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Tectonism and magmatism during NE Atlantic continental break-up: The V??ring Margin
Abstract
The temporal and spatial relationships of tectonic and magmatic features on the Vøring volcanic margin show that continental break-up occurred in association with significant magmatic activity about 18 Ma after initiation of lithospheric extension. From the distribution of extension across the margin and the volumes of melt produced, a thermal anomaly of 50-80°C is estimated, in agreement with predictions from recent plume models. A tectono-magmatic model is proposed in which the ascending proto-Iceland plume released the rifting, over a > 300 km wide zone, by uplift-induced extension of the NE Atlantic lithosphere already affected by tensional stresses. Initial rifting took place without decompressional melting when the plume, carrying the thermal anomaly, was still 600-700 km beneath the lithosphere. Subsequently, widespread magmatism occurred when the plume impinged on, spread out beneath and infilled the rift-defined relief at the base of the lithosphere. Break-up is suggested to be a consequence of melt-induced weakening of the lithosphere, whereas the anomalously thick igneous crust at the continent-ocean transition along the NE Atlantic margins is explained by melt focusing towards the uplifted break-up axis.
... In 2D seismic data, elliptical structures above sills were first described in the North Atlantic (Skogseid et al., 1992) and were noted to occur at a single stratigraphic level over a widespread area, revealing that they were vents above sills (Skogseid et al., 1992). The offshore structures are analogous to onshore examples, , such as in the Karoo Basin, South Africa, where they have been studied since the early 1900's (e.g., Gevers, 1928). ...
... In 2D seismic data, elliptical structures above sills were first described in the North Atlantic (Skogseid et al., 1992) and were noted to occur at a single stratigraphic level over a widespread area, revealing that they were vents above sills (Skogseid et al., 1992). The offshore structures are analogous to onshore examples, , such as in the Karoo Basin, South Africa, where they have been studied since the early 1900's (e.g., Gevers, 1928). ...
... Initially offshore vent complexes were interpreted as igneous 'volcanic features' (Skogseid et al., 1992;Davies et al., 2002); however, later observations suggested vents above sills formed predominantly through hydrothermal fluid circulation and subsurface remobilization of sediments (Bell and Butcher, 2002;Planke et al., 2005). The sedimentary nature of many vent fills was recognized by limited acoustic impedance contrasts between some vent fills and the surrounding host rocks (Hansen, 2006), and that some vent fills are polygonally faulted, which would not be expected to occur in more competent volcanic rocks (Hansen, 2006). ...
Ancient hydrothermal vent complexes have released large volumes of greenhouse gases in the past causing global warming, and similar modern vent structures are potential geohazards. In the NE Atlantic, thousands of hydrothermal vent complexes were formed during the Paleocene-Eocene Thermal Maximum. In Java, Indonesia, the erupting Lusi sediment-hosted geothermal system caused the displacement of 40,000 people. In order to determine how ancient and modern hydrothermal venting systems are related, we map a well-defined buried hydrothermal vent complex offshore mid-Norway using 3D seismic reflection data and then compare it to the active Lusi eruption (since 2006) and the neighboring inactive Porong Structure. These are characterized using 2D seismic reflection data, borehole data and field observations. The venting structures are subcircular in plan-view and a few kilometers in diameter. They are funnel-shaped in profiles, with inward-dipping beds surrounding the conduits. The hydrothermal vent complex offshore mid-Norway reveals five seismically-distinct vent fill facies units. Importantly, two of the facies units are separated by an angular unconformity, clearly indicating that the depositional events within the vent fill were distinct. Hydrothermal fluids are interpreted to have led to the fluidization of mud-rich sediments which were erupted and deposited in and around the vent complex. Interpretation of a seismically transparent body along the conduit of the Norwegian venting structure, and the abrupt widening of the conduit at the Porong Structure, are interpreted to be caused by changes in fluid-flow dynamics as the fluids rise and get released from the host-rock. The hydrothermal venting systems in Java and offshore mid-Norway are found to be morphologically similar and are interpreted to form as the result of the transport and eruption of fluidized sediments.
... The anatomy of most magma-induced subsurface vent complexes (SVCs) in seismic reflection data includes a root zone, a sub-vertical conduit and an upper terminus (Skogseid et al. 1992;Jamtveit et al. 2004;Planke et al. 2005;Hansen 2006;Svensen et al. 2006;Zhao et al. 2014). The root zone is usually linked to the vertical conduit where low-amplitude seismic reflections are common and is in turn connected to an upper part where fluidized materials have been expelled or accrued on a paleo-surface. ...
... This study area comprises the Naglfar Dome in the Vøring Basin of the Norwegian Sea (Fig. 1) where the interaction between magmatic sills and hydrothermal vents has been Shambel B. Mituku was formerly affiliated with the Department of Petroleum and Geoscience, Norwegian University of Science and Technology, NTNU, Trondheim previously documented (Planke et al. 2005;Kjoberg et al. 2017;Omosanya et al. 2017;Siregar et al. 2019). The Vøring Basin is one of the igneous provinces in the NE Atlantic margins and was highly affected by thermal activity, magmatism, uplift and exhumation processes due to the Paleocene opening of the Norwegian and Greenland Seas (Skogseid et al. 1992;Doré et al. 1999;Buiter and Torsvik 2014). Volcanic (lava flows) and massive magmatic emplacements (dikes and sill complexes) in the basin manifest paleomagmatic activities during the Paleogene (Skogseid et al. 1992;Ren et al. 2003;Torne et al. 2003). ...
... The Vøring Basin is one of the igneous provinces in the NE Atlantic margins and was highly affected by thermal activity, magmatism, uplift and exhumation processes due to the Paleocene opening of the Norwegian and Greenland Seas (Skogseid et al. 1992;Doré et al. 1999;Buiter and Torsvik 2014). Volcanic (lava flows) and massive magmatic emplacements (dikes and sill complexes) in the basin manifest paleomagmatic activities during the Paleogene (Skogseid et al. 1992;Ren et al. 2003;Torne et al. 2003). Like most magma-rich basins, the extensive networks of dikes and sills in the Vøring Basin are intimately connected to the formation of fluid escape structures such as hydrothermal vent complexes (Jamtveit et al. 2004;Hansen 2006). ...
This paper provides a novel classification scheme for magma-induced subsurface vent complexes based on morphometric data and stacking patterns. The study area is the Naglfar Dome in the Vøring Basin where the interaction between magmatic intrusions and vent complexes is well known. Seismic interpretation, characterization and morphometric analyses of 35 vent complexes observed within the Palaeocene-Eocene strata were done from a high-quality, three-dimensional seismic reflection data. The vent complexes have dome-shaped, eye-shaped, fault-controlled and reactivated upper termini, and are linked to their root zones by columnar, downward-tapered and fault-related fluid conduits or pipes. Statistical analyses and cross plots of L-A and Dmax-Hmax allowed the discrimination of vents and pipes into their genetic types. A new classification scheme based on the morphometric sub-division and structures of subsurface vent complexes is introduced to include S-P-V, S-P-F, V-P-V and V-P-F types. These combinations freshly demonstrate the importance of morphometric data at deciphering the nature, timing, classification and activity of subsurface fluid vent complexes.
... In 2D seismic data, elliptical structures above sills were first described in the North Atlantic (Skogseid et al., 1992) and were noted to occur at a single stratigraphic level over a widespread area, revealing that they were vents above sills (Skogseid et al., 1992). The offshore structures are analogous to onshore examples, , such as in the Karoo Basin, South Africa, where they have been studied since the early 1900's (e.g., Gevers, 1928). ...
... In 2D seismic data, elliptical structures above sills were first described in the North Atlantic (Skogseid et al., 1992) and were noted to occur at a single stratigraphic level over a widespread area, revealing that they were vents above sills (Skogseid et al., 1992). The offshore structures are analogous to onshore examples, , such as in the Karoo Basin, South Africa, where they have been studied since the early 1900's (e.g., Gevers, 1928). ...
... Initially offshore vent complexes were interpreted as igneous 'volcanic features' (Skogseid et al., 1992;Davies et al., 2002); however, later observations suggested vents above sills formed predominantly through hydrothermal fluid circulation and subsurface remobilization of sediments (Bell and Butcher, 2002;Planke et al., 2005). The sedimentary nature of many vent fills was recognized by limited acoustic impedance contrasts between some vent fills and the surrounding host rocks (Hansen, 2006), and that some vent fills are polygonally faulted, which would not be expected to occur in more competent volcanic rocks (Hansen, 2006). ...
Hydrothermal vent complexes are common in volcanic sedimentary basins worldwide. The vent complexes have commonly formed in sedimentary basins affected by large igneous provinces due to widespread sill emplacement into organic-rich shales, which caused devolatilization in sill aureoles. The activity of these vents could have contributed to trigger drastic climate changes and extinction events through the Earth history. Continental break-up of the NE Atlantic during the Paleocene to earliest Eocene led to the formation of thousands of hydrothermal vent complexes ultimately leading to the rapid climate changes of the Paleocene-Eocene Thermal Maximum (PETM). Thousands of these hydrothermal vents are scattered within the Møre and Vøring basins where sills and connected vertical conduits are identified respectively as high amplitude reflections and chaotic reflection zones. The north east Java sedimentary basin comprises potential modern analogues for fluid flow-related structures triggered by hydrothermal activity. The spectacular Lusi eruption, ongoing in north east Java since 2006, represents a unique modern analogue of the hydrothermal vent complexes from the geological past. Lusi is fueled by magmatic intrusions and hydrothermal fluids migrating from 4.5 km depth from the neighboring Arjuno-Welirang volcanic complex. Less than 10 km away from Lusi is the buried Porong structure. This kilometers-scaled vent is interpreted as the evidence of a similar palaeo hydrothermal vent activity characterizing the area. Here we present a comparative study of these vent complexes based on 2D and 3D seismic data offshore Norway and onshore Indonesia. The key features that characterize these hydrothermal vent systems are analyzed and compared within a broad context to investigate the mechanisms of fluids migration. The analyzed hydrothermal vents are characterized by distinctive eye-shaped geometries in the upper part, consisting of craters or depressions with mound-like geometries above. The conduits between the sills and the vents are characterized by disrupted seismic reflections and surrounded by inward dipping reflections. The shallow amplitude anomalies are interpreted to be caused by gas due to their vertically clustered geometries, the 'soft' nature of some of the anomalies, and the widespread occurrence of gas in the basin. Mounds suggest that sediments were later remobilized due to fluid flow. These features occur directly above the vents, suggesting that fluids migrated preferentially through the underlying hydrothermal vent complexes. The interpreted fluid flow features show the importance of hydrothermal vent complexes for fluid overpressure development in basins and for potential migration of hydrocarbons from deep structures to shallow reservoirs. The shallow seismic anomalies furthermore suggest that the vent complexes have been reused for focused fluid flow for millions of years after their initial formation.
... Sills and sill-complexes occur in a wide range of sedimentary basins worldwide (e.g. Delaney et al., 1986;Mcclay and Ellis, 1987;Skogseid et al., 1992;Planke et al., 2005;Cartwright and Hansen, 2006;Bédard et al., 2012;Schofield et al., 2012;Magee et al., 2013b;Magee et al., 2013c). Whilst many intrusion-bearing sedimentary basins contain arrays of faults, the interaction between magma plumbing systems and fault networks remains poorly understood. ...
... Here, we address the role of fault-sill interactions in controlling magma plumbing systems by using the Naglfar Dome, offshore Norway as a natural laboratory where several studies have documented sill emplacement (e.g. Skogseid et al., 1992;Skogly, 1998;Svensen et al., 2003;Planke et al., 2005;Omosanya et al., 2017;Omosanya et al., 2018). Sill emplacement in the Naglfar Dome, Mid-Norwegian continental margin (Fig. 1) was associated with the opening of the Norwegian-Greenland Seas during the Early Cenozoic (Lundin and Doré, 2002;Omosanya et al., 2017). ...
... The location of our study area is between latitude 62°and 69°N along the Mid-Norwegian continental margin within a region that consists of the Møre and the Vøring Basins ( Fig. 1; Brekke et al., 1999). We focus on the Vøring Basin, which is a sedimentary basin containing several structural highs, graben, and sub-basins (Skogseid et al., 1992). The Vøring Escarpment and the Vøring Marginal High bound the Vøring Basin in the west, whilst the Nordland Ridge and Trøndelag Platform bound the basin to the east (Brekke, 2000). ...
Pre-existing faults may facilitate subsurface magma transport and sill trangression in sedimentary basins. Although widely recognized, interaction between faults and intrusion networks in sedimentary basins remains poorly imaged by seismic reflection data. To understand how sills interact with pre-existing faults in sedimentary basins, we use high-resolution 3-D seismic reflection data from the Naglfar Dome, Vøring Basin to examine the seismic expression of sills, interpret fault geometries and displacement patterns, and characterize sill-fault interactions. The sills are expressed as tuned reflection packages, meaning they are below ∼<50 ± 5 m thick, with saucer-shaped, transgressive, and strata-concordant morphologies that range in area from 6 km 2 to 180 km 2. The interconnected sills form a sill-complex, which was emplaced during the Eocene and cross-cuts three main strati-graphic intervals (i.e. the Nise, Tang, and Brygge Formations). Faults are of Early Paleocene to Early Eocene age, tectonic in origin and dominated by normal faults that are up to 28 km long. Fault and sill interactions define a spectrum, which we subdivide into five categories (i.e. Type 1a, 1b, 2, 3 and 4); the two main end-member fault-sill relationships documented here are (a) sills stepping up stratigraphy short distances via faults and (b) those with inclined limbs intruded along fault planes. Whilst interactions between the faults and the sills are common, quantitative displacement analysis reveals fault displacement did not influence where sills exploited faults. In the study, the intricate interaction of fault and magmatic sills and its broader implications to structural compartmentalization and outcrop-scale studies in many magma-rich continental margins are demonstrated.
... The first episode reflected the final closure of the Iapetus Ocean during the Caledonian Orogeny in the Late Silurian to Early Devonian (Brekke, 2000). A second episode was marked by extensional events spanning from the Late Devonian to the Palaeocene that culminated in continental separation between Greenland and Eurasia (Brekke, 2000;Eldholm et al., 1989;Skogseid et al., 1992). A third and final episode reflecting seafloor spreading in the North Atlantic between Eurasia and Greenland occurred from the earliest Eocene to the present (Brekke, 2000). ...
... The Early Cretaceous geodynamic evolution of the Vøring Basin is directly linked to continental break-up and the subsequent opening of the Norwegian and Greenland Seas (Fig. 1). Increased magmatism during the Late Palaeocene resulted in the extrusion of flood basalts in the northwestern segment of the study area (Eldholm et al., 1989;Skogseid et al., 1992). Pervasive magmatism followed the latest stage of rifting leading to continental breakup at 55 Ma (Lundin and Doré, 2002). ...
... Pervasive magmatism followed the latest stage of rifting leading to continental breakup at 55 Ma (Lundin and Doré, 2002). This resulted in the emplacement of numerous dolerite intrusions within Cretaceous (and older) rocks in the study area (Ostendorf and Gundersen, 2003;Skogseid et al., 1992). ...
Here, we document a suite of radial faults associated with hydrothermal vent complexes in the Vøring Basin, offshore Norway. These complexes have pyramid-shaped, cylindrical- and conical-shaped conduits, with a dome-, or eye-shaped morphology at their summit, intruding on Paleogene sedimentary rocks. Hydrothermal vents are intimate with the tips of magmatic sills that were emplaced at depths ranging between 1800 and 5800 ms Two Way Travel Time (TWTT). At shallower depths of 1800 to 3000 ms TWTT and intermediate depths of 3000 to 5000 ms TWWT, magmatic sills regularly intersect the lower parts of the vent conduits, which are characterized here as pipes. An important parameter that is used to characterize the morphology of a hydrothermal vent conduit is the width of the conduit, which is defined as the longest axis marking the extent of the vents' conduit within the surrounding host-rock strata. Our findings reveal that radial faults are commonly associated with the summits of hydrothermal vents, implying the existence of local stress fields around the vents, where the maximum compressive stress is radial and minimum stress is circumferential, which overrides the regional stress field and indicate variable stress regimes as opposed to tectonic faults. Importantly, circumferential stretching due to catastrophic plumbing of hydrothermal fluids, differential compaction and intensive fracturing enabled the polygonal faults to realign in a radial pattern resulting in the formation of radial faults at the vent summit. As a corollary of this work, we hypothesize that pyramid-shaped hydrothermal conduits are possibly markers of protracted sill emplacement in sedimentary basins.
... It can be mentioned that the presence of magmatic features and the assumed large volumes are often interpreted to be related to anomalous mantle temperatures (e.g. Skogseid et al. 1992). The elevated temperatures are regularly proposed to be related to a hot-spot or plume (e.g. ...
... Given the complexity of the tectonic processes this region has endured, complex lithological assemblages, such as extremely deformed mid-to lower crustal material and/or altered mantle can be expected. Furthermore, the distal and outer domains are often associated with materials which can have strong masking effects on geophysical signals, such as intrusive and/or extrusive magmatic rocks (e.g. the Møre and Vøring margins; Skogseid et al. 1992) or salt (e.g. the Angola margin; Peron-Pinvidic et al. 2015). These features commonly prevent clear imaging, characterization, modeling and mapping of structural geometries at depth. ...
