Mantle Inflow Research Articles

Surface‐Wave Tomography of the Central‐Western Mediterranean: New Insights Into the Liguro‐Provençal and Tyrrhenian Basins

AbstractThe complex tectonic setting of the central‐western Mediterranean has interested geoscientists for decades, but its geodynamic evolution remains a matter of debate. We rely on 807 seismometers from southern Europe and northern Africa to measure Rayleigh and Love phase velocities in the period range ∼5–200 s, based on teleseismic earthquakes and seismic ambient noise. By nonlinear joint inversion of the phase‐velocity maps, we obtain a 3‐D shear‐wave velocity (VS) model of the study area. At shallow depths, our model correlates with surface geology and reveals the presence of a sedimentary cover in the Liguro‐Provençal basin, as opposed to the Tyrrhenian basin where this is either very thin or absent. At ∼5‐km depth, high velocities below the Magnaghi, Vavilov, and Marsili seamounts point to an exhumed, scarcely serpentinized mantle. These are replaced by lower velocities at larger depths, likely connected to the presence of partial melt. At 50–60‐km depth, a very heterogeneous structure characterizes the Tyrrhenian basin, with low velocities pointing to the presence of fluids due to the lateral mantle inflow from the Ionian slab edges, and higher velocities associated with a relatively dry upper mantle. Such heterogeneity disappears at depths ≳75 km, replaced by more uniform velocities which are ∼2% lower than those found in the Liguro‐Provençal basin. We infer that, at the same depths, the Tyrrhenian basin is characterized by a larger concentration of fluids and possibly higher temperatures.

Subduction-related lamproitic signature in intraplate-like volcanic rocks: the case study of the Tallante alkali basalts, Betic Chain, South-Eastern Spain

South-Eastern Spain is a key locality of the Western Mediterranean to unravel the origin of post-collisional magmatism and its relationships with the multi-stage evolution of the upper mantle in complex geodynamic settings. This study is focused on the Plio-Pleistocene post-collisional intraplate-like alkali basalts cropping out in the Tallante area (South-Eastern Spain) that are erupted in a region previously characterised by subduction-related calc-alkaline to ultrapotassic (i.e., lamproitic) volcanism. Tallante alkali basalts represent a volcanic episode similar to other contemporary small volume intraplate volcanism of Western-Central Europe and North Africa, which are believed to derive from a Common Mantle Reservoir characterising the entire Circum-Mediterranean asthenosphere.We report new major, trace element, and Sr-Nd-Pb isotope data on Tallante alkali basalts, showing compositions broadly similar to the other mafic volcanic rocks derived from the Common Mantle Reservoir and thus suggesting an asthenospheric component similar to other European within plate volcanic rocks. On the other hand, specific trace element ratios, such as Ce/Pb, Nb/U, Th/Yb, along with D207Pb/204Pb and 87Sr/86Sr, clearly indicate the involvement of an enriched, subduction-related component, with a lamproitic geochemical and isotopic flavour. The trace element ratios suggest that low melting degree magmas from the Common Mantle Reservoir infiltrated and triggered melting of the strongly metasomatised sub-continental lithospheric mantle, which produced the lamproitic magmas previously erupted in the same area.The proposed origin of Tallante alkali basalts, located along a large scale left-lateral strike-slip fault, is consistent with analytical, numerical and laboratory models indicating that narrow subduction zones such as the Gibraltar Arc are characterised by mantle inflow around the slab edges determining upwelling and adiabatic melting of the asthenospheric mantle through slab tears.

