A thin mantle transition zone beneath the equatorial Mid-Atlantic Ridge.

The location and degree of material transfer between the upper and lower mantle are key to the Earth's thermal and chemical evolution. Sinking slabs and rising plumes are generally accepted as locations of transfer1,2, whereas mid-ocean ridges are not typically assumed to have a role3. However, tight constraints from in situ measurements at ridges have proved to be challenging. Here we use receiver functions that reveal the conversion of primary to secondary seismic waves to image the discontinuities that bound the mantle transition zone, using ocean bottom seismic data from the equatorial Mid-Atlantic Ridge. Our images show that the seismic discontinuity at depths of about 660 kilometres is broadly uplifted by 10 ± 4 kilometres over a swath about 600 kilometres wide and that the 410-kilometre discontinuity is depressed by 5 ± 4 kilometres. This thinning of the mantle transition zone is coincident with slow shear-wave velocities in the mantle, from global seismic tomography4-7. In addition, seismic velocities in the mantle transition zone beneath the Mid-Atlantic Ridge are on average slower than those beneath older Atlantic Ocean seafloor. The observations imply material transfer from the lower to the upper mantle-either continuous or punctuated-that is linked to the Mid-Atlantic Ridge. Given the length and longevity of the mid-ocean ridge system, this implies that whole-mantle convection may be more prevalent than previously thought, with ridge upwellings having a role in counterbalancing slab downwellings.

Author Correction: Variable water input controls evolution of the Lesser Antilles volcanic arc.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Variable water input controls evolution of the Lesser Antilles volcanic arc.

Oceanic lithosphere carries volatiles, notably water, into the mantle through subduction at convergent plate boundaries. This subducted water exercises control on the production of magma, earthquakes, formation of continental crust and mineral resources. Identifying different potential fluid sources (sediments, crust and mantle lithosphere) and tracing fluids from their release to the surface has proved challenging1. Atlantic subduction zones are a valuable endmember when studying this deep water cycle because hydration in Atlantic lithosphere, produced by slow spreading, is expected to be highly non-uniform2. Here, as part of a multi-disciplinary project in the Lesser Antilles volcanic arc3, we studied boron trace element and isotopic fingerprints of melt inclusions. These reveal that serpentine-that is, hydrated mantle rather than crust or sediments-is a dominant supplier of subducted water to the central arc. This serpentine is most likely to reside in a set of major fracture zones subducted beneath the central arc over approximately the past ten million years. The current dehydration of these fracture zones coincides with the current locations of the highest rates of earthquakes and prominent low shear velocities, whereas the preceding history of dehydration is consistent with the locations of higher volcanic productivity and thicker arc crust. These combined geochemical and geophysical data indicate that the structure and hydration of the subducted plate are directly connected to the evolution of the arc and its associated seismic and volcanic hazards.

Sediment structure at the equatorial mid-atlantic ridge constrained by seafloor admittance using data from the PI-LAB experiment.

Well-constrained marine sediment characteristics (sediment thickness and shear wave velocity) are important not only for the study of climate over geologic times scales but also for correcting and accounting for its presence in seismic data used to investigate deeper structures. We use data from the PI-LAB (Passive Imaging of the Lithosphere Asthenosphere Boundary) experiment, which consisted of 39 broadband ocean bottom seismometers deployed at the Equatorial Mid-Atlantic Ridge near the Chain fracture zone covering 0-80 Myr old seafloor. We compute admittance between the pressure to the vertical displacement at the seafloor at frequencies between 0.1 and 0.2 Hz for microseism-generated Rayleigh waves for 18 stations where data quality is good to determine the sediment thickness and shear wave velocity. We find a general trend of increasing sediment thickness with the seafloor ages, as expected with sediment thicknesses that range from 10-450 m and, shear wave velocities that range from 0.05-0.34 km/s. We find sediment thickness varies almost uniformly across both sides of the ridge, and it indicates that both sides experienced a similar sedimentation process. Our results are in good agreement with the global sediment model that is based on drilling cores and active source experiments, but thinner by up to 50 m at several stations on seafloor older than 25 My. Overlap of the 95% confidence regions between admittance and Ps estimates for thickness and shear velocity is found at 15 stations where we have both Ps and admittance estimates. It suggests that both methods yield accurate estimates for sediment thickness. In addition, our admittance result extends the lateral resolution of sediment characteristics to stations that were not previously resolved by Ps.

