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Abstract

New seismic profiles, bathymetric data and sediment-rock sampling document for the first time the discovery of hydrothermal vent complexes and volcanic cones at 4800-5200 m depth related to recent volcanic and intrusive activity in an unexplored area of the Canary Basin (Eastern Atlantic Ocean, 500 km west of the Canary Islands). A complex of sill intrusions is imaged on seismic profiles showing saucer-shaped, parallel or inclined geometries. Three main types of structures are related to these intrusions. Type I consists of cone-shaped depressions developed above inclined sills interpreted as hydrothermal vents. Type II is the most abundant and is represented by isolated or clustered hydrothermal domes bounded by faults rooted at the tips of saucer-shaped sills. Domes are interpreted as seabed expressions of reservoirs of CH4- and CO2-rich fluids formed by degassing and contact metamorphism of organic-rich sediments around sill intrusions. Type III are hydrothermal-volcanic complexes originated above stratified or branched inclined sills connected by a chimney to the seabed volcanic edifice. Parallel sills sourced from the magmatic chimney formed also domes surrounding the volcanic cones. Core and dredges revealed that these volcanoes, which must be among the deepest in the world, are constituted by OIB-type, basanites with an outer ring of blue-green hydrothermal Al-rich smectite muds. Magmatic activity is dated, based on lava samples, at 0.78±0.05 and 1.61±0.09 Ma (K/Ar methods) and on tephra layers within cores at 25-237 ky. The Subvent hydrothermal-volcanic complex constitutes the first modern system reported in deep-water oceanic basins related to intraplate hotspot activity.

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... The eastern Canary Basin is one of the few places in the world where to study in the Quaternary the interaction and relationship among volcanism, tectonic and giant submarine landslide. Here, an intensive research has been carried out in fields such as the Quaternary or current volcanic and hydrothermal activity (Klügel et al., 2020;van den Bogaard, 2013;Medialdea et al., 2017;Somoza et al., 2017), gravitational instabilities and mass-transport deposits (MTDs) (Georgiopoulou et al., 2009(Georgiopoulou et al., , 2010Hunt et al., 2011Hunt et al., , 2013Hunt et al., , 2014Palomino et al., 2016;León et al., 2019), the relationship between MTDs and volcanic activity (Hunt et al., 2014;Hunt and Jarvis, 2017;León et al., 2019) and the presence of critical metallic elements (Marino et al., 2017). However, only a few of these studies have focused on the relationship between tectonics, volcanic activity and seafloor morphology Sánchez-Guillamón et al., 2018a, 2018b. ...
... This tectonic fabric also controls the bulges, domes and volcanic reliefs (Figs. 12, 13 and 14). In MCS profiles, the NNE-SSW structures controlling the structural reliefs are deep-rooted in the oceanic basement (Fig. 10D) with the downthrown block seaward, as evidenced by Medialdea et al. (2017) and Reston et al. (2004). We infer that the NNE-SSW trend reveals the oceanic fabric related to the ridge axis fabric (the abyssal hill fabric), which reflects the oceanic basement blocks controlled by normal faults perpendicular to the oceanic fracture zone. ...
... In the Lower Pleistocene, this volcano-tectonic activity decreased to practical inactivity in the southern CIVP. The large MTDs in branches V and VI ceased and the tectonic and volcano-tectonic activity receded to a local and latent activity appearing only as local hydrothermal sites (Subvent Area; Medialdea et al., 2017), structural reliefs, local seafloor instabilities (e.g. the San Borondón crest; Fig. 7A) or small flank collapses (e.g. the MTDs to the north of the Echo seamount; Fig. 7D). Based on only geomorphological indicators, the latent volcano-tectonic activity could be possible extended to the Holocene. ...
Article
This paper integrates sedimentary, tectonic and volcanic geological processes inside a model of volcano-tectonic activity in oceanic intraplate domains related to rifted continental margins. The study case, the eastern Canary Basin (NE Atlantic), is one of the few places in the world where giant MDTs and Quaternary volcanic and hydrothermal edifices take place in intraplate domains. In this paper, we analyse how two structural systems (WNW-ESE and NNE-SSW) matching with the oceanic fabric control the location of volcanic systems, seafloor tectonic reliefs and subsequently the distribution of main sedimentary systems. Linear turbidite channels, debris flow lobes and the lateral continuity of structural and volcanic reliefs follow a WNW-ESE trend matching the tracks of the oceanic fracture zones. Furthermore, escarpments, anticline axes and volcanic ridges follow a NNE-SSW trend matching normal faults delimiting blocks of oceanic basement. The morpho-structural analysis of all the above geomorphological features shows evidence of a volcanic and tectonic activity from the middle–upper Miocene to the Lower–Middle Pleistocene spread over the whole of the eastern Canary Basin that reached the western Canary Islands. This reactivation changes the paradigm in the seamount province of Canary Islands reported inactive since Cretaceous. A tecto-sedimentary model is proposed for this period of time that can be applied in other intraplate domains of the world. A tectonic uplift in the study area with a thermal anomaly triggered volcanic and hydrothermal activity and the subsequent flank collapse and emplacement of mass transport deposits on the Western Canary Slope. Furthermore, this uplift reactivated the normal basement faults, both trending WNW-ESE and NNE-SSW, generating folds and faults that control the location of turbidite channels, escarpments, mass transport deposits and volcanic edifices.
... At volcanically active seamounts, vigorous hydrothermal activity can be driven by magmatic heat sources at comparatively shallow levels, which is common at arc settings (Butterfield, 2000;de Ronde and Stucker, 2015;Caratori Tontini et al., 2019) but also at some intraplate volcanoes. Examples are very rare, however, and are mostly confined to active systems (Sakai et al., 1987;Staudigel et al., 2004;German et al., 2020) with few exceptions (Medialdea et al., 2017). ...
... In the SW, locally elevated heat flow values coincide with zones of seismic amplitude blanking (Figure 4A), i.e., reduction of the amplitude of seismic reflections caused, e.g., by the presence of gas hydrates (Lee and Dillon, 2001). A prominent blanking zone 10 km southwest of the seamount summit shows upward bending of adjacent reflectors, resembling doming linked to deeper magmatic intrusions and related hydrothermal activity (Berndt et al., 2016;Medialdea et al., 2017). This feature seems to be rather old because the overlying reflectors are not bent; it is also not accompanied by a heat flow anomaly ( Figure 4B). ...
... If the development of hydrothermal circulation and chemosynthetic communities at Henry Seamount was indeed a consequence of single magmatic pulses, then similar scenarios might be envisaged for many other volcanic seamounts in the deep ocean basins. Whether they are monogenetic or form by a succession of eruptions over a long period of time, each eruption has the potential to drive ephemeral hydrothermal activity (e.g., Medialdea et al., 2017;German et al., 2020), which may provide habitats for chemosynthetic communities. The same holds for the submarine flanks of volcanic islands that are often scattered with presumably monogenetic volcanic cones (e.g., Santana-Casiano et al., 2016). ...
Article
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Our knowledge of venting at intraplate seamounts is limited. Almost nothing is known about past hydrothermal activity at seamounts, because indicators are soon blanketed by sediment. This study provides evidence for temporary hydrothermal circulation at Henry Seamount, a re-activated Cretaceous volcano near El Hierro island, close to the current locus of the Canary Island hotspot. In the summit area at around 3000–3200 m water depth, we found areas with dense coverage by shell fragments from vesicomyid clams, a few living chemosymbiotic bivalves, and evidence for sites of weak fluid venting. Our observations suggest pulses of hydrothermal activity since some thousands or tens of thousands years, which is now waning. We also recovered glassy heterolithologic tephra and dispersed basaltic rock fragments from the summit area. Their freshness suggests eruption during the Pleistocene to Holocene, implying minor rejuvenated volcanism at Henry Seamount probably related to the nearby Canary hotspot. Heat flow values determined on the surrounding seafloor (49 ± 7 mW/m ² ) are close to the expected background for conductively cooled 155 Ma old crust; the proximity to the hotspot did not result in elevated basal heat flow. A weak increase in heat flow toward the southwestern seamount flank likely reflects recent local fluid circulation. We propose that hydrothermal circulation at Henry Seamount was, and still is, driven by heat pulses from weak rejuvenated volcanic activity. Our results suggest that even single eruptions at submarine intraplate volcanoes may give rise to ephemeral hydrothermal systems and generate potentially habitable environments.
... These magmatic intrusions have important implications for hydrocarbon exploration (Hansen et al., 2008;Holford et al., 2012), metal mineralization (Nelson, 2000), global climate change , and basin-scale processes . Examples of sill-dome structures have been well described in the southern Australian margin , the Norwegian Sea (Planke et al., 2005;Omosanya et al., 2017), the eastern central Atlantic (Medialdea et al., 2017;Sánchez-Guillamón et al., 2018a, 2018b and other worldwide magma-rich margins. Igneous intrusions are also widely distributed among the South China Sea (SCS) basins and continental slopes (Yan et al., 2006;Song et al., 2017;Wang et al., 2019). ...
... Igneous intrusions may take various forms when emplaced in sedimentary layers (Lee et al., 2006), among which sills are the most common ones. Emplacement of igneous sills within sediments can result in the development of forced folds (Hansen and Cartwright, 2006;Jackson et al., 2013;Sun et al., 2014;Omosanya et al., 2017;Zhang et al., 2017) and/or formation of hydrothermal vent complexes Svensen et al., 2004;Planke et al., 2005;Hansen et al., 2008;Magee et al., 2015;Medialdea et al., 2017;Omosanya et al., 2018;Wang et al., 2019). These sill-related forced folds typically manifest as domes on the seafloor (Sánchez-Guillamón et al., 2018a, 2018b, and some may be overlain by younger strata, which will date the timing of intrusion event (Trude et al., 2003;Hansen and Cartwright, 2006;Jackson et al., 2013). ...
... These gas-rich fluids will firstly migrate toward the edges of the sill (Iyer et al., 2013) and generate the peripheral faults rooted at both ends of the intrusion (e.g. Medialdea et al., 2017). The gradual accumulation of gas-rich fluids filled in sediments may uplift the overlying strata bound by the peripheral faults, which are shown as parallel convex reflections immediately above the igneous sill, called as forced folds/domes in the study area (Hansen and Cartwright, 2006). ...
Article
Magmatism can exert significant impact on sedimentary basins such as the Zhongjiannan Basin (ZJNB), western South China Sea. We have evaluated multibeam bathymetric and multichannel seismic reflection data acquired by the Guangzhou Marine Geological Survey in recent years, in order to investigate the distribution, the characteristics and the subsurface structures related to seafloor domes found in the northeastern ZJNB. Our data revealed forty-two domes at water depths between 2312 m and 2870 m, which are clustered around volcanic mounds, large seamounts and along the edge of the central depression in the study area. These domes are generally circular to elongate or irregular in plan view with large basal areas, and they also have gentler flanks (dips of 1.46°–7.73°) with vertical reliefs ranging from tens to hundreds of meters. In seismic sections, majority of the domes are underlain by variably shaped and complex magmatic sills, which provide a cause-effect relationship between domes formation and igneous intrusions. These intrusions heat surrounding organic-rich sediments, release hydrocarbons, fluidize sediment pore waters and form gas-rich fluids, which fill in sediment and uplift overlying strata immediately above the sills to form forced folds, which are manifested as seafloor domes. These sill-folds-dome structures have important implications for understanding geomorphologic features caused by sills emplaced at depth.
