Page 199 - Global Tectonics

CONTINENTAL RIFTS AND RIFTED MARGINS 185

successful at explaining another major source of vari- kilometers) faults is a reduction in the cohesion of the
ability in rifts: the degree of strain localization in faults faulted material. During extension, cohesion can be
and shear zones. In some settings normal faulting is reduced by a number of factors, including increased

widely distributed across large areas where many faults fluid pressure (Sibson, 1990), the formation of fault
accommodate a relatively small percentage of the total gouge, frictional heating (Montési & Zuber, 2002),
extension (Section 7.3). However, in other areas or at mineral transformations (Bos & Spiers, 2002), and
different times, extension may be highly localized on decreases in strain rate (Section 2.10). Lavier et al. (2000)
relatively few faults that accommodate a large percent- used simple two-layer models to show that the forma-
age of the total extension. Two approaches have been tion of a large-offset normal fault depends on two
used to explain the causes of this variability. The fi rst parameters: the thickness of the brittle layer and the rate
incorporates the effects of a strain-induced weakening at which the cohesion of the layer is reduced during
of rocks that occurs during the formation of faults and faulting (Plate 7.4a,b between pp. 244 and 245). The
shear zones. A second approach, discussed in Section models include an upper layer of uniform thickness
7.6.6, shows how vertical contrasts in the rheology of overlying a ductile layer having very little viscosity. In the
crustal layers affect the localization and delocalization ductile layer the yield stress is strain-rate- and tempera-
of strain during extension. ture-dependent following dislocation creep fl ow laws
In order for a normal fault to continue to slip as the (Section 2.10.3). In the upper layer brittle deformation is
crust is extended it must remain weaker than the sur- modeled using an elastic-plastic rheology. The results
rounding rock. As discussed in Section 7.6.4, the defl ec- show that where the brittle layer is especially thick
tion of the crust by faulting changes the stress fi eld (>22 km) extension always leads to multiple normal
surrounding the fault. Assuming elastic behavior, faults (Plate 7.4c between pp. 244 and 245). In this case
Forsyth (1992) showed that these changes depend on the width of the zone of faulting is equivalent to the
the dip of the fault, the amount of offset on the fault, thickness of the brittle layer. However, for small brittle
and the inherent shear strength or cohesion of the faulted layer thicknesses (<22 km), the fault pattern depends on
material. He argued that the changes in stresses by how fast cohesion is reduced during deformation (Plate
normal faulting increase the yield strength of the layer 7.4d,e between pp. 244 and 245). To obtain a single large-
and inhibit continued slip on the fault. For example, slip offset fault, the rate of weakening must be high enough
on high-angle faults create surface topography more to overcome the resistance to continued slip on the fault

efficiently than low-angle faults, so more work is that results from fl exural bending.
required for large amounts of slip on the former than These studies provide some insight into how layer
on the latter. These processes cause an old fault to be thickness and the loss of cohesion during faulting
replaced with a new one, leading to a delocalization of control the distribution of strain, its symmetry, and the
strain. Buck (1993) showed that if the crust is not elastic formation of large-offset faults. However, at the scale of

but can be described with a finite yield stress (elastic- rifts, other processes also impact fault patterns. In ductile
plastic), then the amount of slip on an individual fault shear zones changes in mineral grain size may promote
for a given cohesion depends on the thickness of the a switch from dislocation creep to grain-size-sensitive
elastic-plastic layer. In this model the viscosity of the diffusion creep (Section 2.10.3), which can reduce the
elastic-plastic layer is adjusted so that it adheres to the yield strengths of layers in the crust and mantle. In addi-
Mohr–Coulomb criterion for brittle deformation tion, the rate at which a viscous material flows has an

(Section 2.10.2). For a brittle layer thickness of >10 km important effect on the overall strength of the material.

and a reasonably low value of cohesion a fault may slip The faster it flows, the larger the stresses that are gener-
only a short distance (a maximum of several kilome- ated by the flow and the stronger the material becomes.

ters) before a new one replaces it. If the brittle layer is This latter process may counter the effects of cohesion
very thin, then the offset magnitude can increase loss during faulting and could result in a net strengthen-
because the increase in yield strength resulting from ing of the lithosphere by increasing the depth of the
changes in the stress field due to slip is small. brittle–ductile transition (Section 2.10.4). At the scale

Although layer thickness and its inherent shear of the lithosphere, it therefore becomes necessary to
strength play an important role in controlling fault pat- examine the interplay among the various weakening
terns, a key process that causes strain localization and mechanisms in both brittle and ductile layers in order to
may lead to the formation of very large offset (tens of reproduce deformation patterns in rifts.