F. Aumento and H. Loubat, The mid-Atlantic ridge near 45øN: Serpentinized ultramafic intrusions, Can. J. Earth Sci, vol.8, pp.631-663, 1971.

E. Bonatti, Serpentinite protrusions in the oceanic crust, Earth Planet. Sci. Lett, vol.32, pp.107-113, 1976.

E. Bonatti and P. J. Michael, Mantle peridotites from continental riffs to ocean basins to subduction zones, Earth Planet. Sci. Lett, vol.91, pp.297-311, 1989.

W. F. Brace, Some new measurements of linear compressibility of rocks, J. Geophys. Res, vol.70, pp.5669-5678, 1965.

W. F. Brace, Volume changes during fracture and frictional sliding: A review, Pure Appl. Geophys, vol.116, pp.603-614, 1978.

W. F. Brace and D. L. Kohlstedt, Limits on lithospheric stress imposed by laboratory experiments, J. Geophys. Res, vol.85, pp.6248-6252, 1980.

W. F. Brace, B. W. Paulcling, and C. Scholz, Dilatancy in the fracture of crystalline rocks, J. Geophys. Res, vol.71, pp.3939-3953, 1966.

K. H. Brodie and E. H. Rutter, On the relationship between deformation and metamorphism, with special reference to the behavior of basic rocks, Kinematics, Textures and Deformation, vol.4, pp.138-179, 1985.

J. Byedee, M. Cannat, D. Bideau, and H. Bougault, Serpentinized peridotites and gabbros in the Mid-Atlantic Ridge axial valley at 15ø37'N and 16ø522q, Earth Planet. Sci. Lett, vol.116, pp.87-106, 1978.

M. Cannat, Thin crust, ultramafic exposures, and rugged faulting patterns at the Mid-Atlantic Ridge (22ø-24øN), Geology, vol.23, pp.49-52, 1995.

L. J. Caruso and J. V. Chernosky, The stability of lizardite, Can. Mineral, vol.17, pp.757-769, 1979.

W. P. Chen and P. Molnar, Focal depths of intracontinental and intraplate earthquakes and their implications for the thermal str?cture and mechanical properties of the lithosphere, J. Geophys. Res, vol.88, pp.4183-4214, 1983.

R. G. Coleman, Petrological and geophysical nature of serpentinites, Bull. Geol. Soc. Am, vol.82, pp.879-918, 1971.

A. J. Coulton, G. D. Harper, and D. S. O'hanley, Oceanic versus emplacement age serpentinization in the Josephine ophiolite: Implications for the nature of the Moho at intermediate and slow spreading ridges, J. Geophys. Res, vol.100, pp.245-267, 1995.

W. A. Deer, R. A. Howie, and J. Zussman, An Introduction to the RockForming Minerals, vol.528, 1966.

C. A. Dengo and J. M. Logan, Implications of the mechanical and frictional behavior of serpentinite to seismogenic faulting, J. Geophys. Res, vol.86, pp.771-781, 1981.

H. J. Dick, Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism, Magrnatism in the Ocean Basins, vol.42, pp.71-105, 1989.

M. A. Dungan, C. Bastite, . Mineral, B. Evans, J. T. Fredrich et al., The brittle-ductile transition in rocks: Recent experimental and theoretical progress, The brittleductile, vol.17, pp.1-20, 1979.

J. T. Fredrich, B. Evans, T. , and -. F. Wong, Micromechanics of the brittle to plastic transition in Carrara marble, J. Geophys. Res, vol.94, pp.4129-4145, 1989.

K. M. Gillis, G. Thompson, and D. S. Kelley, A view of the lower crustal component of hydrothermal systems at the mid-Atlantic ridge, J. Geophys. Res, vol.98, pp.597-616, 1993.

D. J. Harding, Josephine peridotite tectonites: A record of upper mantle plastic flow, Comell Univ, 1987.

H. H. Hess, History of ocean basins, Petrological Studies: a Volume in Honor ofA. F. Buddington, pp.599-620, 1962.

G. Hirth and J. Tullis, The effects of pressure and porosity on the micromechanics of the brittle-ductile transition in quartzite, Geophys. Res, vol.94, pp.825-842, 1989.

G. Hirth and J. Tullis, The brittle-plastic transition in experimentally deformed quartz, 1. Geophys. Res, vol.99, pp.11731-11747, 1994.

H. Strating, E. H. , and R. L. Vissers, Structures in natural serpentinite gouges, J. Struct. Geol, vol.16, pp.1205-1215, 1994.

D. R. Janecky and W. E. , Hydrothermal serpentinization of peridotite within the oceanic crust: Experimental investigations of mineralogy and major element chemistry, Geochirn. Cosrnochirn. Acta, vol.50, pp.1357-1378, 1986.

J. A. Karson, Along-axis variations in seafloor spreading in the MARK Area, Nature, vol.328, pp.681-685, 1987.

D. L. Kohlstedt, B. Evans, and S. J. Mackwell, Strength of the lithosphere: Constraints imposed by laboratory experiments, J. Geophys. Res, vol.100, pp.587-604, 1995.

J. Lin and E. M. Parmentier, Mechanisms of lithospheric extension at mid-ocean ridges, Geophys. J, vol.96, pp.1-22, 1989.

J. Lin, G. M. Purdy, H. Schouten, J. C. Sempere, and C. Zervas, Evidence from gravity data for focused magmatic accretion along the Mid-Atlantic Ridge, Nature, vol.344, pp.627-632, 1990.

D. A. Lockner and J. D. Byedee, Dilatancy in hydraulically isolated faults and the suppression of instability, Geophys. Res. Lett, vol.21, pp.2353-2356, 1994.

