towards
Finnmark during deposition of the Moine Supergroup and
of the Sørøy Succession as suggested by paleocurrent data
(Roberts, 2007), and by transport to the southeast for the
Sværholt Succession. In addition, U–Pb dating of detrital
zircons show that metasedimentary rocks of the Kalak Nappe
Complex have a Timanian signature (i.e., located near Baltica),
displaying a clear peak at 570 Ma (Andresen et al., 2014;
Gee et al., 2017), thus further supporting a deposition of the
Kalak Nappe Complex near Baltica and the Timanian Orogen.
The Porsanger Orogeny
The intrusion of the Lillefjord Granite and Revsneshamn
Granite at 840 ± 6 Ma and of related pegmatite at 828 ± 4 Ma
was dated through U–Pb geochronological analyses on zircon
(Figure 2) and ascribed these magmatic intrusions to syn-kinematic
deformation of the Porsanger Orogeny, which is constrained
to 1600–840 Ma, i.e., potentially overlapping with the
Grenvillian–Sveconorwegian Orogeny (Corfu et al., 2007;
Daly et al., 1991; Kirkland et al., 2006a). However, recent
K–Ar geochronological analysis of syn-kinematic illite along
brittle faults in Proterozoic basement rocks in adjacent
areas of Finnmark near Alta yielded average 1050 ± 15 Ma,
950 ± 15 Ma and 825–805 ± 15 Ma (Koehl et al.,
2018b). The first two average ages directly correlate with
contraction and late–post-orogenic collapse related to the
Grenvillian–Sveconorwegian Orogeny (Andersen et al., 2007;
Bingen et al., 2008; Corfu et al., 2011; Slagstad et al., 2013;
Viola et al., 2011), whereas the latter average age coincides
with the initial break-up of Rodinia (Hartz & Torsvik, 2002;
Koehl et al., 2018b; Li et al., 2008) and overlaps with the
intrusion of the Lillefjord and Revsneshamn granites and
pegmatites (Kirkland et al., 2006a).
Though the Kalak Nappe Complex is thought to be exotic to
Baltica (Laurentian affinities; Corfu et al., 2007; Kirkland
et al., 2007a; Slagstad et al., 2006) in the Neoproterozoic, the
geochronological ages obtained by Koehl et al. (2018b) are
relevant for these rocks because, regardless of where they were
located at 840–828 Ma, recent paleogeographic reconstructions
show that Baltica and Laurentia where located close to each
other in the latest Mesoproterozoic (Grenvillian–Sveconorwegian
Orogeny; Cawood & Pisarevsky, 2017; Slagstad et al., 2013)
and in the latest Neoproterozoic (Timanian Orogeny; Estrada
et al., 2018a; Estrada et al., 2018b; Estrada et al., 2018c;
Gumsley et al., 2020; Koehl, 2020; Li et al., 2008; Rosa
et al., 2016; Tegner et al., 2019). Therefore, it does not mat-
ter on which side of Iapetus the rocks of the Kalak Nappe
Complex were located because they must have been through
the same tectonic events as Laurentia and Baltica throughout
the Neoproterozoic, which would imply that the tectonother-
mal events recorded by Kirkland et al. (2007a) in the Kalak
Nappe Complex are events of local significance, e.g., related
to the Seiland Igneous Province at 570–560 Ma (Krill &
Zwaan, 1987; Roberts et al., 2010), to the Timanian Orogeny
at 650–550 Ma (Olovyanishnikov et al., 2000), and/or to
regional plume and rifting events at 825–740 Ma (Li et al.,
2008). Thus, it is more likely that the Lillefjord and
Revsneshamn granites, whose age partly overlaps with the ca.
825–805 Ma normal faulting ages in Finnmark (Koehl et al.,
2018b), are related to extensional tectonic processes (as also
considered by Kirkland et al., 2006a). For example, they
may have formed in an arc-related setting if the subduc-
tion related to the Timanian Orogeny had already initiated.
