Anabola

Anabola is a geological term used to describe a type of symmetrical, uniaxial fold found in sedimentary and metamorphic rock layers. The fold is characterized by a concave-up shape that resembles an inverted U, with a relatively planar axis that runs parallel to the direction of maximum compressive stress. The term originates from the Greek root “ana” meaning “upward” and the suffix “‑bola” meaning “to throw or bend,” reflecting the upward bending of the strata.

The defining feature of an anabola is its symmetry about the fold axis, which allows geologists to infer the direction and magnitude of the forces that created it. In a typical anabola, the limbs of the fold are nearly parallel along the same plane and the hinge line may be nearly straight or gently curved. Because the axis is often oriented normal to the main thrust or shear direction, anabolae are sometimes considered structural indicators of later tectonic reactivation or post‑folding uplift.

Anabolae are most commonly associated with large-scale structural deformations such as the development of mountain belts, where regional folding and thrusting occur. Examples include the Appalachian Mountains of eastern North America, the Eastern Alps of Europe, and the Zagros fold and thrust belt of Iran. In some settings, anabolae may form part of more complex fold-and-thrust assemblages, appearing as small “voids” or hinge misalignments within broader, asymmetric folds.

Geologists use anabolae as diagnostic features in structural mapping and in reconstructing the tectonic history of a region. Their presence can indicate periods of active deformation, post‑tectonic erosion that has exposed deep structural elements, or differential compaction of sedimentary layers. By comparing field measurements of anabola geometry and orientation, researchers can estimate basin thickness, sediment load, and the timing of compressional events.