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Folds, Faults, and Geologic MapsFolds, Faults, and Geologic Maps
Chapter 9
Geology TodayBarbara W. Murck & Brian J. Skinner
N. Lindsley-Griffin, 1999
Folds, South Georgia Island
StressStress
Stress - the force acting on a surface, per unit area
N. Lindsley-Griffin, 1999
Fig. 9.2, p. 243
Stress and StrainStress and Strain
N. Lindsley-Griffin, 1999
Strain - the change in shape or volume of a rock in response to stress. (Fig. 9.3, p. 244)
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
N. Lindsley-Griffin, 1999
Types of DeformationTypes of Deformation
Fig. 9.5 A, p. 245
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
Controlling FactorsControlling Factors
N. Lindsley-Griffin, 1999Undeformed Ductile Brittle
(Fig. 9.6, p. 246)
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.
Controlling FactorsControlling Factors
N. Lindsley-Griffin, 1999
(Fig. 9.6, p. 246)
Undeformed Ductile Brittle
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
Controlling FactorsControlling Factors
N. Lindsley-Griffin, 1999
(Fig. 9.5, p. 245)
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)
Strike and DipStrike and Dip
N. Lindsley-Griffin, 1999
Fault - a fracture in
the crust along which
movement has occurred.
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
FaultsFaults
N. Lindsley-Griffin, 1999
Hanging Wall
Footwall
Footwall
Normal Fault -
hanging wall down relative to footwall.
Tensional stress
(Fig. 9.9, p. 250)
FaultsFaults
N. Lindsley-Griffin, 1999
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)
FaultsFaults
N. Lindsley-Griffin, 1999
Reverse Fault -
hanging wall moves up
relative to footwall
Dip is steep
Compressional stress
(Fig. 9.11, p. 251)
N. Lindsley-Griffin, 1999
FaultsFaults
Thrust Fault - hanging wall moves up relative to footwall
Dip is < 30 degrees(Fig. 9.12, p. 251)N. Lindsley-Griffin, 1999
FaultsFaults
Reverse fault for comparison
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
FaultsFaults
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
FoldsFolds
Fig. 9.17 B, p. 255
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
FoldsFolds
Fig. 9.17 B, p. 255
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
FoldsFolds
Fig. 9.17 B, p. 255
Five common types of fold geometries
(Fig. 9.20, p. 257)
N. Lindsley-Griffin, 1999
FoldsFolds
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
FoldsFolds
Fig. 9.17 A, p. 255
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
FoldsFolds
Fig. 9.17 A, p. 255
Here, both limbs dip in the same direction so that one limb is actually upside down:
overturned, symmetrical folds
N. Lindsley-Griffin, 1999
FoldsFolds
Fig. 9.1, p. 242
Here, both limbs dip in the same direction so that one limb is actually upside down:
overturned, symmetrical folds
N. Lindsley-Griffin, 1999
FoldsFolds
Fig. 9.1, p. 242
Open, asymmetrical folds
(Fig. 9.4 A, p. 245)
N. Lindsley-Griffin, 1999
FoldsFolds
Open, asymmetrical folds
(Fig. 9.4 A, p. 245)
N. Lindsley-Griffin, 1999
FoldsFolds
Special fold types
Domes and Basins (Fig. 9.18, p. 256)
N. Lindsley-Griffin, 1999
FoldsFolds
Monocline - upper and lower limbs are horizontal, only the central limb is inclined (Fig. 9.16, p. 254)
N. Lindsley-Griffin, 1999
FoldsFolds Special fold types
Geologic MapsGeologic Maps
N. Lindsley-Griffin, 1999
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.
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
FoldsFolds
Cross sections show how structures and rock layers behave at depth.
Appalachian Mtns.
(Fig. 9.26, p. 266)
Cross SectionsCross Sections
N. Lindsley-Griffin, 1999
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
Geologic MapsGeologic Maps
Topographic maps use contour lines to depict topography. (Fig. 9.23, p. 263)
N. Lindsley-Griffin, 1999
Topographic MapsTopographic Maps