The result predicts initial failure in a uniform material under the assumed principal-stress orientations. The actual Figure 3 is a present-day NW–SE cross section through a thick, heterogeneous Alpine stack, extending to about 50 km depth. It contains near-horizontal basal stretches, curved ramps, tightly bent contacts, and steep structures in the internal zones around Viso and the Dora Maira complex. It does not show a single undeformed conjugate pair newly formed in horizontal uniform rock.
Several processes explain the departures:
- Mechanical layering and inherited weaknesses. Faults can follow weak bedding, old fault planes, or contacts with lower friction or cohesion than the surrounding rock. These can form shallow décollements linked by steeper ramps. Figure 2 explicitly distinguishes the near-surface red “geophysical bedrock” boundary at a few hundred metres from the much deeper crystalline basement beneath the Jurassic, Triassic and Permian cover. An age label or a seismic contrast alone does not establish a friction coefficient; clay-rich or evaporite detachments are possibilities to test, not lithologies proved by that drawing.
- Rotation after formation. Continuing tectonic shortening, folding, stacking and fault slip can rotate an initially optimal fault plane to steeper or shallower present-day fault dips. A steep older structure may be inherited from an earlier deformation episode rather than created at its present orientation.
- Curved faults and local stresses. Faults bend through a layered crust; local principal stress axes rotate near topography, ramps and interacting faults. The optimum angle is measured relative to the local , not necessarily to the regional horizontal direction everywhere.
- Changing strength and deformation mechanism. Spatial changes in pore fluid pressure, coefficient of friction, cohesion and temperature alter which surfaces reactivate. At depth, ductile deformation can replace frictional brittle failure, so the same new-fracture construction need not apply throughout the section.
Finally, a section shows apparent dip unless it is perpendicular to fault strike. For an ideal plane, , where is the angle between the section direction and fault strike. Obliquity can make a plane look shallower, but cannot on its own explain an apparent dip steeper than its true dip. Do not infer that each black curve is a currently active brittle earthquake fault.
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