EFFECT OF PREEXISTING SHEAR ZONE ORIENTATION ON ROCK STRENGTH

Pre-existing faults and shear zones can affect the strength of orogens during subsequent deformations. Mohr-Coulomb theory and experimental results indicate that brittle faults can be reactivated within a range of orientations relative to the compression direction that is dependent on the friction coefficient, but new brittle faults will develop if preexisting faults are outside of this range of “easy” slip orientations in rocks deforming by brittle mechanisms. Experimental evidence shows that pre-existing crystal plastic shear zones also weaken feldspathic aggregates deforming by crystal plastic mechanisms, but nobody has determined the range of preexisting shear zones that will cause weakening in rocks deforming by crystal plastic mechanisms. We are performing an experimental study to determine the range of orientations of pre-existing shear zones that can cause strain localization by deforming pre-cut cores of a foliated gneiss with fine-grained mixed-phase powders added in these cuts to simulate a preexisting shear zone. The foliation of the rock is in a weak orientation, 45 degrees to the compression direction to maximize shear stress on the foliation. The pre-cut shear zones are in orientations of 45-75 degrees, which covers a range of orientations with decreasing shear stress on the pre-cut shear zones. We expect that as the shear stresses decrease at angles greater than 55 degrees to the compression direction, strain will not localize in the pre-cut shear zones.