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Journal ArticleDOI

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

01 Oct 2003-Geochemistry Geophysics Geosystems (John Wiley & Sons, Ltd)-Vol. 4, Iss: 10, pp 8514
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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Citations
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Journal ArticleDOI
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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Journal ArticleDOI
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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Journal ArticleDOI
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

Journal ArticleDOI
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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Journal ArticleDOI
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

References
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Journal ArticleDOI
TL;DR: In this article, trace element and isotope data for basalts from the mid-Atlantic ridge between 31 and 41oN allow a better description of the geochemical gradient south of the Azores triple junction, and the systematics of mantle source heterogeneity.

169 citations


"Interaction between the Mid-Atlanti..." refers background in this paper

  • ...[44] The geochemical signature of the Azores hot spot along the MAR axis is asymmetric [Schilling et al., 1983; Yu et al., 1997; Dosso et al., 1999] (Figure 2): from Terceira Island, this signature extends at least 250 km to the north, and at least 1000 km to the south (to the Hayes FZ, or even…...

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  • ...…elevated spreading ridge [Vogt, 1976; Schilling, 1985; Gente, 1987; Thibaud et al., 1998], basalt geochemistry [Schilling, 1975; White et al., 1976; Bougault and Treuil, 1980; Schilling et al., 1983; Dosso et al., 1999], and gravity anomalies [Detrick et al., 1995; Thibaud et al., 1998] (Figure 2)....

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  • ...This along-axis influence appears asymmetrical to the north and south of the Azores hot spot, with a more limited northward extension [Yu et al., 1997; Dosso et al., 1999; Goslin et al., 1998, 1999]....

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  • ...…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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Journal ArticleDOI
TL;DR: In this paper, the crustal thickness and upper mantle structure along the rift valleys of three segments of the northern Mid-Atlantic Ridge with contrasting morphologies and gravity signatures are determined from a seismic refraction study.
Abstract: The crustal thickness and crustal and upper mantle structure along the rift valleys of three segments of the northern Mid-Atlantic Ridge with contrasting morphologies and gravity signatures are determined from a seismic refraction study. These segments lie between the Oceanographer and Hayes transforms and from north to south have progressively deeper axial valleys with less along-axis relief and smaller mantle Bouguer gravity lows. Major variations in seismic crustal thickness and crustal velocity and density structure are observed along these segments. The thickest crust is found near the segment centers, with maximum crustal thicknesses of 8.1, 6.9, and 6.6±0.5 km, decreasing from north to south. However, the mean crustal thickness is similar for each segment (5.6±0.4, 5.7±0.4 and 5.1±0.3 km). Near the segment ends, crustal thickness is 2.5 to 5±0.5 km with no systematic variation from north to south. At segment ends, both crustal velocities and vertical velocity gradients are anomalous and may indicate fracturing and alteration of thin igneous crust and underlying mantle. Away from segment ends, the thickness of the upper crust is relatively uniform along axis (∼3 km), although its internal structure is laterally heterogeneous (velocity anomalies of ±0.6 km s−1 over distances of 5 km), possibly related to the presence of discrete volcanic centers. The along-axis crustal thickness variations are primarily accommodated in the lower crust. The center of the northern segment (OH-1) has an unusually thick crustal root (excess thickness of 2–4 km and along-axis extent of 12 km). Our results are consistent with an enhanced supply of melt from the mantle to the segment centers and redistribution of magma along axis at shallow crustal levels by lateral dike injection. Along this portion of the Mid-Atlantic Ridge, our results suggest that differences in axial morphology, seismic crustal thickness, and gravity anomalies are correlated and the result of variations in melt flux from the mantle. A surprising result is that the melt flux per segment length is similar for all three segments despite their different morphologies and gravity signatures. This argues against excess melting of the mantle beneath segment OH-1. Instead, we suggest that the thickened crust at the segment center is a result of focusing of melt, possibly due to the influence of the thermal structure of the Oceanographer fracture zone on melt migration in the mantle.

163 citations


"Interaction between the Mid-Atlanti..." refers background in this paper

  • ...While this can be considered a reasonable proxy for ridges away from hot spots [e.g., Hooft et al., 2000], studies on ridge-hot spot interactions showed that in such contexts, part of the RMBA signal is related to mantle density variations due to the thermal and compositional effects of the nearby…...

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Journal ArticleDOI
TL;DR: In this article, the oldest volcanic formations known in the Azores archipelago were estimated to date from 5.5 m.y. to the present day using the K/Ar method.

160 citations

Journal ArticleDOI
TL;DR: In this paper, the dynamics of mantle flow and melting of a ridge-centered plume were investigated with three-dimensional variable-viscosity numerical models, focusing on three buoyancy sources: temperature, melt depletion, and melt retention.

154 citations

Journal ArticleDOI
TL;DR: In terms of current knowledge of crustal genesis in the Atlantic Ocean, several unique or highly anomalous features exist in the vicinity of the Azores, such as the seismically active East Azores Fracture Zone extending from Gibraltar to the Mid-Atlantic Ridge, the seismic inactive West Azores fracture Zone which is offset northwards from the trend of the east-west transverse fracture system as mentioned in this paper.
Abstract: Summary In terms of current knowledge of crustal genesis in the Atlantic Ocean, several unique or highly anomalous features exist in the vicinity of the Azores. These include the seismically active East Azores Fracture Zone extending from Gibraltar to the Mid-Atlantic Ridge; the seismically inactive West Azores Fracture Zone which is offset northwards from the trend of the East Azores Fracture Zone; the transverse island chain of the Azores islands which trends southeast-northwest across the Mid-Atlantic Ridge; the marked change in direction of the Mid-Atlantic Ridge from northeast-southwest to north-south; and the broadening of the Mid-Atlantic Ridge to the east. Bathymetric and magnetic data from surveys of the Mid-Atlantic Ridge in the Azores area by R.V. Trident, the U.S.Naval Oceanographic Office, and other sources have been compiled. These, together with the previously published data are compatible with a basic model consisting of a Mid-Atlantic Ridge migrating eastwards at the local crustal spreading rate, which is greater to the north than to the south of an east-west transverse fracture system. Superimposed on this is the development of a northwest-southeast trending secondary spreading centre or triple junction, within a ‘leaky transform’ system (Menard & Atwater 1968) which would have developed as the result of a change in the local crustal spreading direction south of the east-west transverse fracture system from mainly east-west to mainly northwest-southeast. Simple geometrical considerations, when combined with the length of a newly proposed ‘Terceira Rift’ and independent estimates of local crustal spreading rates suggest that the change in crustal spreading direction and triple junction development began at least 45 My ago.

153 citations