Geological Survey of Denmark and Greenland Bulletin 11, 125-144 125 Presentation and interpretation of structural data from the Nagssugtoqidian orogen using a GIS platform: general trends and features Jeroen A.M. van Gool and Sandra Piazolo In this contribution we present data collected by more than 50 international geologists involved in geological mapping and research projects in the Nagssugtoqidian orogen of West Greenland, organ- ised by the Geological Survey of Denmark and Greenland and the Danish Lithosphere Centre. Using a geographical information system (GIS) as a framework for visualisation and analysis of structural and lithological data, it is now possible to give a unique overview of thousands of data points, em- ployed here within a study area of approximately 160 × 180 km in the central and northern Nagssug- toqidian orogen. The GIS methodology allows comparison, integration and analysis of datasets in terms of subject, space, and scale. This is extremely helpful in the recognition of geological patterns, such as terrain or domain boundaries and map-scale structures. Analysis of the available structural data shows clear differences in deformation patterns between the core and the northern segment of the Nagssugtoqidian orogen. One of the most prominent features is the ENE-striking Nordre Strøm- fjord shear zone, which transects the orogen from the coast to the Inland Ice. The data also clearly document a change from predominantly steeply dipping, ENE–WSW-trending fabrics and large, elongate structural domains in the core of the orogen, to large, open fold patterns and moderately to shallowly dipping fabrics in smaller structural domains in the north. Keywords: geographical information systems, Nagssugtoqidian orogen, West Greenland, structural data, structural do- mains _______________________________________________________________________________________________________________________________________________________________________ J.A.M.v.G. & S.P., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: jvg@geus.dk 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. Over the past ten years the Nagssugtoqidian orogen in central West Greenland (Fig. 1) has been the subject of intense geological research, involving both bedrock map- ping and research into the Palaeoproterozoic and Archae- an tectonic evolution of the region. This has led to a ma- jor improvement in the understanding of the tectonic de- velopment of the Nagssugtoqidian orogen (Kalsbeek & Nutman 1996; Connelly et al. 2000; van Gool et al. 2002). The research was undertaken in two projects, organised by the Danish Lithosphere Centre (DLC) from 1994 to 1999 and the Geological Survey of Denmark and Green- land (GEUS) from 2000 to 2003, respectively. Approxi- mately 35 international geologists from institutions on three continents participated in the field work of these projects, with changing teams from year to year. During these projects a very large amount of data was collected, including structural measurements, lithological observa- tions, intrusive relationships, information about metamor- phic mineral assemblages, etc. Other structural data were collected during previous work in part of the region in © GEUS, 2006. Geological Survey of Denmark and Greenland Bulletin 11, 125–144. Available at: www.geus.dk/publications/bull 126 68° 51° Archaean, variably reworked Metasedimentary rocks Surficial deposits Basalt Quaternary Palaeogene Palaeoproterozoic Sandstone Sisimiut charnockite Arfersiorfik quartz diorite Granodioritic gneiss Orthogneiss Granitic gneiss Dioritic gneiss Anorthosite Orthogneiss, undifferentiated Amphibolite (includes Proterozoic components) Metasedimentary rocks (includes Proterozoic components) (includes Archaean components) Metagabbro Figs 2–8 Qeqertarsuaq N N O C N O SN O NISB Nordre Isortoq Nordre Strømfjord Ussu it NSSZ N ag ss ug to qi di an o ro ge n R in ki an fo ld b el t N or th A tla nt ic cr at on Agto Ataneq map sheet Ussuit map sheet Ilulissat Disko Bugt Disko Inland Ice SydostbugtenAasiaat Naternaq Attu Arfersiorfik Kangerlussuaq ITZ Sisimiut Kangaatsiaq Ikamiut Jakobshavn Isfjord 50 km Inland Ice Greenland Iceland Canada Fig. 1. Schematic geological map of central West Greenland (modified from van Gool et al. 2002), depicting the Nagssugtoqidian orogen. ITZ, Ikertôq thrust zone; NISB, Nordre Isortoq steep belt; NSSZ, Nordre Strømfjord shear zone; SNO, CNO and NNO, respectively, the southern, central and northern Nagssugtoqidian orogen. Small boxes outline the locations of the Agto and Ussuit map sheets; Large box indicates the location of Figs 2–8. 127 the 1960s and 1970s by the former Geological Survey of Greenland (GGU, now part of GEUS). Part of this was used by the collectors in their individual research or in small groups, and for the compilation of a published geo- logical map, However, to date only a fraction of the total dataset has been made accessible in publications, and most of the original data collected prior to 2001 are only avail- able for further analysis if extracted from the individual geologists’ field diaries and field maps. With traditional methods it would be highly impractical and tedious to obtain an overview of all the structural data collected over time from the Nagssugtoqidian orogen in West Green- land, and a rigorous conventional analysis of the com- plete dataset would be close to impossible. Hence, alter- native methods of data compilation and analysis were re- quired. Since 2001 GEUS’ mapping projects have included systematic collection of structural data which are avail- able digitally. The data from all participants are recorded in field diaries and on maps, stored electronically in spread- sheets, and subsequently entered in a geographical infor- mation system (GIS) for further presentation and analy- sis. In this way all data from an entire group of geologists can be accessed as a whole and used for map production and data