A 28-degree fault that cuts a fold: thrust or reactivated normal fault?

The fault is a reverse fault, probably oblique.

• The fault dips north-west and places older sandstone over younger shale. Because older rocks are on the hanging wall, the hanging wall has moved up relative to the footwall. That is reverse displacement.
• The stratigraphic throw of about 120 m is consistent with a moderate reverse fault. The fault dip of 28 degrees is low for a normal fault and typical of a thrust or reverse fault.
• The footwall shale is dragged into a syncline. Drag folds form as the rock adjacent to the fault is sheared; the asymmetry is consistent with the hanging wall moving up and over the footwall.
• The hanging-wall sandstone is cut off sharply, which means the fault propagated through the sandstone after the fold formed. It does not show a gradual flexure, so the fault is a discrete brittle structure.

Caveat: the slickenfibre rake is not given, so a strike-slip component cannot be ruled out. The fault should be described as reverse with possible oblique slip until the rake is measured.

Answer: hanging wall moved up and toward the south-east, footwall moved down; the structure is a low-angle reverse fault.

The present evidence favours a thin-skinned thrust, but inversion cannot be excluded without subsurface data.

• For a thrust: the fault dips 28 degrees, cuts both limbs of the fold, and places older over younger rocks. It contains calcite slickenfibres and clay gouge but no mylonite, which fits a shallow brittle thrust in a sedimentary sequence.
• Against simple inversion: an inverted normal fault would originally have placed younger rocks over older rocks. Positive inversion would reverse that, but it usually leaves a thick fault zone, local collapse breccias or a sharp change in hanging-wall facies. None is described.
• Timing: the fault cuts the fold, so faulting post-dated folding. That is compatible with a thrust breaking through a previously folded sequence, or with later reactivation.

Evidence that would settle it: a trench or drill hole across the fault to see whether the hanging wall contains a thick syn-rift sequence, and whether the fault zone contains earlier normal-slip fabrics overprinted by reverse slip. A kinematic analysis of slickenfibres and fault-rock microstructures would also help.

Judgement: map it as a shallow thrust, but flag it as possibly reactivated. Do not use the word inversion unless syn-rift strata or overprinted fabrics are found.

The fault should be treated as potentially active until proven otherwise.

• The 3 m scarp follows the same strike, lies in alluvium and has a fresh free face. That is strong circumstantial evidence for Holocene surface rupture on the same fault or a splinter of it. A fresh free face is unlikely to survive for long in this setting without erosion.
• The scarp is not proof by itself. It could be a fault-line scarp exhumed by erosion, a lateral spread, or a scarp related to a different structure. The absence of a trench is the critical gap.
• The road cut shows a shallow brittle fault with gouge and calcite, which is capable of slip at shallow depth. The 120 m throw indicates a significant structure, not a minor shear.

Recommendation: dig a trench across the scarp and date the colluvium above and below the faulted horizon with OSL. If the scarp cuts Holocene alluvium, classify the fault as active and design the road and any nearby structure to avoid the fault zone. A geophysical profile should trace the fault beneath the alluvium.

Hazard rating: high, pending trench data. The fault should be zoned as an avoidance zone rather than treated as a stable geological contact.