Geographic boundary of the “Pacific Anomaly” and its geometry and transitional structure in the north

Abstract

[1] We determine the geographical boundary and average shear velocity structure of the Pacific Anomaly at the base of the mantle based on travel time analysis of ScSH-SH and ScS2 (ScSScS)-SS phases and waveform modeling results. We further constrain the detailed geometry of the northern Anomaly around (20 N, À170 E) and its transition to the surrounding high velocity region along three perpendicular cross sections on the basis of forward waveform modeling of the observed direct S and ScS phases. The observed differential travel-time residuals and waveform modeling results allow the whole geographic boundary of the Anomaly to be delineated and the area of the base of the Anomaly is estimated to be 1.9 Â 10 7 km 2 . The maximum shear velocity perturbation inside the Anomaly reaches À5% in the lowermost 500 km of the mantle. Waveform analysis suggests that the northern Anomaly reaches 450 km above the CMB with both steeply and shallowly dipping edges and its basal layer extends beneath the surrounding mantle with the degree of extension changing rapidly across a small distance. The inferred characteristics of the Anomaly support the previous suggestion that the Pacific Anomaly represents a chemical anomaly. However, unlike the inferred features of the African Anomaly pointing to an ancient compositionally distinct and geologically stable anomaly, the existence of several separated piles extending into the mid-lower mantle, the complex morphology of the piles with both steeply and shallowly dipping edges and the presence of many ultra-low velocity zones at its base suggest that the Pacific Anomaly likely possesses varying intrinsic compositions and exhibits complex interaction with the surrounding mantle.

Morphology of seismically slow lower-mantle structures

“…In He & Wen (2012), they confirm the presence of a separate anomaly to the east, 450 km high and bounded by shallowly sloped sides. These two anomalies are the two westernmost anomalies seen in the vote map in Fig.…”

“…4dD). Therefore, the vertical extents inferred by He & Wen (2012) and Zhao et al (2015) would represent apparent heights along the corridor where their data are sensitive to the anomaly, and would not be representative of the maximum height of the anomaly. The height of the Superswell anomaly up to the mantle transition zone beneath the southern Pacific Ocean is independently confirmed by the P-wave tomography models of Tanaka et al (2009) and Suetsugu et al (2009).…”

“…Zhao et al (2015) analyse the same anomaly beneath the northern Pacific using observations of waveform broadening and suggest it extends to 600-900 km above the CMB and also argue that it has shallowly sloped sides. The specific geometries used by He & Wen (2012) and Zhao et al (2015) constrain the shallow slopes only on the northern and eastern edge of this anomaly. Our vote map suggests that this anomaly, which we dubbed the Superswell anomaly, does indeed have shallowly sloped sides, but also continues further to the south where it extends throughout the lower mantle (Fig.…”

Continent-sized anomalous zones with low seismic velocity at the base of Earth's mantle

“…The vertical extent of reduced LLSVP velocities between models is similarly divergent, but extends as far up as 1,200 km above the core-mantle boundary (CMB) in places [5][6][7][8] . Models consistently reveal different shapes for the Pacific and African LLSVPs: the Pacific LLSVP has a relatively round shape whereas the African LLSVP is more elongated in the north-south direction in the north and stretched eastward in the southernmost part.…”

The stability and structure of primordial reservoirs in the lower mantle: insights from models of thermochemical convection in three-dimensional spherical geometry

“…In particular, two large low shear wave velocity provinces (LLSVPs), in which the shear wave velocity decreases by a few per cent compared to the average mantle, are present beneath the Pacific and Africa (e.g. Su et al 1994;Li & Romanowicz 1996;Ni et al 2002;Wang & Wen 2007;Houser et al 2008;Ritsema et al 2011;He & Wen 2012). Palaeomagnetic reconstructions of the locations of large igneous provinces (e.g.…”