Geosciences 2014, 4, 316-334; doi:10.3390/geosciences4040316 OPEN ACCESS geosciences ISSN 2076-3263 www.mdpi.com/journal/geosciences Review Continental Collision Structures and Post-Orogenic Geological History of the Kangerlussuaq Area in the Southern Part of the Nagssugtoqidian Orogen, Central West Greenland Jon Engström 1,* and Knud Erik S. Klint 2 1 Geological Survey of Finland, P.O. Box 96, SF-02151 Espoo, Finland 2 Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark; E-Mail: [email protected] * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +358-29-503-2624; Fax: +358-29-503-2901. External Editors: Ken McCaffrey and Jesus Martinez-Frias Received: 29 August 2014; in revised form: 23 November 2014 / Accepted: 2 December 2014 / Published: 10 December 2014 Abstract: Deep-seated continental collision sutures, formed at a depth of more than 20 km, are exposed near Kangerlussuaq, close to the Greenland ice cap, on the southern margin of the Nagssugtoqidian orogen in Central West Greenland, thus offering a rare opportunity to study the tectonic deformation style of such an orogen. This paper adds new information on the tectonic history of the southern flank of the Nagssugtoqidian orogen. It focuses on (1) the results of a detailed structural investigation of lineament zones revealed from remote sensing of geophysical and topographic data and aerial photo interpretation, (2) detailed geological mapping at key locations and (3) a tectonic structural model describing the geological development of the area. The area has undergone several episodes of deformation, which have been compiled into an event succession that recognizes eight tectonic events overprinting each other: Two stages of folding (F1 and F2) have been identified along with one major episode of intrusion of the Kangâmiut mafic dyke swarm (2.05 Ga) into the Archaean continent. These dyke intrusions are very important, since by examining the style of deformation for these intrusions it is possible to define the transition from the North Atlantic Craton in the south to the mobile belts in the Nagssugtoqidian orogen in the north. Five different types of pronounced lineaments and one less pronounced lineament post-dating the Kangâmiut dykes extending from ductile deformation shearing events to brittle deformation with extensive faulting. These lineaments cover both the collisional and post-collisional Geosciences 2014, 4 317 tectonic history of the area. The study focused on two types of lineaments: one semi-ductile type trending E–W with a dextral sense of shear and a second, a pronounced lineament outlining the Kangerlussuaq–Russell thrust fault. These two features are interpreted to be related to the Nagssugtoqidian orogeny, while the latter lineaments have a more brittle appearance and are regarded to be considerably younger and probably related to post-orogenic tectonic events. Keywords: continental collision structures; Kangerlussuaq–Russell thrust fault; tectonic structural model; Nagssugtoqidian orogeny; West Greenland 1. Introduction The Nagssugtoqidian orogeny of West Greenland mainly occurred between 1.92 and 1.75 Ga [1], as a consequence of the convergence and collision of the North Atlantic Craton and a poorly defined craton to the north [2]. The Palaeoproterozoic reworking during the orogeny extends from Kangerlussuaq to southern Disko Bugt [3–5]. Several studies have been performed in the central and northern areas of the Nagssugtoqidian orogen (e.g., [1,6]), while the southern end of the orogen at the northern margin of the North Atlantic Craton, in the proximity of Kangerlussuaq, has largely remained unexplored. However, since 2008, the international Greenland Analogue Project (GAP) has been conducting intensive research in the area around Kangerlussuaq [7]. The main goal of the GAP has been to investigate the deep groundwater in a glaciated terrain. In order to study and model the hydrogeology, a detailed geological investigation and a conceptual geological 3D model of the fault/fracture zones was required. The GAP also drilled three cored boreholes in the vicinity of the ice margin, which provided data for the 3D geological interpretation. The geological survey revealed a number of structures formed in a plate tectonic collision zone, and the main goal of this paper is to add complementary information to aid in understanding the formation of the Nagssugtoqidian orogen in central West Greenland. 