A review and reinterpretation of the architecture of the South and South-Central Scandinavian Caledonides—A magma-poor to magma-rich transition and the significance of the reactivation of rift inherited structures

Interpretations of the pre-Caledonian rifted margin of Baltica commonly reconstruct it as a simple, tapering, wedge-shaped continental margin dissected by half graben, with progressively more rift-related magmas towards the ocean-continent transition zone. It is also interpreted to have had that simple architecture along-strike the whole length of the margin. However, present-day rifted margins show a more complex architecture, dominated by different and partly diachronous segments both along and across strike. Here, we show that the composition and the architecture of the Baltican-derived nappes of the South and South Central Scandinavian Caledonides are to a large extend rift-inherited. Compositional variations of nappes in similar tectonostratigraphic positions can be ascribed to variations along-strike the rifted margin, including a magma-rich, a magma-rich to magma-poor transition zone, and a magma-poor segment of the margin. The architecture of the nappe stack that includes the Baltican-derived nappes was formed as a result of the reactivation of rift-inherited structures and the stacking of rift domains during the Caledonian Orogeny.


Introduction
The present architecture of the Scandinavian Caledonides is principally the result of the Silurian-Devonian Scandian continental collision of Baltica-Avalonia with Laurentia, the subsequent late-to postorogenic extension, and deep erosion (e.g. Fossen, 2010;Corfu et al., 2014). During the Scandian collision and in parts during the early-Caledonian events affecting the distal margin of Baltica, the rifted continental margin of Baltica was deeply buried beneath Laurentia and a complex stack of nappes was thrust over great distances towards the south-east onto Baltica. The underlying autochthon comprises Archean to Palaeoproterozoic basement in the north and Mesoproterozoic basement in the south that is covered by (par)autochthonous metasediments of Neoproterozoic to latest Silurian age. The nappe-stack comprises allochthons of Baltican, transitional oceanic-continental, oceanic, and Laurentian affinity.
The allochthons of Baltican affinity include Neoproterozoic pre-to post-rift successions as well as post-rift continental margin deposits of Cambrian to Silurian age and foreland basin sediments deposited in front of and incorporated into the advancing thrust sheets during the Scandian Orogeny (e.g. Nystuen et al., 2008). Baltican-derived basement and basement-cover nappes are commonly referred to as the Lower and Middle Allochthons and are interpreted to contain transgressive sequences deposited along the Iapetus margin of Baltica Stephens and Gee, 1989). Ophiolite/island arc assemblages and nappes of Laurentian affinity are commonly referred to as the Upper and Uppermost Allochthons, respectively.
In the traditional tectonostratigraphic scheme, all units with oceanfloor-like lithologies are referred to as ophiolites or dismembered ophiolites and are interpreted to have initially formed in an ocean, outboard of all rocks with continental affinity . The traditional interpretations assume that a mostly uniform and continuous tectonostratigraphy with the same palaeogeographic significance can be traced along the entire length of the Scandinavian Caledonides (e.g. Gee et al., 2016). However, present-day understanding of continental margins and their remnants within mountain belts is that rifted margins have a more complex architecture, dominated by different and partly diachronous segments both along and across strike. Such segmentations may include very different fault geometries and structural styles, producing major variations in width and length of basins and highs as well as more fundamental and largerscale variations with magma-poor and magma-rich segments (e.g. Mohn et al., 2010;Péron-Pinvidic and Manatschal, 2010). On a regional scale, passive margins may also be decorated with
In other areas, mostly in south-western Norway, the fossiliferous Cambrian-Ordovician overlies a glacially striated basement floor in Hardangervidda (Fig. 2) and are in turn overthrust by mica schists (Holmasjø Formation) of unknown age (see Gabrielsen et al., 2015). With the exception of the~616 Ma Egersund mafic dykes, minor volcanics and dykes in the Hedmark basin and a horizon of basaltic volcanics on Hardangervidda (Nystuen, 1987;Bingen et al., 1998;Andresen and Gabrielsen, 1979), mafic magmatic rocks are absent in the Baltican basement and the Neoproterozoic-Ordovician succession in the foreland area of South-Scandinavia.

The Lower Bergsdalen Nappe
The Lower Bergsdalen Nappe includes crystalline basement and Proterozoic metasediments, which are associated with metamorphosed basic to intermediate plutons and volcanics (Kvale, 1945;Fossen, 1993). Interleaved with the coarse-grained metasediments and  Mohn et al. (2014); map originally modified from Péron-Pinvidic and  and Lundin and Doré (2011); profile modified from Welford et al. (2012). Please note the presence of seamounts in the northern Rockall basin. HBk, Hatton Bank; RBk, Rockall Bank; PBk, Porcupine Bank; AD, Anton Dohrn Seamount; HS, Hebrides Seamount. magmatic rocks are phyllites and mica schists (Kvale, 1945). The metasediments are mainly coarse-grained meta-arkoses and quartzites. Some of the granites in the crystalline sheets were dated by the Rb-Sr whole-rock method at 1274-953 Ma (Pringle et al., 1975;Gray, 1978). Kvale (1945) interpreted the quartzites to be the oldest rocks of the Lower Bergsdalen Nappe because mafic and felsic magmas intrude into the metasediments. Consequently, the quartzites were interpreted to be pre-Sveconorwegian in age (> 1274 Ma).
The Lower Bergsdalen Nappe is positioned structurally above the Western Gneiss Region (WGR), a thin discontinuous cover of mica schists (Wennberg et al., 1998) and allochthonous metasediments, which are possibly equivalent to the Synnfjell NC. It is structurally overlain by a unit of metasediments that contain a number of detrital and solitary metaperidotite bodies (see Section 2.1.3). The Lower Bergsdalen Nappe can be traced around the core of the Bjørnafjorden Antiform (Fig. 2), as originally defined by Kvale (1945).