Predictions and Observations for the Oceanic Lithosphere From S-to-P Receiver Functions and SS Precursors.

The ocean lithosphere is classically described by the thermal half-space cooling (HSC) or the plate models, both characterized by a gradual transition to the asthenosphere beneath. Scattered waves find sharp seismic discontinuities beneath the oceans, possibly from the base of the plate. Active source studies suggest sharp discontinuities from a melt channel. We calculate synthetic S-to-P receiver functions and SS precursors for the HSC and plate models and also for channels. We find that the HSC and plate model velocity gradients are too gradual to create interpretable scattered waves from the base of the plate. Subtle phases are predicted to follow a similar trend as observations, flattening at older ages. Therefore, the seismic discontinuities are probably caused by a thermally controlled process that can also explain their amplitude, such as melting. Melt may coalesce in channels, although channels >10 km thick should be resolvable by scattered wave imaging.

Broad fault zones enable deep fluid transport and limit earthquake magnitudes.

Constraining the controlling factors of fault rupture is fundamentally important. Fluids influence earthquake locations and magnitudes, although the exact pathways through the lithosphere are not well-known. Ocean transform faults are ideal for studying faults and fluid pathways given their relative simplicity. We analyse seismicity recorded by the Passive Imaging of the Lithosphere-Asthenosphere Boundary (PI-LAB) experiment, centred around the Chain Fracture Zone. We find earthquakes beneath morphological transpressional features occur deeper than the brittle-ductile transition predicted by simple thermal models, but elsewhere occur shallower. These features are characterised by multiple parallel fault segments and step overs, higher proportions of smaller events, gaps in large historical earthquakes, and seismic velocity structures consistent with hydrothermal alteration. Therefore, broader fault damage zones preferentially facilitate fluid transport. This cools the mantle and reduces the potential for large earthquakes at localized barriers that divide the transform into shorter asperity regions, limiting earthquake magnitudes on the transform.

Slab to back-arc to arc: Fluid and melt pathways through the mantle wedge beneath the Lesser Antilles.

Volatiles expelled from subducted plates promote melting of the overlying warm mantle, feeding arc volcanism. However, debates continue over the factors controlling melt generation and transport, and how these determine the placement of volcanoes. To broaden our synoptic view of these fundamental mantle wedge processes, we image seismic attenuation beneath the Lesser Antilles arc, an end-member system that slowly subducts old, tectonized lithosphere. Punctuated anomalies with high ratios of bulk-to-shear attenuation (Qκ-1/Qµ-1 > 0.6) and VP/VS (>1.83) lie 40 km above the slab, representing expelled fluids that are retained in a cold boundary layer, transporting fluids toward the back-arc. The strongest attenuation (1000/QS ~ 20), characterizing melt in warm mantle, lies beneath the back-arc, revealing how back-arc mantle feeds arc volcanoes. Melt ponds under the upper plate and percolates toward the arc along structures from earlier back-arc spreading, demonstrating how slab dehydration, upper-plate properties, past tectonics, and resulting melt pathways collectively condition volcanism.

A sharp lithosphere-asthenosphere boundary imaged beneath eastern North America.

