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The major features of the upper mantle P-wave velocity structure are
well-resolved fast velocities associated with the Juan de Fuca–Gorda slab (which is traced
down to at least 500 km), beneath the Idaho batholith and beneath Nevada where a slab-like
Fig. 3.89 Depth profiles of average shear velocity anomalies in the upper mantle based on the S20RTS global tomographic
model of Ritsema et al. (2004). Left: average seismic velocity profiles with respect to PREM beneath continents (the
Continental Reference Model, CRM) and beneath those oceanic regions where water depth is 5500–6500 m and
lithosphere is 40–80 My old (the Oceanic Reference Model, ORM; for these ocean regions, cooling plate models explain
the bathymetry and the surface heat flow well); dashed line labeled “Degree–0” – average anomaly of the entire
S20RTS model (Ritsema et al., 2004). Middle: the same reference models converted to absolute velocities; global
models PREM and iasp91 are shown for comparison. CRM and ORM inherit the LVZ at around 220 km depth when
converted to absolute velocities. Right: Depth profiles of average S20RTS shear velocity heterogeneity in the upper
mantle of anomalous oceanic structures (the East Pacific Rise, Indian Rise, and the Mid-Atlantic Ridge) with respect
to ORM (after Ritsema et al., 2004). Below 220 km depth and down to the transition zone, both oceanic and
continental velocity profiles based on the S20RTS model have systematically higher values (by 0.1–0.2 km/s) than
global models.
152 Seismic structure of the lithosphere
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eastward-dipping feature of increased P-wave velocities is observed down to at least 300 km
depth (and perhaps as deep as 700 km). Slow upper mantle velocities are observed beneath
the Yellowstone–Snake River plain, the north-central Oregon, and along the Pacific
coast. Several tomographic studies have imaged fast linear velocity anomalies below a
200–300 km depth that are sub-parallel to the Pacific coast and extend down to c. 1500 km
depth beneath the western-central North American continent. They are interpreted as frag-
ments of the Farallon plate, a huge oceanic plate that has subducted beneath the American
plates during the Mesozoic and Cenozoic eras (van der Lee and Nolet, 1997b; Bunge and
Grand, 2000).
For Greenland, the number of regional-scale tomographic models is so far very limited
and restricted to the GLATIS project (Darbyshire et al., 2004). The analysis of fundamental-
mode Rayleigh wave phase velocity dispersion curves (over the period range 25–160 s)
suggests that in the “lithosphere” the shear wave velocity is 4–12% above global reference
models (for resolution kernels, see Fig. 3.79). The thickness of the lithosphere is estimated
to be c. 100 km along the southeast coast of Greenland increasing to c. 180 km beneath
central-southwestern Greenland where the highest velocities are reported. A body-wave
tomographic model (e.g. Grand, 2002) indicates that the region of high shear velocities
(2% above PREM) may extend down to c. 300 km.
South America
Until recently, regional tomographic models of South America suffered from insufficient ray
coverage, and the upper mantle velocity structure beneath most of the stable part of the
continent to date remained largely unconstrained (Fig. 3.91). Recent tomographic models
provide reliable images for the upper 100–200 km of the mantle, but their resolution is still
limited due to the poorly known crustal structure of the continent. The best known is the
velocity structure of the upper mantle beneath the Andes (both for compressional and for
shear waves) which has been resolved in great detail by numerous studies. They indicate a
complex short-wavelength pattern of fast and slow velocities which suggests that lateral
variations in composition, melting, and water content play an important role in velocity
variations in the mantle wedge.
The major large-scale features of the South American upper mantle revealed by recent
tomographic studies include:
(1) slow subhorizontal shear-velocity anomalies in the upper mantle down to c. 150 km
depth beneath the Andes associated with flat subducting slab;
(2) Eastward-dipping slow-velocity anomaly in the mantle continues beneath the Chaco
Basin and extends as far east as the Paraná Basin where it is imaged at a 200–300 km
depth; low velocities down to c. 200 km beneath the Chaco and the Pantanal Basins can
be associated with the subducting slab;
(3) high Vs velocities beneath the Paraná and the Parnaiba Basins down to 100–150 km
depth and perhaps as deep as 250 km (Lebedev et al., 2009);
(4) fast-velocity anomaly down to c. 200 km depth beneath the Archean Amazonian craton
as consistently imaged by different tomographic models; some regional tomographic
154 Seismic structure of the lithosphere
studies image a reduced-velocity anomaly beneath the Amazon Basin (a paleorift?) which
cuts the oldest part of the continent into two subcratons (the Guayana and the Guapore
Shields);
(5) high-velocity region in the upper mantle (down to 150–200 km depth) beneath the
Archean Saō Francisco craton.
Most tomographic results for South America are based on fundamental mode surface
waves (e.g. Feng et al., 2007) and thus cannot reliably resolve the upper mantle structure
below 200 km depth (Fig. 3.79).
Europe
Tomographic models for Eurasia, one of the best studied continents, are numerous, both on
the continent-scale and at regional scales (e.g. Panza et al., 1980; Spakman, 1991; Zielhuis
and Nolet, 1994; Villaseñor et al., 2001; Piromallo and Morelli, 2003; Kustowski et al.,
Fig. 3.91 Major tectonic provinces of South America (based on Heintz et al., 2005).
155 3.6 Teleseismic seismology
2008b). The major large-scale patterns are as follows (for details