The three-dimensional velocity structure of the upper mantle beneath rifts can be ascertained using teleseis-mic travel-time delays and seismic tomography. Davis & Slack (2002) modeled these types of data from beneath the Kenya Dome using two Gaussian surfaces that separate undulating layers of different velocities (Plate 7.2 between pp. 244 and 245). An upper layer (mesh surface) peaks at the Moho beneath the rift valley and has a velocity contrast of -6.8% relative to 8 km s-1 mantle. A lower layer (grayscale surface) peaks at about 70 km depth and has a -11.5% contrast extending to a depth of about 170 km. This model, which is in good agreement with the results of seismic refraction studies, shows a domal upper mantle structure with sides that dip away from the center of the Kenya Rift. The authors suggested that this structure results from the separation of upwelling asthenosphere into currents that impinge on the base of the lithosphere and form a low velocity, low density zone of melting between 70 and 170 km depth.
Park & Nyblade (2006) used teleseismic P-wave travel times to image the upper mantle beneath the
Eastern branch of the East African Rift system to depths of 500 km. They found a steep-sided, west-dipping low velocity anomaly that is similar to the one modeled by Davis & Slack (2002) above 160 km depth. Below this depth, the anomaly broadens to the west indicating a westerly dip. Similar structures have been imaged below Tanzania (Ritsema et al., 1998; Weeraratne et al., 2003) and parts of Ethiopia (Benoit et al., 2006). Bastow et al. (2005) found that a tabular (75 km wide) low velocity zone below southern Ethiopia broadens at depths of >100 km beneath the more highly extended northern section of the rift (Fig. 7.7c,d). The anomalies are most pronounced at ~150 km depth. These broad, dipping structures are difficult to reconcile with models of a simple plume with a well-defined head and tail. Instead they appear to be more consistent with either multiple plumes or tomographic models (Plate 7.3 between pp. 244 and 245) where the hot asthenosphere connects to a broad zone of anomalously hot mantle beneath southern Africa.
In the deep mantle below South Africa, Ritsema et al. (1999) imaged a broad (4000 by 2000 km2 area) low velocity zone extending upward from the core-mantle boundary and showed that it may have physical links to the low velocity zones in the upper mantle beneath East Africa (Plate 7.3 between pp. 244 and 245). The tilt of the deep velocity anomaly shows that the upwelling is not vertical. Between 670 and 1000-km depth the anomaly weakens, suggesting that it may be obstructed. These observations support the idea that anomalously hot asthenosphere beneath Africa is related in some way to this broad deep zone of upwelling known as the African superswell (Section 12.8.3). Nevertheless, a consensus on the location, depth extent and continuity of hot mantle material below the East African Rift system has yet to be reached (cf. Montelli et al., 2004a).
A comparison of the mantle structure beneath rifts in different settings indicates that the size and strength of mantle upwellings are highly variable. Achauer & Masson (2002) showed that in relatively cool rifts, such as the Baikal Rift and the southern Rhine Graben, low velocity zones are only weakly negative (-2.5% relative to normal mantle P-wave velocities) and occur mostly above depths of 160 km. In these relatively cool settings, the low velocity zones in the uppermost mantle show no continuation to deeper levels (>160 km) and no broadening of an upwelling asthenosphere with depth below the rift. In still other settings, such as the Rio Grande rift, low velocity zones in the upper mantle may form parts of small-scale convection cells where upwell-
ing occurs beneath the rift and downwelling beneath its margins (Gao et al., 2004).
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