Interaction between the Mid-Atlantic Ridge and the Azores hot spot during the last 85 Myr: Emplacement and rifting of the hot spot-derived plateaus
TL;DR: In this paper, multiple and single-beam bathymetric data are compiled over the Azores plateau to produce a 1 km × 1 km grid between latitudes 32°N and 49°N, and longitudes 22°W and 43°W.
Abstract: [1] Multiple- and single-beam bathymetric data are compiled over the Azores plateau to produce a 1 km × 1 km grid between latitudes 32°N and 49°N and longitudes 22°W and 43°W. Mantle Bouguer anomalies are then calculated from this grid and the satellite-derived gravity. These grids provide new insights on the temporal and spatial variations of melt supply to the ridge axis. The elevated seafloor of the Azores plateau is interpreted as resulting from the interaction of a mantle plume with the Mid-Atlantic Ridge (MAR). The presence of a large region of elevated seafloor associated with a thick crust between the Great Meteor Seamounts and the Azores platform on the Africa plate, and less developed conjugate structures on the North America plate, favors genetic relations between these hot spot-derived structures. This suggests that a ridge-hot spot interaction has occurred in this region since 85 Ma. This interaction migrated northward along the ridge axis as a result of the SSE absolute motion of the Africa plate, following a direction grossly parallel to the orientation of the MAR. Kinematic reconstructions from chron 13 (∼35 Ma) to the present allow a proposal that the formation of the Azores plateau began around 20 Ma and ended around 7 Ma. A sharp bathymetric step is associated with the beginning of important melt supply around 20 Ma. The excess of melt production is controlled by the interaction of the ridge and hot spot melting zones. The geometry and distribution of the smaller-scale features on the plateau record episodic variations of the hot spot melt production. The periodicity of these variations is about 3–5 Myr. Following the rapid decrease of widespread volcanism, the plateau was subsequently rifted from north to south by the Mid-Atlantic Ridge since 7 Ma. This rifting begins when the MAR melting zone is progressively shifted away from the 200-km plume thermal anomaly. These results bear important consequences on the motion of the Africa plate relative to the Azores hot spot. They also provide an explanation to the asymmetric geochemical signature of the Azores hot spot along the Mid-Atlantic Ridge.
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TL;DR: Small-scale volcanic systems are commonly monogenetic in the sense that they are represented at the Earth's surface by fields of small volcanoes, each the product of a temporally restricted eruption of a compositionally distinct batch of magma, and this is in contrast to relatively large edifices built by multiple eruptions over longer periods of time involving magmas with diverse origins.
Abstract: Abstract Small-scale volcanic systems are the most widespread type of volcanism on Earth and occur in all of the main tectonic settings. Most commonly, these systems erupt basaltic magmas within a wide compositional range from strongly silica undersaturated to saturated and oversaturated; less commonly, the spectrum includes more siliceous compositions. Small-scale volcanic systems are commonly monogenetic in the sense that they are represented at the Earth's surface by fields of small volcanoes, each the product of a temporally restricted eruption of a compositionally distinct batch of magma, and this is in contrast to polygenetic systems characterized by relatively large edifices built by multiple eruptions over longer periods of time involving magmas with diverse origins. Eruption styles of small-scale volcanoes range from pyroclastic to effusive, and are strongly controlled by the relative influence of the characteristics of the magmatic system and the surface environment.
115 citations
Cites background from "Interaction between the Mid-Atlanti..."
...At the less productive oceanic hotspots of the Azores (Gente et al. 2003) and Canary Islands (Fullea et al....
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...At the less productive oceanic hotspots of the Azores (Gente et al. 2003) and Canary Islands (Fullea et al. 2015), clinopyroxene–melt barometry and petrographical observations show that magma batches partially crystallize and mix with preexisting magma batches in a zone of temporary magma storage…...
