Major disruption of D″ beneath Alaska

Abstract

D″ represents one of the most dramatic thermal and compositional layers within our planet. In particular, global tomographic models display relatively fast patches at the base of the mantle along the circum‐Pacific which are generally attributed to slab debris. Such distinct patches interact with the bridgmanite (Br) to post‐bridgmanite (PBr) phase boundary to generate particularly strong heterogeneity at their edges. Most seismic observations for the D″ come from the lower mantle S wave triplication (Scd). Here we exploit the USArray waveform data to examine one of these sharp transitions in structure beneath Alaska. From west to east beneath Alaska, we observed three different characteristics in D″: (1) the western region with a strong Scd, requiring a sharp δVs = 2.5% increase; (2) the middle region with no clear Scd phases, indicating a lack of D″ (or thin Br‐PBr layer); and (3) the eastern region with strong Scd phase, requiring a gradient increase in δVs. To explain such strong lateral variation in the velocity structure, chemical variations must be involved. We suggest that the western region represents relatively normal mantle. In contrast, the eastern region is influenced by a relic slab that has subducted down to the lowermost mantle. In the middle region, we infer an upwelling structure that disrupts the Br‐PBr phase boundary. Such an interpretation is based upon a distinct pattern of travel time delays, waveform distortions, and amplitude patterns that reveal a circular‐shaped anomaly about 5° across which can be modeled synthetically as a plume‐like structure rising about 400 km high with a shear velocity reduction of ~5%, similar to geodynamic modeling predictions of upwellings.

Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

“…These velocity jumps range from 0.48% (in mod7) to 2.52% (in mod11). The strength of these velocity discontinuities is similar to that reported for downwelling regions (e.g., Helmberger et al, 2005; Sun et al, 2016; Yao et al, 2015; Young & Lay, 1987a, 1987b). …”

Waveform inversion for 3-D S-velocity structure of D′′ beneath the Northern Pacific: possible evidence for a remnant slab and a passive plume

“…Recent waveform forward modeling studies (He et al 2014;Sun et al 2016) estimated depths of the D′′ discontinuity of ∼200 and ∼250 km above the CMB beneath Kamchatka and the Northern Pacific, respectively, which are consistent with the depth of the significant high-velocity anomalies between 200 and 300 km above the CMB in our model (Figs. 2, 3).…”

“…He et al (2014) suggested an 850-kmthick low-velocity anomaly surrounded by a 210-km-thick high-velocity anomaly in D′′ beneath Kamchatka on the basis of forward modeling of seismic waveforms. Sun et al (2016) studied the lowermost mantle beneath Alaska using waveforms recorded at recently deployed USArray stations. They divided the study area into three subregions with lateral scales of ∼15 • (western, middle, and eastern parts).…”

“…They reported that the western, middle, and eastern parts show a sharp D′′ discontinuity with δV s = 2.5 %, no clear evidence for a D′′ discontinuity, and a gradual increase in δV s , respectively. The number of earthquake sources used by the above studies is 10 for Lay and Helmberger (1983), 36 for Young and Lay (1990), 2 for He et al (2014), and 3 for Sun et al (2016), respectively, and waveform stacking or waveform forward modeling was used.…”

Ultra-low velocity zone heterogeneities at the core–mantle boundary from diffracted PKKPab waves

“…These strong lateral heterogeneities are much larger than those calculated from global tomographic models and imply significantly complex structures at the CMB that have long been known to exist [see Garnero (2000); Lay and Garnero (2011) for a review], even on the small-scale level (Ma et al 2016;Sun et al 2016). The explanations for these heterogeneities, however, are still inconclusive.…”