... Submarine magmatic structures such as volcanoes, vents and intrusions often occur in rifted margins, ocean spreading centre, intraplate hot spots and arcs along subduction zones (De Ronde et al., 2005;Harding et al., 2017;Hekinian et al., 1991;Ingebritsen, Geiger, Hurwitz, & Driesner, 2010;Langmuir et al., 1997;Magee, Jackson, & Schofield, 2013;Medialdea et al., 2017;Planke, Symonds, Alvestad, & Skogseid, 2000, Planke, Rasmussen, Rey, & Myklebust, 2005Sun et al., 2014;Wheeler et al., 2013). They have been extensively studied over the past decades due to their importance for influences on basin tectonic evolution, petroleum exploration and submarine mineral deposits (Darros De Matos, 2000;Fjeldskaar, Helset, Johansen, Grunnaleite, & Horstad, 2008;Medialdea et al., 2017;Petersen et al., 2016;Pirajno & Van Kranendonk, 2005;Sun, Wu, Cartwright, Lüdmann, & Yao, 2013). ...
... Submarine magmatic structures such as volcanoes, vents and intrusions often occur in rifted margins, ocean spreading centre, intraplate hot spots and arcs along subduction zones (De Ronde et al., 2005;Harding et al., 2017;Hekinian et al., 1991;Ingebritsen, Geiger, Hurwitz, & Driesner, 2010;Langmuir et al., 1997;Magee, Jackson, & Schofield, 2013;Medialdea et al., 2017;Planke, Symonds, Alvestad, & Skogseid, 2000, Planke, Rasmussen, Rey, & Myklebust, 2005Sun et al., 2014;Wheeler et al., 2013). They have been extensively studied over the past decades due to their importance for influences on basin tectonic evolution, petroleum exploration and submarine mineral deposits (Darros De Matos, 2000;Fjeldskaar, Helset, Johansen, Grunnaleite, & Horstad, 2008;Medialdea et al., 2017;Petersen et al., 2016;Pirajno & Van Kranendonk, 2005;Sun, Wu, Cartwright, Lüdmann, & Yao, 2013). ...
... The magmatic hydrothermal systems were defined as aqueous fluid systems derived from or influenced by magma bodies (Ingebritsen et al., 2010;Pirajno & Van Kranendonk, 2005). They have been well studied at scales ranging from several kilometres to hundreds of kilometres in the mid-ocean ridges, volcanic islands, passive margins and magmatic arcs along subduction zones, and play an important role in linking among the lithosphere, hydrosphere and biosphere (De Ronde et al., 2005;Gay et al., 2012;Hansen, 2006;Haymon, 1996;Ingebritsen et al., 2010;Lowell, 1991;Lowell & Germanovich, 1994;Medialdea et al., 2017;Planke, Rasmussen, Rey, & Myklebust, 2005;Reynolds et al., 2017;Wheeler et al., 2013). Consequently magmatically induced fluid flows may have a role in volcanically active regions such as the SCS. ...
Article
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Submarine magmatism and associated hydrothermal fluid flows has significant feedback influence on the petroleum geology of sedimentary basins. This study uses new seismic profiles and multi‐beam bathymetric data to examine the morphology and internal architecture of post‐seafloor spreading magmatic structures, especially volcanoes of the Xisha uplift, in extensive detail. We discover for the first time hydrothermal systems derived from magmatism in the northwestern South China Sea. Numerous solitary volcanoes and volcanic groups occur in the Xisha uplift and produce distinct seismic reflections together with plutons. Sills and other localized amplitude anomalies were fed by extrusions/intrusions and associated fluid flows through fractures and sedimentary layers that may act as conduits for magma and fluid flows transport. Hydrothermal structures such as pipes and pockmarks mainly occur in the proximity of volcanoes or accompany volcanic groups. Pipes, pockmarks and localized amplitude anomalies mainly constitute the magmatic hydrothermal systems, which are probably driven by post‐seafloor spreading volcanoes/plutons. The hydrothermal fluid flows released by magma degassing or/and related boiling of pore fluids/ metamorphic dehydration reactions in sediments produced local over‐pressures, which drove upward flow of fluid or horizontal flow into the sediments or even seafloor. Results shows that post‐seafloor spreading magmatic activity is more intense during a 5.5 Ma event than one in 2.6 Ma whereas the hydrothermal activities are more active during 2.6 Ma than in 5.5 Ma. Our analysis indicates that post‐seafloor spreading magmatism may have a significant effect on hydrocarbon maturation and gas hydrate formation in the Xisha uplift and adjacent petroliferous basins. Consequently the study presented here improves our understanding of hydrocarbon exploration in the northwestern South China Sea. This article is protected by copyright. All rights reserved.
... Here, the term "mound" is used loosely to refer to all submarine edifices regardless of their basal shape and tentative origin. The genesis of these features was first reported and characterised by Medialdea et al. (2017), and has been associated with buried intrusive complexes accompanying volcanic and hydrothermal activity. ...
... MT1 shows a proportional relationship between size and slope variables, reaching only 30 m in height, having a mean slope of 1.3º, and mean widths of around 2.4 km. The circular shape of these mounds can presumably be linked to the geometry of the underlying systems: dometype forced folds related to magmatic intrusions (Medialdea et al., 2017). A secondary ...
... The main acoustic characteristics of this type of mound are represented by ET1 (Figs. 7, 10). Up-doming of these units is attributed to the progressive generation of overpressure and differential compaction at depth (Williams, 1987), probably related to folds formed due to the inclined sill-type intrusions (Medialdea et al., 2017). Small offsets in these mounds are associated with small-scale faulting observed in HRPP (i.e., M34 in Fig. 7C) distributed preferentially along the summit and flanks. ...
Article
The increasing volume of high-resolution multibeam bathymetry data collected along continental margins and adjacent deep seafloor regions is providing further opportunities to study new morphological seafloor features in deep water environments. In this paper, seafloor mounds have been imaged in detail with multibeam echosounders and parametric sub-bottom profilers in the deep central area of the Canary Basin (~350–550 km west off El Hierro Island) between 4800 and 5200 mbsl. These features have circular to elongated shapes with heights of 10 to 250 m, diameters of 2–24 km and with flank slopes of 2–50°. Based on their morphological features and the subsurface structures these mounds have been classified into five different types of mounds that follow a linear correlation between height and slope but not between height and size. The first, second (Subgroup A), and third mound-types show heights lower than 80 m and maximum slopes of 35° with extension ranging from 2 to 400 km2 and correspond to domes formed at the surface created by intrusions located at depth that have not outcropped yet. The second (Subgroup B), fourth, and fifth mound-types show higher heights up to 250 m high, maximum slopes of 47° and sizes between 10 and 20 km2 and are related to the expulsion of hot and hydrothermal fluids and/or volcanics from extrusive deep-seated systems. Based on the constraints on their morphological and structural analyses, we suggest that morphostructural types of mounds are intimately linked to a specific origin that leaves its footprint in the morphology of the mounds. We propose a growth model for the five morphostructural types of mounds where different intrusive and extrusive phenomena represent the dominant mechanisms for mound growth evolution. These structures are also affected by tectonics (bulge-like structures clearly deformed by faulting) and mass movements (slide scars and mass transport deposits). In this work, we report how intrusive and extrusive processes may affect the seafloor morphology, identifying a new type of geomorphological feature as ‘intrusive’ domes that have, to date, only been reported in fossil environments but might extend to other oceanic areas.
... These seafloor features are both circular and elongated in shape, with diameters ranging from 2 to 24 km, heights of up to 250 m, and flank slopes ranging from 2 to 24 • [40]. In earlier studies, they were characterized by [41] as various types of structures including both hydrothermal domes and different types of volcanoes related to recent volcanic and intrusive activity. The authors of [42] also classified them into five morphostructural types of edifices (MT1 to MT5), intimately linked to specific origins ( Figure 1B). ...
... [42]. The highlighted mounds are categorized according to their origin following [41]. ...
... This basin has been characterized as having a heterogeneous distribution of various volcanic elevations including seamounts, hills, and seafloor mounds [42,44]. Nevertheless, in the central area of this basin, known as the Subvent Area, these seafloor mounds are hydrothermal domes and scattered volcanoes related to Quaternary intrusive activity that gave rise to a huge magmatic sill complex together with volcanic activity [41]. Indeed, [42]. ...
Article
Derived digital elevation models (DEMs) are high-resolution acoustic technology that has proven to be a crucial morphometric data source for research into submarine environments. We present a morphometric analysis of forty deep seafloor edifices located to the west of Canary Islands, using a 150 m resolution bathymetric DEM. These seafloor structures are characterized as hydrothermal domes and volcanic edifices, based on a previous study, and they are also morphostructurally categorized into five types of edifice following an earlier classification. Edifice outline contours were manually delineated and the morphometric variables quantifying slope, size and shape of the edifices were then calculated using ArcGIS Analyst tools. In addition, we performed a principal component analysis (PCA) where ten morphometric variables explain 84% of the total variance in edifice morphology. Most variables show a large spread and some overlap, with clear separations between the types of mounds. Based on these analyses, a morphometric growth model is proposed for both the hydrothermal domes and volcanic edifices. The model takes into account both the size and shape complexity of these seafloor structures. Grow occurs via two distinct pathways: the volcanoes predominantly grow upwards, becoming large cones, while the domes preferentially increase in volume through enlargement of the basal area.
... Seismic profiles across these domes show an uplifted and faulted sedimentary cover overlying an abrupt, high-amplitude terminating reflector, often displaying a saucer-shaped profile (Figure 7a,b). These deep reflectors are consistent with shallow magmatic intrusions (sills or laccoliths), which would induce domal uplift and faulting of the overlying sedimentary cover [Kumar et al., 2022, Medialdea et al., 2017, Montanari et al., 2017, Omosanya et al., 2017. We interpret these domes as forced folds by analogy to the description of Paquet et al. [2019] and other sources [Montanari et al., 2017, and references therein]. ...
Article
Full-text available
Geophysical and geological data from the North Mozambique Channel acquired during the 2020–2021 SISMAORE oceanographic cruise reveal a corridor of recent volcanic and tectonic features 200 km wide and 600 km long within and north of Comoros Archipelago. Here we identify and describe two major submarine tectono-volcanic fields: the N’Droundé province oriented N160°E north of Grande-Comore Island, and the Mwezi province oriented N130°E north of Anjouan and Mayotte Islands. The presence of popping basaltic rocks sampled in the Mwezi province suggests post-Pleistocene volcanic activity. The geometry and distribution of recent structures observed on the seafloor are consistent with a current regional dextral transtensional context. Their orientations change progressively from west to east (${\sim }$N160°E, ${\sim }$N130°E, ${\sim }$EW). The volcanism in the western part appears to be influenced by the pre-existing structural fabric of the Mesozoic crust. The 200 km-wide and 600 km-long tectono-volcanic corridor underlines the incipient Somalia–Lwandle dextral lithospheric plate boundary between the East-African Rift System and Madagascar.
