34 research outputs found
From Caledonian collapse to North Sea Rift: The extended history of a metamorphic core complex
Extensional systems evolve through different stages due to changes in the rheological state of the lithosphere. It is crucial to distinguish ductile structures formed before and during rifting, as both cases have important but contrasting bearings on the structural evolution. To address this issue, we present the illustrative ductileâtoâbrittle structural history of a metamorphic core complex (MCC) onshore and offshore western Norway. Combining geological field mapping with newly acquired 3âD seismic reflection data, we correlate two distinct onshore basement units (BU1 and BU2) to corresponding offshore basement seismic facies (SF1 and SF2). Our interpretation reveals two 40 km wide domes (one onshore and one offshore), which both show characteristic kilometerâscale, westward plunging upright folds. The gneiss domes fill antiformal culminations in the footwall of a >100 km long, shallowly west dipping, extensional detachment. Overlying Caledonian nappes and Devonian supradetachment basins occupy saddles of the hyperbolic detachment surface. Devonian collapse of the Caledonian orogen formed dome and detachment geometries. During North Sea rifting, brittle reactivation of the MCC resulted in complex fault patterns deviating from NâS strike dominant at the eastern margin of the rift. Around 61°N, only minor NâS faults (<100 m throw) cut through the core of the MCC. Major rift faults (â¤5 km throw), on the other hand, reactivated the detachment and follow the steep flanks of the MCC. This highlights that inherited ductile structures can locally alter the orientation of brittle faults formed during rifting.publishedVersio
Architecture, deformation style and petrophysical properties of growth fault systems: the Late Triassic deltaic succession of southern Edgeøya (East Svalbard)
The Late Triassic outcrops on southern Edgeøya, East Svalbard, allow a multiscale study of synâsedimentary listric growth faults located in the prodelta region of a regional prograding system. At least three hierarchical orders of growth faults have been recognized, each showing different deformation mechanisms, styles and stratigraphic locations of the associated detachment interval. The faults, characterized by mutually influencing deformation envelopes over spaceâtime, generally show SWâ to SEâdipping directions, indicating a counterâregional trend with respect to the inferred WâNW directed progradation of the associated delta system. The downâdip movement is accommodated by polyphase deformation, with the different fault architectural elements recording a timeâdependent transition from fluidalâhydroplastic to ductileâbrittle deformation, which is also conceptually scaleâdependent, from the smallerâ (3rd order) to the largerâscale (1st order) endâmember faults respectively. A shift from distributed strain to strain localization towards the fault cores is observed at the meso to microscale (<1 mm), and in the variation in petrophysical parameters of the lithoâstructural facies across and along the fault envelope, with bulk porosity, density, pore size and microcrack intensity varying accordingly to deformation and reworking intensity of inherited structural fabrics. The secondâ and thirdâorder listric fault nucleation points appear to be located above blind fault tipârelated monoclines involving cemented organic shales. Close to planar, throughâgoing, firstâorder faults cut across this boundary, eventually connecting with other favourable lowerâhierarchy fault to create seismicâscale fault zones similar to those imaged in the nearby offshore areas. The inferred largeâscale driving mechanisms for the firstâorder faults are related to the combined effect of tectonic reactivation of deeper Palaeozoic structures in a far field stress regime due to the Uralide orogeny, and differential compaction associated with increased sand sedimentary input in a fineâgrained, waterâsaturated, lowâaccommodation, prodeltaic depositional environment. In synergy to this largeâscale picture, smallâscale causative factors favouring secondâ and thirdâorder faulting seem to be related to mechanicalârheological instabilities related to localized shallow diagenesis and liquidization fronts.publishedVersio
The Svalbard Carboniferous to Cenozoic Composite Tectono-Stratigraphic Element
The Svalbard Composite Tectono-Stratigraphic Element is located on the north-western corner of
the Barents Shelf and comprises a Carboniferous to Pleistocene sedimentary succession. Due to
Cenozoic uplift the succession is subaerially exposed in the Svalbard archipelago. The oldest parts of
the succession consist of Carboniferous to Permian mixed siliciclastic, carbonate and evaporite and
spiculitic sediments that developed during multiple phases of extension. The majority of the
Mesozoic succession is composed of siliciclastic deposits formed in sag basins and continental
platforms. Episodes of Late Jurassic and Early Cretaceous contraction are evident in the eastern part of the archipelago and in nearby offshore areas. Differential uplift related to the opening of the
Amerasian Basin and the Cretaceous emplacement of the High Arctic Large Igneous Province created
a major hiatus spanning from most of the Late Cretaceous and early Danian throughout the Svalbard
Composite Tectono-Stratigraphic Element. The West Spitsbergen Fold and Thrust Belt and the
