GEUS Bulletin (Geological Survey of Denmark and Greenland)
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Radical past climatic changes in the Arctic Ocean and a geophysical signature of the Lomonosov Ridge north of Greenland
No full textThe Arctic Ocean is a landlocked basin, at present covered by perennial sea ice. During the past few decades a significant thinning and shrinking of the sea ice has been observed, and modelling studies indicate that the Arctic Ocean ice cover could, by the end of this century, almost disappear from most parts of the Arctic Ocean during peak summer seasons. It remains uncertain, however, whether the environmental changes are an enhanced greenhouse-warming signal or a result of natural (long-term) variability, but palaeoceanographic studies can contribute to our understanding of the natural variability of environmental parameters, e.g. sea-ice cover and oceanographic changes on time-scales of centuries to millennia. As part of the multidisciplinary EU project Greenland Arctic Shelf Ice and Climate Experiment (GreenICE), sediment coring and seismic reflection measurements have been undertaken in a hitherto unexplored part of the Arctic Ocean, the margin of the Lomonosov Ridge in the Lincoln Sea (Fig. 1). The aim of the project was to study the structure and dynamics of the sea-ice cover and attempt to relate these to longer-term records of climate variability retrieved from sediment cores. The main field work was carried out in May 2004 from an ice camp established by a Twin Otter aircraft on drifting sea ice at 85°N, 65°W, c. 170 km north of Alert, Arctic Canada. The camp was deployed over the shallowest part of the Lomonosov Ridge off the northern Greenland/Canada continental margin (Fig. 1). The sea-ice drift would normally be between east and south, but persistent easterly winds resulted in a fast drift trajectory towards the WSW, such that the camp drifted a distance of approximately 62 km during the two weeks camp period. At present the study area is heavily ice covered, and forecast models of future shrinking Arctic sea-ice cover suggest that this area is one of the least sensitive to warming in the Arctic. The results obtained from the GreenICE project challenge this view
New hornblende and muscovite 40Ar/39Ar cooling ages in the central Rinkian fold belt, West Greenland
No full textThe Palaeoproterozoic Rinkian fold belt in West Greenland consists of reworked Archaean basement, mainly orthogneiss, and the unconformably overlying Palaeoproterozoic Karrat Group. Both parts were intensely deformed and metamorphosed at around 1.87 Ga, at which time the crustal anatectic Prøven igneous complex was emplaced into the northern part of the belt. Seven new hornblende and muscovite 40Ar/39Ar cooling ages are presented from the central–northern parts of the Rinkian fold belt. Four 40Ar/39Ar hornblende ages ranging from 1795 ± 3 to 1782 ± 3 Ma were obtained from amphibolite and hornblendite enclaves in the Archaean orthogneiss, and two from relict dyke fragments in the latter that may be of Palaeoproterozoic age. Three 40Ar/39Ar muscovite ages of 1681 ± 6 Ma, 1686 ± 3 Ma and 1676 ± 3 Ma were obtained from samples of Karrat Group metagreywacke, andalusite schist and metasiltstone. The new 40Ar/39Ar ages, from hornblende and muscovite respectively, are very uniform and probably unrelated to local metamorphic grade and structural history, and are interpreted as regional late orogenic cooling ages. The new hornblende ages are significantly older than those previously obtained from the central and northern parts of the adjacent Nagssugtoqidian orogen to the south, and point to different uplift histories, which may suggest that the orogeny was not synchronous in the two regions
Advanced in situ geochronological and trace element microanalysis by laser ablation techniques
No full textLaser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was developed in 1985 and the first commercial laser ablation systems were introduced in the mid 1990s. Since then, LA-ICP-MS has become an important analytical tool in the earth sciences. Initially, the main interest for geologists was in its ability to quantitatively determine the contents of a wide range of elements in many minerals at very low concentrations (a few ppm and below) with relatively high spatial resolution (spot diameters of typically 30–100 μm). The potential of LA-ICP-MS for rapid in situ U–Th–Pb geochronology was already realised in the early to mid 1990s. However, the full potential of LA-ICP-MS as the low-cost alternative to ion-microprobe techniques for highly precise and accurate in situ U–Th–Pb age dating was not realised until the relatively recent advances in laser technologies and the introduction of magnetic sectorfield ICP-MS (SF-ICPMS) instruments. In March 2005, the Geological Survey of Denmark and Greenland (GEUS) commissioned a new laser ablation magnetic sectorfield inductively coupled plasma mass spectrometry (LA-SF-ICP-MS) facility employing a ThermoFinnigan Element2 high resolution magnetic sectorfield ICP-MS and a Merchantek New Wave 213 nm UV laser ablation system. The new GEUS LA-SF-ICP-MS facility is widely used on Survey research projects in Denmark and Greenland, as well as in collaborative research and contract projects conducted with partners from academia and industry worldwide. Here, we present examples from some of the these ongoing studies that highlight the application of the new facility for advanced geochronological and trace element in situ microanalysis of geomaterials. The application of LASF-ICP-MS based in situ zircon geochronology to regional studies addressing the Archaean geology of southern West Greenland is presented by Hollis et al. (2006, this volume)