... CSF02 is also located east of Mayotte, on the border of a topographic dome 10 km in diameter and 30 m in height (Figure 3). In volcanic areas, such morphology corresponds to a forced fold, often related to the intrusion of a saucer-shaped sill at depth and described in various geological contexts [Jackson et al., 2013, Medialdea et al., 2017, Magee et al., 2017, as well as experimentally reproduced [Galland, 2012]. On a seismic profile across the site, we observe that doming affects the underlying sedimentary succession (0.5 s twtt) down to an older volcanic layer that affects the seismic image and precludes sill localization ( Figure 3). ...
Article
Full-text available
Heat flow in the Northern Mozambique Channel is poorly constrained, with only a few old measurements indicating relatively low values of 55–$62~\text{mW/m}^2$. During the SISMAORE cruise to the Northern Mozambique Channel, we obtained new heat flow measurements at four sites, using sediment corers equipped with thermal probes. Three of the sites yield values of 42–47 $\text{mW/m}^2$, confirming low regional heat flow in this area. Our values are consistent with a Jurassic oceanic lithosphere around Mayotte, although the presence of very thin continental crust or continental fragments could also explain the observed heat flow. Our values do not support a regional thermal anomaly and so do not favor a hotspot model for Mayotte. However, at a fourth site located 30 km east of the submarine volcano that appeared in 2018 east of Mayotte, we measured a very high heat flow value of $235~\text{mW/m}^2$, which we relate to the circulation of hot fluids linked to recent magmatic activity.
... ures 3A, B, 4, 6, and 7) [Medialdea et al., 2017]. We thus interpret the disturbed seismic facies in the seal bypass systems as the result of a network of almost vertical dykes or fractures, not imaged in seismic reflection, in which fluids and/or melt are rising from crustal or sub-crustal levels, up to the submarine volcanic edifices. ...
Article
Full-text available
A multichannel seismic reflection profile acquired during the SISMAORE cruise (2021) provides the first in-depth image of the submarine volcanic edifice, named Fani Maore, that formed 50 km east of Mayotte Island (Comoros Archipelago) in 2018–2019. This new edifice sits on a 140 m thick sedimentary layer, which is above a major, volcanic layer up to 1 km thick and extends over 120 km along the profile. This volcanic unit is made of several distinct seismic facies that indicate successive volcanic phases. We interpret this volcanic layer as witnessing the main phase of construction of the Mayotte Island volcanic edifice. A 2.2–2.5 km thick sedimentary unit is present between this volcanic layer and the top of the crust. A complex magmatic feeder system is observed within this unit, composed of saucer-shape sills and seal bypass systems. The deepest tip of this volcanic layer lies below the top-Oligocene seismic horizon, indicating that the volcanism of Mayotte Island likely began around 26.5 Ma, earlier than previously assumed. https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.5802/crgeos.154/
... Existing seismic examples covering such structures are rare within the literature and mainly rely on the interpretation of post-stack seismic sections in the time domain (e. g., Medialdea et al., 2017). Here, we present depth-migrated seismic data covering several shallow-level magmatic systems, as well as a domal structure related to a magmatic intrusion. ...
Conference Paper
METEOR Cruise M150 BIODIAZ provided material from sublittoral down to deep-sea stations to incorporate innovative aspects into the study of seamount and island productivity and their potential role for the establishment of benthic assemblages comprising all size classes (George et al. 2021). The aim was to get a baseline on the diversity, faunal composition and distribution of shelf and deep-sea taxa and related sediments from three different Azorean islands (Flores, Terceira, and Santa Maria) and two adjacent seamounts (Princess Alice Bank, Formigas Bank). Such baseline shall serve to prove fundamental hypotheses regarding the role of seamounts/islands for marine organisms and the principle (bio)-sedimentary processes in the evolution of seamounts. The significance of potential endemism in zoobenthic communities based on the extensively sampled material is studied in the context of the geologic age, topographic isolation, phytoplankton productivity and diversity of the systems.
... The buried volcanoes and seamounts are generally identified based on their external geometries and internal seismic attributes on seismic profiles (Niyazi et al., 2021). Buried volcanoes and seamounts exist dome-shaped external geometries (Figs. 6, 9 and 10), high-amplitude top reflections, and chaotic or blank internal reflections (Figs. 9 and 10) (Magee et al., 2015;Zhang et al., 2016;Medialdea et al., 2017). Buried volcanoes are covered by sedimentary layers (Figs. 9, and 10), and seamounts have their tops exposed to the seafloor (Fig. 6). ...
Article
The South China Sea, which is located to the southeast of Eurasian continent, developed as the result of intra-continental rifting and seafloor spreading on the South China margin. Passive margins are traditionally classified as one of two end-member types, the magma-rich and magma-poor margins, based on the relative abundance or scarcity of magmatism during rifting and breakup. Previous studies suggest that the northern margin of the South China Sea is a magma-poor margin for lacking abundant magmatic activities during the breakup. A growing body of work is beginning to recognize that significant, widespread magmatism may also be present on margins that are presently considered to be magma-poor margins. In this study, we use high resolution 2D/3D seismic profiles, industrial well data, basalt geochemistry (major oxides, trace elements and isotopes) and published results to outline post-rift magmatism developed within the northern margin of the South China Sea. Four magmatic stages occurred since lithospheric breakup. The first stage, from 32 to 23.6 Ma and mainly within the distal margin of the northern South China Sea, was dominated by magma intrusions and corresponded to the spreading of the East Sub-basin. The second stage, from 23.6–19.1 Ma, was mainly in the Baiyan Sag and surrounding uplifts and explosive eruptions dominated. The third Stage, from 19.1–10 Ma, was east of the Zhu III Depression and west of the Enping Sag and dominated by quiet eruptions. The fourth stage, since 10 Ma, and widely distributed in the southern Dongsha uplift, experienced scattered volcanic eruptions. Two basalt samples from 23.6–19.1 Ma (industrial well HJ1, Stage 2) and seven samples from 19.1–10 Ma (industrial well EP1, Stage 3) are analyzed for major oxides, trace elements and Sr-Nd-Pb-Hf isotope compositions. Geochemical features of trace elements show that these samples are characterized by OIB-like basalts, being highly enriched in LREEs (light rare earth elements) relative to HREEs (heavy rare earth elements). Geochemical features of Sr-Nd-Pb-Hf isotope compositions show that the samples all resemble ocean-island basalts with two mixing endmembers: depleted mid-ocean ridge basalt mantle (DMM) and enriched mantle II (EMII). Pb isotopic characteristics show the Dupal isotope anomaly in the northern margin of the South China Sea. And geochemistry data of all the samples signals the contribution of Hainan Plume. Previous studies have revealed the existing of southeastward mantle flow from Tibet to South China Sea and a branch of Hainan Plume existing beneath the northern South China Sea using geophysical methods by different researchers. Based on our latest research and published geological evidences by other researchers, we propose that Stage 1 magmatism was caused by southeastward mantle flow stemming from Indo-Eurasian collision, the stage 2 and stage 3 magmatism was caused by Hainan Plume and the activation of Yangjiang-Yitongansha Faut Zone, whereas the last stage magmatic activity was mainly related with the combination of Hainan Plume and activation of the faults caused by the subduction of the SCS beneath the Luzon Arc at the Manila trench.
... Alternatively, the acoustically transparent mounds off Madeira could be interpreted as structures linked to the migration and escape of over-pressurized fluids within the sedimentary column, such as mud volcanoes and domes. Mud volcanoes, domes and associated pockmarks are widely recognized features in continental margins (e.g., Judd and Hovland, 2007) and deep-water environments (e.g., Medialdea et al., 2017;Sánchez-Guillamón et al., 2018a;Sánchez-Guillamón et al., 2018b). The isolated mounds recognized off Madeira Island share some characteristics with acoustically transparent mounded features described by Rebesco et al. (2007) in Antarctica's distal sediment drifts. ...
Article
The deep-water sedimentary processes and morphological features offshore Madeira Island, located in the Central-NE Atlantic have been scantly studied. The analysis of new multibeam bathymetry, echo-sounder profiles and few multichannel seismic reflection profiles allowed us to identify the main geomorphologies, geomorphic processes and their interplay. Several types of features were identified below 3800 m water depth, shaped mainly by i) the interplay between northward-flowing Antarctic Bottom Water (AABW) and turbidity currents and ii) interaction of the AABW with oceanic reliefs and the Madeira lower slope. Subordinate and localized geomorphic processes consist of tectono-magmaticslope instability,turbidity currents and fluid migration, . The distribution of the morphological features defines three regional geomorphological sectors. Sector 1 represents a deep-seafloor with its abyssal hills, basement highs and seamounts inherited from Early Cretaceous seafloor spreading. Sector 2 is exclusively shaped by turbidity current flows that formed channels and associated levees. Sector 3 presents a more complex morphology dominated by widespread depositional and erosional features formed by AABW circulation, and localized mixed contourite system developed by the interplay between the AABW circulation and WNW-ESE-flowing turbidite currents. The interaction of the AABW with abyssal hills, seamounts and basement ridges leads to the formation of several types of contourites: patchdrifts, double-crest mounded bodies, and elongated, mounded and separated drifts. The patch drifts formed downstream of abyssal hills defining an previously unknown field of relatively small contourites. We suggest they may be a result of localized vortexes that formed when the AABW's flow impinges these oceanic reliefs producingthe erosional scours that bound these features. The bottom currents in the area are known to be too weak (1–2 cm s⁻¹) to produce the patch drifts and scours. Therefore, we suggest that these features could be relics at present, having developed when the AABW was stronger than today, as during glacial/end of glacial stages.
... Low-T hydrothermal vents after violent submarine volcanic eruptions generate long-term CO 2 inputs to oceans due to the continuous degasification of the magmatic systems mainly placed on hot-spot volcanic islands like Hawaii or Canary Islands. This is due to the high contents in C bearing in the thick oceanic sediments below the submarine volcanoes that are expulsed by low-T hydrothermal vent systems [88]. ...
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In this work, we integrate five case studies harboring vulnerable deep-sea benthic habitats in different geological settings from mid latitude NE Atlantic Ocean (24–42° N). Data and images of specific deep-sea habitats were acquired with Remoted Operated Vehicle (ROV) sensors (temperature, salinity, potential density, O2, CO2, and CH4). Besides documenting some key vulnerable deep-sea habitats, this study shows that the distribution of some deep-sea coral aggregations (including scleractinians, gorgonians, and antipatharians), deep-sea sponge aggregations and other deep-sea habitats are influenced by water masses’ properties. Our data support that the distribution of scleractinian reefs and aggregations of other deep-sea corals, from subtropical to north Atlantic could be dependent of the latitudinal extents of the Antarctic Intermediate Waters (AAIW) and the Mediterranean Outflow Waters (MOW). Otherwise, the distribution of some vulnerable deep-sea habitats is influenced, at the local scale, by active hydrocarbon seeps (Gulf of Cádiz) and hydrothermal vents (El Hierro, Canary Island). The co-occurrence of deep-sea corals and chemosynthesis-based communities has been identified in methane seeps of the Gulf of Cádiz. Extensive beds of living deep-sea mussels (Bathymodiolus mauritanicus) and other chemosymbiotic bivalves occur closely to deep-sea coral aggregations (e.g., gorgonians, black corals) that colonize methane-derived authigenic carbonates.
... level and related to hydrothermal-volcanic activity (Medialdea et al., 2017;Sanchez-Guillamón et al., 2018). Magma injection causes differential uplifting, forced folding and faulting of the overlying sedimentary layers, and can induce the transport of hot fluids to the surface. ...