Rifted margins are major geological objects that mark the transition between continents and oceans, the two first‐order types of land masses present on Earth. Rifted margins and offshore rift basins are of high interest for many various reasons. Rifted margins include several morphologically distinct entities: continental shelf, continental slope, continental rise, and abyssal plain. Rifted margins are extremely diverse, and encompass a variety of geometries, basement compositions and sedimentary architectures that, in turn, indicate differences in temporal and spatial evolution. The chapter reviews the evolution of knowledge on rift and rifted margin evolution and architecture, from the pioneering models that defined the fundamental physical rules governing lithospheric extension to modern conceptual models and today's consensus and debates. Modern concepts now include polyphase, in‐and‐out of sequence fault‐systems, concave‐downward detachments, zones of newly accreted basement with low‐angle top‐basement fault surface, among others.
... It can be mentioned that the presence of magmatic features and the assumed large volumes are often interpreted to be related to anomalous mantle temperatures (e.g. Skogseid et al. 1992). The elevated temperatures are regularly proposed to be related to a hot-spot or plume (e.g. ...
... Given the complexity of the tectonic processes this region has endured, complex lithological assemblages, such as extremely deformed mid-to lower crustal material and/or altered mantle can be expected. Furthermore, the distal and outer domains are often associated with materials which can have strong masking effects on geophysical signals, such as intrusive and/or extrusive magmatic rocks (e.g. the Møre and Vøring margins; Skogseid et al. 1992) or salt (e.g. the Angola margin; Peron-Pinvidic et al. 2015). These features commonly prevent clear imaging, characterization, modeling and mapping of structural geometries at depth. ...
Rifts can be summarized as geographical regions consisting of extensional sedimentary basins of various sizes, with various tectonic and sedimentary geometries that are linked in various structural contexts. This chapter provides a list of the main types of rifts and case examples. It provides the definition of the active and passive rifting categories, as these are regularly mentioned in the literature and are the primary designations of extensional rift settings. The chapter lists the major tectonic structures and basin types encountered in rifts and rifted margins. Subsidence is the stage that leads to the progressive deepening of the basin floor and hence allows the accumulation of sediments in rift basins. It is the process by which the lithosphere regains isostatic equilibrium. Sedimentation, although always present in rift basins, is extremely variable from one system to the other. Traditionally, sedimentary sequences of rift basins are subdivided into three categories: pre‐, syn‐ and post‐rifts.
... Tectonically, the Mid-Norwegian continental margin (Vøring and Møre Basin) was affected by three main periods of rifting, in the Carboniferous to Permian, Late-Jurassic to Early Cretaceous, and the Late Cretaceous to Early Eocene (Brekke 2000). The first two extensional phases affected the entire Vøring Basin, especially in its eastern part while extension during the Late Jurassic-Early Cretaceous markedly produced syn-tectonic listric faulting in the basin (Skogseid et al. 1992). ...
... Nevertheless, intrusive rocks continued to be emplaced into the Cenozoic succession during the Eocene and periodically interacted with a complex array of normal and inverted faults . The Pliocene to Pleistocene were dominated by intense uplift and erosion, which favoured the deposition of sedimentary strata towards the shelf edge (Skogseid et al. 1992). ...
Diatremes and volcanoes have been described largely from rock exposures in magma-rich sedimentary basins. A growing number of research works however describe volcanoes and similar structures (hydrothermal vent complexes and fluid-escape structures) from seismic reflection data. Nevertheless, the detailed timing, composition and dimensions of diatremes and volcanoes from seismic reflection data are sparsely known owing to paucity of direct measurements from wellbores, cores, and limited resolution power of seismic reflection data. In this work, high quality 3-D seismic reflection data and seismic attributes are used to interpret the origin and configuration of diatremes buried beneath 40 paleo-volcanoes. The diatremes are mostly reflected as faulted zones of low amplitude and distorted seismic signal that are depicted as composite columnar, conical, fault-controlled, and bifurcated structures. Above the diatremes are cone- and crater-type volcanoes, which are infilled with syn-eruptive deposits. Volcanoes in the study area, are common in two main domains, representing the boundaries of isolated centres of (a) eruptive volcanoes, and (b) strong material fragmentation and dispersion, common with diatreme volcanoes on the paleo-seafloor. The interaction of magma and fluids at deeper depths caused piercement of the overburden rocks, creating the diatremes, which later acted as conduits for the migration of magma and fluids onto the Palaeocene–Eocene seafloor. Magma and fluid plumbing through the diatremes were principally aided by the complex interactions of pre-existing faults and diatremes. The ensued eruptions predominantly produced small-volume, monogenetic volcanoes (< 1 km3) which are dispersed along the flanks of a NE–SW oriented volcanic field where large-volume, polygenetic volcanoes (> 1 km3) are prevalent. The complex internal configuration of subsurface diatremes and their volcanoes as demonstrated in this work further reinforces the diversities and the caveats in their seismic interpretation along many magma-rich margins.
... Since the study by Talwani & Eldholm (1972), the large-scale geodynamic processes and nature of the breakup in the Norwegian-Greenland Sea have been extensively debated (e.g., Talwani & Eldholm, 1977;Eldholm et al., 1989;Skogseid & Eldholm, 1989;Skogseid et al., 1992Skogseid et al., , 2000Blystad et al., 1995;Bjørnseth et al., 1997;Ren et al., 1998Ren et al., , 2003Swiecicki et al., 1988;Berndt et al., 2001;Lundin & Doré, 2002Mjelde et al., 2001Mjelde et al., , 2005Mjelde et al., , 2007Gernigon et al., 2003Gernigon et al., , 2004Gernigon et al., , 2015Gernigon et al., , 2020Manatschal et al., 2004;Osmundsen & Ebbing, 2008;Péron-Pinvidic et al., 2012Nirrengarten et al., 2014;Osmundsen et al., 2016;Péron-Pinvidic & Osmundsen, 2016Theissen-Krah et al., 2017;Osmundsen & Péron-Pinvidic, 2018;Zastrozhnov et al., 2018Zastrozhnov et al., , 2020. Gernigon et al. (2015) suggested that the magmatic breakup in the Norwegian-Greenland Sea was diachronous and initiated as isolated and segmented sea-floor spreading centres in the Early Eocene, an assumption supported by Péron-Pinvidic & Osmundsen (2018). ...
... Significant uplift and erosion to the northwest have been attributed to the breakup processes and/or the impact of the Iceland plume (e.g., Skogseid et al., 1992;Ren et al., 1998;Mjelde et al., 2007). The outer Vøring Basin is a complex system of faulted ridges, located between a deep Cretaceous basin to the southeast and the Vøring Marginal High to the northwest, which delimits the oceancontinent transition (Fig. 1). ...
... Fluids migrate towards the sea floor through fractured conduits to form HTVCs. These typically occur in basins that experience crustal extension with considerable magmatic input, for example, the Karoo Basin in South Africa (Svensen et al., 2006), magma-rich continental margins such as the Bass Basin offshore Australia (Holfor, , or the Vøring and Møre Basins offshore mid-Norway (Planke, Rasmussen, Rey, & Myklebust, 2005;Skogseid Pedersen, Eldholm, & Larsen, 1992). Hence, rift basins and resulting continental margins are commonly categorised as magma-rich or magma-poor, with magma-rich margins being associated with: a) higher spreading rates than their magma-poor counterparts, and b) large volumes of syn-rift igneous rocks (Franke, 2013;White & McKenzie, 1989). ...
... However, given how clearly other surrounding deep sills are imaged, it is interpreted that dykes are the source points for these HTVCs, which may be expected in such a setting. Skogseid et al. (1992) and Davies, Bell, Cartwright, and Shoulders (2002) also presented examples of dyke-fed volcanic vents or craters at the sea floor within the Vøring and Faeroe-Shetland Basins respectively; therefore, this is also expected to be the case on the Modgunn Arch. ...
Conventional three-dimensional (3D) seismic data reveal abundant igneous activity on the Modgunn Arch, mid-Norwegian margin. Magmatic sills and associated hydrothermal vent complexes located at various depths prove the repeated utilisation of Paleocene-Eocene magmatic conduits. In total, 125 sills and 85 hydrothermal vent complexes were identified and mapped, with vent complexes ranging in diameter from 300 to 3100 m and sills from 0.5 to 50 km. Three examples of stacked vent complexes are presented, revealing large eruptions of hydrothermal fluids vertically through the same conduit, from sills to the palaeo-sea floor. The vent complexes are found throughout Paleocene strata (66-56 Ma), whilst at least ten (10) vents were active during the Eocene. This study emphasises the importance of characterising ancient magmatic structures, as hydrothermal conduits and vent structures were, and may still be, reutilised as preferential fluid flow pathways to shallower strata. A minimum of four (4) phases of hydrothermal vent complex formation are inferred. Cretaceous faults are both bypassed and used for magma and fluid flow. The reutilisation of magmatic structures here described may bring to light previously overlooked plays and renew interest in exploring magma-rich continental margins.
... The process of differential compaction affecting vent morphologies was briefly considered by Skogseid et al. (1992) and Davies et al. (2002). Planke et al. (2005) described the process more thoroughly considering the vent fill to compact less than the surrounding strata causing the dome morphology. ...
... Distinctive other "flows" or "lobes" like Lobe "B", which was discussed in section 4.4.4.1, are not associated with large numbers of vents. Though, it is plausible that there may be more vents which are not resolvable.When it comes to sills which are deeper, it is very common to find vents which occur above the terminations and highest points of sills; these often coincide because of the upward transgressive nature of sill geometries(Joppen and White, 1990;Skogseid et al. 1992;Bell and Butcher, 2002;Davies et al. 2002;Jamtveit et al. 2004; Svensen et al. 2004; Planke et al. 2005; Corfield et al. 2004; Hansen, 2006; Thomson, 2007b; Rocchi et al. 2007; Hansen et al. 2008; Moy and Imber, 2009; Rollet et al. 2012; Magee et al. 2013b; Magee et al. 2013c; Magee et al. 2014a). Because of the large number of vents and the excellent imaging of the sills in this study area, this relationship can be considered further than before.Fig. ...
This thesis reports on over 27 sills and 213 associated vents. The sills and vents were investigated using 3D seismic data, in a ~1000 km2 area, offshore Norway between the Møre and Vøring Basins (the Edvarda survey). A wide range of sill geometries are observed which are interpreted to be the result of five different processes acting on the sills. Three of these processes relate to how the host deforms. If sill intrusion causes deformation of the seafloor, creating folds, or the sills interact with folds created by neighbouring sills, sills are found to cross bedding (transgress) abruptly. Alternatively, if deformation is interpreted to be local, then continuously increasing Young’s Modulus with depth is interpreted to result in sills which transgress continuously upwards, akin to smooth ‘bowls’. At shallow depths the host is interpreted to fluidise, leading to limited transgression or in some cases multiple bowls. The seismic amplitude responses of shallow sills include flow related features such as channels and lobes. The other two processes interpreted to affect sill propagation stem from structures in the host: abrupt changes in lithology and pre-existing faults. Multiple sills are found to terminate, and in some cases form, at sand rich units in the otherwise mudstone dominated host. Additionally, some sills are interpreted to have intruded into a host with pre-existing polygonal faults, which led to angular sill geometries. Vents are found to occur directly above sills, often along the margins of sills, but in some cases over sill interiors, especially where the sills are locally shallower. Additionally, a cluster of 98, relatively small vents occur above the shallowest sill. Differential compaction and slumping are found to affect some larger vent morphologies. Overall, vent size is found to closely follow a power-law such that smaller vents are significantly more numerous than larger vents.
... The tectonic development of the Vøring Basin is strongly connected to the geodynamic evolution of the Mid-Norwegian continental margin (Bukovics and Ziegler, 1985). Its main tectonic evolution started during the Late Jurassic, and its structure was developed through several rifting events that lasted until the Early Cretaceous (Skogseid et al., 1992a) (Fig. 2b). Although the timing of extensional tectonics in the Vøring Basin remains controversial, Brekke (2000) suggested that the three main tectonic phases that affected the basin were a) extension from the Late Jurassic to the Late Cenomanian; b) extension and compression from the Late Cenomanian to the early Palaeocene; and c) extension and compression from the early Palaeocene to the present. ...
... Nonetheless, the Cenozoic tectonic evolution of the Vøring Basin and the entire Norwegian margin is dominated by the opening of the Norwegian and Greenland seas and by uplift and glaciation during the Neogene (Henriksen et al., 2011). This Neogene uplift and glaciation are exemplified by the deposition of thick sedimentary wedges on the mid-Norwegian shelf (Skogseid et al., 1992a). In addition to the extension and compression that affected the Vøring Basin from the Late Jurassic to the Cenozoic (Fig. 2b), transpression, which was coincident with the Alpine orogenies during the Late Eocene and Middle Miocene, was also recorded in the basin (Brekke, 2000). ...
The use of conventional interpretation techniques like manual interpretation, seismic attribute analysis and opacity rendering or geobody extraction have demonstrated significant success in interpreting magmatic sills from seismic reflection data. Nonetheless, the downsides of seismic imaging and interpretation of magmatic sills are that they are rarely drilled, or too thin to be detected. We present an automated approach for interpreting magmatic sills from seismic reflection data by using a newly designed meta-attribute, called as the Sill Cube (SC). The SC meta-attribute is generated by amalgamating a number of different attributes that are trained over the interpreter's knowledge of the sill networks through a supervised scheme of neural learning. The approach has shown its efficiency by delivering enhanced images of magmatic sills from seismic data in three frontier areas i.e., the Kora prospect (offshore Taranaki Basin, New Zealand), the Utgard and the Tulipan prospects (both offshore Vøring Basin, Norway). The SC meta-attribute has completely captured the saucer-shape geometry of most of the sills. Additionally, the meta-attribute has brought out different structural elements such as the limbs, bridges, junctions and fingers and highlighted the overlying forced folds. The interpreted sills cover areas of 6–16 km2 in the Kora prospect, 105–1140 km2 in the Utgard prospect and 15–60 km2 in the Tulipan prospect. Such an interpretational approach does not only honour the interpreter's knowledge of sills networks but also add values in understanding the tectonic architecture.
... Here, we address the role of fault-sill interaction in controlling magma plumbing systems by 60 using the Naglfar Dome, offshore Norway as a natural laboratory where several studies have 61 documented sill emplacement (e.g. Skogseid et al., 1992;Skogly, 1998 sandstones and occasional carbonate stringers (Dalland et al., 1988;Omosanya et al., 2018). 104 ...
... The Kai Formation is comprised of alternating claystone, siltstone and sandstone with 146 limestone stringers that were deposited in marine environments (Dalland et al., 1988;Eidvin 147 et al., 1998). 148 149 Throughout the Pliocene-Pleistocene, the Vøring Basin and the Norwegian mainland 150 experienced intense uplift and erosion, which favoured the deposition of sedimentary strata 151 towards the shelf edge (Skogseid et al., 1992). Uplift and glaciation indicate that deep glacial 152 erosion had removed 1-2 km sedimentary cover from the Norwegian onshore. ...
Pre-existing faults may facilitate subsurface magma transport and sill trangression in sedimentary basins. Although widely recognized, interaction between faults and intrusion networks in sedimentary basins remains poorly imaged by seismic reflection data. To understand how sills interact with pre-existing faults in sedimentary basins, we use high-resolution 3-D seismic reflection data from the Naglfar Dome, Vøring Basin to examine the seismic expression of sills, interpret fault geometries and displacement patterns, and characterize sill-fault interactions. Sills in the area range from 6 km2 to 180 km2 and display saucer-shaped, transgressive, and strata-concordant morphologies. The sills are interconnected and form a sill-complex, emplaced during the post-Early Eocene, spanning three main stratigraphic intervals (i.e. Nise, Tang, and Brygge Formations) from depths of -3000 ms TWTT to -5500 ms TWTT. Faults are tectonic in origin and dominated by normal faults that generally can reach 28 km long in plan-view. Fault and sill interactions are define a spectrum, which we sub-divided into five categories (i.e., Type 1a, 1b, 2, 3 and 4); the two main end-member fault-sill relationships documented here are (a) sills stepping stratigraphy and (b) those with their inclined segments intruded along fault planes. The displacement plots show that sill intrusion into fault planes do not have any influence on fault displacement character. Sill emplacement was along the strike of faults and perpendicular to the least compressive stress. Our work further demonstrates the intricate interaction of fault and magmatic sills and its broader implications for magma plumbing in many magma-rich continental margins.
... The tectonic development of the Vøring Basin is strongly connected to the geodynamic evolution of the Mid-Norwegian continental margin (Bukovics and Ziegler, 1985). Its main tectonic evolution started during the Late Jurassic, and its structure was developed through several rifting events that lasted until the Early Cretaceous (Skogseid et al., 1992a) (Fig. 2b). Although the timing of extensional tectonics in the Vøring Basin remains controversial, Brekke (2000) suggested that the three main tectonic phases that affected the basin were a) extension from the Late Jurassic to the Late Cenomanian; b) extension and compression from the Late Cenomanian to the early Palaeocene; and c) extension and compression from the early Palaeocene to the present. ...
... Nonetheless, the Cenozoic tectonic evolution of the Vøring Basin and the entire Norwegian margin is dominated by the opening of the Norwegian and Greenland seas and by uplift and glaciation during the Neogene (Henriksen et al., 2011). This Neogene uplift and glaciation are exemplified by the deposition of thick sedimentary wedges on the mid-Norwegian shelf (Skogseid et al., 1992a). In addition to the extension and compression that affected the Vøring Basin from the Late Jurassic to the Cenozoic (Fig. 2b), transpression, which was coincident with the Alpine orogenies during the Late Eocene and Middle Miocene, was also recorded in the basin (Brekke, 2000). ...
