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

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

Shear Folds

Kink Folds (photograph Taiwan kink-bands)

Kink Folds (common in shales, slates)

Flexural Slip

Vergence

Flexural Flow

$Flexural Flow (aka Karl’s favorite rock!) Fractures open outward Extensional fractures end at sliding$

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

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

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