Susanne
For oblique basement fabrics to be activated as they
commonly are in rifts there must be some cohesive
strength to the crust during the initiation of the
rift for the strength anisotropy to be apparent. Once
the faults are established and the crust has been
weakened, the intial strength anisotropy becomes less
important. Instead there is competition between
whether the strength of the non-ideally oriented
faults is sufficiently low that they continue to move,
or whether they becomes replaced by more 'ideally'
oriented faults.
Another common pattern in rifts is for the numerous
early faults to shut off and extension become
concentrated on a few larger faults. This presumably
allows annealing and increase of cohesive strength
within the crust containing the abandoned faults.
It is common to see fluid flow up faults, hence
cementation, annnealing and increased cohesion across
inactive faults is very likely. Igneous intrusions up
fault planes can be common in some areas.
That just scratches the surface, but I think the way
the cohesive strength of fault zones evolves during
rifting is quite complex. I don't think you can just
use one number to model a rift from beginning to end.
Inverting an old rift fault, - inversion tends to be
very selective - only a few (presumably weakest, best
oriented) faults in a population tend to be inverted,
suggesting the existance of cohesive strength to the
fault population. Often inverted normal faults show a
lot of brecciation and cataclasis around the fault
zone, and commonly low-angle fault splays indicating
slip was not easy.
The cohesion that is used for inversion would have to
vary because inversion can occur during a phase of
rifting when the faults would be very weak, or
millions of years after rifting when the faults might
be significantly annealed.
Chris Morley
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