Metaperidotite-bearing metasedimentary complexes
Between the Bergen Arcs and Lom ( Fig. 2) a prominent metasediment-dominated complex, which contains numerous mantle-derived metaperidotite lenses (< 2 km long) and local clastic serpentinites, including detrital breccias, conglomerates, and sandstones has been mapped (Andersen et al., 2012). The metasedimentary matrix is dominated by originally fine-grained sediments, now mica-schist and phyllite. Because of its mixed character, this unit has been non-genetically referred to as a mélange by Andersen et al., 2012;Jakob et al., 2017aJakob et al., , 2017b. However, to avoid confusion with other metaperidotitebearing mélanges, such as those that have been formed at the plate interface in subduction zones and because of its resemblance with reworked OCT assemblages in other mountain belts (Andersen et al., 2012;Jakob et al., 2017aJakob et al., , 2017bBeltrando et al., 2014), we refer to this unit as an OCT assemblage.
The OCT assemblage structurally overlies the WGR and Lower Bergsdalen NC. From the Major Bergen Arc and around the Bjørnafjorden Antiform, it can be traced below both the allochthonous crystalline rocks of the Lindås NC (Section 2.1.5), the Upper Bergsdalen NC (Section 2.1.4), as well as the main ophiolite/island-arc nappe complexes of the Iapetus (Section 2.1.6). From Stølsheimen across Sognefjorden, NE-wards to Lom (Fig. 2), the same OCT unit has been mapped continuously below the western flank of the Jotun NC.
The mostly pelitic metasediments also contain lenses of metaconglomerate and metasandstone as well as thin calcareous horizons, up to 40 km long and thin (< 1 km) discontinuous sheets of Proterozoic gneisses, minor gabbro and granodiorite of Late Cambrian to early Middle Ordovician age (487 to 471 Ma) and lenses of undated mafic rocks in the SW (Jakob et al., 2017b). Conglomerates and sandstones with a continental source (quartzite, vein quartz, granite clasts, one dated at 1033 Ma) indicate a Baltican-affine source (Andersen et al., 2012). Detrital zircons show that sedimentation continued at least into the Middle Ordovician (468 Ma) (Slama and Pedersen, 2015).
Late Scandian (~427 to 415 Ma) syn-orogenic granitoids intrude both the metasediments of the Major Bergen Arc, including the OCT assemblage, as well as the Jotun and Lindås NCs in western Norway (Austrheim, 1990;Jakob et al., 2017a;Wennberg et al., 2001;Lundmark and Corfu, 2007). The mafic and granitoid intrusives, which occur near the southern termination of the Jotun NC, in the Major Bergen Arc, and at Stølsheimen are unknown between Stølsheimen and Lom (Fig. 2).
The entire OCT assemblage experienced upper greenschist to amphibolite facies metamorphism during the Scandian Orogeny after 430 Ma (Jakob et al., 2017a(Jakob et al., , 2017b. Because of its characteristic lithological assemblage that resembles those of inverted magma-poor hyperextended margins in other orogens (e.g. Beltrando et al., 2014), the OCT assemblage is interpreted to have formed by pre-Scandian hyperextension and exhumation of subcontinental mantle or by the reworking of a magma-poor hyperextended rifted margin in the Ordovician (Andersen et al., 2012;Jakob et al., 2017b).

The Upper Bergsdalen and Blåmannen nappes
The Upper Bergsdalen NC represents a second sequence of allochthonous Baltican basement gneisses and metamorphosed continental margin sequences that are intercalated with Lower Palaeozoic phyllites and mica schists; similar to the Lower Bergsdalen Nappe (Kvale, 1945). The southern part of the Upper Bergsdalen NC is structurally overlain by the Jotun NC (Section 2.1.5), whereas south of Sognefjorden, the rocks of the Upper Bergsdalen NC trail out into the mica schists of the OCT assemblage (Fig. 2). The metaperidotites of the OCT assemblage, however, consistently occur structurally below the Upper Bergsdalen NC. The lower thrust sheets of the Upper Bergsdalen NC are dominated by crystalline gneisses whereas the upper thrust sheets are mostly composed of metasediments that are locally associated with mafic igneous rocks.
A meta-rhyolite of the Upper Bergsdalen NC was dated at 1219 ± 111 Ma (Rb-Sr whole rock analysis, Gray, 1978). The magmatic history of the Upper Bergsdalen NC is apparently similar to that of the Lower Bergsdalen Nappe. However, a number of undated mafic sheets and dykes cutting metasediments occur in both of the Bergsdalen nappes, and their complete Proterozoic and younger (?) intrusive history is not yet known. The Blåmannen Nappe in the minor Bergen Arc is another sliver of basement-cover rocks that structurally overlies the OCT assemblage. It consists of allochthonous crystalline basement that is unconformably overlain by a sedimentary sequence, including a tillite, that is suggested to have been deposited in the Proterozoic (Fossen, 1988(Fossen, , 1989.
Unlike the continental metasediments in the Osen-Røa, Kvitvola, Synnfjell and Valdres nappes, the Høyvik Group of the Dalsfjord NC contains a mid-ocean ridge-type mafic dyke-swarm and minor pillowbasalts at high stratigraphic levels (Andersen et al., 1998). The Høyvik Group and the dykes were deformed and metamorphosed before the deposition of the Middle Silurian (Wenlock) Herland Group (Andersen et al., 1998). 40 Ar/ 39 Ar cooling ages of phengitic mica in the Høyvik metasediments show that the deformation occurred before 447-449 Ma (Andersen et al., 1998;Eide et al., 1999). The Herland Group metasandstones and metaconglomerates are unconformably overlain by the Sunnfjord obduction mélange and the~443 Ma Solund-Stavfjord Ophiolite Complex (Andersen et al., 1990;Furnes et al., 1990;Dunning and Pedersen, 1988). The Herland Group deposition and transgression, and the deposition of the Sunnfjord mélange is interpreted to herald the obduction and emplacement of the Solund-Stavfjord ophiolite (Andersen et al., 1990;Skjerlie and Furnes, 1990).  Fig. 5. The Sunnfjord region encompasses the fjord region E and NE of the island Atløy, which is marked with "Høyvik Group".
The Lindås NC is another AMCG basement nappe of Baltican affinity, which is structurally positioned above the OCT assemblage (Fig. 2). The composition and age of the Lindås NC is similar to those of the Dalsfjord NCs. The north-western trailing end of the Lindås NC contains~430 Ma eclogites (Austrheim, 1990;Glodny et al., 2008) indicating an early Scandian deep burial and metamorphism of the Lindås NC. Unlike the Dalsfjord NCs, the Lindås NC contains minor Late Scandian (430-418 Ma) syn-orogenic granitoids (Austrheim, 1990;Wennberg et al., 1999, Kühn et al., 2002. The Jotun NC is a large sheet of crystalline mostly AMCG rocks that is similar to those discussed-above. On its western flank, the Jotun NC includes highly strained metasediments, which are referred to as the Turtagrø metasediments. The Turtagrø metasediments are similar to the sparagmites of the Valdres NC and are apparently also free of syn-rift magmatic rocks (Koestler, 1983). Locally, there are abundant Late Scandian syn-orogenic granitoid dykes (~427 Ma Årdal Dyke Complex) intruding the Jotun NC (Lundmark and Corfu, 2007).
In this study, we treat the Jotun, Dalsfjord and Lindås NCs as a large composite unit due to their similar AMCG-lithologies, geochronological fingerprints, and tectonostratigraphic position structurally above the OCT assemblage and below the outboard nappes of Iapetus and Laurentian origin.

Ophiolites and magmatic arc rocks of western Norway
The structurally highest Scandian thrust nappes of the SW Caledonides consist of a complex assemblage of ophiolite-island-arc and magmatic intrusive complexes (e.g. Andersen and Andresen, 1994). In the Major Bergen Arc the~489 Ma Gullfjellet ophiolite (Dunning and Pedersen, 1988) was emplaced above the Lindås NC, as well as the OCT assemblages (Fig. 2). The ophiolite/island-arc complexes occur again structurally above the Baltican-affine continental rocks between Hyllestad and Nordfjord, and structurally above OCT assemblages near the north-eastern termination of the Jotun NCs (see Section 2.2.4). The Dalsfjord NC and its sedimentary cover is structurally overlain by thẽ 443 Ma Solund-Stavfjord ophiolite, which was constructed on the remnants of early Ordovician ophiolite/island-arc in a back-arc basin setting . The Late Cambrian to Early Ordovician ophiolite island-arc complexes in the SW Caledonides are interpreted to have originated along the Laurentian margin of the Iapetus. They record a protracted history of subduction, arc-continent collision, volcanism and sedimentation, as well as Early-Caledonian metamorphism and deformation prior to Scandian thrusting of the nappes onto Baltica (e.g. Andersen and Andresen, 1994;Furnes et al., 2012).

