Page 197 - Global Tectonics

CONTINENTAL RIFTS AND RIFTED MARGINS 183

Models of continental extension that emphasize crustal buoyancy force are important at low strain rates,

crustal buoyancy forces incorporate the effects of ductile when thermal diffusion is relatively efficient (e.g. Fig.

flow in the lower crust. Buck (1991) and Hopper & Buck 7.22h–j), and after long (>30 Myr) periods of time. In
(1996) showed that the pressure difference between addition, thermal buoyancy forces may dominate over
areas inside and outside a rift could cause the lower crust crustal buoyancy forces immediately after rifting when

to flow into the zone of thinning if the crust is thick and strain magnitudes are relatively low. This latter effect

hot. Efficient lateral flow in a thick, hot, and weak lower occurs because variations in crustal thicknesses are rela-

crust works against crustal buoyancy forces by relieving tively small at low stretching (β) factors. This study, and
the stresses that arise from variations in crustal thick- the work of Buck (1991) and Hopper & Buck (1996),
ness. This effect may explain why the present depth of suggests that shifts in the mode of extension are
the Moho in some parts of the Basin and Range Prov- expected as continental rifts evolve through time and
ince, and therefore crustal thickness, remains fairly the balance of thermal and crustal forces within the
uniform despite the variable amounts of extension lithosphere changes.
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 thin- 7.6.4 Lithospheric flexure
ning can remain fixed as high strains build up near the

surface. Buck (1991) and Hopper & Buck (1996) defi ned Border faults that bound asymmetric rift basins with
this latter style of deformation as core complex-mode uplifted flanks are among the most common features in

extension (Fig. 7.24c). Studies of flow patterns in ancient continental rifts (Fig. 7.25). Some aspects of this char-
lower crust exposed in metamorphic core complexes acteristic morphology can be explained by the elastic
(e.g. Klepeis et al., 2007) support this view. response of the lithosphere to regional loads caused by
The relative magnitudes of the thermal and crustal normal faulting.

buoyancy forces may be affected by two other param- Plate flexure (Section 2.11.4) describes how the
5
eters: strain rate and strain magnitude. Davis & Kusznir lithosphere responds to long-term (>10 years) geo-
(2002) showed that the strain delocalizing effects of the logic loads. By comparing the flexure in the vicinity of

(a) Basin width
4 Rift flank uplift
A A
km 0

6

(b)
A flank 10– 100 km A
Uplifted 10–60 km

Monocline
Border fault

Figure 7.25 Generalized form of an asymmetric rift basin showing border fault in (a) cross-section and (b) plan view
(after Ebinger et al., 1999, with permission from the Royal Society of London). Line of section (A–A′) shown in (b).
Shading in (b) shows areas of depression.