Article
A detailed morpho-bathymetric study of the Comoros archipelago, based on mostly unpublished bathymetric data, provides a first glimpse into the submarine section of these islands. It offers a complete view of the distribution of volcanic structures around the archipelago, allowing to discuss the origin and evolution of this volcanism. Numerous volcanic cones and erosional-depositional features have been recognized throughout the archipelago. The magmatic supply is focused below one or several volcanoes for each island, but is also controlled by lithospheric fractures evidenced by volcanic ridges, oriented along the supposed Lwandle-Somali plate boundary. Massive mass-wasting morphologies also mark the submarine flanks of each island. Finally, the submarine geomorphological analysis made possible to propose a new scheme for the succession of the island's growth, diverging from the east-west evolution previously described in the literature.
... TM software for interpretation. Interval seismic velocities calculated by Medialdea et al. (2017) have been used. These authors have checked these data with available regional seismic velocity information and are in accordance with those reported in DSDP Sites 137-139 (Hayes et al., 1972). ...
Article
A new temporal history of mass wasting processes for the west of the Canary volcanic province is presented. Its onset has been estimated in the middle–upper Miocene (∼13.5 ± 1.2 Ma), matching with a critical period of construction for this volcanic province. Seismic profiles show an emplacement longevity (from the Miocene to Quaternary) in multiple events, defined by stacked lobes of debrites, linked to the flank collapses and volcanic avalanches of the volcanic edifices (islands and seamounts). An evolution of pathways and source areas has been detected from east (Miocene) to west (Quaternary); as well as a migration of the activity to the northwest (west of the Canary Islands: e.g. El Hierro and La Palma). Six connected branches (I–VI), three of them described for the first time here, of Quaternary seismic units of mass transport deposits (MTDs) have been characterized. The Pleistocene makes up a huge buried MTDs system, until now unknown, pointing a new mass transport sedimentological scenario. Finally, the two southernmost branches (V–VI), up to now unknown, are a mainly buried system of stacked and terraced lobes of debrites sourced mainly from the flank collapses of the volcanic seamounts of the Canary Island Seamount Province, apparently inactive from upper Cretaceous.
... TM software for interpretation. Interval seismic velocities calculated by Medialdea et al. (2017) have been used. These authors have checked these data with available regional seismic velocity information and are in accordance with those reported in DSDP Sites 137-139 (Hayes et al., 1972). ...
Article
A new temporal history of mass wasting processes for the west of the Canary volcanic province is presented. Its onset has been estimated in the middle–upper Miocene (∼13.5 ± 1.2 Ma), matching with a critical period of construction for this volcanic province. Seismic profiles show an emplacement longevity (from the Miocene to Quaternary) in multiple events, defined by stacked lobes of debrites, linked to the flank collapses and volcanic avalanches of the volcanic edifices (islands and seamounts). An evolution of pathways and source areas has been detected from east (Miocene) to west (Quaternary); as well as a migration of the activity to the northwest (west of the Canary Islands: e.g. El Hierro and La Palma). Six connected branches (I–VI), three of them described for the first time here, of Quaternary seismic units of mass transport deposits (MTDs) have been characterized. The Pleistocene makes up a huge buried MTDs system, until now unknown, pointing a new mass transport sedimentological scenario. Finally, the two southernmost branches (V–VI), up to now unknown, are a mainly buried system of stacked and terraced lobes of debrites sourced mainly from the flank collapses of the volcanic seamounts of the Canary Island Seamount Province, apparently inactive from upper Cretaceous.
... Alternatively, the acoustically transparent mounds off Madeira could be interpreted as structures linked to the migration and escape of over-pressurized fluids within the sedimentary column, such as mud volcanoes and domes. Mud volcanoes, domes and associated pockmarks are widely recognized features in continental margins (e.g., Judd and Hovland, 2007) and deep-water environments (e.g., Medialdea et al., 2017;Sánchez-Guillamón et al., 2018a;Sánchez-Guillamón et al., 2018b). The isolated mounds recognized off Madeira Island share some characteristics with acoustically transparent mounded features described by Rebesco et al. (2007) in Antarctica's distal sediment drifts. ...
... Therefore, the seismicity distribution seems to be correct and may track an inclined magmatic structure. A recent study on seismic images of sills offshore El Hierro by Medialdea et al. (2017) shows the possibility of a wide range of sill shapes including inclined sills at depth. Moreover, a 3D tomography of El Hierro by Martí et al. (2017) revealed an inclined structure of low velocity at a depth of 12-16 km. ...
Article
Six different magmatic intrusions were detected around El Hierro Island in the two years that followed the end of the 2011–2012 submarine eruption. Each intrusion lasted between few days to three weeks and produced intense seismic swarms and rapid ground deformation. We performed a hypoDD relocation of >6000 earthquakes and inverted the GPS data in order to obtain the location of the magma source of each intrusion. Each episode presents a spatial gap between seismicity and magma source of 3–8 km with the earthquakes located always deeper than the deformation sources. We propose a magma plumbing system consisting on a deep structure injecting magma to a more ductile shallower location beneath El Hierro crust. While the seismicity is associated with the deeper structure, the ascent and accumulation of magma at shallower level deforms the crust aseismically. The mechanism of most of these episodes consists of an initial injection of magma producing most of the ground deformation and high b-values of the seismicity indicating fluid fractures during the first days and finishes with high magnitude earthquakes and low b-values indicating an overpressure of the injection process. There is a correlation between the seismic and geodetic moment ratio and the direction of propagation of each intrusion towards one of the volcanic rifts of the island, suggesting the possible existence of a deep structure beneath the island related with to the triaxial origin of the island. This work presents important advances in the knowledge of monogenetic magmatic intrusions and, specifically, in those occurred in El Hierro Island between 2011 and 2014, with important implications for future volcano monitoring in the Canary Islands.
... The sill complex may have fed dykes that locally reached the surface generating volcanoes, which are laterally offset with respect to the two main fault systems. Sill complexes may provide efficient magma flow pathways, transporting magma to the surface over great vertical and lateral distances, as suggested by several authors in different tectonic contexts (e.g., Magee et al., 2016;Medialdea et al., 2017). ...
Article
The tectonic framework of the northern sector of the Capo Granitola-Sciacca Fault Zone (CGSFZ), a NNE-oriented lithospheric strike-slip fault zone located in the Sicilian Channel (southern Italy), has been reconstructed with the aim to clarify the relationships between geometry and kinematics of the structures and the occurrence and distribution of the magmatic manifestations observed in the area. This has been achieved by the interpretation of a large dataset composed of 2-D multichannel seismic profiles, Chirp profiles, magnetic data and borehole information. In addition to the volcanic edifices known in the Graham and Terribile banks, this study has allowed to recognize several other magmatic manifestations. The magmatic occurrences consist of small volcanic cones, buried magma ascents and potential igneous sills. The CGSFZ is bounded by two strike-slip fault systems, the Capo Granitola Fault System (CGFS) to the west and the Sciacca Fault System (SFS) to the east, dominated by positive flower structures generated by tectonic inversion of NNE-oriented late Miocene extensional faults. Only the southern part of the CGFS shows the presence of a sub-vertical, N-S oriented strike-slip master fault. The sector between the two fault systems does not show a significant Pliocene-Quaternary tectonic deformation, except for its southern part hosting the Terribile Bank, which is dissected by WNW to NW-trending normal faults developed during late Miocene and later reactivated. This set of faults is currently active at the Terribile Bank, whereas is buried by Pliocene-Quaternary deposits in the central and northern sectors of the CGSFZ. The observed magmatism is driven by a mechanism of non-plume origin. Magmas have used as open paths the faults of the CGFS and SFS, which cut the whole lithosphere reaching the asthenosphere and producing partial melting by simple pressure release. Most of the magmatism develops along the strike-slip master fault associated with the CGFS and the normal faults affecting the Terribile Bank. The magmatic feeding of the Terribile Bank would be related to lateral magma migration coming from the structures of the SFS, which would use the open pathways represented by active normal faults. In the central-northern part of the CGSFZ, magmas migrate upward along lithospheric faults, then move laterally and rise toward the surface through NNE and NW-trending buried normal faults. These late Miocene faults do not reach the surface, and this may have favoured the emplacement of igneous sills, which in turn may explain the observed volcanic centres.
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This study presented recently reprocessed multi-channel seismic data and multi-beam bathymetric map to reveal the geomorphology and stratigraphic architecture of the Yongle isolated carbonate platform in the Xisha Archipelago, northwestern South China Sea. Our results show that the upper slope angles of Yongle carbonate platform exceed 10° and even reach to ∼32.5° whereas the lower slope angles vary from .5° to 5.3°. The variations of slope angles show that margins of Yongle Atoll belong to escarpment (bypass) margins to erosional (escarpment) margins. The interior of carbonate platform is characterized by sub-parallel to parallel, semi-continuous to continuous reflectors with medium-to high-amplitude and low-to medium-frequency. The platform shows a sub-flat to flat-topped shape in its geometry with aggradation and backstepping occurring on the platform margins. According to our seismic-well correlation, the isolated carbonate platform started forming in Early Miocene, grew during Early to Middle Miocene, and subsequently underwent drowning in Late Miocene, Pliocene and Quaternary. Large-scale submarine mass transport deposits are observed in the southeastern and southern slopes of Yongle Atoll to reshape the slopes since Late Miocene. The magmatism and hydrothermal fluid flow pipes around the Yongle Atoll have been active during 10.5–2.6 Ma. Their activity might intensify dolomitization of the Xisha isolated carbonate platforms during Late Miocene to Pliocene. Our results further suggest that the Yongle carbonate platform is situated upon a pre-existing fault-bounded block with a flat pre-Cenozoic basement rather than a large scale volcano as previously known and the depth of the basement likely reached to 1400 m, which is deeper than the well CK-2 suggested.
Article
Hydrothermal iron (Fe)-rich sediments were recovered from the Tagoro underwater volcano (Central Atlantic) that was built during the 2011–2012 volcanic event. Cruises in 2012 and 2014 enabled the monitoring and sampling of the early-stage establishment of a hydrothermal system. Degassing vents produced acoustic flares imaged on echo-sounders in June 2012, four months after the eruption. In 2014 during a ROV dive was discovered and sampled a novel hydrothermal vent system formed by hornito-like structures and chimneys showing active CO2 degassing and anomalous temperatures at 120–89 m water depth, and along the SE flank at 215–185 m water depth associated with secondary cones. Iron- and silica-rich gelatinous deposits pooled over and between basanite in the hornitos, brecciated lavas, and lapilli. The low-temperature, shallow-water hydrothermal system was discovered by venting of Fe-rich fluids that produced a seafloor draped by extensive Fe-flocculate deposits precipitated from the neutrally buoyant plumes located along the oxic/pHotic zone at 50–70 m water depths. The basanite is capped by mm- to cm-thick hydrothermally derived Fe-oxyhydroxide sediments and contain micro-cracks and degasification vesicles filled by sulfides (mostly pyrite). Mineralogically, the Fe-oxyhydroxide sediments consist of proto-ferrihydrite and ferrihydrite with scarce pyrite at their base. The Fe-rich endmember contains low concentrations of most trace elements in comparison with hydrogenetic ferromanganese deposits, and the sediments show some dilution of the Fe oxyhydroxide by volcanic ash. The Fe-oxyhydroxide phase with a mean particle size of 3–4 nm, low average La/Fe ratios of the mineralized deposits from the various sampling sites, and the positive Eu anomalies indicate rapid deposition of the Fe-oxyhydroxide near the hydrothermal vents. Electron microprobe studies show the presence of various organomineral structures, mainly twisted stalks and sheaths covered by iron-silica deposits within the mineralized samples, reflecting microbial iron-oxidation from the hydrothermal fluids. Sequencing of 16 s rRNA genes also reveals the presence of other microorganisms involved in sulfur and methane cycles. Samples collected from hornito chimneys contain silicified microorganisms coated by Fe-rich precipitates. The rapid silicification may have been indirectly promoted by microorganisms acting as nucleation sites. We suggest that this type of hydrothermal deposit might be more frequent than presently reported to occur in submarine volcanoes. On a geological scale, these volcanic eruptions and low-temperature hydrothermal vents might contribute to increased dissolved metals in seawater, and generate considerable Fe-oxyhydroxide deposits as identified in older hot-spot seamounts.
