Showing papers by "David L. Kohlstedt published in 1980"

Limits on lithospheric stress imposed by laboratory experiments

Abstract: Laboratory measurements of rock strength provide limiting values of lithospheric stress, provided that one effective principal stress is known. Fracture strengths are too variable to be useful; however, rocks at shallow depth are probably fractured so that frictional strength may apply. A single linear friction law, termed Byerlee's law, holds for all materials except clays, to pressures of more than 1 GPa, to temperatures of 500°C, and over a wide range of strain rates. Byerlee's law, converted to maximum or minimum stress, is a good upper or lower bound to observed in situ stresses to 5 km, for pore pressure hydrostatic or subhydrostatic. Byerlee's law combined with the quartz or olivine flow law provides a maximum stress profile to about 25 or 50 km, respectively. For a temperature gradient of 15°K/km, stress will be close to zero at the surface and at 25 km (quartz) or 50 km (olivine) and reaches a maximum of 600 MPa (quartz) or 1100 MPa (olivine) for hydrostatic pore pressure. Some new permeability studies of crystalline rocks suggest that pore pressure will be low in the absence of a thick argillaceous cover.

Deformation‐induced microstructures, paleopiezometers, and differential stresses in deeply eroded fault zones

Abstract: Deformation-induced dislocation densities, subgrain sizes, and grain sizes in rocks from deeply eroded fault zones provide estimates of the differential stresses that existed when the faults were active. Laboratory experiments have demonstrated that the dislocation density, subgrain size, and grain size generated during steady state deformation depend primarily on the magnitude of the applied differential stress; temperature has only a minor influence on these three quantities through materials parameters such as the shear modulus. Theoretical arguments support these experimental results. Differential stresses in fault zones, determined from the empirical stress-microstructure relations, lie in the range 40–200 MPa. In many fault zones, such as the Moine thrust zone in Scotland, the progressive development of the deformation-induced microstructure can be observed, particularly in quartzites. Far from the fault surface, quartz grains are nearly equant but show undulatory extinction. As the central portion of the fault is approached, extinction remains undulatory as the grains become increasingly elongated (aspect ratios of 100∶1 are common) and recrystallized. Quartzites are often 100% recrystallized near the centers of fault zones. This progressive development of the microstructure indicates that recovery at low or zero stress has not erased information on the last tectonic event in the plastic deformation regime.

Faulted dipoles in germanium A high-resolution transmission electron microscopy study

Abstract: Faulted dipoles ranging in height from 3 to 12 nm in high-purity Ge have been studied by lattice-fringe and weak-bean transmission electron microscopy techniques. Only intrinsic, Z-shape faulted dipoles were observed. Lattice-fringe images of large faulted dipoles, weak-beam images of faulted dipoles, and weak-beam images of faulted dipoles, and weak-beam images of near-edge dislocations yield stacking-fault energies of 78±14 mJ m−2, respectively. When determined from the height and width of faulted dipoles viewed end-on in lattice-fringe images, the value for the stacking-fault energy calculated from anisotropic elasticity theory increases from 78 to 160 mJ m−2 as the height decreases from 6 to 3nm.

Electron Diffraction and Microscopy Studies of the Structure of Grain Boundaries in Al2O3

Abstract: Electron diffraction and weak-beam imaging techniques were used to examine the structure and thickness of grain boundaries in polycrystalline Al2O3. Extra diffraction spots from boundaries inclined to the foil surface were detected and related to the periodic structure of the boundary. Relrods from edge-on boundaries were detected and used to estimate the thickness of the boundary region, i.e. the depth that the displacement field of the boundary penetrates into the two crystals adjacent to the interface.