Page 193 - Global Tectonics

CONTINENTAL RIFTS AND RIFTED MARGINS 179

7.6.2 Lithospheric stretching of the initial and final thickness of the crust (McKenzie,
1978).
The thermal and mechanical effects of lithospheric
During horizontal extension, lithospheric stretching stretching at different strain rates are illustrated in Fig.
results in a vertical thinning of the crust and an increase 7.22, which shows the results of two numerical experi-
in the geothermal gradient within the zone of thinning ments conducted by van Wijk & Cloetingh (2002). In
(McKenzie, 1978). These two changes in the physical these models, the lithosphere is divided into an upper
properties of the extending zone affect lithospheric crust, a lower crust, and a mantle lithosphere that have
strength in contrasting ways. Crustal thinning or necking been assigned different rheological properties (Fig.
tends to strengthen the lithosphere because weak crustal 7.22a). Figures 7.22b–d show the thermal evolution
material is replaced by strong mantle lithosphere as the of the lithosphere for uniform extension at a rate of
−1
latter moves upward in order to conserve mass. The 16 mm a . At this relatively fast rate, heating by thermal
upward movement of the mantle also may result in advection outpaces thermal diffusion, resulting in
increased heat fl ow within the rift. This process, called increased temperatures below the rift and strain local-

heat advection, results in higher heat flow in the rift ization in the zone of thinning. As the crust thins,
because the geotherms become compressed rather than narrow rift basins form and deepen. Changes in stretch-
through any addition of heat. The compressed geo- ing factors for the crust (β) and mantle (δ) are shown in
therms tend to result in a net weakening of the litho- Fig. 7.22e,f. The total strength of the lithosphere (Fig.
sphere, whose integrated strength is highly sensitive to 7.22g), obtained by integrating the stress field over the

temperature (Section 2.10). However, the weakening thickness of the lithosphere, gradually decreases with
effect of advection is opposed by the diffusion of heat time due to stretching and the strong temperature
away from the zone of thinning as hot material comes dependence of the chosen rheologies. Eventually, at
into contact with cooler material. If the rate of heat very large strains, the thermal anomaly associated with
advection is faster than the rate of thermal diffusion and rifting is expected to dissipate. These and many other
cooling then isotherms at the base of the crust are models of rift evolution that are based on the principles
compressed, the geotherm beneath the rift valley of lithospheric stretching approximate the subsidence
increases, and the integrated strength of the lithosphere patterns measured in some rifts and at some rifted con-
decreases. If thermal diffusion is faster, isotherms tinental margins (van Wijk & Cloetingh, 2002; Kusznir
and crustal temperatures move toward their pre-rift et al., 2004) (Section 7.7.3).

configuration and lithospheric weakening is inhibited. The experiment shown in Fig. 7.22h–j shows the
England (1983) and Kusznir & Park (1987) showed evolution of rift parameters during lithospheric stretch-
−1
that the integrated strength of the lithosphere in rifts, ing at the relatively slow rate of 6 mm a . During the
and competition between cooling and heat advection first 30 Ma, deformation localizes in the center of the

mechanisms, is strongly influenced by the rate of exten- rift where the lithosphere is initially weakened as iso-
−13 −1
−14 −1
sion. Fast strain rates (10 s or 10 s ) result in larger therms and mantle material move upward. However, in
increases in geothermal gradients than slow rates contrast with the model shown in Fig. 7.22b–d, tem-
−16 −1
(10 s ) for the same amount of stretching. This effect peratures begin to decrease with time due to the effi -
suggests that high strain rates tend to localize strain ciency of conductive cooling at slow strain rates. Mantle
because inefficient cooling keeps the thinning zone upwelling in the zone of initial thinning ceases and the

weak, allowing deformation to focus into a narrow lithosphere cools as temperatures on both sides of the
zone. By contrast, low strain rates tend to delocalize central rift increase. At the same time, the locus of thin-
strain because efficient cooling strengthens the litho- ning shifts to both sides of the first rift basin, which does

sphere and causes the deformation to migrate away not thin further as stretching continues. The mantle
from the center of the rift into areas that are more easily thinning factor (Fig. 7.22l) illustrates this behavior.

deformable. The amount of net lithospheric weakening During the first 45 Ma, upwelling mantle causes δ to be
or strengthening that results from any given amount of larger in the central rift than its surroundings. After this
stretching also depends on the initial strength of the time, δ decreases in the central rift as new upwelling
lithosphere and on the total amount of extension. The zones develop on its sides. The total strength of the
total amount of thinning during extension usually is lithosphere (Fig. 7.22m) for this low strain rate model
described by the stretching factor (β), which is the ratio shows that the central rift is weakest until about 55 Ma.