Plate tectonic theory hinges on the concept of a relatively rigid lithosphere moving over a weaker asthenosphere, yet the nature of the lithosphere-asthenosphere boundary remains poorly understood. The gradient in seismic velocity that occurs at this boundary is central to constraining the physical and chemical properties that create differences in mechanical strength between the two layers. For example, if the lithosphere is simply a thermal boundary layer that is more rigid owing to colder temperatures, mantle flow models indicate that the velocity gradient at its base would occur over tens of kilometres. In contrast, if the asthenosphere is weak owing to volatile enrichment or the presence of partial melt, the lithosphere-asthenosphere boundary could occur over a much smaller depth range. Here we use converted seismic phases in eastern North America to image a very sharp seismic velocity gradient at the base of the lithosphere-a 3-11 per cent drop in shear-wave velocity over a depth range of 11 km or less at 90-110 km depth. Such a strong, sharp boundary cannot be reconciled with a purely thermal gradient, but could be explained by an asthenosphere that contains a few per cent partial melt or that is enriched in volatiles relative to the lithosphere.

Subduction history of the Caribbean from upper-mantle seismic imaging and plate reconstruction.

The margins of the Caribbean and associated hazards and resources have been shaped by a poorly understood history of subduction. Using new data, we improve teleseismic P-wave imaging of the eastern Caribbean upper mantle and compare identified subducted-plate fragments with trench locations predicted from plate reconstruction. This shows that material at 700-1200 km depth below South America derives from 90-115 Myr old westward subduction, initiated prior to Caribbean Large-Igneous-Province volcanism. At shallower depths, an accumulation of subducted material is attributed to Great Arc of the Caribbean subduction as it evolved over the past 70 Ma. We interpret gaps in these subducted-plate anomalies as: a plate window and tear along the subducted Proto-Caribbean ridge; tearing along subducted fracture zones, and subduction of a volatile-rich boundary between Proto-Caribbean and Atlantic domains. Phases of back-arc spreading and arc jumps correlate with changes in age, and hence buoyancy, of the subducting plate.

Scattered wave imaging of the oceanic plate in Cascadia.

Fifty years after plate tectonic theory was developed, the defining mechanism of the plate is still widely debated. The relatively short, simple history of young ocean lithosphere makes it an ideal place to determine the property that defines a plate, yet the remoteness and harshness of the seafloor have made precise imaging challenging. We use S-to-P receiver functions to image discontinuities beneath newly formed lithosphere at the Juan de Fuca and Gorda Ridges. We image a strong negative discontinuity at the base of the plate increasing from 20 to 45 km depth beneath the 0- to 10-million-year-old seafloor and a positive discontinuity at the onset of melting at 90 to 130 km depth. Comparison with geodynamic models and experimental constraints indicates that the observed discontinuities cannot easily be reconciled with subsolidus mechanisms. Instead, partial melt may be required, which would decrease mantle viscosity and define the young oceanic plate.

Seismic Imaging of Thickened Lithosphere Resulting From Plume Pulsing Beneath Iceland.

Ocean plates conductively cool and subside with seafloor age. Plate thickening with age is also predicted, and hot spots may cause thinning. However, both are debated and depend on the way the plate is defined. Determining the thickness of the plates along with the process that governs it has proven challenging. We use S-to-P (Sp) receiver functions to image a strong, persistent LAB beneath Iceland where the mid-Atlantic Ridge interacts with a plume with hypothesized pulsating thermal anomaly. The plate is thickest, up to 84 ± 6 km, beneath lithosphere formed during times of hypothesized hotter plume temperatures and as thin as 61 ± 6 km beneath regions formed during colder intervals. We performed geodynamic modeling to show that these plate thicknesses are inconsistent with a thermal lithosphere. Instead, periods of increased plume temperatures likely increased the melting depth, causing deeper depletion and dehydration, and creating a thicker plate. This suggests plate thickness is dictated by the conditions of plate formation.

The Role of Oceanic Transform Faults in Seafloor Spreading: A Global Perspective From Seismic Anisotropy.