analysis. GIS methods has already proved useful in several disci- plines including mineral exploration on local to global scales (e.g. Bonham-Carter et al. 1990; Goodwin et al. 1996; Knox-Robinson & Wyborn 1997; Harris et al. 2001), palaeontology (e.g. Carrasco & Barnosky 2000), and environmental assessment (e.g. True et al. 1999; Books 2000; Wilson et al. 2000). In this paper we demonstrate the application of GIS data management, visualisation and methods of analysis in a large-scale and long-term international project, in- cluding data from two previous mapping projects in the region. We present for the first time in a digital format a set of more than 10 000 structural orientation measure- ments and observations collected by more than 50 geolo- gists in the Nagssugtoqidian orogen over a period of 40 years. Such a presentation can (a) provide a very helpful overview of the data itself, (b) help to identify where fu- ture research efforts may be scientifically interesting, and (c) show how these together with geological and geophysi- cal maps can define structural domains and illustrate the large-scale structural variations through an important part of the orogen. The Nagssugtoqidian orogen The Nagssugtoqidian orogen in West Greenland is a Palaeoproterozoic collisional belt, dominated by Archae- an gneisses that were reworked at amphibolite and granu- lite facies during the Palaeoproterozoic orogeny (van Gool et al. 2002). It forms the northern boundary of the North Atlantic craton in southern Greenland, and is bound to the north by the contemporaneous Rinkian fold belt. It consists of three tectonic segments, referred to as the south- ern, central and northern Nagssugtoqidian orogen (SNO, CNO and NNO; Fig. 1), which respectively consist of a southern parautochthonous foreland, a high-grade core, and a transition zone to the Rinkian fold belt. Juvenile Palaeoproterozoic magmatic arc rocks and supracrustal sequences occur mainly in narrow belts within the CNO. The Nagssugtoqidian orogen is characterised by a domi- nant ENE–WSW structural trend, which culminates in a number of linear belts: the Ikertôq thrust zone, the Nor- dre Isortoq steep belt, and the Nordre Strømfjord shear zone. These are interpreted as crustal-scale structures and alternate with areas dominated by large fold structures. Detailed investigations in the core of the orogen have shown that during the Nagssugtoqidian orogeny this re- gion originally underwent a phase of NW-vergent thrust- ing, followed by folding now recognised predominantly as isoclinal folds (van Gool et al. 2002). A second fold phase resulted in upright, ENE-trending folds on a scale of tens of kilometres, with associated development of ex- tension lineations plunging shallowly ENE. Finally, a phase of sinistral strike-slip shearing on the steep flanks of the large fold structures resulted in the above mentioned prominent linear belts. It is therefore only the latest defor- mation phases that generated the main ENE–WSW-trend- ing tectonic fabric of the orogen (van Gool et al. 2002). The area discussed in this study extends from 67°N in the Nordre Strømfjord region to 69°10′N at Jakobshavn Isfjord and covers the northern part of the CNO and most of the NNO (Fig. 1). Structural data Origin of the data The structural data have been derived from two different sources. The data north of 68°N were collected during recent GEUS mapping projects (2001–2003), while the data from south of 68°N have been extracted from pub- lished GGU and GEUS maps, collected during previous GGU, DLC and GEUS projects. During the recent GEUS 128 mapping projects, data were collected in the northern Nagssugtoqidian orogen. Structural measurements and other geological data were noted in field diaries together with their geographical coordinates using global position- ing system (GPS) receivers. These data were subsequently entered in spreadsheets and imported into ArcView®. The geographical distribution of the structural data reflects the way they were collected along shorelines and on in- land traverses. There may be several measurements at any one location, whereas no data were obtained in areas between traverses (which were often located many kilo- metres apart). In this study, we have restricted our analy- sis to the structural measurements. However, a combina- tion of these data with other information, e.g. lithologi- cal and geophysical data, would make this GIS-based anal- ysis tool even more powerful. The southern part of the study area, south of 68°, is covered by two 1:100 000 scale maps, which were com- piled prior to the digital storage of field data. The Agto map sheet in the west (Olesen 1984) consists of analogue data (but was recently digitised), whereas the Ussuit map sheet in the east was produced in digital format (Fig. 1; van Gool & Marker 2004). The structural data from the Agto and Ussuit map sheets are stored in GEUS’ Geogreen map database and were extracted from this for the present study. However, these structural data only represent a frac- tion of the original data collected in the field. During the map compilations, the original structural data recorded on field maps or noted field diaries were filtered such that only representative measurements were shown on the fi- nal map; each measurement typically covers an area of a few square kilometres. Thus, the southern part of the da- ta compilation map in this paper shows an even distribu- tion of data, and a much lower data density, compared to the more recently compiled areas in the north. The fact that the Agto map sheet only contains very few lineation measurements compared to the surrounding regions also reflects a change in focus since the 1960s and 1970s, when measurement of lineations was not considered a high pri- ority. A large gap in the data coverage occurs in the east, from Arfersiorfik fjord to the north almost up to Sydost- bugten (Fig. 1); this area was not covered by the mapping programmes