Per Terje Osmundsen

Deep crustal fabrics and a model for the extensional collapse of the southwest Norwegian Caledonides

The exhumed deep crustal rocks in the Western Gneiss Region (WGR) of Norway experienced Caledonian high-pressure metamorphism during the Silurian, Scandian continental collision between Baltica and Laurentia. The record of coesite-bearing eclogites and pressure-temperature estimates from the WGR demonstrate extreme burial of these rocks at Pmax. Eclogite tectonite fabrics record coaxial deformation characterized by bulk horizontal shortening and vertical stretching. Many eclogites, particularly those with a high content of kyanite, quartz, phengite and clinozoisite have constrictional fabrics related to vertical stretching. Fabrics that developed during orogenic extensional collapse are of two main types. The deepest exposed sections are dominated by penetrative coaxial fabrics that are characterized by vertical flattening and horizontal, E-W, stretching. These fabrics developed during rapid decompression and were associated with, and locally enhanced by, partial melting of the deep crust. The collapse-related coaxial vertical shortening and horizontal stretching developed at granulite to amphibolite facies and is overprinted by non-coaxial deformation that formed thick mylonites along extensional detachments. The detachment zones are rooted in the coaxially deformed deep crust, and separate the exhumed deep-crustal rocks of the Lower Plate, from the rocks in the hanging-walls that are characterized by medium- to low-grade Caledonian metamorphism. Devonian basins were formed by extensional faulting in the upper crust, and the faults that controlled the sedimentation were rooted in the extensional detachments.

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

Abstract 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.

The role of fault reactivation and growth in the uplift of western Fennoscandia

New structural data, published AFT data, and topographic data suggest that the latest Cretaceous–Cenozoic uplift of western Norway was associated with normal reactivation of the Møre–Trøndelag Fault Complex. Reactivation focused along the base of todays topographic rise, with maximum displacements in the order of 2–3 km. Structural and AFT data indicate that reactivation occurred along a displacement gradient with least normal displacement in the NE and increasing displacements towards the SW. Reactivation commenced in connection with the Triassic to earliest Cretaceous phases of rifting on the Norwegian margin, and probably continued through most of the Tertiary. The Late Cretaceous to Cenozoic topographic development of the Scandes Mountains followed structurally induced templates, such as those described from other areas of active normal faulting. A fault model for the Cenozoic uplift of the Norwegian mainland provides a framework for provenance, erosion and transport of sediments eventually deposited in the offshore post-rift basins in the Latest Cretaceous, Palaeogene and Neogene. The asymmetric topographic profile of Fennoscandia is reflected in the AFT data and suggests that lithospheric flexure places a first-order control on the shape of the Scandes Mountains and the central Fennoscandian craton.

Permian and Mesozoic extensional faulting within the Caledonides of central south Norway

Palaeomagnetic data from fault rocks along major faults in the Laerdal-Gjende Fault System cutting the Caledonian structure in the Jotunheimen area of central south Norway reveals a multi-component remanence pattern. Sample and site-mean directions from the fault rocks obtained by thermal cleaning demonstrate a simple pattern of normal polarity low blocking components and reverse polarity high-blocking directions. The magnetic signature of the Laerdal-Gjende Fault System fault rocks is identical to that observed on breccias on late faults along the west coast of Norway. Based on available palaeomagnetic reference data, we assign ages of mid-late Permian and late Jurassic-early Cretaceous for important phases of faulting and breccia formation along the Laerdal–Gjende Fault System in central south Norway. Structural windows, partly exposing basement along the axis of the Caledonides in southern Norway were exhumed by footwall uplift on major faults in the Laerdal–Gjende Fault System. The consanguinity of fault rock data from the Laerdal–Gjende Fault System and fault rocks in western Norway, and comparison with displacement on the offshore continuation of the Laerdal–Gjende Fault System along the Hardangerfjorden Shear Zone, indicate that the main tectonic events responsible for the development of the North Sea basin also significantly affected the geology of central south Norway.

Caledonian compressional and late-orogenic extensional deformation in the Staveneset area, Sunnfjord, Western Norway

Abstract The studied area in Western Norway constitutes part of the hangingwall of the extensional Kvamshesten Detachment Zone (KDZ). The KDZ separates eclogite-bearing lower crust from a hangingwall consisting of a Precambrian basement-cover pair, Silurian continental margin sediments, a Silurian obduction melange, a Late-Ordovician ophiolite and the Devonian Kvamshesten Basin, deposited unconformably on the Pre-Devonian rocks. Contractional deformation related to ophiolite obduction and to the Caledonian Scandian Orogeny produced a suite of SE-verging structures developed under greenschist facies conditions. As orogenic collapse commenced, extensional shear zones were formed in the upper and middle crust, re-activating contractional shear zones and fabrics. The direction of transport on the inherited fabrics was reversed from top-to-southeast to top-to-northwest, and structures related to the regional extension were superimposed on the contractional structures. Back-folding of the main contractional foliation by asymmetric W-vergent folds, together with NW and W-directed shearing along weak lithologies and semi-ductile faulting in the melange, are the main structural expressions of early stages of the orogenic collapse in the Staveneset area. There is apparently no metamorphic break between the contractional fabrics and the earliest extensional structures. The extensional structures developed, however, under progressively more brittle conditions. The Devonian sediments were deposited upon a sequence of rocks in the upper plate of the Kvamshesten Detachment that had already undergone significant extensional deformation and tectonic exhumation in the Late-Silurian-Early Devonian. The extensional deformation in the upper plate of the Western Norwegian extensional detachments has up to recently been largely ignored in the discussion of the structural control of the formation of the Devonian basins.