D. A. Lockner, J. D. Byerlee, V. Kuksenko, A. Ponomarev, and A. Sidorin, Observations of quasistatic fault growth from acoustic emissions, Fault Mechanics and Transport Properties of Rocks, pp.3-31, 1992.

A. J. Maltman, Serpentinite textures in Anglesey, Geol. Soc. Am. Bull, vol.89, pp.972-980, 1978.

C. M6vel and M. Cannat, Lithospheric stretching and hydrothermal processes in oceanic gabbros from slow spreading ridges, Ophiolite Genesis and Evolution of the Oceanic Lithosphere, pp.293-312, 1992.

J. B. Moody, An experimental study on the serpentinization of ironbearing olivines, Can. Mineral, vol.14, pp.462-478, 1976.

D. Moore, D. A. Lockner, M. Shengli, R. Summers, and J. D. Byedee, Strength of serpentinite gouges to 200øC, Fall Meet. Suppl, vol.76, issue.46, 1995.

S. A. Murrell and I. A. Ismail, The effect of decomposition of hydrous minerals on the mechanical properties of rocks at high pressures and temperatures, Tectonophysics, vol.31, pp.207-258, 1976.

G. A. Neumann and D. W. Forsyth, The paradox of the axial profile: Isostatic compensation along the axis of the Mid-Atlantic Ridge?, J. Geophys. Res, vol.98, pp.891-908, 1993.

G. T. Norrell, A. Teixell, and G. D. Harper, Microstructure of serpentine mylonites from the Josephine ophiolite and serpentinization in retrogressive shear zones, Geol. Soc. Am. Bull, vol.101, pp.673-682, 1989.

D. S. O'hanley, Serpentinites: Records of Tectonic and Petrological History, Oxford Monogr. Geol. Geophys, issue.34, 1996.

D. S. O'hanley, J. V. Chernosky, and F. J. Wicks, The stability of lizardite and chrysotile, Can. Mineral, vol.27, pp.483-493, 1989.

M. Paterson, Experimental Rock Deformation: The Brittle Field, vol.254, 1978.

C. B. Raleigh and M. B. Paterson, Experimental deformation of serpentinite and its tectonic implications, J. Geophys. Res, vol.70, pp.3965-3985, 1965.

L. A. Reinen and T. E. Tullis, Microstructural evidence of strain localization and distributed strain in serpentinite friction experiments, Fall Meet. Suppl, vol.76, issue.46, 1995.

L. A. Reinen, J. D. Weeks, and T. E. Tullis, The frictional behavior of lizardite and antigorite serpentinites: Experiments, constitutive models, and implications for natural faults, Pure Appl. Geophys, vol.143, pp.318-358, 1994.

O. Sano, I. Ito, and M. Terada, Influence of strain rate on dilatancy and strength of Oshima granite under uniaxial compression, J. Geophys. Res, vol.86, pp.9299-9311, 1981.

P. Segall, J. R. Rice, W. T. Shea, and A. K. Kronenberg, Dilatancy, compaction, and slip instability of a fluid-infiltrated fault, J. Geophys. Res, vol.100, pp.155-177, 1992.

M. Shimada and A. Cho, Two types of brittle fracture of silicate rocks under confining pressures and their implications in the Earth's crust, Tectonophysics, vol.175, pp.221-235, 1990.

N. Sleep and M. L. Blanpied, Creep, compaction and the weak theology of major faults, Nature, vol.359, pp.687-692, 1992.

M. Tolstoy, A. J. Harding, and J. A. Orcutt, Crustal thickness on the Mid-Atlantic ridge: Bull's eye gravity anomalies and focused accretion, Science, vol.262, pp.726-729, 1993.

B. E. Tucholke and J. Lin, A geological model for the structure of ridge segments in slow spreading ocean crust, J. Geophys. Res, vol.99, pp.7-11, 1994.

J. Tullis and R. A. Yund, Experimental deformation of dry Westerly granite, J. Geophys. Res, vol.82, pp.5705-5718, 1977.

J. Tullis and R. Yund, The brittle-ductile transition in feldspar aggregates: An experimental study, Fault Mechanics and Transport Properties of Rock, pp.89-117, 1992.

P. Ulmer and V. Trommsdorff, Serpentine stability to mantle depths and subduction-related magmatism, Science, vol.268, pp.858-861, 1995.

R. H. Vernon, Relationship between microstructures and metamorphic assemblages, vol.39, pp.439-452, 1977.

P. Wessel and W. H. Smith, Free software helps map and display data, Eos Trans. AGU, vol.72, issue.41, pp.445-446, 1991.

F. J. Wicks and D. S. O'hanley, Serpentine minerals: Structures and petrology, Hydrous Phyllosilicates (exclusive of micas, vol.19, pp.91-167, 1988.

F. J. Wicks and E. J. Whittaker, Serpentine textures and serpentinization, Can. Mineral, vol.15, pp.459-488, 1977.

F. J. Wicks, E. J. Whittaker, and J. Zussman, An idealized model for serpentine textures after olivine, Can. Mineral, vol.15, pp.446-458, 1977.

T. Wong, Micromechanics of faulting in Westerly granite, Int. J. Rock Mech. Min. Sci, vol.19, pp.49-64, 1982.

T. Wong, Mechanical compaction and the brittle-ductile transition in porous sandstones, Deformation Mechanisms, Rheology and Tectonics, vol.54, pp.111-122, 1990.

J. Zhang, T. Wong, and D. M. Davis, Micromechanics of pressureinduced grain crushing in porous rocks, J. Geophys. Res, vol.95, pp.345-352, 1990.

J. Escartin, . 3le, and . England,

, Rm 54-718, Cambridge, MA 02139. (email: brievans?mit.edu) G. Hirth, Department of Geology and Geophysics, 1996.