Undisturbed Layers

How is rock deformation discussed?How is rock deformation discussed?How is rock deformation discussed?How is rock deformation discussed?(stress and strain relationships)(stress and strain relationships)

StressStress• force (pressure) acting on the rock surfaceforce (pressure) acting on the rock surface

StressStress• force (pressure) acting on the rock surfaceforce (pressure) acting on the rock surface

StrainStrain• a change in the shape (deformation) to thea change in the shape (deformation) to the response of stressresponse of stress

StrainStrain• a change in the shape (deformation) to thea change in the shape (deformation) to the response of stressresponse of stress

CompressionCompressionCompressionCompression

TensionalTensionalTensionalTensional

ShearShearShearShearundisturbed undisturbed cubecube

Types of stresses and strains Types of stresses and strains

Confined PressureConfined PressureStress is applied to the rockStress is applied to the rock““uniformially” in all directionsuniformially” in all directions

Confined PressureConfined PressureStress is applied to the rockStress is applied to the rock““uniformially” in all directionsuniformially” in all directions

Differential PressureDifferential PressureStress is applied to the rockStress is applied to the rockin various directionsin various directions

Differential PressureDifferential PressureStress is applied to the rockStress is applied to the rockin various directionsin various directions

Uniformly “squeezed”Uniformly “squeezed”Uniformly “squeezed”Uniformly “squeezed” Strained in other directionsStrained in other directionsStrained in other directionsStrained in other directions

If you can see this cube “rotate”, thenyou can picture diagrams in 3-D.If you can see this cube “rotate”, thenyou can picture diagrams in 3-D.

Structural GeologyStructural Geology – –Structural geologists decipher earth history by Structural geologists decipher earth history by

identifying identifying and and mappingmapping deformational structures. deformational structures.

Why understand structural geology?Why understand structural geology?

Decipher geologic history by looking at the subsurfaceDecipher geologic history by looking at the subsurface

Look for oil resourcesLook for oil resources

Look for mining resourcesLook for mining resources

Monitoring groundwater flowMonitoring groundwater flow

How does a geologist measure the orientation of How does a geologist measure the orientation of rock layers and use these measurements to predict therock layers and use these measurements to predict thegeologic structure below the earth’s surface?geologic structure below the earth’s surface?

Strike and dip

StrikeStrikeThe compass orientation of the line of intersection between The compass orientation of the line of intersection between the a horizontal plane and a planer feature (a rock layer)the a horizontal plane and a planer feature (a rock layer)

DipDipThe angle between a tilted surface and a horizontal planeThe angle between a tilted surface and a horizontal plane

Measuring the “strike” of strata

• represents the intersection of the tilted layer and the imaginary horizontal plane

• A MAP symbol – showing the “directional trend” of strata

Measuring the “strike” of strata

• represents the intersection of the tilted layer and the imaginary horizontal plane

• A MAP symbol – showing the “directional trend” of strata

imaginaryplane

imaginaryplane

Map symbol

Measuring the directionaltrendMeasuring the directionaltrend

NorthNorth

SS

EEWW

00

9090

180180

270270

4545

135135225225

315315

Strike is 45 degreesStrike is 45 degrees

How would you “plot” the strike of these rock layersHow would you “plot” the strike of these rock layers

Geologic MapsGeologic Maps

NorthNorth

Strike represents the MAP orientation and“general direction” of strataStrike represents the MAP orientation and“general direction” of strata

Measuring the dip angle with respect to strike ofstrata

• represents the ANGLE (tilt) of strata in reference to the imaginary horizontal plane

• Dip is ALWAYS PERPENDICULAR to strike

Measuring the dip angle with respect to strike ofstrata

• represents the ANGLE (tilt) of strata in reference to the imaginary horizontal plane

• Dip is ALWAYS PERPENDICULAR to strike

Imaginary plane

Dip angleDip angle

Strike and dip map symbolsStrike and dip map symbols

20201515

1212

4747

6262

0-90o0-90o

Let’s add some stresses and bend the rock layers intoLet’s add some stresses and bend the rock layers intofoldsfolds

Let’s add some stresses and fold these rocksLet’s add some stresses and fold these rocksLet’s add some stresses and fold these rocksLet’s add some stresses and fold these rocks

Typical parts of a Fold

The hinge lineapex of fold

Cuts the fold in two equal parts

“leg” of the fold

Interpreting foldsInterpreting folds

Syncline fold Anticline fold

Map Symbols

YoungestYoungest rocks rocksin the middlein the middle

OldestOldest rocks rocksin the middlein the middle

Youngoldest

Map ViewMap View Map ViewMap View

How can you tell the differencebetween an anticline and syncline on the

MAP VIEW????