by GGU and GEUS. Definition of terms In the descriptions below the general orientation of a struc- tural element is its three-dimensional orientation with respect to true north and horizontal, as defined by the combination of strike, dip direction and dip for planar structures, and plunge direction and plunge for linear structures. the term trend is used as the direction along the strike of a planar structure (without indication of dip direction), or the direction of plunge of a linear structure, without distinction between plunges up or down this di- rection. The trend is always expressed as two opposite di- rections (e.g. NE–SW). Methods of data presentation For map presentation of the data we have used ArcView® version 3.2. In addition, stereographic projections and sta- tistical analysis for the determination of great circles and point maxima were prepared with StereoNett (J. Duyster, unpublished freeware). Once the structural dataset has been incorporated into the GIS database, the ArcView® Geoprocessing extension can be used to easily select sub- sets of data in areas with irregular shapes (Fig. 2), or alter- natively functions like ArcView® Query Builder can be used to select data with certain characteristics. The data were plotted on a topographic map using con- ventional structural symbols, whereas orientations and dip angles were colour-coded. For foliations and lineations, four maps with different colour codes were plotted (Figs 3–7). Having attempted several different ways of display- ing variations of dip/plunge directions on maps, we found 51°52°53° 50° 69° 68° 20 km 2 7 11 9 10 14 13 16 4 5 3 6 8 15 12 2 1 Fig. 2. Structural domains in the central and northern Nagssugto- qidian orogen, based on structural orientation data. Domain num- bers refer to those used in Table 1, Fig. 8, and the main text. 129 that the regional trends were best shown using a subdivi- sion into colour-coded quadrants. This gives a clear indi- cation of variations on a regional scale and displays fea- tures that conventional plots of structural data would not have easily revealed. Other features of the data could be highlighted with other methods of coding, or by plotting them on a different scale. The GIS program allows the user to change the coding criteria and colours with a lim- ited number of key strokes, and thus forms a powerful, user-friendly tool of analysis. We plotted one map for each of the planar and linear datasets (Figs 3A, 5A), using differently coloured symbols for dip/plunge directions within each of four different quadrants: directions between 0–90° (NE quadrant) are shown in red, 90–180° (SE quadrant) in orange, 180– 270° (SW quadrant) in green, and 270–360° (NW quad- 69° 51°52°53°54° 50° 68° 20 km Foliation dip directions NENW SW SE Fig. 3A. Foliation data, using four different colours to represent dip directions in the four quadrants of 0–90°, 90–180°, 180–270° and 270–360°. 130 rant) in blue. We also plotted foliations, lineations and fold axes, respectively, in three maps where the colour intensity reflects the steepness of the dip/plunge (Figs 3B, 5B, 7). Here the light orange colour indicates shallow dips/ plunges, and darker brown to black colours indicate pro- gressively steeper dips/plunges. The data were also plotted in a third way by combin- ing the two just described methods. The different colours were maintained for the dip/plunge directions within each of the four quadrants, combined with colour intensity to display the variations in dip. The foliation data were split into two separate plots to show more detail and avoid clut- ter: Fig. 4A shows the overall ENE–WSW-trending struc- tures (blue and orange), whereas Fig. 4B contains the over- all ESE–WNW-trending structures (red and green). A similar method was used for the lineations, however, on 69° 51°52°53°54° 50° 68° 20 km 0° < dip < 19° Foliations 20° < dip < 29° 30° < dip < 49° 50° < dip < 69° 70° < dip < 90° Fig. 3B. Foliation data, using colour intensity to reflect dip angle. Darker colours indicate steeper dip angles. 131 the scale of presentation many of the red and green linea- tions (ENE–WSW-trending) would overlap. Therefore, the green symbols were plotted separately (Fig. 6A), where- as the red symbols were included with the blue and or- ange ones (Fig. 6B); there are relatively few blue and or- ange symbols and therefore less cluttering. There are significantly less measurements of fold axes than of other structural elements. Therefore we were not able to use their orientations for analysis of regional trends, and the fold axes are only colour coded for plunge angle (Fig. 7). In some areas the very high data density causes a satu- ration with the colour of the main orientation on the scale of presentation. Although the main trends can still be seen, minor orientation components may be obscured. This problem can be overcome by zooming in on smaller areas 69° 51°52°53°54° 50° 68° 20 km 0° < dip < 15° Foliation Azimuth and dip 16° < dip < 30° 31° < dip < 45° 46° < dip < 60° 61° < dip < 75° 76° < dip < 90° SE NW SW Fig. 4A. Foliation data, using blue and orange colours for NE–SW trends with NW and SE dips, respectively, combined with colour intensity to reflect dip angle. See Fig. 4B for NW–SE trends. 132 and printing on a different scale, revealing the full range of the data (e.g. Mazur et al. 2006, this volume). Structural domains Apart from the general variations in structural style, it is apparent that there are well defined areas with distinct structural patterns. We therefore divided the whole study area into 16 structural domains (Fig. 2), within each of which the structural characteristics are largely consistent and more or less distinct from those of adjacent domains. this subdivision is exclusively based on visual evaluation of the plotted data. A more rigorous approach for the def- inition of domains would have been possible, for example the method by Vollmer (1990) based on eigenvalue cal- 69° 51°52°53°54° 50° 68° 20 km 0° < dip < 15° Foliation Azimuth and dip 16° < dip < 30° 31° < dip < 45° 46° < dip < 60° 61° < dip < 75° 76° < dip < 90° NE SW Fig. 4B. Foliation data, using red and green colours for NW–SE trends with NE and SW dips, respectively, combined with colour intensity to reflect dip angle. See Fig. 4A for NE–SW trends. 