Subduction-related magmatism in the mid–late Neoproterozoic
may also explain the presence of late Neoproterozoic (680 ±
10 Ma) migmatite in nearby areas of Kvaløya in Finnmark
(Eidvågeid migmatite; Corfu et al., 2007; Figure 2).
Another possibility is that the Porsanger Orogeny reflects
intra-oceanic contraction during the onset of convergence
between Baltica and Laurentia, thus explaining why it is recorded
on both Laurentia and Baltica. However, an extensional set-
ting is generally favored for tectonothermal events between
870 and 650 Ma, e.g., break-up of Rodinia (Callegari et al.,
2023; Koehl et al., 2018b; Li et al., 2008). Notably, the
Knoydartian/Moranian Orogeny was reinterpreted into an
extensional event (Corfu et al., 2007; Cutts et al., 2009; Soper
& Harris, 1997). Thus, it is more likely that the sediments
of the lower and upper Kalak Nappe Complex respec-
tively dated to > 980 Ma (Kirkland et al., 2006a) and ca.
920–910 ± 15–20 Ma to ca. 840 Ma (Kirkland et al.,
2007a), whose age overlaps that of tectonic events related
to Grenvillian–Sveconorwegian contraction or collapse
at 950 ± 15 Ma (Koehl et al., 2018b), were deposited in a
Grenvillian–Sveconorwegian foreland basin and/or in a
late–post Grenvillian–Sveconorwegian collapse basin.
Implications for the Finnmarkian event
Mild diachronous deformation occurred in Finnmark between the
Timanian and Caledonian (Scandian) orogenies at 555–460 Ma
as shown by numerous Cambrian–Early Ordovician geo-
chronological ages for metamorphism in rocks of the Kalak
Nappe Complex (see references in Rice & Frank, 2003).
However, quite a few of these ages were reinterpreted to
reflect partial resetting of the K–Ar and Rb–Sr systems and, as
concluded by Kirkland et al. (2006a; 2008b), there are no evi-
dence of major tectonic movements during that period justifying
a full orogenic event (Finnmarkian Orogeny sensu stricto).
Based on the correlation of the Trollfjorden–Komagelva
Fault Zone on the western Finnmark Platform and reinterpre-
tation of the tectonic setting during intrusion of the Seiland
Igneous Province, the hydrothermal event at 555 ± 15 Ma
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(Kirkland et al., 2008b) is more likely to be related to latest
Neoproterozoic dyke intrusion in Finnmark (Rice et al., 2004)
and therefore part of the backarc extension phase above the
southwest-dipping Timanian subduction zone. Cambrian–Early
Ordovician deformation in Finnmark is therefore likely the
product of partial resetting of Timanian ages by Caledonian
deformation. Alternatively if the ages are genuine, they may
reflect the progressive transition from top-SSW Timanian
contraction to top-southeast Caledonian (Scandian) contrac-
tion, including progressive bending and reorientation of the
western portion of the Trollfjorden–Komagelva Fault Zone
(Sørøya–Ingøya shear zone) from a NNE-dipping (e.g., in
the Varanger Peninsula; Gabrielsen et al., 2022; Herrevold
et al., 2009; Siedlecka & Siedlecki, 1967; Siedlecka, 1975) to a
northwest-dipping geometry (Figure 1a–b and Figure 4a–d;
Koehl et al., 2018a).