Autochthon and Neoproterozoic syn-rift sediments with little to no syn-rift magmatic rocks
In the South-Central Caledonides (Fig. 3), the basement and minor (par)autochthonous metasediments are exposed in a series of tectonic windows, including the WGR, the Atnsjøen Window and the core of the Skardøra Antiform (e.g. Sjöström, 1984;Nystuen, 1987). The structurally lowest nappes are the Osen-Røa and Kvitvola nappes (Fig. 4), which preserve continental margin sequences that contain little to no syn-rift igneous rocks (see also Section 2.1.1). A few isolated minor occurrences of tholeiitic basalt can be found stratigraphically overlying quartzites of the Osen-Røa NC (Nystuen, 1987). Towards the north-east into Sweden, these (par)autochthonous and allochthonous continental margin successions can be correlated with the Dividal Group and the Risbäck NC (Törnebohm, 1896;Føyn and Glaessner, 1979;Gee et al., 1985).

Sheets of crystalline basement gneisses
Structurally above the proximal syn-rift to post-rift sediments of the Osen-Røa and Kvitvola NCs is a series of crystalline basement gneisses that can be traced from Norway across the Skardøra Antiform into Sweden (Fig. 3). In Sweden, east of the Skardøra Antiform (Fig. 3), the gneisses are referred to as the Tännäs Augen Gneiss. The Tännäs Augen Gneiss is Mesoproterozoic in age (~1685-1610 Ma, Claesson, 1980) and is locally mylonitised along tectonic contacts at its base and top. These gneisses are apparently without Ediacaran syn-rift intrusives.
West of the Skardøra Antiform, the gneisses can be traced as a thin band, at a consistent tectonostratigraphic level, along-strike into the Gudbrandsdalen Antiform (Fig. 3). Here, the gneisses are referred to as the Høvringen Gneiss Complex, Rudihø Crystalline Complex and Mukampen Suite (Gjelsvik, 1945;Strand, 1951;Lamminen et al., 2011;Heim and Corfu, 2017). The allochthonous gneisses of the Rudihø and the Mukampen Suite are 1700-1200 Ma and experienced high-grade metamorphism associated with some magmatism at 920 to 900 Ma (Lamminen et al., 2011;Heim and Corfu, 2017). A late tonalitic dyke cutting the Mukampen Suite was dated at~430 Ma (Heim and Corfu, 2017). Similar to the Tännäs Augen Gneiss east of the Skardøra Antiform, no Ediacaran syn-rift intrusives have been reported from these gneisses. Thus, the gneisses at Høvringen, Rudihø and Mukampen are similar in age, composition and metamorphic history to the Tännäs Augen Gneiss and some of the large crystalline nappes of the South Norwegian Caledonides.
The mafic dyke swarms and plutons in the Särv and Seve NCs, including the~596 Ma Ottfjället Dyke Swarm, are interpreted to represent Iapetus break-up magmatism (e.g. Andersen and Andresen, 1994;Kumpulainen et al., 2016, Tegener et al., in press). Regional studies of the Seve NC in Central and North Sweden show that pre-Caledonian continental margin-type metasediments in most parts are densely intruded by pre-Caledonian, Ediacaran mafic dyke swarms. These complexes are interpreted to represent the magma-rich segment of the Baltican rifted margin (e.g. Andersen and Andresen, 1994;Svenningsen, 2001;Tegner et al., 2016Tegner et al., , 2019). The regional geochemistry of the~1000 km long Scandinavian Dyke Swarm indicates that formation of the melts was related to a large igneous province (LIP) formed by a mantle plume associated with the Central Iapetus Magmatic Province (Tegner et al., 2019). The Seve NC also contains a number of solitary metaperidotite bodies and detrital serpentinites (e.g. Stigh, 1979), and the Ediacaran OCT is considered to be represented by the upper sections of the Seve NC (e.g. Andersen et al., 1991;Svenningsen, 2001;Kjøll et al., 2017, in press).
The metasediments of the Hummelfjell Group contain a number of undated mafic intrusives and volcanics, which traditionally have been correlated with the rift-related igneous rocks in the Särv and Seve NCs (Törnebohm, 1896;Holmsen, 1943). The number and volume of mafic igneous rocks within these Neoproterozoic successions decrease in south-westerly direction from the Särv and Seve NCs towards the Heidal Group. However, some mafic intrusives are reported from the upper sections of the Heidal Group (Gjelsvik, 1945;Strand, 1951). Gjelsvik (1945) also reported granitoid dykes cutting the mafic intrusives within the Heidal Group. However, none of these rocks have yet been dated.

Metaperidotite-bearing metasedimentary complexes
Between Vågåmo and the Skardøra Antiform (Fig. 3), metaperidotite-bearing metasediments structurally above the Heidal and Hummelfjell groups are referred to as the Sel Group (Bøe et al., 1993;Nilsson et al., 1997;Sturt et al., 1995) and Aursunden Group (Nilsen, 1988;Nilsen and Wolff, 1989). A lithological assemblage similar to those of the Sel and Aursunden groups also occurs in the Einunnfjellet Dome area (Fig. 3) overlying Neoproterozoic quartzites correlated with the Hummelfjell Group (Nilsen and Wolff, 1989;McClellan, 1994McClellan, , 2004. The mica schist matrix of the metaperidotite-bearing complexes between Vågåmo and the Skardøra Antiform are similar to the OCT assemblages further southwest, and contain both solitary and detrital metaperidotites, siliciclastic metaconglomerates and metasandstones as well as layers and lenses of turbidite-deposits. A major difference to the OCT assemblages between Stølsheimen and Lom is that the metaperidotite-bearing complexes between Vågåmo and the Skardøra Antiform also contain a large number of metamorphosed mafic bodies of unknown age. South of lake Rien (Fig. 3), an undeformed quartz diorite pluton intrudes schists of the Aursunden Group and contains xenoliths of the surrounding schists. Similar to some of the Late Scandian granitoids in the Major Bergen Arc (Jakob et al., 2017b) the granitoid at Rien contains euhedral magmatic epidote indicating emplacement of the granitoid at pressures above 4 kbar (Naney, 1983;Zen and Hammarstrom, 1984;Schmidt and Poli, 2004).
The Sel Group in the Gudbrandsdalen Antiform Ramsay, 1997, 1999) contains numerous discontinuous lenses of monomict detrital serpentinites. Near Otta, one locality also hosts an island-type Dapingian-Darriwilian fauna (Bruton and Harper, 1981;Harper et al., 2008), which shows that sedimentation at this stratigraphic level took place in the Early-Middle Ordovician. The Aursunden Group is also suggested to be of Cambrian-Ordovician age (Nilsen and Wolff, 1989).
Both the Sel and the Aursunden Group are considered to have been deposited on the uppermost metasediments of the Heidal Group as well as on sheets of mafic crystalline rocks at the base of the Ordovician metasedimentary complexes, which, in turn are supposed to have tectonic contacts with the Heidal and Hummelfjell Groups below (e.g. Sturt et al., 1991;Bøe et al., 1993;Nilsson et al., 1997;Sturt and Ramsay, 1999). Apparent depositional contacts between the metaperidotite-bearing complexes and the units structurally below are exposed, e.g., at Vågåmo and Hornsjøhøe (Fig. 3).