A simple picture of mantle wedge flow patterns and temperature variation

The solid-state mantle flow is an important factor that controls the mass and heat transfer in the solid Earth. This study aims to provide a simple picture of three-dimensional (3-D) mantle flow patterns in the sub-arc region of subduction zones based on the results of 3-D steady-state numerical models with varying subduction parameters. Here, the mantle wedge flow pattern is evaluated based on the azimuthal directions of the mantle inflow from the back-arc and the down-dip outflow. The outflow direction generally parallels the subduction direction, but the inflow direction relative to the outflow direction depends on the local subduction obliquity – the angle between the subduction direction and the strike-normal axis of the subducting slab. A change in the strike of the slab leads to a change in the obliquity and thus the inflow direction. Such change is common along curved margins as the strike of the slab tends to follow that of the margin, or vice versa. Along convex-arc-ward margins, the mantle inflow is deflected towards the region of lowest obliquity but with reduced vigor due to lower dynamic pressure gradients that partly drive the flow, resulting in a cooler mantle wedge. Along concave-arc-ward margins, the mantle inflow is deflected away from the region of lowest obliquity but with increased vigor, resulting in a hotter mantle wedge. These effects increase with decreasing radius of curvature. Along-margin change in the dip of the subducting slab also affects the inflow direction through its impact on the strike of the slab, but its effect is relatively small. We express the azimuthal inner angle between the inflow and outflow directions as a function of obliquity and apply the function to predict sub-arc mantle inflow directions in the circum-Pacific and neighboring regions. Within and among these margins, the inner angle varies over its full range of 0–180°. Most of the margins that are 1000s of kilometers in length are either straight or curved concave-arc-ward with large radii of curvature, for which small or gradual along-margin changes in the mantle inflow direction and the mantle wedge temperature are predicted. A large drop in the mantle wedge temperature by up to a couple of hundred degrees is predicted at short convex-arc-ward segments, such as at the Kuril-Japan and Bonin-Mariana junctions. The fringes of flat slab segments are curved with small radii of curvature, likely resulting in sharp lateral changes in the inflow direction and the mantle wedge temperature.

Metamorphic gradient modification in the Early Cretaceous Northern Andes subduction zone: A record from thermally overprinted high-pressure rocks

New field observations and petrological data from Early Cretaceous metamorphic rocks in the Central Cordillera of the Colombian Andes allowed the recognition of thermally overprinted high-pressure rocks derived from oceanic crust protoliths. The obtained metamorphic path suggests that the rocks evolved from blueschist to eclogite facies towards upper amphibolite to high-pressure granulite facies transitional conditions. Eclogite facies conditions, better recorded in mafic protoliths, are revealed by relic lawsonite and phengite, bleb- to worm-like diopside-albite symplectites, as well as garnet core composition. Upper amphibolite to high pressure granulite facies overprinting is supported by coarse-grained brown-colored Ti-rich amphibole, augite, and oligoclase recrystallization, as well as the record of partial melting leucosomes.Phase equilibria and pressure-temperature (P-T) path modeling suggest initial high-pressure metamorphic conditions M1 yielding 18.2–24.5 kbar and 465–580 °C, followed by upper amphibolite to high pressure granulite facies overprinting stage M2 yielding 6.5–14.2 kbar and 580–720 °C. Retrograde conditions M3 obtained through chlorite thermometry yield temperatures ranging around 286–400 °C at pressures below 6.5–11 kbar. The obtained clockwise P-T path, the garnet zonation pattern revealing a decrease in Xgrs/Xprp related to Mg# increment from core to rim, the presence of partial melting veins, as well as regional constraints, document the modification of the thermal structure of the active subduction zone in Northern Andes during the Early Cretaceous. Such increment of the metamorphic gradient within the subduction interface is associated with slab roll-back geodynamics where hot mantle inflow was triggered. This scenario is also argued by the reported trench-ward magmatic arc migration and multiple extensional basin formation during this period. The presented example constitutes the first report of Cretaceous roll-back-related metamorphism in the Caribbean and Andean realms, representing an additional piece of evidence for a margin-scale extensional event that modified the northwestern border of South America during the Early Cretaceous.

Interference of tectonic signals in subsurface hydrologic monitoring through gravity and GPS due to mountain building

Abstract Global Positioning System observations in the Alps have now sufficient precision to reliably observe vertical surface movement rates at the orogen scale. The geodynamic modeling of converging plate margins requires constraints on the origin of orogenic uplift, of which the two end members are pure crustal uplift and crustal thickening, respectively. Gravity change rates joint with uplift measurements allows to distinguish the two mechanisms. We use vertical uplift rates over the Alpine range and the southern foreland basin, to predict the gravity change for different geodynamic hypotheses of pure uplift and mantle inflow, or crustal thickening and isostatic Moho lowering. The sensitivity of gravity as a tool to distinguish the two mechanisms is investigated. This model differs from the predicted isostatic movements, based on the glacial history and the mantle viscosity, since the uplift is measured and not predicted. The estimate of this tectonic signal is important, when gravity change rates, as those obtained from GRACE, are interpreted exclusively in terms of hydrologic changes tied to climatic variation. It has been already shown that in some areas, as the Tibetan plateau and the Himalayas, the tectonic signal is not negligible. Here we estimate the effect of the tectonic signal for the uplift of smaller mountain ranges, as is the Alpine arc. Our results indicate that tectonic and hydrological signals superpose and we cannot ignore the tectonic signal when using GRACE to invert for the equivalent water height (EWH).

