Nordfjord-Sogn Detachment Zone Research Articles

Structural architecture and composition of crystalline basement offshore west Norway

Numerous studies have investigated the geodynamic history and lithological composition of the Proterozoic basement, Caledonian nappes, and Devonian extensional basins and shear zones onshore west Norway. However, the offshore continuation of these structures, into the northern North Sea, where they are suspected to have influenced the structural evolution of the North Sea rift, is largely unknown. Existing interpretations of the offshore continuation of Caledonian and Devonian structures are based on simple map-view correlations between changes in offshore fault patterns and pronounced onshore structures, without providing evidence for the presence, nature, and geometry of offshore, basement-hosted structures. By integrating three-dimensional (3-D) seismic, borehole, and onshore geological and petrophysical data, as well as two-dimensional (2-D) forward modeling of gravity and magnetic data, we reveal the structural architecture and composition of the crystalline basement on the Maloy Slope, offshore west Norway. Based on 3-D mapping of intrabasement reflection patterns, we identified three basement units that can be correlated with the Caledonian thrust belt, and the major Devonian Nordfjord-Sogn detachment zone, located only 60 km to the east, onshore mainland Norway. Similar to that observed onshore, offshore crystalline basement of the Proterozoic basement (Western Gneiss Region) and allochthons is folded into large-scale antiforms and synforms. These units are separated by the strongly corrugated Nordfjord-Sogn detachment zone. Our analyses show that different types of crystalline basement can be distinguished by their seismic reflection character, and density and magnetic properties. We speculate that the main causes of the observed intrabasement reflectivity are lithological heterogeneities and strain-induced structures such as shear and fracture zones. Our interpretation of the architecture of crystalline basement offshore west Norway has important implications for the location of the suture zone between Baltica and Laurentia.

Shear heating in extensional detachments: Implications for the thermal history of the Devonian basins of W Norway

article i nfo The supra-detachment basins in western Norway were formed during Lower to Middle Devonian in the hangingwall of the extensional system of detachments known as the Nordfjord-Sogn Detachment Zone (NSDZ). The basins experienced elevated peak temperatures (up to 345 °C adjacent to the main detachment fault) during the terminal stages of the extension. In this study, we show that the heat generated by the de- formation of the rocks within the shear zone of the detachment, also known as shear heating, played a role in the thermal evolution of the entire region during the extension and could have induced the elevated peak temperatures of the supra-detachment basins. We numerically analyse the influence of the rheology and the deformation style within the shear zone, the rate of exhumation of the footwall, and the thickness of the sedimentary accumulation on the top of the system. The model reproduces the elevated temperatures recorded in the supra-detachment basins where locally 100 °C and 25% of the total heat budget can be attrib- uted to shear heating. We show that this additional heat source may increase the peak temperatures of the hangingwall as far as 5 km away from the detachment.

Thermal structure of supra-detachment basins: a case study of the Devonian basins of western Norway

We investigate the peak temperatures of the Devonian Hornelen, Kvamshesten and Solund basins in SW Norway to constrain their thermal history. These basins are the three largest Devonian units exposed in Norway and were formed as supra-detachment basins in the hangingwall of the Nordfjord Sogn Detachment Zone. The peak temperatures of the basins were obtained using a geothermometer based on Raman spectroscopy of carbonaceous material on detrital carbonaceous plant fossils. The data confirm an anchizone to low greenschist-facies metamorphism with temperatures (±30 °C) of 284–301 °C in the Hornelen and Solund basins and a significantly higher temperature, 345 °C in the Kvamshesten basin. The temperature increases toward the detachment fault and cannot be explained by ordinary burial alone. In the Kvamshesten basin this temperature increase is close to 100 °C. The new data demonstrate that exhumation of high-grade rocks in the footwall in the Nordfjord Sogn Detachment Zone played an important role in controlling temperatures in the hangingwall. We conclude that the dynamic evolution along large-scale detachments may introduce heat at the base of the hangingwall and thereby control the thermal state of supra-detachment basins formed during extension.

