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

Abstract

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.

Variation in slip on intersecting normal faults: Implications for paleostress inversion

“…The 15 largest faults, traceable along strike for 1 to 6 km, displace the Jurassic Navajo Sandstone, a well-sorted, highly porous aeolian sandstone, and the overlying Carmel the odd-axis model, the principal paleostrain orientations and ratios [Krantz, 1988[Krantz, , 1989]. Cowie and Shipton [1998] and Shipton [1999] analyzed the slip profile along one of the main faults and relate the linear slip gradients at the fault tips to a positive stress feedback between sequential slip increments.…”

Bimodal or quadrimodal? Statistical tests for the shape of fault patterns

“…The next three datasets have been digitised from published papers 160 on normal faults in Utah ( Figure 5b; Chimney Rock; Krantz, 1989), northern England ( Figure 161 5c; Flamborough; Peacock & Sanderson, 1992) and Italy ( Figure 5d; Central Italy; Roberts, 162 2007). In each case, the published stereograms were digitised to extract Cartesian (x,y) 163 coordinates of the poles to faults, and these were then converted to plunge and plunge direction 164 using the standard equations for the projection used (e.g.…”

“…290 The Chimney Rock dataset is probably not orthorhombic according to the two tests, and lies 319 close to the line for k=1 on Figure 7. It is interesting to note that the Chimney Rock data, and 320 other fault patterns from the San Rafael area of Utah, are considered as displaying 321 orthorhombic symmetry by Krantz (1989) and Reches (1978). …”

Stress Partitioning: a Practical Concept for Analysing Boundary Conditions of Brittle Deformation

“…Natural examples shown by Aydin and Reches [35], although described as essentially coeval, biconjugate fault patterns, show systematic crosscutting relationships between different fracture sets at each site. Bimodal striation patterns and the inferred 'alternating strain fields' at the Basin and Range [9] also work against a true coeval development.…”

“…After considering that this is a limitation of the stress-based approach, a number of authors have proposed alternative models that try to account for multiple-set fault patterns (able to accommodate 3-D finite strain) on the basis of kinematic concepts (e.g. [7,8,9,10]). Fault patterns predicted by these models are typically made of four sets showing orthorhombic symmetry.…”