Mantle anisotropy beneath mid-ocean ridges and oceanic transforms is key to our understanding of seafloor spreading and underlying dynamics of divergent plate boundaries. Observations are sparse, however, given the remoteness of the oceans and the difficulties of seismic instrumentation. To overcome this, we utilize the global distribution of seismicity along transform faults to measure shear wave splitting of over 550 direct S phases recorded at 56 carefully selected seismic stations worldwide. Applying this source-side splitting technique allows for characterization of the upper mantle seismic anisotropy, and therefore the pattern of mantle flow, directly beneath seismically active transform faults. The majority of the results (60%) return nulls (no splitting), while the non-null measurements display clear azimuthal dependency. This is best simply explained by anisotropy with a near vertical symmetry axis, consistent with mantle upwelling beneath oceanic transforms as suggested by numerical models. It appears therefore that the long-term stability of seafloor spreading may be associated with widespread mantle upwelling beneath the transforms creating warm and weak faults that localize strain to the plate boundary.

Imaging Lithospheric Discontinuities Beneath the Northern East African Rift Using S-to-P Receiver Functions.

Imaging the lithosphere is key to understand mechanisms of extension as rifting progresses. Continental rifting results in a combination of mechanical stretching and thinning of the lithosphere, decompression upwelling, heating, sometimes partial melting of the asthenosphere, and potentially partial melting of the mantle lithosphere. The northern East African Rift system is an ideal locale to study these processes as it exposes the transition from tectonically active continental rifting to incipient seafloor spreading. Here we use S-to-P receiver functions to image the lithospheric structure beneath the northernmost East African Rift system where it forms a triple junction between the Main Ethiopian rift, the Red Sea rift, and the Gulf of Aden rift. We image the Moho at 31 ± 6 km beneath the Ethiopian plateau. The crust is 28 ± 3 km thick beneath the Main Ethiopian rift and thins to 23 ± 2 km in northern Afar. We identify a negative phase, a velocity decrease with depth, at 67 ± 3 km depth beneath the Ethiopian plateau, likely associated with the lithosphere-asthenosphere boundary (LAB), and a lack of a LAB phase beneath the rift. Using observations and waveform modeling, we show that the LAB phase beneath the plateau is likely defined by a small amount of partial melt. The lack of a LAB phase beneath the rift suggests melt percolation through the base of the lithosphere beneath the northernmost East African Rift system.

Marine Geophysical Investigation of the Chain Fracture Zone in the Equatorial Atlantic From the PI-LAB Experiment.

The Chain Fracture Zone is a 300-km-long transform fault that offsets the Mid-Atlantic Ridge. We analyzed new multibeam bathymetry, backscatter, gravity, and magnetic data with 100% multibeam bathymetric data over the active transform valley and adjacent spreading segments as part of the Passive Imaging of the Lithosphere Asthenosphere Boundary (PI-LAB) Experiment. Analyses of these data sets allow us to determine the history and mode of crustal formation and the tectonic evolution of the transform system and adjacent ridges over the past 20 Myr. We model the total field magnetic anomaly to determine the age of the crust along the northern ridge segment to better establish the timing of the variations in the seafloor fabric and the tectonic-magmatic history of the region. Within the active transform fault zone, we observe four distinct positive flower structures with several en échelon fault scarps visible in the backscatter data. We find up to -10 mGal residual Mantle Bouguer Anomaly in the region of the largest positive flower structure within the transform zone suggesting crustal thickening relative to the crustal thinning typically observed in fracture zones in the Atlantic. The extensional/compressional features observed in the Chain Transform are less pronounced than those observed further north in the Vema, St. Paul, and Romanche and may be due to local ridge segment adjustments.

Seismic Imaging of the North American Midcontinent Rift Using S-to-P Receiver Functions.