Origin of contrasting Devonian supradetachment basin types in the Scandinavian Caledonides

Research Article| June 01, 2012 Origin of contrasting Devonian supradetachment basin types in the Scandinavian Caledonides Vegard V. Vetti; Vegard V. Vetti Department of Earth Science, University of Bergen, Postboks 7803, 5020 Bergen, Norway Search for other works by this author on: GSW Google Scholar Haakon Fossen Haakon Fossen Department of Earth Science, University of Bergen, Postboks 7803, 5020 Bergen, Norway Search for other works by this author on: GSW Google Scholar Author and Article Information Vegard V. Vetti Department of Earth Science, University of Bergen, Postboks 7803, 5020 Bergen, Norway Haakon Fossen Department of Earth Science, University of Bergen, Postboks 7803, 5020 Bergen, Norway Publisher: Geological Society of America Received: 30 May 2011 Revision Received: 13 Jan 2012 Accepted: 25 Jan 2012 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2012 Geological Society of America Geology (2012) 40 (6): 571–574. https://doi.org/10.1130/G32512.1 Article history Received: 30 May 2011 Revision Received: 13 Jan 2012 Accepted: 25 Jan 2012 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Vegard V. Vetti, Haakon Fossen; Origin of contrasting Devonian supradetachment basin types in the Scandinavian Caledonides. Geology 2012;; 40 (6): 571–574. doi: https://doi.org/10.1130/G32512.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Supradetachment basins that formed during the Devonian extensional collapse of the Scandinavian Caledonides have been explained as hanging-wall basins formed along the listric breakaway zone of a major detachment fault system. This model involves significant rotation of bedding as the detachment flattens to an approximately horizontal orientation, and explains the large stratigraphic thicknesses of east-dipping layers overlying in tectonic contact the top-to-the-west Nordfjord-Sogn detachment zone (NSDZ). However, it fails to explain one of the basins, the Håsteinen basin, where east-dipping Devonian strata rest unconformably on a metamorphic substrate that forms part of the upper plate of the detachment system. Based on detailed field mapping, we present a model where the Håsteinen forms as a ramp basin that develops on the upper plate above a major west-facing ramp in the NSDZ. We test both of these models by forward modeling using 2DMove (two-dimensional kinematic modeling software). The results show how two different types of supradetachment basins can coexist in a subhorizontal detachment zone in an area that underwent many tens of kilometers of lateral crustal extension. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Structure and 40 Ar/ 39 Ar thermochronology of an ultrahigh-pressure transition in western Norway

Abstract: The Nordfjord region of western Norway hosts an archetypal subducted crustal section, underpinned by ultrahigh-pressure (UHP) eclogite, overlain by Devonian sediments, and cored by a crustal-scale extensional shear zone. Structural mapping reveals two distinct displacement zones that played different roles during the formation and exhumation of this section: (1) the Sandane Shear Zone is a NW-dipping, amphibolite-facies, high-strain zone near the base of the eclogite-bearing crust that separates allochthonous units from underlying crystalline basement; it may have originated during early thrusting, but was overprinted by top-to-the-west extensional fabrics at lower crustal depths; (2) structurally above this, the Nordfjord–Sogn Detachment Zone is a top-to-the-west, amphibolite- to greenschist-facies detachment shear zone within allochthonous units that defines the upper boundary of the eclogitized crust and was responsible for exhumation through at least mid-crustal depths. Muscovite 40 Ar/ 39 Ar ages suggest that amphibolite-facies deformation below the Nordfjord–Sogn Detachment was mostly finished by c . 397 Ma, whereas muscovite ages from the deeper parts of the UHP domain indicate that it cooled after 390 Ma. During exhumation through the middle crust, west-directed stretching was accompanied by north–south folding. Late sinistral transpressional faulting in the middle to upper crust truncated the earlier folds and shear zones. Supplementary material: Complete 40 Ar/ 39 Ar data and a summary geological map of the Nordfjord region are available at http://www.geolsoc.org.uk/SUP18460 .

Strain within the ultrahigh-pressure Western Gneiss region of Norway recorded by quartz CPOs

Abstract Electron back-scatter diffraction (EBSD) was used to measure the crystal preferred orientations (CPOs) from 101 samples across the ultrahigh-pressure Western Gneiss region of Norway to assess slip systems, sense of shear, CPO strength, and strain geometry. The CPOs suggest a dominance of prism 〈a〉 slip, with lesser amounts of prism [c] slip and basal 〈a〉 slip; there are few Type I and Type II girdles. The major structural feature in the study area – the high-strain, top-W, normal-sense Nordfjord–Sogn Detachment Zone – is characterized by asymmetric and strong CPOs; an eastern domain with strong asymmetric CPOs shows top-E shear. Strain throughout the study area was characterized by a mix of plane strain and constriction with no evidence of flattening. Adjacent gneiss and quartzite/vein samples have similar CPOs.

