Geology 3120:Geology 3120:Fault RocksFault Rocks

Brittle shear zone, CO Brittle shear zone, CO NatlNatl MonMon

OutlineOutline

• Fault rocks and strength

• Fault zone processes

• Fault rocks

• Example from Death Valley

Fault Rocks and StrengthFault Rocks and Strength

Brittle Fault Zone ProcessesBrittle Fault Zone Processes

• Stress exceeds rock strength

• Fracturing

• Grinding and crushing

• Friction

• Fluid flow

Plastic Shear Zone ProcessesPlastic Shear Zone Processes

• Crystal plastic processes (dislocation climb and glide)

• Fluid flow

• Neomineralization

Fault RocksFault Rocks

• Breccia

• Cataclasite• Pseudotachylite

• Mylonite

BrittleBrittle

DuctileDuctile

CataclasticCataclastic ProcessesProcesses

• Grain size reduction by brittle fracture

• Randomly oriented fabrics

• Fluid flow + cementation/silicification• Frictional melting (pseudotachyllite)

• Grain size reduction by crystal plastic strain

• Systematically oriented fabrics

• Development of foliation

• Development of stretching lineation

BrittleBrittle

DuctileDuctile

BrecciaBreccia

• Angular clasts in a finer matrix

• Generally no preferred orientation, random fabric

• Noncohesive to compacted

• Cohesive - silicified or mineralized

• Low confining pressure, high fluid pressure

Fault RockFault Rock ClastClast SizeSize

MegabrecciaMegabreccia > 0.5 m> 0.5 mBrecciaBreccia 0.5 m 0.5 m -- 1 mm1 mmMicrobrecciaMicrobreccia 1 1 -- 0.1 mm0.1 mmGougeGouge < 0.1 mm< 0.1 mm

MegabrecciaMegabreccia -- Titus Canyon, Death ValleyTitus Canyon, Death Valley

BrecciaBreccia and Gouge at microscopic scalesand Gouge at microscopic scales

Breccia Granular Gouge Clay Gouge

0.5 mm• Angular clasts in a finer matrix

• Generally no preferred orientation, random fabric

Limestone fault brecciaCalcite veins (composition similar to wallrock)How much rotation of clasts has occurred?

Limestone fault brecciaCalcite veins (composition similar to wallrock)How much rotation of clasts has occurred?

Highly silicified fault brecciaDeath Valley

Clast rotation?

Weakly silicified fault breccia (siltstone), Basin & Range, NVNot much vein infilling.How much rotation of clasts has occurred?

Clay gouge, Maxwell Fault, Fourmile Canyon, Granite protolith

How weak/strong is this fault rock?

CataclasiteCataclasite

• Angular clasts in a finer matrix

• Generally no preferred orientation of clasts

• Cohesive and strongly indurated, more than breccias

• Often highly cemented or silicified

Fault RockFault Rock Clast SizeClast Size

CataclasiteCataclasite 0.1 0.1 -- 10 mm10 mm

UltracataclasiteUltracataclasite < 0.1 mm< 0.1 mm

Cataclasite Cataclasite -- Quartz Sandstone (micro scale) Quartz Sandstone (micro scale)

1 mm

Cataclasite, Maxwell Fault, Fourmile Canyon, Granite protolithHow weak/strong is this fault rock?

Strain Gradient, Maxwell fault (Granite -> Cataclasite -> Gouge)

Strain Gradient across the Laguna Salada fault (Dextral+Normal) (Tonalite -> Fractured Tonalite -> Breccia -> Cataclasite)Hydrothermal minerals include epidote + chlorite

PseudotachylitePseudotachylite

• Tachylite - basaltic volcanic glass

• Pseudotachylite - “fault rock” produced by frictional melting

• Cryptocrystalline - < 1 µm crystals in an isotropic ground-mass (e.g. glass) This example is from VredefortSouth Africa, an asteroid impact ejecta sheet of granitic clasts and glass

Pseudotachyllite vein in a tonaliticprotolith deformed in a brittle low-angle normal fault (West SaltonDetachment) SE Calif.

