The myth of the De Geer Zone: a change of paradigm for the opening of the Fram Strait
Open Research Europe
Abstract
Background
Cenozoic rifting in the Arctic and the resulting opening of the Labrador Sea and the Fram Strait are typically associated with the movement of the Svalbard Archipelago c. 400 km southwards and its separation from Greenland. Thus far, most of this tectonic displacement was ascribed to lateral movement along the N–S-striking De Geer Zone, a thousand-kilometer-long paleo-transform fault believed to extend from northwestern Norway to northern Greenland.
Methods
The study presents a new interpretation of tectonic structures on seismic reflection data north and west of Svalbard.
Results
The present study reports the presence of two km-thick, hundreds of kilometers long, E–W- to WNW–ESE-striking shear zones, northwest and west of the island of Spitsbergen, Svalbard, in the Norwegian Arctic. Contractional structures within the shear zones, their strike, the inferred transport direction, and the great depth at which they are found indicate that they formed during the Timanian Orogeny in the late Neoproterozoic (c. 650–550 Ma). These structures extend at least 80–90 km west of the coastline of Spitsbergen. The presence of continuous, late Neoproterozoic Timanian thrusts this far west of Spitsbergen invalidates the occurrence of c. 400 km lateral movements along the N–S-striking De Geer Zone along the western Barents Sea–Svalbard margin in the Cenozoic.
Conclusions
The present results suggest that the De Geer Zone does not exist and that related fault complexes (e.g., Hornsund Fault Complex) did not accommodate any strike-slip movement. In addition, the formation of major NW–SE-striking transform faults in the Fram Strait was controlled by late Neoproterozoic Timanian thrust systems. The present results call for major revisions of all current plate tectonics models for the opening of the Fram Strait and Arctic tectonics in the Cenozoic and for critical reviews of major fault zones inferred from indirect observations.
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Background Thus far, most earthquakes around the Svalbard Archipelago in the past 75 years, some reaching up to moment magnitude (M W ) 6.1, have been attributed to movements along N–S- to NE–SW-trending fault surfaces, possibly including inherited Caledonian (ca. 460–425 Ma) and late Paleozoic–Cenozoic structures. Recent geophysical studies in the Barents Sea and Svalbard have revealed that active transform faults in the Fram Strait parallel a newly identified structural trend comprising WNW–ESE-striking thrust systems of the Timanian Orogeny (ca. 650–550 Ma) thousands of km long. Methods We used seismicity data from the International Seismological Centre and seismic reflection data from the DISKOS database to correlate recent earthquakes and discrete slip surfaces in the sub-surface. Results Analysis of seismic reflection data in Storfjorden reveals a set of km-scale normal faults, which truncate the seafloor and are located above and parallel to a WNW–ESE-striking Timanian thrust system. Another Timanian thrust system, the Kinnhøgda–Daudbjørnpynten fault zone, coincides with the location of the 2008–2016 Storfjorden seismic sequence. The southwards migration and shallowing of the aftershocks over time is explained by rupture along a deep, gently NNE-dipping slip surface within the basal part of the thrust in the north and ensuing collapse of overlying rocks and thrust sheets along the shallow portion of nearby slip surfaces. Conclusions Seismicity and seismic reflection data in Svalbard indicate that most earthquakes in the last 75 years may be associated with movement along WNW–ESE-striking Timanian thrust systems. The regional stress field causing movement along the inherited Timanian thrusts is likely related to ongoing rifting and transform faulting in the Fram Strait. The present study suggests that linear earthquake swarms may be generated by and occur orthogonal to inherited brittle–ductile thrust systems. The structures responsible for the seismicity around Svalbard may generate earthquakes up to magnitude 8.9.
Over the last decade, new observations and expanded databases have enriched our understanding of Earth structure, dynamics, and, crucially, plate tectonics. For example, features such as the 20-40 km thick crust at the Greenland-Iceland-Faroe Ridge (GIFR), thus far ascribed to voluminous plume magmatism, are now becoming easier to study thanks to advances in seismic imaging. Recent seismic mapping off the Faroe Islands, southeastern Greenland, and within the GIFR shows that late Paleoproterozoic thrust systems of the Ammassalik Belt and Laxfordian Orogen continue beyond the Seaward-Dipping Reflectors, which were previously interpreted as the continent-ocean boundary. A continental nature for the crust under the GIFR and its regional implications must therefore be considered. It is possible to reconcile continental crust under the GIFR with reconstructed plate kinematics and seafloor spreading northeast and southwest of Iceland through the "orogenic bridge" concept. Orogenic bridges are old orogens that strike highly obliquely to active rifts and hinder breakup, forcing the rift to step sideways and locally reorient. Orogenic bridges are not to be mistaken for so-called "Land Bridges", which imply continent fixity. On the contrary, orogenic bridges stretch together with adjacent oceanic crust. At the GIFR, late Paleoproterozoic thrust systems strike NW-SE and are unsuitably oriented to thin the crust efficiently. Thus, while breakup has occurred and oceanic crust is being formed at the Reykjanes and Kolbeinsey mid-ocean ridges, breakup has not yet occurred at the GIFR and the orogenic crust there is still being stretched and thinned.
