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Journal Pre-proofStructural and metamorphic inheritance controls strain partitioning during orogenicshortening (Kalak Nappe Complex, Norwegian Caledonides)A. Ceccato, L. Menegon, C.J. Warren, A.M. HaltonPII:S0191-8141(19)30548-6DOI: 104057To appear in:Journal of Structural GeologyReceived Date:26 December 2019Revised Date:1 April 2020Accepted Date:2 April 2020Please cite this article as: Ceccato, A., Menegon, L., Warren, C.J., Halton, A.M., Structural andmetamorphic inheritance controls strain partitioning during orogenic shortening (Kalak NappeComplex, Norwegian Caledonides), Journal of Structural Geology (2020), doi: is a PDF file of an article that has undergone enhancements after acceptance, such as the additionof a cover page and metadata, and formatting for readability, but it is not yet the definitive version ofrecord. This version will undergo additional copyediting, typesetting and review before it is publishedin its final form, but we are providing this version to give early visibility of the article. Please note that,during the production process, errors may be discovered which could affect the content, and all legaldisclaimers that apply to the journal pertain.© 2020 Published by Elsevier Ltd.
Author Statement Alberto Ceccato: conceptualization, investigation, formal analysis, data curation, visualization, writing – original draft, review & editing; Luca Menegon: conceptualization, investigation, resources, project administration, funding acquisition, writing – original draft, review & editing; Clare J. Warren: investigation, resources, formal analysis, validation, data curation, writing – review & editing; Alison M. Halton: investigation, resources, formal analysis, validation, data curation.
Structural and metamorphic inheritance controls strain partitioning during orogenic shortening 1 (Kalak Nappe Complex, Norwegian Caledonides). 2 A. Ceccatoa,1, L. Menegona, b, C. J. Warrenc, and A. M. Haltond 3 aSchool of Geography, Earth and Environmental Sciences, University of Plymouth, 4 Plymouth, United Kingdom. 5 bThe Njord Centre, Department of Geosciences, University of Oslo, Oslo, Norway 6 cSchool of Environment, Earth & Ecosystem Sciences, Open University, Milton Keynes, 7 United Kingdom. 8 d School of Physical Sciences, Open University, Milton Keynes, United Kingdom. 9 1Present address: Dipartimento di Scienze Biologiche, Geologiche ed Ambientali, Università 10 di Bologna, Italy 11 12 Corresponding author: 13 Authors e-mail:;; 14 15 Keywords: Structural inheritance; Strain partitioning; Kalak Nappe Complex; Norwegian 16 Caledonides; Shear-parallel folds 17 18
Abstract 19 The occurrence of pre-collisional structural and metamorphic fabrics may control the 20 development of new structures during subsequent orogenic deformation. Structural, 21 petrological and geochronological analyses have been performed on selected samples 22 collected along a NW-SE cross section of the Kalak Nappe Complex (KNC) exposed on 23 Kvaløya Island (Finnmark, Norway), in order to define pre-Caledonian or Caledonian affinity 24 of deformation fabrics. Nappes within the KNC experienced different pre-collisional 25 tectonometamorphic histories, resulting in contrasting pre-Caledonian fabrics, which in turn 26 controlled orogen-scale strain partitioning and metamorphic re-equilibration during 27 Caledonian shortening. Caledonian deformation during top-to-SE-directed thrusting occurred 28 at 550-675°C and 0.8-1.0 GPa in the presence of fluid. Suitably-oriented pre-collisional 29 fabrics were firstly exploited as zones of localized shearing internal to the Nappe and 30 subjected to metamorphic re-equilibration during shortening. Fold geometry during 31 Caledonian thrusting was also controlled by the orientation of pre-Caledonian fabrics. SE-32 verging asymmetric folds were developed after minor tilting of pre-Caledonian upright folds 33 with orogen-parallel hinge in the hinterland consistently with top-to-SE shearing. Shear-34 parallel folds displaying orogen-perpendicular hinge lines resulted from top-to-SE general 35 shearing of pre-collisional upright folds showing pre-collisional orogen-perpendicular hinge 36 lines. Caledonian metamorphism appears to have been accompanied by infiltration of 37 radiogenic 40Ar-rich fluids, which affected the Ar isotopic system in synkinematic micas. 38 39 1. Introduction 40 Pervasive ductile deformation in the middle and lower crust controls the evolution of the deep 41 portions of orogenic wedges and the accommodation of deep crustal shortening during large-42