The Trondheim Nappe complex
The rocks of the Trondheim NC are dominated by three separate tectonic units, i.e. the Støren, Gula and Meråker nappes, all of which are composed of oceanic, ophiolite and island-arc assemblages (Guezou, 1978;Wolff, 1979;Gee et al., 1985;Nilsen et al., 2003Nilsen et al., , 2007Slagstad et al., 2014) (Fig. 3). All units of the Trondheim NC are intruded bỹ 440-430 Ma bimodal plutons (Dunning and Grenne, 2000;Nilsen et al., 2003Nilsen et al., , 2007. The Silurian plutons also intrude and are associated with older Cambrian-Ordovician ophiolite/arc rocks, e.g. in the Trondheim area or near Folldal, and with low-grade sediments containing Laurentian fossils as well as a Middle Silurian trondhjemite pluton, which contains inherited zircons of Archean age (Bruton and Bockelie, 1980;Nilsen et al., 2003Nilsen et al., , 2007Slagstad et al., 2014). Thus, the plutonic history of the Trondheim NC is similar to that of the ophiolite/island-arc complexes in the SW Caledonides (Section 2.1.6).
The Gula nappes were commonly believed to be of Baltican origin. However, the assumption of a Baltican origin of the Gula nappes was founded on Tremadocian graptolites and similarities in trace element geochemistry of black shales from the Gula nappes and the Cambrian (par)autochthon of Baltica (e.g. Gee, 1981). The Rhapdinopora flabelliformis sociale fossils in the so-called Dictyonema shales of the Gula nappes have also been described from the Tremadoc in Argentina, China, Belgium, and Newfoundland and are considered to be near cosmopolitan (e.g. Wang and Servais, 2015). Similarly, the high contents of V, Mo and U in black shales from the Gula nappes and the (para)autochthonous Cambrian-Ordovician of Baltica are rather indicators for the depositional environment than of provenance. Because of the lack of evidence for unequivocal Baltican origin and the common intrusive history in all nappes of the Trondheim NC as well as the faunal  indications for a Laurentian affinity in the western Trondheim NC, we consider the entire Trondheim NC to be exotic with respect to Baltica. Sturt et al. (1995) followed by Nilsson et al. (1997) suggested that the Sel and Aursunden groups are also unconformable on the Gula nappes. However, an unconformity below the Ordovician metasediments and on top of the Heidal and Hummelfjell groups, as well as their continuation into the Särv and Seve nappes, would stich the continental margin successions together with the oceanic assemblages of the Trondheim NC nappes as early as the Early-Middle Ordovician. The presence of such a terrane-link would require moving the Neoproterozoic metasediments of the Heidal and Hummelfjell Groups and as well the Ediacaran sediments of the Särv and Seve nappes structurally below the Trondheim NC before the deposition of the Sel and Aursunden groups. Moreover, the Laurentian, Baltican and Celtic faunas were highly diverse at the time of the deposition of the OCT assemblages in the Early-Middle Ordovician and did not unify before the Wenlock (Harper et al., 2009;Torsvik and Cocks, 2017a). We therefore consider it to be highly unlikely, and not demonstrated that the Sel and Aursunden Groups are unconformable on the rocks of the Trondheim NC in the Early-Middle Ordovician.

The Jotun Microcontinent
Because the Jotun as well as the Dalsfjord and Lindås NCs display similar AMGC lithologies and geochronological histories as the Baltican craton (Bingen et al., 2001;Andersen, 2002, 2015;Corfu, 2007, 2008;Roffeis and Corfu, 2014;Corfu and Andersen, 2015), these nappes are all considered to have a Baltican ancestry. However, because the Jotun and Lindås NCs structurally overlie the OCT assemblage, in theory, they could have been detached from the Baltican plate in the Ediacaran and moved independently throughout the Cambrian-Ordovician, or they may even be exotic with respect to Baltica and originate, e.g., from Gondwana or Laurentia. Unfortunately, there are no palaeomagnetic or other constraints on the Cambrian-Ordovician latitudinal position of these units and no fossils have been described from the metasediments associated with the crystalline rocks of the Jotun, Dalsfjord or Lindås NCs. Therefore, their plate tectonic history remains partly speculative and can only be inferred based on the lithological/geochronological dataset, its tectonic relationships with the other nappes and the lithostratigraphic correlations along the mountain belt.
An outboard origin of the large crystalline nappes of Southern Scandinavia would require that these rocks were near the leading edge of the upper plate during the Cambrian-Silurian, either near the Laurentian margin or the peri-Gondwanan terranes, see e.g. Domeier (2015) for a review of the closure of the Iapetus. However, the lack of magmatic arc rocks in the crystalline basement nappes of Southern Norway and the occurrence of Late Ordovician to early Silurian metamorphic rocks including eclogites in the Dalsfjord and Lindås NCs suggest that these NCs were part of the lower plate, i.e. Baltica, during the closure of the Iapetus.
Similar problems arise if we consider a scenario where these large basement nappes rifted off Baltica in the Ediacaran and subsequently moved independently of Baltica for~200 million years, until the Scandian Orogeny, just to be juxtaposed with each other and Baltica after the continental collision in the Silurian. In this context it is important to consider that the Iapetus did not close by simple orthogonal convergence, but involved large-scale clockwise and counter-clockwise rotations of Baltica as well as major changes in plate-motion directions throughout the Cambrian to Silurian (Cocks and Torsvik, 2011;Torsvik and Cocks, 2005;Domeier, 2015).
The radiometric ages of magmatic and metamorphic minerals from the crystalline nappes are similar to those of the Baltican autochthonous basement and are of Gothian age (1.7-1.6 Ga). The Gothian autochthonous domains closest to the crystalline nappes lie partly to the NE of the present-day position of the crystalline nappes. A SE directed emplacement of these NCs agrees with the Scandian kinematics (e.g. Fossen, 1993) and may indicate that the Baltican craton continued to the NW beyond the present-day North-Atlantic continental margin. Such a pre-Caledonian continuation of Baltica has previously been suggested by, e.g., Lamminen et al. (2011Lamminen et al. ( , 2015, but is inherently difficult to test due to pervasive overprint by Caledonian Orogeny and the limits of the present-day continental margin. Although a direct causal relationship is difficult to demonstrate, the NE termination of the Jotun NC near Vågåmo (Figs. 2, 3) correlates remarkably well with a major NW-SE trending change in the Baltican basement structure, which coincides with a Sveconorwegian lineament across southern Scandinavia (e.g. Kolstrup and Maupin, 2013;Frasetto and Thybo, 2013;Olesen et al., 2010). Whereas the outboard Caledonian nappes continue across this boundary, the transition from a magma-rich to a magma-poor domain also coincides with this lineament. We suggest that the magma-rich to magma-poor as well as the termination of the very large Baltican basement NCs both represent primary features of the pre-Caledonian margin of Baltica that most likely were inherited from the Middle Proterozoic structure of Baltica, which has been surprisingly little discussed in the large-scale architecture of the Scandinavian Caledonides.
By using the OCT assemblage as a reference level in the tectonostratigraphy, a first order architecture of the Pre-Caledonian margin of Baltica can be deduced by "unstacking" the nappes. Orthogneisses and metasediments of the Jotun NC structurally overlie the OCT assemblage on the western side of the Jotun NC. The OCT assemblage can be traced into the Gudbrandsdalen area where it structurally overlies the Heidal Group and sheets of basement gneisses (Figs. 3 and 4), which, in turn, structurally overly the proximal rift-basins, including the Osen-Røa, Kvitvola, Synnfjell and Valdres NCs. Therefore, before the inversion of the Caledonian margin of Baltican, a basin, which was floored by transitional crust (the OCT assemblage), separated the proximal basins and thinned continental crust to the SE from the rocks of the Jotun NC.
Whereas, the Neoproterozoic metasediments of the proximal basins structurally below the Jotun NC contain no syn-rift igneous rocks, the rocks of the Høyvik Group and the orthogneisses of the Dalsfjord NC contain mafic dykes and pillow basalts (Andersen et al., 1990(Andersen et al., , 1998Corfu and Andersen, 2002). We suggest that the dyke swarm in the Høyvik Group and other correlative units of the Dalsfjord NC indicate that these rocks represent the ocean facing NW (present-day coordinates) magma-rich rifted segment of a crystalline block(s) outboard of the OCT assemblage. The distal position with respect to Baltica is also indicated by the Middle Ordovician deformation and metamorphism which affected the Dalsfjord-Høyvik basement-cover pair beforẽ 449 Ma (Andersen et al., 1998), whereas the proximal part of the rifted margin in the south apparently was little affected by this event.
With regard to the points discussed above, we support the model that interprets the large crystalline basement nappes of South Norway as a former microcontinent. Although separated from the main Baltica continent by hyperextension and formation of the magma-poor OCTunit, it still formed part of the Baltican lithospheric plate in the period between the Ediacaran and the Silurian. An outboard palaeoposition of these continental blocks was already suggested by Andersen et al. (1991Andersen et al. ( , 2012 and Jakob et al. (2017b), who interpreted these continental units as part of a microcontinent or continental sliver, referred to as the Jotun Microcontinent (Andersen et al., 1991). We suggest that the microcontinent included the Dalsfjord, Lindås, and Jotun NCs, if not all the large AMCG-nappe complexes in Southern Norway.

