Page 174 - Global Tectonics

160 CHAPTER 7

(a) (b)
28 32 36 40 44 48
20° N 20
5000
4000 Central MER A'
3000 A
16° N 16 2000
meters 1000 0

12° N -40 12 –1000
–2000 Topography
-50 -30 -20 –3000
-20 A -90 0 50 100 150 200 250 300 350
8° N -30 -60 8 km
-100
-110 -60 -50 –160
-120
A' -80 -70 -40
-7 0
–180
-80
4° N 4 –200
mGal –220
-90 –240
0° N -100 0
–260
Bouguer anomaly
-110 –280 0 50 100 150 200 250 300 350
4° S km -4 km
0 400 800 0
-120 –10
km subsurface –20
8° S -8
28° E 32° E 36° E 40° E 44° E 48° E –30
–40
Crustal thickness
–50
0 50 100 150 200 250 300 350
km
Figure 7.6 (a) Map showing long-wavelength Bouguer gravity anomalies after removal of the short wavelength

(image provided by A. Tessema and modified from Tessema & Antoine, 2004, with permission from Elsevier). (b) Profiles

(A–A′) of topography, short-wavelength Bouguer gravity anomaly, and crustal thickness estimates of the central Main

Ethiopian Rift (MER) (images provided by C. Tiberi and modified from Tiberi et al., 2005, with permission from Blackwell

Publishing). Profile location shown in (a). Circles in crustal thickness profile indicate depths estimated from receiver
function studies.

In East Africa, relatively slow P n wave velocities more central about the rift axis below this depth (Fig.
−1
of 7.7 km s in the upper mantle beneath the Adama 7.7c). In the more highly extended northern section
Rift Basin in Ethiopia (Fig. 7.5a) suggest elevated of Ethiopian Rift (Fig. 7.7d), the low velocity anomaly
temperatures (Mackenzie et al., 2005). Elsewhere broadens laterally below 100 km and may be con-
upper mantle P n wave velocities are in the range nected to deeper low velocity structures beneath the
−1
8.0–8.1 km s , which is expected for stable areas with Afar Depression (Section 7.4.3). This broadening of
normal heat flow. Tomographic inversion of P- and the low velocity zone is consistent with the propaga-

S-wave data (Fig. 7.7a–c) indicate that the low velocity tion of the Main Ethiopian Rift, during Pliocene–
zone below the rift is tabular, approximately 75 km Recent times, toward the older spreading centers of
wide, and extends to depths of 200–250 km (Bastow the Red Sea and Gulf of Aden.
et al., 2005). The zone is segmented and offset away In addition to the high temperatures, the low
from the rift axis in the upper 100 km but becomes velocity zones beneath rifts may also refl ect the