Page 192 - Global Tectonics

178 CHAPTER 7

the East African Rift system where the rift axis meets (a) Pure shear model
the cool, thick lithospheric root of the Archean
Tanzanian craton (Section 7.8.1). The Tanzanian
example suggests that lateral heterogeneities at
the lithosphere–asthenosphere boundary rather than
(b) Simple shear model
shallow level structures in the crust are required to

significantly alter rift geometry (Foster et al., 1997).

(c) Delamination model
7.6 STRAIN

LOCALIZATION AND

DELOCALIZATION Brittle upper crust Asthenosphere

PROCESSES Lithosphere Ductile crust 20 km Magma
Upper mantle
20 km
7.6.1 Introduction Figure 7.21 Kinematic models of continental extension
(after Lister et al., 1986, with permission from the
Geological Society of America).
The localization of strain into narrow zones during
extension is achieved by processes that lead to a mechan-
ical weakening of the lithosphere. Lithospheric weaken-
ing may be accomplished by the elevation of geotherms margins. Figures 7.4c, 7.10, and 7.11 illustrate the data
during lithospheric stretching, heating by intrusions, types that frequently are used to generate these types
interactions between the lithosphere and the astheno- of models. Among the most common kinematic exam-
sphere, and/or by various mechanisms that control the ples are the pure shear (McKenzie, 1978), the simple shear
behavior of faults and shear zones during deformation. (Wernicke, 1985), and the crustal delamination (Lister et
Working against these strain softening mechanisms are al., 1986) models of extension (Fig. 7.21). The predic-
processes that promote the mechanical strengthening tions from these models are tested with observations of
of the lithosphere. Lithospheric strengthening may be subsidence and uplift histories within rifts and rifted
accomplished by the replacement of weak crust by margins, and with information on the displacement pat-
strong upper mantle during crustal thinning and by the terns recorded by faults and shear zones. This approach
crustal thickness variations that result from extension. has been used successfully to explain differences in the
These and other strain hardening mechanisms promote geometry of faulting and the history of extension
the delocalization of strain during rifting. Competition among some rifts and rifted margins. However, one
among these mechanisms, and whether they result in a major limitation of kinematic modeling is that it does
net weakening or a net strengthening of the lithosphere, not address the underlying causes of these differences.
controls the evolution of deformation patterns within By contrast, mechanical models employ information
rifts. about the net strength of the lithosphere and how it
To determine how different combinations of litho- changes during rifting to test how different physical
spheric weakening and strengthening mechanisms processes affect rift evolution. This latter approach
control the response of the lithosphere to extension, permits inhomogeneous strains and a quantitative eval-
geoscientists have developed physical models of rifting uation of how changes to lithospheric strength and rhe-

using different approaches. One approach, called kine- ology influence rift behavior. The main physical
matic modeling, involves using information on the geom- processes involved in rifting and their effects on the
etry, displacements, and type of strain to make evolution of the lithosphere are discussed in this
predictions about the evolution of rifts and rifted section.