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On the basis of 2D multichannel and very-high-resolution seismic data and swath bathymetry, we report a sequence of giant mass-transport deposits (MTDs) in the Scan Basin (southern Scotia Sea, Antarctica). MTDs with a maximum thickness of c. 300 m extend up to 50 km from the Discovery and Bruce banks towards the Scan Basin. The headwall area consists of multiple U-shaped scars intercalated between volcanic edifices, up to 250 m high and 7 km wide, extending c. 14 km downslope from 1750 to 2900 m water depth. Seismic sections show that these giant MTDs are triggered by the intersection between diagenetic fronts related to silica transformation and vertical fluid-flow pipes linked to magmatic sills emplaced within the sedimentary sequence of the Scan Basin. This work supports that the diagenetic alteration of siliceous sediments is a possible cause of slope instability along world continental margins where bottom-simulating reflectors related to silica diagenesis are present at a regional scale.
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The structure of upper crustal magma plumbing systems controls the distribution of volcanism and influences tectonic processes. However, delineating the structure and volume of plumbing systems is difficult because (1) active intrusion networks cannot be directly accessed; (2) field outcrops are commonly limited; and (3) geophysical data imaging the subsurface are restricted in areal extent and resolution. This has led to models involving the vertical transfer of magma via dikes, extending from a melt source to overlying reservoirs and eruption sites, being favored in the volcanic literature. However, while there is a wealth of evidence to support the occurrence of dike-dominated systems, we synthesize field- and seismic reflection-based observations and highlight that extensive lateral magma transport (as much as 4100 km) may occur within mafic sill complexes. Most of these mafic sill complexes occur in sedimentary basins (e.g., the Karoo Basin, South Africa), although some intrude crystalline continental crust (e.g., the Yilgarn craton, Australia), and consist of interconnected sills and inclined sheets. Sill complex emplacement is largely controlled by host-rock lithology and structure and the state of stress. We argue that plumbing systems need not be dominated by dikes and that magma can be transported within widespread sill complexes, promoting the development of volcanoes that do not overlie the melt source. However, the extent to which active volcanic systems and rifted margins are underlain by sill complexes remains poorly constrained, despite important implications for elucidating magmatic processes, melt volumes, and melt sources.
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Seismic profiles across the Madeira Abyssal Plain show a relatively simple seismic stratigraphy in which an irregular diffractive acoustic basement is overlain by distinctive seismic units, reflecting a great thickness of ponded turbidites overlying pelagic drape. Within the uppermost ponded turbidite unit, a number of distinct, continuous, and laterally extensive reflectors are recognized. Sites 950 through 952 were drilled into these reflectors and allow dating of the beginning of large-scale turbidite emplacement on the abyssal plain and identification and dating of previously recognized seismic reflectors with a good degree of certainty. The extent and probable volume of the distinct turbidite packages can now be quantified. The Madeira Abyssal Plain overlies oceanic crust of Cretaceous age. Five distinct seismic units, separated by prominent, continuous, laterally extensive reflectors, can be identified. The lowermost of these (Unit B), which directly overlies acoustic basement, is a variably stratified unit and contains reflectors that generally show low coherency and onlap onto basement highs. At Site 950, the upper part of Unit B consists of red pelagic clays, with thin calcareous turbidites and ash layers, of late Eocene to Oligocene age. Unit A overlies Unit B with clear unconformity, marked by a conspicuous basinwide seismic reflector (Reflector 4). Unit A is a variably stratified unit and can be divided into four seismic units, A0 through A3, separated by prominent reflectors of regional extent. These units consist of thick, ponded turbidites with pelagic intervals. Many turbidites are basinwide in extent and can be correlated between drill sites. Four main types of turbidites are recognized: volcanic-rich turbidites derived from the Canary Islands, organic-rich turbidites derived from the Northwest African Margin, calcareous turbidites derived from seamounts to the west of the plain, and turbidites of 'intermediate' character. Organic-rich turbidites are the dominant type, although volcanic-rich turbidites are numerous in Units A0 through A2. Conversion of two-way traveltime to depth using shipboard sonic log data suggests that thick volcanic-rich and 'intermediate' character turbidites of wide lateral extent commonly correspond to strong seismic reflectors, and that acoustically transparent intervals within Unit A correspond to intervals of predominantly organic-rich turbidites. The base of Unit A is the regionally important Reflector 4 that correlates with a distinctive calcareous bed at all three drill sites dated at 16 Ma. The seismic units can be laterally mapped using an extensive data set of seismic reflection profiles and the minimum volumes of sediments deposited within individual seismic units calculated, giving values for sediment accumulation on the plain per unit time. The data show that since the inception of the abyssal plain in the middle Miocene (16 Ma), a minimum of 19,000 km3 of sediments (turbidites and hemipelagites) have been deposited.
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Extrusive edifices and structural reliefs, catalogued as mounds and located on the seafloor to the west of Canary Islands were analyzed by acoustic data obtained with multibeam and parametric echosounders during several oceanographic expeditions. They were carried out at deep waters, from 4800 to 5200 m, and they have allowed characterizing 41 newly discovered submarine structures which occur either as isolated edifices or clustered mounds. These features have circular to elongated shapes with diameters of 2-24 km and relief heights of 10 to 250 m, showing different flank slopes of 2-50°. They generally display mounded forms and show morphological elements as ridges, near-circular rock outcrops, depressions and fault scarps together with mass flow and slide deposits located at the vicinity of the edifices. Two types of extrusive features are evidenced by the morphological and seismic data analyses, the first one probably corresponds to high velocity extrusions that reach the seafloor surface and the second one is probably formed by the combination of faulted structures and low velocity extrusions that produce singular domes in the shallower sedimentary records. Based on both analyses, extrusive phenomena represent the dominant mechanism for mound field evolution in the Canary lower slope region.
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During a regional seismic interpretation study of leakage anomalies in the northern North Sea, mounds and zones with a highly chaotic seismic reflection pattern in the Tertiary Hordaland Group were repeatedly observed located above gas chimneys in the Cretaceous succession. The chaotic seismic reflection pattern was interpreted as mobilized sediments. These mud diapirs are large and massive, the largest being 100 km long and 40 km wide. Vertical injections of gas, oil and formation water are interpreted to have triggered the diapirs. On the eastern side of the Viking Graben, another much smaller type of mud diapir was observed. These near-circular mud diapirs are typically 1–3 km in diameter in the horizontal plane. Limited fluid injection from intra-Hordaland Group sands, through sand injection zones, into the upper Hordaland Group is interpreted to have triggered the near-circular diapirs. This observed ‘external’ type of mobilization was generated at shallow burial (<1000 m) and should be discriminated from the more common ‘internal’ type of mud diapirism that is generated in deep basins (>3000 m). The suggested model has implications for the understanding of the palaeofluid system, sand distribution, stratigraphic prediction within the chaotic zone, seismic imaging, and seismic interpretation of the hydrocarbon ‘plumbing’ system.
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The architecture of subsurface magma plumbing systems influences a variety of igneous processes, including the physiochemical evolution of magma and extrusion sites. Seismic reflection data provides a unique opportunity to image and analyze these subvolcanic systems in three dimensions and has arguably revolutionized our understanding of magma emplacement. In particular, the observation of (1) interconnected sills, (2) transgressive sill limbs, and (3) magma flow indicators in seismic data suggest that sill complexes can facilitate significant lateral (tens to hundreds of kilometers) and vertical (<5 km) magma transport. However, it is often difficult to determine the validity of seismic interpretations of igneous features because they are rarely drilled, and our ability to compare seismically imaged features to potential field analogues is hampered by the limited resolution of seismic data. Here we use field observations to constrain a series of novel seismic forward models that examine how different sill morphologies may be expressed in seismic data. By varying the geologic architecture (e.g., host-rock lithology and intrusion thickness) and seismic properties (e.g., frequency), the models demonstrate that seismic amplitude variations and reflection configurations can be used to constrain intrusion geometry. However, our results also highlight that stratigraphic reflections can interfere with reflections generated at the intrusive contacts, and may thus produce seismic artifacts that could be misinterpreted as real features. This study emphasizes the value of seismic data to understanding magmatic systems and demonstrates the role that synthetic seismic forward modeling can play in bridging the gap between seismic data and field observations.
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Seabed fluid flow involves the flow of gases and liquids through the seabed. Such fluids have been found to leak through the seabed into the marine environment in seas and oceans around the world - from the coasts to deep ocean trenches. This geological phenomenon has widespread implications for the sub-seabed, seabed, and marine environments. Seabed fluid flow affects seabed morphology, mineralization, and benthic ecology. Natural fluid emissions also have a significant impact on the composition of the oceans and atmosphere; and gas hydrates and hydrothermal minerals are potential future resources. This book describes seabed fluid flow features and processes, and demonstrates their importance to human activities and natural environments. It is targeted at research scientists and professionals with interests in the marine environment. Colour versions of many of the illustrations, and additional material - most notably feature location maps - can be found at www.cambridge.org/9780521819503.
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The origin and life cycle of ocean islands have been debated since the early days of Geology. In the case of the Canary archipelago, its proximity to the Atlas orogen led to initial fracture-controlled models for island genesis, while later workers cited a Miocene-Quaternary east-west age-progression to support an underlying mantle-plume. The recent discovery of submarine Cretaceous volcanic rocks near the westernmost island of El Hierro now questions this systematic age-progression within the archipelago. If a mantle-plume is indeed responsible for the Canaries, the onshore volcanic age-progression should be complemented by progressively younger pre-island sedimentary strata towards the west, however, direct age constraints for the westernmost pre-island sediments are lacking. Here we report on new age data obtained from calcareous nannofossils in sedimentary xenoliths erupted during the 2011 El Hierro events, which date the sub-island sedimentary rocks to between late Cretaceous and Pliocene in age. This age-range includes substantially younger pre-volcanic sedimentary rocks than the Jurassic to Miocene strata known from the older eastern islands and now reinstate the mantle-plume hypothesis as the most plausible explanation for Canary volcanism. The recently discovered Cretaceous submarine volcanic rocks in the region are, in turn, part of an older, fracture-related tectonic episode.
