Rift Initiation

Continental rifting requires the existence of a horizontal deviatoric tensional stress that is sufficient to break the lithosphere. The deviatoric tension may be caused by stresses arising from a combination of sources, including: (i) plate motions; (ii) thermal buoyancy forces due to asthenospheric upwellings; (iii) tractions at the base of the lithosphere produced by convecting asthe-nosphere; and/or (iv) buoyancy (gravitational) forces created by variations in crustal thickness (Huismans et al., 2001). These stresses may be inherited from a previous tectonic regime or they may develop during extension. Full rupture of the lithosphere leading to the formation of a new ocean basin only occurs if the available stresses exceed the strength of the entire lithosphere. For this reason lithospheric strength is one of the most important parameters that governs the formation and evolution of continental rifts and rifted margins.

The horizontal force required to rupture the entire lithosphere can be estimated by integrating yield stress with respect to depth. The integrated yield stress, or lithospheric strength, is highly sensitive to the geother-mal gradient as well as to crustal composition and crustal thickness (Section 2.10.4). A consideration of these factors suggests that a force of 3 X 1013 N m-1 may be required to rupture lithosphere with a typical heat flow value of 50 mW m-2 (Buck et al., 1999). In areas where lithosphere exhibits twice the heat flow, such as in the Basin and Range Province, it may take less than 1012 N m-1 (Kusznir & Park, 1987; Buck et al., 1999). Several authors have estimated that the tectonic forces available for rifting are in the range 3-5 X 1012 N m-1 (Forsyth & Uyeda, 1975; Solomon et al., 1975). If correct, then only initially thin lithosphere or lithosphere with heat flow values greater than 65-70 mW m-2 is expected to undergo significant extension in the absence of any other weakening mechanism (Kusznir & Park, 1987). Elsewhere, magmatic intrusion or the addition of water may be required to sufficiently weaken the lithosphere to allow rifting to occur.

Another important factor that controls whether rifting occurs, is the mechanism that is available to

(a) Tectonic stretching

Asthenosphere

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(b) Magmatic extension

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0 200 400 600 800 1000 Stress difference (MPa)

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(d)

30

800

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30 40 50 60 70 80 90 100 Heat flow (mW m~2)

Figure 7.20 Sketches showing the difference between extension of thick lithosphere without (a) and with (b) magmatic intrusion by diking. Temperature and yield stress curves for each case are show to the right of the sketches. VE, vertical exaggeration. (c) Example of yield stresses for strain rate 10~'4 s- for 30-km-thick crust. Solid line, stress difference for magmatic rifting; dashed line, stress difference for lithospheric stretching. (d) Tectonic force for rifting with and without magma as a function of heat flow. The bold black line in (d) shows the estimated value of driving forces (from Buck, 2004. Copyright © 2004 from Columbia University Press. Reprinted with permission of the publisher).

1000 0

1000 0

40 km accommodate the extension. At any depth, deviatoric tension can cause yielding by faulting, ductile flow, or dike intrusion, depending on which of these processes requires the least amount of stress. For example, if a magma source is available, then the intrusion of basalt in the form of vertical dikes could permit the lithosphere to separate at much lower stress levels than is possible without the diking. This effect occurs because the yield stress that is required to allow basaltic dikes to accommodate extension mostly depends on the density difference between the lithosphere and the magma (Buck, 2004). By contrast, the yield stresses required to cause faulting or ductile flow depend upon many other factors that result in yield strengths that can be up to an order of magnitude greater than those required for lithospheric separation by diking (Fig. 7.20). High temperatures (>700°C) at the Moho, such as those that can result from the thermal relaxation of previously thickened continental crust, also may contribute to the tectonic forces required for rift initiation. For high Moho temperatures gravitational forces become increasingly important contributors to the stresses driving rifting.

Finally, the location and distribution of strain at the start of rifting may be influenced by the presence of pre-existing weaknesses in the lithosphere. Contrasts in lithospheric thickness or in the strength and temperature of the lithosphere may localize strain or control the orientations of rifts. This latter effect is illustrated by the change in orientation of the Eastern branch of the East African Rift system where the rift axis meets 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 shallow level structures in the crust are required to significantly alter rift geometry (Foster et al., 1997).

7.6 STRAIN LOCALIZATION AND DELOCALIZATION PROCESSES

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