Where does nappe collide?

In structural geology, a nappe is a large body of rock pushed far from its origin along a nearly flat thrust fault or as the overturned limb of a recumbent fold, so that older strata come to lie on top of younger ones across tens of kilometres. Dennis and colleagues' 1981 What is a thrust? What is a nappe? defines a thrust nappe as an allochthonous sheet that has moved along a thrust fault. The name is the French word for a tablecloth or sheet, the same word used in hydraulics for a sheet of falling water; here the referent is a displaced slab of crust, measured by its transport distance and basal thrust geometry.[1]

Unit

The displacement and extent of a rock sheet moved along a basal thrust

Recovery means

Not applicable

Measured by

Reconstructed by mapping thrust contacts, measuring displacement, and restoring balanced cross-sections

In hydraulics, the nappe is the coherent sheet of water that springs free and falls as flow passes over the crest of a weir or dam; its shape, trajectory, and whether its underside is aerated govern the weir's discharge and whether the sheet vibrates or clings. Crookston and Tullis's 2013 study of nappe aeration, instability, and vibration on labyrinth weirs analyses this behaviour. The referent, borrowing the same French word for a sheet, is a free-falling film of water characterised by its ventilation and discharge coefficient, unrelated to a displaced slab of rock.[2]

Unit

The geometry and aeration state of the overflowing water sheet

Recovery means

Not applicable

Measured by

Assessed from the discharge coefficient, the nappe trajectory, and the degree of aeration beneath the sheet