fold anatomy

Folding buckles strata away from the linear, planar, and horizontal.

Folds may be symmetrical or asymmetrical. In a symmetrical fold (image at left), the axial plane is vertical and the limbs (sides) dip symmetrically from the axis. In an asymmetrical fold, the axial plane is tilted from the vertical with one limb dipping more steeply than the other.

▪ the axial plane of a fold is an imaginary plane surface that divides a fold as symmetrically as possible.

▪ the fold axis is a line drawn along the points of maximum curvature of a layer of a fold (parallel to the hinge in anticlines and synclines).

▪ the plunge of a fold is the angle between the horizontal plane and the fold axis (when this is not not horizontal).

▪ the plunge direction is the geographic quadrant towards which the plunge is directed

By convention, plunge direction is expressed as a three-digit number, and the plunge by a two-digit number. Thus, a line plunging 35º toward the azimuth 35º will be noted: 45º->035

Monoclinal folding drapes strata as though over a ledge, whereas hinge folding wraps the limbs of a fold as though hinged around the fold axis.

Because the eroded surface of a plain could fail to indicate the sub-surface folding relationships of ridges of resistant strata, folding is most easily elucidated by examining structures eroded through folds (side view of diagram at left).





However, the apparent shape of a fold may be distorted by the angle of erosional exposure relative to the fold axis (image at left).


(Try this with a stick of celery.)




├ .. On geological maps, the stike and dip of bedding, rather than axes and folds, are indicated.

▪ faulting ▪ fault attributes – net-slip, dip-slip, dip vector, strike-slip, vertical-throw, horizontal-throw, horizontal vector, heave, rake