Anallochthon, or an allochthonous block, is a large block of rock which has been moved from its original site of formation, usually bylow angle thrust faulting.^[1] The term is derived from Greek:allo, meaning other, andchthon, meaning earth. For other possible mechanisms seeobduction.

An allochthon which is isolated from the rock that pushed it into position is called aklippe. If an allochthon has a "hole" in it so that one can view theautochthon beneath the allochthon, the hole is called a "window" (or fenster). In generalized terms, the term is applied to any geologic units that originated at a distance from their present location.^[2]

In the United States there are three notable allochthons, all of which were displaced nearly 50 km (31 miles) along thrust faults. TheGolconda and Robert Mountains allochthons are both found inNevada, a product of the Antler orogeny in the late Devonian period. Another is the Taconic allochthons found inNew York,Massachusetts andVermont formed from the collision of the Taconic magmatic arc with the super-continentLaurentia in the late Cambrian period.^[3]

• Accretion (geology)
• Continental collision
• Orogeny