133 culations of data in small subsets, but is beyond the scope of the present study. The domains are presented schema- tically in Fig. 2, and their outlines are also shown in Figs 3–8. The structural data for each of the domains were extracted and plotted as equal area, lower hemisphere ste- reographic projections (Fig. 8). Foliations were plotted as poles to planes and contoured, and the orientation of the maximum density of data indicated in each plot. Great circles were calculated where visual inspection of the con- toured data suggested that a great circle distribution ex- ists. Calculations of great circle and fold axis orientations are based on the orientations of the three eigenvectors of the data. The large number of data in the contoured plots results in an accentuation of the high concentrations, while smaller populations that define separate structures are less visible. However, these are included in the calculations of 69° 51°52°53°54° 50° 68° 20 km Lineation orientations NENW SW SE Fig. 5A. Lineation data, using four different colours to represent plunge directions in the four quadrants of 0–90°, 90–180°, 180–270° and 270– 360°. See also Fig. 5B. 134 the great circles and fold axes, and therefore the calcula- ted great circle may diverge from the one defined by the maximum orientations, as in domains 7, 9 and 14 (Fig. 8). Lineations were plotted and contoured, with indica- tion of the orientation of the maximum concentration of data (Fig. 8B). Table 1 contains short descriptions of the characteristics of each domain regarding foliation, linear data and general geology. Results The main variations in the structural patterns within the study area are described in the following sections, using structural maps and stereographic projections (Figs 3–8). The structural variations are apparent at a first glance as clustering of data and variations in colours; they reflect the nature of the large-scale tectonic evolution of the oro- gen, which is discussed in a final section. 69° 51°52°53°54° 50° 68° 20 km 0° < dip < 15° Lineations 16° < dip < 30° 31° < dip < 45° 46° < dip < 60° 61° < dip < 90° Fig. 5B. Lineation data, using colour intensity to reflect steepness of plunge. See also Fig. 5A. 135 Foliations Overviews of the orientations of the planar fabrics are shown in Figs 3, 4. The predominant foliation trend is ENE–WSW, shown in orange and blue colours. Linear belts in this direction, dominated by steeply dipping folia- tions, alternate with broader regions characterised by strongly variable orientations. These belts and regions with different structural characteristics have previously been referred to as steep belts and flat belts, respectively (Mark- er et al. 1995). The distinct alternation between such dis- tinct linear belts and folded regions diminishes towards the north, and the predominant general ENE–WSW trend becomes progressively weaker, as reflected by the increase of red- and green-coloured symbols. This is apparent es- pecially in the north-eastern corner of the study region, where the foliations are dominated by NE dip directions 69° 51°52°53°54° 50° 68° 20 km 0° < plunge < 15° Lineation Direction and plunge 16° < plunge < 30° 31° < plunge < 45° 46° < plunge < 60° 61° < plunge < 75° 76° < plunge < 90° SW Fig. 6A. Lineation data, using green colour to represent SW directions and colour intensity to reflect steepness of plunge. See Fig. 6B for NW, NE and SE directions. 136 shown in red. Coupled with this progressive change towards the north there is an overall decrease in the dip angle, as expressed by an increasing amount of light or- ange-coloured symbols. In the south, the two main linear belts, the Nordre Iso- rtoq steep belt in domain 1 and the Nordre Strømfjord shear zone in domain 4, are characterised by a near-uni- form ENE–WSW-trending foliation, a marked absence of NW–SE-trending foliations, and steep dip angles. A third linear belt in the north, the Naternaq belt in domains 12 and 13 and the northern part of domain 7, is discon- tinuous and less well defined. Smaller, discontinuous shear zones also occur in the NNO e.g. in domains 10 and 15; these are indicated by strong clustering and alignment of symbols of the same colour, but not necessarily by steep dips. Dip directions of the overall ENE–WSW-trending 69° 51°52°53°54° 50° 68° 20 km Decreasing colour intensity = decreasing angle of plunge Lineation Direction and plunge SW SE NW NE Fig. 6B. Lineation data, using blue, red and orange colours for NW, NE and SE orientations, respectively, combined with colour intensity to reflect steepness of plunge. See Fig. 6A for SW directions. 137 structures (in blue and orange colours, Fig. 3A) show a clear regional pattern of alternating NW and SE dip di- rections. In the south, switches in dip directions are asso- ciated with the two main linear belts such that the inter- vening area, which forms a large anticlinorium (van Gool et al. 2002), is characterised by predominant SSE dips (in orange), while NNW dips (in blue) prevail to the north and south. NW–SE-trending foliations are predominant in two distinct areas in the north-east: one in the extreme north-eastern corner with predominating NE-dipping foliations (in red), and another around Sydostbugten, char- acterised by SW-dipping foliations (in green). Farther south only two areas of uniform dip directions are recognised, one around Attu with predominant SW dips (in green), and another forming a belt north of the Nordre Strømfjord shear zone, which has uniform NW dips (in blue). 