Kattenhorn, S.A. and Pollard, D.D. (1998)
Effect of the Remote Horizontal Stress Ratio on Joint Orientations Around Normal Faults
As faults slip and grow, they perturb the surrounding stress and pore pressure fields. These perturbations may be great enough to cause rock to fracture in opening mode, forming joints. Dilatant joints produce a fracture permeability that may be the dominant control on fluid flow. Thus knowledge of joint orientations and distribution can play a major role in understanding fluid flow in oil reservoirs and aquifers. The prediction of joint orientations around faults based upon fault configurations and mechanical boundary conditions is useful for identifying the anisotropic nature of permeability.
We use 3D boundary element models of normal faults imbedded in an elastic body to calculate the elastostatic solution to the stress tensor at any arbitrary point around a slipping normal fault. These stress fields are used to predict the orientations of joints forming in association with slipping faults. Numerical results indicate that joint orientations vary around the tipline of an elliptical fault, ranging from fault-parallel (upper and lower tips) to fault-perpendicular (lateral tips).
The ratio of fault-parallel to fault-perpendicular remote stress (remote horizontal stress ratio) determines the distance away from the fault affected by the fault-perturbed stress field and thus the extent of joint growth away from the fault. If a remote tectonic tension acts perpendicular to fault strike in numerical models and the other horizontal principal stress is lithostatic, then joints are predicted to form parallel to fault strike. However, as the fault-parallel remote horizontal principal stress becomes less compressive than lithostatic, joints form at successively higher angles to fault strike, particularly in fault relay zones. This stress condition may result from fault-parallel tectonic tensile stresses or temporal variations in principal stress directions following stress relaxation associated with fault slip.
Analogous field examples occur in sandstones at Arches National Park, Utah. Joints emanate away from normal faults at high angles to fault strike. Field relationships indicate that the joints are genetically related to faulting. Some joints are filled with a clayey vein material and suggest that high fluid pressures in fault-perturbed stress fields may contribute to joint development.
In summary, as the remote horizontal principal stresses change values, so too does the obliquity between fault and joint strikes. Accordingly, the horizontal component of fluid flow rotates from fault-parallel toward fault-perpendicular. This implies a corresponding decreased lateral connectivity of flow domains along fault strike and increased connectivity across interfault blocks.
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