Geologic Structures

Prepared by Betsy Conklin forDr. 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 areastrain: 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 Foldsplunging 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 thesurface 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 basinStructural dome

Interpreting foldsopen folds: a fold with gently dipping

limbsisoclinal fold: a fold in which the limbs are

parallel to one anotheroverturned fold: a fold in which both limbs

dip in the same directionrecumbent 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.