The magma-rich to magma-poor transition zone
Almost all the Neoproterozoic sedimentary sequences that are structurally above the sheets of allochthonous Baltican basement, including the Lower Bergsdalen NC, Tännäs, Høvringen, Rudihø and Mukampen gneisses, contain mafic igneous rocks. Because, these metasediments are interpreted to represent Meso-Neoproterozoic pre-to syn-rift sediments that were deposited on the thinned Baltican craton, the mafic (and minor felsic) intrusions within these sedimentary sequences must be younger. The rocks of the Heidal Group, Hummelfjell Group, as well as the Särv and Seve nappes can be correlated by their petrology, depositional age, and tectonostratigraphic position. Several of them contain diamictites interpreted as tillites and some also contain newly discovered stomatolites (Kjøll et al. in press). Therefore, we follow the classical interpretations of Törnebohm (1896) and Holmsen (1943), that the mafic intrusives in the Neoproterozoic sequences of the Hummelfjell Group can be correlated with those in the Särv and Seve NCs, which were emplaced by LIP-magmatism (Tegner et al., 2019; in press) at~605-596 Ma (see Section 2.2.3 and Fig. 4).
The regional correlation of these units results in a relatively simple tectonostratigraphy for the South-Central Caledonides (Fig. 4), i.e. from base to top: 1) basement with cover (also in windows); 2) Neoproterozoic metasediments mostly without mafic igneous rocks; 3) a level of thin allochthonous basement gneisses; 4) Neoproterozoic metasediments with mafic igneous rocks; 5) Cambro-Ordovician metasedimentary complexes with abundant meta-peridotite bodies; and 6) the outboard ophiolite/island-arc complexes, including the Trondheim NC.
Although masked by some additional complexities, this simple tectonostratigraphy of the South-Central Caledonides can also be recognised in the South Caledonides. The structural position of the Upper Bergsdalen NC between the Jotun NC (above) and the metaperidotitebearing metasediments (below), suggests that it originated outboard of the transitional crust basin. Because the Upper Bergsdalen NC trails out into the Ordovician OCT assemblage near Sognefjorden (Fig. 2), it apparently was also separated from the Jotun Microcontinent; at least during the shortening of the margin but perhaps since the Ediacaran. Because of the presence of a mafic dyke swarm in the Høyvik Group of the Dalsfjord NC and a lack of mafic intrusions in the units structurally below the Jotun NC and in the metasediments of the Blåmannen Nappe, we suggest that the magma rich margin of Baltica was diverted to the outboard side of the Jotun Microcontinent. Because of the similarities of the Neoproterozoic succession, it is possible that mafic dykes in the quartzites of the Lower and Upper Bergsdalen NCs may also have been emplaced during the Ediacaran; although this remains to be substantiated by radiometric dating.
As discussed above, the rift-inherited domains of the Pre-Caledonian margin along strike of the orogen include a magma-rich part preserved in the Särv and Seve NCs a magma-poor part that is presently structurally below the remnants of the Jotun Microcontinent (Fig. 4). The Neoproterozoic continental margin successions between the magmarich part in the north-east and the magma-poor part in the south-west are characterised by a south-westerly decrease in the abundance of synrift mafic plutons and volcanics. We interpret this progressive reduction of mafic igneous rocks in the Hummelfjell and Heidal groups to represent a magma-rich to magma-poor transition zone that stretches for about 200 km from the Särv (Tossåsfjället basin) and Seve NCs to the north-eastern termination of the Jotun NC (Heidal Group). It is also noteworthy that the transition also coincides with the pre-rift Sveconorwegian lineament parallel to Gudbrandsdalen (see above; inset map in Figs. 2 & 6). The radiometric evidence as well as the pre-deformation and metamorphic relative ages of the mafic intrusives within the Seve and Särv NC, which correlates with the Hummelfjell Group, demonstrate that the magma-poor to magma-rich transition zone is a primary rift-inherited feature of the Central Caledonides and Ediacaran in age.

Correlation of Ordovician sequences in the South-Central Caledonides
The correlation of the tectonic units in the South and South-Central Caledonides presented above is further corroborated by the continuity of the peridotite-bearing OCT assemblages from the Bergen Arcs to the Skardøra Antiform (Figs. 2, 3, 4). The cross sections A to C (Figs. 2, 3, 5) demonstrate the consistent organisation of the nappe complexes (see Section 3.2). However, a complexity is added by the presence of the Jotun Microcontinent and the Bergsdalen NCs in the SW. The Neoproterozoic successions are not present between Stølsheimen and Lom. It is likely that these units were excised by the post-orogenic extension during exhumation of the WGR (e.g. Andersen et al., 1991;Fossen, 2010). However, the Cambrian to Ordovician metaperidotite-bearing OCT metasediments can be traced almost seamlessly between Bergen and the Skardøra Antiform. These Cambrian-Ordovician units can be correlated by the litho-and tectonostratigraphy and also a continuous metamorphic signature as well as their depositional age across the Gudbrandsdalen Antiform.

