Geological Survey of Denmark and Greenland Bulletin 11, 163-178 163 Structural analysis of the northern Nagssugtoqidian orogen, West Greenland: an example of complex tectonic patterns in reworked high-grade metamorphic terrains Stanislaw Mazur, Sandra Piazolo and G. Ian Alsop Structural analysis of the deeply eroded northern flank of the Palaeoproterozoic Nagssugtoqidian orogen shows marked regional variations in both the orientation and type of fabrics, as is characteristic of Precambrian high-grade terrains subjected to polyphase deformation. Here we investigate the rela- tionship between strain, metamorphic grade, and the resulting structural patterns. The study area south of Aasiaat in West Greenland consists of amphibolite- to granulite-grade Archaean orthogneisses and relatively thin supracrustal units. The regional foliation displays a WSW–ENE to SW–NE strike associated with steep to moderate dips towards the WNW or SSE. Lineation trends are WSW–ENE and generally plunge gently towards the WSW. Mesoscopic fold hinges are usually colinear with the regional lineation. A systematic change in the plunge of lineations occurs across the south-western part of the study area. Towards the south, the lineation plunge progressively increases, despite the generally uniform strike of foliation. This southward increase of lineation pitch is typically associated with the transition from L > S or L = S shape fabrics in rocks characterised by a low pitch, to S > L or S fabrics in the zone of moderate to high pitch. The structural patterns point to subdivision of the study area into a southern domain mostly characterised by S or S > L shape fabrics and a moderate to high angle of lineation pitch, and a northern domain showing L > S or L = S fabrics and low angles of lineation pitch. This subdivision corresponds well with the map scale boundary between granulite facies rocks in the south and amphibolite facies rocks farther north. The observed structural pattern may be explained by two alternative tectonic models: (1) northward indentation of the previously cooled granulite block into the rheologically weaker amphibolite domain, and (2) strain partitioning within a mid-crustal transpression zone. In model 2 the northern domain represents a localised zone dominated by strike-slip kinematics, whereas the southern domain shows evidence of mostly coaxial shortening. Recent geochronology supports the indentator model in spite of limited available data. Despite the details and structural complexities of the two tectonic models, the granulite and amphi- bolite facies domains seem to form autochthonous segments of a crustal section linked by a transition- al zone that was only reactivated and reworked during indentation or transpression. The Nagssugto- qidian compression was effectively transferred across this zone towards the northern amphibolite do- main that suffered penetrative deformation during the Palaeoproterozoic event. The N–S shortening was accommodated through folding, indentation and/or strike-slip displacements, rather than by thrusting and folding as seen south of the study area. Keywords: deformation, GIS, Nagssugtoqidian orogen, transpression, indentation tectonics, West Greenland ____________________________________________________________________________________________________________________________________________________________________________________ S.M., Institute of Geological Sciences, University of Wroclaw, Maxa Borna 9, 50-204 Wroclaw, Poland. E-mail: smazur@ing.uni.wroc.pl S.P., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. Present address: Department of Geolog y and Geochemistry, Stockholm University, 10691 Stockholm, Sweden. G.I.A., School of Geography and Geosciences, University of St. Andrews, Fife KY16 9AL, UK. © GEUS, 2006. Geological Survey of Denmark and Greenland Bulletin 11, 163–178. Available at: www.geus.dk/publications/bull 164 50 km 69° 67° 54° 50° Disko Bugt B A N ag ss ug to qi di an o ro ge n Inland Ice NNO SNO CNO xxxx 68°15' Kangaatsiaq Tunorsuaq 53° 53° 52°30' 52°30' 53°30' 53°30' 53° 52°30'53°30' Qeqertarsuatsiaq 10 km NaternaqNaternaq Naternaq SaqqarputAtaneq Ar fer s io r f ik Amphibolite facie s Naternaq 500 km Surficial deposits Quaternary Basalt Palaeogene Amphibolite Nagssugtoqidian orogen Sisimiut charnockite Arfersiorfik quartz diorite Archaean orthogneiss, reworked Metasedimentary rocks (mainly Archaean) Metasedimentary rocks (mainly Palaeoproterozoic) Granodioritic-granitic gneiss Orthogneiss (unreworked) Archaean craton Orthogneiss, mainly tonalitic Granitic gneiss Metagabbro-anorthosite Metamorphic transition zones Quartzo-feldspathic metasediment Mica schist and amphibolite Quaternary marine deposits Palaeogene dolerite dyke Granulite fa cie s Granulite fa cie s Tra nsiti on zo ne Tra nsiti on zo ne Fig. 1. A: Schematic geological map of the Nagssugtoqidian orogen and adjacent foreland (modified from van Gool et al. 2002b). Outlined box shows location of the study area. SNO, southern Nagssugtoqidian orogen; CNO, central Nagssugtoqidian orogen; NNO, northern Nagssugto- qidian orogen. B: Simplified geological map of most of the study area (modified from van Gool et al. 2002a). 