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...At the less productive oceanic hotspots of the Azores (Gente et al. 2003) and Canary Islands (Fullea et al. 2015), clinopyroxene–melt barometry and petrographical observations show that magma batches partially crystallize and mix with preexisting magma batches in a zone of temporary magma storage at near and sub-Moho depths of 15–40 km (Hansteen et al. 1998; Schwarz et al. 2004; Klugel et al. 2005; Galipp et al. 2006; Longpré et al. 2009; Stroncik et al. 2009)....
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TL;DR: In this article, He isotope and major, trace and volatile element compositions for basaltic scoriae from five monogenetic cones emplaced along the fissure zone of Pico Island, the youngest island of the Azores archipelago.
Abstract: The concept of an 'Azores mantle plume' has been widely debated, and the existence of an Azores hotspot questioned. In an effort to shed new light on this controversy, we present He isotope and major, trace and volatile element compositions for basaltic scoriae from five monogenetic cones emplaced along the fissure zone of Pico Island, the youngest island of the Azores archipelago. The bulk scoriae and lavas are moderately alkaline basalts, and their He isotope ratios, determined on olivine crystals, vary between 10*2 and 11*1 ± 0*1 Ra. In contrast, melt inclusions hosted in olivine (Fo76-83*5) span a large range of compositions (K2O = 0*7-1*7 wt %; Ce = 32-65 ppm; Nb = 21-94 ppm), which extends the compositional field of lavas erupted along the Pico fissure zone. This chemical evolution is predominantly controlled by polybaric fractional crystallization. Most melt inclusions share similar enrichments in large ion lithophile and light rare earth elements, and trace element ratios (La/Sm, La/Yb, Sr/Nd, Ta/Th, Zr/Y) with their bulk-rocks. Only a few of them differ in their lower contents of incompatible elements and La/Sm, Li/Ta and Na/K ratios, a feature that is ascribed to distinct conditions of melting. As a whole, the melt inclusions preserve high and variable volatile contents, and contain up to 1*8-2*0 wt % of H2O and 0*4 wt % of CO2. The total fluid pressures, retrieved from the dissolved CO2 and H2O concentrations, and the PCO2 from fluid inclusions, indicate magma ponding and crystallization at the crust-mantle boundary (ca. 18 km deep). The H2O/Cl and H2O/Ce ratios in the inferred parental undegassed basalts of the Pico fissure zone average 0*036 ± 0*006 and 259 ± 21, respectively. The latter value is significantly higher than that reported for typical mid-ocean ridge basalts from the southern Mid-Atlantic Ridge, but is similar to published ratios for submarine undegassed basalts from the Azores platform. Combining the calculated compositions of Pico primary magmas formed by low degrees of melting with recent geophysical data for the Azores, we propose a model for Azores magma generation involving the decompression melting of a water-enriched mantle domain (H2O = 680-570 ppm) with an estimated temperature excess of ≤120°C with respect to the Mid-Atlantic Ridge.
92 citations
Cites background from "Interaction between the Mid-Atlanti..."
...At the junction between three major lithospheric platesçthe North American, African and Eurasian platesçthe Azores islands are thus located in a complex tectonic setting characterized by anomalously thick crust ( 8 km; Luis et al., 1998; Gente et al., 2003; Dias et al., 2007; Georgen & Sankar, 2010; Silveira et al., 2010) and the presence of ‘V-shaped’ ridges along the adjacent MAR (e....
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...…platesçthe Azores islands are thus located in a complex tectonic setting characterized by anomalously thick crust ( 8 km; Luis et al., 1998; Gente et al., 2003; Dias et al., 2007; Georgen & Sankar, 2010; Silveira et al., 2010) and the presence of ‘V-shaped’ ridges along the adjacent MAR…...
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Lamont–Doherty Earth Observatory1, University of Bristol2, Tokyo Institute of Technology3, University of Lisbon4, United States Geological Survey5, University of Münster6, University of the Azores7, Leibniz Center for Tropical Marine Ecology8, University of North Carolina at Wilmington9, University of Porto10
TL;DR: In this paper, the authors reconstruct the evolutionary history of Santa Maria with respect to the timing and magnitude of its vertical movements, using detailed field work and 40Ar/39Ar geochronology.
