Geological Survey of Denmark and Greenland Bulletin 23, 2011, 65–68 65 This paper describes structural data collected during field work in southern East Greenland, a region characterised by a complex tectonic history. Here, 3D photogeology based on aerial and oblique photographs using high-resolution photo- grammetry of a 150 km2 area in Sødalen in southern East Greenland shows ESE–WNW-trending faults cross-cutting Paleocene rift structures and f lexure-related normal faults. The kinematic analysis highlights oblique and left-lateral strike-slip movements along faults oriented 120°. Strike-slip and dip-slip kinematic indicators on the walls of the chilled contacts between alkaline E–W-oriented dykes and the vol- canic host rocks suggest that the faults and dykes formed at the same time, or maybe the faults were re-activated at a later stage. Palaeostress analysis, performed by inversion of fault-slip data, shows the presence of three different tectonic events. Coupling the 3D photogeological tool with struc- tural analysis at key localities is a fundamental way to under- stand better the tectonic history of such a large area. Geological setting The Blosseville Kyst in southern East Greenland is charac- terised by a thick sequence of f lood basalts and mafic intru- sions (Fig. 1). The Skaergaard layered gabbro, the Miki Fjord macrodyke and dolerite sill complexes were formed dur- ing the continental break-up and the initial opening of the North–East Atlantic ocean at 55 Ma (Nielsen 1975; Karson & Brooks 1999; Tegner et al. 2008). In the Sødalen region pre-basaltic sediments characterise the Kangerlussuaq Basin and the lower part of the Blosseville Group (Wager 1947; Nielsen et al. 1981). Sedimentological studies recognise different facies associations of late Aptian to late Paleocene age (Larsen, M. et al. 1999). The youngest part of the basin comprises interfingering Paleocene volcanic units. Based on stratigraphy, geochemistry and petrography, the lavas of the Blosseville Group have been divided into two main se- ries: (1) a 2 km thick sequence of volcanic rocks that formed in a continental rift environment (Nielsen et al. 1981), and (2) a 6 km thick sequence of plateau basalts (Larsen, L.M. et al. 1989). Furthermore, the Blosseville Kyst is characterised by different generations of dykes and sills, partly related to the break-up and post-break-up history (Wager 1947; Hanghøj et al. 2003). Southern East Greenland is a type example of a volcanic rifted margin (Geoffroy 2005). The geological evolution of the margin is interpreted as the result of a NE–SW-oriented Late Cretaceous rifting phase that led to the onset of oceanic spreading in the Late Paleocene – Early Eocene (c. 55 Ma) after a period of syn-rift continental tectonism and volcanism. The general south-east dip of the basalts, the presence of landward- dipping normal faults and the coastal dyke swarm suggest a regional lithosphere f lexure (Larsen, H.C. & Saunders 1998). Sødalen region Sødalen is an 8 km long, NW–SE-oriented, U-shaped glacial valley extending SE–NW up to the ‘Sødalengletscher’ (Fig. 2A). The bedrock of the area is characterised by gneiss base- ment, locally overlain by syn-rift sedimentary and volcanic rocks that form a monocline that dips south-eastwards. The late Paleocene syn-rift sedimentary rocks crop out along the western side of the valley; they are unconformably overlain by sedimentary rocks belonging to the Vandfaldsdalen Forma- Analysis of Palaeogene strike-slip tectonics along the southern East Greenland margin (Sødalen area) Pierpaolo Guarnieri Sø d ale n ‘Sødalengletscher’ 68°15´N 31°W Miki Fjord Skaergaard intrusion M ac ro d yk e Fig. 2A Greenland Mainly Palaeogene volcanic rocks Palaeogene gabbro Mesozoic–Palaeogene sedimentary rocks Precambrian basement 10 km B l o s s e v i l l e K y s t Fig. 1. Simplified geological map of the Sødalen region in southern East Greenland. © GEUS, 2011. Geological Survey of Denmark and Greenland Bulletin 23, 65–68. Open Access: www.geus.dk/publications/bull 6666 Main strike-slip fault Faults and dykes Bedding Structural locality 1 10 Sill Miki Fjord macrodyke Mikis Fm Lava flows Breccias Hyaloclastite Volcaniclastics Schjelderup Member Precambrian basement Vandfaldsdalen Formation 300 2 0 0 400 5 0 0 1 0 0 7 0 0 8 0 0 6 0 0 700 6 0 0 5 0 0 500 3 0 0 500 900 60 0 60 0 800 800 400 4 0 0 80 070 0 300 200 900 8 7 4 8 5 9 7 19 20 22 10 15 10 900 Sødalen 1 km 68°14´N 68°12´N 68°14´N 68°12´N 31°25´W 31°20´W 31°20´W31°25´W A B C D Lake Ice cap Quaternary deposits ‘S ød a le n gl et sc h er ’ 5 2 4 3 1 Faults (outcrop data) DykesFaultsFaults and dykes (vertical photographs) reverse faultsnormal faults left-lateral faults all faults (n = 92) N NNN s1> s2> n = 111 max = 13% n = 66 max = 18% n = 82 max = 17% n = 92 max = 15% (oblique photographs) right-lateral faults s3 N NN phase 1 phase 2 phase 3 67 tion (Nielsen et al. 1981). The unconformity may be related to pre-volcanic uplift, coeval with the NE–SW-oriented rift- ing, followed by a rapid subsidence that accommodated the volcanism (Larsen, M. et al. 1999). The continental break- up is contemporaneous with the emplacement of layered gab- bro bodies dated to c. 55 Ma, which formed at c. 2 km depth in the continental crust. The Skaergaard intrusion and the Miki Fjord macrodyke (Nielsen et al. 1981; Tegner et al. 2008) are contemporaneous with the up to 6 km thick se- quence of plateau basalts (Larsen, L.M. et al. 1989). Structural data A total of 350 measurements for structural analysis were col- lected, from two sources: (1) from outcrops (metre scale) at five sites used for kinematic analysis and (2) from 3D photo- geology to evaluate strike and dip direction and cross-cutting relationships of faults and dykes using vertical aerial photo- graphs (kilometre scale) and oblique photographs (100 m scale). A new tool for photogeology and mapping is developed and implemented at GEUS to collect geological features as 3D polylines with a descriptive GIS database suitable for 3D mod- elling (Vosgerau et al. 2010). Dykes – Three main generations of dykes are found in the area. Their relative ages can be established from cross-cutting relationships, which show that the oldest generation (D 1 ) is mainly NE–SW-oriented, orthogonal to bedding or land- ward-dipping; the trend is parallel to the Miki Fjord mac- rodyke. The average trend of the second generation (D 2 ) is ENE–WSW and these dykes are almost vertical (Fig. 2B). The third generation of dykes found in the area (D 3 ) trends E–W (Fig. 2B). Faults – Two main trends of fault traces, up to 2 km long, can be followed on the vertical aerial photographs (Fig. 2B). The oldest generation (F 1 ) is characterised by ENE–WSW- oriented normal faults. These faults are mainly landward- dipping and are interpreted as f lexure-related faults (Wager 1947; Nielsen et al. 1981). At site 3 (Fig. 2A), the Miki Fjord macrodyke contact is downfaulted by a landward-dipping (F 1 ) normal fault with an average vertical offset of 400 m. The youngest (F 2 ) faults trend ESE–WNW. South-east of localities 2 and 4 (Fig. 2A), the fault traces are curved in pla- nar view typical of strike-slip fault systems. Kinematic analysis Field data suitable for fault-slip analysis include measure- ments of fault plane orientations, slip directions, senses of slip and bedding orientations. The slip direction of faults is determined using slickensides and calcite fibres on the fault plane. Sense of slip indicators include tails and scratches and crescentic marks formed by intersection of the fault plane with secondary fractures such as: R, R’, P and T (Petit 1987). Data collected in the canyon at locality 2 (Fig. 2A) define the kinematics of a 120°-trending fault corresponding to a major left-lateral strike-slip fault that cuts the basalts. The fault zone is c. 50 m wide and contains a >50 cm thick cal- cite vein. Double movement along the fault plane with well- developed dip-slip and strike-slip slickensides and calcite fi- bres suggests a