THE STRENGTH PROFILE FOR BIMINERALIC SHEAR ZONES - AN INSIGHT FROM HIGH-TEMPERATURE SHEARING EXPERIMENTS ON CALCITE-HALITE MIXTURES

Shearing experiments on mixed halite-calcite layers (0.7 mm thick) hav e been performed to understand the behavior of bimineralic fault zones , using a high-temperature biaxial testing machine at a slip rate of 0 .3 mu m/s and shear strains to about 30. Temperature was increased to 700 degrees C in linear proportion to the normal stress, with the expe rimental geotherm of about 22 degrees C/MPa, simulating the natural ge othermal gradient. The experimental data clearly demonstrate that the effect of mineral composition on the ultimate frictional strength is d istinctly different from that on the residual frictional strength. Tha rp's framework model (1983) quantitatively accounts for the ultimate f rictional strength, whereas the residual frictional strength at large shear strains can be predicted by Jordan's two-block model (1988). Thu s, the frictional behavior of a bimineralic shear zone changes from th e framework model to the two-block model with increasing displacement. Inclusion of halite, the weaker member, in quantities as small as 5% by volume, reduces the friction almost to the level of pure halite and suppresses stick-slip at large shear strains, because halite grains a re extremely sheared at the zone of strain concentration. The strength profile and the slip mode, including the lower limit of seismic behav ior, of bimineralic shear zones at large shear strains are controlled primarily by the weaker member. The present results disprove Strehlau' s fault model (1986). The role of individual constituent mineral is no t clarified in Scholz's fault model (1988), and his model disagrees wi th existing experimental data. (C) 1998 Elsevier Science B.V. All righ ts reserved.