The use of conventional interpretation techniques like manual interpretation, seismic attribute analysis and opacity rendering or geobody extraction demonstrated significant success in interpreting magmatic sills from seismic reflection data. Nonetheless, the downsides of seismic imaging and interpretation of magmatic sills includes the fact that they are rarely drilled, are too thin and often are beyond seismic resolution to be detected. This work documents a novel automated approach for interpreting magmatic sills from seismic reflection data with the help of a newly designed meta-attribute, called the Sill Cube (SC). The SC is prepared through an amalgamation of different seismic attributes that are trained over interpreter's knowledge of the sill networks following a supervised scheme of neural learning. The new approach revealed its efficiency by producing enhanced images of magmatic sills from seismic data in three frontier areas i.e., the Kora prospect (offshore Taranaki Basin, New Zealand), the Utgard and the Tulipan prospects (both offshore Vøring Basin, Norway). The SC meta-attribute prominently captured the complete saucer-shape geometry of most of the sills. In addition to this, the meta-attribute brought out different structural elements such as the limbs, bridges, junctions and fingers and highlighted the overlying forced folds. Based on SC meta-attribute the interpreted sills cover areas of 6-16 km2 in the Kora prospect, 105-1140 km2 in the Utgard prospect and 15-60 km2 in the Tulipan prospect. We demonstrate that such an interpretational approach does not only honour the interpreter's knowledge of sills networks but also add value in understanding their tectonic architecture.
... The volcanic passive margin of Norway is the result of the last phase of Pangea breakup (55 Ma bp) in the early Cenozoic. It evolved in response to the North Atlantic breakup, presumably initiated by the abnormally hot mantle of the Iceland plume (White, 1989;Skogseid et al., 1992;Ren et al., 1998). The Norwegian continental margin records several pre-breakup rifting phases that played a major role in initiating the formation of deep sedimentary basins (Skogseid, 1994;Blystad et al., 1995;Swiecicki et al., 1998;Doré et al., 1999Doré et al., , 2002. ...
... Proximity to the Iceland mantle plume might also be effective in causing high geothermal gradients in the oceanic crustal domain of the Norwegian margin. The North Atlantic breakup was possibly initiated by the abnormally hot mantle of the Iceland plume (White, 1989;Skogseid et al., 1992;Gernigon et al., 2004Gernigon et al., , 2006Parkin and White, 2008) activated approximately 5 million years earlier than the continental breakup (Saunders et al., 1997). While some studies have shown that the Iceland plume propagated northward (e.g., Ruedas et al., 2007;Steinberger et al., 2015), seismic tomography (Rickers et al., 2013) suggests lateral movement of plume material in addition to the parallel propagation along the mid-ocean ridge. ...
The aim of this study is to investigate the shallow thermal field differences
for two differently aged passive continental margins by analyzing regional
variations in geothermal gradient and exploring the controlling factors for
these variations. Hence, we analyzed two previously published 3-D conductive
and lithospheric-scale thermal models of the Southwest African and the
Norwegian passive margins. These 3-D models differentiate various sedimentary,
crustal, and mantle units and integrate different geophysical data such as
seismic observations and the gravity field. We extracted the
temperature–depth distributions in 1 km intervals down to 6 km below the
upper thermal boundary condition. The geothermal gradient was then calculated
for these intervals between the upper thermal boundary condition and the
respective depth levels (1, 2, 3, 4, 5, and 6 km below the upper thermal
boundary condition). According to our results, the geothermal gradient
decreases with increasing depth and shows varying lateral trends and values
for these two different margins. We compare the 3-D geological structural
models and the geothermal gradient variations for both thermal models and
show how radiogenic heat production, sediment insulating effect, and thermal
lithosphere–asthenosphere boundary (LAB) depth influence the shallow thermal
field pattern. The results indicate an ongoing process of oceanic mantle
cooling at the young Norwegian margin compared with the old SW African
passive margin that seems to be thermally equilibrated in the present day.
... Following Devonian into Carboniferous postorogenic extension, well-documented rift phases affected the Mid-Norwegian margin in Permian (?)-Early Triassic, Late Jurassic-Early Cretaceous, and Late Cretaceous-Paleogene times, culminating with magmatic breakup in the early Eocene at circa 54 Ma (e.g., Blystad et al., 1995;Brekke, 2000;Bugge et al., 2002;Skogseid et al., 1992). Whereas deep, fault-bounded Mid-Triassic and older basins xformed on moderately extended crystalline crust in the margin's proximal domain (e.g., basins deposited during the coupling process. ...
... The volcanic features interpreted in the outermost margin area include intrusions, underplating, seaward dipping reflectors, lava flows, and hyaloclastic deltas (e.g., Berndt et al., 2001) (Figure 5a). Together with significant uplift and erosion, they have been attributed to the breakup processes and/or to the impact of the Iceland plume (e.g., Skogseid et al., 1992). In spite of the magmatic breakup, the NE Atlantic margin appears to share some important architectural characteristics with magma-poor settings such as the Iberian Margin and margins in the South Atlantic (e.g., Osmundsen et al., 2002Osmundsen et al., , 2016Osmundsen & Ebbing, 2008;Péron-Pinvidic et al., 2013;Ren et al., 1998;Rüpke et al., 2013). ...
The large-magnitude faults that control crustal thinning and excision at rifted margins combine into laterally persistent structural boundaries that separate margin domains of contrasting morphology and structure. We term them breakaway complexes. At the Mid Norwegian margin, we identify 5 principal breakaway complexes that separate the proximal, necking, distal and outer margin domains. Down-dip and lateral interactions between the faults that constitute breakaway complexes became fundamental to the evolution of the 3D margin architecture. Different types of fault interaction are observed along and between these faults, but simple models for fault growth will not fully describe their evolution. These structures operate on the crustal scale, cut large thicknesses of heterogeneously layered lithosphere, and facilitate fundamental margin processes such as deformation coupling and exhumation. Variations in large-magnitude fault geometry, erosional footwall incision and subsequent differential subsidence along the main breakaway complexes likely record the variable efficiency of these processes.
... Seismic observations reveal multiple sub-horizontal sheets which cross-cut stratigraphy, and therefore, have transgressive geometries which are interpreted, with wellties, to be sill intrusions into the pre-breakup sedimentary rocks (Skogseid et al., 1992;Planke et al., 2000;2015), and different features imaged beneath the basalt cover (Angkasa et al., 2017). The emplacement of a thick igneous sequence of Mid-Ocean Ridge Basalt (MORB-type) basalts overlying glassy and more evolved flows with distinct crustal contamination signatures, during the Vøring Margin formation, has been recovered from Ocean Drilling Program (ODP) Hole 642E (Viereck et al., 1989;Abdelmalak et al., 2016a). ...
Soft sediment deformation structures may form when denser sediments or fluids are deposited on or flow over unlithified and less dense sediments. This study presents a seismic geomorphological study of the basal contact between an extrusive volcanic sequence and underlying sediments, defining the ‘Base Basalt’ surface, on the Mid-Norwegian Margin. This contribution focuses in particular on the development of geomorphological features related to the rapid loading of a several hundred-meter-thick lava delta package of hyaloclastite onto poorly consolidated sediments of the pre-volcanic sedimentary basin fill. Seismic horizons, sequence boundaries, volcanic facies units, and attribute maps are used to characterize the seismic geomorphological features imaged within a high-quality 3D seismic cube. The ‘Base Basalt’ horizon and attribute maps reveal incised channels and a network of polygonal to irregular depressions and ridges described here as an ‘egg-box network’. More than 150 depressions, with a typical diameter of 1 km and a depth of 100 m, have been mapped. The deformation features, which are restricted to the base of the Lava Delta seismic facies unit, are interpreted to be the result of rapid loading of the Lava Delta onto poorly consolidated unlithified pre-volcanic sediments. This study presents new evidence for the dynamic nature of the transition between sedimentary basins and large-scale volcanism found along volcanic margins and basins associated with rapid volcanic deposition.
Supplementary material at https://doi.org/10.6084/m9.figshare.c.6946691
... The basins located offshore along the west coast of Scotland have been subjected to significant igneous activity, especially during the Paleogene, when the Inshore West Coast Basins were part of the North Atlantic Igneous Provence (England 1992;Skogseid et al. 1992;Bell and Williamson 2002;Emeleus and Bell 2005;Planke et al. 2005;Schofield et al. 2017). During this period, significant magmatic activity occurred within the basins, with the resulting emplacement of central igneous complexes, extensive sill complexes, lava flows and dyke swarms (Fyfe et al. 1993;Emeleus and Bell 2005). ...
Offshore to the west of the Scottish mainland resides a series of Late Palaeozoic to Mesozoic rift basins, which have been the target of small-scale petroleum exploration from 1968 to the present day. The series of 14 basins, which run from the Minch Basin, offshore northwest Scotland to the Portpatrick Basin, located between southwest Scotland and Northern Ireland, have been here collectively named the ‘Inshore West Coast Basins’ and are the primary focus for this PhD. The geological development, structure and stratigraphy of each of the Inshore West Coast Basins has been assessed as well as the basins potential for containing a working petroleum system. The potential for each basin to contain petroleum system elements such as source rock, reservoir rock, seal and traps have been reviewed alongside an assessment of the risks to finding commercial sized petroleum accumulations. To assess the petroleum potential of each basin, well data, borehole data, seismic reflection data and gravity and magnetic data have been evaluated alongside previously published geological studies.
Further in-depth geological and prospectivity studies were undertaken for four of the basins which showed potential for containing petroleum reserves (the Sea of Hebrides Basin, Minch Basin, Larne Basin and the Portpatrick Basin). The conclusion drawn from these studies is that despite each basin containing petroleum shows and a working petroleum system, the potential for the basins to contain commercial sized oil or gas accumulations is low. Overall, the potential for discovering petroleum accumulations within the Inshore West Coast Basins is poor. The main risks to prospectivity are the low maturity of the source rocks, particularly the Jurassic source rocks, the risk of trap breach due to the basins undergoing numerous exhumation events and the potential for biodegradation or flushing of any petroleum accumulations due to the shallow burial of potential reservoirs.
... Surface heat flow variations in an oceanic basin provide insight into its thermal evolution (McKenzie, 1967), the magnitude, timing, and extent of magmatic activity (Skogseid et al., 1992), subsurface hydrothermal circulation patterns (Elder, 1965;Langseth & von Herzen, 1970;Davis et al., 1999;Kolandaivelu et al., 2020), and marine gas hydrate stability (Yamano et al., 1982). The thermal history has implications for hydrocarbon generation from organic-rich sediments (Simoneit and Lonsdale, 1982;Didyk and Simoneit, 1989). ...
Seafloor heat flow provides information about the thermal evolution of the lithosphere, the magnitude and timing of volcanic activity, and hydrothermal circulation patterns. In the central Gulf of California, the Guaymas Basin is part of a young marginal spreading rift system that experiences high sedimentation (1–5 km/Myr) and widespread magmatic intrusions in the axial troughs and the off‐axis regions. Heat flow variations record magmatic and sedimentary processes affecting the thermal evolution of the basin. Here, we present new seismic evidence of a widespread bottom‐simulating reflection (BSR) in the northwestern Guaymas Basin. Using the BSR depths and thermal conductivity measurements, we determine geothermal gradient and surface heat flow variations. The BSR‐derived heat flow values are less than the conductive lithospheric heat flow predictions for mid‐oceanic ridges. They suggest that high sedimentation (0.3–1 km/Myr) suppresses the lithospheric heat flow. In the central and southeastern regions of the basin, the BSR‐derived geothermal gradient increases as the intruded magmatic units reach shallower subsurface depths. Thermal modeling shows that recent (<5,000 years) igneous intrusions (<500 m below the seafloor) and associated fluid flow elevate the surface heat flow up to five times. BSR‐derived geothermal gradients correlate little with the depth of the shallowest magmatic emplacements to the north, where the intrusions have already cooled for some time, and the associated hydrothermal activity is about to shut down.
... Thus, the margin has been studied at a range of scales and serves as a principal natural laboratory for the evolution of rifted margins (e.g. Skogseid et al. 1992;Brekke 2000;Eldholm et al. 2002;Osmundsen et al. 2002;Gernigon et al. 2004;Tsikalas et al. 2005;Faleide et al. 2008Faleide et al. , 2010Osmundsen and Ebbing 2008;Peron-Pinvidic et al. 2013;Abdelmalak et al. 2017;Zastrozhnov et al. 2020). A continued supply of state-of-the-art geophysical data ensures the continuation of scientific contributions from this margin, as well as continued discussions on its evolution. ...
The Norwegian rifted margin consists of an abandoned rift, a hyperextended margin and a sheared margin. The Mid‐Norwegian margin is located between the narrow Lofoten margin and the abandoned North sea rift. Based on its large‐scale structure and general morphology, the Mid‐Norwegian margin was originally subdivided into platform, terrace, deep basin and marginal high areas, as reflected by the formal names used in official maps and communication. The Mid‐Norwegian margin is associated with a suite of syn‐rift basins that generally become younger in the seaward direction overlain diachronously by post‐kinematic strata. Contrary to magma‐poor counterparts such as that of Iberia, the Mid‐Norwegian margin underwent a strongly magmatic breakup in the Eocene around 54 Ma. The Mid‐Norwegian margin is covered by an Eocene and younger post‐rift basin, which records thermal subsidence following Mesozoic‐Earliest Cenozoic rifting and processes related to episodic uplift of the Norwegian mainland.
... Thus, the margin has been studied at a range of scales and serves as a principal natural laboratory for the evolution of rifted margins (e.g. Skogseid et al. 1992;Brekke 2000;Eldholm et al. 2002;Osmundsen et al. 2002;Gernigon et al. 2004;Tsikalas et al. 2005;Faleide et al. 2008Faleide et al. , 2010Osmundsen and Ebbing 2008;Peron-Pinvidic et al. 2013;Abdelmalak et al. 2017;Zastrozhnov et al. 2020). A continued supply of state-of-the-art geophysical data ensures the continuation of scientific contributions from this margin, as well as continued discussions on its evolution. ...
The margins of the North Atlantic Ocean, including the Newfoundland and Iberian Margins, present two distinct episodes of rifting: at Permian–Trias times and in the middle Cretaceous. In the South Atlantic Ocean, rifting occurred on the location of the Pan‐African suture, more than 450 Ma after its formation, and the two events are clearly dissociated. At first order, the geodynamic segmentation of the South and Equatorial Atlantic Oceans leads to the formation of different types of passive margins, showing a relationship between the regional geodynamic context and the structural architecture of passive margins. The Central Segment of the South Atlantic Ocean is characterized by sedimentary basins with pre‐ and syn‐break‐up magmatism, and the presence of an approximately 1–2 km‐thick salt layer in the so‐called continent‐ocean transition, overlying a mainly non‐marine sequence.
... Recognizing study 1 pre-rift lithospheric rheology and its syn-rift changes Buck, 1991;Burov and Diament, 1995;Brun, 1999;Burov and Watts, 2006;Huismans and Beaumont, 2008;Reston, 2009;Masini et al., 2013;Gouiza and Paton, 2019 2 pre-existing anisotropy Molina, 1988;Versfelt and Rosendahl, 1989;De Wit et al., 2018;Gouiza and Paton, 2019 3 thermal regime Goetze and Evans, 1979;Carter and Tsenn, 1987;Ranalli and Murphy, 1987;Gouiza and Paton, 2019 4 strain rate Davis and Kusznir, 2004;Nemčok et al., 2012a;Brune et al., 2016 5 occurrence of depleted versus enriched mantle Holbrook et al., 2001;Nemčok, 2016 6 plume assistance or no assistance Braun and Beaumont, 1987;White and McKenzie, 1989;Griffiths and Campbell, 1991;Hopper et al., 1992;Skogseid et al., 1992;Holbrook and Kelemen, 1993;Bijwaard and Spakman, 1999;van Wijk et al., 2001;Reston, 2009 7 length of the extension time period Huismans, 1999;Davis and Kusznir, 2004 8 presence or absence of magma Ebinger and Casey, 2001;Davison, 2005;Geoffroy, 2005 9 loading stress regime Mandl, 1988;Huismans and Beaumont, 2005 Table 1 A C C E P T E D M A N U S C R I P T ...
Compared segments of East and West Indian passive margins have different evolution and crustal architecture. The East Indian margin is less magmatic. It results from a crust first/mantle second breakup scenario of a continent experiencing two rift events. The West Indian margin is more magmatic. It results from a mantle first/crust second breakup scenario of a continent experiencing four rift events. The architecture across both margins can be divided into stretching, thinning and hyperextension zones. The East Indian margin is characterized by oceanward-dipping listric normal faults accommodating thinning in the thinning and hyperextension zones and a zone of the exhumed mantle separating continental and oceanic crusts. The West Indian margin in contrast is characterized by landward-dipping listric faults accommodating magma-assisted thinning in the thinning and hyperextension zones and no exhumed mantle. The final breakup affects the lithospheric mantle layer in the East Indian case and the crustal layer in the West Indian case. Although the temperature-dependent rheologies of these two last unbroken layers are rather different, seismic interpretation suggests that they are both broken by upward-convex normal faults, which succeed the development of listric faults. They appear to be the first spontaneously formed faults in the breakup-delivering process, although their nucleation may be magma-assisted. The main difference between controlling factors of the aforementioned breakup scenarios affecting similar lithospheres at similar extension rates is the cumulative time length of pre-breakup rift events, being 62 and 115 Ma at East and West Indian margins.
Supplementary material at https://doi.org/10.6084/m9.figshare.c.5912978
... Alves & Cunha, 2018;Gillard et al., 2015;Larsen & Saunders, 1998;Monteleone et al., 2019;Soares et al., 2012); or (v) the presence of substantial synbreakup igneous products, which tend to reduce the quality of seismic reflection data (e.g. Skogseid et al., 1992). ...