69° 51°52°53°54° 50° 68° 20 km 0° < plunge < 15° Fold axes 16° < plunge < 30° 31° < plunge < 45° 46° < plunge < 60° 61° < plunge < 75° 76° < plunge < 90° Fig. 7. Fold axis data, using colour intensity to reflect steepness of plunge. 138 69° 51°52°53°54° 50° 68° 20 km n = 233 max. density at 337/78 2 3 4 6 7 8 11 9 10 15 16 1 12 13 14 n = 231 max. density at 337/78 n = 529 max. density at 346/78 5n = 630 max. density at 347/54 n = 1239 max. density at 323/48 FA 268/27 n = 746 max. density at 335/72 FA 244/26 n=198 max. density at 175/78 n = 829 max. density at 330/24 FA 246/4 n = 343 max. density at 163/42 FA 244/4 n = 569 max. density at 260/18 FA 251/14 n = 384 max. density at 128/48 n = 476 max. density at 153/66 FA 72/33 n = 552 max. density at 180/24 FA 251/6 n = 743 max. density at 129/42 FA 50/20 n = 1152 max. density at 60/24 FA 67/21 n = 1238 max. density at 171/84 FA 80/24 2 1 2 3 4 5 12 6 7 8 1110 Foliations 14 13 15 16 5 4 9 Fig. 8A. Characterisation of structural domains from Fig. 2 with stereographic projections of foliations within each domain. Poles to foliations plotted on lower hemisphere, equal angle nets and contoured at 1, 2, 3, etc. times random distribution. The number of data points (n) and orientation of maximum density are indicated for each plot. FA, calculated fold axis. 139 69° 51°52°53°54° 50° 68° 20 km 2 3 4 5 6 7 8 11 9 10 15 16 1 12 13 14 n = 27 max. density at 60/18 n = 309 max. density at 78/12n = 29 max. density at 72/30 n = 86 max. density at 253/06 n = 59 max. density at 49/24 n = 308 max. density at 267/24 n = 547 max. density at 240/12 n = 92 max. density at 267/24 n = 592 max. density at 244/06 n = 288 max. density at 240/12 n = 224 max. density at 254/12 n = 139 max. density at 129/48 n = 217 max. density at 72/0 n = 465 max. density at 258/12 n = 373 max. density at 60/24 n = 525 max. density at 45/18 2 1 2 3 4 5 6 7 1110 Lineations 14 13 15 16 5 4 9 12 8 Fig. 8B. Characterisation of structural domains from Fig. 2 with stereographic projections of lineations within each domain. Data plotted on lower hemisphere, equal angle nets and contoured at 1, 2, 3, etc. times random distribution. The number of data points (n) and orientation of maximum density are indicated for each plot. 140 Steep N-dipping foliation with consistent orientation. A sharp transition to S-dipping foliations at the northern boundary of the belt. Foliation curved along shallowly ENE-plunging folds on a scale of tens of kilometres. Dips variable, SE dips predominating over NE and less common SW dips. Steep foliation. Both northerly and southerly dips. Few folds on scales of 0.5–1 km. Steep foliations with consistent orientations, slightly oblique to the trend of the linear belt. Northerly dips predominate. Near the southern boundary a sharp transition to SSE-dipping foliations. Gradual northward transition from steep dips close to the NSSZ to shallower NNW dips. The foliation locally curves into the shear zone. NE dips are mixed with the dominant NNW dips in most of the area. The plentiful data points on the stereonet (Fig. 8A)obscure orientations (mainly NE-dipping) away from the maximum. Intensely folded region with 5–10 km large folds, mainly W-plunging. Northerly and westerly dips pre- dominate in contrast to domain 5. Variable strike. NNW dips less predominant than in domain 5. Folding. Dips mainly moderate, but steep in the south-west. Fig. 8A displays a girdle over shallow, WSW-plunging fold structures. A calculated great circle is discordant to the measured maxima, which align on a steeper great circle. Foliations outline the large fold structure visible on Fig. 1. Southerly dips predominate in the northern part (green and orange/brown, Fig. 3A). A zone with NW dips (blue) runs through the domain centre. Mixed dip directions in the south. Dips moderate to steep. Irregular stereonet pattern (Fig. 8A), with remnants of a great circle distribution similar to that in domains 6 and 7. Foliation Lineation Geology SynopsisDomain 1 2 3 4 5 6 7 8 Consistent, predominant shallow ENE plunge. Variable lineations, with predominant shallow ENE plunges, parallel with fold axes. Shallow, SE-plunging lineations in a c. 10 km wide zone south of the NSSZ. Lineations mainly shallowly ENE-plunging. Some variation in fold hinges, especially near the eastern domain boundary. Subhorizontal lineations, the majority WSW-plunging in contrast with surrounding areas. Progressive change from shallow ENE plunges near the NSSZ towards steep NE and N plunges in the north-west. Mixed orientations with a cluster of shallowly SE-plunging lineations. In the west progressive change in orientation continues from domain 5. More variable orientations in the east. Rather consistent ENE- trending lineations. ENE plunges in the east become shallower and mixed with WSW-plunging domains in the west, followed by moderate to steep WSW plunges at the coast. Locally steep lineations in fold cores. Steep SW- and SE-plunging lineations around a large central fold core. Mainly E–W-trending lineations in the west in transition to domain 7. Variable lineations in the east. Two maxima of shallow W plunges and steeper SE plunges (Fig. 8B), the latter possibly with a small circle distribution. Nordre Isortoq steep belt (shear zone), predominant sinistral shear. Mainly paragneiss. Northern CNO flat belt of interleaved Archaean orthogneis- ses and Proterozoic ortho- and paragneis- ses. Steep belt within the northern CNO flat belt; tightly interleaved ortho- and paragneis- ses. NSSZ. Interleaved ortho- and paragneis- ses. Archaean ortho- gneisses and horizons of supracrustal amphibolite. Archaean indentor, ortho- and paragneiss as in domain 5. Large fold structures. Archaean orthogneisses and metasedimentary rocks define a poorly sampled linear zone at the northern boundary of the indentor block of Piazolo et al. (2004). A few kilometre-sized fold structures. Orthogneisses interleaved with supra- crustal amphibolite. A fold interference pattern occurs south of the main fold at the western end of the Naternaq