C A

39 40

Longitude

1-300

1-300

S-wave % velocity anomaly

39 40

Longitude

39 40

Longitude

Figure 7.7 Depth slices through (a) P-wave and (b) S-wave velocity models at 75km depth in the Main Ethiopian Rift. (c,d) Vertical profiles through the P-wave velocity model (images provided by I. Bastow and modified from Bastow et al., 2005, with permission from Blackwell Publishing). Heavy black lines in (a) and (b) are Pleistocene magmatic segments and mid-Miocene border faults (cf. Fig. 7.3). The locations of stations contributing to the tomographic inversions are shown with white squares in (a) and (b). Profile locations shown in (a). Velocity scales in (c) and (d) are same as in (a).

presence of partial melt. Observations of shear wave splitting and delay times of teleseismic waves traveling beneath the Kenya Rift (Ayele et al., 2004) and northern Ethiopian Rift (Kendall et al., 2005) suggest the alignment of partial melt in steep dikes within the upper 70-90 km of the lithosphere or the lattice preferred orientation of olivine in the asthenosphere as hot material flows laterally into the rift zone. These observations indicate that the upper mantle underlying rifts is characterized by low velocity, low density and anomalously high temperature material.

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