Folds and Their Classification Questions and Answers

Answer: A

The inner surface has the greater curvature: curvature of the inner surface > curvature of the outer surface.

Dip isogons join points of equal dip on the two surfaces of a folded layer. In Ramsay's classification, isogons converging towards the core define Class 1 folds, in which the outer arc is more gently curved than the inner arc. In Class 1B, the parallel fold, the layer keeps a constant thickness, so the inner arc has the smaller radius and therefore the larger curvature; Classes 1A and 1C show the same inequality.

Equal curvature on both surfaces describes Class 2, the similar fold, whose isogons are parallel. A greater curvature on the outer surface describes Class 3, whose isogons diverge towards the core. The median surface is not what the question compares.

Answer: B

Large layer-parallel shortening results in buckling.

When a competent layer embedded in weaker material is compressed parallel to its length, it shortens a little and then becomes unstable and deflects into folds with a characteristic wavelength. This buckling (buckle folding) is the main mechanism for folds formed by shortening along the layering.

Convolute folds are small, contorted folds formed in soft sediment by loading or dewatering, not by tectonic shortening. Fracturing is brittle failure rather than folding, and flow folding happens in very ductile rock where layers have little competence contrast, so the layers do not buckle.

Answer: D

A fold with an interlimb angle of zero is Isoclinal.

Folds are classified by the angle between their limbs (Fleuty's scheme): gentle 180-120 degrees, open 120-70, close 70-30, tight 30-0 (exclusive), and isoclinal at 0 degrees, when the two limbs are parallel.

Gentle, open and tight folds all have limbs at an angle to each other, so they cannot have an interlimb angle of zero; a tight fold approaches but does not reach it.