belt. High-strain strike-slip zone constrained by metasedimentary rocks. Anticlinorium between two shear zones. Lineation constant in spite of intense folding. Only one zone where lineations plunge SE, locally steeply. Steep zone with tight folds within the larger domain 2 with large fold structures. Steep zone discontinuous to the east, and concentrated in an area dominated by two metasedimentary belts. Sinistral strike slip zone5 km wide. Obliquity of foliation fits with sinistral shear. Region with shallow, N-dipping structures. Overall W- and WSW-plunging folds. Two sets of lineations, one variable, the other consistently WSW-plunging, parallel with fold axis. Predominantly straight and steep foliations with ENE trends, but variable dip directions, and consistently shallow lineations. Misalignment of foliation girdle and calculated great circle (Fig. 8A) indicates complex fold pattern: along-strike variation of lineations (and presumably fold axes) and local folds with steep axes disturb the stereographic plot. Non-consistent dip directions and dips in the northern linear zone suggest intense folding. Mixed structural patterns including ENE-trending fold limbs and fold interfe- rence patterns. The core of the large fold with the steep lineations is located along the southern extension of a NNE-trending belt of steep lineations on the western limb of the Naternaq supracrustal belt. Table 1. Summary of characteristic features of the structural domains CNO: Central Nagssugtoqidian orogen. NNO: Northern Nagssugtoqidian orogen. NSSZ: Nordre Strømfjord shear zone. 141 Consistent ENE trend with both NNW and SSE dip directions. Large fold structures in the east. Steep dips, shallower towards north. Great circle distribution with shallowly NNW- dipping and subordinate steep, SSW-dipping flanks tentatively interpreted as due to asymmetric S-vergent folds (Fig. 8A). Mainly steep S dips with consistent trend. As in domain 9, ENE-WSW- trending foliation in the west swings towards E–W in the east. At the southern boundary a linear belt with moderate S dip. Partial great circle distribution; no dip directions within the NE quadrant (Fig. 8A). Foliations outline a W-plunging antiform c. 10 km large. The northern part of its northern limb appears overturned towards N. Great circle distribution with predominant shallow to moderate dips on stereonet (Fig. 8A). Foliations outline a fold with a folded, overall moderately SE-dipping western main limb. The high-strain southern limb dips steeply S. Large spread on stereonet (Fig. 8A). Eastern continuation of steep, high-strain southern limb of fold from domain 12 and large E-plunging antiform. NW dips more common in the north, dips shallower near hinge. The northern limb has moderate to steep NE dip. Fig. 8A shows a well-defined great circle and NE-plunging calculated fold axis. Irregular foliation in the core of the eastern Naternaq fold, forming an E–W-trending whaleback structure. Fig. 8A shows a point maximum and partial girdle which do not fit the calculated great circle (see the main text). Foliations define a large, open NE-trending synform with steepest orientations in the core, bounded by straight belts. An antiform occurs in the south-east, with its southern limb in domain 13. A well-defined great circle on Fig. 8A indicates cylindrical, NE-plunging folds. The foliation defines large, open, NE-plunging folds besides a large antiform surrounding the synform of domain 15. The stereonet data (Fig. 8A) display a point maximum with a partial great circle distribution. Foliation Lineation Geology SynopsisDomain 9 10 11 12 13 14 15 16 Predominant subhorizontal ENE–WSW-trending lineations, gradually changing to E–W in the east, where large-scale folds occur. Very strong preferred ENE–WSW subhorizontal orientation, with a tail towards E–W trends displayed on Fig. 8B. Consistent ENE–WSW-trending subhorizontal lineations, with indistinct domains of respectively easterly and westerly plunges. Strong point maximum on stereonet with shallow WSW plunges (Fig. 8B). Shallow plunges of lineations, mainly towards W. Subordinate NW plunges on northern fold limb. Fig. 8B shows strong point maximum parallel with calculated fold axis. Moderately SE-plunging lineations on the western limb, with isolated SSW plunges in hinge zone. Very few measurements on the southern limb, with shallow WSW plunges. Lineations shallow and ENE- or WSW-plunging on the southern limb. Variable plunges on the eastern limb between NE and ESE. The latter orientation most common in hinge areas. Fig. 8B clearly shows these three separate populations. Predominant subhorizontal WSW plunges, except ENE plunges at the eastern domain margin along the eastern limb of the map-scale fold. Fig. 8B shows a single strong point maximum. Strong predominance of ENE-plunging lineations along the synform axis. Consistent SE plunges south-east of the shear zone. Consistent moderate NW plunge in the antiform near head of fjord. Fig. 8B shows a fairly well defined point maximum close to the calculated fold axis. Predominant shallow, NE-plunging lineations and a small population of slightly steeper, E-plunging lineations in the north-eastern domain corner. The lineations swing, following the folds. Region around Kangaat- siaq with metasedimen- tary rocks, amphibolite and granite within the regional grey gneiss. Straight zone of pre- dominant orthogneisses around Aasiaat, bounded to the south by a high-strain zone. Ikamiut supracrustal rocks. Large antiform with 1 km-scale parasitic folds on its southern limb. Northern limb poorly exposed and undersamp- led. Western Naternaq belt. Steep paragneisses folded on 20 km-scale. Mainly data from isoclinal fold on western limb; less from straight southern limb. Steeply dipping, straight gneisses in eastern Naternaq belt. A large E-plunging antiform at the eastern end, and a north-eastern fold limb with only minor meta- sedimentary rocks. Predominantly ortho- gneiss, cut