How can you tell the differencebetween an anticline and syncline on the

MAP VIEW????

11

22

3344 44 5555 6666

OLDEST found in middle

ANTICLINE

OLDEST found in middle

ANTICLINE

11

22 22

33 3344 4455 5566

YOUNGEST found in middle

SYNCLINE

YOUNGEST found in middle

SYNCLINE

Folds are tilted or “plunged”• How would the “map-view” of the plunging fold appear from horizontal to steep tilt???

Folds are tilted or “plunged”• How would the “map-view” of the plunging fold appear from horizontal to steep tilt???

horizontalhorizontal Slightly steepSlightly steep Very SteepVery Steep

Map Views

Direction of plunge

• What happens to the map view of the anticline What happens to the map view of the anticline as the plunge steepens?as the plunge steepens?

Direction of plungeDirection of plunge

Map viewMap view

Direction of plunge for anticlinesand synclinesDirection of plunge for anticlinesand synclines

JointJoint – fracture within a rock body – fracture within a rock bodyFault Fault – movement along a fracture or joint– movement along a fracture or jointJointJoint – fracture within a rock body – fracture within a rock bodyFault Fault – movement along a fracture or joint– movement along a fracture or joint

FaultsFaultsFaultsFaults

JointJoint

faultfault

AA

BB

BA

Determining the type of fault using the hanging-wall (HW)Determining the type of fault using the hanging-wall (HW)and and foot-wallfoot-wall (FW) (FW)Determining the type of fault using the hanging-wall (HW)Determining the type of fault using the hanging-wall (HW)and and foot-wallfoot-wall (FW) (FW)

FWFWHWHW

Normal FaultNormal Fault• HW HW downdown relative to FW relative to FW

Normal FaultNormal Fault• HW HW downdown relative to FW relative to FW

FWFW

HWHW

HWHW

FWFW

Reverse FaultReverse Fault• HW HW upup relative to FW relative to FW

Reverse FaultReverse Fault• HW HW upup relative to FW relative to FW

• Tensional stress• Tensional stress

• Compression stress• Compression stress

FWFW

HWHW

HWHW

FWFW

HWHW

FWFW HWHW

FWFW

Normal faultingNormal faultingNormal faultingNormal faulting Reverse faultingReverse faultingReverse faultingReverse faulting

Vertical type faultsVertical type faults

TensionalTensional

CompressionCompression

Horizontal faulting – two blocks sliding past one anotherHorizontal faulting – two blocks sliding past one anotherHorizontal faulting – two blocks sliding past one anotherHorizontal faulting – two blocks sliding past one another

Map ViewMap ViewMap ViewMap View

Turned to RightTurned to RightTurned to RightTurned to Right Turned to LeftTurned to LeftTurned to LeftTurned to Left

What type of strike-slip fault?What type of strike-slip fault?What type of strike-slip fault?What type of strike-slip fault?

Right-lateral – strike slipRight-lateral – strike slipRight-lateral – strike slipRight-lateral – strike slip

Normal FaultNormal Fault• hanging wall hanging wall DOWNDOWN relative to footwall relative to footwall• tensional forces (pulling apart)tensional forces (pulling apart)• extension of crust (“crust grows”)extension of crust (“crust grows”)

Normal FaultNormal Fault• hanging wall hanging wall DOWNDOWN relative to footwall relative to footwall• tensional forces (pulling apart)tensional forces (pulling apart)• extension of crust (“crust grows”)extension of crust (“crust grows”)

Reverse Fault • hanging wall UP relative to footwall• compressional forces (pushing together)• shorting of crust (“crust shrinks”)

Reverse Fault • hanging wall UP relative to footwall• compressional forces (pushing together)• shorting of crust (“crust shrinks”)

Strike-Slip fault• horizontal movement (sliding past)• moves rocks along a horizontal plane

Strike-Slip fault• horizontal movement (sliding past)• moves rocks along a horizontal plane