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the most reliable in tomographic modeling.
Furthermore, Dziewonski (2003) argues that
the observed anomalies in seismic data result from integration of perturbation in structure
either along the ray path, for travel times, or volume for splitting of low order normal
modes. Perturbation is a smoothing operation and, assuming constant amplitude, the effect
of the component of the structure with a wavelength λ1 compared to that of λ2 is equal to the
ratio λ1/λ2. Thus long-wavelength components of the structure are much easier to recover
than short-wavelength ones.
Significant discrepancies in the amplitudes and in the radial distributions of velocity anomalies,
in particular in the uppermost mantle, result, in part, from incomplete crustal corrections,
unaccounted variations in topography of first-order seismic discontinuities in the mantle, and
from incorrect assumptions on the relations between Vp and Vs heterogeneity.
The most important robust result of global tomography is the observation of a strong
correlation of velocity structure of the upper 200–300 km with surface tectonics observed in
all tomographic models (Fig. 3.87). Strong (~15%) velocity heterogeneity in the upper
mantle (down to c. 300 km depth) is related to plate tectonics and the ocean–continent
variation (Nataf and Ricard, 1996). The ocean–continent variation is the first-order feature
revealed by all types of seismic data. In the upper 200 km, P-velocity heterogeneity in body-
wave tomography models is well correlated with major surface tectonics elements such as
148 Seismic structure of the lithosphere
cratons and subduction zones, where peak anomalies exceeding 2% are observed (Vasco
et al., 2003). Low-velocity anomalies associated with back arc basins can be traced down to
200–400 km depth, while some cratons are seen as high-velocity anomalies (although of a
smaller lateral extent) down to 400 km depth.
Similarly, surface-wave tomography indicates the existence of a strong correlation
between surface tectonics and the patterns of velocity heterogeneity in the upper mantle:
fast regions (with positive velocity anomalies with respect to PREM) are observed beneath
the cratons, while slow regions are observed beneath the oceans. Except for tectonically
active regions such as western USA, regional continental models have high shear velocities
throughout the entire upper mantle (higher than 4.5–4.6 km/s) and, in a good agreement
with PREM, flat-gradient velocity profiles between 200 km and 410 km depth (Fig. 3.88).
Linear belts of high mantle velocities clearly correlate with major continent–continent and
continent–ocean collisional zones (e.g. the Andes, the Himalayas–Tibet, the Altaids of
Central Asia, and Java–Sumatra) and are associated with subducting slabs. Tectonically
active regions, although not so well resolved due to their smaller size, show slow velocities
in the upper mantle. Similarly, the oceanic upper mantle has slow velocities, in particular at
shallow depth along mid-ocean ridges (Fig. 3.87).
In the cratonic regions high upper mantle velocities may persist down to 200–300 km
depth (Fig. 3.89). As discussed earlier, the apparent termination of high-velocity anomalies
at these depths can be a systematic artifact due to:
(1) use of PREM in tomographic inversions for the continents (Fig. 3.86);
(2) use of fundamental modes that (for periods <40 sec) cannot resolve upper mantle deeper
than ~200 km (Fig. 3.79);
(3) a general decrease in resolution of tomographic models at 300–400 km depth.
Global observations of the velocity structure of the upper mantle are supported by recent
high-resolution continent-scale and regional tomographic models. Seismic velocity struc-
ture of the continents and oceans is discussed in more detail below. First, an overview of
continent-scale and ocean-scale patterns is provided, followed by the characteristic velocity
structures for various continental and oceanic tectonic settings.
Continents
Adetailed description of regional studies is outside the scope of this bookwhich aims to discuss
general patterns in lithospheric structure related to the tectonic evolution of the Earth. A brief
(and inevitably incomplete due to the huge number of regional publications) overview of
continent-scale tomographic models is presented below. The limitations on vertical, lateral, and
amplitude resolution of tomographic models are discussed in detail in the previous section.
North America and Greenland
Since the early tomographic models (Grand and Helmberger, 1984), it has been recognized
that the velocity structure of the upper mantle in North America is well correlated with
149 3.6 Teleseismic seismology
Fig. 3.87 Shear-wave velocity structure of the upper mantle at 100 km, 200 km, and 300 km depth based on the global S362ANI
tomography model of Kustowski et al. (2008). In this model, radial anisotropy is confined to the uppermost mantle
(that is it becomes very small below a depth of 250 km and disappears at 410 km). Including anisotropy in
the uppermost mantle significantly improves the fit of the surface-wave data. The inversion is based on a new
spherically symmetric reference model REF.
surface geology (e.g. Grand, 1994; Humphreys and Dueker, 1994; van der Lee and Nolet,
1997a; Nettles and Dziewonski, 2008):
(1) In the Archean–Paleoproterozoic provinces of the Canadian Shield, the strongest shear-
velocity anomaly is observed in the upper 200 km of the mantle with a weaker velocity
anomaly extending down to c. 300 km depth (Fig. 3.90).
(2) A recent high-resolution seismic tomography model indicates the presence of
two distinct lithospheric domains in the Canadian Shield with the boundary at
approximately 86–87°W (the Wabigoon-Wawa/Quetico boundary) (Frederiksen
et al., 2007). To the west of this boundary, the lithospheric mantle has high velocities,
while to the east the average shear-velocity anomaly is c. 2.5% weaker (relative to
iasp91).
Fig. 3.88 Mean isotropic shear velocity profiles for regional and global oceanic (left) and continental (middle) tomographic
models. Regional models commonly provide a better fit to observed seismograms from these regions than the existing
global tomographic models. Right panel: typical shear velocity profiles for normal oceans, cratons, and active
continents. Reference models are shown as a comparison: global model PREM (Dziewonski and Anderson, 1981),
global continental model ak135 (Kennett et al., 1995), and global reference models for normal oceans (ORM) and
continents (CRM) based on S20RTS model (Ritsema et al., 2004). References for regional models: Grand (1994,
1997); Lerner-Lam and Jordan (1987); Kennett et al. (1994); Gaherty et al. (1999); Simmons et al. (2002), Pedersen
et al. (2009), Lebedev et al. (2009). Compare with Fig. 3.101a.
151 3.6 Teleseismic seismology
(3) At present, tomographic models of the Archean Slave craton are largely restricted to
models based on the POLARIS seismic array in the central part of the craton. They
indicate that high shear velocities in the upper mantle persist down to at least a 300 km
depth (Pedersen et al., 2009).
(4) In the terranes of Mesoproterozoic age in south-central North America (Fig. 3.66), a
positive velocity anomaly in the upper mantle is more shallow than beneath the
Archean–Paleoproterozoic terranes and terminates at around 200–250 km depth.
(5) Western North America, as other tectonically active continental regions, is, in
general, characterized by slow upper mantle velocities which already start at c.
50–70 km depth and have a minimum at c. 100–120 km depth (Figs. 3.88, 3.90b).
However, recent high-resolution regional tomographic models for northwestern USA image
a very complex mosaic of velocity anomalies in the upper mantle related to post-Laramide
tectono-magmatic events and the tectonic evolution of the Cascadia Subduction Zone (e.g.
Roth et al., 2008).