High-temperature deformation during continental-margin subduction & exhumation: The ultrahigh-pressure Western Gneiss Region of Norway

A new dataset for the high-pressure to ultrahigh-pressure Western Gneiss Region allows the definition of distinct structural and petrological domains. Much of the study area is an E-dipping homocline with E-plunging lineations that exposes progressively deeper, more strongly deformed, more eclogite-rich structural levels westward. Although eclogites crop out across the WGR, Scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area. The Scandian reworking increases westward, culminating in strong Scandian fabrics with only isolated pockets of older structures; the dominant Scandian deformation was coaxial E–W stretching. The sinistrally sheared Møre–Trøndelag Fault Complex and Nordfjord Mylonitic Shear Zone bound these rocks to the north and south. There was moderate top-E, amphibolite-facies deformation associated with translation of the allochthons over the basement along its eastern edge, and the Nordfjord–Sogn Detachment Zone underwent strong lower amphibolite-facies to greenschist-facies top-W shearing. A northwestward increase in exhumation-related melting is indicated by leucosomes with hornblende, plagioclase, and Scandian sphene. In the western 2/3 of the study area, exhumation-related, amphibolite-facies symplectite formation in quartzofeldspathic gneiss postdated most Scandian deformation; further deformation was restricted to slip along biotite-rich foliation planes and minor local folding. That the Western Gneiss Region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation, implies that continental crust in general need not undergo pervasive deformation during subduction.

Exhuming Norwegian ultrahigh‐pressure rocks: Overprinting extensional structures and the role of the Nordfjord‐Sogn Detachment Zone

The Nordfjord‐Sogn Detachment Zone (NSDZ) is widely cited as one of the primary structures responsible for the exhumation of Norwegian (ultra)high‐pressure (UHP) rocks. Here we review data from the considerable volume of research describing this shear zone, and compile a strike‐parallel cross section along the NSDZ from the Solund Basin in the south to the Sørøyane UHP domain in the north. This cross section highlights several previously unrecognized patterns, revealing a shear zone with top‐to‐the‐west asymmetric fabrics that (1) initiated at amphibolite facies, (2) overprints metamorphic breaks and tectonostratigraphic contacts, and (3) has a gradational continuum of muscovite cooling ages. These patterns constrain the kinematic evolution of the NSDZ and suggest a new three‐step model for the exhumation of Norwegian (U)HP rocks. The initial stages of exhumation were characterized by the rise of crustal rocks from (U)HP depths to the base of the crust by buoyancy‐driven mechanisms not specified in this paper. Mantle exhumation was followed by top‐to‐the‐west, normal‐sense displacement within a broad noncoaxial ductile shear zone near the base of the crust that overprinted tectonostratigraphic contacts formed previously during mantle exhumation. In the final stages of crustal exhumation, top‐W brittle‐ductile detachments soled into and partially excised this ductile shear zone, dropping the Devonian basins into contact with rocks of varying tectonostratigraphic levels. This new interpretation of the NSDZ is significant as it accounts for the extreme crustal excision observed in western Norway using three sequentially overprinting structures active at different stages of UHP rock exhumation.

Exhumation of ultrahigh-pressure rocks beneath the Hornelen segment of the Nordfjord-Sogn Detachment Zone, western Norway

The Nordfjord-Sogn Detachment Zone of western Norway represents an archetype for crustal-scale normal faults that are typically cited as one of the primary mechanisms responsible for the exhumation of ultrahighpressure (UHP) terranes. In this paper, we investigate the role of normal-sense shear zones with respect to UHP exhumation using structural geology, thermobarometry, and geochronology of the Hornelen segment of the Nordfjord-Sogn Detachment Zone. The Hornelen segment of the zone is a 2–6 km thick, top-W shear zone, primarily developed within amphibolite-grade allochthonous rocks, that juxtaposes the UHP rocks of the Western Gneiss Complex in its footwall with lower-grade allochthons and Carboniferous-Devonian Basins in its hanging wall. New thermobarometry and Sm/Nd garnet geochronology show that these top-W fabrics were initiated at lower crustal depths of 30–40 km between 410 Ma and 400 Ma. Structural geology and quartz petrofabrics indicate that top-W shear was initially relatively evenly distributed across the shear zone, and then overprinted by discrete ductile-brittle detachment faults at slower strain rates during progressive deformation and exhumation. These results require a three-stage model for UHP exhumation in which normal-sense shear zones exhumed UHP rocks from the base of the crust along initially broad ductile shear zones that were progressively overprinted by discrete ductile-brittle structures.

