1. Folds II Geometry and classification
Lecture 23
Gly 326

2. Fold geometry Axial plane/hinge line geometry
Not all hinge lines are vertical  folds may be asymmetric about the axis!

3. YOUNG
OLD
YOUNG
OLD

4. Fold geometry Folds can described by hinge geometry
Plunging fold – Has a hinge line that is tilted (plunging) into the subsurface
Non-plunging fold – Has a horizontal hinge
Measure the plunge of the hinge line as a lineation!

5. Fold geometry Dome - Fold with appearance of an overturned bowl
Erode to expose old rocks in center; younger rocks outside - Result from crustal upwarping. The Black Hills of South Dakota are large oval domes
Four way dip closure: Center of dome is a great trap for fluids!
Basin - Fold shaped like a bowl
Erode to expose young rocks in center; older outside - Result from crustal subsidence. Most of Michigan is a large basin
Domes and basins can result from vertical crustal motions and/or from doubly plunging structures!

6. Richat structure (“Eye of the Sahara”): A dome!
From Wiki: The Richat Structure, also known as the Eye of the Sahara and Guelb er Richat, is a prominent circular feature in the Sahara desert near Ouadane, west–central Mauritania. This structure is a deeply eroded, slightly elliptical, 40 km in diameter dome. The sedimentary rock exposed in this dome ranges in age from Late Proterozoic within the center of the dome to Ordovician sandstone around its edges. The sedimentary rocks comprising this structure dip outward at 10°–20°. Differential erosion of resistant layers of quartzite has created high-relief circular cuestas. Its center consists of a siliceous breccia covering an area that is at least 3 km in diameter.[1][2][3]

7. Michigan basin: A basin
Michigan basin: A basin! Note - does not mean that is topographically a “bowl”

8. Fold geometry Monoclines
Simple bends or flexures in otherwise horizontal or uniformly dipping rock layers. (think carpet draped over a stair step)
Generated by blind faults in the basement rock
Recall: Blind faults! They do not cut through to the surface!
Fault displacement folds overlying sedimentary cover (remember fault propagation folds??)

9. Fold classification
For a fold in cross section (profile plane): fold classification by interlimb angle (Tightness)
Recall: Interlimb angle encloses hinge and connects inflection points
Works when we have the profile plane (cross section perpendicular to hinge plane/fold axis

10. Fold classification Classify this fold based on tightness
1) Pick a folded layer to use 2) ID the hinge and inflection points
3) Draw your lines 4) Measure the angle

11. Fold classification
Classification based on hinge line plunge vs. axial surface orientation
Axial surface dip (Recall: Axial surface connects hinge lines for multiple layers!)
90-80°: Upright
80-10°: XXX inclined
≤10°: Recumbent
Fleuty method, 1964

12. Fold classification
Classification based on hinge line plunge vs. axial surface orientation
Hinge line plunge (Recall: Hinge line can be curved or straight, and also inclined relative to the ground)
0-10°: Horizontal
10-80°: XXX plunging
≤80°: Vertical
Fleuty method, 1964

13. Fold classification N 25° Axial surface dip: 75° E
Classify this fold by the Fleuty (1964) method! N
25°
Axial surface dip: 75° E
Is it an anti or syn- form? 2) Is the fold plunging? 3) What angle is the hinge line plunging at? 4) What is the dip of the Axial surface?

14. Gently plunging, Steeply inclined Antiform! Fleuty method, 1964
Strata dip away from fold axis
Plunging 25° at 180 °
Axial surface dip: 75° E
Gently plunging,
Steeply inclined
Antiform!
Fleuty method, 1964

15. Fold classification Ramsay’s (1967) classification
Dip isogons: lines connecting points of identical dip for vertically oriented folds (Upright or steeply inclined)
3 Classes, illustrated at the right:

16. Fold classification Class 1: isogons converge towards inner arc
Class 2: isogons parallel axial trace (similar folds) inner arc = outer arc (variable thickness)
Class 3: isogons diverge towards the inner arc

17. Fold classification Class 1A: hinge thickness < limb
We’ll just consider “similar” and “parallel”
Class 1A: hinge thickness < limb
Class 1B: uniform thickness (parallel folds)
Class 1C: intermediate between Class 1B and Class 2

18. Fold classification Parallel or similar? Which is which?
(Uniform bed thickness or variable?) A B
PARALLEL
SIMILAR

19. Fold classification and symmetry
Folds can be symmetric or asymmetric in cross-section
Symmetric: when looking at a cross-section perpendicular to the axial surface (profile plane!), the two sides are mirror images of each other
Sometimes called M-folds
Asymmetric folds are sometimes called S- or Z-folds
Z-folds have short limbs that appear to have been rotated clockwise (S-folds: counter-clockwise)
Nothing to do with curved vs. angular shape!

20. Fold symmetry
Fold systems consisting of folds with a consistent asymmetry are said to have a vergence
Given by (a) the sense of displacement of the upper limb relative to the lower limb, or (b) the rotation of the inclined short limb  clockwise rotation implies a right-directed vergence

21. Fold symmetry
Large folds tend to have smaller folds occurring in their limbs and hinge zones (first order folds
These smaller folds (second-order) are also called parasitic folds
2nd order fold asymmetry or vergence indicates their position on the large-scale structure!

22. Even though layers retain their original thickness, they nonetheless experience shortening and extension.
outer & inner arc & neutral surface

23. Inner-arc compression
Outer-arc extension
&
Inner-arc compression