Continental breakup involves a transition from rapid, fault-controlled syn-rift subsidence to relatively slow, post-breakup subsidence induced by lithospheric cooling. Yet the stratigraphic record of many rifted margins contain syn-breakup unconformities, indicating that episodes of uplift and erosion interrupt this transition. This uplift has been linked to mantle upwelling, depth-dependent extension, and/or isostatic rebound. Deciphering the breakup processes recorded by these unconformities and their related rock record is challenging because uplift-associated erosion commonly removes the strata that help constrain the onset and duration of uplift. We examine three major breakup-related unconformities and the intervening rock record in the Lower Cretaceous succession of the Gascoyne and Cuvier margins, offshore NW Australia, using seismic reflection and borehole data. These data show the breakup unconformities are disconformable (non-erosive) in places and angular (erosive) in others. Our recalibration of palynomorph ages from rocks underlying and overlying the unconformities shows: (i) the lowermost unconformity developed between 134.98–133.74 Ma (Intra-Valanginian), probably during the localisation of magma intrusion within continental crust and consequent formation of continent-ocean transition zones (COTZ); (2) the middle unconformity formed between ~134–133 Ma (Top Valanginian), possibly coincident with breakup of continental crust and generation of new magmatic (but not oceanic) crust within the COTZs; and (iii) the uppermost unconformity likely developed between ~132.5–131 Ma (i.e. Intra-Hauterivian), coincident with full continental lithospheric breakup and the onset of seafloor spreading. During unconformity development, uplift was focused along the continental rift flanks, likely reflecting flexural bending of the crust and landward flow of lower crust and/or lithospheric mantle from beneath areas of localised extension towards the continent (i.e. depth-dependent extension). Our work supports the growing consensus that the ‘breakup unconformity’ is not always a single stratigraphic surface marking the onset of seafloor spreading; multiple unconformities may form and reflect a complex history of uplift and subsidence during continent-ocean transition.
... We propose that different types of faults (e.g., transfer and transform faults) may have induced and localized crustal thinning (e.g., the thinned crust between the F1 and F2) and as a consequence, lithospheric thinning, which increased the amount of magmatic activity locally. Based upon the sum on this evidence for magmatic activity during the entire evolution of the transform margin, we propose that the Limpopo transform margin be considered as a magma-rich margin which had a continuous of magma, similar to magma-rich rifted margin (Skogseid et al., 1992;White & McKenzie, 1989). Thus, we defined here a new type of margin: the magma-rich transform margin. ...
The rifted continental margins of Mozambique provide excellent examples of continental passive margins with a significant structural variability associated with magmatism and inheritance. Despite accumulated knowledge, the tectonic structure and nature of the crust beneath the South Mozambique Coastal Plain (SMCP) are still poorly known. This study interprets high‐resolution seismic reflection data paired with data from industry‐drilled wells and proposes a structural model of the Limpopo transform margin in a magma‐rich context. Results indicate that the Limpopo transform margin is characterized by an ocean‐continent transition that links the Beira‐High and Natal valley margin segments and represents the western limit of the continental crust, separating continental volcano‐sedimentary infilled grabens from the oceanic crust domain. These basins result from the emplacement of the Karoo Supergroup during a Permo‐Triassic tectonic event, followed by an Early Jurassic tectonic and magmatic event. This latter led to the establishment of steady‐state seafloor spreading at ca.156 Ma along the SMCP. A Late Jurassic to Early Cretaceous event corresponds to formation of the Limpopo transform fault zone. Which accommodated the SSE‐ward displacement of Antarctica with respect to Africa. We define a new type of margin: the magma‐rich transform margin, characterized by the presence of voluminous magmatic extrusion and intrusion coincident with the formation and evolution of the transform margin. The Limpopo transform fault zone consists of several syn‐transfer and ‐transform faults rather than a single transform fault. The intense magmatic activity was associated primarily with mantle dynamics, which controlled the large‐scale differential subsidence along the transform margin.
... However, the progressive intrusion of magma into continental crust during break-up often leads to the development of broad, complex zones, the structural and geochemical character of which can display both continental and oceanic affinities (e.g. Skogseid et al. 1992;Symonds et al. 1998;Planke et al. 2000;Skogseid et al. 2000;Direen et al. 2007;Bastow and Keir 2011;Nirrengarten et al. 2020). ...
Magnetic stripes have long been assumed to be indicative of oceanic crust. However, continental crust heavily intruded by magma can also record magnetic stripes. We re-evaluate the nature of the Cuvier Abyssal Plain (CAP), offshore NW Australia, which hosts magnetic stripes and has previously been defined as oceanic crust. We show chemical data from a basalt within the CAP, previously described as an enriched MORB, could equally be interpreted to contain evidence of contamination by continental material. We also recognise seaward-dipping reflector (SDR) sequences in seismic reflection data across the CAP. Borehole data from overlying sedimentary rocks suggests these SDRs were emplaced in a shallow-water (<200 m depths) or sub-aerial environment. Our results indicate the CAP may not be unambiguous oceanic crust, but may instead comprise a spectrum of heavily intruded continental crust through to fully oceanic crust. If the CAP represents such a continent-ocean transition zone, adjacent unambiguous oceanic crust would be located >500 km further offshore NW Australia than currently thought; this would impact plate tectonic reconstructions, as well as heat flow and basin modelling studies. Our work also supports the growing consensus that magnetic stripes cannot, by themselves, be used to determine crustal affinity.
Supplementary material: Enlarged and uninterpreted versions of the magnetic data and seismic reflection lines are available at https://doi.org/10.6084/m9.figshare.c.5332172 .
... However, the progressive intrusion of magma into continental crust during break-up often leads to the development of broad, complex zones, the structural and geochemical character of which can display both continental and oceanic affinities (e.g. Skogseid et al. 1992;Symonds et al. 1998;Planke et al. 2000;Skogseid et al. 2000;Direen et al. 2007;Bastow and Keir 2011;Nirrengarten et al. 2020). ...
... Extensive Paleocene-Eocene sill complexes, identified within rift basins located along the NE Atlantic margin, are interpreted to have been emplaced as part of the North Atlantic Igneous Provence, which developed in response to the eventual opening of the NE Atlantic Ocean (England 1992;Skogseid et al. 1992;Bell and Williamson 2002;Emeleus and Bell 2005;Planke et al. 2005;Schofield et al. 2017). Within the Sea of Hebrides Basin (SOHB), located on the NE Atlantic margin to the NW of the Scottish mainland, a Paleocene sill complex has been intruded across a 4000 km 2 area, primarily within Mesozoic ( primarily Lower and Middle Jurassic) sedimentary rocks (Harker 1904;Anderson and Dunham 1966;Gibb and Gibson 1989;Fyfe et al. 1993;Bell and Williamson 2002;Schofield et al. 2016). ...
The Little Minch Sill Complex is comprised of a series of stacked, multi-leaved Paleocene aged dolerite sills, which have been primarily intruded into Mesozoic sedimentary rocks and Paleocene tuffs/?hyaloclastites within the Sea of Hebrides Basin, situated on the NE Atlantic margin. Two previously proposed models for the emplacement of the sill complex have opposing ideas for the location of magma input and emplacement mechanisms. Both models have been constructed using data primarily from onshore outcrops, located on the Isle of Skye, Raasay and the Shiant Isles. However, onshore outcrops only represent a quarter (1040 km ² ) of the entire extent of the sill complex, which is largely situated offshore. In order to understand how the sill complex as a whole was emplaced within the basin, both onshore and offshore magma transport needs to be considered. Using high resolution multibeam bathymetry data (up to 2m resolution) obtained between 2008 and 2011 along with supporting seismic reflection, sparker and pinger data, a new assessment of the offshore extent and character of the sill complex has been constructed. Mapping of large-scale relationships between intrusions and the host rock, along with morphological features such as magma lobes, magma fingers, transgressive wings, en-echelon feeder dykes and the axis of saucer/half-saucer shaped intrusions, has indicated magma flow directions within the intrusive network. Assessing the flow kinematics of the sills has provided insights into magma transport and emplacement processes offshore. Combining data from previously mapped onshore sills with data from our newly constructed model for magma emplacement offshore has allowed us to construct a new model for the emplacement of the Little Minch Sill Complex. This model demonstrates that major basin bounding faults may play a lesser role in channelling magma through sedimentary basins than previously thought. Applying the knowledge obtained from this study could further progress understanding of the effect of sill emplacement on fluid flow within volcanic rift basin worldwide, with direct impacts on the exploitation of petroleum and geothermal systems.
... However, the opposite can happen also due to different external and internal processes that can deform and disturb the vents (Grosse et al., 2009). The differential compaction processes (Planke et al., 2005;Skogseid et al., 1992) including the gravity driven and normal stratigraphic loads further modify and thinning the tails and flanks of the vents ( Figure 5-3B & C). (Figure 4-19), respectively. ...
This thesis deals with the seismic interpretation of magma-induced hydrothermal vents and vertical
fluid-flow conduits in the Naglfar Dome, Vøring Basin. The study area is a magma-rich basin in
the Mid-Norwegian Sea, which was formed during the continental break-up and opening of the
Norwegian-Greenland Seas. The aims of this work are to understand the timing, development,
scale variability and the implications of magmatic intrusions, vents and conduits on hydrocarbon
prospectivity in the basin. To achieve these aims, seismic interpretation and detailed statistical
analysis of vents, conduits and faults were done from a high-resolution, high-quality, threedimensional
seismic reflection data.
Forty (40) hydrothermal vents were interpreted within the Late Paleocene to Early Eocene strata,
and are characterized by unique upper part geometries (dome-shaped, eye-shaped, fault-related
and reworked vents). In the study area, laterally- and vertically- stacked vent complexes found at
three different stratigraphic levels permitted the identification of three main episode of magmatic
emplacement from Late Paleocene and Early Eocene. Hydrothermal vents in this study area were
formed due to the direct eruption of magmatic and mobilized hydrothermal materials evidenced
by high and low amplitude reflections inundating the upper parts of the vents. Pipe-like, downward
tapered cone-shaped and fault-related conduits are the three (3) different vertical fluid-flow
conduits identified here based on their geometry.
In this thesis, new classification schemes for conduits are proposed based on their origin,
propagation behaviour, acoustic impedance contrast, relative width, and pipe order. The different
pipe combinations identified and introduced in this study are S-P-V, S-P-GF, V-P-V and V-P-GF.
Statistical analyses of the morphometry of the vents revealed diagnostic relationship for each
hydrothermal vent and conduit type. Significantly, the geometry of the vents and conduits are
distorted and modified by later magmatic emplacement. A new conceptual framework for the
development and modification of the four types of vent geometries is given in this study.
Understanding the scale variability of hydrothermal vents and conduits has broader implications
for tectonic history, hydrocarbon maturation and mineral exploration in the Naglfar Dome and the
entire Vøring Basin.
... Two more characteristic features of volcanic passive margins are Seaward Dipping Reflectors (SDRs; interpreted as the expression of basaltic extrusions (Hinz, 1981;Mutter et al., 1982;White et al., 1987;White and McKenzie, 1989)), and usually more than 10 km thick syn-and post-rift sediments (White and McKenzie, 1989;Franke, 2013). It evolved in response to the North Atlantic breakup, presumably, initiated by the abnormally hot mantle of the Iceland plume (White, 1989;Skogseid et al., 1992;Ren et al., 1998;Tsikalas et al., 2002;Blystad et al., 1995;Gernigon et al., 2004Gernigon et al., , 2006. ...
The aim of this study is to investigate the shallow thermal field differences for two differently aged passive continental margins by analyzing regional variations in geothermal gradient and exploring the controlling factors for these variations. Hence, we analyzed two previously published 3D conductive and lithospheric-scale thermal models of the Southwest African and the Norwegian passive margins. These 3D models differentiate various sedimentary, crustal and mantle units and integrate different geophysical data such as seismic observations and the gravity field. We extracted the temperature-depth distributions in 1 km intervals down to 6 km below the upper thermal boundary condition. The geothermal gradient was then calculated for these intervals between the upper thermal boundary condition and the respective depth levels (1, 2, 3, 4, 5, and 6 km below the upper thermal boundary condition). According to our results, the geothermal gradient decreases with increasing depth and shows varying lateral trends and values for these two different margins. We compare the 3D geological structural models and the geothermal gradient variations for both thermal models and show how radiogenic heat production, sediment insulating effect, and thermal LAB depth influence the shallow thermal field pattern. The results indicate an ongoing process of oceanic mantle cooling at the young Norwegian margin compared with the old SW African passive margin that seems to be thermally equilibrated at the present-day.
... Furthermore, the North Atlantic underwent multi-phase rifting for an extended period of time (over 300 Myrs) after post-Caledonide collapse (e.g. Skogseid et al., 1992 ;Faleide et al., 2008 ;Abdelmalak et al., 2019)the consequences of which for the Barents Sea shelf remain to be reconciled with future work on circum-Greenland motions. On the other hand, mechanical modelling results offer quantitative predictions that can be tested against field observations to improve the existing kinematic models. ...
Late Cretaceous‐Cenozoic contractional structures are widespread in the Barents Sea. While the exact dating of the deformation is unclear, it can only be inferred that the contraction is younger than the early Cretaceous. One likely contractional mechanism is related to Greenland Plate kinematics at Paleogene times. We use a thin sheet finite element modelling approach to compute deformation within the Barents Sea in response to the Greenland‐Eurasia relative motions during the Paleogene. The analytical solution for the 3‐D folding of sediments above basement faults is used to assess possibilities for folding. Two existing Greenland Plate kinematic models, differing slightly in the timing, magnitude and direction of motion, are tested. Results show that the Greenland Plate’s general northward motion promotes growing anticlines in the entire Barents Sea shelf. Our numerical models suggest that the fan‐shaped pattern of cylindrical anticlines in the Barents Sea can be associated with the Eurekan deformation concurrent to the initial rifting and early seafloor spreading in the northeast Atlantic. The main contraction phase in the SW Barents Sea coincides with the timing of continental breakup, whereas the peak of deformation predicted for the NW Barents Sea occurred at later times. Svalbard has experienced a prolonged period of compressional deformation. We conclude that Paleogene Greenland Plate kinematics are a likely candidate to explain contractional structures in the Barents Sea.
... Three episodes of extensional tectonics are commonly agreed upon by most authors in the study area (Fig. 2a). These includes (a) a regional extensional event of Early Cenomanian age that caused relative uplift of the Gjallar Ridge and the development of a regional depocentre in the Rås Basin, (b) a second extensional event of Late Campanian faulting on the Nyk High and along the Fles Fault Complex, and (c) a third regional extensional event associated with voluminous volcanism in the Late Cretaceous-Palaeocene (Skogseid et al., 1992;Hinz et al., 1993;Blystad et al., 1995;Bjørnseth et al., 1997;Ren et al., 1998;Doré et al., 1999;Brekke, 2000;Peron-Pinvidic and Osmundsen, 2016). In parallel, Late Cretaceous-Palaeogene tectonic events cannot explain the extensional deformation observed within the sediments belonging to the Nise Formation to Brygge Formation interval in the study area. ...
Extensional settings with multiple phases of rifting are often intermitted by compressional tectonics or inversion. Differentiating compressional structures/phases in such settings become difficult when magma-induced forced folds are also present. This study uses a high-quality 3D seismic data to interpret tectonic inversion structures in the Naglfar Dome of the Norwegian North Sea. Cenozoic tectonic inversion in the Naglfar Dome is clearly observed in the area as (a) snakehead reflections and (b) trains of fault propagation anticlines along inverted normal faults. These structures selectively affect strata of Early Eocene to Oligocene age, indicating that both basin-wide and localised tectonic inversion have occurred in the Naglfar Dome. Early Miocene inversion in the study area occurred along reactivated, main and boundary faults between the Hel Graben and the Nyk High. Tectonic inversion in a purely extensional domain such as the Naglfar Dome post-dated the formation of a regional magma-induced forced fold. These inversion structures are cumulative consequences of the interaction of a NW-SE compression associated with a reconfiguration of plate motion during the opening of the Norwegian-Greenland Sea in the Eocene to Oligocene, active ridge push from both the Mohns Ridge and the Iceland insular margin, and far-field plate tectonic stresses associated with late stages of the Alpine orogeny. The results presented here demonstrate the rare occurrence of contractional structures within a purely extensional domain, with further implications for understanding the dynamics and evolution of faults along continental margins.
... Nowadays the Vøring passive margin is considered as magma-rich, and the deep structures (LCBs) with high seismic velocities (V P > 7.1 km/s; Gernigon et al., 2004) located in the outer Vøring Basin have generally been interpreted as breakup-linked "underplated" igneous crust (Eldholm and Grue, 1994;Mjelde et al., 2009;Skogseid et al., 1992). However, as recalled by Péron-Pinvidic et al. (2013), alternative hypotheses exist at the scale of the NE Atlantic, including igneous transitional crust or thick oceanic crust (Weigel et al., 1995;Holbrook et al., 2001), dense Caledonian crustal fragments (Gernigon (White et al., 2008), and serpentinized mantle (e.g. ...
