Buoyancy forces and lower crustal flow

In addition to crustal thinning and the compression of geotherms (Section 7.6.2), lithospheric stretching results in two types of buoyancy forces that influence strain localization during rifting. First, lateral variations in temperature, and therefore density, between areas inside and outside the rift create a thermal buoyancy force that adds to those promoting horizontal extension (Fig. 7.23). This positive reinforcement tends to enhance those aspects of lithospheric stretching (Section 7.6.2) that promote the localization of strain. Second, a crustal buoyancy force is generated by local (Airy) isostatic effects as the crust thins and high density material is brought to shallow levels beneath the rift (Fleitout & Froidevaux, 1982). Because the crust is less dense than the underlying mantle, crustal thinning lowers surface elevations in the center of the rift (Fig. 7.23). This subsidence places the rift into compression, which opposes the forces driving extension. The opposing force makes it more difficult to continue deforming in the same locality, resulting in a delocalization of strain as the deformation migrates into areas that are more easily deformable (Buck, 1991).

Several processes may either reduce or enhance the effects of crustal buoyancy forces during lithospheric stretching. Buck (1991) and Hopper & Buck (1996) showed that where the crust is initially thin and cool, and the mantle lithosphere is relatively thick, the overall strength (the effective viscosity) of the lithosphere remains relatively high under conditions of constant strain rate (Fig. 7.24a). In this case, the effects of crustal buoyancy forces are reduced and the thermal effects of lithospheric necking are enhanced. Narrow rifts result because the changes in yield strength and thermal buoyancy forces that accompany lithospheric stretching dominate the force balance, causing extensional strains to remain localized in the region of necking. By contrast, where the crust is initially thick and hot, and the mantle lithosphere is relatively thin, the overall strength of the lithosphere remains relatively low. In this case, crustal buoyancy forces dominate because the amount of possible weakening due to lithospheric necking is relatively small, resulting in strain delocalization and the formation of wide zones of rifting (Fig. 7.24b) as the necking region migrates to areas that require less force to deform. These models illustrate how crustal thickness and the thermal state of the lithosphere at the start of rifting greatly influence the style of extension.

Temperature (°C)

Figure 7.23 Schematic diagram illustrating thermal and crustal buoyancy forces generated during rifting. A and B represent vertical profiles outside and inside the rift valley, respectively. Pressure and temperature as a function of depth for each profile are shown to the right of sketch (modified from Buck, 1991,by permission of the American Geophysical Union. Copyright © 1991 American Geophysical Union). Differences in profiles generate lateral buoyancy forces.

(a) Narrow rift mode

(a) Narrow rift mode

Mantle

Mantle

Lithosphere Asthenosphere i>

0 1200 0 600 17 23

Temperature Yield strength

Viscosity

(b) Wide rift mode

(b) Wide rift mode

Crust

Crust

Mantle

Lithosphere Asthenosphere

0 1200 0 600 17 23

th pt

th pt

Temperature Yield strength

Viscosity

(c) Core complex mode

upper crust m^ ^/¿-Lithosphere

Asthenosphere i>

lower crust

Straining region h

40 km

0 1200 0 600 17 23

0 1200 0 600 17 23

Temperature Yield strength

Viscosity

Temperature Yield strength

Viscosity

Figure 7.24 Sketches of the lithosphere illustrating three modes of extension emphasizing the regions undergoing the greatest amount of extensional strain (modified from Buck, 1991, by permission of the American Geophysical Union. Copyright © 1991 American Geophysical Union). (a) Narrow mode, (b) wide mode, (c) core complex mode. Lithosphere is defined as areas with effective viscosities of >1021 Pa s~'. The plots to the right of each sketch show initial model geotherms, yield strengths (for a strain rate of 8 x TO'5 s~') and effective viscosities for a dry quartz crust overlying a dry olivine mantle. From top to bottom the crustal thicknesses are 30 km, 40km, and 50km. initial surface heat flow. (c) shows layers labeled at two scales: the upper crust and lower crust labels on the left side of diagram show a weak, deforming lower crust (shaded); the lithosphere and asthenosphere labels on the right side of diagram show a scale emphasizing that the zone of crustal thinning (shaded column) is localized into a relatively narrow zone of weak lithosphere.

Models of continental extension that emphasize crustal buoyancy forces incorporate the effects of ductile flow in the lower crust. Buck (1991) and Hopper & Buck (1996) showed that the pressure difference between areas inside and outside a rift could cause the lower crust to flow into the zone of thinning if the crust is thick and hot. Efficient lateral flow in a thick, hot, and weak lower crust works against crustal buoyancy forces by relieving the stresses that arise from variations in crustal thickness. This effect may explain why the present depth of the Moho in some parts of the Basin and Range Province, and therefore crustal thickness, remains fairly uniform despite the variable amounts of extension observed in the upper crust (Section 7.3). In cases where low yield strengths and flow in the lower crust alleviate the effects of crustal buoyancy, the zone of crustal thinning can remain fixed as high strains build up near the surface. Buck (1991) and Hopper & Buck (1996) defined this latter style of deformation as core complex-mode extension (Fig. 7.24c). Studies of flow patterns in ancient lower crust exposed in metamorphic core complexes (e.g. Klepeis et al., 2007) support this view.

The relative magnitudes of the thermal and crustal buoyancy forces may be affected by two other parameters: strain rate and strain magnitude. Davis & Kusznir (2002) showed that the strain delocalizing effects of the crustal buoyancy force are important at low strain rates, when thermal diffusion is relatively efficient (e.g. Fig. 7.22h-j), and after long (>30 Myr) periods of time. In addition, thermal buoyancy forces may dominate over crustal buoyancy forces immediately after rifting when strain magnitudes are relatively low. This latter effect occurs because variations in crustal thicknesses are relatively small at low stretching (P) factors. This study, and the work of Buck (1991) and Hopper & Buck (1996), suggests that shifts in the mode of extension are expected as continental rifts evolve through time and the balance of thermal and crustal forces within the lithosphere changes.

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