Geological evolution of northern Norway from the
latest Mesoproterozoic to the mid-Paleozoic
The proximity of the Kalak Nappe Complex to northern
Norway in the latest Mesoproterozoic to early Neoproterozoic
when deposited between 1025–840 Ma (Kirkland et al.,
2006a; Kirkland et al., 2007a; Kirkland et al., 2008a) and
its truncation by intrusions and migmatization at ca. 980
(Kirkland et al., 2006b; Kirkland et al., 2008a; Figure 2) indi-
cate that northern Norway must also have been affected by
the Grenvillian–Sveconorwegian event (Figure 16a). This is
also supported by possible latest Mesoproterozoic–earliest
Neoproterozoic faulting ages (i.e., coeval with the
Grenvillian–Sveconorwegian Orogeny) in parauthochthonous
basement rocks in Finnmark (Koehl et al., 2018b). The position
of Svalbard and of the Barents Sea at that time is still uncer-
tain despite some promising Neoproterozoic paleomagnetic
Figure 16. Summary model detailing the tectonic evolution of northern Norway and the southern Barents Sea from the latest
Mesoproterozoic to the mid-Paleozoic. (a) Deposition of sedimentary rocks of the Kalak Nappe Complex, possibly in a foreland basin
associated with the Grenvillian–Sveconorwegian Orogeny at 1050–910 Ma and their partial migmatization and intrusion by tonalites at ca.
980 Ma. (b) Normal faulting at 825–735 Ma and felsic magmatism at 840–828 Ma (granite and pegmatite, e.g., Lillefjord and Revsneshamn
granites) in the Kalak Nappe Complex during the breakup of Rodinia. (c) Top-SSW contraction during the Timanian Orogeny at 650–550 Ma
and intrusion of the Seiland Igneous Province in a back-arc basin at 580–520 Ma. (d) Top-southeast thrusting and reworking (folding) and
partial reactivation of NNE-dipping Timanian thrusts during the Caledonian Orogeny in the early–mid Paleozoic.
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poles for rocks eastern Svalbard (Michalski et al., 2022). How-
ever, evidence of Grenvillian–Sveconorwegian-aged granitic
intrusions in Svalbard (Johansson et al., 2004; Johansson
et al., 2005), eastern Greenland (Leslie & Nutman, 2000;
Leslie & Nutman, 2003), the Pearya Terrane (Trettin, 1987;
Trettin et al., 1987), and Taimyr Peninsula (Pease et al., 2001)
suggest that the whole Barents Sea and Svalbard might have
already existed in their present configuration in the latest
Mesoproterozoic.
In the mid-Tonian, 840–828 Ma granitic and pegmatitic intru-
sions in the Kalak Nappe Complex (Kirkland et al., 2006a;
Figure 2), 870–840 Ma granitic and gabbroic intrusions in the
Seve Nappe in northern Sweden (Callegari et al., 2023), and
ca. 825–805 Ma normal faulting in parauthochthonous basement
rocks (Koehl et al., 2018b) suggest that northern Baltica
experienced rifting related to the break-up of Rodinia
(Li et al., 2008; Figure 16b). Rifting must have continued
through the late Tonian as suggested by 790–780 Ma and
740–735 Ma ages for brittle–ductile faults in Paleoproterozoic
basement rocks in northern Norway (Torgersen et al., 2014).
In the late Neoproterozoic, northwestern Russia (Dovzhikova
et al., 2004; Gee et al., 2000; Glodny et al., 2004; Kostyuchenko
et al., 2006; Kuznetsov et al., 2007; Larionov et al., 2004;
Lorenz et al., 2004; Olovyanishnikov et al., 2000; Pease et al.,
2004; Remizov, 2006; Remizov & Pease, 2004; Siedlecka
& Roberts, 1995), northern Norway (Dallmeyer & Reuter,
1989; Gorokhov et al., 2001; Siedlecka, 1975), the Barents Sea
(Klitzke et al., 2019; Koehl, 2020; Koehl et al., 2022a; Koehl
et al., 2023; Korago et al., 2004; Lopatin et al., 2001), Svalbard
(Birkenmajer, 1991; Bjørnerud, 1990; Bjørnerud et al., 1991;
Dallmeyer et al., 1990; Faehnrich et al., 2020; Koglin et al.,
2022; Majka et al., 2008; Manecki et al., 1998; Peucat et al.,
1989), and probably northern Greenland (Estrada et al., 2018a;
Rosa et al., 2016) and the Pearya Terrane (Estrada et al., 2018b)
were involved in top-SSW contraction and southwest-directed
subduction related to the Timanian Orogeny (Figure 16c).
