Transcript 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.

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
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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.