Fold (geology)

Lore & Background

Folds are described using specific terminology. The fold hinge is the line joining points of maximum curvature on a folded surface, which may be straight or curved. A fold surface seen perpendicular to its shortening direction can be divided into hinge and limb portions; the limbs converge at the hinge zone. The axial surface connects all hinge lines of stacked folded surfaces; if planar, it is called an axial plane. A fold axis is the closest approximation to a straight line that, when moved parallel to itself, generates the form of the fold; folds generated this way are called cylindrical folds.

Reader's Guide

They are also described by symmetry—symmetrical folds have limbs of relatively equal length, asymmetrical folds have highly unequal limbs. Deformation style classes include concentric folds (uniform layer thickness) and similar folds (thinning of limbs, thickening of hinge zone). Types of folds include linear forms such as anticlines and synclines, and nonlinear forms such as domes and basins. Folds arise from layer-parallel shortening, fault-related folding (fault-bend, fault propagation, detachment), folding in shear zones, and synsedimentary processes like slumping.

Did You Know?

• Folds in rocks vary in size from microscopic crinkles to mountain-sized folds.
• The interlimb angle defines fold tightness, with isoclinal folds having an angle between 10° and zero.
• Concentric folds maintain uniform layer thickness, while similar folds display thinning of limbs and thickening of the hinge zone.
• Synsedimentary folds are those formed during sedimentary deposition.