Timanian subduction processes led to the intrusion of the
Seiland Igneous Province from ca. 650 Ma to 530–520 Ma
(Krauskopf, 1954; Roberts et al., 2006; Roberts et al., 2010;
Robins & Gardner, 1975; Speedyman, 1983; Sturt & Ramsay,
1965), of dolerite dykes on the Varanger Peninsula and in the
Kalak Nappe Complex at 577 Ma and 550 Ma (Andersen &
Sundvoll, 1995; Nasuti et al., 2015a; Rice et al., 2004), and
to the Finnmarkian hydrothermal event at 555 Ma (Kirkland
et al., 2008b; Figure 2) in a backarc setting, and, eventually,
to the rifting of the Iapetus Ocean (Figure 16c; Bingen et al.,
1998; Gumsley et al., 2020; Higgins & van Breemen, 1998;
Kjøll et al., 2019; Meert et al., 2007; Pease et al., 2008; Tegner
et al., 2019), although with a significantly narrower width than
that proposed by previous studies (e.g., Torsvik & Trench, 1991).
Rifting continued through the early Paleozoic, until
NW–SE-oriented convergence in the Middle Ordovician–Silurian
resulted in the Caledonian Orogeny (e.g., Binns & Gayer,
1980; Corfu et al., 2011; Kirkland et al., 2007b; Townsend,
1987; Figure 16d). Caledonian contraction involved the rework-
ing of Timanian thrusts in the Barents Sea, Svalbard, and
northern Norway into NNE-plunging (Figure 7–Figure 14)
and into dome- and trough-shaped folds (Gabrielsen et al.,
2022; Koehl, 2020; Koehl et al., 2022a; Koehl et al., 2023;
Siedlecka & Siedlecki, 1971), and the reactivation of folded
portion of Timanian thrusts (e.g., Trollfjorden–Komagelva
Fault Zone on the western Finnmark Platform) as
top-southeast thrusts (Figure 4d and Figure 16d).
Conclusions
1) Top-SSW Timanian thrust systems on the Finnmark
Platform were folded into NNE-plunging and dome-
and trough-shaped folds during the Caledonian
Orogeny.
2) On the western Finnmark Platform, the NNE-dipping
Timanian thrust front, potentially the
Trollfjorden–Komagelva Fault Zone, bends into a
northwest-dipping geometry and was locally reacti-
vated as a top-southeast thrust during the Caledonian
Orogeny. Alternatively, the Timanian thrust front is
located just off the coast of Finnmark.
3) The Seiland Igneous Province most likely formed
in a backarc setting near the Timanian thrust front
(Trollfjorden–Komagelva Fault Zone).
4) The Kalak Nappe Complex is intruded by the Seiland
Igneous Province, which reaches a depth superior to
that of the maximum thickness of the Kalak Nappe
Complex. Metasedimentary rocks of the Kalak Nappe
Complex therefore deposited along the Baltican
margin of the Iapetus Ocean.
5) The proximity of the Kalak Nappe Complex both to
Baltica and Laurentia suggests that the Iapetus Ocean
was much narrower than the several thousands of
kilometers width commonly proposed.
6) Magmatism related to the Porsanger Orogeny is
more likely to have occurred in an extensional setting
during the initial breakup of Rodinia at ca. 840–805 Ma,
and lowermost Neoproterozoic sedimentary rocks
of the Kalak Nappe Complex probably deposited in
basins formed during Grenvillian–Sveconorwegian
contraction or related late–post-orogenic collapse.
7) The thermal event at the Neoproterozoic–Cambrian
transition in eastern Finnmark initially associated with
the Finnmarkian event is more likely related to backarc
magmatism over the southwest-directed Timanian sub-
duction zone. Alternatively, the Finnmarkian event
may represent the transition from top-SSW Timanian
to top-southeast Caledonian (Scandian) deformation.
