Devonian core complex exhumation and Cenozoic decollements as alternatives to the Ellesmerian Orogeny

Poster presentation at ARCEx Annual Conference 2018, Longyearbyen, Svalbard, 09.10. - 11.10.18. https://arcex.no/arcex-2018/. The Ellesmerian Orogeny (Piepjohn et al., 2000) is a short-lived contractional–transpressional event that occurred in the Late Devonian–Mississippian, i.e., after Devonian collapse of the Caledonides and prior to Carboniferous rifting. Thus far, this episode of contraction–transpression was required to explain the presence of undeformed Carboniferous–Permian sedimentary rocks on top of folded Upper Devonian strata in central Spitsbergen. The orogen is poorly constrained in other parts of the Arctic due to the lack/poor exposure of Devonian–Carboniferous sedimentary rocks (Rippington et al., 2010). We present an alternative model involving core complex exhumation through continuous, decreasing, Devonian–Carboniferous extension during the collapse of the Caledonides, and (partial) strain decoupling during Cenozoic transpression in Svalbard

Impact of Timanian thrusts on the Phanerozoic tectonic history of Svalbard

Presentation at "Friday seminar" at UiT The Arctic University of Norway, 13.09.2019.Despite more than a century of investigation, the relationship between basement rocks throughout the Svalbard Archipelago is still a mystery. Though these rocks display similar geochronological ages, they show significantly different metamorphic grades and structures. Thus far, Svalbard was believed to be composed of three terranes of rocks formed hundreds–thousands of kilometers apart and accreted in the mid-Paleozoic. New evidence from seismic, gravimetric, aeromagnetic, seismological, bathymetric, and field data show that these terranes might have already been accreted in the late Neoproterozoic. Notably, the data show that at least three–four, crustal-scale, WNW–ESE-striking thrust systems crosscut Spitsbergen and merge with Timanian thrusts in the northern Barents Sea and northwestern Russia. These thrusts were reactivated as sinistral-reverse oblique-slip faults and partly folded during the Caledonian and Eurekan orogenies, and reactivated as sinistral-normal faults during Devonian–Mississippian extensional collapse, thus offsetting N–S-trending Caledonian grain and post-Caledonian basins. The presence of these faults explains the juxtaposition of basement rocks of seemingly different origin throughout the Svalbard Archipelago, the distribution of Mississippian rocks and Early Cretaceous intrusions along a WNW–ESE-trending axis in central Spitsbergen, the west vergence of Cenozoic folds in Devonian rocks in central–northern Spitsbergen (previously ascribed to Late Devonian Ellesmerian contraction), the arch shape of the Cenozoic West Spitsbergen Fold-and-Thrust Belt in Brøggerhalvøya, and the strike and location of transform faults west of Spitsbergen. Further implications of this work might be that the tectonic plates constituting present-day Norwegian Arctic regions (Laurentia and Baltica) have retained their current geometry and alignment for the past 600 Ma, that the Timanian Orogeny and associated WNW–ESE-striking faults extend from northwestern Russia to Svalbard, and, possibly, to Greenland and Arctic Canada, and that the transport of Svalbard from next to Greenland in the early Cenozoic to its present position (ca. 400 km southwards) might have been accommodated exclusively by break-up and displacement along transform faults (strike-slip movements), and by top-SSW thrusting and folding (horizontal shortening) along inherited Timanian grain instead of dextral strike-slip movement along the De Geer Zone

The Billefjorden Fault Zone north of Spitsbergen: a major terrane boundary?

