Kinematics of folding

taupeselectionMechanics

Nov 14, 2013 (3 years and 9 months ago)

94 views

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

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

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:




Are flexural slip folds Cylindrical?

What class of fold are flexural slip folds?

What kind of rocks do you think make flexural slip folds?

Parallel fold

(individual layers do not thin or thicken)

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 zone
-


extension in outer arc


contraction in inner arc


neutral surface in between



Are Neutral surface folds Cylindrical?

What class of fold are neutral surface folds?

What kind of rocks do you think make neutral surface folds?

Note: Neutral surface does not have to be
in the middle of the layer

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?

Outer
-
arc
extension

&

Inner
-
arc
compression

Kinematics of folding
-

Neutral Surface folding

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

Kinematics of folding: Outer
-

arc extension

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





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)


Kinematics of folding
-
Shear folding

(
Passive folds)

Combinations of fold kinematics

What is the evidence of
flexural folding?

What is the evidence of
neutral surface folding?

Extensional fractures
end at sliding
surface on
fold limb

Fractures open
outward at fold hinge

Combinations of fold kinematics

Layer thickness and
rheology
:
both define fold wavelength

What color strata are weakest?
Light or Dark?

Interpreting folds

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