Early-Middle Ordovician reworking of an older rifted margin vs. an Ordovician extensional formation of the Ordovician units
Whereas the depositional and magmatic history of the Neoproterozoic metasedimentary complexes is relatively well-constrained, the origin and significance of the Cambro-Ordovician OCT assemblages is more uncertain due to the paucity of datable rocks in this unit. For the origin of the OCT assemblages three key characteristics must be addressed: (1) the resemblance with other OCT assemblages; (2) the duration of deposition of the matrix sediments into the Middle Ordovician (~470 Ma; Slama and Pedersen, 2015) and (3) the intrusion of minor mafic to granitoid plutons dated at 487-471 Ma (Jakob et al., 2017b). Two scenarios for the formation of the metaperidotite-bearing metasedimentary units might be proposed: (1) The OCT assemblage was formed by reworking of an older Ediacaran basin and OCT zone in the Late Cambrian to Middle-Ordovician; or (2) it was formed by thinning of the crust in the Late Cambrian to Middle Ordovician, which was accompanied or followed by minor intrusions.
In the first scenario, the reworking of an older OCT zone assemblage may have been linked to compression along the Baltican margin in the Late Cambrian to Middle Ordovician. The reworking of transitional crust inboard of the Jotun Microcontinent was accompanied by the emplacement of minor mafic to felsic igneous rocks into older sediments at 487, 476 and 471 Ma (Jakob et al., 2017b) and continued sedimentation with detrital zircons as young as 468 Ma into the Dapingian-Darriwilian (Bruton and Harper, 1981;Slama and Pedersen, 2015). Resetting of zircons at 482 Ma in the Øygarden basement window (Fig. 2) west of the Lindås NC (Wiest et al., 2018) may also be linked to this event.
Because, there is no radiometric evidence for Pre-Scandian penetrative deformation and metamorphism in the Baltican autochthon of South Norway except at Øygarden (Wiest et al., 2018), the Early-Caledonian reworking likely involved only the outermost part of the Baltica margin, including nappes that comprise the OCT in the magmarich part of the margin, e.g. the Seve NC, and along the western margin (Høyvik-Dalsfjord segment) of the Jotun Microcontinent.
Other indications for compression, uplift and erosion along the Baltican margin in the Early Ordovician are provided by 482 Ma eclogites in the northernmost Seve NC (Root and Corfu, 2012), the occurrences of turbidites that overly and are intercalated with Early-Middle Ordovician metapelites, which also include the Cr-and Ni-rich Elnes Formation in the Oslo region (Bjørlykke and Englund, 1979;Bruton et al., 2010), the Föllinge Formation in Sweden (Greiling and Garfunkel, 2007) and Cambrian-Ordovician successions of the proximal basins (Nickelsen et al., 1985). Moreover, from the Gudbrandsdalen area towards the north-east (Fig. 3), the OCT assemblage contains an increasing number of mafic bodies. Thus, the Ordovician units may reflect the increase of mafic igneous rocks of the underlying Neoproterozoic successions (Section 3.2) and may further support the notion that the metasedimentary complexes between Gudbrandsdalen and the Skardøra Antiform represent the remnants of the reworked outermost rifted margin of Baltica. However, except for one 618 Ma garnet (Cutts and Smit, 2018) no Ediacaran crystallisation ages have been reported from the OCT assemblage.
The closure of the OCT basin inboard of the Jotun Microcontinent and the reworking of the OCT assemblage, is comparable with the closure of narrow oceanic basins in the Alpine Tethys realm as described by Chenin et al. (2017). The difference in style of the pre-Scandian deformation and metamorphism in the South and the Central Caledonides may be directly linked to the presence of the large, strong and mostly intact Mesoproterozoic continental crust of the Jotun Microcontinent, which thwarted pre-Scandian deep burial and deformation compared to deep burial and high-pressure metamorphism of rocks in the Seve NC and along the westernmost Dalsfjord-Høyvik area.
However, except for the 618 Ma garnet, no other rocks in the OCT assemblages yielded Ediacaran ages that could be linked to the opening of the Iapetus whereas Lower Ordovician ages abound. And, because, the minimum age of some of the peridotite-bearing metasediments predate a minor 487 ± 2 Ma gabbro in the Bergen Arcs (Jakob et al., 2017a), these assemblages may have formed during a second phase of rifting in the Cambrian to Middle Ordovician (≥487-468 Ma). A modern-day analogue for this scenario could be the Tyrrhenian basin, that opened in the Pliocene-Quaternary during a phase of hyperextension and rifting after initial phase of opening of the Sardinia Province Basin in the Oligocene-Miocene (e.g. Prada et al., 2016;Savelli and Ligi, 2017). However, the Tyrrhenian opened in an upper plate, back-arc setting, for which there is little evidence in the Caledonides. None of the Baltican nappes are associated with an arc of that age and the (HPLT) metamorphism in the Høyvik-Dalsfjord and Seve NC rather indicate a lower plate configuration for the distal margin of Baltica.
The OCT assemblage may also have formed by thinning of a forearc basin and subsequent obduction of the Ordovician units onto the Ediacaran sequences. Forearc extension has been suggested for the highly dismembered south Tibetan ophiolites (Maffione et al., 2015). However, because of the lack of evidence for an (intraoceanic) arc along the Baltican margin of that time, the Baltican affinity of discontinuous slivers of crystalline gneisses within the OCT assemblage metasediments (Jakob et al., 2017a) and the structural position of the large crystalline NCs, it is difficult to explain the formation of the OCT assemblages in a forearc setting.
As an alternative to an upper plate configuration of Baltica, the OCT assemblage may have been formed with Baltica being the lower plate. On these terms the second stage of rifting and thinning may also have been related to the subduction of the northern part of the Baltica margin at about 482 Ma and may be comparable with the opening of the South China Sea (e.g. Morley, 2002;Clift et al., 2008;Bai et al., 2015;Larsen et al., 2018).

