Arild Andresen

Caledonian sole thrust of central East Greenland: A crustal-scale Devonian extensional detachment?

Structural observations of the basement-cover contact in the Central Fjord region of the East Greenland Caledonides suggest Silurian to Devonian crustal thinning with top-to-the-east displacement of the cover sequence. The east-dipping, low-angle shear zone separating the Late Proterozoic (Eleonore Bay Group) to Lower Ordovician cover sequence from the underlying high-grade gneisses has previously been interpreted as the Caledonian sole thrust in the region, displacing the cover sequence toward the Caledonian foreland in the west. However, emplacement of younger, low greenschist facies rocks on top of Archean to Middle Proterozoic gneissic rocks across the shear zone instead favors an extensional origin. This interpretation is supported by ductile and brittle shear-sense indicators in the footwall formed under progressively lower temperature conditions, consistently showing top-to-the-east displacement. The cover sequence in the hanging wall is cut by numerous extensional faults. The inferred earliest fault set developed contemporaneously with the deposition of the unconformably overlying Devonian deposits, whereas the youngest set crosscut the entire Devonian stratigraphy. Neither of these late brittle extensional faults are found in the basement, suggesting that they merge with the extensional shear zone at depth. Collectively, these observations suggest that the Devonian basin formed as a supradetachment basin during collapse of the Caledonian orogen.

Interaction of Basement-Involved and Thin-Skinned Tectonism in the Tertiary Fold-Thrust Belt of Central Spitsbergen, Svalbard

The Tertiary fold-thrust belt in Oscar II Land, central Spitsbergen, consists of three major zones of distinct structural style: (1) a western basement-involved fold-thrust complex, (2) a central zone of thin-skinned fold-thrust units above a decollement in Permian evaporites, and (3) an eastern zone characterized by a frontal duplex system in the fold-thrust belt, bounded eastward by steep, basement-rooted reverse faults (Billefjorden and Lomfjorden fault zones) beneath subhorizontal platform strata. Offshore seismic data from Isfjorden (Statoil) confirm the threefold zonation and document thick-skinned and thin-skinned structural interactions in both the fold-thrust belt and the foreland section. An admissible cross section yields about 45%, or 20 km, of shortening in Oscar II Land. Deeper parts of the seismic profiles show fault-bounded Devonian (central and east) and Carboniferous (west) basins. The structural grain of the Tertiary fold-thrust belt partly coincides with the margin-bounding normal faults of these basins, suggesting that preexisting structures and stratigraphy controlled the Tertiary fold-thrust belt development. A kinematic evolution of the fold-thrust belt is invoked: (1) north-northeast-directed, bedding-parallel shortening, (2) major west-southwest-east-northeast shortening, with in-sequence foreland fold-thrust propagation, (3) basement-involved, west-southwest-east-northeast uplift in the eastern foreland zone, (4) eastward out-of-sequence propagation of thrusts, and (5) west-east extension in the hinterland. Our regional structural compilation map and synthesis of the central Spitsbergen transect advocates structural variation and linked basement-involved thrusting in the hinterland and thin-skinned/thick-skinned reactivation and out-of-sequence thrusting in the east (foreland), and is new compared with previous work of the region. The synthesis also raises several important new structural play concepts for investigating hydrocarbon prospects in Spitsbergen and adjacent regions; for example, inverted Carboniferous basins, and traps produced by Tertiary thin- and thick-skinned contraction and reactivation structures.

The tectonic significance of pre-Scandian 40Ar/39Ar phengite cooling ages in the Caledonides of western Norway