The outer Vøring Basin, Norwegian Sea, is an abnormally deep and distal part of the Norwegian passive margin, known as “magma-rich”. Yet, the lastest borehole drilled in its northernmost part led to a drastic change in the interpretation of seismic reflection data. This update motivates a new model for the entire Mesozoic-Eocene rifting period. Here I evaluate the link between tectonics, serpentinization and magmatism in the Norwegian margin from the Ryazanian-Valanginian to the Palaeocene-Eocene magmatic breakup. Maps of the regional Nise sandstones and main regional unconformities specifically highlight the fundamental role of the Nyk-Vema Structure to understand the basin and to question the deformation timing related to deep structures. The top of partially serpentinized mantle is supported by high-amplitude reflectors clearly visible on certain seismic lines, known as “T-Reflector”, covered with gravity-driven pre-exhumation sediments and further deposits. A new structural scheme based on the interpretation of 2D and 3D seismic data shows the complex organization of fault networks and constrains a major deformation period between the Late Campanian and the Late Palaeocene. Ocean-scale complementary arguments are in good agreement with Lower-Cretaceous mantle exhumation. Before becoming a volcanic margin in the Palaeocene, this segment of the Norwegian margin appears as amagmatic until the Late Campanian, at least. I propose an integrated model since the onset of hyperextension in the end of the Jurassic until the cessation of activity of the Aegir Ridge in the Miocene. This model has implications regarding the context-dependent interpretation of lower-crustal bodies, the ubiquity of serpentinization and mantle exhumation worldwide, and on the hydrocarbon system.
... The second phase began at ca. 56 Ma (at the Paleocene-Eocene boundary) and is represented by seaward-dipping reflector sequences along the continental margins, and it includes the Main Series Basalts in Eastern Greenland, and the Greenland-Faeroes Ridge (Saunders et al., 1997). At that time, a large magmatic complex (covering at least ~10 5 km 2 ) of dominantly subhorizontal sills of basaltic composition was intruded into the Vøring and Møre Basins in the eastern North Atlantic (Skogseid et al., 1992;Svensen et al., 2004;Angkasa et al., 2017). ...
... Literature on the origin of magma-rich margins in the North Atlantic is dominated by the plume 1089 concept; in this hypothesis, the impingement of the Icelandic plume on the base of the1090 lithosphere has variously been implicated in raised mantle temperatures, elevated margins,1091 voluminous magmatism and break-up itself. A full description of this model is beyond the 1092 scope of this paper; it has been well-described in (for example)(White and McKenzie 1989; 1093 White 1992;Skogseid et al. 1992Skogseid et al. , 2000 while problems with the hypothesis have been 1094 highlighted by (for example)(Foulger 2002(Foulger , 2010Lundin and Doré 2005). Other ideas exist 1095 to explain the anomalous magmatism. ...
The North Atlantic, extending from the Charlie Gibbs Fracture Zone to the north Norway-Greenland-Svalbard margins, is regarded as both a classic case of structural inheritance and an exemplar for the Wilson-cycle concept. This paper examines different aspects of structural inheritance in the Circum-North Atlantic region: 1) as a function of rejuvenation from lithospheric to crustal scales, and 2) in terms of sequential rifting and opening of the ocean and its margins, including a series of failed rift systems. We summarise and evaluate the role of fundamental lithospheric structures such as mantle fabric and composition, lower crustal inhomogeneities, orogenic belts, and major strike-slip faults during breakup. We relate these to the development and shaping of the NE Atlantic rifted margins, localisation of magmatism, and microcontinent release. We show that, although inheritance is common on multiple scales, the Wilson Cycle is at best an imperfect model for the Circum-North Atlantic region. Observations from the NE Atlantic suggest depth dependency in inheritance (surface, crust, mantle) with selective rejuvenation depending on time-scales, stress field orientations and thermal regime. Specifically, post-Caledonian reactivation to form the North Atlantic rift systems essentially followed pre-existing orogenic crustal structures, while eventual breakup reflected a change in stress field and exploitation of a deeper-seated, lithospheric-scale shear fabrics. We infer that, although collapse of an orogenic belt and eventual transition to a new ocean does occur, it is by no means inevitable.
... Three models can explain the temperature increase described above: 1) Burial under the typical 434 thickness of extrusive and intrusive volcanic rocks generally found at magma-rich rifted margins 435 (seaward dipping reflector sequences of 3-7 km; Skogseid et al., 1992;Menzies et al., 2002), which, 436 with a moderate geothermal gradient of 35°C/km would give a temperature increase of 105 to 245°C. 2) 437 ...
During the terminal stages of Wilson cycles, remnants of magma‐poor margins may be incorporated into the orogens, whereas the magma‐rich margins often are lost in subduction due to low buoyancy. The understanding of magma‐rich margins is therefore mostly based on drill holes and geophysical observations. In this contribution, we explore the temporal evolution and the ambient conditions of a magma‐rich rifted margin preserved within the Scandinavian Caledonides. The Scandinavian Dike Complex was emplaced into a sedimentary basin during the initial breakup and opening of the Iapetus Ocean 615 to 590 million years ago. The dike complex constitutes 70–90% of the magma‐rich, syn‐rift basins and is locally well preserved despite the complex Caledonian history. This contribution provides new observations about the geometry, relative timing, and development of the margin. Jadeite‐in‐clinopyroxene geothermobarometry, titanium‐in‐biotite geothermometry, and garnet isopleth modeling show that the ambient pressure and temperature conditions were similar for the entire dike complex at 0.25 to 0.45 GPa, with contact metamorphic temperatures up to approximately 700 °C. In the northernmost part of the study area, U‐Pb dating of magmatic zircon shows that partial melting of the sedimentary host rock, at relatively shallow levels, occurred at 612 Ma. This shows that the crust was molten already 6 million years before the northernmost dike swarm was emplaced at 605.7 ± 1.8 Ma. We propose that the locally pervasive partial melting occurred due to high geothermal gradients and introduction of mafic melt in the lower crust. These processes significantly reduced the strength of the crust, eventually facilitating continental breakup.
... As elsewhere in the NE-Atlantic and Baffin Bay system, syn-magmatic (syn-SDRs) extension was Paleocene to Eocene in age (e.g. Skogseid et al., 1992;Mjelde et al., 2001;Gernigon et al., 2015;Schiffer et al., 2018). Like elsewhere in the NE-Atlantic (section 2.a), this breakup occurred within a previously stretched and thinned continental crust which experienced major wrench and extensional tectonic events (Ziegler, 1989;Doré et al., 1999;Skogseid et al., 2000;Mosar et al., 2002). ...
... intrusions, underplating, SDRs, lava flows, lava deltas), together with the uplift and erosion, are traditionally attributed to the impact of the Icelandic plume and/or to the breakup processes (e.g. Eldholm et al., 1987;Skogseid et al., 1992). The Lofoten-Vesterålen margin extends between the Bivrost Lineament and the Senja Fracture Zone, from the northern end of the Vøring margin segment up to the connection to the SW Barents Sea and its sheared margin (Fig. 1). ...
In this contribution we review our knowledge of the regional first order geometry of the Norwegian - Greenland Sea rift system, focusing on the evolution of the distal margins and on breakup. We include a description of selected published and unpublished geoseismic profiles considered as representative of the margins architecture, from the Møre - Jan Mayen conjugates to the Lofoten - North-East Greenland segment. The selected transects illustrate the geometry of the extensional system and outline the main structural units. Based on these transects, we discuss rifting evolution at the regional scale, focusing on along-strike architectural variability and on the relationship between the distal and outer margin domains and breakup. In that framework, we examine the definition of breakup, the lateral variations in terms of structures and magmatism, discuss the possible nature and role of transfer features, and the formation and isolation of microplates. We propose to consider the distal margin domains as segmented and multiphased basins, floored by exhumed altered crustal and/or mantle rocks, variously affected by magmatic additions in diverse forms, and capped by crustal allochthons of different dimensions. We favour the role of tectonic processes in the separation of the Jan Mayen microcontinent from the Møre and Vøring margins. And, along the same line of thought, we propose that it was the lateral change of structural roles of large-magnitude fault complexes that became responsible for the along-strike segmentation of the margin.
... It is suggested that continental break-up is a result of meltinduced weakening of the lithosphere (Skogseid et al., 1992). The intrusion of mantle-derived mafic magmas may trigger melting of the continental crust, leading to felsic magmatism, which forms a bimodal magmatic sequence (Pin and Paquette, 1997). ...
Understanding the dynamics of Large Igneous Provinces (LIPs) is critical to deciphering processes associated with rupturing continental lithosphere. Microcontinental calving, the rifting of microcontinents from mature continental rifted margins, is particularly poorly understood. Here we present new insights into these processes from geochronological and geochemical analyses of igneous rocks from the Tethyan Himalaya. Early Cretaceous mafic dikes are widely exposed in the eastern and western Tethyan Himalaya, but no such rocks have been reported from the central Tethyan Himalaya. Here we present an analysis of petrological, geochronological, geochemical, and Sr–Nd–Hf–Os isotopic data for bimodal magmatic rocks from the center-east Tethyan Himalaya. Zircon U–Pb dating yields six weighted-mean concordant ²⁰⁶Pb/²³⁸U ages of 118±1.2 to 115±1.3 Ma. Mafic rocks display MORB-like compositions with flat to depleted LREE trends, and positive εNd(t) (+2.76 to +5.39) and εHf(t) (+8.0 to +11.9) values. The negative Nb anomalies and relatively high ¹⁸⁷Os/¹⁸⁸Os ratios (0.15–0.19) of these rocks are related to variable degrees (up to 10%) of crustal contamination. Geochemical characteristics indicate that mafic rocks were generated by variable degrees (2–20%) of partial melting of spinel lherzolites in shallow depleted mantle. Felsic rocks are enriched in Th and LREE, with negative Nb anomalies and decoupling of Nd (εNd(t) = −13.39 to −12.78) and Hf (εHf(t) = −4.8 to −2.0), suggesting that they were derived mainly from garnet-bearing lower continental crust. The geochemical characteristics of the bimodal magmatic associations are comparable to those of associations that form in a continental rift setting. Results indicate that Early Cretaceous magmatism occurred across the whole Tethyan Himalaya, named here as the “Tethyan Himalaya igneous province”. Separation of the Tethyan Himalaya from the Indian craton may have occurred during ongoing Early Cretaceous extension related to the Kerguelen mantle plume during the nascent stages of a global plate-reorganization event. If this is the case, our findings provide clues to the nature of the Tethyan Himalaya, challenging traditional view of the India–Asia single-stage collision model.
... The tectonic development of the Vøring Basin is strongly connected to the geodynamic evolution of the Mid-Norwegian continental margin (Bukovics & Ziegler, 1985). Its main tectonic evolution started during the Late Jurassic, and its structure was developed through several rifting events that lasted until the Early Cretaceous (Skogseid, Pedersen, Eldholm, & Larsen, 1992). Although the timing of extensional tectonics in the Vøring Basin remains controversial, Brekke (2000) suggested that the three main tectonic phases that affected the basin were a) extension from the Late Jurassic to the Late Cenomanian; b) extension and compression from the Late Cenomanian to the Early Palaeocene; and c) extension and compression from the Early Palaeocene to the present. ...
Mass wasting is triggered on many continental slopes by a number of mechanisms, including seismic shaking, high sedimentation rates, the presence of weak geological units and gas hydrate dissociation. In this study, the morphology of a Late Miocene–Early Pliocene mass transport complex (MTC) on the Utgard High is unravelled and discussed in relation to possible trigger mechanisms. The approach used here includes 3‐D seismic interpretation and the analysis of variance attribute maps. The interpreted MTC is located on the crest and flanks of the Utgard High and is composed of three mass transport deposits with seismic characters varying from transparent and chaotic seismic facies at the base to slightly deformed layers composed of mounds and rafted blocks in the middle and chaotic to transparent reflections at the top. Lithologically, the MTC consists predominantly of claystone with high gamma ray and low density and resistivity values, demonstrating that the associated mounds represent remobilized ooze sediments. A vertical stack of six magmatic sills emplaced from 55.6–56.3 Ma into the Upper Cretaceous shales is interpreted at depths of 3000–5500 ms Two way Travel Time (TWTT). In association with these magmatic sills are several hydrothermal vent complexes that interacted with the top MTC horizon, signifying that episodic and secondary fluid‐venting events might be the principal mechanism facilitating mass wasting in the study area. In addition, the remobilization of ooze sediments into mounds is hypothesized to be dependent on fluids and clayey layers. As a corollary of this work, the importance of relict and recurrent episodes of fluid flow in the Vøring Basin and their influence on the geotechnical integrity of the overburden and later mass wasting is established.
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... In the Cretaceous, the supply of coarse clastics slowed and the deposition of a large chalk deposit occurred during a major transgression in the Late Cretaceous. The Late Cretaceous to Palaeocene was a period of continued extension culminating in the formation of deep basins and eventually continental break-up and the formation of oceanic crust at the beginning of the Eocene (Skogseid et al., 1992;Srivastava and Tapscott, 1986). The Cenozoic has seen two episodes of uplift and tectonic activity in the North Atlantic region. ...
... This phenomenon, referred to as the extension discrepancy, is attributed to subseismic faulting (Marrett & Allmendinger, 1992;Crossby et al., 2008), polyphase faulting (Reston 2005;Reston & McDermott, 2014), sequential faulting (Ranero & Perez-Guissinyé, 2010) and/or to higher extension rates in the lower crust than in the upper crust, implying lateral flows of crustal material along shear zones (Sibuet, 1992;Davis & Kushnir, 2004;Egan & Meredith, 2007;Jolivet at el., 2015). Magma-rich margins are characterized by the presence of voluminous magmatic intrusion and extrusion contemporaneous with extensional tectonics (White and McKenzie, 1989;Skogseid et al., 1992). The main features of magma-rich rifted margins are: i) the presence of highvelocity seismic zones at the base of the crust: ii) a continental crust strongly intruded by sills and dikes and iii) covered with thick flood basalts and tuffs formation often imaged in seismic profiles as strongly reflective Seaward Dipping Reflectors (SDR; Barton and White, 1997;Mutter et al., 1982;White et al., 1987;White & McKenzie, 1987). ...
The stunningly increased resolution of the deep crustal levels in recent industrial seismic profiles acquired along most of the world's rifted margins leads to the unraveling of an unexpected variety of structures. It provides unprecedented access to the processes occurring in the middle and lower continental crust. We present a series of so far unreleased profiles that allows the identification of various rift-related geological processes such as crustal boudinage, ductile shear and low-angle detachment faulting, and a rifting history that differs from the classical models of oceanward-dipping normal faults. The lower crust in rifted margins appears much more intensely deformed than usually represented. At the foot of both magma-rich and magma-poor margins, we observe clear indications of ductile deformation of the deep continental crust along large-scale shallow dipping shear zones. These shear zones generally show a top-to-the-continent sense of shear consistent with the activity of Continentward Dipping Normal Faults (CDNF) observed in the upper crust. This pattern is responsible for a migration of the deformation and associated sedimentation and/or volcanic activity toward the ocean. We discuss the origin of these CDNF and investigate their implications and the effect of sediment thermal blanketing on crustal rheology. In some cases, low-angle shear zones define an anastomosed pattern that delineates boudin-like structures. The maximum deformation is localized in the inter-boudin areas. The upper crust is intensely boudinaged and the highly deformed lower crust fills the inter-boudins underneath. The boudinage pattern controls the position and dip of upper crustal normal faults. We present some of the most striking examples from the margins of Uruguay, West Africa, South China Sea and Barents Sea, and discuss their implications for the time-temperature history of the margins.
... In the Upper Cretaceous, extension continued making pre-existing Jurassic faults to be reactivated, with subsidence and uplift leading to the development of the Trøndelag Platform and the Gjallar Ridge (Brekke et al., 1999). Prior to the Eocene break-up, the Vøring Marginal High was uplifted and increased heat flow during the Late Palaeocene caused the emplacement of subaerial flood basalts during the Late Palaeocene to Early Eocene (Eldholm et al., 1989;Skogseid et al., 1992). Pervasive magmatic activities, which was concurrent with the late stages of rifting led to continental break-up (Breivik et al., 2014;Mjelde et al., 2008). ...
In this study, three-dimensional seismic reflection and borehole data from the Vøring Basin, offshore Norway have been used to characterize a supra-sill related forced fold to understand its evolution and relevance in the context of regional tectonics. Magmatic sills were recognised to be positive high-amplitude anomalies with similar polarity to the seabed reflection. The seismic dataset reveals two groups of sills in the study area comprising interconnected sills beneath the regional forced fold, and those intruded into the overburden. Magmatic sills forming the interconnected sill complex are emplaced at a depth of about 5.5 s TWTT below the modern seafloor. Aspect ratio (length/width), A for the sills ranges from 1.63–6.90. The regional forced fold is interpreted based on its bathymetric and seismic-stratigraphic expression on horizon H7, which is part of the Palaeocene to Eocene Tang Formation. Amplitude of the accommodation fold is about 780 km². Hydrothermal vent complexes and fluid-flow conduits in the study area develop above the sill edges and on the flanks of the interconnected sill complex extending from the lower part of the Tang Formation to the uppermost section of the Brygge Formation evidencing vertically focussed fluid flow in the study area. The overlying overburden is in turn deformed and structurally compartmentalized through forced folding and Late Cenozoic tectonics. We demonstrate that accommodation folding is formed in response to the emplacement of several interconnected sills during the opening of the Norwegian-Greenland Seas. Sill emplacement in the study area causes uplift of the Cretaceous to Palaeocene depocentre prior to further restructuration during Cenozoic tectonic inversion. Magmatic intrusions documented in this study have wider implications for understanding supra-sill deformations along volcanic margins with well-developed emplaced sills at depth and likewise hydrocarbon prospectivity in the study area.