Mantle flow influence on subduction evolution

The impact of remotely forced mantle flow on regional subduction evolution is largely unexplored. Here we investigate this by means of 3D thermo-mechanical numerical modeling using a regional modeling domain. We start with simplified models consisting of a 600 km (or 1400 km) wide subducting plate surrounded by other plates. Mantle inflow of ∼3 cm/yr is prescribed during 25 Myr of slab evolution on a subset of the domain boundaries while the other side boundaries are open. Our experiments show that the influence of imposed mantle flow on subduction evolution is the least for trench-perpendicular mantle inflow from either the back or front of the slab leading to 10–50 km changes in slab morphology and trench position while no strong slab dip changes were observed, as compared to a reference model with no imposed mantle inflow. In experiments with trench-oblique mantle inflow we notice larger effects of slab bending and slab translation of the order of 100–200 km. Lastly, we investigate how subduction in the western Mediterranean region is influenced by remotely excited mantle flow that is computed by back-advection of a temperature and density model scaled from a global seismic tomography model. After 35 Myr of subduction evolution we find 10–50 km changes in slab position and slab morphology and a slight change in overall slab tilt. Our study shows that remotely forced mantle flow leads to secondary effects on slab evolution as compared to slab buoyancy and plate motion. Still these secondary effects occur on scales, 10–50 km, typical for the large-scale deformation of the overlying crust and thus may still be of large importance for understanding geological evolution.

Numerical models of the magmatic processes induced by slab breakoff

After the onset of continental collision, magmatism often persists for tens of millions of years, albeit with a different composition, in reduced volumes, and with a more episodic nature and more widespread spatial distribution, compared to normal arc magmatism. Kinematic modelling studies have suggested that slab breakoff can account for this post-collisional magmatism through the formation of a slab window and subsequent heating of the overriding plate and decompression melting of upwelling asthenosphere, particularly if breakoff occurs at depths shallower than the overriding plate.To constrain the nature of any melting and the geodynamic conditions required, we numerically model the collision of two continental plates following a period of oceanic subduction. A thermodynamic database is used to determine the (de)hydration reactions and occurrence of melt throughout this process. We investigate melting conditions within a parameter space designed to generate a wide range of breakoff depths, timings and collisional styles.Under most circumstances, slab breakoff occurs deeper than the depth extent of the overriding plate; too deep to generate any decompressional melting of dry upwelling asthenosphere or thermal perturbation within the overriding plate. Even if slab breakoff is very shallow, the hot mantle inflow into the slab window is not sustained long enough to sufficiently heat the hydrated overriding plate to cause significant magmatism. Instead, for relatively fast, shallow breakoff we observe melting of asthenosphere above the detached slab through the release of water from the tip of the heating detached slab. Melting of the subducted continental crust during necking and breakoff is a more common feature and may be a more reliable indicator of the occurrence of breakoff. We suggest that magmatism from slab breakoff alone is unable to explain several of the characteristics of post-collisional magmatism, and that additional geodynamical processes need to be considered when interpreting magmatic observations.

Focusing of upward fluid migration beneath volcanic arcs: Effect of mineral grain size variation in the mantle wedge

AbstractWe use numerical models to investigate the effects of mineral grain size variation on fluid migration in the mantle wedge at subduction zones and on the location of the volcanic arc. Previous coupled thermal‐grain size evolution (T‐GSE) models predict small grain size (<1 mm) in the corner flow of the mantle wedge, a downdip grain size increase by ∼2 orders of magnitude along the base of the mantle wedge, and finer grain size in the mantle wedge for colder‐slab subduction zones. We integrate these T‐GSE modeling results with a fluid migration model, in which permeability depends on grain size, and fluid flow through a moving mantle matrix is driven by fluid buoyancy and dynamic pressure gradients induced by mantle flow. Our modeling results indicate that fluids introduced along the base of the mantle wedge beneath the fore arc are initially dragged downdip by corner flow due to the small grain size and low permeability immediately above the slab. As grain size increases with depth, permeability increases, resulting in upward fluid migration. Fluids released beneath the arc and the back arc are also initially dragged downdip, but typically are not transported as far laterally before they begin to travel upward. As the fluids rise through the back‐arc mantle wedge, they become deflected toward the trench due to the effect of mantle inflow. The combination of downdip migration in the fore arc and trench‐ward migration in the back arc results in pathways that focus fluids beneath the arc.