Pre-Silurian continental-margin deposits in western Norway, non-conformably overlying allochthonous continental orthogneisses retain Ordovician 40Ar/39Ar cooling ages for phengites, implying either rapid cooling immediately after a Late Ordovician orogenic event, or less likely, a slow cooling following an Early Ordovician or older orogeny. The Dalsfjord Suite–Høyvik Group basement–cover pair are probably a lateral equivalent to Late Proterozoic sandstones (‘sparagmites’) covering the Jotun Nappe gneisses of the Middle Allochthon in central-south Norway. The Høyvik Group underwent polyphase deformation, greenschist-facies metamorphism (Tmax<450°C) and exhumation prior to deposition of the unconformably overlying Wenlockian continental-margin deposits of the Herland Group. The Høyvik Group was only weakly metamorphosed during obduction of the Solund–Stavfjord Ophiolite and the Scandian continental collision between Baltica and Laurentia. Phengitic white micas from the Høyvik Group yield cooling ages of 446.1± 3.0, 449.1±2.2 and 447.5±4.0 Ma, respectively, identical within experimental error. One sample gives a plateau over 72% of the gas analysed, whereas the other samples were slightly disturbed after initial cooling, as indicated by systematically lower apparent ages at low experimental extraction temperatures. Minor 40Ar loss probably occurred during subsequent Scandian deformation and late to post-orogenic extension.The Høyvik Group rocks were unroofed before the Wenlock time (423–428 Ma) and cooled through the temperature for argon retention in phengite at c. 447±4 Ma, indicating a maximum cooling rate between 14 and 22C/Ma-1 through Ashgill and Llandovery times before being subjected to low-grade metamorphism during the Scandian orogeny. Rapid pre-Scandian cooling, combined with peak metamorphic conditions of 450C or less, may indicate that the Dalsfjord–Høyvik basement–cover pair were aVected by an orogenic event during the Late Ordovician (Caradoc) time. The data also suggest that the Caledonian margin of Baltica may have experienced a more protracted tectonism during the Caledonian cycle than previously models focusing on Early Caledonian and Tremadoc (or older) ophiolite obduction and the Scandian continental collision between Baltica and Laurentia.

Nature and distribution of deep crustal reservoirs in the southwestern part of the Baltic Shield : Evidence from Nd, Sr and Pb isotope data on late Sveconorwegian granites

Late Sveconorwegian (‘postorogenic’) granites (c. 1.0–0.93 Ga) make up a voluminous and widespread suite of intrusions across south Norway. From radiogenic isotope data, three groups of late Sveconorwegian granites can be distinguished: (1) granite with more than 150 ppm Sr, 87Rb/86Sr<5,87Sr/86Sr0.93 Ga<0.710 and εNd<0; (2) granite with less than 150 ppm Sr, 87Rb/86Sr>5, 87Sr/86Sr0.93 Ga>0.710 and εNd<0; (3) juvenile granite with 87Sr/86Sr0.93 Ga<0.705 and εNd>0. Granite plutons belonging to Group 1 (‘normal-Sr concentration granite’) occur all over south Norway and include the largest batholiths (Østfold, Flå, Herefoss). Granite plutons of Group 2 (‘low-Sr concentration granites’) are restricted to north-central Telemark (Rjukan rift), and are associated with c. 1.5 Ga Rjukan Group rhyolite. Group 3 is represented by one intrusion only, but still suggests input of mantle-derived magma, or the presence of young, mantle derived rocks in the deep crust in the region at c. 0.93 Ga. The Group 1 granites are similar in Sr and Nd characteristics to some of the older (1.05 Ga) Sveconorwegian granitic intrusions (‘augen gneisses’) in the region in terms of radiogenic isotope systematics, but Pb isotopes suggest that the magmas did not form by simple remelting of augen gneiss. The Nd, Sr, and Pb isotopic systematics of the late Sveconorwegian granites indicate mixing between a depleted-mantle derived component and two or more major components with an extended crustal history. One of the crustal end members is present throughout south Norway and has an isotopic signature similar to older granitic rocks of the Trans Scandinavian Igneous Belt (TIB). The other crustal end member is indistinguishable from Rjukan Group rhyolites and slightly younger intrusions associated with these, and is restricted to areas within the mid-Proterozoic Rjukan rift. The available data suggest that the deep continental crust of south Norway, both east and west of the Permian Oslo Rift, is an integral part of the Baltic Shield, with a common history back to 1.7–1.9 Ga, that is, to the end of the Svecofennian orogeny and the TIB magmatism.

TIMING OF PALEOZOIC OROGENY AND EXTENSION IN THE CONTINENTAL SHELF OF NORTH-CENTRAL NORWAY AS INDICATED BY LASER 40AR/39AR MUSCOVITE DATING

Metamorphic rocks of Lofoten, Norway, represent a leading edge of the Baltic craton that was partly subducted beneath Laurentia during Caledonian orogeny. We find that muscovite porphyroblasts in Lofoten record a remarkably complete history of cooling and unroofing and yield data about Lofoten9s tectonic evolution in the interval from 425 to 265 Ma. These data confirm that culminating metamorphism of basement and allochthonous rocks in Lofoten occurred during the Scandian phase of orogeny. Permian ages for muscovite are the youngest obtained for regional metamorphic rocks anywhere in Scandinavia and appear to record significant basement unroofing during extensional evolution of the Norwegian-Greenland seaway. Regional age variations of muscovite in Lofoten can be related to present crustal thickness, Devonian to Permian sedimentation in the Norwegian seaway, and extensional structures, and they indicate the timing of multiple extension events in the evolution of Norway9s passive margin. The results of this study show that 40 Ar closure profiles in porphyroblasts can form even in active tectonic settings characterized by rapid and episodic unroofing.

