The transition from high temperature creep to fracture in Maryland diabase

The transition from high‐temperature creep to brittle fracture in Maryland diabase was investigated as a function of confining pressure and strain rate. Experiments were conducted at 1000°C. Confining pressure was varied to 450 MPa and strain rates from 2×10 −3 s −1 to 4×10 −6 s −1 At fixed strain rate, the rock strength first increased with pressure, reached a maximum, and then decreased with increasing pressure. Finally, with high pressures, the strength reached an asymptotic value which was the steady state creep strength at that temperature and strain rate. The positive pressure sensitive domain corresponded to brittle behavior, the negative pressure sensitivity domain to a transitional behavior, and the pressure insensitive domain to dislocation creep. The boundary between the last two domains occurred where the strength of the rock was about equal to the confining pressure. Similar variations in strength have been reported in the literature for carbonates and silicates, although not associated with a transitional behavior. Unfaulted specimens deformed in the transitional field showed microcracks and plastically deformed minerals. The boundaries of the transitional domain were extrapolated to geological conditions corresponding to the oceanic lithosphere. The transition depth near ridges is in close agreement with the base of the seismogenic layer (Tapponnier and Francheteau, 1978). However, the old oceanic lithosphere is probably totally brittle over its whole thickness.