The Nordfjord-Sogn Detachment, W. Norway MIKE G. NORTON Norton, M. G.: The Nordfjord-Sogn Detachrnent, W. Norway. Norsk Geologisk Tidsskrift, Vol. 67, pp. 93-106. Oslo 1987. ISSN 0029-196X. In western Norway thick continental sediments of Devonian age are found in four basins each bounded to the east by major fault zones. Shallowly dipping fault segments previously identified as thrusts have recently been reinterpreted (Hossack 1984) as parts of a single large extensional fault. Detailed fieldwork along this structure shows it to be a classic detachment (here redefined and renamed as the Nordfjord­ Sogn Detachment) similar to those described from the Basin and Range province of the western USA (e.g. Davis 1983). Most of the features attributed to a late Devonian 'Svalbardian' 'Solundian' orogeny are explained as related to irregularities in the detachrnent surface. Other possible Devonian extensional structures in southern Norway are evaluated. M. G. Norton, DepartmentofGeology, UniversityofKeele, Keele, Staffs. , ST55BG, UK. Present address: Department of Geology, Royal Holloway and Bedford New College, Egham Hill, Egham, TW20 OEX, UK. Rocks of Devonian age in western Norway are Range region of the USA. Other possible detach­ found in four major areas interpreted as separate ments within the southern Norwegian Cale­ basins (Fig. 1), from north to south: Hornelen, donides are then evaluated. Håsteinen, Kvamshesten and Solund. These basins have been interpreted as forming in a domi­ nantly strike-slip regime (Steel et al. 1977; Basin development models Roberts 1983; Steel et al. 1985). Syn- to post­ depositional folding has been observed in all the Before considering the main models for basin basins and they all have an unconformable west­ development, details of published sedimentary ern margin and all except Håsteinen a tectonic and tectonic observations are reviewed which pro­ eastern margin. This eastern contact in each case vide constraints on these models. brings the Devonian sediments against a con­ Sedimentation in the preserved Devonian has siderable thickness (>100 m) of mylonites and been related to the development of long-lived the low-angle segments have been interpreted as alluvial fan systems along parts of their present thrusts (Høisæter 1971; Nilsen 1968; Bryhni margins. In Hornelen small debris-ftow fans and 1978; Roberts 1983; Sturt 1983; Torsvik et al. fan deltas formed along the northern margin while 1986); a similar thick mylonite zone is present larger stream-flowfans formed on the eastern and just to the northeast of the Håsteinen outcrop. southeastern margins (Steel et al. 1977; Steel & Hossack (1984) has reinterpreted all the tectonic Gloppen 1980). Apart from consisting mainly of contacts as representing a single irregular exten­ conglomerate, probably of alluvial fan origin, sional fault, which he calls the Måløy Fault. He nothing has been published on the sedimentology identified this structure as an extensional fault of Håsteinen. Kvamshesten has alluvial fan from a consideration of the apparent omission of deposits along its northern (breccia dominated) structural section across it as indicated on pub­ and southern (conglomerate dominated) margins lished geological maps of the area. (Bryhni & Skjerlie 1975). Solund consists almost All existing models for the development of the entirely of alluvial fan deposits (Nilsen 1968) with basins are here reviewed taking account of both at least one major intercalated sedimentary slide sedimentological and tectonic constraints. The unit (M ichelsen et al. 1987). In Hornelen and field characteristics of the extensional structure, in the centre of Kvamshesten the marginal fan here redefined and renamed the Nordfjord-Sogn deposits pass laterally into alluvial plain/flood­ Detachment, are described and compared with basin deposits forming an axial system. Sediment similar structures developed in the Basin and dispersal directions are from south to southwest 94 M. G. Norton NORSK GEOLOGISK TIDSSKRIFT 67 (1987) l Upper Plate � Devonian sandstone & tonglomerate �V Devonian volcanics Granodiorite Stavfjord Nappe, ophiolite Jotun Nappe, bastment & Eotambrian- Ordovician cover Lowtt Plate Eocambrian- ?Silurian cover Anorthosite with granite gneiss Undifferentiated basement •' ,. l Unconformable boundary to Devonian � Nordfjord-.Sogn DetQChment 10km .? Thrust ./ High-angle normal fault /�l'Folds affecting Devonian strata NORSK GEOLOGISK TIDSSKRIFT 67 (198) Nordfjord-Sogn detachment 95 along 'northern' margins, north to northwest mentary evidence for faulting active during sedi­ along 'southern' margins and west to southwest mentation near the position of all of the present in the axial systems (Steel et al. 1985). It is clear faulted boundaries. More direct sedimentary evi­ that throughout the sedimentation recorded by dence comes fromthe southwest of Solund in the the Devonian strata strong source area top­ Utvaer region (Indrevaer Steel 1975), where & ography was maintained. NNW-SSE trending extensional faults may have . All of the sediments are organized into coar­ been active forming