Faults

BASIC DEFINITIONS► FAULT: A surface or narrow zone along which one side

has moved relative to the other. A fault may form through either brittle (fractures) or ductile (shear) deformation. Faults are classified based upon the orientation of the displacement vector along the fault surface.

DIP-SLIP FAULTS: Faults along which the movement is approximately parallel to the dip

STRIKE-SLIP FAULTS: Faults along which the movement is approximately parallel to the strike

OBLIQUE-SLIP FAULTS: Faults along which the movement is inclined to both the strike and dip of the fault surface. It may be described as the sum of a strike-slip and a dip-slip vector

HANGING WALL vs FOOTWALL

► A fault divides rock into two fault blocks

► For any inclined fault, the block above the fault is the hanging wall block, and the block below the fault is the footwall block

www.data.scec.org/Module/footnt02.html

CLASSIFICATION of DIP-SLIP FAULTS

► NORMAL FAULTS: Dip-slip faults on which the hanging wall moves down relative to the footwall

Place younger rocks on older rocks

May result in omission of strata

Form in regions of lateral extension

www.data.scec.org/

Module/footnt02.html

EXAMPLES OFNORMAL FAULTS

Conjugate Normal Faults,

Canyonlands National Park

darkwing.uoregon.edu/~millerm/conjN1.html

Normal Fault Scarps, Turkey

www.msnucleus.org/.../pt/hazards/4/pth4_1a.html

Outcrop scale normal faults

EXAMPLES OFNORMAL FAULTS

Conjugate Normal Faults,

Canyonlands National Park

darkwing.uoregon.edu/~millerm/conjN1.html

Normal Fault Scarps, Turkey

www.msnucleus.org/.../pt/hazards/4/pth4_1a.html

Outcrop scale normal faultsFW

HW

FWHW

FW

HW

FWHW

FWHW

CLASSIFICATION of DIP-SLIP FAULTS

► THRUST or REVERSE FAULTS: Dip-slip faults on which the hanging wall moves up relative to the footwall

Place older rocks on younger rocks

May result in repetition of strata

Form in regions of lateral compression

www.data.scec.org/

Module/footnt02.html

EXAMPLES OFTHRUST FAULTS

Thrust Fault in Concrete from 1964 Quake,

Anchorage, Alaska

www.ucmp.berkeley.edu/.../alaska/0709log.html

Thrust Fault in Sediments

www001.upp.so-net.ne.jp/fl-fg/05-01.htm

Outcrop scale thrust faultswww.pitt.edu/.../7Structures/ReverseFaults.html

EXAMPLES OFTHRUST FAULTS

Thrust Fault in Concrete from 1964 Quake,

Anchorage, Alaska

www.ucmp.berkeley.edu/.../alaska/0709log.html

Thrust Fault in Sediments

www001.upp.so-net.ne.jp/fl-fg/05-01.htm

Outcrop scale thrust faultswww.pitt.edu/.../7Structures/ReverseFaults.html

FW

HW

FW

HW

FW

HW

CLASSIFICATION of STRIKE-SLIP FAULTS

► RIGHT-LATERAL (DEXTRAL) FAULTS: Strike-slip faults across which the block moves to the right

► LEFT-LATERAL (SINISTRAL) FAULTS: Strike-slip faults across which the block moves to the right

EXAMPLES OFSTRIKE-SLIP FAULTS

San Andreas Fault, CA

http://education.usgs.gov/california/

pp1515/chapter2/fig2-21.jpgRight Lateral Slip, Izmit, Turkey, 1999 Quake

http://www.geo.uib.no/jordskjelv/

index.php?topic=earthquakes&lang=en

Right Lateral Fault in Asphalt

www.uwsp.edu/.../fault_transform_photo.html

EXAMPLES OFSTRIKE-SLIP FAULTS

San Andreas Fault, CA

http://education.usgs.gov/california/

pp1515/chapter2/fig2-21.jpgRight Lateral Slip, Izmit, Turkey, 1999 Quake

http://www.geo.uib.no/jordskjelv/

index.php?topic=earthquakes&lang=en

Right Lateral Fault in Asphalt

www.uwsp.edu/.../fault_transform_photo.html

STRATIGRAPHIC INDICATORS OF FAULTS

►Truncation of Units

Unconformity? Intrusion?

►Omission of units

Unconformity?

►Repetition of Units

Folding?

PHYSIOGRAPHIC INDICATORS OF FAULTS

► Scarps

Terraces? River Valley?

► Linear Valleys

Erodable Strata?

► Pull-Apart Basins or Push-Up Ridges

http://www.bedford.k12.ny.us/flhs/

science/images/sanandreas1.jpg

MESOSCOPIC STRUCTURALINDICATORS OF FAULTS

► Slickensides

►Drag Folds

►Vein Sets

Horizontal Slickensideshttp://wso.williams.edu/~cgarvin/photos/

slickensides.jpg

Drag Fold Associated with Thrust Faultweb.umr.edu/.../205-mile_landslides.htm

MESOSCOPIC STRUCTURALINDICATORS OF FAULTS

► Slickensides

►Drag Folds

►Vein Sets

Horizontal Slickensideshttp://wso.williams.edu/~cgarvin/photos/

slickensides.jpg

Drag Fold Associated with Thrust Faultweb.umr.edu/.../205-mile_landslides.htm

Why do certain types of faults have

predictable dips?

FRACTURE EXPERIMENTS

►To understand the mechanics of fracturing, geologists perform controlled experiments on rock cores

►Rock is wrapped in a metal jacket

Provides confining pressure

σ2 and σ3 are perpendicular to core length

► Piston presses along the length of the core

σ1 is parallel to core length

FRACTURE EXPERIMENTS

► Core fractures with a consistent pattern

FRACTURE EXPERIMENTS

► Core fractures with a consistent pattern

► Two sets of fractures with an acute angle of approximately 60º between them

Conjugate fractures

FRACTURE EXPERIMENTS

► Core fractures with a consistent pattern

► Two sets of fractures with an acute angle of approximately 60º between them

Conjugate fractures

►σ1 bisects the acute

angle between the conjugate fractures

σ1

σ1

FRACTURE EXPERIMENTS

►σ1 bisects the acute angle between the conjugate set

►σ3 bisects the obtuse angle between the conjugate set

►σ2 runs parallel to the intersection of the conjugate set

σ1

σ1

σ3

ANDERSON’S THEORY OF FAULTING

►The surface of Earth is a “free surface”, that is, it is a surface that cannot support shear stress Shear stress = 0

► Shear stress = 0 along principal stress planes Principal stress directions must be parallel or

perpendicular to a principal stress plane

►The surface of Earth is a principal stress plane At the surface of Earth, the principal stress directions

must be parallel or perpendicular to a principal stress plane

COMBINING FRACTURE EXPERIMENTS WITH

ANDERSON’S THEORY

►σ1 must be either parallel or perpendicular to Earth’s surface

► Fractures form in conjugate sets around σ1

►The orientation of fractures (faults) will vary with the orientation of σ1 and the other principal stresses

►There are only 3 possible orientations of the principal stresses relative to the Earth’s surface

σ1 Vertical…

► Produces fractures/faults that dip at 60º

►Motion is along dip with hanging-wall down

►NORMAL FAULTS

σ1 Horizontal, σ3 Vertical…

► Produces fractures/faults that dip at 30º

►Motion is along dip with hanging-wall up

►THRUST FAULTS

σ1 Horizontal, σ2 Vertical…

► Produces fractures/faults that dip at 90º

►Motion is along strike

► STRIKE-SLIP FAULTS