A late orogenic extensional origin for the infracrustal gneiss domes of the East Greenland Caledonides (72–74°N)

Abstract The Caledonides in the Fjord Region of East Greenland are made of a NS-trending basement dome of Archaean to Mesoproterozoic migmatites and gneisses, bordered to the east and to the west by Neoproterozoic (Eleonore Bay Supergroup) metasediments. Emplacement of younger rocks on older ones, abrupt break in metamorphic grade, and kinematic indicators, demonstrate that the east-dipping high-strain zone along the eastern margin of the dome is an extensional detachment (the Fjord Region Detachment) and not a west-directed thrust. The detachment zone cuts across contractional structures in the hanging wall. Amphibolite-grade mylonites, up to 1–2 km thick, developed during the initial stage of movement on the Fjord Region Detachment. P-T estimates from less deformed lenses or layers within the mylonite zone indicate that the mylonite protolith originated from a depth of more than 25 km (>7 kbar). Subsequent deformation occurred under progressively lower P-T conditions. A 10–50-m-thick brecciated, ultracataclasite zone marks the upper part of the high-strain zone. Stratigraphical evidences and isotopic data ( 40 Ar 39 Ar ) on lamprophyres suggest that extension started in early Late Silurian times (ca. 420 Ma) and continued into the Upper Devonian. The Middle to Upper Devonian molasse basin is an upper crustal response to the extensional movement on the Fjord Region Detachment. The structural and metamorphic relationships observed along and across the Fjord Region Detachment in central East Greenland is comparable to the situation encountered below the Neoproterozoic metasediments in the Ardencaple Fjord region to the north (75–76°N). In the light of this similarity we suggest that the Fjord Region Detachment (or some other extensional shear zone) continues northward to the Ardencaple Fjord region. If so, it seems reasonable to relate exhumation of the eclogitic basement terrain around Dove Bugt to a late Caledonian extensional event. Such a scenario is comparable to existing models for the Western Gneiss Region in the Scandinavian Caledonides, where formation of the Devonian basins is related to orogenic extensional collapse and exhumation of coesite-bearing eclogites. Neither a high-strain zone comparable to the Fjord Region Detachment nor a metamorphic or structural break was observed along the eastern boundary of the gneiss dome. Instead a gradual downward increase in metamorphic grade is observed in the Neoproterozoic metasediments (Peterman Bjerg Group) reaching high amphibolite grade, with local migmatization, at the contact.

Late Caledonian extension in the Ofoten area, northern Norway

Abstract Structural observations along the base of the Caledonian nappe pile in the Ofoten region of northern Norway demonstrate a complex interaction of compressional and extensional structures. Top-to-the-west extensional structures are consistently superimposed on top-to-the-east-southeast compressional structures and control the basement-cover relationships across the orogen. Extensional shear zones offsetting the basement-cover contact appear to be restricted to the area east of the Ofoten synform. The eastern flank of the synform is an approximately 1-km-wide W-dipping shear zone, with more than 30 km of dip-slip offset. The basement culimination (“roll-over”) west of the synform, is most likely controlled by the listric geometry of this shear zone at depth. This major shear zone, and other minor extensional shear zones within the Rombak Window, have a ductile shear fabric with post-kinematic recovery fabrics developed under middle to upper greenschist facies conditions, suggesting that the footwall and hanging-wall rocks were still at a mid-crustal level some time after the extensional event. The extensional shear zone flattens upsection and splays into a network of anastomosing detachment zones eastwards, cutting both upsection and downsection in the direction of westward tectonic transport. Development of steeply dipping extensional basement faults and subhorizontal detachment faults in the cover have resulted in a complex basement-cover relationship and reworking of the nappe stratigraphy established during the preceeding compressional event. These relationships make restoration of the Caledonian nappe pile to a pre-orogenic configuration uncertain, and shortening estimates difficult. The recognition of the extensional event as a distinct post-contractional tectonic event, suggests a link with the Devonian extensional collapse in southern Norway. A major difference between southern and northern Norway, however, is the lack of brittle extensional features in the Often area, suggesting different amounts of extension and basement uplift in the two regions.