Data availability
Underlying data
The seismic reflection data used in the present study are from
the Norwegian National Data Repository for Petroleum Data
(DISKOS database), the Norwegian Defense Research Establish-
ment, and TGS (FP13). The data may be accessed for research
purpose by contacting the Norwegian Offshore Directorate at
https://www.sodir.no/om-oss/kontakt-oss/, the Norwegian Defense
Page 35 of 47
Open Research Europe 2024, 4:30 Last updated: 15 OCT 2024
Research Establishment at https://www.ffi.no/en/about-ffi/contact-
us, and TGS at https://www.tgs.com/contact-us.
The interpreted magnetic data are from the Geological Survey
of Norway. Access to the onshore data (MINN project) can
be obtained by contacting the Geological Survey of Norway
at https://www.ngu.no/om-ngu/kontakt-ngu. Regarding the off-
shore dataset, in addition to contacting the NGU, a small part of
the data (over the Hammerfest Basin) is the property of Equinor,
which may be contacted at https://www.equinor.com/about-us/con-
tact-us to get access to the entire dataset (BASAR project).
The onshore topographic and nearshore bathymetric data
of the Norwegian Mapping Authority may be obtained by
contacting the organization at https://www.kartverket.no/en/
about-kartverket/contact-us.
Extended data
DataverseNO: Extended data for “Caledonian reactivation and
reworking of Timanian thrust systems and implications for
latest Mesoproterozoic to mid-Paleozoic tectonics and magma-
tism in northern Baltica”, doi.org/10.18710/YQNSGQ (Koehl,
2024)
The project contains the following extended data:
-00_ReadMe.txt
-Koehl_Stokmo_(Extended_data).docx (high-resolution versions
of the figures)
-Koehl_Stokmo_(Extended_data).pdf (pdf version of the
above-mentioned document)
The data are available under the terms of the Creative Commons
Zero “No rights reserved” data waiver (CC0 1.0 Public
domain dedication).
Acknowledgements
The author of the present manuscript thanks the Norwegian Off-
shore Directorate, the Geological Survey of Norway, and the
Norwegian Mapping Authority for granting access and
allowing publication respectively of seismic data from the
DISKOS database, magnetic data from the MINN and BASAR
projects, and bathymetry–topography data in Finnmark. The
authors would like to thank Laurent Gernigon (Geological
Survey of Norway) and Frances Cooke (UiT The Arctic
University of Norway in Tromsø) for constructive discussion.
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Open Peer Review
Current Peer Review Status:
Version 1
Reviewer Report09 July 2024
https://doi.org/10.21956/openreseurope.18407.r40249
© 2024 Xiao W. This is an open access peer review report distributed under the terms of the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the
original work is properly cited.
Wenjiao Xiao
1 University of Chinese Academy of Sciences, Beijing, China
2 University of Chinese Academy of Sciences, Beijing, China
The results of this manuscript challenge the existing models regarding the extension of the
Trollfjorden–Komagelva Fault Zone, offering a clearer and more definitive understanding of its
continuation through reinterpreting previous geophysical results including seismic reflection,
bathymetric, topographic, and magnetic data analysis. I think the novelty of the manuscript is
stated very obviously. However, it needs a major revision. There are some reasons as outlined
below:
Major concerns:
1. The authors stated that the seismic data were interpreted using Petrel. Did the authors use
seismic data sourced from relevant database and utilize open-source software for data
processing, or did they merely reinterpret seismic reflection profiles that were originally derived
from previous studies conducted by other researchers? Please clarify this in the main text. If the
geophysical findings presented in this manuscript originate from previous research publications
by other scholars, please give a reference to relevant literature.
2. I am uncertain if the reflection profiles presented in Figures 2 to 4 can be consistently
reproduced through repetition of the same methodology or experiments.
