Strong, Multi-Scale Heterogeneity in Earth's Lowermost Mantle

The core mantle boundary (CMB) separates Earth’s liquid iron outer core from the solid but slowly convecting mantle. The detailed structure and dynamics of the mantle within ~300 km of this interface remain enigmatic: it is a complex region, which exhibits thermal, compositional and phase-related heterogeneity, isolated pockets of partial melt and strong variations in seismic velocity and anisotropy. Nonetheless, characterising the structure of this region is crucial to a better understanding of the mantle’s thermo-chemical evolution and the nature of core-mantle interactions. In this study, we examine the heterogeneity spectrum from a recent P-wave tomographic model, which is based upon trans-dimensional and hierarchical Bayesian imaging. Our tomographic technique avoids explicit model parameterization, smoothing and damping. Spectral analyses reveal a multi-scale wavelength content and a power of heterogeneity that is three times larger than previous estimates. Inter alia, the resulting heterogeneity spectrum gives a more complete picture of the lowermost mantle and provides a bridge between the long-wavelength features obtained in global S-wave models and the short-scale dimensions of seismic scatterers. The evidence that we present for strong, multi-scale lowermost mantle heterogeneity has important implications for the nature of lower mantle dynamics and prescribes complex boundary conditions for Earth’s geodynamo.

Seismic structure, crustal architecture and tectonic evolution of the Anatolian-African Plate Boundary and the Cenozoic Orogenic Belts in the Eastern Mediterranean Region

The modern Anatolian–African plate boundary is represented by a north-dipping subduction zone that has been part of a broad domain of regional convergence between Eurasia and Afro–Arabia since the latest Mesozoic. A series of collisions between Gondwana-derived ribbon continents and trench-roll-back systems in the Tethyan realm produced nearly East–West-trending, subparallel mountain belts with high elevation and thick orogenic crust in this region. Ophiolite emplacement, terrane stacking, high‐P and Barrovian metamorphism, and crustal thickening occurred during the accretion of these microcontinents into the upper plates of Tethyan subduction roll-back systems during the Late Cretaceous–Early Eocene. Continued convergence and oceanic lithospheric subduction within the Tethyan realm were punctuated by slab breakoff events following the microcontinental accretion episodes. Slab breakoff resulted in asthenospheric upwelling and partial melting, which facilitated post-collisional magmatism along and across the suture zones. Resumed subduction and slab roll-back-induced upper plate extension triggered a tectonic collapse of the thermally weakened orogenic crust in Anatolia in the late Oligocene–Miocene. This extensional phase resulted in exhumation of high‐P rocks and medium- to lower-crustal material leading to the formation of metamorphic core complexes in the hinterland of the young collision zones. The geochemical character of the attendant magmatism has progressed from initial shoshonitic and high‐K calc‐alkaline to calc‐alkaline and alkaline affinities through time, as more asthenosphere-derived melts found their way to the surface with insignificant degrees of crustal contamination. The occurrence of discrete high-velocity bodies in the mantle beneath Anatolia, as deduced from lithospheric seismic velocity data, supports our Tethyan slab breakoff interpretations. Pn velocity and Sn attenuation tomography models indicate that the uppermost mantle is anomalously hot and thin, consistent with the existence of a shallow asthenosphere beneath the collapsing Anatolian orogenic belts and widespread volcanism in this region. The sharp, north-pointing cusp (Isparta Angle) between the Hellenic and Cyprus trenches along the modern Anatolian–African plate boundary corresponds to a subduction-transform edge propagator (STEP) fault, which is an artifact of a slab tear within the downgoing African lithosphere.

Seismostratigraphy and thermal structure of Earth's core-mantle boundary region

We used three-dimensional inverse scattering of core-reflected shear waves for large-scale, high-resolution exploration of Earth's deep interior (D'') and detected multiple, piecewise continuous interfaces in the lowermost layer (D'') beneath Central and North America. With thermodynamic properties of phase transitions in mantle silicates, we interpret the images and estimate in situ temperatures. A widespread wave-speed increase at 150 to 300 kilometers above the coremantle boundary is consistent with a transition from perovskite to postperovskite. Internal D'' stratification may be due to multiple phase-boundary crossings, and a deep wave-speed reduction may mark the base of a postperovskite lens about 2300 kilometers wide and 250 kilometers thick. The core-mantle boundary temperature is estimated at 3950 +/- 200 kelvin. Beneath Central America, a site of deep subduction, the D'' is relatively cold (DeltaT = 700 +/- 100 kelvin). Accounting for a factor-of-two uncertainty in thermal conductivity, core heat flux is 80 to 160 milliwatts per square meter (mW m(-2)) into the coldest D'' region and 35 to 70 mW m(-2) away from it. Combined with estimates from the central Pacific, this suggests a global average of 50 to 100 mW m(-2) and a total heat loss of 7.5 to 15 terawatts.

Evidence for a chemical-thermal structure at base of mantle from sharp lateral P-wave variations beneath Central America

Compressional waves that sample the lowermost mantle west of Central America show a rapid change in travel times of up to 4 s over a sampling distance of 300 km and a change in waveforms. The differential travel times of the PKP waves (which traverse Earth's core) correlate remarkably well with predictions for S-wave tomography. Our modeling suggests a sharp transition in the lowermost mantle from a broad slow region to a broad fast region with a narrow zone of slowest anomaly next to the boundary beneath the Cocos Plate and the Caribbean Plate. The structure may be the result of ponding of ancient subducted Farallon slabs situated near the edge of a thermal and chemical upwelling.

Seismological constraints on a possible plume root at the core-mantle boundary

Recent seismological discoveries have indicated that the Earth's core-mantle boundary is far more complex than a simple boundary between the molten outer core and the silicate mantle. Instead, its structural complexities probably rival those of the Earth's crust. Some regions of the lowermost mantle have been observed to have seismic wave speed reductions of at least 10 per cent, which appear not to be global in extent. Here we present robust evidence for an 8.5-km-thick and approximately 50-km-wide pocket of dense, partially molten material at the core-mantle boundary east of Australia. Array analyses of an anomalous precursor to the reflected seismic wave ScP reveal compressional and shear-wave velocity reductions of 8 and 25 per cent, respectively, and a 10 per cent increase in density of the partially molten aggregate. Seismological data are incompatible with a basal layer composed of pure melt, and thus require a mechanism to prevent downward percolation of dense melt within the layer. This may be possible by trapping of melt by cumulus crystal growth following melt drainage from an anomalously hot overlying region of the lowermost mantle. This magmatic evolution and the resulting cumulate structure seem to be associated with overlying thermal instabilities, and thus may mark a root zone of an upwelling plume.