Fold Types in Geology

Study Notes

Fold Types in Geology

Definition: Folds are wave-like structures in rocks caused by bending, not breaking, during deformation. They may appear as mountains or valleys on the surface, identified by parallel rock layers dipping in opposite directions.
Ductile Behavior: Rocks must be ductile (able to deform without breaking) to form folds. This ductile behavior is controlled by pressure and temperature conditions, causing rocks to bend gradually under compressional stress. Brittle materials break instead.
Compressional Stress Causes: Factors contributing to compressional stress leading to folding include changes in rock type (horizons/bedding surfaces), faults, intrusive igneous rock, and the intrusion of a stronger layer into a softer one.

Linear Fold Types

General: Common fold types include anticlines, synclines, antiforms, synforms, and monoclines, all considered linear folds with a central horizontal axis.
Anticline: Arch-shaped fold with the oldest rocks at its core. Rock layers dip away from the center axis (hinge). Steepest dip near the hinge, decreasing outward. Often formed by thrust faults (reverse faults with a low angle).
Syncline: Trough-shaped fold with the youngest rocks at its core. Rock layers dip towards the center axis. Often form valleys. Primarily formed during mountain-building events.
Antiform/Synform: More general categories than anticline/syncline. Antiform = any upward convex fold; synform = any downward convex fold. The oldest/youngest rock isn't always near the center. Overturned folds can become antiforms or synforms.
Monocline: Step-like fold in rock layers, with a zone of steeper dips, followed by gentler dips or horizontals. Often the result of localized deformation or faulting.
Asymmetrical/Symmetrical Folds: Defined by hinge angle. Asymmetrical have an inclined hinge, different dip angles. Symmetrical have a vertical hinge, identical dip angles.
Isoclinal Folds: Symmetrical folds with identical angles on each limb. Often occur in sets, and limbs are parallel, aligned.
Overturned/Recumbent Folds: Overturned folds rotate > 180 degrees. Recumbent folds rotate partially, with a nearly horizontal hinge. Both caused by continued deformation after initial folding.
Domes/Basins: Semi-spherical to semi-ellipsoidal features formed by intersecting folds. Domes have oldest rocks in the center, youngest at the outer rim. Basins have youngest in the center, oldest at the outer rim.
Chevron Folds: Defined by straight limbs and sharp hinges, caused by compressive stress. Often found in alternating rock layers of varying deformability.
Ptygmatic/Disharmonic Folds: Ptygmatic are irregular, chaotic folds, often caused by sediment slumping. Disharmonic have significant differences in shape, symmetry, and wavelength between folds.
Parasitic Folds: Secondary, smaller folds within larger folds. Typically asymmetric due to the larger fold's geometry.
Plunging Folds: Folds with an inclined hinge line, created by compressional stress from multiple directions, The incline is referred to as the nose of the fold.

Classification of Folds

Tightness: Describing the angle between the fold limbs (open, tight, isoclinal).
Axial Plane: Describing the relative orientation of the axial plane and limbs (upright, overturned, recumbent).
Bed Thickness: Describes the thickness of rock layers (thick and thinly bedded). Thickly bedded tend to form concentric folds.

Description

This quiz explores the different types of folds in geology, including definitions and examples of linear folds such as anticlines and synclines. Understand the conditions under which folds are formed and the significance of ductile behavior in rocks during deformation.