Plate-driven Flow Research Articles

Upper mantle seismic anisotropy as a constraint for mantle flow and continental dynamics of the North American plate

The alignment of intrinsically anisotropic olivine crystals under convection is typically invoked as the cause of the bulk of seismic anisotropy inferred from shear-wave splitting (SWS). This provides a means of constraining the interplay between continental dynamics and the deep mantle, in particular for densely instrumented regions such as North America after USArray. There, a comparison of “fast orientations” from SWS with absolute plate motions (APM) suggests that anisotropy is mainly controlled by plate motions. However, large regional misfits and the limited realism of the APM model motivate us to further explore SWS based anisotropy. If SWS is estimated from olivine alignment in mantle circulation instead, plate-driven flow alone produces anisotropy that has large misfits with SWS. The addition of large-scale mantle density anomalies and lateral viscosity variations significantly improves models. Although a strong continental craton is essential, varying its geometry does, however, not improve the plate-scale misfit. Moreover, models based on higher resolution tomography degrade the fit, indicating issues with the flow model assumptions and/or a missing contributions to anisotropy. We thus compute a “lithospheric complement” to achieve a best-fit, joint representation of asthenospheric and frozen-in lithospheric anisotropy. The complement shows coherent structure and regional correlation with independently imaged crustal and upper mantle anisotropy. Dense SWS measurements therefore provide information on depth-dependent anisotropy with implications for tectonics, but much remains to be understood about continental anisotropy and its origin.

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.

Interfacial instability in two-layer Couette–Poiseuille flow of viscoelastic fluids

The temporal linear stability analysis of shear flow of two-layer viscoelastic fluids, both described using the upper convected Maxwell (UCM) model, is carried out. The plane Couette flow under creeping flow conditions both in the absence and presence of pressure gradient is examined to study the role of unmatched elasticities in the interfacial instability. As the most critical disturbance is known to be of finite wavelength of the order of channel width, the numerical analysis examines the entire range of disturbance wavenumbers and the stability map is constructed in the parametric space of fluid Weissenberg numbers to indicate the region of stable interface. With a focus on optical fiber coating process, the analysis investigates the role of pressure gradient imposed on the plate-driven flow. Both the adverse and favorable pressure gradients are analyzed to identify the region for stable coating flow. In the presence of pressure gradient, the region of stable interface broadens when the high elasticity fluid occupies the region of low shear rate, which is the bottom (top) layer for the adverse (favorable) pressure gradient. For the two fluids of unmatched rheology, in addition to elastic instability, the role of viscosity stratification leading to a jump in the shear rate at interface is also examined. The adverse pressure gradient has a stabilizing effect when more viscous fluid is more elastic, whereas the favorable pressure gradient tends to stabilize the interface when the less viscous fluid has higher elasticity than the more viscous fluid.

Seismic anisotropy predicted for 2-D plate-driven flow in the Lau back–arc basin

The Lau back–arc basin and spreading centers exhibit significant along-strike variations in plate motion rates and distance between the spreading centers and the Tonga trench. The mantle processes are expected to be complex in this region and seismic anisotropy offers a key constraint. Linked geodynamic and mineral alignment models of the mantle wedge predict the scale of anisotropic variation and underlying assumptions can be tested by comparison to observed anisotropy. Three cross-basin transects allow us to isolate the effects of trench-spreading center distance as well as rates of convergence and spreading. Lattice preferred orientations (LPO) corresponding to each transect’s 2-D flow field are calculated for peridotite polycrystals. Predicted P-wave anisotropy of several percent and shear-wave split times up to 3.5s are determined, with measurable differences predicted between transects. Variation in the 2-D estimates of flow driven by each transect’s plate kinematics will induce along-strike flow in the mantle wedge. We find that the rate of such flow would likely not alter the subduction/spreading induced LPO sufficiently to explain the reported trench/arc parallel fast seismic directions in the Lau Basin; additional geodynamic factors must play a role.