Kinematics of the Høybakken detachment zone and the Møre–Trøndelag Fault Complex, central Norway

The tectonic disintegration of the Caledonian orogen through combined extension, contraction and strike-slip was characterized by spatial and temporal strain partitioning through a period of at least 30 Ma. Early to Mid-Devonian exhumation of the Central Norway basement window was associated with retrograde, top-to-the-SW extensional shearing in the Høybakken detachment zone, sinistral shearing along the Møre–Trøndelag Fault Complex, and formation of extension-parallel folds. Progressive exhumation led to increasing strain localization and to the transition from ductile to brittle deformation. In the interval between c. 370 and 320 Ma, the Høybakken detachment fault cut previously folded detachment mylonites, capturing mylonites in its hanging wall. 40Ar/39Ar mica and K-feldspar ages indicate a Late Devonian or younger age for the uppermost parts of the adjacent ‘Old Red’ basin. Gentle folding of this stratigraphic level attests to the continuation of shortening and orogen-oblique extension into Late Devonian–Carboniferous time. Shortening was intensified along strands of the Møre–Trøndelag Fault Complex, as shown by mutually cross-cutting reverse and strike-slip faults. ‘Flower structures’ may be particularly common in constrictional strain systems where strike-slip faults develop parallel to the principal elongation trend, but normal to the principal axis of shortening.

Systematic Mapping of Large Unstable Rock Slopes in Norway

Historically, large rock slope failures impacting into a fjord and causing a several tens of metre high displacement wave have been one of the natural hazards in Norway claiming most lives. In the last 7 years, the Geological Survey of Norway has implemented a systematic mapping approach to characterize unstable rock slopes prone to catastrophic failures, so that future events can be recognized beforehand and society can adapt to the hazard. Systematic mapping has been carried out in three countries and more than 285 unstable slopes have been found. Of these sites, 62 are monitored periodically and 4 have been characterized as high risk objects with continuous monitoring systems installed. In order to classify the likelihood of a future event, rock slope mapping of each object includes the analyses of slide kinematic, velocity of the slide accompanied with other indicators of slide activity and an analysis of recurrence of previous events along the slope.

Metamorphic core complexes and gneiss-cored culminations along the Mid-Norwegian margin: an overview and some current ideas

From the Palacozoic to the Cretaceous, crustal thinning in the Mid Norway area was associated with the denudation of gneiss-cored culminations and metamorphic core complexes in the footwalls of major extensional faults. The development of the culminations led to warping and deactivation of early detachments, to the nucleation of new faults in more distal positions and to the exhumation of highgrade metamorphic rocks to more shallow levels in the crust. Some of the culminations and core complexes became part of the erosional template in Mid-Late Palaeozoic time, some were probably exhumed in the Mesozoic, whereas some may never have reached the surface. We present an overview of five types of gneiss-cored culminations and core complexes that have been identified in the field, through the interpretation of offshore, long-offset seismic reflection data. We furthermore address their mechanism(s) of formation, and their role in the progressive evolution of the Mid-Norwegian margin.

Complex fault interaction controls continental rifting

Rifted margins mark a transition from continents to oceans and contain in their architecture a record of their rift history. Recent investigations of rift architecture have suggested that multiphase deformation of the crust and mantle lithosphere leads to the formation of distinct margin domains. The processes that control transitions between these domains, however, are not fully understood. Here we use high-resolution numerical simulations to show how structural inheritance and variations in extension velocity control the architecture of rifted margins and their temporal evolution. Distinct domains form as extension velocities increase over time and deformation focuses along lithosphere-scale detachment faults, which migrate oceanwards through re-activation and complex linkages of prior fault networks. Our models demonstrate, in unprecedented detail, how faults formed in the earliest phases of continental extension control the subsequent structural evolution and complex architecture of rifted margins through fault interaction processes, hereby creating the widely observed distinct margin domains.Continental rifting and break up processes are poorly constrained in the early stages. Here, the authors using high-resolution numerical simulations to show how early formed faults in continental extension can then control subsequent structure evolution of rifts.

The Role of Inherited Structures in Deep Seated Slope Failures in Kåfjorden, Norway

From studies of orthophotos and through field work, a complex deformation pattern has been recognized in the Lyngen area, Troms, Norway. The area is among the most alpine in Norway and contains a strong clustering of rock slope failures. The rock slope failures are characterized by two different deformation styles, and the difference in style is geographically separated by a fjord and valley lineament. Field studies suggest that two directions of tension oriented almost perpendicular to each other, utilize pre-existing brittle to brittle/ductile fabrics inherited from much older deformation events. The NE-SW direction of tension is parallel to the average displacement vector pointing down-dip along inherited faults. This vector is gravitationally controlled. The NW-SE displacement vector trends strike-parallel along the inherited faults. The presence of the latter appears to be confined geographically.