Mantle wedge flow pattern and thermal structure in Northeast Japan: Effects of oblique subduction and 3-D slab geometry

We develop a 3-D thermal model for the Northeast Japan subduction margin, using a realistic slab geometry for the subducting Pacific plate, and investigate the effects of oblique subduction and 3-D slab geometry on the mantle wedge flow pattern and the thermal structure. In the Tohoku region, the mantle wedge flow pattern is nearly two-dimensional resulting in a thermal structure similar to those obtained by a 2-D model, owing to the simple slab geometry and subduction nearly perpendicular to the margin. However, in Hokkaido, oblique subduction leads to 3-D mantle wedge flow with northerly inflow and west–northwestward outflow and also results in lower temperatures in the shallow part of the mantle wedge than in Tohoku due to lower sinking rate of the slab. Between Hokkaido and Tohoku, the slab has a hinge-like shape due to a relatively sharp change in the dip direction. In this hinge zone, northerly mantle inflow from Hokkaido and westerly mantle inflow from Tohoku converge, discouraging inflow from northwest and resulting in a cooler mantle wedge. The model-predicted mantle wedge flow patterns are consistent with observed seismic anisotropy and may explain the orientations of volcanic cross-chains. The predicted 3-D thermal structure correlates well with the along-arc variations in the location of the frontal arc volcanoes and help to provide new insights into the surface heat flow pattern and the down-dip extent of interplate earthquakes.

Lithospheric structure beneath the High Lava Plains, Oregon, imaged by scattered teleseismic waves

We compute high‐resolution seismic images from scattered wavefield to detect discontinuities beneath the High Lava Plains (HLP), using data recorded at a dense broadband array. Our images of the HLP and surrounding regions reveal (1) a prominent Moho discontinuity with varying depth, with thinnest crust of 35 km beneath the volcanic track, and thickened crust of ∼45 km beneath the Owyhee Plateau (OP); (2) distinct intracrustal velocity reversals beneath regions of pre‐2.0 Ma volcanism and within the OP; and (3) intermittent negative velocity discontinuities in the uppermost mantle beneath regions of Holocene volcanism and volcanic centers near Steens Mountain and Newberry volcano. These features exhibit remarkable similarity with those seen in the surface wave tomography and Ps receiver functions. We fail to find evidence for a ubiquitous regional lithosphere‐asthenosphere boundary (LAB). In concert with petrological constraints on the equilibration depths of primitive basaltic melts, our results suggest that the present‐day HLP mantle lithosphere is thin or absent, perhaps a consequence of episodes of extensive mantle inflow, lithospheric extension, and possibly melting induced by rapid slab rollback and trench retreat. It remains possible, however, that strong E‐W seismic anisotropy reported across this region may reduce the effective S‐wave velocity contrast to render the LAB less detectable. In contrast, the Owyhee Plateau exhibits a clear LAB, consistent with it being a block of older preexisting lithosphere. Our images demonstrate the complexity of mantle dynamics in the Cascadian back‐arc and the close casual link between subduction‐related processes and the origin of HLP volcanism.

