[show abstract][hide abstract] ABSTRACT: Recent work has suggested that the Scandinavian ice sheet was much more dynamic than previously believed, and its western marine-based margin can provide an analogue to the rapid-paced fluctuations and deglaciation observed at the margins of the Antarctic and Greenland ice sheets.In this study we used a complimentary dating technique, OSL (Optically Stimulated Luminescence dating), to support the existence of the Trofors interstadial in central Norway; an ice-free period that existed from ∼25 to 20 ka recorded at multiple sites throughout Norway (cf. Andøya interstadial) and that divides the Last Glacial Maximum (LGM) into two stadials. OSL signal component analysis was used to optimize data analysis, and internal (methodological) tests show the results to be of good quality. Both large and small aliquots gave consistent OSL ages (22.3 ± 1.7 ka, n = 7) for sub-till glaciofluvial/fluvial sediments at the Langsmoen stratigraphic site, and an apparent old age (∼100 ka) for a poorly-bleached sample of glaciolacustrine sediment at the nearby stratigraphically-related Flora site. Eight radiocarbon ages of sediment from the Flora site gave consistent ages (20.9 ± 1.6 cal ka BP) that overlap within 1σ with OSL ages from the nearby Langsmoen site.The similarity in age within and between these stratigraphically-related sites and using different geochronological techniques strongly suggests that this area was ice-free around ∼21 or 22 ka. The existence of the Trofors interstadial along with other interstadials during the Middle and Late Weichselian (MIS 3 and MIS 2) indicates that not only the western margin, but the whole western part of the Scandinavian ice sheet, from the ice divide to the ice margin was very dynamic. These large changes in the ice margin and accompanying drawdown of the ice surface would have affected the eastern part of the ice sheet as well.
[show abstract][hide abstract] ABSTRACT: In this paper, regional and high-resolution bathymetric data are used to reconstruct the pattern and dynamics of ice-sheet flow in the fjord and shelf areas of northern Norway. Interpretation of the morphology of the sea floor reveals a series of glacigenic features, which are mainly related to the most recent glaciation of the area. The shelf area of Troms county comprises four cross-shelf troughs with intermediate shallow banks. Glacial features within the cross-shelf troughs (mega-scale glacial lineations (MSGL), lateral shear margin moraines, grounding zone wedges) are interpreted to have been formed under fast-flowing ice streams. On the shallow bank areas, there are far fewer ice-flow indicators and here, relatively passive ice acted as interstream ridges. Outside the Finnmark coast, to the north and east, a coast-parallel depression (Djuprenna) deflected rapidly flowing ice from the fjords in a generally east to west direction, and then farther out through Ingøydjupet and into the Bear Island Trough. The fjords guided most of the ice flow and functioned as tributaries to the major ice streams on the shelf, including the vast Bear Island Trough Ice Stream at the Last Glacial Maximum. The landform distribution and their cross-cutting relationships indicate that major switches in ice-stream flow direction took place during deglaciation. Additionally, grounding zone wedges are found within the cross-shelf troughs and these reveal temporal variability in ice-stream dynamics in different troughs during deglaciation. The distribution of glacigenic features on the ice-stream beds also reveals a downstream evolution that has been reported by other workers, whereby small bedrock features progressively merge into more elongate forms and MSGL. This downstream evolution in subglacial geology/subglacial bedforms implies that the force balance (e.g. basal shear stress) of the ice streams also evolves in the downstream direction and, more crucially, that the flow mechanism of the ice stream is spatially variable along its length.
[show abstract][hide abstract] ABSTRACT: An ice stream in the Vestfjorden-Trænadjupet depressions off northern Norway transported glacial debris to the shelf edge and into the deep sea during the last glaciation. Our interpretation is based mainly on regional and detailed bathymetry, and 2D and 3D seismic data. This palaeo-ice stream was approximately 400 km long and covered an area of ∼20,000 km2. Including a complex system of tributary fjords and valleys in Nordland county, the drainage basin had an area of ca. 150,000 km2 within the Fennoscandian Ice Sheet (FIS) during the Last Glacial Maximum (LGM). The location of the palaeo-ice stream prevented large ice masses from reaching the montainous areas of Lofoten and Vesterålen. A local ice dome was established in these areas. The palaeo-ice stream eroded extensively into both Mesozoic and Cenozoic sedimentary rocks, and Quaternary sediments. A pattern of well-developed, subglacial, sedimentary bedforms was produced (e.g., mega-scale lineations, drumlins). Ice-stream shear margin moraines are located on both sides of Trænadjupet, defining the width of the palaeo-ice stream (∼ 90 km at the shelf edge). These moraines are commonly 10–20 m high, a few kilometres wide, and reach several tens of kilometres in length. A large recessional moraine (the Tennholmen Ridge) has been mapped in Vestfjorden. It is 80 m high, 20 km wide and 60 km long. We have interpreted this ridge as a grounding-line moraine, formed during a halt or a readvance of the ice stream during the last deglaciation. The large transport of glacial debris towards the grounding line occurred during this stage. Northeast of the Tennholmen Ridge, several smaller moraine ridges were developed at still-stands during deglaciation.
