Folds, Faults, and Geologic Maps Chapter 9 Geology Today Barbara W. Murck & Brian J. Skinner N....

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

Fig. 9.2, p. 243

Stress and StrainStress and Strain

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

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)

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)

Undeformed Ductile Brittle

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)

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

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

Hanging Wall

Footwall

Normal Fault -

hanging wall down relative to footwall.

Tensional stress

(Fig. 9.9, p. 250)

FaultsFaults

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

Reverse Fault -

hanging wall moves up

relative to footwall

Dip is steep

Compressional stress

(Fig. 9.11, p. 251)

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)

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

FoldsFolds

Fig. 9.17 B, p. 255

Fold limbs - the two sides of the fold that are separated by the axial plane

Anticline - limbs point down

Syncline - limbs point up

FoldsFolds

Fig. 9.17 B, p. 255

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

FoldsFolds

Fig. 9.17 B, p. 255

Five common types of fold geometries

(Fig. 9.20, p. 257)

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

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

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

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

FoldsFolds

Fig. 9.1, p. 242

Open, asymmetrical folds

(Fig. 9.4 A, p. 245)

FoldsFolds

Open, asymmetrical folds

(Fig. 9.4 A, p. 245)

FoldsFolds

Special fold types

Domes and Basins (Fig. 9.18, p. 256)

FoldsFolds

Monocline - upper and lower limbs are horizontal, only the central limb is inclined (Fig. 9.16, p. 254)

FoldsFolds Special fold types

Geologic MapsGeologic 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.

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.

FoldsFolds

Cross sections show how structures and rock layers behave at depth.

Appalachian Mtns.

(Fig. 9.26, p. 266)

Cross SectionsCross Sections

Geologic maps - show topograpic contours, major folds and faults, contacts between rock units, age and type of rock.

(Fig. 9.22, p. 259)

Geologic MapsGeologic Maps

Topographic maps use contour lines to depict topography. (Fig. 9.23, p. 263)

Topographic MapsTopographic Maps

Folds, Faults, and Geologic Maps Chapter 9 Geology Today Barbara W. Murck & Brian J. Skinner N....

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