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CONTINENTAL RIFTS AND RIFTED MARGINS 159
NW SE
(a) SP11 SP12 SP13 SP14 SP15/25 SP16 SP17 SP18
3.3
5.0 5.2
0 4.8 6.13
6.10 6.08 6.51 6.07
6.15
10 6.24 6.52 6.23 6.31
6.16
6.54 6.38
6.33
20
Depth (km) 30 6.64 6.40 7.38 7.70 6.73 8.05 6.63
6.65
6.82
6.83
40
50
60
0 50 100 150 200 250 300 350
Distance (km)
Rift volcanic and
(b) sedimentary rock Rift valley
0
Pre-rift basalt and
10 sedimentary rock
Depth (km) 20 Mafic intrusion Sills
Lower crustal reflectors
30
High velocity lower crust
? Hot mantle Moho
40 Igneous underplate Normal mantle
Moho
50
0 50 100 150 200 250 300 350
Distance (km)
Figure 7.5 (a) P-wave velocity model and (b) interpretation of the Main Ethiopian Rift (after Mackenzie et al., 2005,
with permission from Blackwell Publishing). Location of profile (B–B′) shown in Fig. 7.4c.
4 High heat flow and low velocity, low density as in East Africa, domal uplifts and
upper mantle. Heat fl ow measurements pervasive volcanism result. Nevertheless,
−2
averaging 70–90 mW m and low seismic there is a large degree of variability in
velocities in many rift basins suggest temperature and volcanic activity among
−1
temperature gradients (50–100°C km ) rifts. The Baikal Rift, for example, is much
that are higher than those in the adjacent cooler. This rift displays low regional heat
−2
rift fl anks and nearby cratons. Where the flow of 40–60 mW m (Lysack, 1992) and
asthenosphere is anomalously hot, such lacks volcanic activity.