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It is well known that seawater that migrates deep into the Earth’s crust will pass into its supercritical domain at temperatures above 407°C and pressures above 298 bars. In the oceanic crust, these pressures are attained at depths of 3 km below sea surface, and sufficiently high temperatures are found near intruding magmas, which have temperatures in the range of 800°C to 1200°C. The physico-chemical behaviour of seawater changes dramatically when passing into the supercritical domain. A supercritical water vapour (ScriW) is formed with a density of 0.3 g/cc and a strongly reduced dipolar character. This change in polarity is causing the ScriW to lose its solubility of the common sea salts (chlorides and sulphates) and a spontaneous precipitation of sea salts takes place in the pore system. However, this is only one of many cases where the very special properties of ScriW affect its surroundings. The objective of this paper is to increase awareness of the many geological processes that are initiated and governed by ScriW. This includes interactions between ScriW and its geological surroundings to initiate and drive processes that are of major importance to the dynamics and livelihood of our planet. ScriW is the driver of volcanism associated with subduction zones, as ScriW deriving from the subduction slab is interacting with the mantle rocks and reducing their melting point. ScriW is also initiating serpentinization processes where olivines in the mantle rocks (e.g. peridotite) are transformed to serpentine minerals upon the uptake of OH-groups from hydrolysed water. The simultaneous oxidation of Fe2+ dissolved from iron-bearing pyroxenes and olivines leads to the formation of magnetite and hydrogen, and consequently, to a very reducing environment. ScriW may also be the potential starter and driver of the poorly understood mud and asphalt volcanism; both submarine and terrestrial. Furthermore, the lack of polarity of the water molecules in ScriW gives the ScriW vapour the potential to dissolve organic matter and petroleum. The same applies to supercritical brines confined in subduction slabs. If these supercritical water vapours migrate upwards to reach the critical point, the supercritical vapour is condensed into steam and dissolved petroleum is partitioned from the water phase to become a separate fluid phase. This opens up the possibility of transporting petroleum long distances when mixed with ScriW. Therefore, we may, popularly, say that ScriW drives a gigantic underground refinery system and also a salt factory. It is suggested that the result of these processes is that ScriW is rejuvenating the world’s ocean waters, as all of the ocean water circulates into the porous oceanic crust and out again in cycles of less than a million years. In summary, we suggest that ScriW participates in and is partly responsible for: 1) Ocean water rejuvenation and formation; 2) Fundamental geological processes, such as volcanism, earthquakes, and meta-morphism (including serpentinization); 3) Solid salt production, accumulation, transportation, and (salt) dome formation; 4) The initiation and driving of mud, serpentine, and asphalt volcanoes; 5) Dissolution of organic matter and petroleum, including transportation and phase separation (fractionation), when passing into the subcritical domain of (liquid) water.
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[1] Large volumes of magma emplaced within sedimentary basins have been linked to multiple climate change events due to release of greenhouse gases such as CH4. Basin-scale estimates of thermogenic methane generation show that this process alone could generate enough greenhouse gases to trigger global incidents. However, the rates at which these gases are transported and released into the atmosphere are quantitatively unknown. We use a 2D, hybrid FEM/FVM model that solves for fully compressible fluid flow to quantify the thermogenic release and transport of methane and to evaluate flow patterns within these systems. Our results show that the methane generation potential in systems with fluid flow does not significantly differ from that estimated in diffusive systems. The values diverge when vigorous convection occurs with a maximum variation of about 50%. The fluid migration pattern around a cooling, impermeable sill alone generates hydrothermal plumes without the need for other processes such as boiling and/or explosive degassing. These fluid pathways are rooted at the edges of the outer sills consistent with seismic imaging. Methane venting at the surface occurs in three distinct stages and can last for hundreds of thousands of years. Our simulations suggest that although the quantity of methane potentially generated within the contact aureole can cause catastrophic climate change, the rate at which this methane is released into the atmosphere is too slow to trigger, by itself, some of the negative δ13C excursions observed in the fossil record over short time scales (<10,000 years).
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Low-temperature hydrothermal alteration of basement from Site 801 was studied through analyses of the mineralogy, chemistry, and oxygen isotopic compositions of the rocks. The more than 100-m section of 170-Ma basement consists of 60 m of tholeiitic basalt separated from the overlying 60 m of alkalic basalts by a >3-m-thick Fe-Si hydrothermal deposit. Four alteration types were distinguished in the basalts: (1) saponite-type (Mg-smectite) rocks are generally slightly altered, exhibiting small increases in H 2 O, δ 18 θ, and oxidation; (2) celadonite-type rocks are also slightly altered, but exhibit uptake of alkalis in addition to hydration and oxidation, reflecting somewhat greater seawater/rock ratios than the saponite type; (3) Al-saponite-type alteration resulted in oxidation, hydration, and alkali and 18 O uptake and losses of Ca and Na due to the breakdown of plagioclase and clinopyroxene; and (4) blue-green rocks exhibit the greatest chemical changes, including oxidation, hydration, alkali uptake, and loss of Ca, Na, and Mg due to the complete breakdown of plagioclase and olivine to K-feldspar and Phyllosilicates. Saponite-and celadonite-type alteration of the tholeiite section occurred at a normal mid-ocean ridge basalt spreading center at temperatures <20°C. Near-or off-axis intrusion of an alkali basalt magma at depth reinitiated hydrothermal circulation, and the Fe-Si hydrothermal deposit formed from cool (<60°C) distal hydrothermal fluids. Focusing of fluid flow in the rocks immediately underlying the deposit resulted in the extensive alteration of the blue-green rocks at similar temperatures. Al-saponite alteration of the subsequent alkali basalts overlying the deposit occurred at relatively high water/rock ratios as part of the same low-temperature circulation system that formed the hydrothermal deposit. Abundant calcite formed in the rocks during progressive "aging" of the crust during its long history away from the spreading center.
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Polygonal faults, mainly oriented N50, N110 and N170, are abundant in the upper part of the mud-dominated Kai Formation (upper Miocene-lower Pliocene) of the Voting Basin. A second, less-developed tier of polygonal faults, oriented N20, N80 and N140, exists at the base of the overlying Naust Formation (upper Pliocene-Present). The faults abruptly terminate upward below a thick interval of debris flows. We propose a dynamic model in which: (1) the development of polygonal faults discontinues temporarily as a result of a change in regional sedimentation, leading to inactive polygonal faults; (2) rapid emplacement of debris flows in the late Pleistocene creates a new interval of polygonal faults in the lower part of the Naust Formation immediately beneath the debris flow and some faults penetrate into the underlying Kai Formation; (3) some polygonal faults within the Kai Formation are reactivated and propagated upward into the base of the Naust Formation. The high interconnectivity between faulted layers allows the fluids to reach shallower depths, forming well-expressed pipes and pockmarks on the sea floor. The model of cessation/reactivation of polygonal faults constrains the sealing capacity of sedimentary cover over the reservoirs and helps to reconstruct the fluid migration history through the sedimentary column.
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Structure of mud volcano systems and pockmarks in the region of the Ceuta Contourite Depositional System (Western Alborán Sea), Marine Geology (2012), doi: 10.1016/j.margeo.2012.06.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Voluminous volcanism characterized Early Tertiary continental break-up on the mid-Norwegian continental margin. The distribution of the associated extrusive rocks derived from seismic volcanostratig-raphy and potential field data interpretation allows us to divide the Møre, Vøring and Lofoten–Vesterålen margins into five segments. The central Møre Margin and the northern Vøring Margin show combinations of volcanic seismic facies units that are characteristic for typical rifted volcanic margins. The Lofoten– Vesterålen Margin, the southern Vøring Margin and the area near the Jan Mayen Fracture Zone show volcanic seismic facies units that are related to small-volume, submarine volcanism. The distribution of subaerial and submarine deposits indicates variations of subsidence along the margin. Vertical movements on the mid-Norwegian margin were primarily controlled by the amount of magmatic crustal thickening, because both the amount of dynamic uplift by the Icelandic mantle plume and the amount of subsidence due to crustal stretching were fairly constant along the margin. Thus, subaerial deposits indicate a large amount of magmatic crustal thickening and an associated reduction in isostatic subsidence, whereas submarine deposits indicate little magmatic thickening and earlier subsidence. From the distribution of volcanic seismic facies units we infer two main reasons for the different amounts of crustal thickening: (1) a general northward decrease of magmatism due to increasing distance from the hot spot and (2) subdued volcanism near the Jan Mayen Fracture Zone as a result of lateral lithospheric heat transport and cooling of the magmatic source region. Furthermore, we interpret small lateral variations in the distribution of volcanic seismic facies units, such as two sets of Inner Seaward Dipping Reflectors on the central Vøring Margin, as indications of crustal fragmentation.
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A voluminous magmatic complex was emplaced in the Vøring and Møre basins during Paleocene/ Eocene continental rifting and break-up in the NE Atlantic. This intrusive event has had a significant impact on deformation, source-rock maturation and fluid flow in the basins. Intrusive complexes and associated hydrothermal vent complexes have been mapped on a regional 2D seismic dataset (c.150 000 km) and on one large 3D survey. The extent of the sill complex is at least 80 000 km2, with an estimated total volume of 0.9 to 2.8 × 104 km3. The sheet intrusions are saucer-shaped in undeformed basin segments. The widths of the saucers become larger with increasing emplacement depth. More varied intrusion geometries are found in structured basin segments. Some 734 hydrothermal vent complexes have been identified, although it is estimated that 2-3000 vent complexes are present in the basins. The vent complexes are located above sills and were formed as a direct consequence of the intrusive event by explosive eruption of gases, liquids and sediments, forming up to 11 km wide craters at the seafloor. The largest vent complexes are found in basin segments with deep sills (3-9km palaeodepth). Mounds and seismic seep anomalies located above the hydrothermal vent complexes suggest that the vent complexes have been re-used for vertical fluid migration long after their formation. The intrusive event mainly took place just prior to, or during, the initial phase of massive break-up volcanism (55.0-55.8Ma). There is also evidence for a minor Upper Paleocene volcanic event documented by the presence of 20 vent complexes terminating in the Upper Paleocene sequence and the local presence of extrusive volcanic rocks within the Paleocene sequence.
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A new polygonal fault system has been identified in the Lower Congo Basin. This highly faulted interval (HFI), 700±50 m thick, is characterized by small extensional faults displaying a polygonal pattern in plan view. This kind of fracturing is attributed to volumetric contraction of sediments during early stages of compaction at shallow burial depth. 3-D seismic data permitted the visualization of the progressive deformation of furrows during burial, leading to real fractures, visible on seismic sections at about 78 m below seafloor. We propose a new geometrical model for volumetrical contraction of mud-dominated sediments. Compaction starts at the water–sediment interface by horizontal contraction, creating furrows perpendicular to the present day slope. During burial, continued shrinkage evolves to radial contraction, generating hexagonal cells of dewatering at 21 m below seafloor. With increasing contraction, several faults generations are progressively initiated from 78 to 700 m burial depth. Numerous faults of the HFI act as highly permeable pathways for deeper fluids. We point out that pockmarks, which represent the imprint of gas, oil or pore water escape on the seafloor, are consistently located at the triple-junction of three neighbouring hexagonal cells. This is highly relevant for predictive models of the occurrence of seepage structures on the seafloor and for the sealing capacity of sedimentary cover over deeper petroleum reservoirs.