3. Another main problem of the manuscript is the introduction of the results and data is in a mess,
such as some of the locations cannot be found in the figure, and the introduction of the results is
not presented in temporal and spatial order to describe, making the paper difficult to read.
Minor comments:
1) The abstract should be presented as a paragraph and should introduce the background, data
and results of the manuscript and the conclusion. However, the present manuscript does not meet
this standard.
2) The conclusions in the “abstract” are not based on the data of this study. Therefore, it is not
acceptable.
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3) There are too many keywords for a published paper. In general, a published paper has less than
5-6 keywords.
4) The fourth and fifth paragraphs of the “Introduction” should be presented in the Discussion
section.
5) The legends in the Figure 1 are too small to read.
6) The structural legend for possible post-Caledonian dykes/sills is not shown in Figure 2.
7) The background of the regional geology is too much. Shorten it.
Is the work clearly and accurately presented and does it cite the current literature?
Partly
Is the study design appropriate and does the work have academic merit?
Partly
Are sufficient details of methods and analysis provided to allow replication by others?
Partly
If applicable, is the statistical analysis and its interpretation appropriate?
Partly
Are all the source data underlying the results available to ensure full reproducibility?
Partly
Are the conclusions drawn adequately supported by the results?
Partly
Competing Interests: No competing interests were disclosed.
Reviewer Expertise: Geodynamics, tectonics
I confirm that I have read this submission and believe that I have an appropriate level of
expertise to confirm that it is of an acceptable scientific standard, however I have
significant reservations, as outlined above.
Author Response 13 Sep 2024
Jean-Baptiste Koehl
Dear Prof. Xiao, thank you very much for your input on the manuscript, it is highly
appreciated. Here is our reply to your comments. We hope the changes we implemented
improve the shortcomings of the manuscript highlighted by your comments and
suggestions. Please do not hesitate to contact us shall this not be the case for some
comments.
Comments by the reviewer
Comment 1: 1. The authors stated that the seismic data were interpreted using Petrel. Did
the authors use seismic data sourced from relevant database and utilize open-source
software for data processing, or did they merely reinterpret seismic reflection profiles that
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were originally derived from previous studies conducted by other researchers? Please clarify
this in the main text. If the geophysical findings presented in this manuscript originate from
previous research publications by other scholars, please give a reference to relevant
literature.
Response: As stated in the method chapter, the authors of the present manuscript
interpreted existing (private) seismic reflection data from TGS and public data from the
Norwegian Defense Research Establishment and the DISKOS database. Changes: None
required. Comment 2: 2. I am uncertain if the reflection profiles presented in Figures 2 to 4
can be consistently reproduced through repetition of the same methodology or
experiments. Response: Agreed. See response to comment 22 by Dr. Simon Stewart.
Changes: See response to comment 22 by Dr. Simon Stewart.
Comment 3: 3. Another main problem of the manuscript is the introduction of the results
and data is in a mess, such as some of the locations cannot be found in the figure, and the
introduction of the results is not presented in temporal and spatial order to describe
Response: Agreed. Changes: Added location of Bjørnøya (Bj), Båsnæringsfjellet (Bn),
Berlevåg (Bv), Ragnarokk Anticline (RA), Reinøykalven (Rk), Rolvsøya (Rv), Stikonjargga (Sk),
and Tanafjorden (Tn) to Figure 1b.
Comment 4: 1) The abstract should be presented as a paragraph and should introduce the
background, data and results of the manuscript and the conclusion. However, the present
manuscript does not meet this standard.
Response: Agreed. The present format of the abstract follows the standard of the journal.
Changes: None.
Comment 5: 2) The conclusions in the “abstract” are not based on the data of this study.
Therefore, it is not acceptable.
Response: Partly agreed. However, the anchoring of the Seiland Igneous Province and of
the metasedimentary rocks of the Kalak Nappe Complex, which they intrude, is indeed a
conclusion from the present study, and so is the probable overestimation of the width of
the Iapetus Ocean.
