allochthon

Allochthon, from Greek allos meaning "other" and chthon meaning "earth," refers to a body of rock or sediment that did not originate at its present location but has been displaced there by tectonic or other geological processes.

In geology, allochthonous units are rocks that have been moved from their original place and thrust onto autochthonous rocks, which formed and remained in place. Allochthonous rocks are often found in nappes or thrust sheets formed during mountain-building events. The distinction between allochthonous and autochthonous units is central to understanding orogenic belts, and it can be extended to subunits or blocks within a larger tectonic stack. Examples are commonly cited in the context of the Alps and other major mountain belts, where stacked allochthonous sheets overlie autochthonous basement rocks. Obducted oceanic lithosphere and other transferred blocks are also described as allochthonous in some contexts.

In addition to geology, the term has ecological and sedimentological uses. Allochthonous input refers to materials and nutrients that originate outside a given ecosystem and enter it, such as leaf litter or woody debris carried into a stream. By contrast, autochthonous production arises within the ecosystem itself, such as algae or aquatic plants generating biomass.

Overall, allochthonous describes something that was derived from elsewhere and relocated, whereas autochthonous denotes origin in the current location. The concept helps explain the assembly of complex geological terrains and the flow of external energy and material in ecological systems.