2. Background: The Development of the Nagssugtoqidian Orogen and the Study Area The Nagssugtoqidian orogen is interpreted as a collisional orogenic setting where juvenile Palaeoproterozoic igneous rocks intruded Archaean units and where Palaeoproterozoic sedimentary volcanic rocks were deposited. The orogen is now seen as an approximately 250-km-wide, south- to southeastward-trending belt that includes diverse Archaean and Palaeoproterozoic rocks affected by polyphase deformation and high-grade metamorphism [8]. Along the northern margin of the North Atlantic craton, within the southern Nagssugtoqidian orogen, Archaean dioritic, granodioritic, tonalitic and granitic orthogneiss of the North Atlantic craton predominate, albeit in a reworked state [2,9–12]. The orthogneisses have been dated to 2.87–2.81 Ga, with the plutonic protoliths deformed and metamorphosed between 2.81 and 2.72 Ga, immediately after their emplacement [11,12]. The orthogneisses are cut by several sets of mafic dykes, the most voluminous of which is the E–W- to NE–SW-trending, 2.05–2.04 Ga Kangâmiut dyke swarm [13–21]. The Kangâmiut dyke swarm, which extends from the Archaean foreland into the centre of the orogen, has become reworked across a sharp transition at the southern Geosciences 2014, 4 318 Nagssugtoqidian front. Its northern extent is abruptly terminated by the Ikertoôq thrust zone (Figure 1), which is a south-vergent ductile shear zone [2]. Dykes from the Kangâmiut swarm occur in the Archaean gneiss complex for approximately 150 km south of the thrust zones traditionally taken as marking the southern Nagssugtoqidian front in SW Greenland, and continue as deformed bodies in the reworked Archaean gneisses for approximately 50–100 km to the north of the front [17]. The granulite-facies thermal peak of metamorphism in the core of the orogen was dated at ca. 1.86–1.84 Ga [11,20,22,23]. This age interval coincides with the timing of thrusting and is interpreted as documenting the main phase of collision across the Nagssugtoqidian orogen. The Nagssugtoqidian orogen is interpreted as being associated with the closure of two south-dipping Palaeoproterozoic sutures [6,24,25]: (1) a northern suture, the Disko Bugt suture, which separates the lower plate Rae craton from the middle Aasiaat domain; and (2) a southern suture rooting in the Nordre Isortoq steep belt that separates the southerly-located North Atlantic Craton from the Aasiaat domain [26]. This model was further developed by Garde and Hollis [1], who concluded there are two south- to SSE-dipping subduction zones in the central and northern Nagssugtoqidian orogen, which were separated by the previously unidentified Aasiaat domain or microcontinent. Figure 1. (A) Geological map of the central West Greenland and the Nagssugtoqidian orogen, indicating the location of Kangerlussuaq. Map modified from van Gool et al. [2]. (B) A lineament and geological map of the study area, in front of the Greenland ice sheet. Map modified from Garde and Marker [27]. The Study Area The study area is located at the southern end of the orogen in the vicinity of the North Atlantic Craton (Figure 1). It covers a domain of 60 km × 80 km with a special focus on an area at the front of the Geosciences 2014, 4 319 Greenland ice sheet (Figure 1A). Most of the mapping, including the drilling of three deep boreholes, was conducted in area A (Figure 1B) during field campaigns in 2008–2009, while area B was surveyed during mapping campaigns in 2010–2011. A geological 3D model close to the boreholes was constructed for groundwater modelling purposes. The mapping at other locations primarily focused on the nature of the lineaments and their crosscutting relations in order to identify lineaments with potential for hydraulically conductive fractures/faults. A preliminary geological event succession based on lineament interpretation of the area was presented by Klint et al. [28] in 2013, complementary data were collected in the field, and this paper presents an updated geological model of the Kangerlussuaq area. The study area, extending from Kangerlussuaq Airport to the ice sheet, is dominated by primarily felsic banded gneisses and mafic garnet-rich orthogneisses, intruded in places by pronounced 1–25-m-wide Kangâmiut dykes, especially in the southwestern part of the study area. Detailed geological mapping focused on selected sites from Kangerlussuaq Airport to the margin of the ice sheet (Figures 1B and 2). The Nagssugtoqidian structures reflect the general ductile nature of the regional deformation and include a penetrative gneissic fabric, macro-scale folds, and evidence of shearing in pronounced shear zones. Brittle structures such as faults and fractures are abundant and were probably formed in a younger, shallow, colder and hence more rigid environment. Figure 2. Geological map illustrating the distribution of the lineaments in the ice-margin valley area as well as the main general structures and rock fabric. Seven key locations were investigated in 2011 and three more (Figures 3–5) in 2013. The key locations described below are highlighted along with the different lineament systems defined by Klint et al. [28]. Geosciences 2014, 4 320 Figure 3. Geological map illustrating the geology around the two boreholes, DH-GAP03 and DH-GAP04. The cross-section illustrates how the folding pattern changes from a larger open fold in the E towards tight to isoclinal folding in the W. The same can be observed in the stereographic plots. Geosciences 2014, 4 321 Figure 4. Location B. Overview of the BBQ location. Detailed measurements of faults and fractures were conducted along 3 transects, BBQ 1–3.