1. Room: 407 prey@es.usyd.edu.au Tel: 12067 Patrice Rey

2. Lecture 4: Folds Aim: To characterize the structures that accommodate the deformation of rocks. Along with faulting, folding accommodates deformation of the earth crust. Folds involves the wavelike contortions of rock layering. In contrast to faults (brittle deformation), folding corresponds to more continuous deformation (plastic deformation) as it does not involve fracturation. As any other rock deforming structures, folds exist in a wide range of scale from microscopic crinkles to domes and basins hundreds of kilometers wide. Folds may overturn large package of sedimentary strata, they may form traps for hydrocarbone and mineral resources, and they help field geologist to unravel the tectonics history of mountain belts.

3. Under the action of stresses, geological layers bend and fold.

4. Folded Jurassic limestones (South France). 0.5m

5. Folded quartz vein in a phyllite. 1mm

6. Folds: Monocline, Anticline, and Syncline Anticline: upward bending, the limbs dip away from the hinge. Erosion reveals older layers in the hinge zone. Syncline: downward bending, the limbs dip toward the hinge. Erosion reveals younger layers in the hinge zone. Monocline: There is no change in dip direction across the fold hinge, the layers dip in the same direction. Limb Hinge Axial plane

7. Hinge Folds: Fold system, Fold axis, Axial plane

8. Those animation were created by John Waldron (Saint Mary's University, Canada). Check for more at: http://www.stmarys.ca/academic/science/geology/structural/

9. Folds: Fold closure, and axial trace Axial trace

10. Synclinal fold in the Kimberley (Margaret River, WA)

11. Folds: Symmetry related classification Classification based on the dip of the axial plane and the dip of the limbs Symmetrical: vertical axial plane Asymmetrical: dipping axial plane, limbs dip in opposite direction Overturned: dipping axial plane, limbs dip in same direction=>overturn limb Recumbent: near-horizontal axial plane

12. Large-scale recumbent fold in the Caledonides belt of northeast Greenland.

14. Profile plane The reference plane used to describe fold shape is called the profile plane. It is perpendicular to the hinge line. Profile plane True shape of the fold Apparent shape 2D sections through a 3D object do not usually give a correct understanding of the geometry of this object. This is perhaps the most common mistake that field geologists do. So be aware...

15. Folds from space: Sulaiman Range of Pakistan.

17. Anticlinal fold in Wyoming

18. Folds from space: Superimposed folds in the Kimberley (WA) N

19. Fold related structures: Axial planar cleavage When folding occur at depth>~5km, a planar structure called schitosity or cleavage may develop. This planar structure results from the systematic reorientation of minerals, and micro-folding of microscopic layering. Crenulation cleavage result from the last process.

20. Fold related structures: When soluble minerals are present, the combination of dissolution, driven by compressive stresses, and precipitation contributes to the development of axial planar cleavage.

21. Axial planar cleavage in symmetric folds (Paragon, Broken Hill, NSW) On the right: The successive dark an pale grey layers represent successive sedimentary layers. The bedding surfaces are folded. The folds present a near-vertical penetrative axial planar cleavage. Bottom picture: The cleavage plane is at right angle to the sedimentary layering. This can only happen in the hinge of folds. So S1

22. Lecture 4 at a glance Folds: Monocline, syncline, anticline Measurable structural elements: Axial plane, fold axis, axial planar cleavage, crenulation, intersection lineation. AsymmetricalOverturnedRecumbent You can easily get an apparently overturned fold (or even a recumbent fold) from a 2D section through a symmetric fold. So make sure that the shape of the fold is describe in the plane perpendicular to the fold axis: i.e the profile plane. When folding occurs at depth>5km, a cleavage plane may develop. It tends to be parallel to the folds axial planar surface. You can locate yourself on a fold by looking at the angular relationship between the bedding plane and the cleavage plane. A B C AC B

23. A little Q UIZZZ ZZZ ZZZ zzz zzz zzz zzzzzz Q1} In slide 3 folds are: 1/ Symmetric; 2/ Asymmetric 3/ Overturn ; 4/ Recumbent Q2} Two folds appear on the central part of slide 13: Which one is a syncline, which one is an anticline? Q3} The fold in slide 14 is a: 1/ Syncline; 2/ Anticline; 3/ Monocline Q4} The fold in slide 15 is : 1/ Symmetric; 2/ Asymmetric; 3/ Overturn Q5} The fold in the right column of slide 12 is : 1/ Symmetric; 2/ Asymmetric; 3/ Overturn Q6} Assuming that in slide 8 the North is in front of you would you say that the plunge direction of the fold axis is: 1/ 1/ N, 2/ S, 3/ SE, 4/NW, 5/ SW, 6/ SE, 7/ NE Q7} In slide 14 what is the plunge direction of the fold: 1/ N, 2/ S, 3/ SE, 4/NW, 5/ SW, 6/ SE, 7/ NE Q8} In slide 17 how would you qualify the cleavage on the bottom right picture: 1/ Crenulation; 2/ Slatty; 3/ Horizontal; 4/Vertical