Folds con’t
Review session for final exam
Thursday afternoon?
Thursday evening? 7:30 -???
Friday morning?
Kinematics of folding- outline
Active folds:
Flexural folds
Neutral surface folds
Passive folds:
Shear folds
Interpreting folds:
Kinematics of folding
Inner workings of folds
and strain patterns:
Three main models:
-Flexural folding
-Neutral-surface
folding
-Shear folding
(passive)
Kinematics of folding
Two end-member ways to produce active folds:
Both produce parallel folds
Kinematics of folding- Flexural slip
Flexural folding occurs where there is
significant mechanical influence of layering
in the rock.
Flexural slip folding accommodates
folding by layer-parallel slip between
layers.
The layers actively participate in folding
by bending and flexing.
Kinematics of folding- Flexural slip
slip between layers.
No strain (shear) at fold hinge
Strain (shear) occurs on fold
limbs
Kinematics of folding- Flexural slip
Inner workings of folds and strain patterns:
Parallel fold
Are flexural slip folds Cylindrical?
(individual layers do not thin or thicken)
What class of fold are flexural slip folds?
What kind of rocks do you think make flexural slip folds?
Kinematics of folding- Flexural slip
If all of the strain is in the
limbs, what would strain
pattern look like?
Kink and Chevron
folds form via
flexural slip
Kinematics of folding- Neutral Surface folding
Neutral surface folds:
Strain within hinge zoneextension in outer arc
contraction in inner arc
neutral surface in between
Note: Neutral surface does not have to be
in the middle of the layer
Are Neutral surface folds Cylindrical?
What class of fold are neutral surface folds?
What kind of rocks do you think make neutral surface folds?
Kinematics of folding- Neutral Surface folding
Inner workings of folds and
strain patterns:
What type of structures
would you expect to see
on the outer arc?
On the inner arc?
Kinematics of folding- Neutral Surface folding
Outer-arc extension
&
Inner-arc compression
Kinematics of folding: Outer- arc extension
Possible structures w/in outer-arc
extension.
Boudinage: Pinch-n-swell structure
1) Folding of layers of different stiffness or
competency, the competent layers will responding
differently to the soft layers. Stiff layers resist
deflection.
2) Layer parallel stretching
Kinematics of folding: Outer- arc extension
Boudinaged stiff-layer that is subsequently rotated & extended.
Common stiff-soft pairs
•sandstone-shale
•limestone-shale
•chert-shale
•sandstone-limestone
•dolomite-limestone
•gneiss-schist
•calcsilicate-marble
More boudinage examples
Review session: Thursday, Dec. 1, 7:30 pm Lind 103
Final, Thursday Dec. 8, 10:00 am
Kinematics of folding- outline
Active folds: What do we mean by active folds?
-What are the two kinematic styles of active folds?
-Where is the strain (shear) accomodated in each style?
Passive folds:
Shear folds
Interpreting folds:
Kinematics of folding-Shear folding
(Passive folds)
Shear folding:
Layer has no mechanical significance=>
passive fold
Kinematics of folding-Shear folding
(Passive folds)
Common in rocks at high
temperatures
Can yield useful kinematic
information (sense of shear,
flow direction, etc)
Kinematics of folding-Shear folding
(Passive folds)
Passive shear folding is
favored when the mechanical
influence of layering in a rock is
very weak.
Little to no mechanical contrast
between the layers.
The layer is passively carried
along.
Kinematics of folding-Shear folding
(Passive folds)
Found in igneous and metamorphic rocks
Requires slip (or flow) along planes parallel to
axial plane
Like pushing your finger into a deck of cards
Produce similar folds
Kinematics of folding-Shear folding
(Passive folds)
Produce similar folds (thinning and
thickening of individual layers)
Combinations of fold kinematics
What is the evidence of
flexural folding?
What is the evidence of
neutral surface folding?
Combinations of fold kinematics
Extensional fractures
end at sliding surface on
fold limb
Fractures open
outward at fold hinge
Interpreting folds
Layer thickness and rheology:
both define fold wavelength
What color strata are weakest?
Light or Dark?
Interpreting folds
Parasitic fold:
Felsic dike within diorite. Which is
more “competent” (viscous)
How many deformation “events”
occurred?
Interpreting folds
Chevron fold:
Where did slip occur? (hinge or
limb?)
Interpreting folds
Flexural-slip folding is
enhanced by low cohesion
between bedding layers – e.g.,
well bedded strata such as
cherts & limestones.
Interpreting folds
If there is no fault on this mountain:
How was shortening accomodated
-in rocks on the left side of the
mountain?
- In rocks on the right side of the
mountain?
Interpreting folds
If there is no fault on this
mountain,
What can you say about
-the relative competence of the
cliff-forming layers on the left?
-the relative competence of the
thinly bedded units on the right