Deformational Mode Fields in Experimentally Deformed Rock
Results from an experimental study of the deformational behavior of an oolitic limestone and a marble indicate that the mode of deformation changes systematically as a function of both confining pressure and total strain. With increasing pressure, the mode changes from extension fracture to brittle faulting to ductile faulting to uniform flow; with increasing strain, the change is from uniform flow to faulting with or without loss of cohesion, depending upon the confining pressure. The results reported here are for 79 tests run on the limestone and for 32 tests on the marble at confining pressures from 1 to 2000 bars, room temperature, dry, and a strain rate of 3.9 × 10 −5 per second. The systematic variation of deformational mode in experimentally deformed rock as a function of rock and environmental factors allows fields to be defined for the individual modes. In this study, the deformational mode fields thus defined for the limestone are: uniform flow, at low strains and low confining pressure to strains up to 15 percent at 2000 bars pressure; extension fracture, below 50 bars confining pressure; brittle faulting, initiated at generally low strains between 50 and 650 bars confining pressure; and ductile faulting, initiated at pressures above 650 bars and at total strains from a few percent at the lower pressures to 15 percent at 2000 bars. In the marble, the fields of ductile faulting and uniform flow have generally similar relationships as those in the limestone, except that they have been shifted laterally with respect to the pressure axis of the field diagram. The boundary between brittle faulting and ductile faulting occurs at 50 bars, rather than at 650 bars as in the limestone. Because of its significantly higher ductility, the marble has no field of extension fracture, and the field of brittle faulting has almost been eliminated for the test conditions cited. The DMF diagram appears to be a potentially valuable tool in the interpretation of natural deformational environments by relating deformational modes to environmental conditions and rock factors.