Ch. 11 Folds

Front 1

anticline

Back 1

fold that is convex in the direction of the youngest beds

Front 2

syncline

Back 2

Fold that is convex in the direction of the oldest beds

Front 3

synformal anticline

Back 3

upside down anticlinewith the oldest layers in the core

Front 4

antiformal syncline

Back 4

upside down synclinewith the youngest layers in the core

Front 5

monocline

Back 5

half fold with blind thrust fault

Front 6

anatomy of folds

Back 6

Front 7

inflection point

Back 7

point of opposing convexity

Front 8

median surface

Back 8

imaginary surface connecting inflection lines

Front 9

hinge point

Back 9

point of maximum curvature, all points on a folded surface form a line

Front 10

axial surface

Back 10

imaginary surface formed by connecting hinge lines of all folded surfaces and bisects the fold

Front 11

interlimb angle

Back 11

measure of angle between limbs of fold

Front 12

symmetrical fold

Back 12

median surface and axial surface are perpendicular

Front 13

asymmetrical fold

Back 13

median and axial surfaces intersect at an oblique angle

Front 14

overturned fold

Back 14

one limb is facing down

Front 15

fold tightness

Back 15

Gental: 180-170

Open: 170-90

Tight: 90-10

Isoclinal: 10-0

Front 16

orientation of axial fold

Back 16

Know this graph!!!!

Front 17

similar fold

Back 17

rocks folded at higher temp where ductile possible

bed thinning in limbs, thickness in hinges

Front 18

concentric folds

Back 18

layer thickness does not change (lower T)

Front 19

cylindirical fold

Back 19

along-strike shape that approximatesa portion of a cylinder. a well-defined,linear fold axis

Front 20

non-cylindrical

Back 20

too complex to have asingle well-defined axial surface

Front 21

interference folds

Back 21

multiple generations of folds

Front 22

parasitic fold

Back 22

same orientation and geometry as big folds

asymmetry of folds on limbs are mirror images

symmetrical in center

Front 23

dip isogons

Back 23

connect equal dip on vertically oreinted folds

Class 1: Isogons converge toward inner arc

Class 2: Isogons parallel to axial trace (similar)

Class 3: Isogons diverge toward inner arc

Front 24

flexural slip folding

Back 24

Folding is accommodated by slip along layer/bedding interfaces. The magnitude of slip increase from zero at the hinge to a maximum at the inflection point in the limbs

Front 25

Flexural Flow Folding

Back 25

Folding is accommodated by the plastic/ductile migration of material out of the high shear zones of the limbs into the low shear hinges.

Front 26

active folding or buckling

Back 26

fold profiles are based entirely on physical-mechanical properties of the layers

Bucking occurs when a competentlayer in a less competent matrix is shortened parallel to the lengthof the layer. Buckling implies that there is a layer parallelshortening and a viscosity contrast involved, and alsoirregularities on which folds can nucleate.

Front 27

passive folding

Back 27

Generated by simple shearing are perfectly similar folds (class 2)

Front 28

forced folds or bending

Back 28

they just go along for the ride,and some of the beds happen to fined themselves in awkward places and are required to stretch or bend"

2-prominent types of forced folds:fault-bend folds and fault-propagation folds

Front 29

Passive folding

Back 29

Passive folding produces harmonic folds where thelayering plays no mechanical role and therefore noinfluence on the fold shape.

Front 30

mechanisms for folding

Back 30

1. Flectural Folding: folding phone book

-beds above move relative to beds below

-Thin, low temp

2. Fectural Flow Folding:

-similar folds

-ductile shear

-material migrate to low shear zones (hinges)

3. Active Folding/Buckling:

-end loading & geometry is mechanical physical properties

-competent layer in less competent matrix is shortened parallel to length of layer

4. Passive Folding:

-folds occur to simple shear

-velocity gradient

Ex. glacier moving

5. Forced Folds:

-Fault Bend Folds

-Fault Propagation Folds

- form as a result of motion upon faults. Beds are not free to fold, and they may or may not have significant layer-parallel stresses. Rather, they are somewhat passively going along for the ride - most often associated with faulting - all types.

Front 31

fault bend folds

Back 31

-ramp through comepetent units

- thrust sheet migrates over plain

- under fault doesn't move

- slip is conserved.

- Fold amplitude is determined by the thickness above the lower hanging wall flat.- Once the fold has achieved maximum amplitude, it only grows in width.

!MOVIE!

Front 32

Fault propogation folds

Back 32

- blind fault propogates through surface

- HW drives triangular zone over, pushes fold over

- triangular zone is where deformation happens

-FW is slightly deformed

- after fault cuts through, stops deforming

-fault slip is not conserved. Fault displacement is taken up by folding. Fault tip is marked by a ductile ‘process zone’. Ideally, FPF’s evolve into FBF’s

Front 33

thrust tip

Back 33

edge of fault plane where displacement become zero