Under high pressure rocks deform according to
Mohr Coulomb plasticity and so the deformations generally localize
into shear bands (if the angle of friction is different
from the angle of dilatancy which is usually the case).
The figure shows a 2d system under horizontal compression
and vertical extension
simulated with Alex Poliakov.
The colors show the local shear rate (red = high rate).
For more details see our
paper published in Geophys. Res. Lett. 21, 2143-2146 (1994).
The network is fractal and its dimension agrees well with
that of that of
shear bands on granite which Alex and myself
photographed in the Pyrenees as has been shown in our
in Fractals 2 , 567-581 (1994).
One of the big questions about shear bands is to understand
their finite width.
With Bertrand Francois and Frederic Lacombe we proposed a model
that can give an estimate on the width in units of grain diameters
based on the combinatorics of stress paths. We also did a simple
two dimensional experiment to verify the predicted probability
functions. All this is published in a
published in Phys. Rev. E, Vol. 65, 031311 (2002)