Sublithospheric Small-scale Convection Research Articles

Sublithospheric small-scale convection—A mechanism for collision zone magmatism

We studied the effect of increased water content on the dynamics of the lithosphere-asthenosphere boundary in a postsubduction setting. Results from numerical mantle convection models show that the resultant decrease in mantle viscosity and the peridotite solidus produce small-scale convection at the lithosphere-asthenosphere boundary and magmatism that follows the spatially and temporally scattered style and volumes typical for collision magmatism, such as the late Cenozoic volcanism of the Turkish-Iranian Plateau. An inherent feature in small-scale convection is its chaotic nature that can lead to temporally isolated volcanic centers tens of millions of years after initial continental collision, without evident tectonic cause. We also conclude that water input into the upper mantle during and after subduction under the circum-Mediterranean area and the Tibetan Plateau can account for the observed magmatism in these areas. Only fractions (200–600 ppm) of the water input need to be retained after subduction to induce small-scale convection and magmatism on the scale of those observed from the Turkish-Iranian Plateau.

Lithosphere-asthenosphere interaction beneath Ireland from joint inversion of teleseismic P-wave delay times and GRACE gravity

The nature and extent of the regional lithosphere-asthenosphere interaction beneath Ireland and Britain remains unclear. Although it has been established that ancient Caledonian signatures pervade the lithosphere, tertiary structure related to the Iceland plume has been inferred to dominate the asthenosphere. To address this apparent contradiction in the literature, we image the 3-D lithospheric and deeper upper-mantle structure beneath Ireland via non-linear, iterative joint teleseismic-gravity inversion using data from the ISLE (Irish Seismic Lithospheric Experiment), ISUME (Irish Seismic Upper Mantle Experiment) and GRACE (Gravity Recovery and Climate Experiment) experiments. The inversion combines teleseismic relative arrival time residuals with the GRACE long wavelength satellite derived gravity anomaly by assuming a depth-dependent quasilinear velocity-density relationship. We argue that anomalies imaged at lithospheric depths probably reflect compositional contrasts, either due to terrane accretion associated with Iapetus Ocean closure, frozen decompressional melt that was generated by plate stretching during the opening of the north Atlantic Ocean, frozen Iceland plume related magmatic intrusions, or a combination thereof. The continuation of the anomalous structure across the lithosphere-asthenosphere boundary is interpreted as possibly reflecting sub-lithospheric small-scale convection initiated by the lithospheric compositional contrasts. Our hypothesis thus reconciles the disparity which exists between lithospheric and asthenospheric structure beneath this region of the north Atlantic rifted margin.

Full waveform tomography for radially anisotropic structure: New insights into present and past states of the Australasian upper mantle

We combine spectral-element simulations and adjoint techniques in a non-linear full seismic waveform tomography for the radially anisotropic structure of the Australasian upper mantle. Our method allows us to explain 30 s waveforms in detail, and it yields tomographic images with locally 2 ∘ lateral resolution. In the course of 19 conjugate-gradient iterations the total number of exploitable waveforms increased from 2200 to nearly 3000. The final model, AMSAN.19, thus explains data that were not initially included in the inversion. This is strong evidence for the effectiveness of the inversion scheme and the physical consistency of the tomographic model. AMSAN.19 confirms long-wavelength heterogeneities found in previous studies, and it allows us to draw the following inferences concerning the past and present states of the Australian upper mantle and the formation of seismic anisotropy: (1) Small-scale neutral to low-velocity patches beneath central Australia are likely to be related to localised Paleozoic intraplate deformation. (2) Increasing seismic velocities between the Moho and 150 km depth are found beneath parts of Proterozoic Australia, suggesting thermochemical variations related to the formation and fragmentation of a Centralian Superbasin. (3) Radial anisotropy above 150 km depth reveals a clear ocean–continent dichotomy: We find strong v sh > v sv beneath the Coral and Tasman Seas. The anisotropy is strongest at the top of the inferred asthenospheric flow channel, where strain is expected to be largest. Radial anisotropy beneath the continent is weaker but more variable. Localised patches with v sh < v sv appear, in accord with small-scale intraplate deformation. (4) The ocean–continent dichotomy disappears gradually between 150 and 250 km depth, where the continental lithospheric mantle and the oceanic asthenosphere pass into the underlying convecting mantle. (5) Significant anisotropy exists below 250 km depth. Its character can be explained by sublithospheric small-scale convection and a change in the dominant glide system of olivine.