The East Scotia Sea: Mantle to Microbe

The East Scotia Sea is an important geological, oceanographic, and biological gateway between the Pacific and Atlantic realms, containing a particularly good example of back-arc spreading, the East Scotia Ridge. Recent surveys show that the ridge consists of nine segments, separated by nontransform offsets. The five middle segments (E3-E7) have the morphology of slow-intermediate spreading centers and erupt mainly normal mid-ocean ridge basalt lavas. End segments, E2 and E9, are much shallower, have the inflated appearance of faster-spreading ridges, and erupt slightly enriched, plume-influenced rocks with small to moderate subduction components. Spreading rates vary from 62 mm/yr in the north to 70 mm/yr in the south and cannot explain the along-axis variations. Rapid slab retreat is believed to cause flow of Atlantic asthenosphere around the ends of the subducting slab at the South Sandwich Trench, advecting fertile mantle into the back-arc region. Eastward rollback of the slab brings this mantle within 150 km of the volcanic front, where excess melting occurs as a result of fluxing by subduction-related hydrous fluids. This leads to the initiation of magmatically inflated end segments, characterized by thicker crust and elevated topography. Reconnaissance work has detected hydrothermal plumes above the most inflated parts of segments E2 and E9, suggesting a close relationship between mantle inflow, magmatism, and hydrothermal venting. Vent sites at the East Scotia Ridge may be affected by currents from the Weddell Sea and the South Pacific. Hence, the at present unknown vent fauna could be of either Pacific or Atlantic type, or conceivably a combination of both.

Chapter 15 Compositional variations of magmas in the Aeolian arc: implications for petrogenesis and geodynamics

Abstract The volcanic rocks of the Aeolian arc exhibit important within-island and along-arc compositional variations that testify to both geochemical heterogeneous mantle sources and different roles and intensities of shallow-level magmatic evolution processes. Calc-alkaline magmas are present on all islands, but dominate in the western arc and at Lipari and Panarea. Shoshonitic rocks are present on the central-eastern islands and are particularly abundant at Vulcano and Stromboli. Mafic and intermediate rocks comprise the bulk of older volcanic sequences for most islands. Rhyolites are restricted to younger activity of the central arc, and become particularly abundant at Lipari and Vulcano. Regional variations of incompatible trace element ratios and Sr-, Nd-, and Pb-isotope signatures in mafic-intermediate rocks document the variable composition of mantle sources, which were contaminated by different types of metasomatic fluids released from an oceanic slab in the western-central sectors and from oceanic slab plus sediments in the east. This metasomatism was superimposed over a heterogeneous mantle wedge, which had a mid-ocean-ridge basalt (MORB-) to ocean-island basalt (OIB)- type character passing from the centre to the margins of the arc. The OIB-type component in the external arc is attributed to asthenospheric mantle inflow from the Africa foreland, around the borders of a narrow slab during rollback.

Sismicité et volcanisme dans le Sud-Ouest du bassin arrière-arc d’Okinawa (Nord-Est Taiwan)

AbstractLa plupart des activités volcaniques observées dans le bassin arrière-arc d’Okinawa s’attachent à la subduction de la plaque des Philippines. Afin d’étudier les mécanismes liés à l’ouverture du bassin d’Okinawa, nous avons analysé les données provenant d’une campagne d’écoute passive de la sismicité et les données géophysiques existantes (données magnétiques et sismiques) en utilisant plusieurs méthodes.Les estimations des profondeurs du toit et de la base du socle magnétique en utilisant les données des anomalies magnétiques ont été effectuées dans le bassin d’Okinawa. La croûte est amincie le long du méridien 123,3oE et est épaissie sous la région volcanique du « cross backarc volcanic trail » (CBVT). La ride de Gagua située le long du méridien 123oE sépare le bassin de Huatung et le bassin ouest-Philippin. Les lithosphères de chaque côté de cette structure sont d’âges différents. L’angle de la plaque Philippine plongeante est plus fort à l’Ouest qu’à l’Est suggérant la présence d’une déchirure du slab des Ryukyus alignée le long de 123,3oE.De fin novembre à début décembre 2003, 15 OBSs ont été déployés dans le sud-ouest du bassin d’Okinawa. Plus de 3300 séismes ont été localisés, dont 20 enregistrés simultanément par les réseaux à terre. En analysant la répartition de 2823 séismes sélectionnés et relocalisés, quatre essaims de séismes ont été identifiés. La plupart des séismes se ré-partissent dans l’axe du bassin. La faille de Lishan, une structure géologique majeure à terre, se prolonge dans le bassin d’Okinawa jusqu’à 123,6oE. Au nord de l’extension de la faille de Lishan, peu de séismes sont enregistrés. En analysant les spectres de vitesse des ondes P, un groupe de séismes de basse fréquence a été identifié sous la chaîne de volcans CBVT, ce qui suggère une origine magmatique pour ces séismes.En partant des nombreux séismes enregistrés par les réseaux terrestres et pendant cette campagne, deux inversions tomographiques ont été réalisées. (1) A l’extrémité ouest du slab des Ryukyus, une zone d’anomalie de faibles vitesses Vp, Vs et de fort rapport Vp/Vs, de 30 km d’épaisseur en forme de saucisse a été localisée sur le rebord du slab, à une profondeur variant de 20 à 125 km. Elle correspond à une zone de fusion partielle ou enrichie en eau provenant de la déshydratation du slab. Un chenal remonte de cette anomalie depuis une profondeur de 40 km en direction de l’île andé-sitique de Kueishantao, matérialisant l’alimentation magmatique de cette île. (2) Dans le bassin d’Okinawa, trois trajectoires magmatiques caractérisées par de faibles vitesses Vp, Vs et un fort rapport Vp/Vs ont été localisées vers 50 km de profondeur, au niveau de la déchirure du slab: le chenal principal remonte en direction de la chaîne volcanique transver-sale (CBVT) située à l’axe du bassin ; le deuxième chenal remonte en direction de l’arc des Ryukyus et s’arrête à une profondeur de 30 km sous l’île d’Iriomote ; le troisième chenal remonte parallèlement à la déchirure du slab. En outre, une remontée asthénosphérique en forme de langue se dirige vers l’axe du bassin. Nous suggérons que la déchirure de la plaque permet la remontée de composants exotiques et/ou d’une source thermique anormale en provenance du manteau située sous la plaque subduite, ce qui se traduit par une augmentation de l’activité volcanique dans la région.

