Page 217 - Global Tectonics

CONTINENTAL RIFTS AND RIFTED MARGINS 203

thickness (T e ) of the lithosphere (Section 7.6.4) com-
Archean crust pared to the rift in northern Tanzania. Although both
Lake
Turkana Proterozoic crust the mantle and crust have thinned, the thinning of
K E N Y A
3 N
U G A N D A
Cenozoic volcanic mantle lithosphere outpaces crustal thinning. This
and sedimentary asymmetry occurs because a sufficient amount of

rock
magma has accreted to the base of the crust, resulting
2 N Reworked craton in a degree of crustal thickening. It also results
margin
Major rift fault because the mantle lithosphere is locally weakened by

0 100 interactions with hot magmatic fluids, which further
1 N Lake km localizes stretching.
Baringo Extension in the central and southern part of the
Main Ethiopian Rift began between 18 and 15 Ma and,
in the north, after 11 Ma (Wolfenden et al., 2004). The
0 N deformation resulted in the formation of a series of
high-angle border faults that are marked by chains of
N
volcanic centers (Fig. 7.38a). Since about 1.8 Ma the loci
of magmatism and faulting have become progressively
1 S
more localized, concentrating into ∼20-km-wide, 60-
km-long magmatic segments (Fig. 7.38b). This localiza-
tion involved the formation of new, shorter and
Buried
2 S Archean narrower rift segments that are superimposed on old
craton long border faults in an old broad rift basin. This nar-

rowing of the axis into short segments reflects a plate
KENY A
Lake TANZAN IA
Natron whose effective elastic thickness is less than it was when
3 S
the long border faults formed (Ebinger et al., 1999). The
extrusion of copious amounts of volcanic rock also has
Lake

Manyara modified both the surface morphology of the rift and
its internal structure. Relationships in this rift segment
35 E 36 E 37 E
indicate that magma intrusion in the form of vertical

Figure 7.37 Structural map of the Eastern branch of dikes first becomes equally and then more important
the East African Rift system in Kenya and northern than faulting as rifting approaches sea fl oor spreading

Tanzania showing the deflection of faults at the (Kendall et al., 2005). Repeated eruptions create thick
boundary of the Archean Tanzanian craton (after piles that load the weakened plate causing older lava
Macdonald et al., 2001, with permission from the flows to bend down toward the rift axis. This process

Journal of Petrology 42, 877–900. Copyright © 2001 by creates the seaward-dipping wedge of lavas (Section
permission of Oxford University Press, and Smith & 7.7.1) that is typical of rifted volcanic margins (Section
Mosely, 1993, by permission of the American Geophysical
7.7.1).
Union. Copyright © 1993 American Geophysical Union).
The rift segments in the Afar Depression illustrate
that, as extension increases and the thickness of the
lithosphere decreases, the asthenosphere rises and
decompresses, and more melt is generated. Eventually
The increase in magmatic activity that accompa- all the border faults in the rift are abandoned as mag-
nies a shallowing of the asthenosphere–lithosphere matism accommodates the extension (Fig. 7.38c). At
boundary beneath the Kenya Rift also results in this stage the rift functions as a slow-spreading mid-
increased crustal heating and contributes to a decrease ocean ridge that is bordered on both sides by thinned
in lithospheric strength (Section 7.6.7). This effect is continental lithosphere (Wolfenden et al., 2005). As the
indicated by a progressive decrease in the depth of melt supply increases and/or strain rate increases, new
earthquake hypocenters and in the depth of faulting oceanic lithosphere forms in the magmatic segments
from 35 km to 27 km (Ibs-von Seht et al., 2001). These and the crust and mantle lithosphere subside below sea
patterns suggest a decrease in the effective elastic level. This transition has occurred in the Gulf of Aden