Page 172 - Global Tectonics

158 CHAPTER 7

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less than 14 km (Fig. 7.4d), probably are 5–10% higher (>6.5 km s ) than
refl ecting magma movement in dikes. In the those outside the rift (Fig. 7.5a). These
rift fl anks, seismic activity may refl ect fl exure differences probably refl ect the presence of

of the crust (Section 7.6.4) as well as mafic intrusions associated with magmatic
movement along faults. The orientation of centers. A nearly continuous intracrustal
the minimum compressive stress determined refl ector at 20–25 km depth and Moho depths
from earthquake focal mechanisms is of 30 km show crustal thinning beneath the
approximately horizontal, parallel to an rift axis. The western fl ank of the rift is
azimuth of 103°. This stress direction, like underlain by a ∼45 km thick crust and
that in Afar, is consistent with determinations displays a ∼15 km thick high velocity
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of extension directions derived from tension (7.4 km s ) lower crustal layer. This
fractures in young <7000 year old lavas, layer is absent from the eastern side,
geodetic measurements, and global plate where the crust is some 35 km thick.
kinematic data (Fig. 7.4c). Mackenzie et al. (2005) interpreted the high
velocity lower crustal layer beneath the
western fl ank as underplated material
3 Local crustal thinning modified by magmatic associated with pre-rift Oligocene fl ood

activity. Geophysical data indicate that basalts and, possibly, more recent magmatic
continental rifts are characterized by thinning activity. Variations in intracrustal seismic
of the crust beneath the rift axis. Crustal refl ectivity also suggest the presence of
thicknesses, like the fault geometries in rift igneous intrusions directly below the rift
basins, are variable and may be asymmetric. valley (Fig. 7.5b).
Thick crust may occur beneath the rift fl anks Gravity data provide additional evidence
as a result of magmatic intrusions indicating that the crustal structure of rift zones is

that crustal thinning is mostly a local permanently modified by magmatism
phenomenon (Mackenzie et al., 2005; Tiberi that occurs both prior to and during
et al., 2005). Variations in crustal thickness rifting. In Ethiopia and Kenya, two long-

may also reflect inherited (pre-rift) structural wavelength (>1000 km) negative Bouguer
differences. gravity anomalies coincide with two major
Mackenzie et al. (2005) used the results of ∼2 km high topographic uplifts: the
controlled-source seismic refraction and Ethiopian Plateau and the Kenya Dome,
seismic refl ection studies to determine which forms part of the East African
the crustal velocity structure beneath the Plateau (Figs 7.2, 7.6a). The highest parts
Adama Rift Basin in the northern part of the Ethiopian Plateau are more than
of the Main Ethiopian Rift (Fig. 7.5a). Their 3 km high. This great height results
velocity model shows an asymmetric crustal from the eruption of a large volume of
structure with maximum thinning occurring continental fl ood basalts (Section 7.4)
slightly west of the rift valley. A thin low between 45 and 22 Ma, with the majority of
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velocity layer (3.3 km s ) occurs within the volcanism coinciding with the opening of the
rift valley and thickens eastward from 1 to Red Sea and Gulf of Aden at ∼30 Ma
2.5 km. A 2–5-km-thick sequence of (Wolfenden et al., 2005). The negative gravity
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intermediate velocity (4.5–5.5 km s ) anomalies refl ect the presence of
sedimentary and volcanic rock lies below the anomalously low density upper mantle and
low velocity layer and extends along the elevated geotherms (Tessema & Antoine,
length of the profile. Normal crustal 2004). In each zone, the rift valleys display

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velocities (P n = 6.0–6.8 km s ) occur to depths short-wavelength positive Bouguer gravity
of 30–35 km except in a narrow 20–30 km anomalies (Fig. 7.6b) that refl ect the presence

wide region in the upper crust beneath the of cooled, dense mafic intrusions (Tiberi et
center of the rift valley where P n velocities al., 2005).