Geological Structures

Geological Structures (Folds, Faults and Joints)

1. GEOLOGICAL STRUCTURES Myra Karl Elise Arevalo Kessey Joy Santerva

2. Geological structures are structures in the Earth's crust that have geological causes. There are many types of geological structures and these can have several causes. For example, tectonics caused widespread deformation of the crust like fractures and folds.

3. FOLDS F FAULTS J JOINTS

4. A wave-like geologic structure that forms when rocks deform by bending instead of breaking under compressional stress. FOLD

5. Types of folds

6. ANTICLINE An anticline is a type of fold that is an arch-like shape and has its oldest beds at its core. A typical anticline is convex up in which the hinge or crest is the location where the curvature is greatest, and the limbs are the sides of the fold that dip away from the hinge. Anticline at Calico Ghost Town Location: San Bernardino County, California, United States. Photo Copyright © Garry Hayes

7. Anticlines can be recognized and differentiated from antiforms by a sequence of rock layers that become progressively older toward the center of the fold. Therefore, if age relationships between various rock strata are unknown, the term antiform should be used.

8. SYNCLINE A syncline is a fold with younger layers closer to the center of the structure. A synclinorium (plural synclinoriums or synclinoria ) is a large syncline with superimposed smaller folds.

9. Synclines are typically a downward fold, termed a synformal but synclines that point upwards, or perched, can be found when strata have been overturned and folded (an antiformal syncline).

10. DOME A dome is a feature in structural geology consisting of symmetrical anticlines that intersect each other at their respective apices. Intact, domes are distinct, rounded, spherical-to-ellipsoidal-shaped protrusions on the Earth’s surface.

11. However, a transect parallel to Earth’s surface of a dome features concentric rings of strata. Consequently, if the top of a dome has been eroded flat, the resulting structure in plane view appears as a bullseye, with the youngest rock layers at the outside, and each ring growing progressively older moving inwards. These strata would have been horizontal at the time of deposition, then later deformed by the uplift associated with dome formation.

12. BASIN A structural basin is a large-scale structural formation of rock strata formed by tectonic warping of previously flat lying strata. Structural basins are geological depressions, and are the inverse of domes. Some elongated structural basins are also known as synclines. Structural basins may also be sedimentary basins, which are aggregations of sediment that filled up a depression or accumulated in an area; however, many structural basins were formed by tectonic events long after the sedimentary layers were deposited.

13. Basins appear on a geologic map as roughly circular or elliptical, with concentric layers. Because the strata dip toward the center, the exposed strata in a basin are progressively younger from outside-in, with the youngest rocks in the center. Basins are often large in areal extent, often hundreds of kilometers across.

14. A monocline (or, rarely, a monoform ) is a step-like fold in rock strata consisting of a zone of steeper dip within an otherwise horizontal or gently-dipping sequence. MONOCLINE

15. Chevron folds are a structural feature characterized by repeated well behaved folded beds with straight limbs and sharp hinges. Well developed, these folds develop repeated set of v-shaped beds. They develop in response to regional or local compressive stress. Inter-limb angles are generally 60 degrees or less. Chevron folding preferentially occurs when the bedding regularly alternates between contrasting competences. Turbidites, characterized by alternating high-competence sandstones and low-competence shales, provide the typical geological setting for chevron folds to occurs. CHEVRON

16. Chevron folds with flat-lying axial planes, Millook Haven, North Cornwall, UK Credit: Smalljim

17. An overturned fold, or overfold, has the axial plane inclined to such an extent that the strata on one limb are overturned. A recumbent fold has an essentially horizontal axial plane. When the two limbs of a fold are essentially parallel to each other and thus approximately parallel to the axial plane   Recumbent fold at Godrevy in Cornwall in England. Credit: mwcarruthers RECUMBENT

18. SLUMP Typically monoclinal, result of differential compaction or dissolution during sedimentation and lithification.       Slump Fold An almost isoclinal fold (coin, seaweed and shells for scale) formed as wet layers of mud settled and solidified in Triassic times. Credit: Anne Burgess