Results of U-Pb geochronology of fault-related carbonates in Kongsfjorden, western Svalbard, and its implications for Orogenic Bridge Theory
Miocene breakup of Svalbard from Greenland formed a deep oceanic gateway that enabled circulation between the Arctic and Atlantic Oceans, significantly changing the global climate. However, the timing of events remains unclear. An excellent opportunity to constrain this timing is found onshore western Svalbard, where the Sarsbukta fault forms the eastern margin of the Eocene−Oligocene Forlandsundet basin. Here, we present new results from U-Pb dating of calcite precipitated in fault-related veins to constrain the timing of Sarsbukta fault deformation and the evolution of the basin. Our oldest calcite age is Permo-Triassic, suggesting long-lived deformation along the fault. A cluster of ages between 41 and 33 Ma overlaps with fossil-based depositional ages from parts of the Forlandsundet basin. These data indicate that onshore transtension partly pre-dated the well-established Chron 13 (magnetic polarity time scale; 35.5−33.7 Ma) reorganization of spreading ridges in the North Atlantic. Our youngest age of 13 Ma indicates that faulting persisted long after the preserved basin fill was deposited. If seafloor spreading marked the end of extension of continental crust, Molloy Ridge spreading during Chron 5 (19.6−9.8 Ma) may have initiated after 13 Ma.
The present study is a detailed structural analysis of 3D seismic data in the Selis Ridge in the western Loppa High in the Norwegian Barents Sea, to which seismic attributes and spectral decomposition were applied. The analysis reveals that pre-Devonian basement rocks are crosscut by a 40-50 kilometers wide, several kilometers thick, E-W-to WNW-ESE-striking fold-and-thrust belt, including a steep, kilometer-thick, top-SSW shear zone. The folds display dome-and trough-shaped geometries, the thrusts appear to have been reactivated dominantly as top-west structures, and the main shear zone warps over the top of the Selis Ridge. The fold-and-thrust belt is interpreted as part of the latest Neoproterozoic (ca. 650-550 Ma) Timanian Orogeny, which was reworked during E-W Caledonian contraction in the Ordovician-Silurian. The results are analogous to recent findings in the northern Norwegian Barents Sea and Svalbard. The presented interpretation provides the basis for discussing Neoproterozoic-Paleozoic plate tectonics reconstructions, the influence of Precambrian-early Paleozoic structures on post-Caledonian fault complexes, and the location of the Timanian and Caledonian suture zones.
The Southwestern Basement Province of Svalbard extends northward from Sørkapp Land in the south to Oscar II Land. In the north, the Müllerneset Formation characterized by polymetamorphosed Proterozoic sedimentary rocks crops out. In this study we used an integrated tectonic and petrochronological approach to gain an insight into the structural and metamorphic evolution of the unit and surrounding basement. The Müllerneset Formation consists of two separate tec-tonic blocks. NNW-SSE trending retrograde foliation is associated with mineral and stretching lineation and kinematic indicators consistent with left-lateral to oblique sinistral shearing in the western block. The eastern block is characterized by the opposite sense of shear that was overturned during the Eurekan event as evidenced by unconformably overlaying Carboniferous sedimentary rocks. Conventional geothermobarometry yields the prograde peak pressure metamorphic conditions of 6.6 - 7.1 kbar at 480 - 520°C followed by peak temperature at 5.1 - 5.9 kbar and 530 - 560°C. Subsequent retrograde greenschist facies overprint is related to left-lateral NNW-SSE trending shearing. Tiny monazite occurs within foliation or overgrows allanite-(Ce), thus is interpreted as growth along a retrograde path. Th-U-total Pb dating of monazite-(Ce) provided an early Caledonian age (ca. 450 Ma) and younger population of ca. 410 ± 8 Ma. This age is consistent with previously reported 40Ar/39Ar cooling ages (410 ± 2 Ma) of muscovite supporting a retrograde growth of monazite. Petrochronological evidence combined with structural observations suggests that the Müllerneset Formation has been tectonically exhumed in the Early Devonian due to the NNW-SSE trending left-lateral shearing. Coeval folding and thrusting in the remaining basement of Oscar II Land to the east indicate a transpressional regime of the deformation in the Early Devonian. Similarly oriented contemporaneous tectonic zones within the Southwestern Basement Province of Svalbard may account for the same set of shear zones dispersing the Ordovician subduction complexes along western Spitsbergen.