The Billefjorden Fault Zone is a major terrane boundary in the Norwegian Arctic. The fault separates basement rocks of Svalbard’s north-eastern and north-western terranes that recorded discrete Precambrian tectonothermal histories and were accreted, intensely deformed and metamorphosed during the Caledonian Orogeny. Although the fault represents a major, crustal-scale tectonic boundary, its north-ward extent is not well constrained. The present short contribution addresses this issue and presents new seismic mapping of structures and rock units north of Wijdefjorden, where the Billefjorden Fault Zone may continue. This study shows that there is no evidence for major faulting of the top-basement reflection, and therefore, that the Billefjorden Fault Zone may die out within Wijdefjorden–Austfjorden, step ≥ 20 km laterally, or be invisible on the presented seismic data. Seismic data also suggest that Caledonian basement rocks in Ny-Friesland (north-eastern terrane) are not significantly different from basement rocks below the Devonian Graben in Andrée Land (north-western terrane). Potential implica-tions include the absence of a major terrane boundary in northern Spitsbergen

Post-Caledonian Brittle Fault Zones on the Hyperextended SW Barents Sea Margin: New Insights into Onshore and Offshore Margin Architecture

Onshore-offshore correlation of brittle faults and tectonic lineaments has been undertaken along the SW Barents Sea margin off northern Norway. The study has focused on onshore mapping of fault zones, the mapping of offshore fault complexes and associated basins from seismic interpretation, and the linkage of fault complexes onshore and offshore by integrating a high-resolution DEM, covering both onshore and offshore portions of the study area, and processed magnetic anomaly data. This study shows that both onshore and offshore brittle faults manifest themselves mainly as alternating NNE–SSW- and ENE–WSW-trending, steeply to moderately dipping, normal fault zones constituting at least two major NE-SW-trending fault complexes, the Troms-Finnmark and Vestfjorden-Vanna fault complexes. These fault complexes in western Troms bound a major basement horst (the West Troms Basement Complex), run partly onshore and offshore and link up with the offshore Nysleppen and Måsøy fault complexes. Pre-existing structures in the basement, such as foliation, lithological boundaries and ductile shear zones are shown, at least on a local scale, to have exerted a controlling effect on faulting. On a larger scale, at least two major transfer fault zone systems, one along the reactivated Precambrian Senja Shear Belt and the other, the Fugløya transfer zone, accommodate changes in brittle fault polarity along the margin. Our results suggest that distributed rifting during Carboniferous and Late Permian/Early Triassic time was followed by a northwestward localisation of displacement to the Troms–Finnmark and Ringvassøy–Loppa fault complexes during the Late Jurassic/Early Cretaceous, resulting in the formation of a short-tapered, hyperextended margin with final break-up at ~55 Ma. An uplift of the margin and preservation of the West Troms Basement Complex as a basement outlier is suggested to be due to unloading and crustal flexure of the short-tapered margin in the region

Mid/Late Devonian-Carboniferous extensional faulting in Finnmark and the SW Barents Sea

In northern Norway, late/post-Caledonian extension initiated along inverted, brittle-ductile thrusts, e.g. Sørøya-Ingøya shear zone, which truncated existing, margin-oblique, late Neoproterozoic, Timanian faults and margin-parallel, latest Mesoproterozoic-mid Neoproterozoic faults related to the opening of the Asgard Sea and Iapetus Ocean. Inverted thrusts were active through the Devonian and early Carboniferous, accommodating the deposition of thick, Mid/Upper Devonian growth strata and of subsequent, lower Carboniferous sedimentary rocks in spoon-shaped basins on the Finnmark Platform and in the southwesternmost Nordkapp basin. Simultaneously, NE-SW trending basement ridges exhumed as part of a regional metamorphic core complex along bowed portions of the Sørøya-Ingøya shear zone and in the footwall of high-angle, zigzag-shaped normal faults following pre-existing basement fabrics, such as the Troms-Finnmark Fault Complex. Late/post-Caledonian exhumation is also documented onshore adjacent areas in NW Finnmark by retrograde fault-rock mineral assemblages and K/Ar faulting ages along the Langfjord-Vargsund fault, indicating rapid exhumation from > 10 km to 2-8 km depth in the Late Devonian-early Carboniferous. Inherited, margin-oblique brittle faults such as the Trollfjorden-Komagelva Fault Zone acted as minor strike-slip transfer faults, segmenting the margin and offsetting major zigzag-shaped fault complexes like the Langfjord-Vargsund fault. Bathymetry data further show sigma-shaped mini-basins with dense, internal, fault-fracture networks (e.g. Ryggefjorden trough), potentially representing small-scale analogs to major offshore basins such as the Nordkapp Basin. In the late Carboniferous, faulting activity declined and extension localized along a few major faults. In addition, Caledonian rocks were slowly exhumed to depths < 3.5 km., and thick, upper Carboniferous evaporites deposited in the southwesternmost Nordkapp basin. The margin was tectonically quiet by the end of the Carboniferous and was only subjected to minor, Permian-Cenozoic tectonic adjustments. Alternatively, late Carboniferous-mid Permian K/Ar ages obtained in NW Finnmark reflect an episode of margin weathering

