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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).