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Continental rifting is often associated with voluminous magmatism and perturbations in the Earth's climate. In this study, we use 2D seismic data from the northeast Greenland margin to document two Paleogene-aged sill complexes and km² in size. Intrusion of the sills resulted in the contact metamorphism of carbon-rich shales, producing thermogenic methane which was released via 52 newly discovered hydrothermal vent complexes, some of which reach up to 11 km in diameter. Mass balance calculations indicate that the volume of methane produced by these intrusive complexes is comparable to that required to have caused the negative isotope excursion associated with the PETM. Combined with data from the conjugate Norwegian margin, our study provides evidence for margin-scale, volcanically-induced greenhouse gas release during the late Paleocene/early Eocene. Given the abundance of similar-aged sill complexes in Upper Paleozoic–Mesozoic and Cretaceous–Tertiary basins elsewhere along the northeast Atlantic continental margin, our findings support a major role for volcanism in driving global climate change.
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During opening of a new ocean, magma intrudes into the surrounding sedimentary basins. Heat provided by the intrusions matures the host rock, creating metamorphic aureoles potentially releasing large amounts of hydrocarbons. These hydrocarbons may migrate to the seafloor in hydrothermal vent complexes in sufficient volumes to trigger global warming, e.g., during the Paleocene-Eocene Thermal Maximum (PETM). Mound structures at the top of buried hydrothermal vent complexes observed in seismic data off Norway were previously interpreted as sediment volcanoes, and the amount of released hydrocarbon was estimated based on this interpretation. Here, we present new geophysical and geochemical data from the Gulf of California suggesting that such mound structures could in fact be edifices constructed by the growth of black smoker–type chimneys rather than sediment volcanoes. We have evidence for two buried and one active hydrothermal vent systems outside the rift axis. The active vent releases fluids of several hundred degrees Celsius containing abundant methane, mid-ocean ridge basalt–type helium, and precipitating solids up to 300 m high into the water column. Our observations challenge the idea that methane is emitted slowly from rift-related vents. The association of large amounts of methane with hydrothermal fluids that enter the water column at high pressure and temperature provides an efficient mechanism to transport hydrocarbons into the water column and atmosphere, lending support to the hypothesis that rapid climate change such as during the PETM can be triggered by magmatic intrusions into organic-rich sedimentary basins.
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Initially recognised in the Hawaiian Islands, volcanic rift zones and associated giant landslides have been extensively studied in the Canaries, where several of their more significant structural and genetic elements have been established. Almost 3,000 km of water tunnels (galerías) that exist in the western Canaries provide a unique possibility to access the deep structure of the island edifices. Recent work shows that rift zones to control the construction of the islands, possibly from the initial stages of island development, form the main relief features (shape and topography), and concentrate eruptive activity, making them crucial elements in defining the distribution of volcanic hazards on ocean islands.
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The island of El Hierro is formed by materials of three volcanic cycles, which can be clearly separated although they show no discontinuity. The oldest formation consists of approx 1400 m- thick subaerial lava flows; the upper limit of this series is marked by some trachytic episodes. The lower part of the formation comprises deposits of special interest due to their phreatomagmatic character and the existence of pseudosedimentary structures with gradual stratification. The intermediate series covers most of the island. The existence of explosive features as well as the presence of basaltic hornblende as a stable mineral indicate that this magmatic cycle is characterized by a high volatile content. The Recent series is formed by sub-historical lava flows. The well-preserved morphology of these flows allows a comparative study of the two principal types, aa and pahoehoe. The three series mainly comprise basalts with scarce coarse-grained xenoliths. There are some trachytic episodes at the top of the old series and one at the top of the intermediate series where some carbonized wood has been found. Erosive action along tectonic lines is proposed to explain the semicircular cliffs of El Golfo and Las Playas.-R.R.C.
Chapter
The Canary Islands, a group of seven major volcanic islands, extends for almost 500 km roughly east-west 100 km off Northwest Africa. The islands formed chiefly during the last 20 Ma, although volcanic activity started during the Oligocene and possibly Eocene in the eastern island of Fuerteventura. Ages of the rapidly formed sub-Canarian mantle are presently active across the entire belt. Total volumes of individual islands are about 10 to 20 x 106 km3 of which the subaerial part generally makes up less than 10%. turated to moderately undersaturated alkali basalt with local tholeiite. Low pressure fractionation of olivine, clino- pyroxene, and plagioclase was generally moderate, owing to rapid replenishment of the fast upward growth of the shield volcanoes and their magma chambers. Highly differentiated magma columns developed chiefly during the waning stages resulting in minor (quartz)-trachyte in the eastern and phonolitic plugs in the central and western islands. Major differentiated magma reservoirs on Gran Canaria and Tenerife culminated in large caldera-forming ash flow eruptions. Surface eruption of basalt magmas was generally inhibited during evolution and periodic partial emptying of such large differentiated zoned magma columns. Late stage basanites to nephelinites are locally nodule-bearing, of small volume, and are only slightly fractionated. High Ca/Al ratios and variable K-contents of these primitive magmas suggest garnet and phlogopite as residual phases during very low degrees of partial melting. Multiphase episodic magmatic evolution consisting of two or more magmatic phases is characteristic of most Canary Islands and is best developed on Gran Canaria where two major multiphase cycles are distinguished. Multiphase magmatic evolution is common on other islands in the Central North Atlantic with alkali basalt shield magmas being broad-ly similar. It is less well developed on smaller islands and those close to the Mid-Atlantic Ridge. Highly alkalic, mafic, under- saturated magmas appear to be restricted to (large volume?) islands on thicker lithosphere (Canaries and Cape Verde Islands), presumably due to low heat flow and thus small degrees of partial melting at greater depth. Intra-archipelago differences in melting conditions and mantle composition are reflected by consistently higher alkalinity and different trace element ratios between the western and central islands contrasted with Lanzarote and Fuerteventura to the east. Canary Island magmas on the whole are richer in Ti, Fe, and Zr and lower in A1 than Azorean and Madeira magmas. Canary Island magmas may be derived from garnet-bearing manle leaving residual garnet. The mantle beneath the Canaries is not very radiogenic with respect to 87Sr/86Sr as is characteristic for the eastern central Atlantic en-compassing the Cape Verde Islands and Madeira. The mantle area south of about 30 to 35 N may be distinct from, and less heterogeneous than the mantle farther north. There is no geological or geochemical evidence for the existence of continental crust beneath any of the Canary Islands. The origin of the Canary Island melting domain is not adequately explained by (a) an oceanic fracture zone, (b) extension of the South Atlas fault, (c) mantle plume and (d) propagating fracture zone. Unspecified mantle instabilities along the boundary between oceanic and continental lithosphere may have been instrumental in generating the unusually long-lived mantle anomaly with west to east translation of the lithosphere leading to an irregular non-linear age progression. Age data presently available for island volcanism in the Eastern Central North Atlantic suggest episodes of high activity between about 18 and 10 Ma and 5 Ma to the present, separated by a period of lesser magmatic productivity.
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Geological, biological, morphological, and hydrochemical data are presented for the newly discovered Moytirra vent field at 45oN. This is the only high temperature hydrothermal vent known between the Azores and Iceland, in the North Atlantic and is located on a slow to ultraslow-spreading mid-ocean ridge uniquely situated on the 300 m high fault scarp of the eastern axial wall, 3.5 km from the axial volcanic ridge crest. Furthermore, the Moytirra vent field is, unusually for tectonically controlled hydrothermal vents systems, basalt hosted and perched midway up on the median valley wall and presumably heated by an off-axis magma chamber. The Moytirra vent field consists of an alignment of four sites of venting, three actively emitting “black smoke,” producing a complex of chimneys and beehive diffusers. The largest chimney is 18 m tall and vigorously venting. The vent fauna described here are the only ones documented for the North Atlantic (Azores to Reykjanes Ridge) and significantly expands our knowledge of North Atlantic biodiversity. The surfaces of the vent chimneys are occupied by aggregations of gastropods (Peltospira sp.) and populations of alvinocaridid shrimp (Mirocaris sp. with Rimicaris sp. also present). Other fauna present include bythograeid crabs (Segonzacia sp.) and zoarcid fish (Pachycara sp.), but bathymodiolin mussels and actinostolid anemones were not observed in the vent field. The discovery of the Moytirra vent field therefore expands the known latitudinal distributions of several vent-endemic genera in the north Atlantic, and reveals faunal affinities with vents south of the Azores rather than north of Iceland.
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An extensive suite of igneous sills, collectively known as the Faroe-Shetland Sill Complex, has been intruded into the Cretaceous and Tertiary sedimentary section of the Faroe-Shetland Channel area. These sills have been imaged offshore by three-dimensional (3D) reflection seismic surveys and penetrated by several exploration boreholes. Data from wireline log measurements in these boreholes allow us to characterize the physical properties of the sills and their thermal aureoles. The borehole data has been compiled to produce new empirical relationships between sonic velocity and density, and between compressional and shear sonic velocities within the sills. These relationships are used to assist in calculation of synthetic seismic traces for sills intruded into sedimentary section, in order to calibrate the seismic response of the sills as observed in field data. This paper describes how the seismic amplitude response of the sills can be used to predict sill thickness where there is some nearby well control, and use this technique to estimate the volume of one well-imaged sill penetrated by Well 205/10-2b. Since the sills have a high impedance contrast with their host rocks, they return strong seismic reflections. 3D seismic survey data allow mapping of the morphology of the sills with a high level of confidence, although in some instances disruption of the downgoing seismic wavefield causes the seismic imaging of deeper sills and other structures to be very poor. Examples of sub-circular and dish-shapes sills, and also semi-conical and sheet-like intrusions, which are highly discordant are shown. The introduction of intrusive rocks can play an important role in the subsequent development of the sedimentary system. An example is shown in which differential compaction or soft sediment deformation around and above the sills appears to have controlled deposition of a reservoir quality sand body. The positioning of the sills within sedimentary basins is discussed, by constructing a simple model in which pressure support of magma from a crustal magma chamber provides the hydrostatic head of magma required for intrusion at shallow levels. This model is made semi-quantitative using a simple equation relating rock densities to intrusion depth, calibrated to observations from the Faroe-Shetland area. The model predicts that sills can be intruded at shallower levels in the sedimentary section above basement highs, which agrees with observations detailed in this paper.
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By combining surface mapping with marine reflection and refraction seismics it is possible to construct a composite image of the entire crustal structure in this region. During Tertiary break-up the basin was intruded by basaltic sills and dykes, and basaltic flood basalts flowed over the basin with decreasing thickness to the north. It seems that magmas were intruded as sills up to 300 m thick in the deep (10-15 km) central parts of the basin. Their geometry and possible volume makes them potential candidates as mid-crustal magma chambers and crustal magma pathways for the flood basalts. There is a general rather conformable relationship between the basin stratigraphy and the gross stratigraphy of the flood basalts, suggesting limited or no initial uplift prior to flood basalt volcanism. The apparent guidance exerted by the basin on the break-up magmatic activity without renewed rifting of the basin itself, the apparent lack of a broad initial uplift during break-up, and the later regional margin uplift, all seem at odds with several current plume models. -from Authors
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Geochemical investigations of dolerite cores from four intrusions in the recently discovered Faeroe-Shetland sill complex have established that the sills are of transition (T) mid-ocean ridge basalt (MORB)-type composition. Some uncertainty surrounds the age of the complex, but there is no doubt that it is, at least in part, of Tertiary age. Comparisons with previously proposed models for the development of sill-sediment complexes during initial stages of seafloor spreading suggests that the Faeroe-Shetland sills may represent an intrusive episode associated with a spreading axis that eventually produced oceanic crust W of the Faeroes.
