Orthorhombic fault patterns: The odd axis model and slip vector orientations

Conjugate fault theory cannot explain all observed patterns of faults and slip vectors. New fault models suggest that orthorhombic fault patterns represent the general, three‐dimensional strain case and relegate conjugate faulting to the special case of plane strain. The odd‐axis model relates orthorhombic fault geometry to principal strain orientations and ratios. The model also predicts the orientations of slip vectors for orthorhombic faults, expressed as the rake of the slip vector on the fault plane. Assuming that one principal strain is vertical, the model predicts steeply or shallowly raking slip for faults developed under strain fields with relatively small magnitudes ofintermediate strain. Oblique‐raking slip is possible only where the relative magnitude of the intermediate strain is large. The odd‐axis model has been tested on five sets of data from Utah, Nevada, Israel and Crete. Predicted slip vector orientations lie close to the observed slip in all cases. The model also suggests that where orthorhombic faults are reactivated in a three‐dimensional strain field, slip vectors may match those of the prior episode of faulting or be distinctly different in orientation, producing faults with bimodal slip patterns, as seen in parts of the Basin and Range province. This last observation suggests that three‐dimensional strains may have played a significant role in Basin and Range development.