Creep and Strain Hardening of Lizardite: Implications for Transient Creep in the Lithosphere

Serpentinites are formed at convergent plate boundaries through hydrothermal metamorphism. Hence, understanding the mechanical properties of serpentine minerals near the brittle-plastic transition can reveal information about fault zone earthquake properties, as well as improve post-seismic deformation models. Lizardite is the most prevalent polytype of serpentine in the oceanic lithosphere. Heretofore, deformation experiments performed on lizardite have mostly focused on studying mechanical behavior in pressure regimes that do not exceed 500 MPa. We investigated sample response in a stiff high pressure apparatus (Griggs-Type) at 1 GPa confining pressures to isolate ductile/plastic deformation mechanisms. We were able to pair creep and constant rate deformation data from experiments on intact ODP lizardite cores. Strengths agree with prior results of Escartin and Hirth (1997). In addition, we observed strain hardening below 10% strain for both creep and constant strain rate (10-5-10-7 1/s) tests. The hardening rates fall between those observed during control experiments on copper and previous results on antigorite. The observed hardening could play an important geological role because serpentinites are often found in very seismically active areas (e.g. subduction zones, oceanic transform faults, San Andreas fault). The transient, decreasing, strain rate at constant stress shows surprising resemblance to geodetic records after earthquakes and could fill gaps in our understanding of afterslip processes in the earthquake cycle.