Page 190 - Global Tectonics

176 CHAPTER 7

Eastern branch of the East African Rift system to depths ing occurs beneath the rift and downwelling beneath its
of 500 km. They found a steep-sided, west-dipping low margins (Gao et al., 2004).
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 7.5 RIFT INITIATION
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 Continental rifting requires the existence of a horizon-

zone below southern Ethiopia broadens at depths of tal deviatoric tensional stress that is sufficient to break
>100 km beneath the more highly extended northern the lithosphere. The deviatoric tension may be caused
section of the rift (Fig. 7.7c,d). The anomalies are most by stresses arising from a combination of sources,
pronounced at ∼150 km depth. These broad, dipping including: (i) plate motions; (ii) thermal buoyancy forces

structures are difficult to reconcile with models of a due to asthenospheric upwellings; (iii) tractions at the

simple plume with a well-defined head and tail. Instead base of the lithosphere produced by convecting asthe-
they appear to be more consistent with either multiple nosphere; and/or (iv) buoyancy (gravitational) forces
plumes or tomographic models (Plate 7.3 between pp. created by variations in crustal thickness (Huismans et
244 and 245) where the hot asthenosphere connects to al., 2001). These stresses may be inherited from a previ-
a broad zone of anomalously hot mantle beneath ous tectonic regime or they may develop during exten-
southern Africa. sion. Full rupture of the lithosphere leading to the
In the deep mantle below South Africa, Ritsema et formation of a new ocean basin only occurs if the avail-
2
al. (1999) imaged a broad (4000 by 2000 km area) low able stresses exceed the strength of the entire litho-
velocity zone extending upward from the core–mantle sphere. For this reason lithospheric strength is one of
boundary and showed that it may have physical links to the most important parameters that governs the forma-
the low velocity zones in the upper mantle beneath East tion and evolution of continental rifts and rifted
Africa (Plate 7.3 between pp. 244 and 245). The tilt of margins.
the deep velocity anomaly shows that the upwelling is The horizontal force required to rupture the entire
not vertical. Between 670 and 1000–km depth the lithosphere can be estimated by integrating yield stress
anomaly weakens, suggesting that it may be obstructed. with respect to depth. The integrated yield stress, or
These observations support the idea that anomalously lithospheric strength, is highly sensitive to the geother-
hot asthenosphere beneath Africa is related in some way mal gradient as well as to crustal composition and
to this broad deep zone of upwelling known as the crustal thickness (Section 2.10.4). A consideration of
13
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African superswell (Section 12.8.3). Nevertheless, a con- these factors suggests that a force of 3 × 10 N m may
sensus on the location, depth extent and continuity of be required to rupture lithosphere with a typical heat
−2

hot mantle material below the East African Rift system flow value of 50 mW m (Buck et al., 1999). In areas

has yet to be reached (cf. Montelli et al., 2004a). where lithosphere exhibits twice the heat flow, such as
A comparison of the mantle structure beneath rifts in the Basin and Range Province, it may take less than
−1
12
in different settings indicates that the size and strength 10 N m (Kusznir & Park, 1987; Buck et al., 1999).
of mantle upwellings are highly variable. Achauer & Several authors have estimated that the tectonic forces
−1
12
Masson (2002) showed that in relatively cool rifts, such available for rifting are in the range 3–5 × 10 N m
as the Baikal Rift and the southern Rhine Graben, low (Forsyth & Uyeda, 1975; Solomon et al., 1975). If correct,
velocity zones are only weakly negative (−2.5% relative then only initially thin lithosphere or lithosphere with
−2
to normal mantle P-wave velocities) and occur mostly heat flow values greater than 65–70 mW m is expected

above depths of 160 km. In these relatively cool settings, to undergo significant extension in the absence of any

the low velocity zones in the uppermost mantle show other weakening mechanism (Kusznir & Park, 1987).
no continuation to deeper levels (>160 km) and no Elsewhere, magmatic intrusion or the addition of water
broadening of an upwelling asthenosphere with depth may be required to sufficiently weaken the lithosphere

below the rift. In still other settings, such as the Rio to allow rifting to occur.
Grande rift, low velocity zones in the upper mantle may Another important factor that controls whether
form parts of small-scale convection cells where upwell- rifting occurs, is the mechanism that is available to