U-Pb and 40Ar/39Ar constraints on the Fjord Region Detachment Zone : a long-lived extensional fault in the central East Greenland Caledonides

The high strain zone separating infracrustal ortho- and paragneisses from supracrustal cover rocks in the East Greenland Caledonides has been interpreted variously as (1) a combined Grenvillian and Caledonian thrust, (2) a Vendian extensional shear zone reactivated as a Caledonian thrust, (3) a Caledonian extensional detachment reactivated as a late Caledonian thrust or (4) a late Caledonian extensional detachment. In this study we present new kinematic and geochronological data (U–Pb and 40Ar/39Ar) from a well-studied segment of the high strain zone in Kejser Franz Joseph Fjord, demonstrating top-to-the-east, normal-sense displacement. Syntectonic peraluminous granites in the infrastructure are interpreted to have formed by decompressional anatexis (c. 430–425 Ma) and were later deformed to ultramylonites, cataclasites and psudotachylites along the high strain zone. Extension related fabrics overprint earlier and are synchronous with contractional top-to-the-NW thrust faults and associated structures. Muscovite records progressively younger 40Ar/39Ar cooling ages (from 408 to 388 Ma) downward in the extensional footwall, whereas feldspar from the mylonites yield a closure age of 349 Ma. Foreland-directed shortening in the East Greenland Caledonides lasted until at least Early Devonian times, while orogenic extensional collapse initiated along the zone of high strain during the Late Silurian in an overall collisional setting. Upper crustal thinning continued well into the Devonian along increasingly more narrow and brittle extensional faults.

An 40Ar/39Ar thermochronology of the Ofoten‐Troms region: Implications for terrane amalgamation and extensional collapse of the northern Scandinavian Caledonides

Fifteen 40Ar/39Ar cooling ages are reported for metamorphic hornblende and muscovite from far traveled terranes constituting the Ofoten nappe stack of northern Norway. Eight cooling ages on hornblende range from 425 to 394 Ma and seven muscovite ages, from the same or nearby outcrops as the hornblendes, range from 400 to 373 Ma. These data are compared with 40Ar/39Ar ages from over a large part of the northern Caledonides to evaluate regional mineral cooling patterns. Results indicate that (1) Scandian (Silurian-Devonian) metamorphism was predominant; (2) most of the nappes investigated contain some vestige of pre-Scandian tectonism and/or metamorphism; (3) hornblende and muscovite cooling ages are progressively younger to the west and south, which suggests a hinged-to-the-east mineral cooling pattern; and (4) a late, out-of-sequence thrust is the only disruption of this cooling pattern. Synmetamorphic amalgamation of the nappes resulted from Scandian A type subduction. The hinged-to-the-east mineral cooling pattern implies isostatic adjustment and exhumation of the footwall of a west dipping, crustal-scale extensional fault, located somewhere west of the present Norwegian coast, during late synorogenic gravitational collapse. The late out-of-sequence fault formed contemporaneously with uplift in the hinterland, implying a kinematic and temporal connection with east directed contractional faulting in the foreland.

Evidence for ophiolite obduction, terrane accretion and polyorogenic evolution of the north Scandinavian Caledonides

Abstract Recent work on the thrust sheets of the Ofoten-Troms region, north Norwegian Caledonides, has resulted in revision of the tectonostratigraphic correlations providing new constraints on models for terrane accretion and poly-orogenic Cambro-Ordovician and Siluro-Devonian development. Previous studies on Late Ordovician-Early Silurian fossils in metasediments unconformably overlying the Lyngen Ophiolite complex within one of the Troms nappes documented a period of ophiolite obduction prior to the major Siluro-Devonian Scandian orogenic event. Correlation of these units and underlying and overlying thrusts sheets southward into the Ofoten area, 150 km to the south, is demonstrated. This implies that an extensive, completely transported, Upper Ordovician-Lower Silurian basin, developed on top of the obducted ophiolite, composes the bedrock of much of northern Norway. Metamorphism and deformation within these basinal rocks resulted from the Scandian phase of the Caledonian orogeny. The relatively weak metamorphic and deformational imprint on the ophiolitic basement provides a rare opportunity to evaluate the pre-Scandian tectonic evolution of northern Scandinavia. Poly-orogenic evolution, a controversial subject in northern Norway, is documented. The character and relative extent of Cambro-Ordovician and Siluro-Devonian orogenic zones in northern Norway are elucidated. The Balsfjord/Ofoten unconformity formed approximately concomitant with other post-ophiolite obduction unconformities in central and southern Norway, implying an orogen-wide Late Cambrian-Early Ordovician event.