Rapid mantle flow beneath the Tonga volcanic arc

Flow in the Earth's asthenospheric mantle is typically assumed to be driven by motions of the overlying and adjacent plates, suggesting flow rates comparable to plate velocities. However, arc-parallel shear-wave splitting and geochemical observations imply along-strike flow in the Lau basin, presenting a challenge to geodynamic models of plate-driven subduction systems as plate-driven flow models predict arc-perpendicular fast direction with simple (A-type) mineral texturing. Although B-type mineral texturing would result in the fast directions developing 90° relative to A-type and likely occurs in the forearc mantle, such texturing is unlikely in the asthenosphere wedge where higher temperatures and weak rheology likely prevail. By tracking mineral texturing development within geodynamic flow models patterned after the Tongan system in the Lau basin that include along-strike flow, we find that the observations are best explained by models including rapid along-arc flow within the subarc mantle. The effect of melt on the anisotropy does not appear to be a primary control, but needs to be investigated more fully. The best fit to the observations is a model with along-strike flow within a low-viscosity channel beneath the arc at a rate of nearly 50 cm/yr, several times the plate motion velocities.

On the steady shear flow of a dipolar fluid in a porous half-space

The plate-driven flow of an incompressible, homogeneous, dipolar fluid in a porous half-space is studied. The exact solution to this problem, which corresponds to the porous media version of the steady-state Stokes’ first problem for dipolar fluids, is obtained and an in-depth analytical and numerical investigation is carried out. Specifically, special cases of the physical parameters are determined and investigated, asymptotic results are given, physical interpretation(s) are discussed, and connections to other areas are noted. Most significantly, the analysis shows the following: (I) that a back flow can occur; (II) the flow velocity is predicted to exceed that of the (driving) plate for certain values of the physical parameters; (III) that a connection exists between the golden ratio and the solution presented here; (IV) that the flow is mathematically equivalent to the unforced motion of a damped harmonic oscillator.

Shear-wave polarization anisotropy and subduction-induced flow in the mantle wedge of northeastern Japan

Shear-wave polarization anisotropy in the central part of northeastern (NE) Japan was investigated. We analyzed S phases of 293 intermediate-depth earthquakes recorded at 77 stations and obtained 1286 splitting parameters. The obtained results show the systematic variation in splitting parameters across the arc. The leading shear-wave polarization directions (fast direction) obtained at stations in the western side of the study area are oriented nearly E–W, which is sub-parallel to the direction of relative plate motion between the Pacific plate and the North American/Okhotsk plate. In contrast, fast directions obtained at stations in the eastern side of the study area are oriented approximately N–S. Stations in the western side have a larger average delay time compared to those in the eastern side. These variations suggest that the nature of anisotropy is quite different between the eastern and the western sides. Both experimental studies on the deformation of olivine and numerical simulations of plate-driven flow predict that the lattice-preferred orientation of olivine causes the fast direction sub-parallel to the flow direction for the conditions in the mantle wedge of NE Japan. We thus infer that the lattice-preferred orientation of olivine generated by flow-induced strain is the most likely candidate for anisotropy that produces splitting with E–W fast direction. Three possible causes of anisotropy in the eastern side are considered: deformation of water-rich olivine in the mantle wedge, trench-parallel flow in the mantle wedge due to along-strike dip variations in the slab and anisotropy in the crust and the slab. We could not, however, determine which causes are dominant. Further systematic study is required to reveal the cause of anisotropy in the eastern side.

Back-arc spreading and mantle flow in the East Scotia Sea

Abstract The East Scotia Ridge exhibits systematic variations in axial morphology and basalt geochemistry. Central segments have morphology typical of intermediate-rate spreading centres and erupt mainly normal mid-ocean ridge basalt (N-MORB). Segments near the ridge ends exhibit anomalous, inflated, axial morphology and erupt more evolved basalts, influenced by the Bouvet plume in the north. As the end segments lie closer to the volcanic arc, these variations could be caused by coupled flow within the mantle wedge, as inferred from similar studies in the Lau Basin. Three of the four zones of crustal accretion defined from the Lau Basin may be identified in the East Scotia Sea, although there is no counterpart to a zone of diminished magma supply observed at the East Lau Spreading Centre. Superimposed on the pattern of plate-driven flow is a ridge-parallel flow related to inflow of Atlantic mantle into the East Scotia Sea back-arc region at both ends of the South Sandwich slab. The inflow causes enhanced magmatism and propagation of the end segments towards the middle of the back-arc region, and may be related to trench-parallel flow beneath the rapidly retreating slab. Alternatively, it may be driven by buoyancy flux from Atlantic hot spots. There is no evidence that retreat was ever driven by escape flow of Pacific mantle.

