Projekter i kortlægningsafdelingen
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1:100 000 scale mapping of the Ussuit map sheet
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Project leader:
Jeroen van Gool and Mogens Marker
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The Ussuit map area is situated in the inner part of Nordre Strømfjord, Central West Greenland, in the core of the Nagssugtoqidian Orogen. The geology of this area has proven to be essential in the understanding of the Palaeoproterozoic tectonic evolution of the region. Previously, the area has been investigated in the Nagssugtoqidian project of the Danish Lithosphere Centre (1994-1999). Subsequently mapping of the 1:100 000 scale Ussuit map sheet was completed under GEUS regime in 2000. This project is part of the main research focus area of the Ore and Mapping departments of GEUS, 2000-2003 and the Ussuit map sheet will be the first of four 1:100 000 scale map sheets to be produced in the area between Nordre Strømfjord and Ilulissat Ice fjord.
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Figure 1
Tectonic overview map of the Nagssugtoqidian Orogen with the location of the Ussuit map sheet.
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Location and topography of the Ussuit area:
The location of the map sheet in West Greenland is indicated in Figure 1. The Inner Nordre Strømfjord region is cut by a branched fjord system, one of which is named Ussuit, which gives relatively easy access to a large area (Fig. 2 and Fig. 3). The shorelines, cleaned by wave acivity, give excellent exposures of the often complex geology, and combined with the relatively low metamorphic grade and locally weak deformation in the area, is this area ideally suited for a study of the early collisional features of the Nagssugtoqidian Orogen.
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Figure 4
Photo and interpretation of thrust structures in allochthonous Palaeoproterozoic metasediments and arc rocks, overlying Archaean gneisses with a tectonic contact. Note a lense of ultramafic rocks of dunnitic composition on the thrust contact between Archaean and Proterozoic rocks.
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Geology of the Ussuit area
The recognition that the Arfersiorfik quartz diorite in the Ussuit area (Fig. 3) was Palaeoproterozoic and calc-alkaline in composition (Kalsbeek et al. 1987), led to the interpretation of this suite of intrusions as a subduction-related magmatic arc within the Nagssugtoqidian Orogen. This implied that the orogen is the site of continental collision and contains a suture. Subsequent fieldwork during the DLC Nagssugtoqidian project led to the reconstruction of a tectonic cycle including continental break-up, and formation of an oceanic basin, convergence with subduction, collision leading to high grade metamorphism and intense deformation, and finally slow uplift. The Ussuit region preserves relics of a period of northwest-directed thrusting, resulting in the interleaving of allochthonous Palaeoproterozoic metasediments and arc rocks, with kilometre-thick sheets of Archaean orthogneisses. Post-collisional deformation intensely folded and refolded these thrust structures and was followed by sinistral strike slip on the Nordre Strømfjord and Nordre Isortoq shear zones. The map area occupies a large-scale anticlinorium in between these two shear zones.
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Participants in the GEUS fieldwork in 2000:
Jeroen van Gool – GEUS - project leader
Mogens Marker – Norges Geologiske Undersøkelse
Adam Garde – GEUS
Andrew Cook – Baker Atlas Geoscience
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Figure 5
Simplified block diagram showing the geometry of the folded thrust structures in the Eastern part of the Ussuit map, forming an anticlinorium in between the Nordre Isortoq and Nordre Strømfjord shear zones.
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References:
Kalsbeek, F. 2001: Geochemical comparison between Archaean and Proterozoic orthogneisses from the Nagssugtoqidian orogen, West Greenland. Precambrian Res. 105, 165-181.
Kalsbeek, F. and Manatschal, G. 1999: Geochemistry and tectonic significance of peridotitic and meta-komatiitic rocks from the Ussuit area, Nagssugtoqidian orogen, West Greenland. Precambrian Research 94, 101-120.Kalsbeek, F. and Nutman, A.P. 1996: Anatomy of the Early Proterozoic Nagssugtoqidian orogen, West Greenland, explored by reconnaissance SHRIMP U-Pb dating. Geology., v.24, p. 515-518.
Kalsbeek, F., Taylor, P. N. and Henriksen, N. 1984: Age of rocks, structures, and metamorphism in the Nagssugtoqidian mobile belt, West Greenland - field and Pb-isotope evidence. Can. J. Earth Sci. 21, 1126-1131. Kalsbeek, F., Pidgeon, R. T. and Taylor, P. N. 1987: Nagssugtoqidian mobile belt of West Greenland: cryptic 1850 Ma suture between two Archaean continents - chemical and isotopic evidence. Earth Planet. Sci. Lett. 85, 365-385.
Manatschal, G., Ulfbeck, D., and van Gool, J.A.M., 1998: Change from thrusting to syn-collisional extension at a mid-crustal level: an example from the Palaeoproterozoic Nagssugtoqidian orogen (West Greenland). Canadian Journal of Earth Sciences v. 35, p. 802-819.
Nutman, A.P., Kalsbeek, F., Marker, M., van Gool, J.A.M. and Bridgwater, D., 1999: U-Pb ages for Kangâmiut dykes and detrital zircons in metasediments in the Paleoproterozoic Nagssugtoqidian Orogen (West Greenland): Clues for the pre-collisional history of the orogen. Precambrian Research v. 93, p. 87-101.
Passchier, C.W., Den Brok, S., van Gool, J.A.M., Marker, M. and Manatschal, G., 1997: A laterally constricted shear zone system - the Nordre Strømfjord steep belt, Nagssugtoqidian Orogen, W. Greenland. Terra Nova, v. 9, p.199-202.
van Gool, J.A.M., Connelly, J.N., Marker, M. and Mengel, F.C., 2002: The Nagssugtoqidian Orogen of West Greenland: tectonic evolution and regional correlations from a West Greenland perspective. Canadian Journal of Earth Sciences, 39, p. 665-686.
van Gool, J.A.M., Kriegsman, L., Marker, M. and Nichols, G.T., 1999: Thrust stacking in the inner Nordre Strømfjord area, West Greenland: significance for the tectonic evolution of the Palaeoproterozoic Nagssugtoqidian orogen. Precambrian Research v. 93, p. 71-86.
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