Changes in seismic anisotropy shed light on the nature of the Gutenberg discontinuity

Influence of the asthenosphere on earth dynamics and evolution

The existence of a thin, weak asthenospheric layer beneath Earth's lithospheric plates is consistent with existing geological and geophysical constraints, including Pleistocene glacio-isostatic adjustment, modeling of gravity anomalies, studies of seismic anisotropy, and post-seismic rebound. Mantle convection models suggest that a pronounced weak zone beneath the upper thermal boundary layer (lithosphere) may be essential to the plate tectonic style of convection found on Earth. The asthenosphere is likely related to partial melting and the presence of water in the sub-lithospheric mantle, further implying that the long-term evolution of the Earth may be controlled by thermal regulation and volatile recycling that maintain a geotherm that approaches the wet mantle solidus at asthenospheric depths.

On the Relationship Between Oceanic Plate Speed, Tectonic Stress, and Seismic Anisotropy

Seismic radial anisotropy (the squared ratio between the speeds of horizontally and vertically polarized shear waves,ξ = V S H 2 V S V 2 ) is a powerful tool to probe the direction of mantle flow and accumulated strain. While previous studies have confirmed the dependence of azimuthal anisotropy on plate speed, the first order control on radial anisotropy is unclear. In this study, we develop 2D ridge flow models combined with mantle fabric calculations to report that faster plates generate higher tectonics stresses and strain rates which lower the dislocation creep viscosity and lead to deeper anisotropy than beneath slower plates. We apply the SGLOBE-rani tomographic filter, resulting in a flat depth-age trend and stronger anisotropy beneath faster plates, which correlates well with 3D global anisotropic mantle models. Our predictions and observations suggest that as plate speed increases from 2 to 8 cm/yr, radial anisotropy increases by ∼0.01-0.025 in the upper 100-200 km of the mantle between 10 and 60 Ma.

Discovery of distinct lithosphere-asthenosphere boundary and the Gutenberg discontinuity in the Atlantic Ocean

The plate tectonic theory requires a rigid lithosphere floating over a weak asthenosphere, separated by the lithosphere-asthenosphere boundary, which has been sometimes interpreted as the Gutenberg discontinuity. Using a deep seismic reflection technique, we report the presence of two continuous reflections covering 27 Ma to 58 Ma oceanic lithosphere in the Atlantic Ocean. We find that the upper reflection deepens with age and follows the ~1250°C isotherm, whereas the deeper reflection lies at a constant depth of ~75 km. We suggest that the upper reflection represents the thermally controlled lithosphere-asthenosphere boundary, whereas the lower reflection is the Gutenberg discontinuity, a frozen-in dehydration boundary separating the dry mantle melting region above from the hydrated mantle below formed at the ridge axis. We also find that thermal mantle anomalies rejuvenate the lithosphere, uplift the lithosphere-asthenosphere boundary, and destroy the Gutenberg discontinuity.

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.

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.

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.

Evidence for frozen melts in the mid-lithosphere detected from active-source seismic data

The interactions of the lithospheric plates that form the Earth's outer shell provide much of the evidentiary basis for modern plate tectonic theory. Seismic discontinuities in the lithosphere arising from mantle convection and plate motion provide constraints on the physical and chemical properties of the mantle that contribute to the processes of formation and evolution of tectonic plates. Seismological studies during the past two decades have detected seismic discontinuities within the oceanic lithosphere in addition to that at the lithosphere-asthenosphere boundary (LAB). However, the depth, distribution, and physical properties of these discontinuities are not well constrained, which makes it difficult to use seismological data to examine their origin. Here we present new active-source seismic data acquired along a 1,130 km profile across an old Pacific plate (148-128 Ma) that show oceanic mid-lithosphere discontinuities (oceanic MLDs) distributed 37-59 km below the seafloor. The presence of the oceanic MLDs suggests that frozen melts that accumulated at past LABs have been preserved as low-velocity layers within the current mature lithosphere. These observations show that long-offset, high-frequency, active-source seismic data can be used to image mid-lithospheric structure, which is fundamental to understanding the formation and evolution of tectonic plates.

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.

Layering of subcontinental lithospheric mantle

Recent seismic studies reveal a sharp velocity drop mostly at ∼70-100km depth within the thick mantle keel beneath cratons, termed the mid-lithosphere discontinuity (MLD). The common presence of the MLD in cratonic regions indicates structural and property layering of the subcontinental lithospheric mantle (SCLM). The nature and origin of the MLD, and many issues associated with the layering of the SCLM are essential to understand the formation and evolution of continents, and have become frontier subjects in the Earth sciences.

Olivine anisotropy suggests Gutenberg discontinuity is not the base of the lithosphere

Tectonic plates are a key feature of Earth's structure, and their behavior and dynamics are fundamental drivers in a wide range of large-scale processes. The operation of plate tectonics, in general, depends intimately on the manner in which lithospheric plates couple to the convecting interior. Current debate centers on whether the transition from rigid lithosphere to flowing asthenosphere relates to increases in temperature or to changes in composition such as the presence of a small amount of melt or an increase in water content below a specified depth. Thus, the manner in which the rigid lithosphere couples to the flowing asthenosphere is currently unclear. Here we present results from laboratory-based torsion experiments on olivine aggregates with and without melt, yielding an improved database describing the crystallographic alignment of olivine grains. We combine this database with a flow model for oceanic upper mantle to predict the structure of the seismic anisotropy beneath ocean basins. Agreement between our model and seismological observations supports the view that the base of the lithosphere is thermally controlled. This model additionally supports the idea that discontinuities in velocity and anisotropy, often assumed to be the base of the lithosphere, are, instead, intralithospheric features reflecting a compositional boundary established at midocean ridges, not a rheological boundary.

High-resolution seismic constraints on flow dynamics in the oceanic asthenosphere

Convective flow in the mantle and the motions of tectonic plates produce deformation of Earth's interior, and the rock fabric produced by this deformation can be discerned using the anisotropy of the seismic wave speed. This deformation is commonly inferred close to lithospheric boundaries beneath the ocean in the uppermost mantle, including near seafloor-spreading centres as new plates are formed via corner flow, and within a weak asthenosphere that lubricates large-scale plate-driven flow and accommodates smaller scale convection. Seismic models of oceanic upper mantle differ as to the relative importance of these deformation processes: seafloor spreading fabric is very strong just beneath the crust-mantle boundary (the Mohorovičić discontinuity, or Moho) at relatively local scales, but at the global and ocean-basin scales, oceanic lithosphere typically appears weakly anisotropic when compared to the asthenosphere. Here we use Rayleigh waves, recorded across an ocean-bottom seismograph array in the central Pacific Ocean (the NoMelt Experiment), to provide unique localized constraints on seismic anisotropy within the oceanic lithosphere-asthenosphere system in the middle of a plate. We find that azimuthal anisotropy is strongest within the high-seismic-velocity lid, with the fast direction coincident with seafloor spreading. A minimum in the magnitude of azimuthal anisotropy occurs within the middle of the seismic low-velocity zone, and then increases with depth below the weakest portion of the asthenosphere. At no depth does the fast direction correlate with the apparent plate motion. Our results suggest that the highest strain deformation in the shallow oceanic mantle occurs during corner flow at the ridge axis, and via pressure-driven or buoyancy-driven flow within the asthenosphere. Shear associated with motion of the plate over the underlying asthenosphere, if present, is weak compared to these other processes.