associated foreland basin in central Spitsbergen (Central Tertiary Basin) formed as a response to the
Eurekan orogeny and the progressive northward opening of the North Atlantic during the
Palaeogene. This event was followed by formation of yet another major hiatus spanning the
Oligocene to Pliocene. Multiple reservoir and source rock units are exposed in Svalbard providing
analogues to the offshore prolific offshore acreages in southwest Barents Sea and are important for
de-risking of plays and prospects. However, the archipelago itself is regarded as high-risk acreage for
petroleum exploration. This is due to Palaeogene contraction and late Neogene uplift of particularly
the western and central parts. In the east there is an absence of mature source rocks, and the entire
region is subjected to strict environmental protection
From orogeny to rifting: insights from the Norwegian âreactivation phaseâ
Based on observations from the Mid-Norwegian extensional system, we describe how, when and where the post-Caledonian continental crust evolved from a context of orogenic disintegration to one of continental rifting. We highlight the importance of a deformation stage that occurred between the collapse mode and the high-angle faulting mode often associated with early rifting of continental crust. This transitional stage, which we interpret to represent the earliest stage of rifting, includes unexpected large magnitudes of crustal thinning facilitated through the reactivation and further development of inherited collapse structures, including detachment faults, shear zones and metamorphic core complexes. The reduction of the already re-equilibrated post-orogenic crust to onlyâ~â50% of normal thickness over large areas, and considerably less locally, during this stage shows that the common assumption of very moderate extension in the proximal margin domain may not conform to margins that developed on collapsed orogens
Structural comparison of archetypal Atlantic rifted margins: A review of observations and concepts.
International audienc
Deep Crustal Flow Within Postorogenic Metamorphic Core Complexes: Insights From the Southern Western Gneiss Region of Norway
Viscous crustal flow can exhume once deeply buried rocks in postorogenic metamorphic core complexes (MCCs). While migmatite domes record the flow dynamics of anatectic crust, the mechanics and kinematics of solidâstate flow in the deep crust are poorly constrained. To address this issue, we studied a deeply eroded and particularly wellâexposed MCC in the southern Western Gneiss Region of Norway. The Gulen MCC formed during Devonian transtensional collapse of the Caledonian orogeny in the footwall of the NordfjordâSogn detachment zone. We developed a semiquantitative mapping scheme for ductile strain to constrain microâ to megascale processes, which brought eclogiteâbearing crust from the orogenic root into direct contact with Devonian supradetachment basins. The Gulen MCC comprises different structural levels with distinct metamorphic evolutions. In the highâgrade core, amphiboliteâfacies structures record fluidâcontrolled eclogite retrogression and coaxial flow involving vast extensionâperpendicular shortening. Detachment mylonites formed during ductileâtoâbrittle noncoaxial deformation and wrap around the core. We present a sequential 3âD reconstruction of MCC formation. In the detachment zone, the combined effects of simple shearing, incision/excision, and erosion thinned the upper crust. Internal necking of the ductile crust was compensated by extensionâperpendicular shortening within the deep crust and resulted in differential folding of distinct crustal levels. We identify this differential folding as the main mechanism that can redistribute material within solidâstate MCCs. Our interpretation suggests a continuum of processes from migmatiteâcored to solidâstate MCCs and has implications for postorogenic exhumation of (ultraâ)highâpressure rocks
Deep Crustal Flow Within Postorogenic Metamorphic Core Complexes: Insights From the Southern Western Gneiss Region of Norway
Viscous crustal flow can exhume once deeply buried rocks in postorogenic metamorphic core complexes (MCCs). While migmatite domes record the flow dynamics of anatectic crust, the mechanics and kinematics of solidâstate flow in the deep crust are poorly constrained. To address this issue, we studied a deeply eroded and particularly wellâexposed MCC in the southern Western Gneiss Region of Norway. The Gulen MCC formed during Devonian transtensional collapse of the Caledonian orogeny in the footwall of the NordfjordâSogn detachment zone. We developed a semiquantitative mapping scheme for ductile strain to constrain microâ to megascale processes, which brought eclogiteâbearing crust from the orogenic root into direct contact with Devonian supradetachment basins. The Gulen MCC comprises different structural levels with distinct metamorphic evolutions. In the highâgrade core, amphiboliteâfacies structures record fluidâcontrolled eclogite retrogression and coaxial flow involving vast extensionâperpendicular shortening. Detachment mylonites formed during ductileâtoâbrittle noncoaxial deformation and wrap around the core. We present a sequential 3âD reconstruction of MCC formation. In the detachment zone, the combined effects of simple shearing, incision/excision, and erosion thinned the upper crust. Internal necking of the ductile crust was compensated by extensionâperpendicular shortening within the deep crust and resulted in differential folding of distinct crustal levels. We identify this differential folding as the main mechanism that can redistribute material within solidâstate MCCs. Our interpretation suggests a continuum of processes from migmatiteâcored to solidâstate MCCs and has implications for postorogenic exhumation of (ultraâ)highâpressure rocks