Abstract: The growth and decay of ocean-island volcanoes are intrinsically linked to vertical movements. While the causes for subsidence are better understood, uplift mechanisms remain enigmatic. Santa Maria Island in the Azores Archipelago is an ocean-island volcano resting on top of young lithosphere, barely 480 km away from the Mid-Atlantic Ridge. Like most other Azorean islands, Santa Maria should be experiencing subsidence. Yet, several features indicate an uplift trend instead. In this paper, we reconstruct the evolutionary history of Santa Maria with respect to the timing and magnitude of its vertical movements, using detailed field work and 40Ar/39Ar geochronology. Our investigations revealed a complex evolutionary history spanning ∼6 m.y., with subsidence up to ca. 3.5 Ma followed by uplift extending to the present day. The fact that an island located in young lithosphere experienced a pronounced uplift trend is remarkable and raises important questions concerning possible uplift mechanisms. Localized uplift in response to the tectonic regime affecting the southeastern tip of the Azores Plateau is unlikely, since the area is under transtension. Our analysis shows that the only viable mechanism able to explain the uplift is crustal thickening by basal intrusions, suggesting that intrusive processes play a significant role even on islands standing on young lithosphere, such as in the Azores.
83 citations
TL;DR: In this paper, the authors used bathymetry and high-resolution seismic reflection profiles to characterize the morphology of the insular shelves adjacent to each volcanic edifice of Terceira Island in order to improve the understanding of its evolution.
Abstract: Shelves from volcanic ocean islands result from the competition between two main processes, wave erosion that forms and enlarges them and volcanic progradation that reduces their dimension. In places where erosion dominates over volcanism, shelf width can be used as a proxy for the relative age of the subaerial volcanic edifices and reconstruction of their extents prior to erosion can be achieved. In this study, new multibeam bathymetry and high-resolution seismic reflection profiles are exploited to characterize the morphology of the insular shelves adjacent to each volcanic edifice of Terceira Island in order to improve the understanding of its evolution. Subaerial morphological and geological/stratigraphic data were also used to establish the connection between the onshore and offshore evolution. Shelf width contiguous to each main volcanic edifice is consistent with the known subaerial geological history of the island; most of the older edifices have wider shelves than younger ones. The shelf edge proved to be a very useful indicator in revealing the original extent of each volcanic edifice in plan view. Its depth was also used to reconstruct vertical movements, showing that older edifices like Serra do Cume-Ribeirinha, Guilherme Moniz, and Pico Alto have subsided while more recent ones have not. The morphology of the shelf (namely the absence/presence of fresh lava flow morphologies and several types of erosional, depositional, and tectonic features) integrated with the analysis of the coastline morphology allowed us to better constrain previous geological interpretations of the island evolution.
79 citations
Cites background from "Interaction between the Mid-Atlanti..."
...…interaction between the triple junction of the Eurasian (Eu), Nubian (Nu), and North American (NA) plates [e.g., Laughton and Whitmarsh, 1974; Saemundsson, 1986], and a magmatic anomaly that some authors consider to be the Azores hotspot [Cannat et al., 1999; Gente et al., 2003; Schilling, 1975]....
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TL;DR: In this paper, the authors used the satellite-derived gravity anomaly to predict the bathymetry for different values of Te, which is a proxy for the long-term strength of the lithosphere.