Submarine landslides are widely recognised around the Northeast Atlantic margin, some of which forming vast slide complexes. On the mid-Norwegian margin, the Storegga Slide complex has been active since the early Pleistocene, first as a regional event (Slide W) at 1.7 Ma, and later culminating in the Storegga Slide sensu stricto at 8.2 ka. By interpreting high-quality seismic data, this work investigates slope instability styles, and their geneses, on the northwest flank of the Storegga Slide complex. Seismic and borehole data prove the presence of i) two new landslides formed prior to Slide W, ii) mass-transport complexes generated during the evacuation of sediment oozes, iii) a series of cracks on the seafloor denoting modern slope instability. Fluid accumulation within glacial-marine deposits was a primary factor promoting early instability; fluid pipes increase in number below the oldest landslide deposits, with most pipes terminating at their glide planes. Furthermore, mathematical models show that vertically stacked intervals with weak layers, and older landslide deposits, are able to promote further instability. This work thus suggests episodic fluid flow as the primary factor promoting long-term instability near the Storegga Slide complex. As a corollary, we reveal the stability of the continental slope to still be precarious, at present, on the northwest flank of the Storegga Slide.
Continental breakup involves a transition from rapid, fault-controlled syn-rift subsidence to relatively slow, post-breakup subsidence induced lithospheric cooling. Yet the stratigraphic record of many rifted margins contain syn-breakup unconformities, indicating episodes of uplift and erosion interrupt this transition. This uplift has been linked to mantle upwelling, depth-dependent extension, and/or isostatic rebound. Deciphering the breakup processes recorded by these unconformities and their related rock record is difficult because associated erosion commonly removes the strata that help constrain the onset and duration of uplift. We examine three major breakup-related unconformities and intervening rock record in the Lower Cretaceous succession of the Gascoyne and Cuvier margins, offshore NW Australia, using seismic reflection and borehole data. These data show the breakup unconformities are disconformable (non-erosive) in places and angular (erosive) in others. Our recalibration of palynomorph ages from rocks underlying and overlying the unconformities shows: (i) the lowermost unconformity developed between 134.98–133.74 Ma (Intra-Valanginian), probably during the localisation of magma intrusion within continental crust and consequent formation of continent-ocean transition zones (COTZ); (2) the middle unconformity formed between ~134–133 Ma (Top Valanginian), possibly coincident with breakup of continental crust and generation of new magmatic (but not oceanic) crust within the COTZs; and (iii) the uppermost unconformity likely developed between ~132.5–131 Ma (i.e. Intra-Hauterivian), coincident with full breakup of continental lithosphere and the onset of seafloor spreading. During unconformity formation, uplift was focused along the continental rift flanks, likely reflecting landward flow of lower crustal and/or lithospheric mantle from beneath areas of localised extension towards the continent (i.e. depth-dependent extension). Our work supports the growing consensus that the ‘breakup unconformity’ is not always a single stratigraphic surface marking the onset of seafloor spreading; multiple unconformities may form and reflect a complex history of uplift and subsidence during the development of continent-ocean transition.
Diatremes and volcanoes have been described largely from rock exposures in magma-rich sedimentary basins. Although a growing number of research works have described volcanoes and similar structures (hydrothermal vent complexes and fluid-escape structures) from seismic reflection data. However, their timing, composition and dimensionalities from seismic reflection data are sparsely known owing to paucity of direct measurements from wellbores, cores, and limited resolution power of seismic reflection data. In this work, high quality 3-D seismic reflection data and seismic attributes are used to interpret the origin and configuration of diatremes buried beneath 40 paleo volcanoes. The diatremes are mostly reflected as faulted zones of low amplitude and distorted seismic signal, that are depicted as composite columnar, conical, fault-controlled, and bifurcated structures. Above the diatremes are cone- and crater-type volcanoes, which are infilled with syn-eruptive deposits. Volcanoes in the study area, are common in two main domains, representing the boundaries of isolated centres of (a) eruptive volcanoes, and (b) strong material fragmentation and dispersion, common with diatreme volcanoes on the paleo seafloor. The interaction of magma and fluids at deeper depths caused piercement of the overburden rocks, creating the diatremes, which later acted as conduits for the migration of magma and fluids onto the Palaeocene-Eocene seafloor. Magma and fluid plumbing through the diatremes were principally aided by the complex interactions of pre-existing faults and diatremes. The ensued eruptions predominantly produced small-volume, monogenetic volcanoes (<1 km3) which are dispersed along the flanks of a NE-SW oriented volcanic field where large-volume, polygenetic volcanoes (>1 km3) are prevalent. Importantly, the complex internal configuration of subsurface diatremes and their volcanoes as demonstrated in this work further reinforces the diversities and caveats in their seismic interpretation along many magma-rich margins.
Une marge passive est une zone de transition non-active entre lithosphère continentale et lithosphère océanique. De nombreuses marges passives présentent un fort développement magmatique (>50%). Ces marges passives volcaniques (MPVs) marquent la rupture lithosphérique au-dessus d’un manteau en fusion (partielle) et sont typiquement caractérisées par l’intrusion et l’extrusion d’un volume significatif de produits magmatiques dans la croûte lors des périodes ante-rift, syn-rift et post-rift. A partir d’une compilation bibliographique, de données sismiques (profils de sismique réflexion ION-GXT, sismique 3D) et d’observations réalisées sur le terrain à l’Est et à l’Ouest du Groenland, les objectifs de cette thèse étaient : (1) de mieux caractériser les modes tectoniques d’accommodation des flexures de la croûte supérieure sous les SDRs (seaward dipping reflectors) et l’interprétation des SDRs externes et, (2), de placer la rupture magmatique à l’échelle de la fragmentation d’un supercontinent. Les principaux résultats obtenus sont: 1) La rupture d’un supercontinent est toujours synmagmatique. Cette rupture se propage ensuite de manière non-magmatique (article en préparation); 2) Les SDRs externes sont découplés tectoniquement d’une croûte inférieure d’origine continentale exhumée. Du matériel d’origine continental pourrait exister en profondeur de manière continue au niveau de rides asismiques transverses (comme GIFR) (article soumis); 3) La flexure crustale est aussi accommodée par du magma qui circule dans les failles de détachement sous SDRs. Un découplage existe à l’extrados des flexures accommodé par des injections de magma syn-tectoniques sous forme de laccolithes à la base des SDRs internes.
Subsurface pipes related to fluid flow are observed in seismic reflection data as vertical to sub-vertical features of low-amplitude reflections, linking chaotic reflections at the base (root zones) of volcanic constructs to their summits which can comprise craters, mounds or eye-shaped vents. To date, uncertainties remain regarding the linear correlation and scaling relationship between pipes and their overlying vent structures. Using 3D seismic reflection data from a magma-rich basin, the Vigrid Syncline in the Vøring Basin (offshore Norway), this study provides a seismic, morphometric and statistical description of forty (40) pipes and their associated structures. These pipes include conical, bifurcated, fault-controlled and columnar types, which are a consequence of the intrusion of two mappable magmatic sills of Early Eocene age. The heights (av. 1098 m), widths (av. 1740 m) and slenderness ratios (i.e. the heights/widths (Ω)) of the pipes (av. 0.6) show low to moderate correlation coefficients with vent parameters such as height (av. 258 m), area (av. 2.3 km2) and ellipticity (av. 1.5). The morpho-statistical analyses provided here show that ‘pipe’ formation mechanisms are mutually exclusive and that vent geometries are not particularly systematic in nature.
Large Igneous Province (LIP) formation involves the generation, intrusion, and extrusion of significant volumes (typically > 1 Mkm³) of mainly mafic magma and is commonly associated with episodes of mantle plume activity and major plate reconfiguration. Within LIPs, magma transport through Earth’s crust over significant vertical (up to tens of kilometres) and lateral (up to thousands of kilometres) distances is facilitated by dyke swarms and sill-complexes. Unravelling how these dyke swarms and sill-complexes develop is critical to: (i) evaluating the spatial and temporal distribution of contemporaneous volcanism and hydrothermal venting, which can drive climate change; (ii) determining melt source regions and volume estimates, which shed light on the mantle processes driving LIP formation; and (iii) assessing the location and form of associated economic ore deposits. Here, we review how seismic reflection data can be used to study the structure and emplacement of sill-complexes and dyke swarms. We particularly show that seismic reflection data can reveal: (i) the connectivity of and magma flow pathways within extensive sill-complexes; (ii) how sill-complexes are spatially accommodated; (iii) changes in the vertical structure of dyke swarms; and (iv) how dyke-induced normal faults and pit chain craters can be used to locate sub-vertical dykes offshore.
Between 1086.6 and 1229.4 m below seafloor at Site 642 on the Outer Voring Plateau, a series of intermediate volcanic extrusive flow units and volcaniclastic sediments was sampled. A mixed sequence of dacitic subaerial flows, andesitic basalts, intermediate volcaniclastics, subordinate mid-ocean ridge basalt, (MORB) lithologies, and intrusives was recovered, in sharp contrast to the more uniform tholeiitic T-type MORB units of the overlying upper series. This lower series of volcanics is composed of three chemically distinct groups, (B, A2, A1) rather than the two previously identified. Flows of the dacitic group (B) have trace-element and initial Sr isotope signatures which indicate that their source magma derived from the partial melting of a component of continental material in a magma chamber at a relatively high level in the crust. The relative proportions of crustal components in this complex melt are not known precisely. The most basic group (A2) probably represents a mixture of this material with MORB-type tholeiitic melt. A third group (A1), of which there was only one representative flow recovered, is chemically intermediate between the two groups above, and may suggest a repetition of, or a transition phase in, the mixing processes. -Authors
The Voring Basin was structured by Late Jurassic-Early Cretaceous extension and subsequent subsidence in the Cretaceous. It is divided into two basin provinces by the Molde-Bodo high. The opening of the Norwegian-Greenland Sea during the early Tertiary was preceded by uplift of the outer Voring Basin accompanied by extensional listric faulting in the Cretaceous sediments and intrusive activity. The early Tertiary extension was restricted to the outer Voring Basin, subsequently forming the Tertiary marginal basin. Initial subaerial sea-floor spreading created the upper volcanic series at ODP Site 642, parts of the flow-sill complex landward of the escarpment and also caused deposition of ashes and tuffs in the Voring Basin. The post-opening margin history is characterized by the progressive submergence of the Voring Plateau marginal high which for long periods was elevated with respect to the Voring Basin. We propose that early Tertiary strike-slip motion, which triggered mass movements lasting into the early Miocene, is responsible for the spectacular north-south domes in the Voring Basin. Finally, a renewed major pulse of sedimentation and subsidence centered on the shelf edge commenced in latest Miocene time. -from Authors
Seismic sections and ODP drilling information at the Voring Plateau volcanic passive margin have provided the framework for modelling the generation of basaltic magma and uplift associated with the early Tertiary rifting event. A simple uniform lithosphere stretching model of subsidence, incorporating partial melting in the upper mantle, accounts for both the volume of liquid basalt produced and the amount of uplift, when a minor temperature excess of approximately 50°C is assumed at the base of the lithosphere. -Authors
During Ocean Drilling Program Leg 104 a 900-m-thick sequence of volcanic rocks was drilled at Hole 642E on the Voring Plateau. This sequence erupted in two series upon continental basement. The upper series corresponds to the seaward-dipping seismic reflectors and comprises a succession of about 122 flows of transitional oceanic tholeiite composition. They have been subdivided into several formations consisting of flows related to each other by crystal fractionation processes, magma mixing, or both. Major- and trace-element chemistry indicates affinities to Tertiary plateau lavas of northeast Greenland and to Holocene lavas from shallow transitional segments of the Mid-Atlantic Ridge, such as Reykjanes Ridge. The tholeiitic magmas have been derived from a slightly LREE-depleted mantle source. Two tholeiitic dikes that intruded the lower series derive from an extremely depleted mantle source. Interlayered volcaniclastic sediments are dominantly ferrobasaltic and more differentiated. They appear to come from a LREE-enriched mantle source, and may have been erupted in close vicinity of the Voring Plateau-during hydroclastic eruptions. The two tholeiitic dikes that intruded the lower series as well as some flows at the base of the upper series show evidence of assimilation of continental upper crustal material. -Authors
The Voring Margin experienced crustal uplift and extension prior to breakup, restricted to a Tertiary marginal basin west of the present shelf edge. Initial volcanic surge and shallow extrusion level are related to a higher than normal temperature at the base of the lithosphere, inducing partial melting combined with opening in previously thinned crust. The commonly described non-extensional nature of this margin is only an apparent phenomenon. Except for the outer basin, extension by dike injection coupled with high strength of the thin, pre-opening crust in the Voring Basin precluded the formation of a faulted rift unconformity. We believe these observations have relevance for volcanic margins elsewhere, but infer that seaward-dipping reflectors can form in many environments. The Voring Plateau marginal high and other similar features in the North Atlantic are an integrated part of the North Atlantic Volcanic Province, which extends 2000 km longitudinally. Compared to the central, transverse, part of the province in the vicinity of the Iceland hotspot that has been active for 60 m.y., marginal volcanism was transient, related to breakup and lasting only for a maximum of about 3 m.y. -from Authors
The role of partial melting in the uniform lithospheric stretching model of continental margin formation is explored. It is shown that the transition from continental lithosphere stretching to oceanic accretion is most probably controlled by the production of a significant amount of partial melting in the asthenosphere immediately below the lithosphere, which requires stretching factors larger than 3. It is also shown that, at stretching factors exceeding 2, the law of subsidence is significantly changed by the presence of partial melt in the underlying asthenosphere. The implications for the existence of deep continental margin basins on thinned continental crusts are examined. The Armorican deep continental margin basin is taken as an example.
Regional averages of the major element chemistry of ocean ridge basalts, corrected for low-pressure fractionation, correlate with regional averages of axial depth for the global system of ocean ridges, including hot spots, cold spots, and back arc basins, as well as "normal" ocean ridges. Quantitative consideration of the variations of each major element during melting of the mantle suggests that the global major element variations can be accounted for by -8-20% melting of the mantle at associated mean pressures of 5-16 kbar. The lowest extents of melting occur at shallowest depths in the mantle and are associated with the deepest ocean ridges. Calculated mean primary magmas show a range in composition from 10 to 15 wt % MgO, and the primary magma compositions correlate with depth. Data for Sm, Yb, Sc, and Ni are consistent with the major elements, but highly incompatible elements show more complicated behavior. In addition, some hot spots have anomalous chemistry, suggesting major element heterogeneity. Thermal modeling of mantle ascending adiabatically beneath the ridge is consistent with the chemical data and melting calculations, provided the melt is tapped from throughout the ascending mantle column. The thermal modeling independently predicts the observed relationships among basalt chemistry, ridge depth, and cmstal thickness resulting from temperature variations in the mantle. Beneath the shallowest and deepest ridge axes, temperature differences of approximately 250'C in the subsolidus mantle are required to account for the global systematics.
The magmatism occurring when a hot convective mantle plume is sited beneath a lithospheric plate may be more complex if the latter is continental, rather than oceanic, because of the characteristic local physical inhomogeneity of continents. Thus, the surface volcanic expression of the plume (hotspot) may be displaced from immediately above its rising stem, if the continent has been previously locally thinned nearby. The magmatism of the British Tertiary Igneous Province and Parana basin, South America, appears to fit this model, and it may also explain why the Miocene Columbia River basalts of the NW United States overlie an Early Tertiary sedimentary basin.
Large igneous provinces (LIPs) include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts. In some cases transient episodes of voluminous magmatism are temporally and spatially related to continental break-up, eg North Atlantic Volcanic Province, Deccan Traps, Parana-Etendeka basalts. In other cases, however, no relationships are apparent, eg Siberian flood basalts, Columbia River flood basalts. LIPs worldwide are reviewed in order to better understand their relationship to the break-up and separation of lithospheric plates. Observational data suggest that existing models do not adequately explain all LIPs; it is suggested that a thermally and chemically heterogeneous asthenosphere, occasionally penetrated by deep mantle plumes, can account for their origin. -from Authors
The extensive plateau basalt lava pile in the Scoresby Sund region has a stratigraphic thickness of 3200 m and an overall average thickness of 1500 m. The pile thins inland from the Atlantic coast and laps onto basement gneisses and Jurassic sediments in the inner fjord region. The lavas are divisible into five formations which form two separate lava sequences. The lower sequence is best developed in the inner fjord region, while the upper sequence dominates the regions near the Atlantic coast. The sequences are interpreted as produced in two vo\canic episodes in connection with failed rifting episodes during the opening of the North Atlantic Ocean. At the Atlantic coast remains of a third separate lava sequence apparently forrned during active spreading.