scale thrusting and nappe stacking and folding (Escher and Beaumont, 1997; Williams and 43 Jiang, 2005; Beaumont et al., 2006; Culshaw et al, 2010; Bastida et al., 2014; Wex et al., 44 2017). During orogenesis, thick-skinned tectonics involves the deformation of crystalline 45 basement units that are commonly characterized by a multi-stage pre-collisional 46 tectonometamorphic history (Audet and Bürgmann, 2011; Faber et al., 2019). The prolonged 47 deformation and metamorphic history of basement units is reflected in the development of 48 multiple and variably oriented fabrics. The occurrence of multiple different fabrics formed 49 under different conditions at different times makes untangling the tectonometamorphic 50 history of basement units very difficult, however this is critical for constraining the different 51 spatial and temporal tectonic processes that take place during either (multiple) orogenic 52 cycles or progressive deformation during the same orogenic cycle. These basement units 53 provide an ideal natural laboratory to analyse how pre-collisional inherited deformation 54 fabrics influence strain accommodation and partitioning during a subsequent orogenic 55 episode (Audet and Bürgmann, 2011; Mouthereau et al., 2013). 56 Structural inheritance has been shown to affect orogenic shortening both at brittle upper 57 crustal levels (Butler et al., 2006; Massironi et al., 2011; Beltrando et al., 2014; Fossen et al., 58 2017), as well as at ductile, mid- to lower-crustal conditions (Vauchez and Barruol, 1996; 59 Jammes and Huismans, 2012; Mouthereau et al., 2013). Similarly, pre-collisional 60 metamorphic history may strongly influence the behaviour of basement units during later 61 continental collision, especially those typically characterized by dry and rheologically strong 62 lithologies (Yardley and Valley, 1997; Austrheim, 2013). The definition of the pre-63 collisional tectonometamorphic history of basement units and related deformation-64 metamorphic fabrics is therefore of fundamental importance for the analysis and 65 interpretation of strain partitioning and accommodation processes during continental 66 collision. Careful investigations of low-strain domains within nappe complexes can provide 67
information on the different P-T conditions and timing of fabric development, as well as on 68 the geometry of strain. Such analyses are crucial to the understanding of the evolution of 69 polyorogenic basement units and of the role of the pre-collisional history on the architecture 70 of mountain belts (Ridley, 1989; Babist et al., 2006; Kirkland et al., 2006b; Manzotti and 71 Zucali, 2013; Gasser et al., 2015; Faber et al., 2019). In addition, petrological and 72 geochronological analysis of regional-scale fabrics are pivotal for the correlation of 73 deformation stages, the correct interpretation of geochronological age dating and for the 74 inference of orogen-scale tectonic processes (Warren et al., 2012; Skipton et al., 2018). 75 Here we present the results of field structural, petrological and in-situ 40Ar/39Ar 76 geochronological analysis on a NW-SE cross section of the Kalak Nappe Complex (KNC) of 77 the Norwegian arctic Caledonides in the Kvaløya Island (Finnmark, Norway). This cross 78 section exposes polyorogenic units of the KNC that were deformed under mid-lower crustal 79 conditions during the Caledonian orogeny. The units were only partially re-equilibrated and 80 reworked during Caledonian shortening, and therefore preserve large-scale low-strain 81 domains (similar to those reported by Gasser et al., 2015). The studied section is parallel to 82 the Caledonian top-to-SE thrusting direction, and provides an ideal natural laboratory for 83 investigating how pre-collisional fabrics affected the accommodation of increasing strain 84 during shortening. We integrated field observations and structural analysis of different pre- 85 and collisional fabrics with P-T-fluid condition estimates in order to define: (i) the 86 metamorphic evolution of the KNC in the study area, and (ii) how the strain distribution and 87 the geometric style of shortening of a polyorogenic basement nappe depend on the orientation 88 and metamorphic history of pre-collisional fabrics. We find that the KNC nappes underwent 89 two different cycles of tectonometamorphic evolution prior to the Caledonian orogeny. The 90 orientation of the previous fabric affected how strain and metamorphism occurred in the 91 KNC units during the Caledonian collision. The study area shows an abrupt change in 92
