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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

Geologic Structures

Prepared by Betsy Conklin for

Dr. Isiorho

tectonic forces at work
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
stress and strain in the earth s crust
Stress and Strain in the Earth’s Crust
  • compressive stress: a stress due to a force pushing together on a body
stress and strain in the earth s crust4
Stress and Strain in the Earth’s Crust
  • tensional stress: caused by forces pulling away from one another in opposite directions
stress and strain in the earth s crust5
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
behavior of rocks to stress and strain
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
present deformation of the crust
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
geologic maps and field methods
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
strike and dip
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
folds
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
plunging folds
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

structural domes and structural basins
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

interpreting folds
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
fractures in rock
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
faults
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
dip slip faults
Dip-Slip Faults
  • footwall: the underlying surface of an inclined fault plane
  • hanging wall: the overlying surface of an inclined fault plane
dip slip faults cont
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
dip slip faults cont19
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.

strike slip faults
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
pictures
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.