Mesozoic rifting and associated decompression melting in the North Sea basin gave rise to igneous rocks which vary from mildly to highly undersaturated in character. Recent theoretical developments based on a parameterization of melting experiments on dry peridotite (McKenzie and Bickle 1988) suggest that there should be quantitative relationships between the compositions and volumes of magmas generated in rift environments, degrees of partial melting of the asthenospheric mantle, mantle temperature, and amounts of extension. In order to test the quantitative model of McKenzie and Bickle, observed and expected major element compositions are compared, and trace element models are used to make estimates for degrees of partial melting represented by the North Sea rocks. This analysis suggests that such models have at present extremely limited application when melt fractions are small, except to provide upper limits on combinations of asthenosphere temperature and extension. -from Authors
The post-Caledonian history of the Voring Basin has been examined by tectonic subsidence and a uniform lithosphere extension model. The study is based on depth converted seismic reflection lines integrated with seismic velocities and well data. Tectonic subsidence calculated for the pre-Cretaceous, Cretaceous and Cenozoic time intervals shows that the main episode of late Jurassic-early Cretaceous extension was centred in the eastern Voring Basin, but affected the entire basin province, whereas the early Cenozoic rifting, associated with high magmatic activity and continental separation, was restricted to the central and western parts of the basin. Source rocks for petroleum hydrocarbons include the Kimmeridgian Clay formation and possibly early Cretaceous and Paleocene sediments believed to have been deposited under conditions of restricted water circulation in the western Voring Basin. A potential reservoir unit may occur in redeposited sediments derived from a 250 km wide Paleocene-earliest Eocene landmass along the line of continental separation. -from Authors
The development of the continental margin of West Greenland is closely related to the processes that led to the opening of the Labrador Sea. The opening of the Labrador Sea began in the Early Paleocene (anomaly 27N), and not in the Late Cretaceous as previously supposed. Modelling of magnetic data and new interpretation of seismic data indicate that a large area previously regarded as underlain by oceanic crust is in fact underlain by block-faulted continental crust overlain by syn- and post-rift sedimentary sequences. The ocean-continent transition is now placed 100-150 km southwest of the foot of the continental slope instead of at the foot of this slope. Rifting in the Labrador Sea area began, however, in the Early Cretaceous. The earliest sediments are the syn-rift lower and upper members of the Bjarni Formation on the Labrador shelf and their likely equivalents, the pre- to syn-rift Kitsissut and Appat sequences on the Greenland margin. The age of these units is Barremian (or older) to Albian. The units are overlain by widespread mudstone-dominated units, the Markland Formation of the Labrador shelf and the Kangeq Sequence on the Greenland margin. The former is Cenomanian-Danian in age. By analogy the base of the Kangeq Sequence is probably Cenomanian (or Turonian), while the top is known from well ties to be at the Cretaceous-Tertiary boundary. Rifting was subdued during deposition of these mudstone units. Rifting was renewed in the Early Paleocene, and mudstones, siltstones and very fine sandstones were deposited. With the initiation of sea-floor spreading there was considerable igneous activity at the ocean-continent transition, as well as in the onshore area where picrites followed by plagioclase-porphyritic basalts were erupted. After the end of the Paleocene there was little rifting in the region, but compressional structures were formed locally as a response to transpression related to strike-slip movements that transferred plate motion from the Labrador Sea to Baffin Bay. A marked Early Oligocene unconformity separates the syn-drift Paleocene-Eocene succession from the post-drift middle Oligocene-Quaternary sediments. Sediments deposited since the Paleocene are dominated by sands. The main hydrocarbon play types offshore West Greenland are related to tilted fault blocks. Source rocks are anticipated near the base of the Kangeq Sequence, which is also the seal, and reservoirs are sandstones in the Appat and Kitsissut sequences. These two sequences were not reached by any of the exploration wells drilled in the 1970s.
Massive igneous activity across a 2000-km-diameter region accompanied the continental breakup that generated the North Atlantic Ocean. I outline the evidence which suggests that the magmatism was caused by passive upwelling and decompression melting of asthenospheric mantle which was 150-200°C hotter than normal. The anomalously hot mantle was introduced by the initiation of the Iceland plume shortly before continental breakup. Although the melt was caused by passive upwelling beneath the rifts, initiation of the mantle plume probably triggered the breakup by introducing regional uplift. -Author
Contrasting styles of lithospheric extension are investigated with two-dimensional plane-strain finite-element models. Emphasis is placed on a comparison between the postcompressional extension of tectonically thickened lithosphere and the extension of normal thickness lithosphere containing a weak zone. The former may represent extension of the Basin and Range province, whereas the latter may correspond to extension at a simple rifted continental margin. -from Authors
Simple arguments show that ascending thermal plumes will entrain their surroundings as the result of coupling between conduction of heat and laminar stirring driven by the plume motion. In the initial stages of ascent of a plume fed by a continuous buoyancy flux (a starting plume) the plume consists of a large buoyant head followed by a narrow vertical conduit. Laboratory experiments reported here show that the spherical head entrains ambient material as it rises, while the axial conduit carries hot source material to the stagnation point at the cap of the plume, from where it spreads laterally into thin laminae. We develop an analysis of the effects of entrainment on the structure and dynamics of starting plumes. The analysis predicts that under conditions appropriate to the earth's mantle large volumes of cooler lower mantle will be stirred into the head of a plume by the time it reaches the top of the mantle if it originates at the core-mantle boundary. The result is a major cooling and enlargement of the head. Source material ascending in the trailing conduit will undergo little contamination or cooling until the conduit is deflected from the vertical by large scale shear associated with plate motion. This plume structure explains the close association of high-temperature melts (komatiites or picrites) with more voluminous, lower temperature basalts in Archaean greenstones and modern continental flood basalt provinces: the picrites can be produced by melting in the hot axial conduit and the basalts from the cooler bulk of the head. More generally, we put forward stirring in plumes as one plausible mechanism contributing to compositional heterogeneity in hotspot melts.The predicted diameter of plume heads originating at the core-mantle boundary is ∼ 1000 km and this is expected to enlarge to ∼ 2000 km when the plume collapses beneath the lithosphere. This result is in excellent agreement with the observed extent of volcanism and uplift associated with continental flood volcanism. It also provides support for the hypothesis that at least some plumes originate at the core-mantle boundary.
The Rockall Trough is the most southerly and widest part of the pre-Tertiary "proto North Atlantic' rift zone running from the Porcupine Bank region west of Ireland to the Voring Plateau off central Norway. Although the northerly part of this rift is intra-continental, the southern part is quasi-oceanic in origin, but of uncertain age. The many arguments for Cretaceous sea floor spreading are evaluated in turn; the few apparently valid ones remaining are shown to relate not directly to the Rockall Trough, but to a newly-identified spreading phase of approximately mid-Cretaceous age which partially opened up the Hatton-Rockall Basin. The opening of the Rockall Trough predates this. Stratigraphic evidence suggests that the proto North Atlantic is pre-Cretaceous, and probably even pre-Jurassic, in age. The rival arguments for late Palaeozoic (late Carboniferous-early Permian) sea floor spreading are all consistent with an important phase of rifting at that time, but they provide no direct evidence for opening. A tentative conclusion is that the Rockall Trough was initiated as a rift, and then opened by a quasi- sea floor spreading mechanism, during the late Carboniferous to early Permian. -from Author
Recent experiments demonstrate that solution-precipitation (pressure-solution) processes in the liquid basalt-olivine system are exceedingly rapid. The effect of liquid basalt on the diffusional (Newtonian) creep rate of polycrystalline olivine is significant; fine-grained, chemically and texturally equilibrated, partially molten assemblages of olivine plus basalt deform at rates which are a factor of 2-5 faster than polycrystalline olivine specimens without the liquid basalt second phase. Measured values of the activation energy for creep suggest that while the kinetics of deformation in this partially molten system are affected by short-circuit diffusion through melt-filled triple junctions, the deformation process is rate-limited by matter transport through melt-free grain boundaries. A simple geometrical model, based on a dynamic equilibrium partial-melt morphology which consists of melt-free olivine grain boundaries coupled with an interconnected melt network along triple junction channnels, adequately describes our deformation results. The extent to which the liquid phase penetrates from the triple junctions into the grain boundaries of texturally equilibrated partial melts determines the contribution of solution-precipitation mechanisms to the creep rate; the critical physical parameters, therefore, are those which affect the solid-liquid and solid-solid interfacial energies, such as various composition-related chemical activities in the liquid phase and the composition and spatial distribution of mineral phases in the crystal residuum.
The oceanic crust created during this first few million years of accretion in the Norwegian-Greenland Sea lies at an unusually shallow depth for its age, has a smooth upper surface, and in many places the results of multichannel seismic reflection profiling reveal that its upper layers comprise a remarkable sequence of arcuate, seaward-dipping reflectors. These have been attributed to lava flows generated during brief period of subaerial seafloor spreading. We describe the results of inversions of digitally recorded sonobuoy measurements and two-ship expanded spread profiles collected over the oceanic crust adjacent to the Norwegian passive margin. We find that the crust of the deep Lofoten Basin is indistinguishable from normal oceanic crust in thickness and structure. Closer to the margin we observe up to a four times expansion in thickness of layers with velocities equal to those of oceanic layer 2, while the layer 3 region retains approximately the same thickness. The area over which the seaward-dipping reflectors can be observed on reflection profiles corresponds to the region of greatest expansion in ``Layer 2'' thickness. In the very oldest crust immediately adjacent to an excarpment that probably marks the continent-ocean boundary, we see evidence for a low velocity zone overlying an indistinct reflector that may mark the dyke-lava interface in the thick crust. Comparing the structure of the thick crust to that of eastern Iceland, we find a strong resemblance, especially in the expansion in thickness of material with layer 2 velocities. These results support the suggestion that during the earliest stages of spreading extursive volcanism at the ridge crest was unusually voluminous, building a thick pile of lavas erupted from a subaerial spreading center.
A complete set of two-dimensional equations for conservation of mass, momentum and energy of a two-phase system of melt in a deformable matrix is used to derive analytically a model for the vertical distribution of the melt fraction. Also, a set of asymptotic relations for parameters in the velocity field of the matrix and the melt in the neighbourhood of the symmetry axis of sea-floor spreading is obtained. These relations have been used to derive solutions appropriate to the melt extraction from the partially molten regions beneath mid-ocean ridges and the conditions for their existence. The results show the convergence of the melt streamlines towards the spreading center. It is caused by: (1) a horizontal gradient of the piezometric pressure associated with the deformations of the matrix, and (2) an upward increase of the porosity due to melting. For accepted values of the relevant parameters, a focusing effect of melt towards spreading centers exists, thereby providing an explanation for the mid-ocean ``paradox'' of a narrow region of ridge axis volcanism overlying a broad melt production zone. Contribution no. 414 from Institut für Geophysik, Kiel.
We present the first internally consistent calculation which leads to a narrow ‘conduit’ of rapid vertical advection and melting of mantle under a spreading center. In this model, mantle flow is driven by plate separation and compositional buoyancy. Melt segregation is described as flow through a permeable media. The major new feature is that the viscosity of the mantle is considered to be a strong function of the amount of partial melt present. Experiments show that the bulk viscosity of a partially molten rock is sharply reduced when the melt fraction exceeds a critical value. In the model, the viscosity is reduced as the critical melt fraction is approached. Whether or not a critical melt fraction can be reached under a spreading center depends on the mantle permeability for melt flow. The width of the upwelling area is controlled by the magnitude of the melt related viscosity reduction. Crust should be formed above the focused upwelling. Seismic observations show that the region of crustal accretion is only a few kilometers wide at fast spreading centers. With a viscosity reduction of three orders of magnitude the model predicts a zone of crustal accretion of this width.
Marine mudstones intercalated with Early Tertiary volcanic hyaloclastites in West Greenland contain dinoflagellate cyst assemblages corresponding to nannoplankton zones NP4-NP8 (Danian to Thanetian). Sediments correlated with NP3 are known from the bottom of the voclanic sequence, whereas the top of the sequence is non-marine. A horizon with normally-magnetized subaerial lavas correlated with hyaloclastites within the NP4 interval. This horizon must correspond to magnetic polarity zone 27N; the overlying reversely-magnetized lavas are assumed to belong to both polarity zones 26R and 25R. The main plateau-building volcanic phase in West Greenland is thereby constrained between "top polarity chron C28N' and "top polarity chron C25R', a time interval of 5-6 Ma. In comparison with other parts of the North Atlantic Tertiary Igneous Province, the volcanism in West Greenland started at an early stage. In terms of an impinging mantle plume in the North Atlantic, the volcanism started over the peripheral parts and not over the central part. The differences in timing and character of the volcanic products in the North Atlantic indicate a considerable lithospheric control. -from Authors
Extensional syn-rift structures at passive margins indicate that continental break-up occurs in response to horizontal deviatoric tension in the continental lithosphere. An abrupt change in the state of stress occurs at the onset of the post-rift (break-up) unconformity when the large tension ceases. The stress regime at the time of continental break-up has been modelled by finite element analysis with a view to simulating the observed features. A local compression is associated with thinned crust and an opposing local tension is produced by hot, low density upwelled asthenosphere. The resulting stress associated with stretching and thinning of the lithosphere is relatively insignificant and is likely to be compressive unless the normal continental crust is very thin. One possible source of break-up tension is the large tensional loading stress associated with an underlying hot, low density upper mantle, such as now occurs in the present day uplifted continental rift systems. It is uncertain whether such local tension could readily cause break-up within the present compressional stress regime of normal continental regions. Another possible source of wide-spread continental tension would be the occurrence of subduction on opposite sides of a large continental mass such as Pangaea, giving rise to instability. At such times, splitting could more readily propagate outwards from a domed plume region leading to break-up, thus involving both active and passive factors. The radical change in stress regime at the onset of seafloor spreading is readily modelled as the change in stress associated with the development of a new weak plate boundary.
Coincident multichannel seismic reflection and refraction profiles acquired northwest of Britain during a two-ship seismic experiment in the southern Rockall Trough provide evidence for a thin (6 km) syn-rift crust below Rockall Trough. The variation of seismic velocity with depth is consistent with either oceanic crust similar to that found in other parts of the North Atlantic or with thinned continental crust heavily intruded by syn-rift igneous rocks. The syn-rift crust beneath Rockall Trough is now buried by nearly 5 km of sediments that are intruded by Tertiary volcanics varying in extent from isolated sills to a large sill complex extending 100 km laterally. The basement imaged on seismic profiles exhibits a band of subhorizontal reflectors that extend laterally over distances of up to 40 km which we interpret as sediments intercalated with submarine lava flows generated during the opening of the Trough. Estimates of stretching from crustal thinning and from subsidence indicate that Rockall Trough has undergone extensive rifting (B>6). Upwelling asthenosphere beneath the thinned lithosphere generated at least 1-3 km thickness of melt as it decompressed. The molten rock rose upward from the mantle until it was in part extruded as lava flows and in part intruded into the crust.
Multichannel seismic profiles at the Vøring Plateau Margin off Norway and the northern Jan Mayen Ridge have provided a framework for the early Tertiary evolutionary history of these areas which were adjacent prior to the opening of the Norwegian Sea. We propose that their Paleogene evolution is quite similar and that the two acoustic basement reflectors, EE and JO, were formed in the arly Eocene by extrusion of flow basalts. From the time of opening to about anomaly 23 time, oceanic seafloor was generated by Icelandic-type spreading. During that time, the Central Jan Mayen Fracture was the main transform. The seaward dipping reflector sequences are flow basalts generated by the Icelandic spreading. The existence of less well developed dipping sequences seaward of the primary one probably reflects that these regions submerged later because of higher initial elevation. During the early Eocene subaerial spreading episode, which is related to the North Atlantic Volcanic Province, basalt flows also covered the adjacent thinned and heavily intruded continental crust. It is proposed that the continent-ocean boundary is located just landward of anomaly 24B, a short distance seaward of the Vøring Plateau Escarpment. Reflector K is only recognized landward of this boundary in a region of ``transitional crust.'' The escarpment experienced syndepositional faulting, and the most elevated parts of the Vøring Marginal High did not subside below shallow water depths before Oligocene/Miocene time. We suggest that the Vøring Margin and the Jan Mayen Ridge represent ``volcanic'' passive margins. The evolution of this margin type may relate to initial uplift due to rifting in previously thinned crust.
Stretching and thinning of continental lithosphere during rifting and
initiation of seafloor spreading cause upwelling of mantle rocks into a
pressure/temperature environment where partial melting occurs. The
amount of melt liberated is, however, insufficient to account for the
thickness of igneous crust emplaced near the continent-ocean boundary at
some passive margins. These igneous structures are known from seismic
reflection and deep refraction studies on numerous passive margins
worldwide. Seismic studies also suggest that the transition from
continental to oceanic crust on these "volcanic margins" is relatively
abrupt. Their evolution apparently does not include a prolonged
extensional phase preceding the inception of spreading. To account for
these observations, we propose a model for the generation of partial
melt in which the lateral temperature contrasts evoked by rifting and
asthenospheric upwelling drive convection in the upper mantle. Deep, hot
mantle material is transported upward by the convection, and hence much
larger volumes flow through the region where partial melting occurs than
during passive upwelling. Numerical calculations show that "convective
partial melting" can provide substantial additions to melting due to
passive upwelling alone. Using this model, we propose an evolutionary
scheme for "volcanic" passive margins, emphasize the contrast with
"nonvolcanic" margins, and describe conditions which might lead to the
development of both types.
The pattern of depth and gravity anomalies in the North Atlantic Ocean was examined by using 1°×1° and 5°×5° averages. The gravity field is dominated by two features: a broad high in the northern and central portion of the ocean and a large area of very negative anomalies in the western basin. The negative anomaly in the western North Atlantic has no expression in residual depth anomalies and does not appear to be related to surface features. The North Atlantic Gravity high is bounded on the south by an important and distinct boundary near 30°N which is present in both depth and gravity anomalies. North of this latitude, residual gravity anomalies (corrected for the 'ridge anomaly') take the form of a very broad high of about 20 mGal with the values along isochrons nearly constant from 30°N to at least as far north as 75°N. The depths within this region are consistently shallow with 5°×5° average residual depth anomalies which vary greatly from a few hundred meters near the Charlie Gibbs fracture zone to a few thousand meters near Iceland and the Azores. We used three-dimensional spherical earth models to investigate simple compensated mass distributions which could explain the observed depth and gravity anomalies. The acceptable family of models has the common characteristics that within the the anomalous region north of the 30°N boundary a portion of the compensation must be distributed to depths of several hundred kilometers and that this deep mass deficiency must be nearly uniform throughout the area. However, the compensation for more local features within this region must be shallow (within the lithosphere). Thus the topographic highs surrounding Iceland and the Azores are compensated within the lithosphere, but the overall elevation is maintained at some greater depth by a mass deficiency in the asthenosphere. This mass deficiency can be explained by an increase in temperature of about 75°C. Thus the gravity data enable us to establish the presence of a very large upper mantle hot spot that might be associated with the broad overall compensation of the entire North Atlantic north of 30°N. Whether this upper mantle hot spot is associated with a deep mantle plume is uncertain, since the gravity effect of a plume is undetectable. However, its presence suggests that the immediate source of the unusual amount of material erupted at Iceland and the Azores is within the upper mantle.
