Seismic tomography based on P-wave travel times and improved earthquake locations provides further evidence for mantle-wide convective flow. The use of body waves makes it possible to resolve long, narrow structures in the lower mantle some of which can be followed to sites of present-day plate convergence at the Earth's surface. The transition from subduction-related linear structures in the mid-mantle to long-wavelength heterogeneity near the core-mantle boundary remains enigmatic, but at least some slab segments seem to sink to the bottom of the mantle.

/pdf/evidence-for-deep-mantle-circulation-from-global-tomography-3unqcjsa05.pdf

Evidence for deep mantle circulation from global tomography

A boundary between compositionally distinct regions at a depth of about 1600 kilometers may explain the seismological observations pertaining to Earth's lower mantle, produce the isotopic signatures of mid-ocean ridge basalts and oceanic island basalts, and reconcile the discrepancy between the observed heat flux and the heat production of the mid-ocean ridge basalt source region. Numerical models of thermochemical convection imply that a layer of material that is intrinsically about 4 percent more dense than the overlying mantle is dynamically stable. Because the deep layer is hot, its net density is only slightly greater than adiabatic and its surface develops substantial topography.

/pdf/compositional-stratification-in-the-deep-mantle-so21hlts35.pdf

Compositional stratification in the deep mantle

Two new global high-resolution models of the P-wave and S-wave seismic structure of the mantle were derived independently using different inversion techniques and different data sets, but they show excellent correlation for many large-scale as well as smaller scale structures throughout the lower mantle. The two models show that high-velocity anomalies in the lower mantle are dominated by long linear features that can be associated with the sites of ancient subduction. The images suggest that most subduction-related mantle flow continues well into the lower mantle and that slabs may ultimately reach the core-mantle boundary. The models are available from anonymous ftp at maestro.geo.utexas.edu in directory pub/grand and at brolga.mit. edu in directory pub/GSAtoday.

https://courses.washington.edu/ess502/GrandEtAl_GSA_1997.pdf

Global seismic tomography: A snapshot of convection in the earth

Maps of lateral variation in shear velocity within the mantle beneath North and South America, their surrounding oceans, and parts of Africa and Eurasia are produced from inversion of travel times of horizontally polarized shear body waves. The data consist of S and ScS waves as well as multibounce phases SS, SSS and SSSS. Waves that bottom within the upper mantle are modeled using synthetic seismograms in order to estimate travel times for each of the multiple arrivals caused by velocity discontinuities near 400 and 660 km depth. The model consists of blocks with uniform slowness anomalies relative to a one-dimensional starting model and extends from the surface to the core-mantle boundary. The blocks have horizontal dimensions of roughly 275 by 275 km and vary in the vertical dimension from 75 to 150 km. The data are inverted using a simultaneous iterative reconstruction technique algorithm. The upper 400 km of the model is dominated by lateral variations that correspond to surface tectonic environments. Three shields on three separate continents have higher than average velocities down to between 320 and 400 km depth. Young tectonically active regions are very slow in the upper 250 km. The transition zone from 400 to 660 km depth is the most poorly resolved region. High velocity beneath western South America in the transition zone is probably associated with subducting slab. The transition zone velocity beneath the western and central part of North America also appears to be slightly faster than average. The lower mantle is dominated by large-scale sheets of higher than average velocity and more equidimensional regions of slow velocity. From South America to Siberia, sheet-like high-velocity anomalies are observed from 750 km depth to the core-mantle boundary. Another lower mantle high-velocity anomaly is seen beneath southern Eurasia. The high-velocity lower mantle anomalies seem to be associated with subduction during the last 150 Ma. Comparing the location of past subduction with the location of lower mantle anomalies, the identification of lower mantle anomalies with old subducted slabs suggests slow sinking of slabs in the lower mantle (about 1 to 2 cm/yr). If this interpretation is correct then high velocity in the deepest mantle off the west coast of South America through the western United States requires significant subduction from 120 to 150 Ma a few thousand kilometers off the coast of the Americas. The slowest deep region found in this study is at the base of the mantle beneath the eastern Atlantic Ocean and may be associated with hotspots in that region. Other hotspots do not appear to be associated with slow lower mantle velocity.

Mantle shear structure beneath the Americas and surrounding oceans

The thermal structure of a generic subduction zone is investigated to elucidate the source region of subduction zone volcanics. The steady state thermal field is evaluated for a model subduction zone where the plates are prescribed by kinematic boundary conditions, such that the subducting slab induces a flow in the mantle wedge. The resulting model suggests that the oceanic crust of the downgoing slab is not melted extensively, if at all, and hence is not the source of subduction zone magmatism (with the possible exception of the special case of very young oceanic crust). The temperature in the mantle wedge is high enough to produce melting at the amphibole-buffered peridotite solidus. It is proposed that the combination of vertical motion of water as a free phase and the transport of hydrous phases (e.g., amphiboles) by the slab-induced mantle wedge flow lead to the net transport of water being horizontal, across the mantle wedge from the slab. Provided the subducting oceanic crust enters the asthenosphere at a velocity > 6(±2) cm/yr, the mantle wedge will be hot enough at the limit of the lateral water transport mechanism to generate melting at the amphibole-buffered solidus. The model was then extended to include the effect of localized sources of buoyancy (melt, residue, etc.) as a stationary body force, to investigate the possibility of reversing the slab-induced flow. Best estimates of the buoyancy sources and appropriate viscosity in the wedge suggest that there is likely to be only a weak modulation of the slab-induced flow unless the slab and wedge are locally decoupled, for instance by shear heating, the presence of water, or dehydration/hydration reactions. If there is decoupling, then it is possible for there to be an appreciable reversal of the slab-induced flow. Such an appreciable reversal of flow, if it persists, leads to cooling of the mantle wedge. Hence flow reversal cannot be a steady state mechanism. Instead it would lead to a cycle in the melting with a period of O(1 m.y.). The time dependence of a model with appreciable flow reversal would be reinforced by the need to clear the wedge of infertile material.

/pdf/physical-model-of-source-region-of-subduction-zone-volcanics-3siohh0fiw.pdf

Physical model of source region of subduction zone volcanics

Three-dimensional, spherical models of mantle convection in the earth reveal that upwelling cylindrical plumes and downwelling planar sheets are the primary features of mantle circulation. Thus, subduction zones and descending sheetlike slabs in the mantle are fundamental characteristics of thermal convection in a spherical shell and are not merely the consequences of the rigidity of the slabs, which are cooler than the surrounding mantle. Cylindrical mantle plumes that cause hotspots such as Hawaii are probably the only form of active upwelling and are therefore not just secondary convective currents separate from the large-scale mantle circulation. Active sheetlike upwellings that could be associated with mid-ocean ridges did not develop in the model simulations, a result that is in agreement with evidence suggesting that ridges are passive phenomena resulting from the tearing of surface plates by the pull of descending slabs.

https://science.sciencemag.org/content/sci/244/4907/950.full.pdf

Three-Dimensional Spherical Models of Convection in the Earth's Mantle.

Model calculations are described for a compressible fluid in a three-dimensional spherical shell with 80 percent of the surface heat flow generated within the model mantle. The numerical solutions are strongly chaotic, with surface planforms dominated by long curvilinear downflows reminiscent of the descending slabs in the earth's mantle. The results suggest that descending slabs play an important part in driving mantle convection, and that their chaotic evolution may influence the spatial and temporal behavior of plates and thus the dispersal and aggregation of continents.

Dave Bercovici

Papers

Three-Dimensional Spherical Models of Convection in the Earth's Mantle.

Chaotic, subduction-like downflows in a spherical model of convection in the Earth's mantle