geologic structures prepared by betsy conklin for dr. isiorho
TRANSCRIPT
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.