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  strain: the change in size (volume) or shape, or both, while an object is undergoing stress

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 Crust
 tensional stress: caused by forces pulling away from one another in opposite directions

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
 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
 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 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: 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
     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: 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 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
 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

Interpreting folds & Unconformities

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
   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
 footwall: the underlying surface of an inclined fault plane  hanging wall: the overlying surface of an inclined fault plane

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 (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 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
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 .