Geochemical consequences of melt transport in 2-D: The sensitivity of trace elements to mantle dynamics

Dynamic models are presented to investigate the consequences of melting and melt transport for stable trace element geochemistry in open systems. These models show that including explicit melt transport in 2-D adds non-trivial behaviour because melts and residues can travel and mix along very different trajectories. Calculations are presented for both equilibrium and disequilibrium transport, and passive and active mid-ocean ridge flows. These calculations demonstrate that trace elements are sensitive to mantle dynamics and can readily distinguish between different end-member flow fields. Passive, plate-driven flow with strong melt focusing produces enrichments of incompatible elements. Active small-scale solid convection within the partially molten region, however, can lead to extreme dilution of incompatible elements, suggesting that this form of convection may not be significant beneath normal ridges. These calculations provide additional predictions about across-axis trends of geochemical variability and estimate the variation in concentrations that can occur even for a constant source. Many of these results are not seen in geochemical models that neglect melt transport and we discuss how this new behaviour affects the inferences drawn from simpler models.

Upper mantle peridotites in the Bay of Islands Ophiolite, Newfoundland: Formation during the final stages of a spreading centre?

The structure of upper mantle rocks in ophiolites reflects the high-temperature flow beneath an oceanic spreading centre, locally overprinted by detachment-related strain. The structure of the mantle section in the ophiolitic Table Mountain massif, Bay of Islands Complex (BOIC), Newfoundland, was investigated in detail. It is correlated with other massifs in the BOIC and the spreading-related mantle flow history is reconstructed. Six structural units can be recognized on the basis of changes in the attitude of the high-temperature mineral stretching lineation. The relative age relationships between the structural units can partly be established. They support the plate-thickening model for oceanic lithosphere formation. The structural history of the mantle section suggests that early plate-driven flow was followed by forced flow, underplating of an off-axis diapir to the base of the lithosphere, and detachment. This complex history is explained in a model of a decreasing spreading rate which tracks the waning stages of spreading before detachment of the ophiolite. During this final history, the rate of spreading was decoupled from the rate of mantle upwelling so that a large forced flow field could build up. In the Blow Me Down (BMD) massif of the BOIC, all spreading-related flow structures are parallel to the ridge, suggesting that the structures and the massif were frozen in near the ridge itself. The presence of several kilometres of massive dunites in the BMD massif might indicate that in the final stages of spreading, melt production was higher than could be accommodated by spreading so that melt ponded at the base of the crust. Lherzolites occurring at the base of the BOIC are considered as product of non-adiabatic mantle upwelling prevalent in the final stages of spreading.

Origin of complex upper mantle structures in the southern Lewis Hills (Bay of Islands Ophiolite, Newfoundland)

The Springers Hill area (Lewis Hills, Bay of Islands Ophiolite) may represent oceanic lithosphere created in close proximity to the nontransform segment of an oceanic fracture zone. Upper mantle rocks exposed on Springers Hill were investigated to establish whether their development was affected by the thermal and rheological changes associated with oceanic fracture zones. Harzburgites of the Springers Hill area reveal complex structural patterns. On a small scale, foliations defined by orthopyroxene grains intersect foliations defined by spinel grains at various angles. Olivine petrofabric work demonstrates that only the spinel foliation is related to the preserved flow plane. The orthopyroxene foliation appears to be the result of pull-apart of formerly larger grains during high-temperature deformation. On the larger scale, orientation patterns of foliation, lineation, and dykes suggest that strike-slip movement occurred parallel and at high angle to the fracture- zone contact at various stages of a complex flow history. Given its location adjacent to a nontransform segment of oceanic lithosphere, the origin of the strike-slip movement parallel to the fracture zone must be clarified. It can be accounted for by movement of the older lithosphere past asthenosphere of the young spreading ridge during plate-driven flow.