Early Scandian shortening of a wide Baltica rifted margin
The main large-scale nappe translation onto Baltica took place during the final continent-continent collision, and the penetrative deformation and (U)HP metamorphism of the Baltican basement occurred in the Late Silurian to Early Devonian, as demonstrated by the continuous SE-NW metamorphic gradient along the floor thrust and into the WGR (e.g. Hacker et al., 2010;Fauconnier et al., 2014;Jakob et al., 2017b). The outermost parts of the Baltican margin, however, may have experienced shortening as early as~450 Ma (see above). In Figs. 6 and 7 the palaeogeographic position of the basin with the OCT assemblage is constrained by the island-type Otta fauna, for which we estimate a minimum distance to the Baltican craton of about 1000 km, a distance great enough for the Otta fauna not to mix with the Baltican (or Laurentian) cratonic faunas. The Jotun Microcontinent is estimated to have had a minimum size of about 200 × 300 km based on the present extent of the Jotun, Lindås and Dalsfjord NCs. Thus, the distance between the outboard margin of the Jotun Microcontinent and the cratonic margin of Baltica was in the order of 1200 km.
Palaeo-plate tectonic models for the closure of the Iapetus Ocean (e.g. Domeier, 2015) indicate that a far outboard Jotun Microcontinent inboard of a seaway as well as hyperextended to rifted segments would have been in contact with the Laurentian cratonic margin at~450 Ma. The arrival of the Jotun Microcontinent at the Iapetan/Laurentian subduction zone is constrained by the deformation of the Høyvik-Dalsfjord and Seve NC at~450 Ma as well as by the eclogitisation of the Lindås NC at~430 Ma. The age constraints for the Scandian deformation are based on the obduction and thrusting of the~443 Ma Solund-Stavfjord back-arc ophiolite onto the fossil-bearing Wenlockian (433-427 Ma) Herland Group (see summary in Fig. 7). The shortening of the thinned margin was completed at the time the two necking domains of the Laurentian and Baltican continents collided, which coincided with the cessation of subduction-related magmatism, the earliest subduction of the WGR and the emplacement of syn-collisional granitoids in Baltican and Laurentian nappes, including the 430-415 Ma granitoids in the Norwegian allochthons (described-above) and 435-415 Ma granitoids on Greenland and Svalbard (Kalsbeek et al., 2008;Gasser, 2013).
The shortening of~1200 km of the Baltican margin between~450 and 435 Ma would have required convergence rates between the Laurentian and Baltican cratons of about 8 cm/yr, which is well within the limits of those of published plate tectonic models (e.g. Torsvik et al., 1996;Torsvik and Cocks, 2005;Domeier, 2015). Crustal thickening with maximum burial of the WGR and the thrusting onto the foreland, however, continued into the Lower Devonian.
The eclogites (446 Ma) in mafic dyke swarms hosted by the continental sediments of the Seve NC in Jämtland, Sweden, indicate that this part of the Seve NC was in a similar outboard position (≥1200 km) as the Jotun Microcontinent (Fig. 4). The even older HP metamorphic ages in the Seve NC further north (~480 to 460 Ma; Root and Corfu, 2012;Klonowska et al., 2017) may indicate that the onset of deformation along of the Baltican margin was oblique and diachronous, and that the northern part of the Baltican margin was affected before   Nystuen et al. (2008). Purple ellipses represent mantle exposed at the sea floor. The island type Otta fauna is placed 1000 km away from the Baltic craton margin. The size of the Jotun Microcontinent is approximately 200 × 300 km. Black circles indicate the palaeoposition of thrust nappes. Open circles indicate the palaeoposition of tectonic units and geological events at some time after onset of earliest Scandian inversion of the margin. EA, extensional allochthon; Eb, Engerdalen basin; HG, Heidal Group; Hb, Hedmark basin; JM, Jotun Microcontinent; NC, nappe complex; Tb, Tossåsfjället basin; Tm, Turtagrø metasediments; Vb, Valdres basin. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) the segments in the south. However, Early-Middle Ordovician faunas of Laurentia, Baltica and at Otta are distinct and the Iapetus probably was at its widest at this time (Torsvik and Cocks, 2017b). Therefore, alternative to an Early-Middle Ordovician incipient oblique closure of the Iapetus, the outermost Baltica margin may have experienced a collision (arc-continent?) in the late Cambrian-early Middle Ordovician. However, direct evidence for an arc arriving at the pre-Caledonian Baltica margin at that time is lacking.

Rift-inherited domains across-strike in the Scandinavian Caledonides
It is commonly suggested that rift-inherited structures in continental margins are reactivated during collision and have paramount influence on the architecture in mountain belts (e.g. Mohn et al., 2011Mohn et al., , 2014Vitale Brovarone et al., 2013;Epin et al., 2017). It is therefore, important to identify possible rift-inherited structures in the Caledonides and to include those into the tectonic evolution of the orogen.
The rift-inherited magma-rich and magma-poor segments are linked by a strike-parallel transition zone of approximately 200 km width (Section 3.2). Rift inheritance is also seen in transverse sections of the mountain belt. The consistency of the tectonostratigraphy and the characteristic lithological assemblages within the main tectonic units play a key-role in this interpretation (Figs. 4,5,6). In particular, the sediment-hosted metaperidotite-bearing assemblages represent a 'marker horizon' that links the South-West with the Central Caledonides. This OCT zone-remnants are at a consistent structural level and allow for a re-interpretation of the across-strike architecture of the mountain belt. The traditional use of Lower, Middle and Upper Allochthon is inadequate as previously outlined by Corfu et al. (2014), because, the tectonostratigraphy is inherited from the highly irregular rifted margin and is not a result of shortening of a continuous and uniform rifted margin. Therefore, the nappe stack is better described in terms of rift-domains defined by comparison with present-day margins, including the proximal and necking domains and as well as hyperextended and distal domains, with or without major magmatic components (e.g. Péron-Pinvidic et al., 2013).
The proximal/necking domain of the Scandinavian Caledonides includes the (par)autochthonous and allochthonous Neoproterozoic successions that contain little to no syn-rift magmatism, e.g. the Osen-Røa, Synnfjell, Dividal and Risbäck NCs (see also Fig. 4). These proximal rift basins record a dominantly siliciclastic input until the occurrence of minor mafic plutons and volcanics (Nystuen, 1983;Nystuen et al., 2008;Lamminen et al., 2011). After the early rift-phase and minor mafic magmatism, the sediment system changes from siliciclastic dominated to carbonate and carbonate-shale dominated (e.g. the Biri Formation). Similar carbonate and carbonate-shale successions are also reported from the rift basins of eastern Laurentia (e.g. Nystuen et al., 2008), which indicate a comprehensive rift-wide change of the system.
Relative changes in sea level move the sedimentary depo-centres either continent-ward or ocean-ward during transgression or regression events, respectively. However, changes in the tectonic activity comprehensively changes the sediment influx into the rift system (e.g. Mohn et al., 2010). For example, the cessation of tectonic activity in proximal rifted-margin basins is believed to coincide with and to be linked to the development of so-called thinning faults due to localization of extension in the future necking and distal domains and the onset of lithospheric break up (Mohn et al., 2010;Mohn et al., 2011). Therefore, the contemporaneous occurrence of carbonate and shale formations, immediately after a phase of minor mafic magmatism, that seal the previously deposited, siliciclastic, main-rift sequences in many proximal rift basins along the Baltican and Laurentian margins, may indicate the cessation of tectonic activity within these proximal basins. We suggest that the proximal basins record an early rift-phase of initial distributed extension until the localization of extension in the future necking and distal domains and that the localization of extension was broadly contemporaneous with the syn-rift magmatism.
With the exception of the nappes comprised of continental metasediments structurally below the Jotun NC, the proximal basins are consistently overthrust by a series of thin crystalline basement nappes with Baltican affinity (Lower Bergsdalen Nappe, Tännäs, Høvringen, Rudihø and Mukampen gneisses). A simple restoration of these nappes require that these gneisses originally were positioned outboard of the proximal domain of the margin. Moreover, their consistent structural position indicates that they represent a regional structural element in the continental margin rather than local imbrications. In present-day passive margins, the hyperextended domain (if identified) is positioned between the necking domain and the zone of exhumed mantle in magma-poor margins, or inboard the zone of main syn-rift magmatism in magma-rich margins (e.g. Péron-Pinvidic et al., 2013;Abdelmalak et al., 2017). Because there is little evidence for Ediacaran magmatism reported from these basement nappes, and because of their structural position between the proximal basins and the Neoproterozoic successions, in which syn-rift igneous rocks abound, we suggest that these gneisses represent rift-inherited thinned continental crust (≤10 km), that were outboard of the necking domain after rifted margin formation.
In the magma-poor to magma-rich transition zone and the magmarich segment of the margin, these gneisses are overlain by Neoproterozoic successions containing abundant syn-rift magmatic rocks, which we interpret as the distal domain of the rifted margin. In the magma-poor segment of the margin the distal domain is characterised by metaperidotite bearing units that are dominantly composed of fine grained metasediments but also include coarser grained metasediments and slivers of continental crust (extensional allochthones). In the South Caledonides, those distal domain assemblages are structurally overlain by the Jotun microcontinent (Figs. 3, 4, 5, 6).