reactivation of the fault (Fig. 3). The estimated vertical offset, based on the tectonic contact between two stratigraphic markers, is around 250 m, whereas the horizon- tal offset is estimated to 500 m. This results in a more than 50–150 m wide, 120°-trending, rhomb-shaped fault zone, 1 km long in map view (Fig. 2A, locality 2) and with a nega- tive f lower structure in cross-section. To the south-east, the fault trace disappears below an ice cap and to the north-west it is covered by the moraine in front of ‘Sødalengletscher’, but it is exposed on the western side of Sødalen, where a well- Facing page: Fig. 2. Structural data analysis. A: Geological map of the Sødalen area (modified from Nielsen et al. 1981). Arrows show the direction of move- ment along strike-slip faults; contour lines 100 m. B: Rose diagrams for orientation of faults and dykes. C: Lower hemisphere stereographic pro- jection of faults grouped by kinematics; arrows show the slip vector. D: Palaeostress analysis of 92 fault-slip measurements. Black arrows indicate maximum horizontal shortening/extension; σ1, σ2, σ3 = principal axes of stress. Visualisation of the right dihedral method (red = pressure, blue = tension) shows planes that are likely to have been re-activated (the three diagrams to the right). dip slipdip slip strike-slipstrike-slipstrike-slip dip slip Fig. 3. Evidence of multiple re-activation of a fault testified by well-developed dip-slip and strike-slip slickensides on a fault plane (locality 2 in Fig. 2A). 6868 developed vertical cleavage, locally with strike-slip slicken- sides, cross-cuts the Miki Fjord macrodyke. At locality 5 (Fig. 2A), a 4 km long E–W-oriented dyke crosses Sødalen; it is an example of the latest dyke genera- tion (D 3 ). Slickensides are found on the chilled margins of the dyke, which show that both dip-slip and strike-slip move- ments have taken place. The trend of the dykes, coupled with evidence of multiple reactivation of the contact, suggests a relationship between strike-slip faults and dykes in which normal faults intruded by dykes were re-activated as left- lateral faults in a NNE–SSW extensional regime associated with the ESE–WNW-trending shear-zone (Fig. 2A). Palaeostress analysis Palaeostress analysis of the heterogeneous fault-slip data set was performed using integrated software for structural anal- ysis (Žalohar 2009). More than 90 fault-slip measurements were taken at five sites (localities 2A–C) and used for inver- sion to obtain palaeostress values. The Gauss Method as- sociated with visualisation of P&T dihedra (Žalohar 2009) distinguishes three superimposed tectonic phases in the area (Fig. 2D): (1) a phase with strike-slip regime and a 20–30°- trending maximum horizontal shortening interpreted as oblique rifting; (2) a phase with a SSE–NNW-trending maximum horizontal extension that corresponds to the coastal f lexure and (3) a phase with strike-slip regime and a 95°-trending maximum horizontal shortening that caused the inversion and uplift of the entire area. Conclusions The structural data collected in Sødalen indicate the pres- ence of strike-slip faults related to two tectonic events sepa- rated in time by the coastal f lexure. The youngest structures and dykes (phase 3; Fig. 2D) are associated with a NW–SE left-lateral shear zone that cross-cuts the Paleocene rift and the structures related to the coastal f lexure of the continental margin (phase 2). The evidence of dyke intrusions related to N–S extension compatible with the strike-slip tectonic re- gime of phase 3, suggests a coexistence of the two phenomena as a superficial expression of deep-seated crustal structures. The oldest structures and dykes of phase 1 show a maximum horizontal extension coherent with the trend of the Miki Fjord macrodyke. This strike-slip tectonic regime could be related to an oblique rifting stage in Paleocene time. 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Author’s address Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: pgua@geus.dk