Folds, Faults, and Geologic Maps
Chapter 9
Geology Today
Barbara W. Murck & Brian J. Skinner
Folds, South Georgia Island
N. Lindsley-Griffin, 1999
Stress
Stress - the force acting on a surface, per unit area
Fig. 9.2, p. 243
N. Lindsley-Griffin, 1999
Stress and Strain
Strain - the change in shape or volume of a rock in
response to stress.
(Fig. 9.3, p. 244)
N. Lindsley-Griffin, 1999
Types of Deformation
Elastic - nonpermanent; rock returns to original shape
when stress is removed
Ductile (plastic) - permanent; rock flows or bends
when stressed beyond its elastic limit
Brittle - rock breaks or cracks
Fig. 9.5 A,
p. 245
N. Lindsley-Griffin, 1999
Controlling Factors
Factors that influence
how rocks deform:
Temperature - increase in T makes solids more ductile
Confining Pressure - increase in P inhibits formation of
fractures; solids resist breaking
(Fig. 9.6, p. 246)
N. Lindsley-Griffin, 1999
Undeformed
Ductile
Brittle
Controlling Factors
Factors that influence
how rocks deform:
Rate of deformation - stress applied quickly enhances
brittle behavior. Stress applied slowly over a long period of
time promotes ductile behavior.
(Fig. 9.6, p. 246)
N. Lindsley-Griffin, 1999
Undeformed
Ductile
Brittle
Controlling Factors
Factors that influence
how rocks deform
Composition - some minerals are very brittle (quartz,
garnet, olivine) as are rocks that contain them.
Minerals that behave ductilely: mica, clay, calcite, gypsum
Rocks that behave ductilely: shale, slate, limestone, marble
(Fig. 9.5, p. 245)
N. Lindsley-Griffin, 1999
Strike and Dip
Strike - orientation
of a horizontal line
in a plane.
(Expressed as a
compass direction)
Dip - angle between
a tilted surface and
a horizontal plane.
(Expressed as an
angle and direction)
N. Lindsley-Griffin, 1999
Faults
Fault - a fracture in
the crust along which
movement has occurred.
Hanging Wall
Footwall
Footwall
Normal Fault (Fig. 9.4, p. 245)
Faults are classified by how steeply they dip and relative
movement of the blocks:
Hanging wall - block on top of the fault
Footwall - block below the fault
N. Lindsley-Griffin, 1999
Faults
Normal Fault hanging wall down
relative to footwall.
Tensional stress
(Fig. 9.9, p. 250)
N. Lindsley-Griffin, 1999
Faults
Horst - two normal
faults dip away from each
other, block between
them is elevated.
Graben - two normal
faults dip towards each
other, block between
them drops down.
Tensional stress
(Fig. 9.10, p. 250)
N. Lindsley-Griffin, 1999
Faults
Reverse Fault hanging wall moves up
relative to footwall
Dip is steep
Compressional stress
(Fig. 9.11, p. 251)
N. Lindsley-Griffin, 1999
Faults
Reverse fault for
comparison
N. Lindsley-Griffin, 1999
Thrust Fault - hanging wall moves up
relative to footwall
Dip is < 30 degrees
(Fig. 9.12, p. 251)
Faults
Strike-slip fault movement is mostly
horizontal and parallel to
strike of fault.
Left-lateral - block
opposite you moves left.
Right-lateral - block
opposite you moves right.
(Fig. 9.13, p. 251)
N. Lindsley-Griffin, 1999
Folds
How do you describe something as
complex as a fold?
Fold axis - the line
along which the
axial plane
intersects a single
rock layer
Axial plane (axial
surface) - the plane
that divides the fold
in half as
symmetrically as
possible
N. Lindsley-Griffin, 1999
Fig. 9.17 B, p. 255
Folds
How do you describe something as
complex as a fold?
Fold limbs - the two
sides of the fold that
are separated by
the axial plane
Anticline - limbs
point down
Syncline - limbs
point up
N. Lindsley-Griffin, 1999
Fig. 9.17 B, p. 255
Folds
How do you describe something as
complex as a fold?
Orientation of both
axial plane and fold
axis are used.
Horizontal fold axis
with vertical axial
plane = upright fold
Plunging fold fold axis is not
horizontal
N. Lindsley-Griffin, 1999
Fig. 9.17 B, p. 255
Folds
N. Lindsley-Griffin, 1999
Five common types of fold geometries
(Fig. 9.20, p. 257)
Folds
To describe a fold, first determine
orientation of axial planes, limbs, axes
Here, axial planes are not quite vertical, axes point straight out,
limbs dip at different angles:
open, asymmetrical, anticline - syncline
N. Lindsley-Griffin, 1999
Fig. 9.17 A, p. 255
Folds
To describe a fold, first determine
orientation of axial planes, limbs, axes
Here, axial planes are not quite vertical, axes point straight out,
limbs dip at different angles:
open, asymmetrical, anticline - syncline
N. Lindsley-Griffin, 1999
Fig. 9.17 A, p. 255
Folds
Here, both limbs dip in the same direction so that one limb is
actually upside down:
overturned, symmetrical folds
N. Lindsley-Griffin, 1999
Fig. 9.1, p. 242
Folds
Here, both limbs dip in the same direction so that one limb is
actually upside down:
overturned, symmetrical folds
N. Lindsley-Griffin, 1999
Fig. 9.1, p. 242
Folds
N. Lindsley-Griffin, 1999
Open, asymmetrical folds
(Fig. 9.4 A, p. 245)
Folds
N. Lindsley-Griffin, 1999
Open, asymmetrical folds
(Fig. 9.4 A, p. 245)
Folds
Special fold types
Domes and Basins (Fig. 9.18, p. 256)
N. Lindsley-Griffin, 1999
Folds
Special fold types
Monocline - upper and lower limbs are horizontal,
only the central limb is inclined
(Fig. 9.16, p. 254)
N. Lindsley-Griffin, 1999
Geologic Maps
Showing strata on maps
(Fig. 9.21, p. 258)
Contacts - boundaries between distinct rock types
Block diagram shows eroded strata, cross section
Geologic map shows contacts with strike and dip symbols.
N. Lindsley-Griffin, 1999
Folds
Showing folds on maps
(Fig. 9.19, p. 256)
Plunging folds make horseshoe patterns on surface
Block diagram shows pattern projected to horizontal surface,
as if strata were eroded flat
Map shows how folds are depicted on geologic maps.
N. Lindsley-Griffin, 1999
Cross Sections
Cross sections show how
structures and rock layers
behave at depth.
Appalachian Mtns.
(Fig. 9.26, p. 266)
N. Lindsley-Griffin, 1999
Geologic Maps
Geologic maps show topograpic
contours, major
folds and faults,
contacts between
rock units, age
and type of rock.
(Fig. 9.22, p. 259)
N. Lindsley-Griffin, 1999
Topographic Maps
Topographic maps use contour lines to depict
topography. (Fig. 9.23, p. 263)
N. Lindsley-Griffin, 1999