Early Cenozoic Eurekan strain partitioning and decoupling in central Spitsbergen, Svalbard

The present study of field, petrological, exploration well, and seismic data describes backward-dipping duplexes comprised of phyllitic coal and bedding-parallel décollements and thrusts localized along lithological transitions in tectonically thickened Lower Devonian to lowermost Upper Devonian; uppermost Devonian–Mississippian; and uppermost Pennsylvanian–lowermost Permian sedimentary strata of the Wood Bay and/or Wijde Bay and/or Grey Hoek formations; of the Billefjorden Group; and of the Wordiekammen Formation, respectively. The study shows that these structures partially decoupled uppermost Devonian–Permian sedimentary rocks of the Billefjorden and Gipsdalen groups from Lower Devonian to lowermost Upper Devonian rocks of the Andrée Land Group and Mimerdalen Subgroup during early Cenozoic Eurekan deformation in central Spitsbergen. Eurekan strain decoupling along these structures explains differential deformation between Lower Devonian to lowermost Upper Devonian rocks of the Andrée Land Group and/or Mimerdalen Subgroup and overlying uppermost Devonian–Permian sedimentary strata of the Billefjorden and Gipsdalen groups in central–northern Spitsbergen without requiring an episode of (Ellesmerian) contraction in the Late Devonian. Potential formation mechanisms for bedding-parallel décollements and thrusts include shortcut faulting and/or formation as a roof décollement in a fault-bend hanging wall (or ramp) anticline, as an imbricate fan, as an antiformal thrust stack, and/or as fault-propagation folds over reactivated or overprinted basement-seated faults. The interpretation of seismic data in Reindalspasset indicates that Devonian sedimentary rocks of the Andrée Land Group and Mimerdalen Subgroup might be preserved east of the Billefjorden Fault Zone, suggesting that the Billefjorden Fault Zone did not accommodate reverse movement in the Late Devonian. Hence, the thrusting of Proterozoic basement rocks over Lower Devonian sedimentary rocks along the Balliolbreen Fault and fold structures within strata of the Andrée Land Group and Mimerdalen Subgroup in central Spitsbergen may be explained by a combination of down-east Carboniferous normal faulting with associated footwall rotation and exhumation, and subsequent top-west early Cenozoic Eurekan thrusting along the Billefjorden Fault Zone. Finally, the study shows that major east-dipping faults, like the Billefjorden Fault Zone, may consist of several discrete, unconnected (soft-linked and/or stepping) or, most probably, offset fault segments that were reactivated or overprinted to varying degrees during Eurekan deformation due to strain partitioning and/or decoupling along sub-orthogonal NNE-dipping reverse faults

Mid/Late Devonian-Carboniferous extensional faulting in Finnmark and the SW Barents Sea