165 The deeply eroded northern flank of the Palaeoproterozoic Nagssugtoqidian orogen is exposed south of Aasiaat in central West Greenland (Fig. 1). These rocks bear record of tectono-thermal processes that operated at mid-crustal levels in a collisional setting and controlled the distribu- tion of strain and metamorphic facies. The area shows a complex structural pattern that varies significantly from south to north. The aim of this work is to describe the regional variation of structural elements and to investi- gate the relationship between strain, metamorphic grade and the orientation of deformational structures. This then leads to the consideration of two different tectonic models, which have been developed to account for the observed structural pattern: (1) an indentor model proposed origi- nally by Piazolo et al. (2004), and (2) a transpression zone model. The study area covers the Kangaatsiaq geological map sheet at scale 1:100 000 (Garde 2004), mapped in 2001– 2002 by the Geological Survey of Denmark and Green- land (GEUS; van Gool et al. 2002b). The directional and fabric type structural data sets were analysed using Geo- graphic Information Systems (GIS) based techniques that proved to be a powerful tool in the investigation of com- plex structural patterns. Some of the structural data col- lected during this field campaign were presented by Pia- zolo et al. (2004) to illustrate the application of GIS in a multidisciplinary approach to survey high-grade terrains. Our present study focuses on a more detailed analysis and interpretation of the structural relationships of the inves- tigated area, a high-grade Precambrian terrain affected by more than one deformation phase, and whose interpreta- tion is not unequivocal. Geological setting The study area covers over 3000 km2, and extends from 68°N–68°30′ N and 52°W–53°15′ W, between the fjord of Ataneq in the south and the island of Qeqertarsuatsiaq in the north (Fig. 1). The area comprises the northern, c. 300 km wide exposure of the roughly E–W-trending Nags- sugtoqidian orogen (Fig. 1). In the broadest sense, this tectonic belt resulted from a continent–continent colli- sion between the Archaean North Atlantic Craton to the south and an Archaean continental mass to the north (e.g. Kalsbeek et al. 1987; Connelly et al. 2000; van Gool et al. 2002a). The orogen is generally characterised by E–W- trending kilometre-scale folds and ENE–WSW-trending linear belts which overprint an Archaean fabric. On the basis of the grade of metamorphic reworking, Ramberg (1949) and later Marker et al. (1995) distinguished south- ern, central and northern segments of the Nagssugtoqidian orogen (SNO, CNO and NNO respectively; Fig. 1). De- tailed structural investigations within the CNO show that deformation in this area is dominated by thrust tectonics (Manatschal et al. 1998; van Gool et al. 1999). The investigated area lies within the NNO and is com- posed of amphibolite- to granulite-grade Archaean ortho- gneisses interlayered with relatively thin metasedimentary units (Fig. 1). This region is transected by major fjord systems that allow data collection along well-exposed coast- al sections. Reconnaissance studies (Noe-Nygaard & Ram- berg 1961; Henderson 1969; Marker et al. 1995; Kalsbeek & Nutman 1996; Mengel et al. 1998; Connelly et al. 2000) provided initial information on the structural style of the study area. A comprehensive description of the structural pattern was recently presented by Piazolo et al. (2004) and interpreted in terms of indentor tectonics with a rigid granulite-grade domain moving northwards into a rheo- logically weaker amphibolite facies domain. Although quartzofeldspathic orthogneiss dominates in the investigated area, the overall map pattern is governed by discontinuous NE–SW-trending supracrustal belts (Fig. 1). These 2–3 km thick sequences comprise several dis- tinct lithological types: (a) monotonous, garnet-bearing quartzofeldspathic paragneiss locally containing subordi- nate mafic volcanic and metapelitic intercalations; (b) pelitic to semipelitic schist with or without garnet and sillimanite, including thin quartzofeldspathic layers and rarely quartzite; and (c) layered mafic to intermediate meta- volcanic successions with calc-silicate bands and/or pods. Previous studies have demonstrated that the metamor- phic grade of the NNO decreases northwards and is pre- dominantly amphibolite facies, with granulite facies rocks preserved only in the south-western corner of the study area near the boundary with the CNO (e.g. Marker et al. 1995). The contact between these two facies is transition- al over a distance of 10–12 km and forms a zone nearly parallel to the strike of the regional foliation (Fig. 1). The granulite facies gneisses are typically pyroxene-bearing and enclose frequent melt pockets and cross-cutting veins. Thermobarometric analyses point to a peak temperature of 800 ± 30°C at medium pressures of 6–7.5 kbar (Piazo- lo et al. 2004). The amphibolite facies gneisses are lighter coloured and contain fewer biotite-bearing melt veins. They reveal peak metamorphism conditions of 650 ± 30°C at 4–5 kbar (Piazolo et al. 2004). Within