[show abstract][hide abstract] ABSTRACT: Reports of neotectonic deformation in Norway, including Svalbard and offshore areas, have been graded into five classes depending on the quality of documentation and their most likely origin. A large number of the mainland locations have been visited and careful field investigations carried out, while offshore localities have been analysed by 2D and 3D seismic and multibeam data. After a critical evaluation of the 79 neotectonic claims in Norway as a whole, we have classified three of these as 'A - Almost certainly neotectonics' and another seven as 'B - Probably neotectonics.' The majority of the reports are attributed to effects other than tectonic. The present grade A claims include two postglacial faults in northern Norway and one postglacial fault in southern Norway The NE-SW oriented, reverse Stuoragurra Fault in western Finnmark constitutes the Norwegian part of the postglacial Lapland Fault Province. The NW-SE striking Nordmannvikdalen fault in northern Troms is a normal fault trending perpendicular to the extensive system of NE-SW trending reverse faults in northern Fennoscandia. The grade 13 claims include Supposed secondary effects of large-magnitude earthquakes such as abundant liquefaction structures, rock-slope failures and other collapse structures in northern and western Norway.
There are indications that three separate, large-magnitude earthquakes affected the Finnmark-Troms region during the period 11,000-9,000 BP. Palaeoseismic events have also been postulated in western Norway. There is, for example, evidence of three regional slide events in western Norway, including one episode shortly after the deglaciation and two events at c. 8,000 and c. 2,000 calendar years BP. The 8,000 y. BP event has been interpreted as the effect of a tsunami generated by the Storegga slide while another 8,000 y. 1311 liquefaction event, in Nord-Trondelag, may also be related to an earthquake.
The offshore investigations have not confirmed any firm evidence of neotectonic deformation events, although several distortions in Quaternary reflectors have been mapped in the northern North Sea area, where Subtle features may represent faults associated with gas leakage. A major seismic pulse most likely accompanied each of the deglaciations Following the multiple glaciation cycles in mainland Fennoscandia and Scotland during the last 600,000 years. The interaction of the contraction and dilation of fissures associated with these glaciation cycles may have facilitated fluid and gas leakage through the reservoir seals and gas chimneys, ultimately forming pockmarks on the sea floor. This mechanism could also have contributed to the concentration and pumping of hydrocarbons from their source rocks to reservoir formations. A possible example of this mechanism is shown by a recent earthquake along the postglacial Stuoragurra Fault, which has significantly influenced the groundwater circulation. Our understanding of past and future tectonic activity is especially important for the evaluation of hazard risk related to rock-slope stability.
Norwegian Journal of Geology. 01/2004; 84(1):3-34.
[show abstract][hide abstract] ABSTRACT: A systematic compilation and characterisation of many reports of neotectonic crustal deformation in Norway (both on local and regional scales) has identified two neotectonic faults in northern Norway. The Stuoragurra Fault is a large reverse fault in Finnmark County. The Nordmannvikdalen fault is a much smaller normal fault in Troms County. The Stuoragurra postglacial fault can be followed, in several discontinuous sections, for 80 km, in a NE–SW direction. The fault has up to 10 m of displacement. During 1998, two trenches were made across the fault. The hanging wall was seen to be thrust upwards over the footwall, with 7 m vertical displacement evident from displaced glacial contacts. The fault did not penetrate the overlying glacial materials, but rather folded them, forming a blind thrust. Large liquefaction and other deformation structures were found in the glaciofluvial sediments in both trenches. Veins of angular and subangular pebbles from the local bedrock penetrate more than 10 m laterally from the thrust plane and into the sediments in the footwall. It is thought that these veins were injected during the fault activity. The major deformation of the sediments has a décollement plane that continues laterally in the E/B horizon contact of the modern soil on top of the footwall. This may indicate that an initial pedogenesis had taken place before the fault activity occurred, however no macro plant fossils to support this were found in the possible buried soil. Deformational structures seen in the trench can be explained as a result of one major fault event. The Nordmannvikdalen postglacial fault is a NW–SE trending normal fault, dipping to the NE. The fault offsets till on the NW slope of Nordmannvikdalen. The escarpment varies in height from 0.5 to 1.5 m, with a trench often present between the hanging wall and the footwall. The fault locally splits into two subparallel branches, however this is probably only in the glacial overburden. Ground penetrating radar (GPR) profiles show two prominent bedrock reflectors, dipping approximately 40° to the NE. An upward extrapolation of the northernmost reflector through the overburden coincides with the position of the surface scarp. The bedrock is well exposed along a streamed parallel to the GPR profiles. Bedding and foliation cannot be responsible for the reflectors visible in the GPR profiles. Gravitational forces have been ruled out as the origin of the fault. Although both faults probably formed shortly after Weichselian deglaciation, there is evidence that isostatic uplift was not the only force acting. Quaternary tilting of Fennoscandia due to ridge push, or mantle convection, may also play a role in neotectonic activity in Norway.