by flat-lying shear zones. Synform with strongly migmatitic paragneiss and a core of migmatitic orthogneiss. High-strain zones to the north-west and south-east. Archaean orthogneiss with thin sheets of pelitic rocks and supracrustal amphibolite. Also areas of very weakly foliated porphyritic granodiorite. Transition from predominant steep, northerly dips in the CNO to shallower, variable dips and large fold structures in the north. From east to west a large swing from ENE–SSW to E–W trends. Lineations uniform also in the area of large folds. Very consistent foliation trends, the northernmost widespread steeply dipping foliations, and very persistent lineations. Data consistent with a shallowly W-plunging antiform becoming progressively tighter westward (but not easily traced into domain 10). Western limb of Naternaq supra- crustal belt, forming a distinct zone, apparently transecting the overall ENE-trending fabric, and with uncommon SE-plunging lineations and fold axes. Consistent lineation trend in antiform, but opposite plunge directions on the limbs. Steeper lineations in the fold hinge. Very consistent lineation trends, also through the antiform in the east. Direction of plunge flips over in the east, perhaps indicating two generations of lineations. The south-eastern limb of the synform, overturned to the north-west and becoming very tight towards north-east. Distinctly different lineations in the underlying, folded shear zone. The NE-dipping orientations and open folds are significantly different from elsewhere in NNO. The shear zone exposed on either side of the synform in domain 15 does not continue in domain 16. Table 1 (continued) 142 The stereographic projections of the foliations clearly reflect two trends (Fig. 8A). The plots from the southern part of the study area define alternating point maxima and great circle distributions, reflecting, respectively, the linear belts and the fold-dominated regions. In contrast, in most of the NNO the plots mainly display (partial) great circle distributions or otherwise irregular patterns, indicating the lack of extensive linear belts in the north. Furthermore, the stereographic projections reflect the northward decrease in dip angle: in the CNO the point maxima indicate dip angles around 80°, whereas the NNO is characterised by point maxima indicating dip angles in the range 20–40°. Lineations Overviews of the linear fabrics are shown in Figs 5, 6. The highly uneven data density in the lineation maps (Fig. 5A, B) reflects that the southern and northern parts of these maps have been compiled from different sources, i.e. pub- lished maps in the south and complete field datasets in the north. Nevertheless, it is apparent that the dominant trend is ENE–WSW, as indicated by the predominant red and green colours (Fig. 5). Outside the linear belts, gra- dual changes in the lineation trends on a scale of 10–50 km or more are seen, for example between Attu and Nor- dre Strømfjord, at western Ussuit, and in the north-east of the study area. Several smaller areas are dominated by E–W trends, for example east of Attu, south-east of Sydost- bugten, and south of Jakobshavn Isfjord. Most lineations plunge 0–30°, with markedly steeper plunges in regions of map-scale fold interference struc- tures and fold hinges. This is prominent west of Ataneq, north-west of Attu, and in the eastern part of domain 13. These regions are also characterised by orientations that diverge from the general ENE–WSW trend. The structural maps (Fig. 6) stereographic projections (Fig. 8B) clearly show that WSW-plunging lineations pre- dominate in most of the NNO (north of the Nordre Strømfjord shear zone) except for small clusters of NE- plunging lineations and the area east and north-east of Sydostbugten, where the plunge is towards ENE (domains 13, 15 and 16); there is a sharp break between these two plunge directions east of Sydostbugten. The same pattern is shown by the fold axes calculated from the great circle girdles of the foliations (Fig. 8A). South of the Nordre Strømfjord shear zone (in the CNO), the point maxima of the lineations and calculated fold axes consistently in- dicate shallow ENE plunges. Fold axes Orientations of fold axes are shown on Fig. 7. The sym- bols are colour-coded for plunge angle in order to facili- tate comparison with the lineation data. The orientations of the fold axes mimic the general characteristics of the lineations, being generally subparallel with the latter. Their distribution in clusters reflects a higher density of meas- urements in areas of map-scale fold hinges, where out- crop-scale folds are more common. Tectonic implications The structural data presented in this paper show that the tectonic style changes significantly across the central and northern parts of the Nagssugtoqidian orogen. While the CNO is dominated by steep, linear and continuous belts separated by well-defined areas of large-scale folding, the NNO does not contain such continuous, linear belts, whereas 20–80 km-scale folds are abundant. The main change in tectonic style occurs across the Nordre Strøm- fjord shear zone. More specifically, the southern part of the study area (the CNO and the Nordre Strømfjord shear zone) is dominated by alternating linear belts and folded regions. The corresponding structural domains follow the main strike of the linear belts and are continuous from the coast to the Inland Ice. The linear belts themselves are dominated by sinistral strike-slip deformation. In contrast, the domains in the NNO are generally less elongate and reflect the lack of linear belts of similar dimensions as in the CNO. Small high-strain zones are observed locally, e.g. along the northern and southern margins of domain 15 and with several examples in domains 7, 9 and 10. Shear sense indicators are rare and inconsistent in the NNO, and the overall deformation in this region seems to be predominantly coaxial (Piazolo et al. 2004; Mazur et al. 