the supracrustal amphibolites, crystallisation of amphibole, usually devel- oped along foliation planes, indicates a syntectonic fluid flux and associated metamorphism. No relics of earlier granulite facies assemblages are preserved in these rocks. Thrane & Connelly (2006, this volume) carried out seve- 166 10 cm 50 cm 50 cm 30 cm 50 cm 50 cm A N S N S S N S N N S S N B C D E F Fig. 2. Examples of mesoscopic folds in the orthogneisses and metasediments of the study area. A: Main foliation S 1 of orthogneiss developed parallel to the axial plane of isoclinal F 1 fold. B: Lithological boundaries and originally cross-cutting basic dykes folded by F 1 isoclinal fold and showing extensive migmatisation. C: S 1 foliation folded into isoclinal folds during the same progressive D 1 event. D: S 1 foliation and its subse- quent folding accompanied by pervasive migmatisation. E: S 1 foliation locally folded by F 3 folds of variable geometry with fold axes developed subparallel to the lineation. F: S 1 foliation of the orthogneiss folded by the F 3 fold with fold axes parallel to the lineation and S-directed asymmetry. N, north; S, south. 167 ral laser ablation and ion probe age determinations of zir- con from within and adjacent to the present study area. Deformation in the south-western part of the area is con- strained by the emplacement age of 2748 ± 19 Ma for a synkinematic granite which intrudes the orthogneiss. A Palaeoproterozoic deposition age of c. 1950 Ma was ob- tained from a metasediment within the Naternaq supra- crustal belt (Fig. 1), and broad rims of zircons from an Archaean sediment from Kangersuneq yielded a metamor- phic age of c. 1850 Ma, suggesting that major Nagssugto- qidian deformation and metamorphism occurred at around this time. Thrane & Connelly (2006, this volume) also obtained an age of 1837 ± 12 Ma for a vertical, straight pegmatite north-east of Kangaatsiaq trending 020° and displaying sinistral shear along its margins, that is thought to date a late phase of overall N–S-directed Palaeoproter- ozoic shortening. Characteristics of directional structures The main foliation (S 1 ) is axial planar to rare isoclinal F 1 folds that fold lithological boundaries as well as cross-cut- ting basic dykes (Fig. 2A). These dykes have been rotated into parallelism with the foliation on the fold limbs (Fig. 2B). The S 1 foliation was itself later folded into isoclinal folds, although this refolding may reflect the same pro- gressive D 1 deformation event since no overprinting fab- ric is associated with it (Fig. 2C). A characteristic feature of these folds is the broad parallelism of their axial planes to the regional foliation S 1 . A L 1 mineral lineation is de- veloped on the foliation planes and is usually defined by a parallel alignment of amphibole crystals and/or elongat- ed quartz-feldspar and biotite aggregates. The lineation is well developed in the amphibolite facies rocks but rather weak in the granulite facies gneisses. In the transition zone between the amphibolite and granulite domains (Fig. 1B), no mutually cross-cutting mineral lineations were detect- ed and no evidence for fabric superimposition was ob- served. The L 1 lineation is only rarely associated with kin- ematic indicators that are commonly symmetric and must have resulted from coaxial strain and/or a finite strain com- bining the effects of several strain increments. Asymmet- ric fabrics have been observed only in zones of steeply dip- ping foliation, and typically indicate a sinistral rotational shear component in the present-day coordinates (Fig. 3). Restoration of the steep foliation attitude to more gentle regional dips would result in the same indicators imply- ing a top-to-the-west or WSW sense of shear. The S 1 foliation and its subsequent folding during the presumed progressive D 1 event, were accompanied by a long-lasting pervasive migmatisation. This is demonstrated by the common occurrence of migmatite layers or patches that are variably deformed and show mutually cross-cut- ting relationships. Some of them are parallel to the main foliation S1 (Fig. 2D) whereas others define discordant veins or dykes oblique to the regional fabric. Between these two end members are a range of cross-cutting veins that are deformed and reoriented to varying degrees. In the metasedimentary rocks of the Naternaq (Lersletten) area, the S1 regional foliation is refolded by F2 folds character- ised by steep to subvertical fold hinges. These folds are developed at kilometre- to centimetre-scale and, in a few cases, map scale F2 folds can be seen refolding F1 (A.A. Garde & J.A. Hollis, personal communication 2003). F2 folds are found exclusively within the metasedimentary belts and at their contacts with the adjacent gneisses. At a mesoscopic scale, they are represented by folds plunging steeply towards the SE and in few cases towards the north. A moderate to strong, SE-plunging mineral lineation is associated with the hinges of F2 folds, locally deviating A B 20 cm 5 cm WSW ENE WSW ENE Fig. 3. Examples of sinistral (top-to-the WSW) kinematic indicators in the orthogneisses of the study area. The lineation is plunging to- wards WSW on north-dipping foliation planes. A: Sheared, asym- metric amphibolite enclave. B: Sigmoidal K-feldspar porphyroclast. 