[show abstract][hide abstract] ABSTRACT: We have graded the neotectonic claims in Norway into five classes depending on the quality of documentation and most likely cause of formation. A large number of the mainland locations have been visited for careful field investigations. After a critical evaluation of the 79 neotectonic claims in Norway, we have classified three claims as 'A - Almost certainly neotectonics' and another seven as 'B - Probably neotectonics'. The majority of the claims can therefore be attributed to effects other than tectonic. The present grade A claims include two postglacial faults in northern Norway and one postglacial fault in southern Norway. The grade B claims include areas with secondary effects of large-magnitude earthquakes such as abundant liquefaction structures, rock avalanches and other collapse structures in northern and western Norway. The Stuoragurra Fault in western Finnmark constitutes the Norwegian part of the postglacial Lapland Fault Province. The Nordmannvikdalen fault in northern Troms is a normal fault trending perpendicular to the extensive system of NE-SW trending reverse faults in northern Fennoscandia. There is evidence for three separate large-magnitude earthquakes in the Finnmark-Troms region during the period 9,000-11,000 BP. Indications of large earthquakes have also been observed in western Norway. There is, for example, evidence of two separate events 2,000 and 7,000 years ago in Møre &Romsdal. The latter, that is also recorded in Nord-Trøndelag and possibly further north, may be related to the triggering of the Storegga avalanche. Detailed analysis of offshore 2D and 3D seismic data has not clearly indicated any neotectonic deformation. Several distortions in the Quaternary reflectors have, however, been mapped in the northern North Sea area, where subtle features may represent faults which are often associated with gas leakage. Ground-water fluctuations are common during earthquakes as indicated for the Stuoragurra Fault. It contains an anomalous high content of water, and further the water level in an adjacent river seems to have been influenced by a M 3.8 earthquake in 1996. Regional data suggest that a major seismic pulse in mainland Fennoscandia and Scotland accompanied each of the deglaciations following the c. 20 glaciation cycles during the last 600,000 years. Possibly, the interaction of the contraction and dilation of fissures associated with these seismic cycles may have assisted fluids to leak through the reservoir seals towards pockmarks at the sea floor and in concentrating hydrocarbons from their source rocks and pumping them to reservoir formations. It has been shown from earthquakes along the Stuoragurra Fault that they significantly influence the groundwater circulation. The understanding of past and future tectonic activity are especially important for the evaluation of hazard risk related to rock-slope stability.
[show abstract][hide abstract] ABSTRACT: The large-scale form of the mid-Norwegian shelf (62°N-68°N) is due to the thick late Pliocene/Pleistocene prograding wedges comprising up to 1500 m of sediments at the shelf break. These units are interpreted mainly to be of glacial origin. Above these sequences, separated by an angular unconformity, the sediments represent the last few interglacial/glacial cycles. A regional bathymetric dataset of the Norwegian shelf reveals a picture with shallow bank areas separated by depressions. The erosion which has taken place is caused by palaeo-ice streams comparable to those operating along the Siple coast in West Antarctica today. The depressions generally start west of the crystalline bedrock and end near the shelf edge. In the glacially eroded depressions, extensive linear elements (megaflutes etc.) parallel to the trough axis, reflect the flow direction of the palaeo-ice streams. From the bathymetric data set and seismic profiles, the flow pattern of the western part of the Scandinavian ice sheet during the Late Weichselian is constructed. The largest ice stream followed the Norwegian Channel along the southern and western coast of Norway, on the Mid-Norwegian shelf the ice streams followed major bathymetric depressions. In intermediate bank areas, ice domes probably existed. Vestfjorden and Andfjorden have existed during many glaciations as drainage route for a large area of the Scandinavian ice sheet. These two ice streams effectively hindered ice to reach the Lofoten/Vesteraalen areas. Thus, these areas were partly covered by a passive ice dome during large parts of many glacial cycles. On some of the shallow bank areas, lateral ridges several tens of kilometres in length are inferred to mark the shear margins of palaeo-ice streams. The understanding of ice stream operation is important to both contemporary and palaeo-glacialogy, and has implications for mechanisms of abrupt climate change. Implications for ice stream functioning and basal processes will be discussed.