North America's ~1.1-Ga failed Midcontinent Rift (MCR) is a striking feature of gravity and magnetic anomaly maps across the continent. However, how the rift affected the underlying lithosphere is not well understood. With data from the Superior Province Rifting Earthscope Experiment and the USArray Transportable Array, we constrain three-dimensional seismic velocity discontinuity structure in the lithosphere beneath the southwestward arm of the MCR using S-to-P receiver functions. We image a velocity increase with depth associated with the Moho at depths of 33-40 ± 4 km, generally deepening toward the east. The Moho amplitude decreases beneath the rift axis in Minnesota and Wisconsin, where the velocity gradient is more gradual, possibly due to crustal underplating. We see hints of a deeper velocity increase at 61 ± 4-km depth that may be the base of underplating. Beneath the rift axis further south in Iowa, we image two distinct positive phases at 34-39 ± 4 and 62-65 ± 4 km likely related to an altered Moho and an underplated layer. We image velocity decreases with depth at depths of 90-190 ± 7 km in some locations that do not geographically correlate with the rift. These include a discontinuity at depths of 90-120 ± 7 km with a northerly dip in the south that abruptly deepens to 150-190 ± 7 km across the Spirit Lake Tectonic Zone provincial suture. The negative phases may represent a patchy, frozen-in midlithosphere discontinuity feature that likely predates the MCR and/or be related to lithospheric thickness.

A unified continental thickness from seismology and diamonds suggests a melt-defined plate.

Thick, rigid continents move over the weaker underlying mantle, although geophysical and geochemical constraints on the exact thickness and defining mechanism of the continental plates are widely discrepant. Xenoliths suggest a chemical continental lithosphere ~175 kilometers thick, whereas seismic tomography supports a much thicker root (>250 kilometers) and a gradual lithosphere-asthenosphere transition, consistent with a thermal definition. We modeled SS precursor waveforms from continental interiors and found a 7 to 9% velocity drop at depths of 130 to 190 kilometers. The discontinuity depth is well correlated with the origin depths of diamond-bearing xenoliths and corresponds to the transition from coarse to deformed xenoliths. At this depth, the xenolith-derived geotherm also intersects the carbonate-silicate solidus, suggesting that partial melt defines the plate boundaries beneath the continental interior.

Constraints on the anisotropic contributions to velocity discontinuities at ∼60 km depth beneath the Pacific.

Strong, sharp, negative seismic discontinuities, velocity decreases with depth, are observed beneath the Pacific seafloor at ∼60 km depth. It has been suggested that these are caused by an increase in radial anisotropy with depth, which occurs in global surface wave models. Here we test this hypothesis in two ways. We evaluate whether an increase in surface wave radial anisotropy with depth is robust with synthetic resolution tests. We do this by fitting an example surface wave data set near the East Pacific Rise. We also estimate the apparent isotropic seismic velocity discontinuities that could be caused by changes in radial anisotropy in S-to-P and P-to-S receiver functions and SS precursors using synthetic seismograms. We test one model where radial anisotropy is caused by olivine alignment and one model where it is caused by compositional layering. The result of our surface wave inversion suggests strong shallow azimuthal anisotropy beneath 0-10 Ma seafloor, which would also have a radial anisotropy signature. An increase in radial anisotropy with depth at 60 km depth is not well-resolved in surface wave models, and could be artificially observed. Shallow isotropy underlain by strong radial anisotropy could explain moderate apparent velocity drops (<6%) in SS precursor imaging, but not receiver functions. The effect is diminished if strong anisotropy also exists at 0-60 km depth as suggested by surface waves. Overall, an increase in radial anisotropy with depth may not exist at 60 km beneath the oceans and does not explain the scattered wave observations.

A global view of the lithosphere-asthenosphere boundary.

The lithosphere-asthenosphere boundary divides the rigid lid from the weaker mantle and is fundamental in plate tectonics. However, its depth and defining mechanism are not well known. We analyzed 15 years of global seismic data using P-to-S (Ps) converted phases and imaged an interface that correlates with tectonic environment, varying from 95 +/- 4 kilometers beneath Precambrian shields and platforms to 81 +/- 2 kilometers beneath tectonically altered regions and 70 +/- 4 kilometers at oceanic island stations. High-frequency Ps observations require a sharp discontinuity; therefore, this interface likely represents a boundary in composition, melting, or anisotropy, not temperature alone. It likely represents the lithosphere-asthenosphere boundary under oceans and tectonically altered regions, but it may constitute another boundary in cratonic regions where the lithosphere-asthenosphere boundary is thought to be much deeper.