1. A bend in rock strata or in any planar feature. The feature (e.g. bedding, cleavage, or layering) is deflected sideways and the amount and direction of dip is altered. Four principal regimes are responsible for folding: layer-parallel or lateral compression; differential vertical subsidence; differential shearing; and thrusting. In a simple anticline-syncline fold pair, an individual fold consists of a curved hinge zone and two planar limbs. An imaginary fold axis lies parallel to the hinge zone (line) and marks the intersection of the axial plane (or surface) with this zone. This basic geometric form gives rise to many fold profiles, including parallel, similar, concentric, open, and isoclinal fold types. To define the attitude of a fold accurately, the orientation of both the hinge line and the axial plane have to be measured. Varying orientations of the hinge line and axial plane may give rise to widely differing fold attitudes, thus vertical, upright and inclined (horizontal and plunging, see plunge), and reclined and recumbent forms may be described. See amplitude; wavelength.

2. In seismic reflection sampling, the number of offset distances which sample one common depth point (CDP). For example, if one CDP is sampled at 24 offset distances it is referred to as ‘24-fold’ coverage. The signals recorded for the CDP on each separate trace are then summed by stacking to improve the signal-to-noise ratio.

Fold