Frictional faulting in polycrystalline halite: Correlation of microstructure, mechanisms of slip, and constitutive behavior Chapter uri icon

abstract

  • The relation between friction constitutive behavior and the mechanisms of shearing in a monomineralic, polycrystalline material that deforms by combined cataclasis and crystal-plasticity has been investigated by shearing thin layers of halite between blocks of quartz sandstone at room temperature (22C), constant normal stresses between 20 and 70 MPa, and shear-displacement rates between 300 and 0.030 ym s1 in a triaxial rock deformation apparatus. Constant and stepping displacement rate modes of testing were utilized, and some tests were preceded by a confining pressure (Pc) reduction load path involving the reduction of Pc after establishing a differential axial load. Pc-reduction causes the shear stress to increase as the normal stress approaches zero, and promotes compaction and the formation of a slip surface in the halite at small shear displacements relative to that for constant normal stress and constant Pc load paths. Discrete changes in the steady state microstructure and constitutive behavior of halite with normal stress and displacement rate define distinct mechanism-fields of frictional faulting. Cataclastic mechanisms dominate at normal stresses less than 40 MPa and at all displacement rates tested, and the coefficient of friction varies from 0.6 to 1.0 as a function of displacement rate. Relatively large magnitude rate-weakening is observed at high displacement rates and is associated with stick slip sliding on a discrete surface. Because steady state shear involves localized slip at these conditions, the use of Pc-reduction prior to shearing reduces the magnitude of displacement necessary to achieve steady state. Small magnitude rate-dependence at low displacement rates is associated with distributed cataclastic flow and stable shearing behavior. Both the comminution rates and stability transition at low normal stresses suggest that the microstructural state is similarly affected by an increase in normal stress and a decrease in displacement rate. At normal stresses greater than 40 MPa and at all displacement rates tested, halite undergoes nearly homogeneous simple shear by dislocation mechanisms. However, the frictional behavior displayed at these conditions implies that stable microfracturing or some other pressure-sensitive process is operative, and that flow is semi-brittle.

author list (cited authors)

  • Chester, F. M., & Logan, J. M.

Book Title

  • The BrittleDuctile Transition in Rocks

publication date

  • January 1, 1990 11:11 AM