HR: 1340h
AN: T23B-0548    [Abstracts]
TI: Three-Dimensional Brittle Shear Fracturing by Tensile Crack Interaction
AU: * Healy, D
EM: dhealy@liv.ac.uk
AF: Rock Deformation Research Group, Department of Earth & Ocean Sciences, University of Liverpool, Liverpool, L69 3GP United Kingdom
AU: Jones, R R
EM: r.r.jones@durham.ac.uk
AF: e-Science Research Institute, University of Durham, Durham, DH1 3LE United Kingdom
AU: Holdsworth, R E
EM: r.e.holdsworth@durham.ac.uk
AF: Reactivation Research Group, Department of Earth Sciences, University of Durham, Durham, DH1 3LE United Kingdom
AB: Faults in granular materials such as rock are composite brittle shear fractures formed through the interaction and coalescence of many tensile microcracks. The geometry of these microcracks and their surrounding elastic stress fields exert a fundamental control on the orientation of the final shear fracture planes. The Coulomb-Mohr brittle failure criterion predicts the development of conjugate bimodal shear planes inclined at an acute angle to the maximum compressive principal stress and parallel to the intermediate principal stress. However, Coulomb-Mohr theory is incapable of explaining more complex three-dimensional fracture geometries that are widely observed in rocks, such as polymodal faulting, in which multiple sets of brittle shear fractures are oriented oblique to the intermediate principal stress direction. Here we show that the distribution of elastic stress around tensile microcracks in three-dimensions promotes interaction and coalescence to form brittle shear planes oriented oblique to the remote principal stresses, and can therefore account for polymodal fault patterns. Previous models of tensile microcrack interaction have employed some form of simplifying two-dimensional approximation. Our fully three-dimensional model is based on the solution of Eshelby. The commonly observed spread in orientations of apparently conjugate bimodal faults in published field data has previously been ascribed to noise caused by some combination of measurement error, material anisotropy (e.g. due to layering), or heterogeneous remote stress fields. However, these variations in the orientation of brittle shear fractures may be of primary importance, and at least partially reflect the oblique nucleation of many brittle shear fractures under triaxial stress conditions in the lithosphere. An improved understanding of the controls on the orientations of brittle shear planes in three-dimensions has important implications for earthquake seismology, rock-mass stability and hazard assessment, as well as fluid migration and the efficient recovery of mineral resources from fractured host rocks.
DE: 8110 Continental tectonics: general (0905)
DE: 8164 Stresses: crust and lithosphere
DE: 8168 Stresses: general
DE: 8199 General or miscellaneous
SC: Tectonophysics [T]
MN: Fall Meeting 2005