NONDILATANT BRITTLE DEFORMATION OF SERPENTINITES - IMPLICATIONS FOR MOHR-COULOMB THEORY AND THE STRENGTH OF FAULTS

We conducted deformation experiments to investigate the strength, defo rmation processes, and nature of the brittle-ductile transition of liz ardite and antigorite serpentinites. A transition from localized to di stributed deformation occurs as confining pressure increases from simi lar to 200 to similar to 400 MPa at room temperature. Deformation in b oth brittle (localized) and ductile (distributed) regimes is accommoda ted by shear microcracks, which form preferentially parallel to the (0 01) cleavage. Axial microcracks (mode I) are infrequently observed. Vo lumetric strain measurements demonstrate that brittle deformation is m ostly nondilatant, consistent with the shear-dominated microcracking. Three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow: (1) a lack of evidence for crystal p lastic deformation, (2) a positive pressure dependence of the maximum differential stress, and (3) abundant evidence for brittle microcracki ng. The weakness of serpentinites relative to other brittle rocks is e xplained by a low fracture strength along the (001) cleavage, combined with the low pressure dependence of strength. The transition from bri ttle to ductile deformation occurs at the crossover between the streng th of intact serpentinite and the friction law unique to each type of serpentinite, rather than the more general Byerlee's law. If brittle d eformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity, serpentinites define an end-m ember style of nondilatant brittle deformation. This deformation style may result in extremely weak faults in nature, and it may also strong ly influence the tectonic evolution of the oceanic lithosphere where s erpentinite is present.