19. PTYGMATIC Folds are chaotic, random and disconnected. Typical of sedimentary slump folding, migmatites and decollement detachment zones.     Ptygmatic folding, Broken Hill Credit: Monash University

20. PARASITIC Short wavelength folds formed within a larger wavelength fold structure – normally associated with differences in bed thickness

21. DISHARMONIC Folds in adjacent layers with different wavelengths and shapes

22. Flexural slip allows folding by creating layer-parallel slip. Folding mechanisms between the layers of the folded strata, which, altogether, result in deformation. The fold formed by the compression of competent rock beds is called “flexure fold”. FLEXURAL FOLD

23. FAULTS A fracture surface in rock across which there is relative motion parallel to the surface between the adjacent blocks of the rock.

24. Types of faults

25. NORMAL FAULTS - Normal faults form when the hanging wall drops down REVERSE FAULTS - Reverse faults form when the hanging wall moves up STRIKE SLIP FAULTS - Transcurrent or Strike-slip faults have walls that move sideways, not up or down.

26. Classification of faults on the basis of net slip

27. DIP SLIP FAULT The faults in which the slip takes place along the direction of the slip is called dip slip fault in the dip slip fault net slip is parallel to the dip of fault

28. STRIKE SLIP FAULT The faults in which the slip takes place along the direction of the strike is called dip slip fault in the dip slip fault net slip is parallel to the strike fault

29. OBLIQUE FAULT When the net slip is neither parallel to strike nor parallel to the dip of fault is called Oblique strike fault.

30. Classification of faults on the basis of DIP ANGLE

31. HIGH ANGLE FAULT A high angle fault is one that dips at angle greater than 45° LOW ANGLE FAULT A low angle fault is one that dips at angle smaller than 45°

32. A  joint  is a fracture dividing rock into two sections that moved away from each other. A joint does not involve shear displacement, and forms when tensile stress breaches its threshold. In other kinds of fracturing, like in a fault, the rock is parted by a visible crack that forms a gap in the rock. Joints push out in various directions, usually vertically. They can have smooth, clean surfaces, or they can be scarred from sliding against another joint. Joints usually occur as sets, with each set made up of joints that are parallel to each other. Joints become more and more obvious when the rock is weathered (eroded by the elements). When water gets into the joints, this can lead to the formation of big caves and underground rivers. JOINT

33. TYPES OF JOINTS WITH RESPECT TO FORMATION

34. SHEETING JOINTS When magma cools fast, cooling is done towards country rocks and size of material become courser at center due to slow cooling and cause shrinkage of layers. These joints are more or less parallel to the surfaces of ground.

35. TECTONIC JOINTS Tectonic joints are formed during deformation episodes whenever the differential stress is high enough to induce tensile failure of the rock. They will often form at the same time as faults. Measurement of tectonic joint patterns can be useful in analyzing the tectonic history of an area because they give information on stress orientations at the time of formation.

36. UNLOADING JOINTS (RELEASE JOINTS) Joints are most commonly formed when uplift and erosion removes the overlying rocks thereby reducing the compressive load and allowing the rock to expand laterally. Joints related to uplift and erosional unloading have orientations reflecting the principal stresses during the uplift. Care needs to be taken when attempting to understand past tectonic stresses to discriminate, if possible, between tectonic and unloading joints.

37. EXFOLIATION JOINTS Exfoliation joints may be a special case of unloading joints formed at, and parallel to, the current land surface in rocks of high compressive strength, although there is as yet no general agreement on a general theory of how they form.

38. COOLING JOINTS Joints can also form via cooling of hot rock masses, particularly lava, forming cooling joints, most commonly expressed as vertical columnar jointing. The joint formation associated with cooling is typically polygonal because the cooling introduces stresses that are isotropic in the plane of the layer.

39. MASTER JOINTS Meters of long joints having splay joints are know as “Master Joints”. These joints may be open, closed or may be filled with secondary minerals. Behavior of these joints depends upon mineralogy; if rocks is fine grained , Joints surface morphology will be smooth and if rocks is coarse grained , Joints surface morphology will be rough. These Joints helps us to develop secondary porosity and help in oil accumulation.