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) have 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 experimental geotherm of about 22 degrees C/MPa, simulating the natural geothermal gradient. The experimental data clearly demonstrate that the effect of mineral composition on the ultimate frictional strength is distinctly different from that on the residual frictional strength. Tharp's framework model (1983) quantitatively accounts for the ultimate frictional strength, whereas the residual frictional strength at large shear strains can be predicted by Jordan's two-block model (1988). Thus, the frictional behavior of a bimineralic shear zone changes from the 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 are extremely sheared at the zone of strain concentration. The strength profile and the slip mode, including the lower limit of seismic behavior, 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 not clarified in Scholz's fault model (1988), and his model disagrees with existing experimental data. (C) 1998 Elsevier Science B.V. All rights reserved.