Applications of inclusion behaviour models to a major shear zone system: The Nordfjord-Sogn Detachment Zone in western Norway

Rigid inclusion models have reached a stage where one should be able to use them to obtain quantitative values from ductile shear zones. We used natural data collected in three sites and combined analogue and theoretical modelling to assess vorticity, strain, nature of rigid inclusion/matrix interface and confinement in the large-scale, ductile Nordfjord-Sogn Detachment Zone (NSDZ) of the Caledonides of western Norway. Our study shows that: (1) the observed shape preferred orientation (SPO) at higher structural levels of the NSDZ at Site 1, Gjervika, can be explained by simple shear (pure shear/simple shear ratio S r = 0) associated with a slipping inclusion/matrix interface. (2) The observed SPO at deeper structural levels of the NSDZ at Site 2, near Sandane can be produced by simple shear associated with a significant amount of shortening across the shear zone ( S r ≈ 1), acting upon rigid inclusions in slipping contact with the enclosing matrix. (3) Observed back rotated boudins deeper in the NSDZ at Site 3, Biskjelneset, can form in confined flow associated with a considerable amount of shortening across the shear zone ( S r ≥ 0.4). (4) The observed tails of porphyroclasts indicate a minimum (at least local) strain of ca. γ ≈ 20. (5) The clasts in the studied shear zones strongly depart from Jeffery's model [Jeffery, G. B., 1922. The motion of ellipsoidal particles immersed in a viscous fluid. Proceedings of the Royal Society of London A102, 161–179]. The large scale extensional NSDZ under investigation shows evidence of strain partitioning: rocks vary from protomylonites to ultramylonites, and the simple shear and pure shear components are heterogeneously distributed. Therefore, we conclude that flow in the NSDZ was very heterogeneous both at the kilometre and the metre scale. However, the present study suggests that the amount of shortening across the shear plane throughout the NSDZ increases with depth, and the flattening component contributes to exhumation of the eclogite facies rocks in its footwall.

Exhumation of high‐pressure rocks beneath the Solund Basin, Western Gneiss Region of Norway

AbstractThe Solund–Hyllestad–Lavik area affords an excellent opportunity to understand the ultrahigh‐pressure Scandian orogeny because it contains a near‐complete record of ophiolite emplacement, high‐pressure metamorphism and large‐scale extension. In this area, the Upper Allochthon was intruded by thec. 434 Ma Sogneskollen granodiorite and thrust eastward over the Middle/Lower Allochthon, probably in the Wenlockian. The Middle/Lower Allochthon was subducted to c. 50 km depth and the structurally lower Western Gneiss Complex was subducted to eclogite facies conditions at c. 80 km depth by c. 410–400 Ma. Within < 5–10 Myr, all these units were exhumed by the Nordfjord–Sogn detachment zone, producing shear strains > 100. Exhumation to upper crustal levels was complete by c. 403 Ma. The Solund fault produced the last few km of tectonic exhumation, bringing the near‐ultrahigh‐pressure rocks to within c. 3 km vertical distance from the low‐grade Solund Conglomerate.

The middle Devonian basins of western Norway: sedimentary response to large-scale transtensional tectonics?

The Devonian basins of western Norway were formed during late- to post-orogenic extension of overthickened Caledonian crust. The basins are situated in the hanging wall of the extensional Nordfjord–Sogn Detachment Zone (NSDZ) and display extensional half-graben geometries in sections parallel to the local direction of principal extension. Based on overall facies configurations, paleocurrent patterns and intrabasinal structures, we infer an anticlockwise rotation of the syndepositional extension direction from NW–SE in the south (Solund basin) to WSW–ENE in the north (Hornelen basin). The axes of folds that are roughly parallel to the local extension direction are rotated correspondingly. The Kvamshesten basin is located between the Solund and Hornelen basins. Sedimentological and structural data show evidence of an early, southeastwards tilt direction followed by a more eastwards tilt and associated E–W flowing paleodrainage. Correspondingly, NW–SE trending folds and reverse faults are superposed by E–W trending ones at low to intermediate stratigraphic levels. The variations in apparent tilt direction for the basins together with variations in intrabasinal structure is interpreted to reflect an anticlockwise rotation of the regional syndepositional strain field. The above observations and inferences indicate that the Devonian basins in western Norway formed in a strain field dominated by regional transtension, accommodated by extension along the NSDZ and sinistral strike–slip along orogen-parallel shear zones and faults to the north of the basins; alternatively, NW-directed extension preceded the introduction of a sinistral strike–slip component. The models are in accordance with recent work carried out in the footwall of the NSDZ and illustrates the tectono-sedimentary response to a complex interplay between extension and strike–slip that appears to have been fundamental in the late-stage disintegration of the Caledonian orogen.

