Geology 3120 Folding Mechanisms Outline Decollement Buckle Folds

Geology 3120 Folding Mechanisms

Outline • Decollement Buckle Folds • Passive (a. k. a. Free) Folds • Kink Folds • Shear Folds • Flexural Slip • Sheath Folds

Decollement Buckle Fold Zagros salt folds

Passive-Free-Buckle Folds m 2 m 1 If m 1 > m 2

Passive, or Free Folds

Mullions

Mullions

Multi-Layer Folds

Layer thickness and rheology define folddefine wavelength thickness and rheology both fold wavelength What color strata are weakest? Light or Dark?

Multi-Layer Folds

Parasitic Folds

Parasitic Folds

Shear Folds

Shear Folds

Kink Folds (photograph Taiwan kink-bands)

Kink Folds (common in shales, slates)

Flexural Slip

Vergence

Flexural Flow

Flexural Flow

Flexural Flow (aka Karl’s favorite rock!) Fractures open outward Extensional fractures end at sliding surface Offset fracture shows vergence

Tangential Longitudinal Strain

Tangential Longitudinal Strain

Tangential Longitudinal Strain

Sheath Folds n High Shear Strain Folds (i. e. shear zones) n Sheath, or Tongue Shaped n Stretching Lineations parallel to fold axis’

Sheath Folds n High Shear Strain Folds (i. e. shear zones) n Sheath, or Tongue Shaped n Stretching Lineations parallel to fold axis’ Sheath Fold in Calcite Marble

Examples

Examples

Examples

In class stunts Use Fig Newtons as an analgue to illustrate extension on outer arc of fold; Wear two old shirts, wrap with rubber band cut off sheath fold