168 from its regional trend although it remains the only line- ation present. An S 2 cleavage axial planar to F 2 is rarely, and only weakly, developed. Beyond Naternaq, the S 1 fo- liation is refolded by F 3 folds of variable, often complex geometry with gently plunging fold axes developed sub- parallel to the L 1 lineation (Fig. 2E). The frequency of such folds increases northwards within the amphibolite- grade rocks. Associated F 3 axial planes are locally marked by a subtle S 3 cleavage, accentuated by local mica aggre- gates and discrete joints which cut the main S 1 foliation. The distinction between F 2 and F 3 folds is primarily based on their different geometries since they both fold the re- gional fabric S 1 and are not associated with penetrative axial cleavages or intersection lineations. The hinges of F 2 folds are relatively steep and oblique to the regional linea- tion, whereas F 3 axes are gently inclined and run parallel to the lineation L 1 . The F 3 mesoscopic folds are frequent- ly asymmetric with fairly uniform SSE vergence in the area south and west of Kangaatsiaq (Fig. 2F). Orientation of directional structures On the map scale, the regional foliation displays a WSW– ENE to SW–NE strike associated with steep to moderate dips towards the NNW or SSE (Fig. 4). Shallow-dipping foliations (< 30°) are rare and randomly distributed throughout the study area. Their variable directions sug- gest that they are associated with the hinge zones of F3 folds developed at different scales (Fig. 4). Moderately and steeply dipping foliations show a distinctly discrete group- ing within the investigated area. The former predominate in the south-eastern corner of the area, corresponding with the granulite-grade block, whereas the steeply dipping foliations are developed in its central part, forming a wide belt along Tunorsuaq (Fig. 4). This belt coincides with a transition zone between the granulite and amphibolite facies domains and partly with the south-eastern margin of the latter (see Figs 1, 4). The strike of foliation dipping steeper than 30° remains fairly consistent throughout the area, while the dip direction varies only in the case of sub- 68°30' 68° Tunorsuaq Ataneq Naternaq 53°53° 52°30'52°30' 52°52°53°30' 53° 52°30' 52° AB TZ TZ GB 68°15' 10 km ≤ 30° Foliation 30–60° > 60° Fig. 4. Representation of foliation trends and dip directions in the study area. GB, granulite block; TZ, transition zone; AB, amphibolite block. 169 vertical planes (> 60°). On stereoplots, the poles to folia- tion are scattered along a regular girdle produced by the F3 folds (Fig. 5). In the southern granulite facies block the vast majority of foliation measurements cluster in one maximum, sug- gesting that late folding is insignificant or absent in this area (Fig. 5). This maximum corresponds to the foliation dipping moderately to steeply towards the NNW, while the axis of the foliation girdle plunges gently to the WSW at 20°. In the northern, amphibolite facies block meas- urements are more evenly distributed along the foliation girdle, providing evidence for the regional importance of F3 folding (Fig. 5). In contrast to the southern block, the stereographic girdle axis is almost subhorizontal. Although the foliation patterns in the southern and northern blocks are fairly similar, a striking difference occurs within the latter between the rheologically competent orthogneisses and relatively incompetent supracrustal formations (Fig. 6). The orthogneisses reveal a regular girdle perfectly con- trolled by a cylindrical geometry of the F3 folds. In con- trast, the less competent supracrustal rocks display a pro- nounced foliation scatter, reflecting a complex interfer- ence between the effects of the (possibly noncylindrical) F2 and F3 folding. The lineation trends WSW–ENE over the whole study area, and frequently plunges gently towards the WSW (Fig. 7). Shallow lineations (< 15°) are concentrated in the northern and central parts of the investigated area (Fig. 7), and commonly coincide with the subvertical foliation within the amphibolite facies block and the associated tran- sition zone towards the granulite facies block. Conversely, lineations plunging steeper than 15° group largely in the south-eastern corner of the area, corresponding with the granulite block, and show mostly WSW-directed plung- es. Mesoscopic F3 fold hinges are usually colinear with the regional lineation, and this is also the case at the larger scale since the lineation maxima are located near the pole of the foliation girdle on stereoplots (cf. Fig. 5). In the amphibolite facies block, the lineation is shallow and rel- atively well grouped in the maximum representing the subhorizontal WSW–ENE trend. A significant scatter occurs only in the Naternaq area, which is displayed on the map by relatively steeply plunging lineations (Fig. 7) and on stereoplots by data distribution along a small cir- A B C D NORTHERN BLOCK SOUTHERN BLOCK N N N N N = 794 N = 616Lineation Lineation N = 794 N = 616Foliation Foliation Lineation maximum 245/05 Lineation maximum 245/05 245/05 F3 245/20 F3 F2 F2? 