Abstract: [1] The seafloor is characterized by numerous seamounts and oceanic islands which are mainly volcanic in origin. Relatively few of these features (<∼0.1%), however, have been dated, and so little is known about their tectonic setting. One parameter that is sensitive to whether a seamount formed on, near, or far from a mid-ocean ridge is the elastic thickness, Te, which is a proxy for the long-term strength of the lithosphere. Most previous studies are based on using the bathymetry to calculate the gravity anomaly for different values of Te and then comparing the calculated and observed gravity anomaly. The problem with such an approach is that bathymetry data are usually limited to single-beam echo sounder data acquired along a ship track and these data are too sparse to define seamount shape. We therefore use the satellite-derived gravity anomaly to predict the bathymetry for different values of Te. By comparing the predicted bathymetry to actual shipboard soundings in the vicinity of each locality in the Wessel global seamount database, we have obtained 9758 Te estimates from a wide range of submarine volcanic features in the Pacific, Indian, and Atlantic oceans. Comparisons where there are previous estimates show that bathymetric prediction is a robust way to estimate Te and its upper and lower bounds. Te at sites where there is both a sample and crustal age show considerable scatter, however, and there is no simple relationship between Te and age. Nevertheless, we are able to tentatively assign a tectonic setting to each Te estimate. The most striking results are in the Pacific Ocean where a broad swath of “on-ridge” volcanism extends from the Foundation seamounts and Ducie Island/Easter Island ridge in the southeast, across the equator, to the Shatsky and Hess rises in the northwest. Interspersed among the on-ridge volcanism are “flank ridge” and “off-ridge” features. The Indian and Atlantic oceans also show a mix of tectonic settings. Off-ridge volcanism dominates in the eastern North Atlantic and northeast Indian oceans, while flank ridge volcanism dominates the northeastern Indian and western south Atlantic oceans. We have been unable to assign the flank ridge and off-ridge estimates an age, but the on-ridge estimates generally reflect, we believe, the age of the underlying oceanic crust. We estimate the volume of on-ridge volcanism to be ∼1.1 × 106 km3 which implies a mean seamount addition rate of ∼0.007 km3 yr−1. Rates appear to have varied through geological time, reaching their peak during the Late/Early Cretaceous and then declining to the present-day.
75 citations
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TL;DR: In this article, the major tectonic elements of the Azores triple junction have been mapped using long-range side-scan sonar data and the data enable the Mid-Atlantic Ridge axis to be located with a precision of a few kilometres.
254 citations
"Interaction between the Mid-Atlanti..." refers background in this paper
...Finally, no clear evidence for a discrete triple junction has been observed west of Faial island, where the diffuse plate boundary separating Eurasia-Iberia andAfrica intersects theMAR axis [Searle, 1980; Frere Luis et al., 1994; Lourenço et al., 1998; Miranda et al., 1998]....
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...arating Eurasia-Iberia andAfrica intersects theMAR axis [Searle, 1980; Frere Luis et al., 1994; Lourenço et al., 1998; Miranda et al., 1998]....
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TL;DR: This paper analyzed the focal mechanisms and depths of 10 moderately sized earthquakes along the Azores-Gibraltar plate boundary by a variety of methods including formal inversion of the waveform and amplitude of teleseismic P and SH waves, first motion readings, and the identification of depth phases.
Abstract: We have analyzed the focal mechanisms and depths of 10 moderately sized earthquakes along the Azores-Gibraltar plate boundary by a variety of methods including formal inversion of the waveform and amplitude of teleseismic P and SH waves, first motion readings, and the identification of depth phases. Our data, together with a compilation of results reported for very large events from the past 30 years and for two recent events in 1983, place new constraints on the present-day deformation of this boundary separating the Eurasian and African plates. The slow (∼2 to 3 mm/yr) divergent plate motion near the Azores Islands at the western end of the boundary appears to be very similar to that of a typical ridge-transform-ridge configuration. A currently aseismic segment with presumed transcurrent motion separates the western end from a broad zone of ocean-ocean convergence to the east. This convergence zone is characterized by scattered seismicity, complex bathymetry, and large positive geoid and gravity anomalies. The dispersed locations of the earthquakes and the lack of consistency in the orientation of their nodal planes and slip vectors all suggest that a single major plate boundary is not present there. We note that a regional stress field of north-northwest-south-southeast compression can be inferred from the consistent P axis directions. The focal depths for earthquakes in this region reach a maximum of 50±5 km beneath the seafloor. These events are among the deepest oceanic earthquakes which are apparently unrelated to a Benioff zone and their maximum depths are approximately limited by the position of the 600°C isotherm. We interpret the regional stress field and the focal depths as the result of the interaction of two sections of strong, cold oceanic lithosphere with nearly identical thermal structure. As the plate boundary crosses the passive continental margins of Iberia and Africa, the zone of ocean-ocean convergence becomes that of continental collision. Two unusually deep events, one at 100 and another at 640 km, have occurred beneath southern Spain since the occurrence of the deep Spanish earthquake of 1954 in the same area. The configuration of a seismic upper crust and uppermost mantle straddling an aseismic lower crust resembles the distribution of focal depths in other areas of recent continental convergence. Based on the distribution of focal depths along the entire plate boundary and recent reconstructions of the history of relative motions between Eurasia and Africa, the very deep events beneath southern Spain are likely to have occurred within a detached piece of lithosphere which sank to its present depth during the current phase of plate convergence.
