Multiple seismic reflectors in Earth’s lowermost mantle

Abstract

The modern view of Earth's lowermost mantle considers a D″ region of enhanced (seismologically inferred) heterogeneity bounded by the core-mantle boundary and an interface some 150-300 km above it, with the latter often attributed to the postperovskite phase transition (in MgSiO 3 ). Seismic exploration of Earth's deep interior suggests, however, that this view needs modification. Socalled ScS and SKKS waves, which probe the lowermost mantle from above and below, respectively, reveal multiple reflectors beneath Central America and East Asia, two areas known for subduction of oceanic plates deep into Earth's mantle. This observation is inconsistent with expectations from a thermal response of a single isochemical postperovskite transition, but some of the newly observed structures can be explained with postperovskite transitions in differentiated slab materials. Our results imply that the lowermost mantle is more complex than hitherto thought and that interfaces and compositional heterogeneity occur beyond the D″ region sensu stricto.seismic imaging | mineral physics | mantle convection T he lowermost mantle, extending several hundred kilometers above the ∼2,900-km-deep core-mantle boundary (CMB), is of considerable interest because it includes the boundary layer of thermochemical mantle convection across which heat is conducted from the core into the mantle. Almost three decades after the detection of an interface some 150-300 km above the CMB (1), the base of the mantle is still a challenging target for crossdisciplinary research. Both the seismic discontinuity that marks the top of the so-called D″ region (1) and the heterogeneity below it (2, 3) have been attributed to a perovskite (Pv) to postperovskite (pPv) transition in the dominant mantle silicate (4-8), and this association has inspired estimation of temperatures above and heat flux across the CMB (9, 10). The D″ interface remains enigmatic, however, and recent high pressure-temperature experiments suggest that seismic observations concerning its depth and thickness are inconsistent with those expected for a pPv transition unless the chemical composition of the regions where they occur differs significantly from standard bulk composition models such as pyrolite (11-13). The transition thickness could be reconciled with nonlinearity in the phase fraction profile (11) or lattice preferred orientation in pPv (14), but the depth is a concern because the pPv transition pressure in pyrolite may be too high for it to occur in the lower mantle (13). Candidate compositions for a seismically detectable pPv transition at mantle pressures include midoceanic ridge basalt (MORB) and harzburgite components of subducted and differentiated oceanic lithosphere. Furthermore, silica may transform from modified stishovite to seifertite in Si-rich parts of the lowermost mantle (15). Inspired by these results, we search for multiple interfaces in and above the conventional D″ region, using seismic waves that sample the lowermost mantle beneath geographic regions where seismic...

Seismic array constraints on the D″ discontinuity beneath Central America

“…Shang et al . [] used a Radon Transform technique to stack ScS arrivals. They found a relatively flat discontinuity ranging between 250 and 300 km in height.…”

“…The overall shape of discontinuity topography uncovered by Shang et al . [] appears similar to the present study, however, muted in amplitude.…”

“…[], Shang et al . [] image a negative impedance reflector at roughly 200 km above the CMB consistently across the entire Central American region. Using a waveform inversion approach, Kawai et al .…”

“…[] also infer a decrease in S wave velocities at a height of roughly 100 km above the CMB. Additional intermittent reflectors of unknown impedance polarity have been detected between roughly 50 and 100 km beneath the Dʺ discontinuity [ van der Hilst et al ., ] and at about 150 km above the D ″ discontinuity [ Shang et al ., ].…”

“…Some of these earliest efforts utilized seismic array processing methods in order to boost the signal‐to‐noise ratio of low‐amplitude arrivals associated with the discontinuity [e.g., Krüger et al ., ; Weber , ; Weber and Davis , ; Yamada and Nakanishi , ]. Many recent efforts have used increasingly sophisticated methods to detect the discontinuity including, 3‐D grid migration and double‐array stacking [ Kito et al ., ], Kirchhoff migration [ Hutko et al ., ], full waveform inversion [ Kawai and Geller , ], and the generalized radon transform technique [ Ma et al ., ; Shang et al ., ; van der Hilst et al ., ; Wang et al ., , ]. Array processing approaches continue to be utilized [e.g., Chaloner et al ., ; Cobden and Thomas , ] and one recent effort has applied array processing techniques to noise correlograms to image the discontinuity [ Poli et al ., ].…”