Reduced oceanic seismic anisotropy by small-scale convection

Amplitudes of azimuthal seismic anisotropy below the Pacific plate are observed to decrease with lithospheric age, whereas an increase is expected due to ongoing shearing between the plate and the underlying mantle. We illustrate that the convective disturbance of this laminar flow field by sub-lithospheric small-scale convection (SSC) provides an explanation for this anisotropy reduction. By combining numerical flow models with forward seismic anisotropy calculations we show that small-scale convection can disturb the anisotropy pattern significantly. When such disturbed signal is smoothed over > ~ 500 km length scale (the approximate present-day seismic resolution for azimuthal anisotropy within the Pacific lithosphere and upper mantle), our results show a significant reduction in anisotropy amplitudes of about a factor two, similar to observed reductions. Perturbations of the fast polarization direction, however, remain relatively small (< ~ 20°). This suggests that, despite the influence of any SSC, observed long-wavelength LPO orientations still represent the large-scale shearing between the lithosphere and the underlying mantle. For higher resolution LPO data, SSC can significantly complicate the interpretation of sub-lithospheric mantle flow from azimuthal seismic anisotropy data.

Small-scale gravitational instabilities under the oceans: Implications for the evolution of oceanic lithosphere and its expression in geophysical observables

Sublithospheric small-scale convection (SSC) is thought to be responsible for the flattening of the seafloor depth and surface heat flow observed in mature plates. Although the existence of SSC is generally accepted, its ability to effectively produce a constant lithospheric thickness (i.e. flattening of observables) is a matter of debate. Here we study the development and evolution of SSC with a 2D thermomechanical finite-element code. Emphasis is put on (i) the influence of various rheological and thermophysical parameters on SSC, and (ii) its ability to reproduce geophysical observables (i.e. seafloor depth, surface heat flow, and seismic velocities). We find that shear heating plays no significant role either in the onset of SSC or in reducing the lithospheric thickness. In contrast, radiogenic heat sources and adiabatic heating exert a major control on both the vigour of SSC and the thermal structure of the lithosphere. We find that either dislocation creep, diffusion creep, or a combination of these mechanisms, can generate SSC with rheological parameters given by laboratory experiments. However, vigorous SSC and significant lithospheric erosion are only possible for relatively low activation energies. Well-developed SSC occurs only if the first ∼300 km of the mantle has an average viscosity of ≲1020 Pa s; higher values suppress SSC, while lower values generate unrealistic high velocities. Seismic structures predicted by our models resemble closely tomography studies in oceanic mantle. However, the fitting to observed seafloor topography and surface heat flow is still unsatisfactory. This puts forward a fundamental dichotomy between the two datasets. This can be reconciled if most of the observed flattening in seafloor topography is influenced by processes other than SSC.

Sublithospheric small‐scale convection and its implications for the residual topography at old ocean basins and the plate model

The seafloor topography and heat flux differ significantly from the predictions of the half‐space cooling (HSC) model at old ocean basins. Understanding the deviations has important implications for thermal evolution of oceanic lithosphere and large‐scale mantle dynamics. A widely used model that explains significant fraction of the deviations is the plate model, but the dynamical feasibility of the plate model has never been demonstrated. In this study, we investigated the effects of sublithospheric small‐scale convection (SSC) and of internal heating on seafloor heat flux and topography and mantle thermal structure, and we examined the dynamic feasibility of the plate model by formulating high‐ resolution two‐dimensional numerical models of mantle convection with strongly temperature‐ and depth‐dependent rheology. We found that mantle convection with tectonic plates often leads to formation of a broad thermal anomaly below old lithosphere where the mantle is not cooled by subducted slabs and heat transfer is less efficient because of thick lithosphere, especially when significant internal heating is present. This trapped heat may exist in the middle mantle when the SSC is absent, and it may also be redistributed by the SSC to shallow depths to reheat the lithosphere and to homogenize mantle temperature. When internal heating accounts for &gt;∼60% of the total heat output, the trapped heat may provide sufficient heat supply to preferentially reheat old lithosphere via SSC while maintaining uniform mantle temperature. We suggest that the trapped heat and the SSC are responsible for the residual heat flux and topography at old ocean basins relative to the HSC model predictions. Our models also show that for the plate model to be dynamically viable, both the SSC and significant internal heating (&gt;60%) are necessary. This is because only the SSC in a mantle with significant internal heating can erode and reheat the lithosphere while maintaining a nearly constant mantle temperature below lithosphere, which is the basic assumption of the plate model. With the viscosity structure and internal heating rate for the present‐day mantle, we think that the plate model is dynamically viable.