What sudden dynamic process might cause granitic rocks to melt along a brittle fault zone?What P/T/Fluid conditions are likely to promote this process?

Pseudotachyllite vein in a tonaliticprotolith deformed in a brittle low-angle normal fault (West SaltonDetachment) SE Calif.

What sudden dynamic process might cause granitic rocks to melt along a brittle fault zone? (Large Earthquakes)What P/T/Fluid conditions promote this process? 5km < X < 20 km, < 300C, no H2O

MyloniteMylonite • Shear-induced foliation

• Grain size reduction by shear

• Ductile regime

MyloniteMylonite • For a quartz rich protolith, what temperature is required for this rock to form?

Fault Rocks and StrengthFault Rocks and Strength

Death ValleyDeath Valley

Death ValleyDeath Valley

Badwater normal detachment fault

Death ValleyDeath Valley

Ductile Flow FabricCoarse BrecciaFractured Footwall

Metamorphic Core ComplexesMetamorphic Core Complexes

Deeply penetrating faults span the brittle to plastic regimesDeeply penetrating faults span the brittle to plastic regimes

Brittle faults are typically narrower than plastic shear zonesBrittle faults are typically narrower than plastic shear zones

Deeper fault rocks are overprinted by shallower onesDeeper fault rocks are overprinted by shallower ones

Weakly Deformed QuartziteWeakly Deformed Quartzite

Deformation bands within quartz clasts (G)

Moderately Deformed QuartziteModerately Deformed Quartzite

Quartz grains begin to form a foliation

Foliation DevelopmentFoliation Development

Continued development of foliation due mostly to

flattening of quartz crystals and some rotation

MyloniteMylonite

Elongated quartz crystals and mylonitic foliation

MyloniticSilver Plumegranite fromBig Elk Meadowsshear zone nearPinewood SprgsCO (near RMNP)

K-spars brittlelydeformedQuartz is highlystrained by crystal plastic processes

Shear Zone Temp =300C (qtz), < 450C(K-spar)

MyloniticSilver Plumegranite fromBig Elk Meadowsshear zone nearPinewood SprgsCO (near RMNP)

K-spars brittlelydeformedQuartz is highlystrained by crystal plastic processes

Shear Zone Temp =300C (qtz), < 450CK-spar

How is foliationoriented?

K-spar

Black = Qtz K-spar

Ultramylonite fromSilver Plume graniteBig Elk Meadowsshear zone

Ultramylonite

Mylonite

Ultramylonite fromSilver Plume graniteBig Elk Meadowsshear zone

Mylonitic orthogneiss - Nash Fork Shear Zone, Medicine Bow Mtns, So. Wyoming.

K-sparBio+Qtz

K-spar

Ultramylonite Rocky Mtn Natl Park, orthogneiss protolith. Note brittle overprinting (randomly oriented clasts in breccia layer) on mylonitic foliation

Ultramylonite Rocky Mtn Natl Park, orthogneiss protolith. Note brittle overprinting (randomly oriented clasts in breccia layer) on mylonitic foliation

ReferencesReferenceshttp://www.geology.washington.edu/~cowan/faultrocks.html#photo

Scholz, C. H., The Mechanics of Earthquakes and Faulting, 2nd. ed., Cambridge University Press, 471 p., 2002Coney, P. J., Cordilleran metamorphic core complexes; an overview, in: Crittenden, M.,D., Jr., Coney, P. J., and G. H. Davis (eds), Cordilleran metamorphic core complexes, Geological Society of America Memoir 153, 7-31. 1980.

http://home.earthlink.net/~rhaughy/ROCKS.HTM

http://earth.leeds.ac.uk/assynt/quartzmfr.htm

http://www.lpl.arizona.edu/~rlorenz/pseud.html

http://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/mylonite.X.html

http://www.nps.gov/deva/pphtml/maps.html

http://www.geophysics.rice.edu/department/research/julia1/julia1.html

http://earth.leeds.ac.uk/mtb/background/nwmap.htmhttp://earth.leeds.ac.uk/assynt/quartzmfr.htm