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Multichannel seismic reflection and gravity data define the structure of Mesozoic ocean crust of the Canary Basin, formed at slow spreading rates. Single and multichannel seismics show a transition from smooth to rough basement topography from Jurassic to Cretaceous crust and a coeval change in crustal structure. Internal reflectivity of the rough basement area comprises upper, upper middle or whole crust cutting discrete dipping reflections. Lower-crustal reflectivity is almost absent and reflections from the crust-mantle transition are short and discontinuous or absent for several kilometers. In contrast, crust in the smooth basement area is characterized by sparse lower crustal events and common reflections from the crust-mantle boundary. The crustal structure of fracture zones in the rough basement area is associated with depressions in the basement top and in most cases with thin crust. In the smooth basement area, fracture zones exhibit neither a clear topographic expression nor crustal thinning. We interpret these characteristics as indicative of an increase in extensional tectonic activity and decrease in magmatic activity at the spreading ridge associated with a general decrease of spreading rate from Jurassic to Cretaceous times. In addition, the crust imaged across the path of the Cape Verde Hot Spot in the Canary Basin exhibits a widespread lower crustal reflectivity, very smooth topography and apparently thick crust. Our data document significant changes in the structure of crust formed at slow spreading rates which we attribute to thermal changes in the lithosphere due either to variations in spreading rate or to the presence of a hot spot beneath the Mesozoic Mid-Atlantic Ridge.
Article
Hydrothermal fluxes of heat and mass at mid-ocean ridges and on ridge flanks estimated using different approaches are reviewed. Heat and fluid fluxes in high temperature axial systems are best determined by geophysical methods, and are then combined with vent fluid compositions to derive chemical fluxes. Axial chemical fluxes calculated by mass balance using data from hydrothermally altered rocks sampled by deep ocean drilling are generally small, probably because of loss of material during drilling. Most of the hydrothermal heat anomaly in ocean crust occurs at low temperatures in off-axis flank systems, and a significant fraction of this must occur at very low temperatures (
Article
Development of the rifted continental margins and subsequent seafloor spreading in the North Atlantic was dominated by interaction between the Iceland mantle plume and the continental and oceanic rifts. There is evidence that at the time of breakup a thin sheet of particularly hot asthenospheric mantle propagated beneath the lithosphere across a 2500 km diameter region. This event caused transient uplift, massive volcanism and intrusive magmatism, and a rapid transition from continental stretching to seafloor spreading. Subsequently, the initial plume instability developed to an axisymmetric shape, with the c. 100 km diameter central core of the Iceland plume generating 30-40 km thick crust along the Greenland-Iceland-Faroes Ridge. The surrounding 2000 km diameter region received the lateral outflow from the plume, causing regional elevation and the generation of thicker and shallower than normal oceanic crust. We document both long-term (10-20 Ma) and short-term (3-5 Ma) fluctuations in the temperature and/or flow rate of the mantle plume by their prominent effects on the oceanic crust formed south of Iceland. Lateral ridge jumps in the locus of rifting are frequent above the regions of hottest asthenospheric mantle, occurring in both the early history of seafloor spreading, when the mantle was particularly hot, and throughout the generation of the Greenland-lceland-Faroes Ridge.
Article
Polygonal fault arrays have been documented in sedimentary basins from around the world and several theories exist as to how they initiate and propagate. Three-dimensional seismic data from polygonal fault arrays from offshore Norway are used to develop a new process model for polygonal fault development. We propose that in siliceous sediment, polygonal fault arrays can be triggered thermally, due to the conversion of opal-A to opal-CT at depths of 100–1000m. This conversion causes differential compaction and shear failure and therefore fault initiation. The location of the earliest faults is dependent on where opal-A to opal-CT conversion and compaction occur first. This is controlled by which strata have a favourable bed composition, local fluid chemistry and temperature or because the strata reach the depth of the reaction front first due to the presence of pre-existing structural relief (folds or faults). Subsidence of biosiliceous sediment through the opal-A to opal-CT reaction front causes fault propagation because of continued localised differential compaction. Fault initiation and propagation due to silica conversion generate polygonal fault arrays at significantly deeper burial depths than previously thought possible.
Article
In contrast to mature mid-oceanic ridges, where magmatic activity is little affected by the slow accumulation of sediments, in young spreading centers (such as that of the Guaymas Basin in the Gulf of California) the basaltic magma of ``great magmatic pulses'' forms dikes and sills within the uppermost few hundred metres of soft sediments. In general, younger dikes and sills are injected next to or on top of the contact zone of the older ones. In this manner a distinctive sill-sediment complex is built up, the sediments of which are rather compacted and partly metamorphosed despite the low burial depth. The thickness of this transitional zone between the sheeted dike complex (seismic layer 2) and younger sediment (seismic layer 1) is controlled chiefly by the rates of sedimentation and spreading. If the half-spreading rate is approximately one order of magnitude greater than the sedimentation rate, the sill-sediment complex can reach a thickness of only a few hundred metres, and the depth of the spreading trough remains approximately constant. Sedimentation rates approaching or surpassing the spreading rate cause filling up of the basins, which probably hampers the injection of magma into sediments. Sill-sediment complexes similar to those in the Gulf of California are also expected to occur at the passive margins of older oceanic basins as well as orogenic belts.
Article
The results of 64 new KAr age determinations, together with 32 previously published ages, show that after a period of erosion of the basal complex, Miocene volcanic activity started around 20 Ma in Fuerteventura and 15 Ma in Lanzarote, forming a tabular succession of basaltic lavas and pyroclastics with a few salic dykes and plugs. This series includes five separate volcanic edifices, each one with its own eruptive history. In Fuerteventura, several Miocene eruptive cycles have been identified: in the central edifice one around 20–17 Ma, followed by two others centred around 15 and 13 Ma; in the southern edifice the maximum of activity took place around 16–14 Ma, whereas in the northern one the main activity occurred between 14 and 12 Ma. In Lanzarote a first cycle of activity took place in the southern edifice between 15.5 and 14.1 Ma, followed by another between 13.6 and 12.3 Ma. In the northern edifice three pulses occurred: 10.2–8.3, 6.6–5.3 and 3.9–3.8 Ma. An important temporal gap, greater in Fuerteventura than in Lanzarote, separates Series I from the Plio-Quaternary Series II, III and IV, formed by multi-vent basaltic emissions. In Fuerteventura the following eruptive cycles have been identified: 5, 2.9–2.4, 1.8–1.7, 0.8–0.4 and <0.1 Ma. In Lanzarote, the activity was fairly continuous from 2.7 Ma to historic times, with a maximum in the Lower Pleistocene.
Article
Magma is transported in the crust by blade-like intrusions such as dykes, sills, saucers, and also collects in thicker laccoliths, lopoliths and plutons. Recently, the importance and great number of shallow (< 5 km) saucer-shaped intrusions has been recognized. Lopoliths and cup-shaped intrusions have also been reported in many geological contexts. Our field observations indicate that many intrusions, especially those emplaced into breccias or fractured rocks, have bulging, lobate margins and have shear faults at their bulbous terminations. Such features suggest that magma can propagate along a self-induced shear fault rather than a hydraulic tension-fracture. To investigate this we use analogue models to explore intrusion propagation in a brittle country rock. The models consist of the injection of analogue magma (honey or Golden syrup) in a granular material (sand or sieved ignimbrite) that is a good analogue for brittle or brecciated rocks. These models have the advantage (over other models that use gelatin) to well represent the properties of brittle materials by allowing both shear-faults and tension fractures to be produced at suitable stresses. In our experiments we mainly obtain vertical dykes and inverted-cone like structures that we call cup-shaped intrusions. Dykes bifurcate into cup-shaped intrusions at depths depending on their viscosity. All cup-shaped intrusions uplift a central block. By injecting against a vertical glass plate we obtain detailed observations of the intrusion propagation style. We observe that dykes commonly split and produce cup-shaped intrusions near the surface and that shear zone-related intrusions develop at the dyke tip. We conclude that many dykes propagate as a viscous indenter resulting from shear failure of host rock rather than tensional hydraulic fracturing of host rocks. The shear propagation model provides an explanation for the shape and formation of cup-shaped intrusions, saucer-sills and lopoliths.
Article
Forced folds formed at the seabed immediately overlying shallow (<1 km) saucer-shaped sills along the NE Atlantic Margin during the early Paleogene. Examples of this sill-fold relationship are exceptionally well imaged by high-resolution 3D seismic datasets from the NE Rockall Basin. The forced folds are domal in shape, 2e4 km in diameter, exhibit a structural relief of up to 350 m, and comprise sediment volumes of ca. 1 km 3 . A comparison of the thickness distribution across and volume of a saucer-shaped sill with a high intrusion diameter to depth ratio and the structural relief and volume of its associated forced fold shows a remarkable equivalence. This has the important implication that the structural relief on intrusion-related forced folds can be used as an estimate of the thickness of the underlying sill. The analysed forced folds are interpreted to have formed through three continual growth stages that are directly linked to the mechanical emplacement of the underlying saucer-shaped sills. Their growth was associated with an increase in faulting of the overlying strata and influenced coeval or subsequent development of polygonal fault systems within the overburden. These structures represent a new type of four-way dip closed hydrocarbon trap.
Article
Doleritic sill complexes, which are an important component of volcanic continental margins, can be imaged using 3D seismic reflection data. This allows unprecedented access to the complete 3D geometry of the bodies and an opportunity to test classic sill emplacement models. The doleritic sills associated with basaltic volcanism in the North Rockall Trough occur in two forms. Radially symmetrical sill complexes consist of a saucer-like inner sill at the base with an arcuate inclined sheet connecting it to a gently inclined, commonly ragged, outer rim. Bilaterally symmetrical sill complexes are sourced by magma diverted from a magma conduit feeding an overlying volcano. With an elongate, concave upwards, trough-like geometry bilaterally symmetrical sills climb away from the magma source from which they originate. Both sill complex types can appear as isolated bodies but commonly occur in close proximity and consequently merge, producing hybrid sill complexes. Radial sill complexes consist of a series of radiating primary flow units. With dimensions up to 3km, each primary flow unit rises from the inner saucer and is fed by primary magma tube. Primary flow units contain secondary flow units with dimensions up to 2km, each being fed by a secondary magma tube branching from the primary magma tube. Secondary flow units in turn are composed of 100-m scale tertiary flow units. A similar branching hierarchy of flow units can also be seen in bilaterally symmetrical sill complexes, with their internal architecture resembling an enlarged version of a primary flow unit from a radial sill complex. This branching flow pattern, as well as the interaction between flow units of varying orders, provides new insights into the origin of the structures commonly seen within sill complexes and the hybrid sill bodies produced by their merger. The data demonstrate that each radially symmetrical sill complex is independently fed from a source located beneath the centre of the inner saucer, grows by climbing from the centre outwards and that peripheral dyking from the upper surface is a common feature. These features suggest a laccolith emplacement style involving peripheral fracturing and dyking during inner saucer growth and thickening. The branching hierarchy of flow units within bilaterally symmetrical sill complexes is broadly similar to that of primary flow units within a radially symmetrical sill complex, suggesting that the general features of the laccolith emplacement model also apply.