Orientation-tensor eigenanalysis, Fisher statistics with a confidence cone, Kamb density contouring, and cylindrical best-fit — computed in the browser, offline, on any number of measurements. Everything below is checkable against Stereonet, OpenStereo or Dips. Publication SVG and a methods paragraph come out the other end.
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The intersection is the fold axis where two limbs meet, the hinge line of a bedding–cleavage pair, and the sliding direction of a rock wedge. One cross product, three uses.
Slickenlines are measured as a rake in the fault plane but plotted as a trend and plunge. This is the conversion, and it is easy to get wrong by hand on a steep plane.
Restore tilted bedding to horizontal and see what the other structures looked like before the folding. Rotate about the fold axis by the amount that flattens the bedding.
The rotated set replaces what is in the box above, so every statistic below recomputes. Rotation is a hypothesis — it assumes the folding was cylindrical and rigid, which is a real assumption, not a formality.
Give a fault plane and the rake of its slickenlines, and say which way the hanging wall went. The shortening and extension axes follow. Rake +90 is pure down-dip slip; −90 is up-dip.
P and T are kinematic axes, not stress axes. They describe the movement on this one fault. Calling them σ₁ and σ₃ assumes the fault slipped at 45° to the principal stresses, which is rarely exactly true and is the commonest overreach in fault analysis.
A picture for the paper, the numbers for the co-author.