In the Late Devonian to earliest Mississippian, Svalbard was affected by a short-lived episode of deformation named the Svalbardian Orogeny. This event resulted in intense folding and thrusting in Devonian sedimentary successions. Deformation stopped prior to the deposition of Carboniferous to Permian sedimentary strata of the Billefjorden and Gipsdalen groups, which lie unconformably over folded Devonian strata. Later on, presumed Svalbardian structures were reworked during Eurekan tectonism in the early Cenozoic and partly eroded. At present, records of Svalbardian deformation are only preserved in narrow N–S-trending belts in central, northern, western, and southern Spitsbergen. Despite extensive field studies, the timing of the Svalbardian Orogeny is poorly constrained and remains a matter of debate in places because of conflicting ages and because of the complex tectonic history of Svalbard. The present contribution aims at reviewing and discussing all available age constraints for Svalbardian tectonism, including notably palynological, paleontological, and geochronological evidence. This has great implications for the plate tectonic reconstructions of Arctic regions and for the tectonic history of Svalbard. Palynological and paleontological evidence suggest that the Mimerdalen Subgroup is upper Givetian to lower Frasnian (ca. 385–380 Ma) in age and that the Billefjorden Group is mid-Famennian to Upper Mississippian (ca. 365–325 Ma) in age, constraining the Svalbardian event in central and northern Spitsbergen to 383–365 Ma if it ever occurred. Palynological ages indicate that the Adriabukta Formation in southern Spitsbergen is Middle Mississippian and therefore cannot have been involved in the Svalbardian event, thus suggesting that all the deformation in southern Spitsbergen is early Cenozoic in age and that strain-partitioning processes had a major role in localizing deformation in weaker stratigraphic units. The few geochronological age constraints yielding Late Devonian–Mississippian ages in Svalbard may reflect either Svalbardian contraction or extensional processes and are therefore of no use to validate or invalidate the occurrence of the Svalbardian event. On the contrary, the contradicting lines of evidence used to support the occurrence of the Svalbardian event and new regional geophysical studies suggest that Svalbard was subjected to continuous extension from the late Silurian to early Permian times.
On Varanger Peninsula, the c. NW-SE-trending Trollfjorden-Komagelva Fault Zone separates Neoproterozoic successions that accumulated in a shallow-marine platformal domain to the southwest of the fault from deep-marine basinal to deltaic sediments to the northeast. In Ediacaran time the fault scarp acted as a buttress in a period of basinal inversion during the top-SW, contractional Timanian orogeny. During the subsequent, top-SE to ESE, polyphase Caledonian orogenesis the fault acted as a dextral strike-slip megafracture and lateral ramp. In the northeastern terrane, Timanian and Caledonian fold interference has produced several examples of double-folding and intersecting cleavages. In the southwestern terrane, the Lower Allochthon Gaissa Thrust Belt overlies the Parautochthon, below which a 45 km-long deformation front has been mapped. Tectonic shortening within this frontal zone varies from top-SSE in the west to top-ESE in the east. Imbricate thrust sheets disrupt the succession in the northeastern terrane, below which a floor décollement is speculated to emerge as a frontal fault along a prominent footwall of the precursor fault acted on the seabed in outermost Varangerfjorden. By use of potential field data, the Caledonian structures on Varanger can be followed to the north into the Barents Sea where the nappe pile in the pre-Carboniferous basement reaches a thickness of several kilometres.
Thematic collection: This article is part of the Fold-and-thrust belts collection available at: https://www.lyellcollection.org/cc/fold-and-thrust-belts
Svalbard’s Northwestern Basement Province is traditionally divided into the Albert I Land and the Biscayarhalvøya terranes. New U-Pb age data on zircon and monazite, structural and geochemical data provide first evidence of early Palaeozoic deposits south of the Biscayarhalvøya Terrane indicating the possible existence of a third terrane: the Germaniahalvøya Terrane. This area is represented by a Cambro-Ordovician succession of mica schist and marble (Lernerøyane Group) and its higher-grade metamorphic equivalent (Liefdefjorden Migmatite Complex), which were affected by the Taconian phase (migmatization at c. 469 Ma) and the Scandian phase ( c. 422–415 Ma) of the Caledonian Orogeny. During the Scandian phase, the ductile Lerner Deformation Zone was formed. New isotopic data from the eclogite-bearing Richarddalen Complex of the Biscayarhalvøya Terrane imply the formation as Ordovician–Silurian collision-related mélange dominantly composed of c. 730 to 600 Ma Timanian island-arc derived detritus and igneous rocks, partly eclogite-facies metamorphosed at c. 656 Ma, and Tonian meta-igneous rocks. After amphibolite-facies metamorphism of the mélange matrix at c. 423 Ma, the Richarddalen Complex and the Stenian–Tonian Biscayarfonna Group were juxtaposed and mylonitized by the dextral Biscayarhalvøya Deformation Zone.
Supplementary material: https://doi.org/10.6084/m9.figshare.c.5778735
New analytical and field techniques, as well as increased international communication and collaboration, have resulted in significant new geological discoveries within the Appalachian-Caledonian-Variscan orogen. Cross-Atlantic correlations are more tightly constrained and the database that helps us understand the origins of Gondwanan terranes continues to grow. Special Paper 554 provides a comprehensive overview of our current understanding of the evolution of this orogen. It takes the reader along a clockwise path around the North Atlantic Ocean from the U.S. and Canadian Appalachians, to the Caledonides of Spitsbergen, Scandinavia, Scotland and Ireland, and thence south to the Variscides of Morocco.