The importance of the rift-inherited margin architecture during the Scandian orogeny
By comparing our observations from the Caledonides with studies in the Alps, we find that structures inherited from the rifted margins were reactivated and developed as major 1st order thrust systems during the orogenic shortening of the Baltica margin (e.g. Jakob et al., 2017b;Manatschal, 2004;Mohn et al., 2011Mohn et al., , 2014Epin et al., 2017) (Fig. 8). An imbrication of the rift domains was likely accommodated by smaller 2nd order thrusts exploiting discontinuities within the units, e.g. changes in rheology or along rift-inherited faults. The thrusting during the main orogenic events which probably were separated in time (Ordovician and Silurian to Devonian) was apparently in sequence, because, the stacking-order of Baltican nappes reflects cross-sections of the pre-Caledonian margin. Therefore, for simplicity, the shortening of the Baltica margin is in the following depicted as a single phase of shortening, neglecting possible pre-Scandian tectonism and metamorphism of the outermost margin.
In the Caledonides, nappes that contain the outermost margin of Baltica including the OCT, the extensional basement allochthons, exhumed meta-peridotites, probably also embryonic oceanic crust (or seamounts) at Vågåmo and Røros, as well as other dismembered ophiolites were emplaced onto the Neoproterozoic successions that host the rift-related mafic dyke swarms. Consecutively, the assemblages of the magma-rich and the magma-rich to magma-poor transition zone were thrust over thinned continental crust of the distal domain. The nappes of the distal domain were, in turn, thrust over the Neoproterozoic successions of the proximal/necking domain by a thrust system, which may represent the reactivated thinning faults of the necking domain. Internal imbrication of the individual domains was accommodated by sub-sets of thrust with smaller offsets.

Summary and conclusions
New data and field observations as well as re-interpretations based on a modern understanding of present-day continental margins, put new constraints on the evolution and architecture of the pre-Caledonian margin of Baltica. We suggest that the major differences along strike in the mountain belt originated by the highly irregular and discontinuous template related to the formation of the pre-Caledonian margin of Baltica. The most important change occurred where the large (> 200 × 300 km) Jotun Microcontinent rifted away from Baltica in the Neoproterozoic. The NE-termination of the microcontinent may have been inherited from a Middle Proterozoic basement structure, because, the termination of the crystalline nappes correlates with the trace of the Sveconorwegian lineament across southern Scandinavia (Figs. 1 and 7). This structure appears to be a fundamental lithospheric lineament in Scandinavia as seen by the change from shallow to deeper MOHO from SW to NE as well as magnetic anomaly studies (Kolstrup and Maupin, 2013;Frasetto and Thybo, 2013;Olesen et al., 2010). This pre-Caledonian lineament also coincides with the magma-poor to magma-rich segmentation along the continental margin as describedabove. We suggest that large-scale discontinuities in the Sveconorwegian basement across south Scandinavia were important structural elements both during the construction of the pre-Caledonian margin of Baltica as well as during the Caledonian plate-convergence and Scandian collision.
This study shows that the present-day tectonostratigraphy of the South and South-Central Caledonides was formed by the orogenic shortening of a highly irregular, Ediacaran, pre-Caledonian, rifted margin of Baltica. The nappe stack from its base to the top reflects a cross section from proximal to distal rift domains. A summary of observations and interpretations presented above include: a) After the post-Sveconorwegian assembly of Rodinia, followed a long (~200 Ma) period of attempted continental rifting, widespread stretching of the shield area and deposition of thick sedimentary successions through the Cryogenic and into the Ediacaran as described by Nystuen et al. (2008). b) The continental break-up and the eventual formation of the pre-Caledonian continental margin of Baltica may have been associated with the arrival of a mantle plume and widespread plume-magmatism at~615-595 Ma (e.g. Bingen et al., 1998;Svenningsen, 2001;Baird et al., 2014;Tegner et al., 2018, in press;Kjøll et al., in press). c) Most of the pre-Caledonian margin of Baltica facing the Iapetus Ocean, including the less-well preserved westernmost margin of the Jotun Microcontinent was apparently magma-rich. However, inboard of the Jotun Microcontinent opened a magma-poor basin and seaway that was floored by hyperextended to transitional crust (Andersen et al., 2012;Jakob et al., 2017b). Rift-related mafic igneous rocks have not been identified in this basin or in the adjacent autochthon of Baltica except for the mafic~615 Ma dyke swarm in the Egersund area (Bingen et al., 1998). d) The along-strike transition from the magma-rich to the magma-poor part took place over an approximately 200 km long orogen-parallel zone between Røros and Vågåmo. This magma-rich to magma-poor transition zone is preserved in the Neoproterozoic successions of the Hummelfjell and Heidal Groups, which represent a continuation of the Särv and Seve NCs into Norway. Additional elements of the magma-rich to magma-poor transition are the incipient formation of oceanic crust in the OCT zone, which locally may be preserved between Vågåmo and Røros as well as in the continuation of OCT assemblage into Sweden (Nilsson and Roberts, 2014). e) A poorly-understood early subduction and shortening of the outermost Baltica margin may have occurred already during latest Cambrian to the Middle Ordovician and affected mainly the Seve NC. This pre-Scandian event may have been associated with or coincident with the reworking of the older hyperextended margin or a second phase of extension in the South Caledonides. f) Major shortening of the Baltican margin started at about 450 Ma when the outermost parts of the very wide (≥1200 km) Baltican rifted margin entered subduction zone(s) in front of Laurentia. g) Deformation in the proximal/necking domains as well as the largescale nappe translation over the Baltican craton, Scandian metamorphism and associated granite magmatism took place during the Scandian Orogeny (continent-continent collision) in the late Silurian (after~430 Ma) into the Early Devonian. h) The across-strike architecture of the nappe stack can be attributed to the stacking of rift domains. In the Central Caledonides, the stacked rift domains, from top to base, include the distal margin with the fossil OCT and break-up magmatism, remnants of the hyperextended domain and proximal rift basins. In the South Caledonides, the nappe stack also includes the Jotun Microcontinent thrust over the remnants of a failed rift hyperextended basin, floored by transitional crust. In the NE it is transitional into magma-poor to magma-rich transition zone and overlies the proximal Neoproterozoic basins. In the SW, the Upper and Lower Bergsdalen NCs, near the southern termination of the Jotun Microcontinent, were originally outboard and inboard of the hyperextended basin, respectively, and all units were thrust over the proximal basins. All of the rift-inherited tectonic units are structurally overlain by the outboard nappes with origins in the Iapetus and Laurentia.