Evolution of Neoarchaean supracrustal belts at the northern margin of the North Atlantic Craton, West Greenland
No full textThe Archaean North Atlantic Craton of West Greenland collided at c. 1.9 Ga with a lesser-known Archaean craton to the north, to form the Nagssugtoqidian orogen. The Palaeoproterozoic metamorphic grade and strain intensity decrease northward through the orogen, allowing investigation of the reworked Archaean components in its northern part. Two Archaean supracrustal belts in this region – the Ikamiut and Kangilinaaq belts – are investigated here using field mapping, aeromagnetic data, zircon geochronology, and geochemistry. Both belts comprise quartzo-feldspathic and pelitic metasedimentary rocks, amphibolite, and minor calc-silicate rocks, anorthosite and ultramafic rocks. Pb-Pb and U-Pb dating of detrital zircons and host orthogneisses suggest deposition at c. 2800 Ma (Kangilinaaq belt) and after 2740 Ma (Ikamiut belt); both belts have zircons with Neoarchaean metamorphic rims. Metasedimentary rocks and orthogneisses at Ikamiut share similar steep REE signatures with strong LREE enrichment, consistent with local derivation of the sediment and deposition directly onto or proximal to the regional orthogneiss precursors. Zircon age data from Kangilinaaq indicate both local and distal sources for the sediment there. Geochemical data for Kangilinaaq amphibolites indicate bimodal, mixed felsic–mafic source rocks with island-arc basaltic affinities, consistent with a shelf or arc setting. Both belts experienced a similar tectono-metamorphic history involving Neoarchaean amphibolite facies peak metamorphism at c. 2740–2700 Ma, possibly due to continued emplacement of tonalitic and granodioritic magmas. Nagssugtoqidian lower amphibolite facies metamorphism at c. 1850 Ma was associated with development of the large-scale F2 folds and shear zones that control the present outcrop pattern. The observed differences in the sources of the Kangilinaaq and Ikamiut belts and their shared post-Archaean history suggest they were formed in different Neoarchaean environments proximal to and on a continental plate, and were amalgamated in a convergent margin setting shortly after their deposition
Structural analysis of the northern Nagssugtoqidian orogen, West Greenland: an example of complex tectonic patterns in reworked high-grade metamorphic terrains
No full textStructural 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 relationship 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 amphibolite facies domains seem to form autochthonous segments of a crustal section linked by a transitional zone that was only reactivated and reworked during indentation or transpression. The Nagssugtoqidian compression was effectively transferred across this zone towards the northern amphibolite domain 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
Palaeomagnetic results from the Lopra-1/1A re-entry well, Faroe Islands
No full textThe palaeomagnetic dating and evolution of the Faroe Islands are discussed in the context of new density and rock magnetic results from the deepened Lopra-1/1A well. The reversal chronology of the c. 6½ km thick basalt succession is also described. The polarity record of the Faroe Islands may now be correlated in detail with the Geomagnetic Polarity Time Scale. The lowermost (hidden) part of the lower basalt formation correlates with Chron C26r (Selandian age), the top (exposed) part of the lower basalt formation correlates with Chrons C26n, C25r and C25n (Selandian and Thanetian age) and the middle and upper basalt formations correlate with Chron C24n.3r (Ypresian). Inclinations indicate a far-sided position of the palaeomagnetic poles, which is characteristic of results from most Palaeogene volcanics from the northern North Atlantic region. The density, magnetic susceptibility and magnetic remanence of 20 specimens from one solid core (1½ m in length) and 26 sidewall cores from the well between –2219 and –3531 m below sea level (b.s.l.) suggest that the volcanic materials can be divided into two characteristic groups: solid unaltered basalts and altered basalts and tuffs. The magnetic properties are typically log-normally distributed and the carriers of remanence are Ti-poor Ti-magnetites with Curie temperatures close to 580°C. The inclination of the 1½ m core at 2380 m b.s.l. is dominantly negative (two plugs at the very top of the core do show normal polarity, but they are likely to be misoriented as all specimens appear to be from one flow). Magnetic logging (magnetic susceptibility and field intensity) down to 3515 m b.s.l. was made in Lopra-1/1A together with other geophysical logs but did not yield conclusive inclination data
Earthquake seismology in Greenland – improved data with multiple applications