6% 4% 2% 0.5% 8% 6% 4% 2% 0.5% Fig. 5. Attitudes of foliation S 1 and stretching lineation L 1 in the northern amphibolite facies block (A, C), and southern granulite facies block (B, D). The positions of the poles to the foliation girdles and maximum of lineation measurements are indicated in stereoplots A, B and C, D, respectively. 170 cle (Fig. 5). A dispersal of lineation measurements is pro- duced in that area by steeply plunging F 2 fold hinges whose orientation corresponds to the centre of a small circle de- fined by the scatter of the lineation (Fig. 5). Two different small circles can be delineated on the stereoplot (Fig. 5) based on the lineation scatter. One is centred at the orien- tation of F 2 hinges steeply inclined to the SE that are rel- atively frequent as mesoscopic structures. The second is developed around the nearly vertical north-plunging F 2 hinges rarely found in the outcrops but probably impor- tant at the map scale. The lineation scatter induced by the F 2 folding is very clear in the supracrustal rocks, where- as it is almost absent in the orthogneisses (Fig. 6). This relationship is consistent with the field observation that the F 2 folds are developed almost exclusively in the meta- sediments. In the southern block the lineation is slightly steeper than in the north and shows a mean plunge of c. 20°. Its maximum is more diffuse than in the amphibo- lite-grade block and more lineations are relatively steep (> 15°). Nevertheless, the average lineation trend defined by the position of maxima on stereoplots is exactly the same for the amphibolite and granulite facies domains (Fig. 5). In order to better understand the geometric relation- ships between planar and linear fabric elements during deformation, they may be directly compared on fabric topology plots in terms of fabric trends and lineation pitch (see Alsop & Holdsworth 2004 for a review). The angle of pitch may be defined as the angle that a line makes with the strike of a surface, when measured within that plane (Fig. 8A). A significant variation in the angle of lin- eation pitch on the regional foliation surface is observed (Fig. 8B); this may be caused by two independent factors: (1) variable plunge of the lineation, and (2) variable dip direction of the foliation. The latter feature seems to be a consequence of local folding, since a high pitch angle (> 45°) is most characteristic for SW-dipping foliations (Fig. 9) that represent the hinges of F 3 folds (Fig. 5). On the other hand, a majority of measurements correspond to foliation dipping to the NW or SE that reveals a low or moderate pitch angle (Fig. 9). Such a foliation pattern is consistent with the regional attitude of foliation inclined towards the NW and only locally reoriented on limbs of the F 3 folds. Since the F 3 folds are only of minor signifi- cance in the southern part of the study area characterised by higher pitch values (Fig. 8), and the majority of linea- tions were measured on steep to subvertical foliation sur- A B C D ORTHOGNEISSES SUPRACRUSTAL ROCKS N N N N N = 554 N = 237 Lineation Lineation N = 554 N =237 Foliation Foliation Lineation maximum 245/05 Lineation maximum 245/05 6% 4% 2% 0.5% 8% 6% 4% 2% 0.5% 245/05 F2 F2? Fig. 6. Attitudes of foliation (S 1 ) and stretching lineation (L 1 ) in the orthogneiss- es (A, C) and supracrustal rocks (B, D) of the northern amphibolite facies block. The position of the pole to the foliation girdle and maximum of lineation measurements are indicated in stereoplots A and C, D, respectively. 171 faces, the broad scatter of the pitch angles can be attribut- ed to variations in the lineation plunge (Fig. 10). Only a minority of measurements plot away from the line that indicates the similarity of pitch and plunge angles (Fig. 10). Thus, the change of foliation strike plays a less im- portant role in the distribution of pitch angles. The low pitch angle (< 15°) is characteristic of the central and northern parts of the study area (Fig. 8), corresponding to the amphibolite facies block and the transition zone to the granulite facies block. This is the area that is addition- ally characterised by the steep foliations and shallow line- ation plunge. The higher pitch angles (exceeding 15°) are more common in the south-east corner of the study area within the granulite facies gneisses, where they are related to relatively steep lineations occurring on the moderately to steeply inclined foliation. A systematic change in the plunge of lineations can be observed across the south-western part of the study area (Fig. 7). The steep SW–NE-striking foliation around Tu- norsuaq is associated with the gently plunging lineation that defines a low angle of pitch on the foliation. Towards the south, however, the plunge of lineation becomes pro- gressively steeper despite the generally uniform strike of foliation (Fig. 4). A pitch versus lineation trend diagram (Fig. 11) demonstrates that the increase in pitch is unre- lated to the change of lineation trend. This means that lineations are typically not reoriented on fold limbs, and that folds, if present, are mostly colinear with lineation. Furthermore, in