fault scarps during sedi­ sening upward cyclothems, c. 150 m thick in Hor­ mentation. nelen (Steel et al. 1977), c. 200 m thick in Solund Steel et al. (1985) have recently reviewed the (Steel et al. 1985) and c. 20 m thick in Kvams­ main models proposed for the formation of the hesten (Bryhni Skjerlie 1975) (note that cycles Devonian basins and I will use their nomen­ & of this smaller scale are also recognized in Home­ clature. len). In Hornelen these units are traceable from the marginal fans into the axial sandstones and Model Hinge Faulting. This model is essen­ A. they appear to also be basin-wide in the other tially a propagating half-graben. It explains the sub-basins. This shows clear tectonic control on eastward migration of the depocentre but fails to sedimentation with a sustained, regular, periodic explain (as pointed out by Steel et al. 1985) either rejuvenation of the source region leading to alluv­ the migration of the western end of the basin or ial fan progradation. the eastward tilting of the strata. There is also a clear relative eastward migration of the depocentre with respect to previously Model Strike-slip Faulting. In this model, uplift B. deposited cycles, with progressive overlap of suc­ on a restraining bend on a strike-slip fault pro­ ceeding units. Finally, in some parts of the sub­ vides a sediment source for deposition in small basins there are marked discrepancies between half-grabens formed successively at a releasing clast lithologies in marginal fanglomerates and bend. Despite the observations of Steel et al. adjacent basement rocks across the faulted con­ (1985), the main objection to this model is the lack tacts (Hornelen: Steel Gloppen 1980; Solund: of evidence of a strike-slip continuation of the & Nilsen 1968). steep northern margin contact into basement. In The tectonic constraints show the character of Hornelen, the best described basin, the contact deformation going on during sedimentation. In appears instead to continue across the mouth of most cases it is impossible to prove that faulting Ålfotfjorden, where it enters Nordfjord (Fig. l) or folding were going on during sedimentation directly into the thrust contact as shown by Steel and some published models treat virtually all the et al. (1985). Steel Gloppen (1980) have shown & deformation of the sediments as being a distinct a series of fault lineaments in the adjacent base­ later event (or series of events), e.g. Roberts ment which trend subparallel to this northern Torsvik et al. Other models (Hossack margin and continue to the east of the present 1983; 1986. 1984, Norton 1986 and this paper, also Steel basin margin. However, the age, amount and & Glopen 1980 froma different standpoint) attempt direction of displacement on these zones is not to explain most of the deformation of the sedi­ known. In some cases they are coincident with ments as being a result of the same process as major lithological boundaries; they all follow basin formation. The features that they must basement trends. There is no evidence that any explain are: the overall eastward tilt of the strata, of these fault lineaments are major strike-slip folding or varying style and trend, and the nature faults. of the tectonic margins. This model does attempt to explain many of The only unequivocal evidence of synsedi­ the structural features affecting the basin sedi­ mentary deformation is from the western end ments by appealing to complex transpression and of the K vamshesten Devonian where the basal transtension. unconformity is seen to be progressively rotated during deposition of later sediments (Bryhni Model Listric Normal Faulting. I am in general & C. Skjerlie 1975). There is also the general sedi- agreement with this model as originally proposed Fig. Geological map of the Sogn og Fjordane area, west Norway. 1. 96 M. G. Norton NORSK GEOLOGISK TIDSSKRIFT 67 (1987) by Hossack (1984), although some detailed criti­ Devonian sediments (Roberts 1983; Torsvik cisms will be made in the next section. The main et al. 1986). objection to this model raised by Steel et al. (4) The formation of a cleavage in the Devonian (1985) is that it does not possess a dynamic hinter­ sediments (Roberts 1983; Torsvik et al. 1986). land. This problem is particularly acute in (5) The types and orientation of magnetic fabric Hossack's (1984) model due to the subhorizontal developed in hanging wall sediments, mylon­ fault geometry on which negligible footwall uplift ites and footwall rocks (Torsviket al. 1986). is predicted, leading to a progressive degradation of fault scarps. However, in a model in which the Before considering these point by point, I will extensional structure is relatively planar and dips present a model of a large-scale extensional struc­ steadily at c.20°-30°, continuous footwall uplift ture (Fig. 2) adapted from Sibson (1977) and would be expected to provide the necessary Davis (1983). Fig. 2(a) shows an extensional fault/ source rejuvenation. shear zone passing through the whole crust at the beginning of its activity. Fig. 2(b) shows the same Model Migrating Listric Fault System.