Three-dimensional modelling of upper mantle structure under the Pacific Ocean and surrounding area

Abstract

Long-period (75-250 s) fundamental-mode Love and Rayleigh waves are inverted for the seismic velocity variations in the upper mantle under the Pacific Ocean and surrounding area. The data are inverted to map the regional variation of phase velocity, and then the velocity structure of the upper mantle. No a priori regionalization is used in this study, and the lateral heterogeneity is expanded horizontally into blocks (lo' X 10'). To estimate the reliability of the velocity distribution, we have calculated the resolving kernels. In the central, NW, NE and SW regions, the resolving kernels have fairly sharp peaks. In the SE region, however, resolution is not so good. The results show good correlation with surface tectonics. In the velocity map, shields and ridges are the major features in the depth interval 50-100km. Anomalies associated with the East Pacific Rise and the shields persist to 300 km. At 400 and 500 km, high-velocity anomalies appear under the eastern and southern Pacific ridges, and a low-velocity anomaly appears beneath Australia. The present study provides better resolution than previous studies because of the better path coverage. For example, a large low-velocity anomaly emerges under Hawaii down to the depth of 200 km for SH velocity. The horizontal extent of this anomaly is comparable to the s u e of the Hawaiian swell. This feature supports that the Hawaii hotspot is an isolated, truly anomalous feature in the Pacific Ocean.

High‐resolution, two‐dimensional vertical tomography of the central Pacific mantle using ScS reverberations and frequency‐dependent travel times

“…Global tomographic models, as well as higherresolution regional studies of the Pacific upper mantle [Zhang and Tanimoto, 1989], have thus far revealed no indications of structural variations in the upper mantle with the magnitude and scale needed to explain the data in Figure 3. Evidence has also accumulated for significant heterogeneity in the D" layer at the base of the mantle beneath this region [Dziewonski et al, 1996;Garnero and Helmberger, 1996;Wysession, 1996], raising the possibility of a deep mantle origin.…”

Structure and dynamics of the Pacific upper mantle

“…They found that the shear velocities are consistent with a normal lithosphere of 50-11 0 Ma with no indication for lithospheric rejuvenation, as suggested by Detrick and Crough (1978). While did not compare the mantle beneath the swell to adjacent mantle regions, other regional (Zhang and Tanimoto, 1989) and global studies (Su et ai., 1994;Masters et ai., 1996; Ekström and Dziewonski, 1997) showed no indication for either high or low velocity anomalies associated with the swell.…”

“…The problem of associating the Hawaiian swell with a strictly thermal origin, such as lithospheric rejuvenation (Detrick and Crough, 1978; Crough, 1983) has already been raised by others. Heat flow measurements taken along a profile transverse to the swell found anomaly which does not correlate with the swell location (Von Herzen et ai., 1989), surface-wave tomography does not reveal any lowvelocity anomaly below the swell (Zhang and Tanimoto, 1989;, and two-station measurements of Rayleigh-wave dispersion found no evidence for lithospheric thinning between Midway and Oahu (Woods et ai., 1991;.…”

“…This variation requires strong lateral shear-velocity gradients in the upper mantle regions sampled by their surface reflections. The global tomographic models, as well as the higher-resolution regional studies of the Pacific upper mantle (Zhang and Tanimoto, 1989), have thus far revealed no indications of structural variations in the upper 9 mantle with the magnitude and scale needed to explain these data. The second corridor extends from the Ryukyu and Izu-Bonin seismic zones to Oahu, where it is oriented almost parallel to the plate motion, as it crosses the Tonga-Hawaii corridor.…”

“…Global tomographic models, as well as higher-resolution regional studies of the Pacific upper mantle (Zhang and Tanimoto, 1989), have thus far revealed no indications of structural variations in the upper mantle with the magnitude and scale needed to explain the data in Figure 4.3. Evidence has also accumulated for significant heterogeneity in the D" layer at the base of the mantle beneath this region (Dziewonski et ai., 1996;Garnero and Helmberger, 1996;Wysession, 1996), raising the possibility of a deep-mantle origin.…”