In northern Norway, late/post-Caledonian extension initiated along inverted, brittle-ductile thrusts, e.g. Sørøya-Ingøya shear zone, which truncated existing, margin-oblique, late Neoproterozoic, Timanian faults and margin-parallel, latest Mesoproterozoic-mid Neoproterozoic faults related to the opening of the Asgard Sea and Iapetus Ocean. Inverted thrusts were active through the Devonian and early Carboniferous, accommodating the deposition of thick, Mid/Upper Devonian growth strata and of subsequent, lower Carboniferous sedimentary rocks in spoon-shaped basins on the Finnmark Platform and in the southwesternmost Nordkapp basin. Simultaneously, NE-SW trending basement ridges exhumed as part of a regional metamorphic core complex along bowed portions of the Sørøya-Ingøya shear zone and in the footwall of high-angle, zigzag-shaped normal faults following pre-existing basement fabrics, such as the Troms-Finnmark Fault Complex. Late/post-Caledonian exhumation is also documented onshore adjacent areas in NW Finnmark by retrograde fault-rock mineral assemblages and K/Ar faulting ages along the Langfjord-Vargsund fault, indicating rapid exhumation from > 10 km to 2-8 km depth in the Late Devonian-early Carboniferous. Inherited, margin-oblique brittle faults such as the Trollfjorden-Komagelva Fault Zone acted as minor strike-slip transfer faults, segmenting the margin and offsetting major zigzag-shaped fault complexes like the Langfjord-Vargsund fault. Bathymetry data further show sigma-shaped mini-basins with dense, internal, fault-fracture networks (e.g. Ryggefjorden trough), potentially representing small-scale analogs to major offshore basins such as the Nordkapp Basin. In the late Carboniferous, faulting activity declined and extension localized along a few major faults. In addition, Caledonian rocks were slowly exhumed to depths < 3.5 km., and thick, upper Carboniferous evaporites deposited in the southwesternmost Nordkapp basin. The margin was tectonically quiet by the end of the Carboniferous and was only subjected to minor, Permian-Cenozoic tectonic adjustments. Alternatively, late Carboniferous-mid Permian K/Ar ages obtained in NW Finnmark reflect an episode of margin weathering

Late Paleozoic-Cenozoic fault correlation and characterization of fault rocks in western Troms, North Norway

The present work focuses on the mapping and description of onshore brittle fault zones on the SW Barents Sea Margin, within gneisses and granitic intrusions belonging to the West Troms Basement Complex. The description of the brittle structures includes the geometry, kinematics and fault rock characteristics, using DEM satellite imaging, structural field work and a microstructural analysis of the fault rocks. As a result of the present study,two major sets of onshore brittle faults have been observed, trending NNE-SSW and ENE-WSW,and they are tentatively correlated in two, NE-SW trending fault complexes: the Rekvika and the Vestfjorden-Vanna fault complexes. These fault complexes run parallel to the Troms-Finnmark fault complex that borders major offshore sedimentary basins in the NW. On the one hand, the Vestfjorden-Vanna fault complex defines the southeastern boundary of the West Troms Basement Complex towards the Caledonian nappes, and is made of wide fault zones that have experienced major displacement (ca. 1-3km). On the other hand, the Rekvika fault complex is considered as an intra-horst fault system composed of narrow fault zones that are thought to have accommodated low amounts of displacement (> 250 m). On a local scale, the fault zones display similarities in attitude (trend and dip) with the Caledonian and Precambrian fabrics, indicating a possible influence of pre-existing zones of weakness on brittle faulting. The dominant deformation mechanism (cataclastic flow) indicates pressure range about 0.2-0.3 GPa, i.e. 5-10 km depth for the formation of the cataclastic fault rocks, and these fault rocks contain mineral assemblages that generally indicate temperatures about 350-500ᵒC. This suggests that the West Troms Basement Complex has been largely uplifted later on, until present level. The NNE-SSW and ENE-WSW trending fracture sets are believed to have formed synchronously due to WNW-ESE extension, during an early stage of rifting in the Permian-Early Triassic. The NNE-SSW trending faults likely represent the main fault system, orthogonal to the extension direction, and the ENE-WSW trending faults may correspond to oblique transfer zones that link the NNE-SSW fault segments. An alternative model implies NW-SE trending transfer zones to link the NNE-SSW trending faults. A late stage of minor reverse reactivation of the NNE-SSW and ENE-WSW trending faults, and the development of NW-SE striking fractures are tentatively correlated to ridge-push forces during the opening of the North Atlantic in the Eocene

From widespread Mississippian to localized Pennsylvanian extension in central Spitsbergen, Svalbard

