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Chapter 20 Geologic structures

Chapter 20 Geologic structures. How rocks respond to tectonic forces!. Tectonic forces Forces generated inside the earth are called tectonic forces. Tectonic forces are expressed in terms of Stress and the resulting Strain. Stress: force applied per unit area.

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Chapter 20 Geologic structures

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  1. Chapter 20 Geologic structures

  2. How rocks respond to tectonic forces!

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

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

  5. 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.

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

  7. 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.

  8. 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.

  9. 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

  10. Geologic structures • They may be; • Folds • Faults • Joints

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

  12. Fold geometry: • 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.

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

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

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

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

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

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

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

  20. Types of faults • 3 types: • Dip slip: • Strike slip: • Oblique slip:

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

  22. 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.

  23. 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.

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