Magmaassisted rifting

Most quantitative treatments of continental rifting focus on the effects of variations in lithospheric conditions. This emphasis reflects both the success of these models at explaining many aspects of rifting and the relative ease at which geoscientists can constrain the physical properties of the lithosphere compared to those of the asthenosphere. Nevertheless, it is evident that interactions between the asthenosphere and the lithosphere form crucial components of rift systems (Ebinger, 2005). One of the most important aspects of these interactions involves magmatism (Section 7.4), which weakens the lithosphere and causes strain localization.

Among its possible effects, mafic magmatism may allow rifting to initiate in regions of relatively cold or thick continental lithosphere (Section 7.5). In addition to its weakening effects, the availability of a significant source of basaltic magma influences the thickness, temperature, density, and composition of the lithosphere. The presence of hot, partially molten material beneath a rift valley produces density contrasts that result in thermal buoyancy forces (Section 7.6.3). As the two sides of the rift separate, magma also may accrete to the base of the crust where it increases in density as it cools and may lead to local crustal thickening (Section 7.2, Fig. 7.5). These processes can create bending forces within the lithosphere as the plate responds to the changing load, and affect the manner in which strain is accommodated during rifting. The changes may be recorded in patterns of uplift and subsidence across rifts and rifted margins.

Buck (2004) developed a simple two-dimensional thermal model to illustrate how rifting and magma intrusion can weaken the lithosphere and influence subsidence and uplift patterns. The emplacement of large quantities of basalt in a rift can accommodate extension without crustal thinning. This process has been observed in the mature rift segments of northern Ethiopia (Section 7.8.1) where strain accommodation by faulting has been greatly reduced as magmatism increased (Wolfenden et al., 2005). If enough material intrudes, the crustal thickening that can result from magmatism can lessen the amount of subsidence in the rift and may even lead to regional uplift. This effect is illustrated in Fig. 7.30, which shows the average isostatic elevation through time for magma-assisted rifting compared to a typical subsidence curve for lithospheric stretching due to thermal relaxation (McKenzie, 1978). The uplift or subsidence result from changes in density related to the combined effects of crustal thinning, basalt intrusion and temperature differences integrated over a 100 km wide rift to a depth of 150 km. Buck (2004) suggested that this process might explain why some continental margins, such as those off the east coast of Canada (Royden & Keen, 1980), show less initial tectonic subsidence related to crustal thinning compared to the

Figure 7.30 Comparison of the predicted average regional isostatic elevation changes for magma-assisted rifting (solid line) and pure shear necking (dashed line) (from Buck, 2004. Copyright © 2004 from Columbia University Press. Reprinted with permission of the publishers).

Figure 7.30 Comparison of the predicted average regional isostatic elevation changes for magma-assisted rifting (solid line) and pure shear necking (dashed line) (from Buck, 2004. Copyright © 2004 from Columbia University Press. Reprinted with permission of the publishers).

long-term (tens of millions of years) subsidence induced by cooling.

Two other problems of rift evolution that also might be resolved by incorporating the effects of magmatism and/or flow of the asthenosphere include the extra subsidence observed at some rifted margins and the lack of magma that characterize nonvolcanic margins (Buck, 2004). These effects are discussed in the context of the evolution of rifted continental margins in Section 7.7.3.

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