\Changes: Deleted “The NNE-dipping Trollfjorden–Komagelva Fault Zone merges with a
recently identified northwest-dipping brittle–ductile thrust, the Sørøya–Ingøya shear zone,
which was previously thought to have formed during the Caledonian Orogeny.” from the
“result” section in the abstract because redundant with the following sentence. Also
replaced “present study suggests” by “Trollfjorden–Komagelva Fault Zone may continue
offshore as a NE–SW-striking folded structure. This has the following implications:”, and
added “possibly” and “likely” in the following phrases.
Comment 6: 3) There are too many keywords for a published paper. In general, a published
paper has less than 5-6 keywords.
Response: Open Research Europe seems to allow high numbers of keywords. Thus, unless
the journal editorial team recommends reducing the number of keywords, the authors of
the present manuscript would prefer to keep them all. Changes: None. Awaiting feedback
from the editorial team.
Comment 7: 4) The fourth and fifth paragraphs of the “Introduction” should be presented in
the Discussion section.
Response: Partly agreed regarding the fourth paragraph. However, the fifth paragraph
must remain in the introduction since it presents some of the broad implications of the
study, which is relevant information to add to the introduction of a scientific article. Having
said that, it is only logical that the fourth paragraph, which is a direct implication of the
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results of the present study, also is mentioned in the introduction. However, the authors of
the present manuscript concede that the models mentioned in paragraph four of the
introduction should be better discussed and set in contrast to the present results in the
discussion.
Changes: Added “bends into a WNW–ESE strike in the northeast and” and “rather than the
commonly proposed WNW–ESE-striking segment of the Troms–Finnmark Fault Complex
(Gabrielsen, 1984; Gabrielsen & Færseth, 1989; Lea, 2016; Roberts et al., 2011” in the first
paragraph of the “The western continuation of the TKFZ offshore” section of the discussion.
Added “in the field”, “and on bathymetric data (Figure 6)”, and “In this case, truncation of
WNW–ESE-striking Timanian fabrics by a top-southeast Caledonian shear zone (e.g., Koehl et
al., 2018a) is not required.” in the second paragraph. Also added a new paragraph
thereafter about the model by Koehl et al. (2019).
Comment 8: 5) The legends in the Figure 1 are too small to read.
Response: Agreed.
Changes: The authors have asked the journal to enlarge Figure 1a and 1b as much as
possible.
Comment 9: 6) The structural legend for possible post-Caledonian dykes/sills is not shown in
Figure 2.
Response: Disagreed.
Changes: The legend for post-Caledonian dykes/sills is included in Figure 2d.
Comment 10: 7) The background of the regional geology is too much. Shorten it.
Response: Disagreed.
Changes: The authors of the present manuscript concede that the geological setting section
is relatively long. However, it is necessary to acknowledge relevant previous works in the
study area and around to set the stage for the issues approached in the discussion.
Competing Interests: No competing interests were disclosed.
Reviewer Report28 April 2024
https://doi.org/10.21956/openreseurope.18407.r38897
© 2024 Stewart S. This is an open access peer review report distributed under the terms of the Creative
Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.
Simon Stewart
1 Saudi Aramco, Dhahran, Saudi Arabia
2 Saudi Aramco, Dhahran, Saudi Arabia
Please see the review in the separate PDF document linked here.
Is the work clearly and accurately presented and does it cite the current literature?
Partly
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Is the study design appropriate and does the work have academic merit?
Yes
Are sufficient details of methods and analysis provided to allow replication by others?
No
If applicable, is the statistical analysis and its interpretation appropriate?
Not applicable
Are all the source data underlying the results available to ensure full reproducibility?
No
Are the conclusions drawn adequately supported by the results?
Partly
Competing Interests: No competing interests were disclosed.
Reviewer Expertise: Structural geology, reflection seismic interpretation.
I confirm that I have read this submission and believe that I have an appropriate level of
expertise to state that I do not consider it to be of an acceptable scientific standard, for
reasons outlined above.
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