246 citations
"Interaction between the Mid-Atlanti..." refers background in this paper
...…from east to west, a compression domain, the Horseshoe seamounts, abutting the Iberian margin; the Gloria transform fault; and a transtensional domain, the Azores region [McKenzie, 1972; Laughton and Whitmarsh, 1974; Grimison and Chen, 1986; Buforn et al., 1988; Madeira and Ribeiro, 1990]....
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...It includes, from east to west, a compression domain, the Horseshoe seamounts, abutting the Iberian margin; the Gloria transform fault; and a transtensional domain, the Azores region [McKenzie, 1972; Laughton and Whitmarsh, 1974; Grimison and Chen, 1986; Buforn et al., 1988; Madeira and Ribeiro, 1990]....
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TL;DR: In this article, the authors show that the Azores mantle plume is 10 to 30 times enriched in LIL elements with very small (⪢ 0.1 < D < 1) bulk crystal/melt partition coefficients (Rb, Cs, Ba, La).
241 citations
"Interaction between the Mid-Atlanti..." refers background in this paper
...…et al., 1973; Le Douaran and Francheteau, 1981; Vogt, 1976; Gente, 1987; Thibaud et al., 1998], geoid topography [Bowin et al., 1984; Cazenave et al., 1992] and geochemical anomalies [Schilling, 1975; White and Schilling, 1978; Bougault and Treuil, 1980; Yu et al., 1997; Dosso et al., 1999]....
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TL;DR: A new global S-wave velocity model reveals that although mid-ocean ridges and hotspots are both underlain by low-velocity anomalies in the mantle, these have distinctly different structures as mentioned in this paper.
Abstract: A new global S-wave velocity model reveals that although mid-ocean ridges and hotspots are both underlain by low-velocity anomalies in the mantle, these have distinctly different structures. This implies that there are differences between the upwelling mechanisms under ridges and under hot-spots. The velocity model also shows that there may be interactions between ridges and hotspots near Afar and St Helena.
227 citations
TL;DR: In this paper, a bathymetric grid with all the available data sources in an area comprised between 24°W to 32°W and 36°N to 41°N is presented.
Abstract: The existing studies of the Azores triple junction, although based on specific geological or geophysical data, largely rely upon morphological considerations. However, there is no systematic bathymetric coverage of this area, and the quality of the available bathymetric charts does not allow consistent morpho-structural analysis. In this work we present a new bathymetric grid elaborated with all the available data sources in an area comprised between 24° W to 32° W and 36° N to 41° N. The basic data set corresponds to the merge of NGDC data with new swath profiles. This new map, included as an Appendix, combined with other results from seismology and neotectonics, is the basis for the study of the morpho-structural pattern of the Azores area, the present day stress field and its implications on the current view of the Azores geodynamics. As a major result, we conclude that the Azores region is controlled by two sets of conjugated faults with 120° and 150° strikes that establish the framework for the onset of volcanism, expressing as linear volcanic ridges or as point source volcanism. This interaction develops what can be considered as the morphological signature of the Azores Spreading axis segmentation. We argue that the Azores domain, presently in a broad transtensional regime, is acting simultaneously as a ultra slow spreading centre and as a transfer zone between the MAR and the dextral Gloria Fault, as it accommodates the differential shear movement between the Eurasian and African plates.
207 citations