No full textEarthquake seismology is a rapidly evolving field that has provided a wealth of new information about deep geological structures on a regional scale over the last decade as well as information about dynamic processes in the Earth. A major leap forward was the development of portable digital broad band (BB) seismographs around 1990. Without any changes in configuration, these are able to record the signals from large distant earthquakes, as well as the signals from weak local events. BB seismographs typically cover a frequency range from 0.0083 Hz to 50 Hz, making them useful for studies ranging from the high frequency signals from explosions to the very low frequency oscillations following major earthquakes. The first seismological observatory in Greenland was established in 1907 in Qeqertarsuaq (GDH) and was in service for about five years (Hjelme 1996). Later, seismographs were established in Ivittut (1927) and Illoqqortoormiut (1928; SCO), and the network has been regularly upgraded and expanded ever since (Fig. 1). Prior to the development of BB seismographs, each station was equipped with a set of seismographs with different frequency sensitivities in an attempt to cover both distant and local earthquakes. Now just one small instrument is needed at each location. The Geological Survey of Denmark and Greenland (GEUS) operates four permanent BB seismographs in Greenland (Fig. 1), two of them in collaboration with foreign institutions. In addition to the permanent network, there are currently 13 temporary BB seismographs active in Greenland, of which eight are operated by GEUS. Three of the temporary seismograph stations were established as part of the Danish Continental Shelf Project (Marcussen et al. 2004), and the remainder in connection with research projects. Three temporary seismographs were deployed during 2005 as part of a research project aiming to resolve very deep regional structures in North Greenland: the Citronen Fjord station (CFJ, Continental Shelf Project), and the stations in Kullorsuaq (KUL) and Daneborg (DBG)
A lead isotope study of an Archaean gold prospect in the Attu region, Nagssugtoqidian orogen, West Greenland
No full textThis paper presents a lead isotope investigation of a gold prospect south of the village Attu in the northern part of the Nagssugtoqidian orogen in central West Greenland. The Attu gold prospect is a replacement gold occurrence, related to a shear/mylonite zone along a contact between orthogneiss and amphibolite within the Nagssugtoqidian orogenic belt. The mineral occurrence is small, less than 0.5 m wide, and can be followed along strike for several hundred metres. The mineral assemblage is pyrite, chalcopyrite, magnetite and gold. The host rocks to the gold prospect are granulite facies ‘brown gneisses’ and amphibolites. Pb-isotopic data on magnetite from the host rocks yield an isochron in a 207Pb/204Pb vs. 206Pb/204Pb diagram, giving a date of 3162 ± 43 Ma (MSWD = 0.5). This date is interpreted to represent the age of the rocks in question, and is older than dates obtained from rocks elsewhere within the Nagssugtoqidian orogen. Pb-isotopic data on cataclastic magnetite from the shear zone lie close to this isochron, indicating a similar origin. The Pb-isotopic compositions of the ore minerals are similar to those previously obtained from the close-by ~2650 Ma Rifkol granite, and suggest a genetic link between the emplacement of this granite and the formation of the ore minerals in the shear/mylonite zone. Consequently, the age of the gold mineralisation is interpreted to be late Archaean
Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic
No full textExtruded basalt flows overlying sedimentary sequences present a challenge to hydrocarbon exploration using reflection seismic techniques. The Lopra-1/1A re-entry well on the Faroese island of Suðuroy allowed us to study the seismic characteristics of a thick sequence of basalt flows from well logs and borehole seismic recordings. Data acquired during the deepening operation in 1996 are presented here. The re-entry well found that the seismic event at 2340 m, prognosed from the pre-drill Vertical Seismic Profile (VSP) as a decrease in impedance, was not base basalt and the deepened well remained within the lower series basalts. Nonetheless, compressional and shear sonic logs and a density log were recorded over the full open hole interval. These allowed a firm tie to be made with the reflected wavefield from a new VSP. The sonic logs show a compressional to shear wavespeed ratio of 1.84 which is almost constant with depth. Sonic compressional wavespeeds are 3% higher than seismic velocities, suggesting dispersion in the basalt flows. Azimuthal anisotropy was weakly indicated by the shear sonic log but its orientation is consistent with the directions of mapped master joints in the vicinity of the well. The VSP downgoing compressional wavelet shows good persistence, retaining a dominant period of 28 ms at 3510 m depth. Average vertical velocity is 5248 m/s, higher than previously reported. Attenuation can largely be modelled by geometrical spreading and scattering loss, consistent with other studies. Within the piled flows, the effective Q from scattering is about 35. Elastic layered medium modelling shows some hope that a mode-converted shear wave may be observed at moderate offsets. Like its predecessor, the 1996 VSP indicates a decrease in impedance below the final depth of the well. However, it is unlikely to be basement or sediment and is probably an event within the volcanic sequence