The localization of strain into narrow zones during extension is achieved by processes that lead to a mechanical weakening of the lithosphere. Lithospheric weakening may be accomplished by the elevation of geotherms during lithospheric stretching, heating by intrusions, interactions between the lithosphere and the astheno-sphere, and/or by various mechanisms that control the behavior of faults and shear zones during deformation. Working against these strain softening mechanisms are processes that promote the mechanical strengthening of the lithosphere. Lithospheric strengthening may be accomplished by the replacement of weak crust by strong upper mantle during crustal thinning and by the crustal thickness variations that result from extension. These and other strain hardening mechanisms promote the delocalization of strain during rifting. Competition among these mechanisms, and whether they result in a net weakening or a net strengthening of the lithosphere, controls the evolution of deformation patterns within rifts.

To determine how different combinations of litho-spheric weakening and strengthening mechanisms control the response of the lithosphere to extension, geoscientists have developed physical models of rifting using different approaches. One approach, called kinematic modeling, involves using information on the geometry, displacements, and type of strain to make predictions about the evolution of rifts and rifted

Pure shear model

Pure shear model

(c) Delamination model

Brittle upper crust | | Asthenosphere Ductile crust Magma

| Upper mantle J

Figure 7.21 Kinematic models of continental extension (after Lister et al., 1986, with permission from the Geological Society of America).

margins. Figures 7.4c, 7.10, and 7.11 illustrate the data types that frequently are used to generate these types of models. Among the most common kinematic examples are the pure shear (McKenzie, 1978), the simple shear (Wernicke, 1985), and the crustal delamination (Lister et al., 1986) models of extension (Fig. 7.21). The predictions from these models are tested with observations of subsidence and uplift histories within rifts and rifted margins, and with information on the displacement patterns recorded by faults and shear zones. This approach has been used successfully to explain differences in the geometry of faulting and the history of extension among some rifts and rifted margins. However, one major limitation of kinematic modeling is that it does not address the underlying causes of these differences. By contrast, mechanical models employ information about the net strength of the lithosphere and how it changes during rifting to test how different physical processes affect rift evolution. This latter approach permits inhomogeneous strains and a quantitative evaluation of how changes to lithospheric strength and rhe-ology influence rift behavior. The main physical processes involved in rifting and their effects on the evolution of the lithosphere are discussed in this section.

20 km

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