the north-western part of the area the lineations plunge gently to the SW and NE to define a series of culminations and depressions, that are clearly il- lustrated by the opposing plunges on the south side of Tunorsuaq and the outer islands to the west-south-west (Fig. 7). This sinuosity of lineations and associated fold hinges defines a large-scale whaleback pattern consistent with a dominantly subhorizontal and approximately N–S contractional strain (cf. Piazolo et al. 2004). The finite planar (S) and linear (L) shape fabrics within a high-strain rock may be qualitatively described (Flinn 1978). Consequently, the relative dominance of these re- 68°30' 68° Tunorsuaq Ataneq Naternaq 53°53° 52°30'52°30' 52°52°53°30' 53° 52°30' 52° AB TZ TZ GB 68°15' 10 km < 15° Lineation 15–35° > 35° Fig. 7. Representation of lineation trends and plunge directions of the study area. GB, granulite block; TZ, transition zone; AB, amphibolite block. 172 68°30' 68° Tunorsuaq Ataneq Naternaq 53°53° 52°30'52°30' 52°52°53°30' 53° 52°30' 52° AB TZ TZ GB 68°15' 10 km ≤ 15° Pitch 15–40° > 40° B strik e dip angle lin ea tio n pitch angle dip foliatio n A 0 NE 90° SE 180° Dip direction 270° 360°NWSW 90° 60° 30° 0 Pi tc h an gl e Fig. 8. Pitch data. A: schematic diagram illustrating the pitch of a lineation. B: representation of lineation pitch angles of the study area. GB, granulite block; TZ, transition zone; AB, amphibolite block. Fig. 9. Plot of lineation pitch angle vs. dip direction of foliation, showing the change of pitch as a consequence of local folding. 173 spective components enables a distinction to be made between fabrics that are foliation dominated (S tectonite), lineation dominated (L tectonite) or contain a combina- tion of foliation and lineation (SL tectonite). The pattern shown by the spatial distribution of fabric types in the study area is clearly differentiated. The north-western part of the area is dominated by LS and L > S fabric types whereas the south-eastern part reveals a vast preponder- ance of S or S > L types (Fig. 12). The southward increase of lineation pitch is typically associated with the transi- tion from L > S or LS shape fabrics in rocks characterised by a low pitch, to S > L or S fabrics in the zone of moder- ate to high pitch. Constrictional fabrics, i.e. L and L > S fabrics, are typical of rocks with shallowly plunging linea- tions on the steep foliation belonging to the amphibolite facies block and the transition zone to the granulite facies block. The latter is dominated by the flattening fabric types, i.e. S and S > L fabrics, which are common in rocks containing steeply plunging lineations. Discussion Three main observations can be made concerning the data presented above: (1) structural elements, i.e. lineations, foliations, folding and fabric type, vary with lithology, (2) two broad domains with different structural patterns can be distinguished, and (3) the structural style in the NNO is markedly different to that previously described from the CNO, e.g. by Manatschal et al. (1998) and van Gool et al. (1999). The observed scarcity of mineral lineations in the gran- 90° 60° 30° 0 Pi tc h an gl e 0 30° Lineation plunge 60° 90° Pitch an gle = plunge angle 0 NE 90° SE 180° Lineation trend 270° 360°NWSW 90° 60° 30° 0 Pi tc h an gl e 0 NE 90° SE 180° Lineation trend 270° NWSW Fig. 10. Plot of lineation pitch angle vs. lineation plunge, showing the contribution of changing lineation plunge to the total variation of pitch. Measurements located on the straight line connecting the lower left and upper right corners of the plot have been taken on vertical foliation planes. Fig. 11. Plot of lineation pitch angle vs. lineation trend, showing the variation of pitch despite the uniform trend of lineation. 174 ulite facies gneisses could be attributed to the absence of minerals with a high shape aspect ratio. If such minerals are lacking, lineations of the type that forms by rigid rota- tion or due to growth parallel to the maximum extension direction or transport direction can hardly develop. Never- theless, aggregate lineations that form by dynamic recrys- tallisation and material transfer should be expected in the granulite facies gneisses. Since even these lineations are lacking in the southern block, it seems that the scarcity of linear structures in this area is not only a function of li- thology but also reflects specific features of deformation history. In the northern block, a significant strain parti- tioning between orthogneisses and supracrustal formations is indicated by the effects of F2 folding in metasedimenta- ry rocks and the resultant scatter of foliation and linea- tions in these rocks (Fig. 6). This points to the concentra- tion of deformation in rheologically weak metasedimen- tary belts that accommodate the bulk of strain induced during F2 folding. The competent orthogneisses experi- enced only subsequent F3 folding, when they were de- formed together with associated less competent supracru- stal rocks. At the western limb of the Naternaq supracrustal belt, there is a structural discordance with another E–W-trend- ing supracrustal unit farther to the west. This structural discordance