Origin of the southern Okinawa Trough volcanism from detailed seismic tomography

Magmatism associated with subducting plate edges or slab tears has been suggested in the southern Okinawa Trough. The cross back‐arc volcanic trail, which consists of a cluster of about 70 seamounts, is located above a Ryukyu slab tear lying along the 123.3°E meridian. In November 2003, more than 3300 earthquakes recorded in this area by 15 ocean bottom seismometers and surrounding land stations during a period of 12 days were used to determine the three‐dimensional Vp and Vs velocity structures and Vp/Vs ratios. A mantle inflow characterized by low Vp and Vs and high Vp/Vs passing through the slab tear is imaged. The fluid and/or melt component is rising obliquely from the slab tear in the directions of the cross back‐arc volcanic trail, the northern slope of the southern Okinawa Trough and to north of Iriomote Island. The asthenospheric intake is also imaged by an inclined chip‐like high Vp/Vs and low Vp and Vs body dipping northerly, which might be linked to the slab retreat. West of the slab tear, most of the earthquakes are located around low Vp and Vs and high Vp/Vs bodies, which suggests that the seismicity is related to magmatic and/or fluid activities. East of it, earthquakes are concentrated in an area characterized by high Vp and Vs velocities and low Vp/Vs, suggesting that the magma chamber is absent beneath the axial part of the trough and that normal faulting is the main factor controlling the seismicity.

Trace element and isotopic variations from Mt. Vulture to Campanian volcanoes: constraints for slab detachment and mantle inflow beneath southern Italy

New Sr–Nd–Pb isotopic ratios and trace element data for volcanic mafic rocks outcropping along a E–W transect in southern Italy, from Mt. Vulture to Neapolitan volcanoes, are reported. The variation of LILE/HFSE, HFSE/HFSE and radiogenic isotopes along this transect indicates that all of these volcanoes contain both intra-plate and subduction-related signatures, with the former decreasing from Mt. Vulture to Campanian volcanoes. New data are also reported for the Paleocene alkaline rocks from Pietre Nere (Apulia foreland), which show isotopic ratios mostly overlapping the values for Mediterranean intra-plate volcanoes as well as the Eocene–Oligocene alkaline mafic lavas from the northern Adria plate. Pietre Nere provides evidence for an OIB mantle composition of FOZO-type, free of subduction influences, that is present beneath the Adria plate (Africa) before its collision with Europe. After this collision, and formation of the southern Apennines, westward inflow of mantle from the Adria plate to the Campanian area occurred, as a consequence of slab break off. Interaction of subduction components with inflowing Adria mantle generated hybrid sources beneath the Vulture–Campania area, which can explain the compositional features of both Mt. Vulture and the Campanian mafic rocks. Therefore, mafic magmas from these volcanoes represent variable degrees of mixing between different mantle components.