2006, this volume). The change in tectonic style is interpreted to be a result of (a) differences in localisation of strain as high-strain, steep belts, (b) different deforma- tion kinematics (strike-slip wrench tectonics in the south, coaxial deformation in the north), and (c) variations in the intensity of deformation. We consider that the overall Palaeoproterozoic strain is significantly lower in the NNO than in the CNO. The NNO commonly preserves shal- low dips, which presumably predominated after the origi- nal phase of thrusting. Besides, the two latest deforma- tion phases, which are responsible for the steep foliation in the CNO, are less intense in the NNO (van Gool et al. 2002; Piazolo et al. 2004; Mazur et al. 2006, this vol- ume). These observations may account for the previously 143 outlined differences in the mode of strain localisation. The data presented here furthermore show that the change in style is rather abrupt across the Nordre Strømfjord shear zone, and thus support the interpretation by Sørensen (1983) and Sørensen et al. (2006, this volume) that this structure is of crustal scale and has a significant offset – a notion that has previously been questioned by Hanmer et al. (1997). It is well established in the literature that the structural pattern of the CNO is fully attributed to Nagssugtoqidian deformation (van Gool et al. 2002). In contrast, the NNO is currently interpreted as having only in part been affect- ed by Palaeoproterozoic deformation, and the Nagssugto- qidian strain is furthermore partitioned into smaller re- gions (Piazolo et al. 2004; Mazur et al. 2006, this vol- ume). A significant part of the deformation in the NNO thus seems to be of Archaean age, and its overall structur- al style defined by interference between Archaean and Palaeoproterozoic structures. Therefore, like Mazur et al. (2006, this volume) we suggest that the change in tecton- ic style from south to north reflects partitioning of Nags- sugtoqidian strain. This is clearly illustrated by the small- er and less elongate structural domains in the NNO, and by the significant change of the general trend of both fo- liations and lineations towards the north-eastern corner of the study area, where we consider that the influence of the Palaeoproterozoic deformation diminishes rapidly. This interpretation is supported by the relatively low metamor- phic temperatures recorded by Hollis et al. (2006, this volume) in some parts of the NNO. It is beyond the scope of this contribution to explore the details of the structural domains that we have out- lined. However, three other contributions in the present volume of Geological Survey of Denmark and Greenland Bulletin deal with the specific nature of some of these domains. Sørensen et al. (2006, this volume) investigate the character of the Nordre Strømfjord shear zone and adjacent areas in domains 4 and 5, Mazur et al. (2006, this volume) focus on the partioning of structures within domains 6, 7 and 8, and Hollis et al. (2006, this volume) describe structures within domains 11 and 15. Conclusions The application of a GIS computer program enables us to visualise large amounts of structural data in a variety of ways. Thus, we can rapidly obtain an overview of large datasets that are otherwise difficult to manage, and delin- eate areas with consistent tectonic trends. Although the methods applied here de not reveal features that are not present in the original geological maps, they can substan- tially facilitate the detection and description of structural trends and variations. In addition, stereographic plots of each of the domains can quickly be produced and ana- lysed. In the present case, the methods greatly helped to subdivide the central and northern Nagssugtoqidian oro- gen into distinct structural domains with specific individ- ual characters. The investigation of the large-scale structural trends in the central and northern Nagssugtoqidian orogen revealed distinct changes in the tectonic style from south to north. In the core of the orogen, the ENE-striking Nordre Strøm- fjord shear zone from the coast to the Inland Ice forms the most prominent feature. The tectonic character of the orogen changes across this structure from predominantly ENE-tending, steep fabrics in the south to large fold pat- terns and generally flat structures in the north, with a marked decrease in the main dip angle from c. 80° south of the shear zone, to c. 20–40° in the north. In addition, a significant decrease in the intensity of deformation is ap- parent, coupled with a decreasing proportion of the strain localised in linear belts. We interpret these patterns as re- flecting a general northward decrease in the Palaeoproter- ozoic tectonic overprint on Archaean structures, as well as strain partitioning in smaller regions. Hence, some of the structural domains in the NNO are largely unaffect- ed by pervasive Palaeoproterozoic deformation. Acknowledgements Reviews by John Grocott and Ken McCaffrey are grate- fully acknowledged. References Bonham-Carter, G.F., Agterberg, F.P. & Wright. D.F. 1990: Weights of evidence modelling: a new approach to mapping mineral potential. Geological Survey of Canada Paper 89, 171–183. Books, C.J. 2000: Defining groundwater system recharge and vulnera- bility areas in regions of suburban expansion; overview of the north- ern Illinois example. Abstracts with Programs – Geological Society of America 33, 45 only. Carrasco, M.A. & Barnosky, A.D. 2000: MIOMAP: a GIS-linked da- tabase to assess the effects of tectonic and climatic changes on mam- malian evolution. 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Abstracts with Programs, Geological So- ciety of America 32, 45 only. __________________________________________________________________________________________________________________________________________________________________________________ Manuscript received 4 November 2004; revision accepted 20 December 2005