Effects of sub-lithospheric small-scale convection with a Newtonian rheology on the seafloor topography and heat flux

Seafloor topography and heat flux show clear dependence on the age of seafloor. A half-space cooling (HSC) model can reproduce seafloor topography and heat flux data for younger seafloor, but for older seafloor the observations show reduced variations with the age in comparison with the HSC model predictions. The deviation was attributed to the sub-lithospheric small-scale (SSC) convection first by Parsons and McKenzie (1978). While there is little doubt that the SSC can enhance heat flux at relatively old seafloor, questions were raised as to whether or not the SSC can actually lead to a reduced topography. In this study, the effects of SSC on seafloor topography and heat flux are investigated by formulating a 2-D thermal convection model that is parallel to plate motion. Instead of using closed boundary conditions, which will bring large pressure effects because of return flow, a flow through boundary condition is adopted. The results show that although the SSC enhances the surface heat flux, it has little effects on topography for the fluids with a more realistic rheology. The reason for this is that the SSC transports the heat from the bottom to the top and cools down the whole fluids, and with the existence of a stagnant lid, the whole effects on topography are negligible.

The effect of shearing on the onset and vigor of small‐scale convection in a Newtonian rheology

As an oceanic plate cools with time, the bottom part of the plate may undergo gravitational instabilities and lead to sub‐lithospheric small‐scale convection (SSC). While numerous previous studies have examined the dynamics of SSC below a fixed top boundary, in this study we investigate the effects of shearing due to plate motion on the onset and time evolution of SSC with 2‐D and 3‐D finite element models with a prescribed surface plate motion and a Newtonian rheology. Our 3‐D models show that plate motion moderately enhances the SSC that displays plate‐motion‐parallel convective roll‐structures. However, plate motion significantly hinders SSC in 2‐D models with plate motion that produces plate‐motion‐perpendicular convective roll‐structures. In spite of moderate influences of plate motion on the dynamics of SSC, our results suggest that for Newtonian rheology the onset time of SSC predicted from previous studies with a fixed top boundary is to the first order valid.

Controls on sublithospheric small‐scale convection

The Pacific upper mantle structures revealed from recent seismic studies prompt us to study the dynamics of sublithospheric small‐scale convection (SSC) derived from thermal boundary layer instabilities of cooling lithosphere. As oceanic lithosphere cools and thickens, its sublayer may go unstable, thus producing SSC in the asthenosphere. By formulating two‐dimensional (2‐D) and three‐dimensional (3‐D) numerical models with realistic mantle rheology, we examine the controls on the onset time of SSC and its dynamic consequences. The onset of SSC is mainly controlled by two parameters: activation energy and asthenospheric viscosity, which can be recast as the Frank‐Kamenetskii parameter θ and a Rayleigh number Rai, respectively. Our models show that the onset time of SSC, τc, scales as Rai−0.68θ0.74, independent of 2‐D or 3‐D geometry. Our scaling coefficient for θ is significantly smaller than that from previous studies, but the weaker dependence on activation energy confirms the result of Korenaga and Jordan [2003]. We found that thermal structure associated with age offset across fracture zones has significant effects on the onset of SSC, and it causes the SSC to occur always first near the fracture zones. Asthenospheric thickness and plate motion may also have significant effects on the onset of SSC. When the thickness of asthenosphere is sufficiently small to be comparable with the wavelength of the SSC, the onset may be delayed significantly. Plate motion also tends to delay the onset of the SSC in our 2‐D models. Although at the onset of SSC surface heat flux Q is consistent with the half‐space cooling model prediction, Q may eventually deviate from the half‐space cooling model prediction as thermal perturbations associated with SSC diffuse through the stable part of lithosphere or stagnant lid to the surface. We found that the time it takes for Q to deviate from the half‐space cooling model after the onset of SSC, Δτ, scales as Rai−0.65θ1.52, while the thickness of the stagnant lid at the onset time, δ, scales as Rai−0.33θ0.78, which is consistent with Δτ ∼ δ2 for thermal diffusion. At the onset of SSC, Q scales as Rai0.34θ−0.37 or τc−0.5 as expected from the half‐space cooling model. However, these scaling coefficients change significantly with time. After nine onset times Q scales as Rai0.28θ−0.7, which although showing the trend toward the scaling for steady state convection is still far from the predictions for steady state convection, thus suggesting a fundamentally transient nature of the SSC.