Geology Chapter on Folds and Faults

Folds

• Folds are bends or flexures in layered rock, formed due to forces like faulting, diapirism, compaction, ductile flow, or regional subsidence/uplift.
• They are expressed as one or more dipping features that correspond to inclined foliations (typically bedding).
• Folds can form under various pressure and temperature conditions. Some examples include: high temperature and pressure in meta-greywacke (New Zealand), moderately-low pressure and temperature in Raplee Ridge Monocline (near Mexican Hat, UT), intermediate pressure and temperature in West Castleton Syncline (Vermont).

Fold Terminology

• Fold Limb: A sloping side of a fold.
• Fold Axis/Hinge: The line of maximum curvature that separates the fold limbs.
• Fold limb (dip domain): An area where rocks all have a relatively consistent dip.
• Rocks can fold in cylindrical or non-cylindrical patterns
• Cylindrical folds maintain parallel hinge lines when drawn through the fold.
• Non-Cylindrical folds require rotation or distortion of a line to trace the fold form.

Elements of a Fold

• Hinge line: The line of maximum curvature on a fold (parallel to the fold axis in cylindrical folds).
• Axial surface: A plane connecting hinge lines for different layers within a fold, separating regions with different dips.

Fold Tightness

• Fold tightness is described using angles.
  • Unfolded: 180 degrees
  • Gentle: 170-180 degrees
  • Open: 90-170 degrees
  • Tight: 10-90 degrees
  • Isoclinal: 0-10 degrees

Fold Curvature

• Curved-hinge folds: Circular and elliptical variations.
• Angular-hinge folds: Chevron and box variations.

Examples of Folds

• Angular Chevron Folds: Folding under moderate-low pressure and temperature conditions in interbedded shales and limestones (Kaikoura, New Zealand).
• Tight Chevron Fold: Barnhardt Canyon, Mazatzal Wilderness, Arizona.
• Kink Bands: Parallel folding, parallel axial surfaces in well-bedded rocks (thin layers, high contrast) Southern Rincons Mountains, Arizona.
• Harmonic Folding: Canadian Rockies Front Ranges, Lake Minnewanka, Alberta, Asturias, Spain
• Disharmonic Folding: layered carbonates and evaporites in the core of the Barbastro Anticline, Aragon, Spain

Intersection of Axial Surfaces

• As axial surfaces converge, the formation of a new axial surface occurs (angle bisector, maintaining layer thickness for map-scale structures formed under low to moderate pressure and temperature).

Characterizing Folds

• Folds can be characterized by describing either their axial surface orientation or hinge line orientation. This is required to uniquely define a fold's geometry.

Fold Classification

• Folds can be categorized based on the dip of the hinge and the axial surface. This includes upright horizontal, inclined horizontal, recumbent, upright plunging, inclined plunging, recumbent inclined plunging.

Ramsay Classification of Folds

• Ramsay classifies folds based on dip isogons (lines connecting points with equal dip between top and bottom layer). Pattern of dip isogons helps classify the fold type. This includes Class 1 (inner curvature > outer curvature), Class 2 (inner curvature = outer curvature), and Class 3 (outer curvature > inner curvature).

Fold Mechanics

• Layer-active folding: Folding that involves buckling and bending of layers.
• Layer-passive folding: Folding that doesn't involve significant bending, primarily through passive amplification.
• Harmonic folding: Gradual changes in fold shape between layers.
• Disharmonic folding: Pronounced changes in fold shape between layers.

Parasitic Folds

• Minor folds that form on the flank of larger folds, indicating the overall shear sense in the fold limb. (Dextral "Z" and Sinistral "S" folds are examples).

Anticlines and Synclines

• Anticline: Concave downward folds with older rocks in the center.
• Syncline: Concave upward folds with younger rocks in the center.
• Plunging synclines/anticlines involve an incline in the folds.