A simple model is developed which explains the occurrence of volcanic continental margins and flood basalts as a consequence of their association with nearby plumes that were active at the time of rifting. In the model, asthenosphere temperatures are increased by 100-150 C over large regions of the earth by heat advected upward in mantle plumes. The amount of partial melt generated by the asthenosphere as it wells up beneath rifts in these hot areas is calculated. Observational constraints from all known examples of volcanic continental margins are reviewed and the model is used to explain these observations.
A characteristic structural feature of the Norwegian continental margin that has been described from multichannel seismic data, as well as DSDP and ODP drilling, is a major buildup of layered extrusive basaltic rocks forming wedges that dip seaward along the entire margin. Models for their origin describe them as either products of a late-stage intra-continental rift phenomenon, with the volcanic rocks overlying attenuated continental crust, or as an integral component of the oceanic crust formed during the first few million years of seafloor spreading in the Norwegian-Greenland Sea. From studies of the Norwegian margin alone it was not possible to unequivocally distinguish between the two proposed models so an investigation of the conjugate portions of the East Greenland margin was undertaken which used two-ship multichannel seismic acquisition as its focus. We found that the wedges were not only present on the East Greenland margin, but that details of their along-strike variability are mirrored on both margins, together with their spatial relationship to the oldest magnetic lineation pattern. The wedges are relatively small in the northern part of the margin where they occur almost entirely landward of lineation #24. Moving south we see that the wedges broaden, encroach into the pattern of lineation #24, then develop into two separate wedges, the seaward of which is associated with lineation #23. Lineations #24 and #23 on both margins are distorted when associated with wedges. We believe that these data lend strength to a tectonic association of the wedges with nascent plate boundary processes. Their observation in many margins throughout the world suggest that a prolific volcanic phase may be commonly associated with the inception of a divergent plate boundary, and we believe that the most likely mechanism to generate the excess volcanism is the ‘convective partial melting’ scheme recently proposed by Mutter and Buck (1986) and Mutter et al. (in press).
Extensive geophysical surveys have been undertaken on the volcanic passive continental margin offshore Norway between 62° and 70°N during the last 25 years. Three main margin segments have been identified, the Lofoten-Vesteralen Margin, the Vøring Margin and the Møre Margin. The main features of the margins are prominent marginal highs, including seaward dipping reflector sequences and an up to 22 km thick volcanic and transitional crust, prominent escarpments (the Vøring Plateau Escarpment and the Faeroe-Shetland Escarpment), and up to 12 km deep post-Jurassic sedimentary basins east of the escarpments. Velocity-depth solutions from about 250 sonobuoys, expanding spread profiles and refraction profiles have been compiled and contoured. Isovelocity horizon contour maps and velocity transects outline a crust which broadly thickens from an oceanic crust with a normal oceanic-type velocity structure to a ca. 35 km thick continental crust with a continental velocity structure, beneath the Norwegian coast. Anomalous features include local crustal thickening below the Møre and Vering marginal highs, and high-velocity bodies in the lower crust in the extension of the Precambrian Lofoten-Vesterålen archipelago. The free-air SEASAT-derived gravity anomalies show a good correlation with the high-velocity bodies, and show prominent NE-trending highs from the Rockall Plateau/Porcupine Plateau region, over the Møre, Vøring and Lofoten-Vesterålen margins, to the southwestern Barents Sea.
The time and duration of igneous activity in the separate component areas of the British Tertiary Igneous Province (BTIP) are investigated using radiometric dates that pass internal consistency tests, used in conjunction with palaeomagnetic polarities and the available stratigraphic information. Reliable results are available only for the currently subaerial parts of the Province. It is found that activity occurred within the approximate interval 63-52 Ma, with most activity at about 59 Ma. Later activity was predominantly of acid magmas though basic rocks preponderate in the province as a whole. The types and span of igneous activity in the separate areas followed no common pattern but areas of more complex geology tend to have had a longer span of activity. Magnetic polarities are predominantly reversed, with all the lavas having this polarity. This predominance is probably largely, but not entirely, due to reversed polarity intervals being longer than normal ones at this time, and with much activity occurring in a single reversed interval. The sequence of polarities found within the BTIP cannot be fully reconciled with the polarity timescales of either Harland et al. (1982) or Berggren et al. (1985).
The addition of crustal material to the base of the continental crust, crustal underplating, may play an important role in the evolution of the continental crust and lithosphere. We have evaluated the potential for this mechanism to increase the total thickness of the crust, and using petrologic and experimental data determined the likely seismic velocity structure which results. Modeling results indicate that mantle-derived crustal material can add more than 10 km to the thickness of the crust. Depending on the composition of this underplated material and the local thermal structure, seismic velocities in this added layer will be in a range of values intermediate between velocities appropriate for crust and mantle materials (7.0–7.8 km/s), or values interpreted to be more typical of mantle materials (7.8–8.1 km/s). Comparison of the predicted seismic signature with velocity data from refraction experiments and laboratory determinations of inferred lower crustal samples indicates a strong correlation between our predictions and observation. If a significant volume of the continental crust has been added by this mechanism, the age and isotopic composition of near-surface crustal rocks may be a relatively poor indicator of lower crustal and uppermost mantle evolution.
Laboratory studies of fluid dynamic analogs of mantle plumes have led to important advances in our understanding of the life cycle of hotspots. Melting within the large volume heads of starting plumes gives rise to flood basalt provinces, while uplift of the surface above plumes results in horizontal deviatoric stresses that may precipitate considerable continental extension. However, for modern-day plumes the magnitude of these stresses appears incapable of actually initiating true continental break-up; rather, the rise of a new plume may lead to the local reorganisation of plate-scale motions (including ridge jumping), enhanced propagation of an existing ridge system, or provide sufficient extra force to drive a weak plate-scale system from slow spreading through to continental rifting. Thus, although plumes do not provide the ultimate driving force for continental break-up (these come from the plate-scale motions), the extra gravitational potential that they impose means that they play an important role in determining both where and when continental break-up does occur.
Calculation of the volume and composition of magma generated by lithospheric extension requires an accurate mitial geotherm,
and knowledge of the variation and composition of the melt fraction as a function of pressure and temperature. The relevant
geophysical observations are outlined, and geotherms then obtained from parameterized convective models. Experimental observations
which constrain the solidus and liquidus at various pressures are described by simple empirical functions. The variation in
melt fraction is then parameterized by requiring a variation from 0 on the solidus to 1 on the liquidus.
The composition of the melts is principally controlled by the melt fraction, though those of FeO, MgO, and SiO2 in addition vary with pressure. Another simple parameterization allows the observed compositions of major elements in 91
experiments to be calculated with a mean error of 1.1%, and those of TiO2 and Na2O to 0.3%. These expressions are then used to calculate the expected compositions of magma produced by adiabatic upwelling.
The mean major element composition of the most magnesium-rich MORB glasses resemble the mean composition calculated for a
mantle potential temperature Tp of 1280°C. Adiabatic melting during upwelling of mantle of this temperature generates a melt thickness of 7 km. The observed
variations of the MgO and TiO2 concentrations in a large collection of MORB glass compositions suggest that extensive low pressure fractional crystallization
occurs, but that its effect on the concentrations of SiO2, Al2O3, and CaO is small. There is no evidence that normal oceanic crust is produced from magmas containing more than 11% MgO. The
mantle potential temperature within hot rising jets is about 1480°C and can generate 27 km of magma containing 17% MgO.
Extension of the continental lithosphere generates little melt unless β> 2 and Tp> 1380°C. The melts generated by larger values of β or of Tp are alkali basalts, and change to tholeiites as the amount of melting increases. Large quantities of melt can be generated,
especially at continental margins, where estimates of β obtained from changes in crustal thickness will in general be too
small.
The question of whether basaltic rocks in continental flood basalt provinces are primary magmas or whether they are descended in general from picritic parent magmas is reviewed. It is suggested that the latter is more likely to be correct on the evidence of phase relations and the relative rareness of mantle materials with appropriate Fe/Mg ratios. Major element variations in the residual liquids of fractional and equilibrium crystallization of basaltic magmas are modelled for a variety of crystallizing assemblages. It is concluded that crystallization of olivine, clinopyroxene, and plagioclase has a marked effect on buffering chemical change in many important elements. It is this effect which accounts for the apparent uniformity of large volumes of flood basalts, not, as has sometimes been supposed, a series of implausible coincidences in the amount of material fractionated from each magma batch. It is further argued that much of the variation seen in basalts may be imposed by polybaric fractionation operating throughout crustal depths, that is at pressures up to at least 12 kb. Parental picritic magmas rising from the mantle reach the surface in exceptional areas of crustal thinning. More usually, however, it is suggested that they intrude the base of the crust as a series of sills which differentiate into upper gabbroic and lower ultramafic portions. Much of the 'low pressure' fractionation of basaltic magmas may take place in this deep crustal sill complex and evolved liquids are transmitted to the surface as their density becomes sufficiently low. This implies that in areas of flood vulcanism a potentially large new contribution to the crust is made by under- plating, the volumes of concealed cumulates being at least as large as the amount of erupted surface lava.
Like other great tholeiite sheets of the world, the Midland Valley Sill and Whin Sill were emplaced in thick accumulations of sediments which, at the time of intrusion, were not much older nor had been orogenically deformed. However, the Carboniferous sediments of northern Britain had been deposited under the influence of continuous differential subsidence, so that they were already disposed in syn-sedimentary basins and swells when intrusion began. By drawing palinspastic contours of the sills relative to a late Westphalian marine band (approximating to the surface at the time of emplacement) it is established that the dolerites mainly follow bedding planes, dipping at angles of as much as 5o down to the bottoms of the basins. Isopachs of the sills indicate a further relationship, for the intrusions are thickest at the bottoms of the basins. These facts are not explained by any intrusion mechanism previously postulated, It is proposed that emplacement must have been controlled in part by gravitational flow down dip from feeder dykes which extended to between 0.5 and 1.0km below the contemporary surface. Thereafter hydrostatic equilibrium was attained first by accumulation at the bottom of the basins and then by further advance up-dip under pressure of head. The sheets are so similar in aspect to flood basalts that it must be assumed that the sediments offered only low frictional resistance to splitting and flow.-Author
A 700 km wide-angle reflection/refraction profile carried out in the central North Atlantic west of Ireland crossed the Erris Trough, Rockall Trough and Rockall Bank, and terminated in the western Hatton-Rockall Basin. The results reveal the presence of a number of sedimentary basins separated by basement highs. The Rockall Trough, with a sedimentary pile up to 5 km thick, is underlain by thinned continental crust 8- 1 0 km thick. Some major fault block structures are identified, especially on the eastern margin of the Rockall Trough and in the adjacent Erris Trough. The Hatton-Rockall Basin is underlain by westward-thinning continental crust 22- 10 km thick. Sedimentary strata are up to 5 km thick. The strata in the Rockall Trough and Hatton-Rockall Basin probably range in age from Late Palaeozoic to Cenozoic. However, the basins have different sedimentation histories and differ in structural style. The geometry of the crust and sediments suggests that the Rockall Trough originated by pure shear crustal stretching, associated with rift deposits and Cenozoic thermal sag strata. In contrast, the development of the Erris Trough, located on unthinned continental crust, was facilitated by shallow, brittle extension with little deep crustal attenuation. A two-layered crust occurs throughout the region. The lower crustal velocity in the Hatton-Rockall Basin is higher than that in the Rockall Trough. The velocity structure shows no indication of crustal underplating by upper mantle material in the region.
Sea-floor spreading magnetic lineations 25 (59 Ma) and older have been
reidentified in the Labrador Sea by using previous magnetic compilations
and some recently acquired data. The higher density of these
identifications enabled the calculation of a new set of better
constrained rotation poles that describes the sea-floor spreading
history of the Labrador Sea and Baffin Bay in a way that is somewhat
different from previously published reconstructions. The most important
inference that emerges from this work is that the change in spreading
direction between Greenland and North America after anomaly 25 time is
larger than previously recognized. As a result, the position of
Greenland at the time of initial opening (92 Ma) may have been about 100
km farther south than obtained in earlier reconstructions.
Stretching and thinning of the Laurasian continental lithosphere, which had proceeded intermittently from late Palaeozoic through the Mesozoic, reached a climax in early Tertiary times with copious generation of basalt magma, invasion of the attenuated crust as dyke swarms and surface eruption, principally by fissure volcanism. Continued basin subsidence allowed basaltic lavas, erupted at near sea-level, to accumulate to thicknesses which in places (e.g. W Greenland, E Greenland, Faeroes) attained several kilometres. In some zones dyke-swarm injection and crustal attenuation led to sea floor spreading and generation of ocean crust. Fissure swarms not infrequently changed position so that there could be one or more 'unsuccessful' rifting events prior to establishment of a 'successful' spreading axis. While some uncertainty still exists regarding the precise timing of the onset of magmatism in the various zones of failure, the bulk occurred between 60 and 50 Ma. Across the British Isles basalt magmatism occurred early (ca. 60-59 Ma) whereas the great volumes of the E Greenland basalts appear to have erupted rapidly over the interval 54-52 Ma. The onset of magmatism in the Faeroes may have preceded that in E Greenland and possibly that of W Greenland-Baffin Island. Major sill swarms developed, commonly within the Mesozoic sedimentary strata. At favoured channels for magma ascent, commonly controlled by major faults, longer-lived central-vent volcanoes developed. Slow cooling of large magma bodies at such foci produced mafic and ultramafic cumulates; production of salic magmas was mainly confined to these central complexes, the majority of which are located close to the E Greenland coast between ca. 66° and 74°N and a N-S zone through the British Isles from ca. 58°-51°N. Although the salic magmas were predominantly silicic (rhyolitic), feldspathoidal (phonolitic) magmas were important in some of the E Greenland centres. Generation of salic magmas was generally late with respect to the main phases of basalt eruption in the various sectors of activity. However, notable sequences of peraluminous silicic lavas may have preceded basaltic eruption in the Rockall Trough and Vøring Plateau areas. It is probable that magmatic activity in some of the sectors that had experienced the most intense early-phase volcanism persisted, on a small-scale, long after the sea floor spreading had been initiated and activity on the passive margins of both W and E Greenland may not have finally terminated until ca. 30 Ma. The total compositional range of early Tertiary magmas associated with the N Atlantic marginal regions was extensive. Whereas tholeiites, varying from FeTi-rich to N-type MORB, were erupted in greatest volume, compositions varied from picrites, through tholeiites to ferro-basalts, icelandites, dacites and rhyolites. More alkalic mafic magmas included alkali olivine-basalt (and its differentiated products), nephelinites, lamprophyres and melilitites, with some associated carbonatites. Salic magmas ranged widely from silicic to feldspathoidal and from peraluminous to peralkaline.
Volcaniclastic deposits in sedimentary sequences of the North Sea Basin and adjacent areas indicate that two phases of early Palaeogene explosive volcanism took place in the north-eastern Atlantic region. The earlier, late Palaeocene (NP5-NP6) phase involved significant activity along a N-S trend that included both the British and Faeroe-Greenland Tertiary volcanic provinces. The later phase spanned the latest Palaeocene and early Eocene (NP9 to NP13), with much or all of the activity taking place in the Faeroe-Greenland Province. Early ashfalls of mixed basaltic to silicic compositions may have included contributions from the final phase of British volcanism, but were followed by a series of 200 or more tholeiitic ashfalls of Faeroe-Greenland provenance. These tholeiitic eruptions appear to have marked the onset of separation of Greenland from Europe in mid NP10 times. A subsequent return to pyroclastic activity of more variable compositions appears to have marked the re-establishment of stresses within the E Greenland crust that continued throughout the early Eocene (mid NP10 to end NP13). The mechanism of eruption of the tholeiitic ashes, which are equivalent to a magma volume of several thousand cubic kilometres, is uncertain, but they would appear to involve hydrovolcanic processes.
THE concept of crustal plate motion over mantle hotspots has been advanced1 to explain the origin of the Hawaiian and other island chains and the origin of the Walvis, Iceland-Farroe and other aseismic ridges. More recently the pattern of the aseismic ridges has been used in formulating continental reconstructions2. I have shown3 that the Hawaiian-Emperor, Tuamotu-Line and Austral-Gilbert-Marshall island chains can be generated by the motion of a rigid Pacific plate rotating over three fixed hotspots. The motion deduced for the Pacific plate agrees with the palaeomagnetic studies of seamounts4. It has also been found that the relative plate motions deduced from fault strikes and spreading rates agree with the concept of rigid plates moving over fixed hotspots. Fig. 1 shows the absolute motion of the plates over the mantle, a synthesis which satisfies the relative motion data and quite accurately predicts the trends of the island chains and aseismic ridges away from hotspots.
The deep crustal structure of the Norwegian and East Greenland conjugate passive margins has been investigated with two-ship multichannel reflection and refraction techniques in the region from the Iceland-Færøe Ridge to the Greenland Fracture Zone. The data demonstrate the presence of thick (15–20 km) igneous crust that was emplaced along the conjugate margins during the initiation of seafloor spreading. Seaward dipping units comprise the uppermost 3–6 km of the thick crust. The thickness of igneous crust diminishes seaward, with oceanic layer 2 approaching normal thickness nearer to the margin than does oceanic layer 3. Wide-angle reflection techniques, used in conjunction with expanded spread profiling, reveal deep Moho reflections beneath 20 km thick oceanic crust that continue seaward into the ocean basin.