In the Devonian–Carboniferous, a rapid succession of clustered extensional and contractional tectonic events is thought to have affected sedimentary rocks in central Spitsbergen, Svalbard. These events include Caledonian post-orogenic extensional collapse associated with the formation of thick Early–Middle Devonian basins, Late Devonian–Mississippian Ellesmerian contraction, and Early–Middle Pennsylvanian rifting, which resulted in the deposition of thick sedimentary units in Carboniferous basins like the Billefjorden Trough. The clustering of these varied tectonic settings sometimes makes it difficult to resolve the tectono-sedimentary history of individual stratigraphic units. Notably, the context of deposition of Mississippian clastic and coal-bearing sedimentary rocks of the Billefjorden Group is still debated, especially in central Spitsbergen. We present field evidence (e.g., growth strata and slickensides) from the northern part of the Billefjorden Trough, in Odellfjellet, suggesting that tilted Mississippian sedimentary strata of the Billefjorden Group deposited during active (Late/latest?) Mississippian extension. WNW–ESE-striking basin-oblique faults showing Mississippian growth strata systematically die out upwards within Mississippian to lowermost Pennsylvanian strata, thus suggesting a period of widespread WNW–ESE-directed extension in the Mississippian and an episode of localized extension in Early–Middle Pennsylvanian times. In addition, the presence of abundant basin-oblique faults in basement rocks adjacent to the Billefjorden Trough suggests that the formation of Mississippian normal faults was partly controlled by reactivation of preexisting Neoproterozoic (Timanian?) basement-seated fault zones. We propose that these preexisting faults reactivated as transverse or accommodation cross faults in or near the crest of transverse folds reflecting differential displacement along the Billefjorden Fault Zone. In Cenozoic times, a few margin-oblique faults (e.g., the Overgangshytta fault) may have mildly reactivated as oblique thrusts during transpression–contraction, but shallow-dipping, bedding-parallel, duplex-shaped décollements in shales of the Billefjorden Group possibly prevented substantial movement along these faults

Devonian–Carboniferous extension and Eurekan inversion along an inherited WNW–ESE-striking fault system in Billefjorden, Svalbard

&lt;p&gt; Background: The Billefjorden area in central Spitsbergen hosts thick Lower–lowermost Upper Devonian, late–post-Caledonian collapse deposits presumably deformed during the Late Devonian Svalbardian Orogeny. These rocks are juxtaposed against Proterozoic basement rocks along the Billefjorden Fault Zone and are overlain by uppermost Devonian–early Permian deposits of the Billefjorden Trough, a N–S-trending Carboniferous rift basin bounded by the Billefjorden Fault Zone.&lt;/p&gt;&lt;p&gt; Methods: We interpreted seismic reflection (also depth-converted), bathymetric, and exploration well data.&lt;/p&gt;&lt;p&gt; Results: The data show abundant Early Devonian, WNW–ESE-striking (oblique-slip) normal faults segmenting the Billefjorden Trough, and a gradual decrease in tectonic activity from the Early Devonian (collapse phase) to early Permian (post-rift phase). Early Devonian–Middle Pennsylvanian WNW–ESE-striking faults were mildly reactivated and overprinted and accommodated strain partitioning and decoupling in the early Cenozoic. This resulted in intense deformation of Lower Devonian sedimentary rocks and in the formation of bedding-parallel décollements, e.g., between the Lower Devonian Wood Bay and the uppermost Pennsylvanian–lowermost Permian Wordiekammen formations. This suggests that intense deformation within Devonian rocks in Dickson Land can be explained by Eurekan deformation alone. Eurekan deformation also resulted in the formation of WNW–ESE- and N–S- to NNE–SSW-trending, kilometer-wide, open folds such as the Petuniabukta Syncline, and in inversion and/or overprinting of Early Devonian to Early Pennsylvanian normal faults by sinistral-reverse Eurekan thrusts. WNW–ESE-striking faults merge at depth with similarly trending and dipping ductile shear zone fabrics in Proterozoic basement rocks, which likely formed during the Timanian Orogeny.&lt;/p&gt;&lt;p&gt; Conclusions: A NNE-dipping shear zone, which is part of a large system of Timanian thrusts in the Barents Sea, controlled the formation of WNW–ESE-striking Devonian–Mississippian normal faults and syn-tectonic sedimentary rocks in Billefjorden. Eurekan strain partitioning and decoupling suggest that the Svalbardian Orogeny did not occur in Svalbard.&lt;/p&gt