1. Chapter 20
Geologic structures

2. How rocks respond to tectonic forces!

4. Tectonic forces
Forces generated inside the earth are called tectonic forces.
Tectonic forces are expressed in terms of Stress and the resulting Strain.

5. Stress: force applied per unit area.
Strain: resulting change in size or shape or both.

6. Stress: 3 types
Compressive: when a body is pushed together or squeezed from opposite directions.
Tensional: when a body is pulled away in opposite directions
Shear: when two bodies slide past one another.

9. How rocks respond to stresses?
Rock behavior depends on;
Amount and rate of applied stress
Type of rock
T&P conditions

10. Rocks behavior is of 3 types;
Elastic: deformed body recovers its original shape after the stress is released.
Ductile: deformed body does not return to its original position and its bends.
Brittle: deformed body will fracture at stresses higher than the elastic limit.

14. Geologic field methods
Geologic maps
What is a map?
Representation of the 3D world on a 2D flat surface.
Maps are designed to show where things are.
A geologic map shows the distribution of geologic features, including different kinds of rocks and structures.

15. Strike is the compass direction of a linear or planar feature.
What is Strike and Dip?
Strike is the compass direction of a linear or planar feature.
N30°E
N45°W
Dip is the angle an inclined plane makes with the horizontal.
30°NE

20. Geologic structures
They may be;
Folds
Faults
Joints

21. Folds: bends or curved features in layered rocks. Best displayed in sed. rocks.
Folds are the result of ductile deformation.

23. Fold geometry: Parts of a fold: Anticline: arching upwards
Syncline: arching downwards
Parts of a fold:
Limb: side of a fold
Hinge point: highest point on a fold
Hinge line: line joining hinge points. Also called fold axis.
Axial plane: plane joining all hinge lines.

27. Dome and basin:
Dome: a structure in which beds dip away from a central point.
Basin: beds dip towards a central point.

29. Types of folds
Open
Isoclinal
Overturned
Recumbent
Open: limbs have gentle dips indicating moderate stresses.

31. Isoclinal: limbs parallel to one another indicating intense stresses.

33. Overturned: axial plane inclined and the limbs dip in the same direction. Indicate unequal compressive stresses.

35. Recumbent: the limbs and axial planes are essentially horizontal
Recumbent: the limbs and axial planes are essentially horizontal. Extreme stresses are indicated.

37. Geologic structures and OIL
Oil and gas are concentrated in the core of an anticline where they are trapped by an impermeable layer.

42. Faults
Fracture in rocks along which there has been some movement.
Parts of a fault:
Hanging wall
Footwall
Fault plane

44. Types of faults
3 types:
Dip slip:
Strike slip:
Oblique slip:

45. Dip-slip faults: movement is parallel to the dip of the fault plane.

46. Two types of dip slip faults: normal and reverse
Normal fault: H.W. moves downwards relative to the F.W.
Reverse fault: H.W. moves upwards relative to the F.W. A thrust fault is a low angle reverse fault.

48. Graben and Horst:
A graben is a block of rock which is moved downwards due to two normal faults.
A horst is an uplifted block of rocks due to two normal faults.

53. Strike-slip faults: movement is parallel to the strike of the fault surface.
San Andeas fault in CA.

58. Joints
Joints are regular fractures along which there has been negligible movement.

59. Joints
Joint sets form where they are oriented parallel to one another.
Joints indicate the direction of applied stresses.