Greenland â Norway separation: A geodynamic model for the North Atlantic
Combining information from onshore and offshore Mid-Norway, we propose a structural model for the Scandinavian North Atlantic passive margin from Permo- Carboniferous through Present. We re-examine the role of post-Permo-Carboniferous normal faults and define an innermost boundary fault system forming the continentward limit of the rifted margin. Crustal-scale cross-sections of the Greenland- Norway passive margins show the asymmetric nature of crustal extension between the two conjugate margins. On both margins the upper plate/lower plate geometry and the dip of major extensional normal faults change across the broad width of the Jan Mayen Fracture Zone. South of this zone on the Møre Margin (Norway), the dip of the major faults is towards the west, defining a lower plate â tilted block margin. North of the transform on the Vøring-Trøndelag Margin (Norway), the major faults dip to the east, defining an upper plate or flexural margin. In the Norwegian passive margin, the transition occurs as a progressive change of vergence of normal faults between the northern Vøring-Trøndelag Platform area and the Møre Basin. The model shows continuous separation between the two conjugate margins of Greenland and Norway, starting with a very tight fit in Late Permian time. The rifting events between Late Permian and Late Cretaceous are associated with a broadly WSW-ENE- to W-E- oriented extension while Late Cretaceous to Early Tertiary extension directions are oriented NNW-SSE. These plate separation directions and the subsequent plate motion can be related to the important basin development within, and probably, to the structural evolution and geometry of, the conjugate passive margins
Segmentation of the Caledonian orogenic infrastructure and exhumation of the Western Gneiss Region during transtensional collapse
The (ultra)high-pressure Western Gneiss Region of the Norwegian Caledonides represents an archetypical orogenic infrastructure of a continentâcontinent collision zone. To test established exhumation models, we synthesize the geochronology and structures of major basement windows and provide new ages from poorly dated areas. Migmatite UâPb zircon samples date melt crystallization at c. 405â
Ma in the Ăygarden Complex, expanding the spatial extent of Devonian migmatization. Micas from shear zones in the Ăygarden and Gulen domes yield 40Ar/39Ar ages mostly between 405 and 398â
Ma, recording the exhumation of metamorphic core complexes. On a larger scale, the youngest ages of various geochronometers in different segments of the Western Gneiss Region show abrupt breaks (10â30â
myr) across low-angle detachments and sinistral transfer zones, which also correspond to metamorphic and structural discontinuities. We explain the segmentation of the orogenic infrastructure by partitioned post-orogenic transtension due to lateral and vertical rheological contrasts in the orogenic edifice (strong cratonic foreland and orogenic wedge v. soft infrastructure). Differential crustal stretching dragged out deep levels of the orogenic crust below low-angle detachments and became progressively dominated by sinistral transfer zones. Collapse obliterated the syn-collisional structure of the orogenic root and resulted in the diachronous exhumation of distinct infrastructure segments
Segmentation of the Caledonian orogenic infrastructure and exhumation of the Western Gneiss Region during transtensional collapse
The (ultra)high-pressure Western Gneiss Region of the Norwegian Caledonides represents an archetypical orogenic infrastructure of a continentâcontinent collision zone. To test established exhumation models, we synthesize the geochronology and structures of major basement windows and provide new ages from poorly dated areas. Migmatite UâPb zircon samples date melt crystallization at c. 405â
Ma in the Ăygarden Complex, expanding the spatial extent of Devonian migmatization. Micas from shear zones in the Ăygarden and Gulen domes yield 40Ar/39Ar ages mostly between 405 and 398â
Ma, recording the exhumation of metamorphic core complexes. On a larger scale, the youngest ages of various geochronometers in different segments of the Western Gneiss Region show abrupt breaks (10â30â
myr) across low-angle detachments and sinistral transfer zones, which also correspond to metamorphic and structural discontinuities. We explain the segmentation of the orogenic infrastructure by partitioned post-orogenic transtension due to lateral and vertical rheological contrasts in the orogenic edifice (strong cratonic foreland and orogenic wedge v. soft infrastructure). Differential crustal stretching dragged out deep levels of the orogenic crust below low-angle detachments and became progressively dominated by sinistral transfer zones. Collapse obliterated the syn-collisional structure of the orogenic root and resulted in the diachronous exhumation of distinct infrastructure segments