Tectonics and sedimentation in the hangingwall of a major extensional detachment: the Devonian Kvamshesten Basin, western Norway

The Middle Devonian Kvamshesten Basin in western Norway is a late‐orogenic basin situated in the hangingwall of the regional extensional Nordfjord–Sogn Detachment Zone. The basin is folded into a syncline with the axis subparallel to the ductile lineations in the detachment zone. The structural and stratigraphic development of the Kvamshesten Basin indicates that the basin history is more complex than hitherto recognized. The parallelism stated by previous workers between mylonitic lineation below the basin and intrabasinal fold axes is only partly reflected in the configuration of sedimentary units and in the time‐relations between deposits on opposing basin margins. The basin shows a pronounced asymmetry in the organization and timing of sedimentary facies units. The present northern basin margin was characterized by bypass or erosion at the earliest stage of basin formation, but was subsequently onlapped and eventually overlain by fanglomerates and sandstones organized in well‐defined coarsening‐upwards successions. The oldest and thickest depositional units are situated along the present southern basin margin. This as well as onlap relations towards basement at low stratigraphic level indicates a significant component of southwards tilt of the basin floor during the earliest stages of deposition. The inferred south‐eastwards tilt was most likely produced by north‐westwards extension during early stages of basin formation. Synsedimentary intrabasinal faults show that at high stratigraphic levels, the basin was extending in an E–W as well as a N–S direction. Thus, the basin records an anticlockwise rotation of the syndepositional strain field. In addition, our observations indicate that shortening normal to the extension direction cannot have been both syndepositional and continuous, as suggested by previous authors. Through most of its history, the basin was controlled by a listric, ramp‐flat low‐angle fault that developed into a scoop shape or was flanked by transfer faults. The basin‐controlling fault was rooted in the extensional mylonite zone. Sedimentation was accompanied by formation of a NE‐ to N‐trending extensional rollover fold pair, evidenced by thickness variations in the marginal fan complexes, onlap relations towards basement and the fanning wedge geometry displayed by the Devonian strata. Further E–W extension was accompanied by N–S shortening, resulting in extension‐parallel folds and thrusts that mainly post‐date the preserved basin stratigraphy. During shortening, conjugate extensional faults were rotated to steeper dips on the flanks of a basin‐wide syncline and re‐activated as strike‐slip faults. The present scoop‐shaped, low‐angle Dalsfjord fault cross‐cut the folded basin and juxtaposed it against the extensional mylonites in the footwall of the Nordfjord–Sogn detachment. Much of this juxtaposition may post‐date sedimentation in the preserved parts of the basin.Basinal asymmetry as well as variations in this asymmetry on a regional scale may be explained by the Kvamshesten and other Devonian basins in western Norway developing in a strain regime affected by large‐scale sinistral strike‐slip subparallel to the Caledonian orogen.

Structural observations adjacent to a large-scale extensional detachment zone in the hinterland of the Norwegian Caledonides

Structural and lithological observations in a N-S traverse across Bremangerlandet, western Norway, demonstrate that the region is separated into two plates by the E-W-striking, S-dipping ductile-brittle Vetvika Shear Zone. The Vetvika Shear Zone places Caledonian high-grade gneisses, with a Precambrian protolith age of the lower plate, in tectonic contact with heterogeneous gneisses of the upper plate. The Vetvika Shear Zone is the uppermost part of the Nordfjord-Sogn Detachment Zone. A gently W-plunging stretching lineation within the S-dipping lower-plate augen gneisses is interpreted as having formed within a several kilometers thick ductile W-dipping extensional shear zone that has undergone later rotation around an E-W axis. A highly attenuated sequence of nappes occur structurally on top of a heterogeneous basement and cover quartzite of the upper plate. Pre-Caledonian structures are preserved locally in the gneissic upper plate basement. The nappes at Bremangerlandet are correlated with the Caledonian Middle and Upper Allochthons, representing Baltic/suspect and exotic terranes, respectively. One large and several medium- to small-scale, brittle, top-to-the-south extensional faults are superimposed on ductile contractional structures in the upper plate. Some of the extensional faults merge downward with the Vetvika Shear Zone. The contractional structures include E-W-trending folds and S-directed thrusts in the cover quartzite, and are superimposed on a pronounced W-E stretching lineation. Conjugate sets of fractures and faults related to E-W extension represent the youngest structures in the upper plate. The Old Red Sandstones (Devonian) of the Hornelen basin rest unconformably on top of extensionally faulted upper plate rocks on Bremangerlandet. The unconformity is partly faulted at Bremanger, and truncated by a regionally extensive late, brittle fault along the basin margin further east. This suggests a long-lived tectonic history for the Vetvika Shear Zone and its eastward continuation, with movement pre- and post-dating the filling of the collapse basin, possibly as an eastward migration of the break-away fault.