Page 216 - Global Tectonics

202 CHAPTER 7

slow-spreading systems (Section 6.6) where large relatively thick lithosphere. They also illustrate that the

fluctuations in melt supply occur in transient magma cross-sectional geometry and the along-axis segmenta-
chambers during the early stages of sea fl oor tion in youthful rifts are controlled by the fl exural
spreading. strength of the lithosphere (Section 7.6.4).
The effects of pre-existing weaknesses on the geom-
etry of rifting are also illustrated in the southern
7.8 CASE STUDIES: segment of the Eastern Rift in Tanzania. Border faults
and half graben preferentially formed in a zone of
weakness created by a contrast between thick, cool
THE TRANSITION lithosphere of the Archean Tanzanian craton and thin,
weak Proterozoic lithosphere located to the east (Foster
FROM RIFT TO RIFTED et al., 1997). From north to south, the axis of the rift
diverges from a single ∼50 km wide rift to a ∼200 km
MARGIN wide zone composed of three narrow segments (Fig.
7.2b). This segmentation and a change in orientation of
faults occurs where the rift encounters the Archean
7.8.1 The East African Tanzanian craton (Fig. 7.37), indicating that the thick
lithosphere has deflected the orientation of the rift.

Rift system These observations illustrate that lateral heterogene-
ities at the lithosphere–asthenosphere boundary exert a
The East African Rift system (Fig. 7.2) is composed of strong control on the initial location and distribution of
several discrete rift segments that record different stages strain at the start of rifting (Section 7.4).
in the transition from continental rift to rifted volcanic An example of a rift that is slightly more evolved
margin (Ebinger, 2005). The Eastern Rift between than the Tanzanian example occurs in central and
northern Tanzania and southern Kenya is an example northern Kenya where rifting began by 15 Ma. In this
of a youthful rift that initiated in thick, cold and strong rift segment the crust has been thinned by up to 10 km
continental lithosphere. Volcanism and sedimentation and the thickness of the lithosphere has been reduced
began by ∼5 Ma with the largest fault escarpments to about 90 km (Mechie et al., 1997). A progressive shal-
forming by ∼3 Ma. Strain and magmatism are localized lowing of the Moho occurs between central and north-
within narrow asymmetric rift basins with no detect- ern Kenya where the rift widens from ∼100 km to
able deformation in the broad uplifted plateau adjacent ∼175 km (Fig. 7.2b). In northern Kenya, crustal thick-
to the rifts (Foster et al., 1997). Earthquake hypocenters ness is about 20 km and the total surface extension is
occur throughout the entire 35 km thickness of the about 35–40 km (β = 1.55–1.65) (Hendrie et al., 1994). In
crust, indicating that crustal heating is at a minimum the south, crustal thickness is 35 km with estimates of
(Foster & Jackson, 1998). The basins are shallow (∼3 km total extension ranging from 5 to 10 km (Strecker et al.,
deep) with 100-km-long border faults that accommo- 1990; Green et al., 1991). As the amount of crustal
date small amounts of extension. The border faults have stretching increases, and the lithosphere–asthenosphere
grown from short fault segments that propagated along boundary rises beneath a rift, the amount of partial
their lengths to join with other nearby faults, creating melting resulting from decompression melting also
linkages between adjacent basins (Foster et al., 1997). increases (Section 7.4.2). Young lavas exposed in central
Faults that were oriented unfavorably with respect to and northern Kenya indicate source regions that are
the opening direction were abandoned as strain pro- shallower than those in Tanzania (Furman et al., 2004).
gressively localized onto the border faults (Ebinger, High velocity, high density material is present in the
2005). Geophysical (Green et al., 1991; Birt et al., 1997) upper crust and at the base of the lower crust, suggest-
and geochemical (Chesley et al., 1999) data show that ing the presence of cooled basaltic intrusions (Mechie
the mantle lithosphere has been thinned to about et al., 1997; Ibs-von Seht et al., 2001). These relationships
140 km. Elsewhere the lithosphere is at least 200 km and indicate that as a continental rift enters maturity mag-

possibly 300–350 km thick (Ritsema et al., 1998). These matic activity increases and a significant component of
patterns conform to the predictions of lithospheric the extension is accommodated by magmatic intrusion
stretching models (Section 7.6.2, 7.6.3) in regions of below the rift axis (Ebinger, 2005).