orientation of a stretching lineation from orogen-parallel to orogen-perpendicular towards 93 the foreland. We show that this switch is due to the different orientation of pre-collisional 94 fabrics and of the resulting Caledonian folds, with shear-parallel folds dominating in the 95 foreland part of the section. Finally, we infer that in-situ 40Ar/39Ar dates of synkinematic 96 micas from both pre-collisional and collisional fabrics do not directly reflect the timing of 97 crystallization, deformation or cooling related to fabric development or tectonic exhumation. 98 Instead, they appear to have been affected heterogeneously by Ar-rich fluid infiltration 99 during Caledonian metamorphism. 100 101 2. Geological setting 102 The Scandinavian Caledonides formed by the convergence and continental collision between 103 Baltica and Laurentia, which followed the subduction and closure of the Iapetus Ocean, 104 during the Ordovician to Devonian (Roberts, 2003). Ocean closure and continental collision 105 resulted in the stacking of different allochthonous nappes with, from bottom to top, Baltica, 106 Iapetus and Laurentia affinity (Roberts, 2003). Traditionally, the nappe stack has been 107 subdivided into Lower, Middle, Upper and Uppermost Allochthon units (Roberts, 2003; Gee 108 et al. 2008). The deep erosional level of the hinterland of the Scandinavian Caledonides 109 provides direct access to middle- and lower crustal sections subjected to intense collisional 110 deformation and metamorphism in an orogen of Alpine-Himalayan style (Gee et al., 2008). 111 The geology of the Scandinavian Caledonides in Finnmark, northern Norway, consists of 112 windows of Baltica basement with its metasedimentary autochthonous units, overlain by 113 parauthochtonous metasediments (Fig. 1a) (Roberts, 2003). Above the parautochthonous 114 units, the stack of allochthonous nappes that were thrusted eastward over Baltica is 115 dominated by the Kalak Nappe Complex (KNC), a composite nappe pile consisting of several 116
(Kirkland et al., 2006a; 2007b). The contact between the two units is probably of tectonic 140 origin, as supported by recent studies on zircon provenance (Kirkland et al., 2007b, 2008b). 141 The upper nappe is composed of paragneisses and metasediments of the Sørøy succession, 142 which includes the Eidvågeid series (dominated by migmatitic paragneisses), the Klubben 143 psammites, and the Storelv schists (Kirkland et al., 2005). The Sørøy succession was 144 deposited between 910-840 Ma (Kirkland et al., 2007b), and was deformed, metamorphosed 145 and intruded by granitic melts during the Porsanger Orogeny at c. 850-820 Ma and during the 146 Snøfjord event at c. 710 Ma (Kirkland et al., 2006a, 2006b, 2007b; Corfu et al., 2007; Gasser 147 et al., 2015).The Sørøy succession was also intruded and locally migmatised and deformed 148 between 580-520 Ma by the Seiland Igneous Province (SIP), a series of mafic to ultramafic 149 and alkaline intrusions related to the rifting of the Iapetus Ocean (Daly et al., 1991; Elvevold 150 et al., 1994; Roberts et al., 2006; Menegon et al., 2011). 151 The exact timing and the origin of the juxtaposition between the lower and the upper nappe of 152 the KNC are still debated (e.g. Corfu et al., 2007). It has been suggested that the original 153 unconformity (now tectonized) between the underlying Sværholt succession and the 154 overlying Sørøy succession is preserved on Hjelmsøy (Kirkland et al., 2008b), and that the 155 juxtaposition was pre-Caledonian, as both successions were affected by the c. 710 Ma 156 Snøfjord tectonomagmatic event (Kirkland et al., 2006a). 157 The internal structure of the KNC has been traditionally described to result from several 158 discrete deformation stages (“5 or 6 folding events” of Zwaan and Roberts, 1978; Rhodes and 159 Gayer, 1977; Rice, 1998) during Neoproterozoic and Caledonian orogenesis. Gayer et al. 160 (1985) and Kirkland et al. (2006b) identified 5 deformation stages (D1 – D5) based on fold 161 geometry and orientation, and on crosscutting relationships between fabrics and granitic 162 intrusions of known age. Whilst D1 is only very rarely preserved, D2 is widely preserved in 163
