I found an interesting link:
http://info.lu.farmingdale.edu/depts...formation.html
It leads me to think that the deformation is a slip "since this requires the least amount of energy".
Here's a 3 page PDF:
http://alspubs.lbl.gov/AbstractManag...oads/01114.pdf
on grain direction under stress, and the slip effect.
I haven't been able to find any references (online) to a measurement of the necessary forces, but expert Googling should do it.