Page 206 - Global Tectonics

192 CHAPTER 7

After ∼25 km of extension, the lower crust pulls apart Buck (2004) developed a simple two-dimensional
and displacements on the normal faults lead the col- thermal model to illustrate how rifting and magma
lapse and dismemberment of the upper crust at the intrusion can weaken the lithosphere and infl uence sub-
margins of the rift. Mantle material wells upward into sidence and uplift patterns. The emplacement of large
the zone of thinning where the collapsing upper crust quantities of basalt in a rift can accommodate extension
is placed in direct contact with mantle rocks. After without crustal thinning. This process has been observed
40 km of extension, the array of normal faults is aban- in the mature rift segments of northern Ethiopia
doned and upper crustal deformation is concentrated in (Section 7.8.1) where strain accommodation by faulting
the center of the rift. Finally, after ∼75 km, new ocean has been greatly reduced as magmatism increased
lithosphere is generated, leaving behind two tectoni- (Wolfenden et al., 2005). If enough material intrudes,
cally quiet passive margins. This, and the other physical the crustal thickening that can result from magmatism
models described in this section, show how combina- can lessen the amount of subsidence in the rift and may
tions of competing processes that either weaken or even lead to regional uplift. This effect is illustrated in
strengthen the crust can be used to explain much of the Fig. 7.30, which shows the average isostatic elevation
variability in deformation patterns observed in rifts. through time for magma-assisted rifting compared to a
typical subsidence curve for lithospheric stretching due
to thermal relaxation (McKenzie, 1978). The uplift or
7.6.7 Magma-assisted rifting subsidence result from changes in density related to the
combined effects of crustal thinning, basalt intrusion
Most quantitative treatments of continental rifting and temperature differences integrated over a 100 km
focus on the effects of variations in lithospheric condi- wide rift to a depth of 150 km. Buck (2004) suggested

tions. This emphasis reflects both the success of these that this process might explain why some continental
models at explaining many aspects of rifting and the margins, such as those off the east coast of Canada
relative ease at which geoscientists can constrain the (Royden & Keen, 1980), show less initial tectonic sub-
physical properties of the lithosphere compared to sidence related to crustal thinning compared to the
those of the asthenosphere. Nevertheless, it is evident
that interactions between the asthenosphere and the
lithosphere form crucial components of rift systems 100
(Ebinger, 2005). One of the most important aspects of
these interactions involves magmatism (Section 7.4),
0 Magma
which weakens the lithosphere and causes strain
localization.
Among its possible effects, mafi c magmatism may –100
allow rifting to initiate in regions of relatively cold or Regional uplift (m) Stretch
thick continental lithosphere (Section 7.5). In addition
to its weakening effects, the availability of a signifi cant –200

source of basaltic magma influences the thickness, tem-
perature, density, and composition of the lithosphere.
The presence of hot, partially molten material beneath –300
a rift valley produces density contrasts that result in
thermal buoyancy forces (Section 7.6.3). As the two –400
sides of the rift separate, magma also may accrete to 0 2 4 6 8 10
the base of the crust where it increases in density as it Time (Ma)
cools and may lead to local crustal thickening (Section
7.2, Fig. 7.5). These processes can create bending forces Figure 7.30 Comparison of the predicted average
within the lithosphere as the plate responds to the regional isostatic elevation changes for magma-assisted
changing load, and affect the manner in which strain is rifting (solid line) and pure shear necking (dashed line)
accommodated during rifting. The changes may be (from Buck, 2004. Copyright © 2004 from Columbia
recorded in patterns of uplift and subsidence across rifts University Press. Reprinted with permission of the
and rifted margins. publishers).