may suggest the presence of some kind of ‘stockwerke tectonics’ (Wegmann 1935), in addition to the inferred strain partitioning between the orthogneisses and metasedimentary rocks. The application of the ‘stock- werke’ model to the Naternaq belt itself remains an in- triguing problem that cannot be resolved at present, due to the lack of sufficient information on the time relation- ship between the structural discordance and the main phase(s) of folding and metamorphism. Disregarding the local complexity at Naternaq, the gen- eral structural pattern described in this paper allows sub- division of the study area into two main domains: (1) a southern domain mostly characterised by S or S > L shape fabrics and a moderate to high angle of pitch, and (2) a northern domain showing L > S or LS fabrics and low 68°30' 68° Tunorsuaq Ataneq Naternaq 53°53° 52°30'52°30' 52°52°53°30' 53° 52°30' 52° AB TZ TZ GB 68°15' L or L > S Fabric type L = S S or S > L 10 km Fig. 12. Spatial distribution of fabric types (terminology af Flinn 1978). GB, granulite block; TZ, transition zone; AB, amphibolite block. 175 angles of lineation pitch. This subdivision compares well with the map scale variation of the metamorphic grade from granulite facies in the south to amphibolite facies further north. The contact between these two facies is tran- sitional and forms a gently curved boundary that is sub- parallel to the strike of the regional foliation (Piazolo et al. 2004). This division of the study area into two con- trasting blocks, indicated by the structural data and vari- ation of metamorphic grade, can be explained by two models, namely (1) an indentor model, and (2) a transpres- sive deformation model, the relevance of which is briefly discussed below. The indentor model, originally developed by Piazolo et al. (2004), invokes a twofold deformation history. Dur- ing the first event the southern block was subjected to a coaxial flattening under granulite facies conditions (Fig. 13B – time A). A fairly uniform strain developed at that time and is manifested by prevailing S or S > L shape fab- rics. During the second event (Fig. 13B – time B) the previously cooled granulite grade block acted as an inden- tor, with the amphibolite facies domain being plastered and moulded around the rigid block. The structural pat- tern and mineral assemblages of the southern domain were only modified in a transition zone that experienced retro- gression to the amphibolite facies. At the same time, the amphibolite grade northern domain was subjected to mostly coaxial strain, including an important constric- tional component. The resultant structural grain in the amphibolite facies rocks mimics the geometry shown by the boundaries of the southern block. The alternative model (Fig. 13C) explains the observed structural relationships in terms of strain partitioning with- in a transpression zone (for a review of transpression see Holdsworth et al. 2002, and references therein). In this model, the different structural patterns documented in the northern and southern domains were produced dur- ing a single deformation event. Consequently, the model implies a continuity of the structural grain across the tran- sitional zone as well as a gradual change in the orientation of directional structures, fabric type and kinematics of strain. The southern domain was mostly subjected to co- axial flattening, resulting in S or S > L shape fabrics with a moderately plunging lineation. The northern domain was deformed in a wrench-dominated regime, characterised by a constrictional or plane strain with a significant rota- tional strike-slip component. Hence, the northern block reveals L > S and LS shape fabrics and shallowly plunging lineations. A sinistral sense of displacement assumed in the model is consistent with rare observations in the field of kinematic indicators with the appropriate asymmetry. The indentor model explains the arcuate swing of the structural grain around the granulite facies block, the map scale geometry of which may appear to control the folia- tion and lineation patterns within the amphibolite facies block further north. A serious weakness of the indentor model is the lack of evidence for fabric overprinting in the boundary zone between the two domains despite the assumed twofold deformation. However, high strain in the transitional zone could account for the obliteration of interference patterns and the apparent continuity of struc- tural grain. The transpression zone model accounts for the presence of steep (F 2 ) fold hinges in the Naternaq supra- crustal belt that are expected to develop within the wrench- dominated part of the transpressive system. The model also explains the contrast of structural style between the northern and southern domain without a detectable tec- tonic boundary or an overlap of structural patterns. Con- sequently, it is consistent with an apparent continuous transition linking the structural patterns within the gran- ulite and amphibolite blocks. The somewhat steeper plunge of lineations within the granulite facies domain may indicate a greater component of coaxial strain in that area. However, the potential strain partitioning, as