Page 200 - Global Tectonics

186 CHAPTER 7

Huismans & Beaumont (2003, 2007) extended the symmetric necking of the lower lithosphere and contin-
work of Lavier et al. (2000) by investigating the effects ued motion on the asymmetric shear zones results in the
of strain-induced weakening in both brittle (frictional- vertical transport of point P until mantle lithosphere is
plastic) and ductile (viscous) regimes on deformation exposed. The model shown in Fig.7.26f and g combines
patterns in rifts at the scale of the lithosphere and over both frictional-plastic and viscous weakening mecha-
time periods of millions of years. This study showed nisms. The early evolution is similar to that shown in Fig.
that strain softening in the crust and mantle can produce 7.26d, except that S1B continues into the ductile mantle.
large-offset shear zones and controls the overall sym- The two softening mechanisms combine to make defor-
metry of the deformation. Figure 7.26a shows a simple mation asymmetric at all levels of the lithosphere where
three-layer lithosphere where brittle deformation is displacements are mostly focused onto one shear zone.
modeled by using a frictional-plastic rheology that, as These models show how a softening of the dominant
in most physical experiments, is adjusted so that it rheology in either frictional-plastic or viscous layers
adheres to the Mohr–Coulomb failure criterion. Ductile infl uences deformation patterns in rifts through a posi-
deformation is modeled using a thermally activated tive feedback between weakening and increased strain.
power law rheology. During each experiment, ambient The effect of strain-dependent weakening on fault
conditions control whether the deformation is fric- asymmetry also is highly sensitive to rift velocity. This
tional-plastic (brittle) or viscous (ductile). Viscous fl ow sensitivity is illustrated in the models shown in Fig. 7.27.

occurs when the state of stress falls below the frictional- The first model (Fig. 7.27a) is identical to that shown in
plastic yield point. Variations in the choice of crustal Fig. 7.26d and e except that the velocity is decreased by

−1
rheology also allow an investigation of cases where the a factor of five to 0.6 mm a . Reducing the velocity has
crust is either coupled or decoupled to the mantle lith- the effect of maintaining the thickness of the frictional-
osphere. Coupled models involve deformation that is plastic layer, which results in deformation that is more
totally within the frictional-plastic regime. Decoupled strongly controlled by the frictional regime than that
models involve a moderately weak viscous lower crust. shown in Fig. 7.26e. The overall geometry matches a

Strain-induced weakening is specified by linear changes lithospheric-scale simple shear model (cf. Fig. 7.21b) in
in the effective angle of internal friction (Section 2.10.2) which the lower plate has been progressively uplifted
for frictional-plastic deformation and in the effective and exhumed beneath a through-going ductile shear
viscosity for viscous deformation. The deformation is zone that remains the single major weakness during
seeded using a small plastic weak region. rifting. By contrast, a velocity that is increased to
−1
A reference model (Fig. 7.26b,c) shows how a sym- 100 mm a (Fig. 7.27b) results in deformation that is
metric style of extensional deformation results when more strongly controlled by viscous flow at the base of

strain softening is absent. An early phase of deformation the frictional layer than that in the model involving slow
is controlled by two conjugate frictional-plastic shear velocities. However, at high velocities the strain soften-
zones (S1A/B) that are analogous to faults and two ing does not develop in part because of the high viscous
forced shear zones in the mantle (T1A/B). During a stresses that result from high strain rates. The model
subsequent phase of deformation, second generation shows no strong preference for strain localization on
shear zones develop and strain in the mantle occurs as one of the frictional fault zones. The deformation
focused pure shear necking beneath the rift axis. Figures remains symmetrical as the ductile mantle undergoes
7.26d and e show the results of another model where narrow pure shear necking. These results suggest that
frictional-plastic (brittle) strain softening occurs and the increasing or decreasing rift velocities can either
resulting deformation is asymmetric. An initial stage is promote or inhibit the formation of large asymmetric
very similar to the early stages of the reference model, structures because varying the rate changes the domi-
but at later times strain softening focuses deformation nant rheology of the deforming layers.
into one of the conjugate faults (S1B). The asymmetry These experiments illustrate the sensitivity of defor-
is caused by a positive feedback between increasing mation patterns to strain-induced weakening mecha-
strain and the strength reduction that results from a nisms during faulting and ductile flow. The results

decreased angle of internal friction (Section 2.10.2). suggest that extension is most likely to be asymmetric
Large displacements on the S2A and T1B shear zones cut in models that include frictional-plastic fault zone weak-
out a portion of the lower crust (LC) at point C (Fig. 7.26, ening mechanisms, a relatively strong lower crust, and
insert) and begin to exhume the lower plate. By 40 Ma, a slow rifting velocities. However, before attempting to