Geomechanics & fault zones
Understanding the geologic structure, slip history, and stress-related permeability evolution of fault zones is of great interest for evaluating seismic hazards, studying regional tectonic histories, and producing oil, gas, and geothermal resources. I recently conducted a detailed study of a rare exposure of a fault zone in the footwall of the Alpine Fault system in the South Island of New Zealand with Virginia Toy and Klaus Gessner, to understand its permeability evolution and slip history. We found that the slip vector and associated mineralized microfaults have kinematics compatible with slip in the current stress field. Despite this, the principal slip surface is poorly oriented for slip in the present stress state, and it appears that small, well-oriented, mesoscale faults have subsequently developed that cross the main fault zone. This fault zone thus represents rare evidence of plastic strain consistent with the present stress field occurring in the Pacific Plate footwall of the Alpine Fault in this area.
Fractures expected to slip (red) and not slip (black) during hydraulic fracture stimulation of a horizontal well in the Barnett Shale, Fort Worth Basin, Texas. Fractures are from an image log. Stress conditions are from Lund Snee and Zoback (2016), except that a hypothetical strongly reverse faulting stress state is considered. Unlike in case of normal and/or strike-slip faulting, bedding planes (horizontal fractures, colored green) may be possible to make slip in this stress state. This figure is from Zoback and Lund Snee (2018).
Fractures expected to slip (red) and not slip (black) during hydraulic fracture stimulation of a horizontal well in the Barnett Shale, Fort Worth Basin, Texas. Fractures are from an image log. Stress conditions are from Lund Snee and Zoback (2016). Note that horizontal fractures (bedding planes, colored green) are impossible to make slip in this stress state, except in cases of extremely high ambient pore pressure, which would cause the Mohr circle to shrink. This figure is from Zoback and Lund Snee (2018).
Looking east from near Okarito, New Zealand, across the Alpine Fault, to the Southern Alps. Mount Cook is the highest peak, near the right side.
Annotated images extracted from a 3D whole-outcrop photo stitch depicting the principal slip zone, fault core, and damage zone of the Smithy Creek fault, South Island, New Zealand (from Lund Snee et al., 2014).