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The titles of many geology articles contain words like "nappes and thrust sheets". Basically these are rather similar and in certain languages can be denoted with one term. The dictionaries tend to mix them giving the same translation for both. Is there any (subtle) difference or can they be considered synonims?

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Nappes and thrust sheets are very similar terms.

I agree with definition found on wikipedia:
(except for the 2 km requirement of movement, I would replace 2 km with significant distance. I am sure you still have a Nappe with only 1.5 km of movement is measured.)

In geology, a nappe or thrust sheet is a large sheetlike body of rock that has been moved more than 2 km (1.2 mi) or 5 km (3.1 mi) above a thrust fault from its original position. Nappes form in compressional tectonic settings like continental collision zones or on the overriding plate in active subduction zones. Nappes form when a mass of rock is forced (or "thrust") over another rock mass, typically on a low angle fault plane. The resulting structure may include large-scale recumbent folds, shearing along the fault plane, imbricate thrust stacks, fensters and klippe.

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"Nappe" is the French word for "tablecloth"; think of what happens when you try to push a tablecloth any distance across a table. The resulting folded (and inverted folding) structure is very different to that created if you were to push a thick layer of dirt across the table.

The term was likely used to describe this folding over low angle thrusts as they were observed in the Alps, and then corrupted over time after being used to describe a range of low angle thrust faulted structures.

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