muscovite grains, after which gases were cleaned with 2 SAES AP-10 getters running at 211 450°C and room temperature. Detailed description of analytical conditions and data 212 processing for all analytical techniques can be found in the Supplementary Material. 213 4. Field observations: the Kalak Nappe Complex on Kvaløya 214 4.1. Lithologies and tectonostratigraphy 215 The map of the study area (Fig. 1b) is a revision of existing map material (1:50000 216 Hammerfest sheet: Jansen et al., 2012) based on our field observations. The lower nappe on 217 Kvaløya (Fig. 1b-c) is mainly composed of: (i) the Fagervik gneissic complex, (ii) 218 metasediments of the Sværholt succession (metapsammites/metapelites), and (iii) an 219 undifferentiated banded gneiss unit, composed predominantly of high-grade metapelites, 220 amphibolites and metapsammites. The Fagervik complex (Fig. 2a-b) is characterized by 221 distinctive banded quartzo-feldspathic gneisses intercalated with amphibolites and with rare 222 ultramafic layers and pods. Quartzo-feldspathic gneisses include paragneisses, and 223 orthogneisses, locally showing mylonitic fabrics. Up to 2 metres thick bands of garnet-rich 224 amphibolites are common (Figs. 2b, SM1a). The metasediments of the Sværholt succession 225 are dominated by metapsammites and minor biotite-rich metapelites (Fig. SM1b). The 226 banded gneiss unit has a distinctive rusty colour on outcrop and includes different lithologies 227 arranged in bands that range in thickness from < 5 cm to 1-2 m (Fig. 2c). The most common 228 lithologies of this unit are quartzites, psammites, high-grade (aluminosilicate-bearing) 229 micaschists, migmatitic garnet- and biotite-rich schists, and garnet-rich amphibolites (Fig. 230 SM1c). The banded gneiss unit is laterally discontinuous, and it is observed at both contacts 231 between the lower- and the upper nappe (Fig. 1b). 232 The Fagervik complex is the lowermost unit of the area and represents the base of the lower 233 nappe. We grouped the “rusty” banded gneiss unit together with the Fagervik complex, based 234
on the occurrence of distinctive layers of garnet amphibolites of similar thickness in the two 235 units (Fig. SM1a,c), and on the consistent structural style displayed by all lithologies (see 236 next chapter). Furthermore, the Fagervik gneisses and the banded gneiss units show a strong 237 lithological affinity with the Fennoscandian (Baltica) basement, such as the West Troms 238 Basement Complex as described in Bergh et al. (2014) and Gee et al. (2017). 239 In the study area, the upper nappe is dominated by the migmatitic paragneisses of the 240 Eidvågeid Series (Rice and Roberts, 1988; Rice, 1990; Corfu et al., 2007; Gasser et al., 2015; 241 Faber et al., 2019), with minor bands of psammites tentatively correlated with the Klubben 242 psammite (Fig. 1b). Stromatic migmatites are defined by up to 10-20 cm thick quartzo-243 feldspatic leucosomes alternating with biotite-sillimanite-rich melanosome. The migmatites 244 contain large clusters and individual crystals of dark pink garnet (up to 10 cm in diameter), 245 typically surrounded by thin (< 1 cm thick) feldspar-rich rims. The unit also contains 246 intercalated minor bands of calcsilicates and marbles, as well as several pegmatitic intrusions 247 predominantly discordant to the stromatic banding. Towards the boundary with the lower 248 nappe, migmatitic paragneisses make transition to muscovite-rich paragneisses and minor 249 schists. These rocks contain up to 2-3 cm long mica-fish of muscovite and dark pink garnet 250 porphyroclasts. The stromatic migmatites and the muscovite-rich paragneisses are 251 intercalated with more psammitic bands ranging in thickness from 5 m to about 50 m. 252 253 4.2. Structural domains 254 Along the analysed NW-SE cross section in Kvaløya, the KNC can be subdivided in three 255 main areas, where the lower- and the upper nappe show different structures and internal 256 deformation features (Fig. 1b, d). In northern Kvaløya, the lower nappe is characterized by 257 NE-SW trending folds and by subhorizontal NE-SW trending stretching lineations parallel to 258