Field trip tomorrow in lab
• Bring field books, pencils, ruler, hand lens,
acid, ect.
• Dress appropriately
• Might rain. Bring rain gear.
• Leave at 1PM from same spot
Fold Mechanics
Goal: To understand some basic mechanisms
that operate during fold formation
Lw  2t 3

6 0
Layer thickness, viscosity, and
fold shape
• Thicker layer = greater arc length
• Greater viscosity contrast = greater arc
length
Finite element
model
Data from
rocks
High or low viscosity ratio?
High or low viscosity ratio?
Active vs. Passive folding
•
Passive folding: Layers play no
mechanical role in developing folds.
•
Active folding: Layer thickness and
viscosity determine fold shape
•
Active folding involves buckling of stiff
layers due to applied stress
Some folding mechanisms
• Flexural slip folding
• Neutral surface folding
• Shear folding
Flexural slip folding: Layers slip past one
another during folding. Active folding.
• Generally forms parallel folds or chevron
folds
• Still requires deformation in the hinge
zones of individual layers
Neutral surface folding: Folded layer
contains only one plane with no internal
deformation, the neutral surface. Active
folding.
Shear folding: Rocks are sheared unevenly
like a deck of cards.
Make a shear fold then find a
way to prove that it is a similar
fold
Structures in
neutral
surface fold