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Geologic Structures

Geologic Structures. Prepared by Betsy Conklin for Dr. Isiorho. Tectonic Forces at Work. structural geology: the branch of geology concerned with the shapes, arrangement, and interrelationships of bedrock units and the forces that cause them stress: a force per unit area

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Geologic Structures

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  1. Geologic Structures Prepared by Betsy Conklin for Dr. Isiorho

  2. Tectonic Forces at Work • structural geology: the branch of geology concerned with the shapes, arrangement, and interrelationships of bedrock units and the forces that cause them • stress: a force per unit area • strain: the change in size (volume) or shape, or both, while an object is undergoing stress

  3. Stress and Strain in the Earth’s Crust • compressive stress: a stress due to a force pushing together on a body

  4. Stress and Strain in the Earth’s Crust • tensional stress: caused by forces pulling away from one another in opposite directions

  5. Stress and Strain in the Earth’s Crust • shear stress: due to movement prallel to but in opposite directions along a fulat or other boundary

  6. Behavior of Rocks to Stress and Strain • elastic strain: strain in which a deformed body recovers its original shape after the stress is released (ex: rubber band) • elastic limit: the maximum amount of stress that can be applied to a body before it deforms in a permanent way by bending or breaking • ductile: capable of being molded and bent under stress • brittle strain: cracking or rupturing of a body under stress

  7. Present Deformation of the Crust • Geologists often say the crust of the earth is “mobile” or “restless” because bedrock is moving and being deformed in many parts of the world • fault: a fracture in bedrock along which movement has taken place

  8. Geologic Maps and Field Methods • geologic map: a map which uses standardized symbols and patterns to represent rock types and geologic structures that is typically produced from the field map for a given area • geologic cross section: represents a vertical slice through a portion of the earth

  9. Strike and Dip • strike: the compass direction of a line formed by the intersection of an inclined plane with a horizontal plane • angle of dip: a measurement downward from the horizontal plane to the bedding plane • direction of dip: the compass direction in which the angle of dip is measured

  10. Folds • fold: bends or wave-like features in layered rock • anticline: an upward arching fold • hinge line: the axis of the fold • syncline: a downward-arching counterpart of an anticline • axial plane: a plane containing all of the hinge lines of a fold

  11. Plunging Folds • plunging folds: folds in which the hinge lines are not horizontal Plunging folds: anticline on left and right, syncline in center. The hinge lines are at an angle to the block diagram, penetrating the surface and emerging from the front cross section

  12. Structural Domes and Structural Basins • structural dome: a structure in which the beds dip away from a central point • structural basin: a structure in which the beds dip toward a central point Structural basin Structural dome

  13. Interpreting folds • open folds: a fold with gently dipping limbs • isoclinal fold: a fold in which the limbs are parallel to one another • overturned fold: a fold in which both limbs dip in the same direction • recumbent fold: a fold overturned to such an extent that the limbs are essentially horizontal

  14. Interpreting folds & Unconformities

  15. Fractures in Rock • joint: a fracture or crack in bedrock where essentially no displacement occurs • joint set: where joints are oriented approximately parallel to one another

  16. Faults • dip-slip fault: movement is parallel to the dip of the fault surface • strike-slip fault: horizontal motion parallel to the strike of the fault surface • oblique-slip fault: both strike-slip and dip-slip components

  17. Dip-Slip Faults • footwall: the underlying surface of an inclined fault plane • hanging wall: the overlying surface of an inclined fault plane

  18. Dip-Slip Faults (cont.) • normal fault: a fault where the hanging-wall block has moved downward relative to the footwall block • graben: when a block bounded by normal faults drops down • horst: when a block bounded by normal faults is uplifted

  19. Dip-Slip Faults (cont.) • reverse fault: when the hanging-wall block has moved upward relative to the footwall block • thrust fault: a reverse fault in which the dip of the fault plane is at a low angle to horizontal A reverse fault. The fault is unaffected by erosion. Arrows indicate compressive stress. Diagram shows area after erosion; dashed lines indicate portion eroded away Thrust fault due to horizontal compression.

  20. Strike-slip Faults • strike-slip fault: a fault where the movement is predominantly horizontal and parallel to the strike of the fault • right-lateral fault: a strike-slip fault in which the block seen across the fault appears displaced to the right • left-lateral fault: a strike-slip fault in which the block seen across the fault appears displaced to the left

  21. Pictures All pictures used in this power point presentation were taken from the following: Carlson, Diane H., David McGeary and Charles C. Plummer. Physical Geology: Updated Eighth Edition. New York City, McGraw-Hill Higher Education, 2001.

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