nappe

Nappe is a geological term for a large, sheet-like mass of rock that has been displaced from its region of origin by thrust faulting during crustal shortening, typically in mountain belts. The word derives from the French nappe, meaning a cloth sheet.

Formation and structure: Nappes form when rocks deform along a low-angle detachment, or decollement, within the crust. Under continued compression, slices of crust are thrust over underlying units, creating long, coherent sheets that can extend for hundreds of kilometers and reach significant thickness. The transported rocks may include basement and cover rocks from different crustal depths, leading to imbricated sequences and recumbent folding.

Classification and characteristics: Rocks within a nappe retain a recognizable internal stratigraphy but are displaced from their original positions. When a nappe has moved far from its source region, it is described as allochthonous; if it remains in situ, it is autochthonous. In many orogens, several nappes are stacked in a complex sequence, separated by multiple detachments and faults, producing characteristic overprinting structures such as large-scale folds and thrust belts.

Examples and significance: The Alps provide a classic example, with multiple nappes recognized in the Penninic, Helvetic, and Austroalpine zones. Similar nappe systems occur in the Himalayas, the Pyrenees, and other ancient and active mountain belts. Nappes are central to understanding crustal shortening, metamorphism, and the tectonic evolution of orogenic regions, illustrating how deep rocks can be transported onto higher structural levels during plate collision.