revealed by the lineation pattern, is relatively weak for a transpres- sion zone. The partitioning of strain that takes place between the orthogneisses and supracrustal rocks within the northern amphibolite facies block is consistent with both of the discussed models. In addition, both models are consistent with a dominantly subhorizontal and ap- proximately N–S contractional strain in the northern block causing the small-scale porpoising and large-scale whale- backing of lineations and associated fold hinges. Thus, even a detailed structural analysis does not allow us to determine unequivocally which of the two models is the more appropriate. Nevertheless, the presented struc- tural models can be indirectly verified by geochronologi- cal data, which constrain the age of peak metamorphism and associated deformation in the northern amphibolite facies block as Palaeoproterozoic (Thrane & Connelly 2006, this volume). Furthermore, the late Archaean crys- tallisation age of syndeformational granites emplaced at Saqqarput (Fig. 1) and at the southern margin of the NNO (Connelly & Mengel 2000; Thrane & Connelly op. cit.), points to a lack of significant deformation in the south- ern block from the late Archaean onwards. This is in con- flict with the coincidence of deformation between the northern and southern blocks required by the transpres- sion model and, thus, supports the indentor model. This is corroborated by the occurrence of undeformed but met- amorphosed mafic dykes of likely Palaeoproterozoic origin emplaced at the southern margin of the investigated area (Glassley & Sørensen 1980; Árting 2004). 176 II IV I III I III V VI II IV I II V V II III IV VI II I I III IV VI I II III IV Plan view granulite facies V VI I III II IV I III IV I II V II I III granulite facies amphibolite facies Plan view V VI I II III IV V VI II granulite facies amphibolite facies Plan view V II I II II V A Schematic trace of lineation Schematic trace of folds Principal stress – σ1 Response to stress Model I (Piazolo et al. 2004). Observed fabrics developed during two deformational events Model II (discussed in this paper). Fabrics developed during a single transpressional event Time A, at granulite facies conditions Time B, at amphibolite facies conditions C B Pla ne st ra in A xi al r at io o n X Y p la ne Flattening Constriction Axial ratio on YZ plane S or S > L fabric SL fabric Boundary of deformation Transition zone Fig. 13. Schematic indentor and transpression zone models with expected fabric types and fold patterns. A: Flinn graph with schematic qualita- tive descriptions of the finite planar (S) and linear (L) shape fabrics. B: Indentor model. C: Transpression zone model. 177 The F 3 folding apparently postdates the juxtaposition of the granulite and amphibolite blocks and had a rela- tively minor influence on their contact zone. The north- ward increase in intensity of F 3 folding is readily explained by the rheological weakness of the amphibolite facies do- main during cooling. The origin of F 3 folds provides evi- dence for prolonged, approximately N–S-directed com- pression, the age of which is roughly constrained by the previously mentioned 1837 ± 12 Ma pegmatite (Thrane & Connelly 2006, this volume). Our structural data also show that compressional stress related to the growth of the Nagssugtoqidian orogen was effectively transferred across the lower crust, and that the amphibolite facies domain south of Aasiaat was subjected to the penetrative Palaeoproterozoic deformation. There- fore, this area represents an integral part of the Nagssug- toqidian orogen despite the obvious paucity of Palaeopro- terozoic crustal components. In contrast to the southern part of the orogen, the overall N–S shortening induced by the Nagssugtoqidian collision was accommodated in the study area through indentation and folding, in con- trast to the central part of the orogen that displays signif- icant thrust tectonics. Concluding remarks The indentor and transpression-driven tectonic models discussed in this paper share a number of features which shed a new light on the evolution of the northernmost Nagssugtoqidian orogen. The granulite and amphibolite facies blocks distinguished in the study area appear to be (par)autochthonous, and their boundary is only reacti- vated and reworked during indentation or transpression. The area studied shows excellent examples of Precam- brian deformation that are characterised by significant strain partitioning into less competent metasedimentary rocks. The presence or absence of lineations may be influenced significantly by lithology and metamorphic grade. Com- plex structural patterns may not always conclusively reveal the structural history on their own, hence geochronological data are essential in distinguishing between different de- formation models such as transpression or indentation. 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Geologische Rund- schau 26, 305–350. __________________________________________________________________________________________________________________________________________________________________ Manuscript received 17 June 2004; revision accepted 14 June 2005