Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone...
Transcript of Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone...
![Page 1: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/1.jpg)
Geology 3120:Geology 3120:Fault RocksFault Rocks
Brittle shear zone, CO Brittle shear zone, CO NatlNatl MonMon
![Page 2: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/2.jpg)
OutlineOutline
• Fault rocks and strength
• Fault zone processes
• Fault rocks
• Example from Death Valley
![Page 3: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/3.jpg)
Fault Rocks and StrengthFault Rocks and Strength
![Page 4: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/4.jpg)
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
![Page 5: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/5.jpg)
Fault RocksFault Rocks
• Breccia
• Cataclasite• Pseudotachylite
• Mylonite
BrittleBrittle
DuctileDuctile
![Page 6: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/6.jpg)
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
![Page 7: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/7.jpg)
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
![Page 8: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/8.jpg)
MegabrecciaMegabreccia -- Titus Canyon, Death ValleyTitus Canyon, Death Valley
![Page 9: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/9.jpg)
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
![Page 10: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/10.jpg)
Limestone fault brecciaCalcite veins (composition similar to wallrock)How much rotation of clasts has occurred?
![Page 11: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/11.jpg)
Limestone fault brecciaCalcite veins (composition similar to wallrock)How much rotation of clasts has occurred?
![Page 12: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/12.jpg)
Highly silicified fault brecciaDeath Valley
Clast rotation?
![Page 13: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/13.jpg)
Weakly silicified fault breccia (siltstone), Basin & Range, NVNot much vein infilling.How much rotation of clasts has occurred?
![Page 14: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/14.jpg)
Clay gouge, Maxwell Fault, Fourmile Canyon, Granite protolith
How weak/strong is this fault rock?
![Page 15: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/15.jpg)
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
![Page 16: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/16.jpg)
Cataclasite Cataclasite -- Quartz Sandstone (micro scale) Quartz Sandstone (micro scale)
1 mm
![Page 17: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/17.jpg)
Cataclasite, Maxwell Fault, Fourmile Canyon, Granite protolithHow weak/strong is this fault rock?
![Page 18: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/18.jpg)
Strain Gradient, Maxwell fault (Granite -> Cataclasite -> Gouge)
![Page 19: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/19.jpg)
Strain Gradient across the Laguna Salada fault (Dextral+Normal) (Tonalite -> Fractured Tonalite -> Breccia -> Cataclasite)Hydrothermal minerals include epidote + chlorite
![Page 20: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/20.jpg)
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
![Page 21: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/21.jpg)
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?
![Page 22: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/22.jpg)
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
![Page 23: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/23.jpg)
MyloniteMylonite • Shear-induced foliation
• Grain size reduction by shear
• Ductile regime
![Page 24: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/24.jpg)
MyloniteMylonite • For a quartz rich protolith, what temperature is required for this rock to form?
![Page 25: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/25.jpg)
Fault Rocks and StrengthFault Rocks and Strength
![Page 26: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/26.jpg)
Death ValleyDeath Valley
![Page 27: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/27.jpg)
Death ValleyDeath Valley
Badwater normal detachment fault
![Page 28: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/28.jpg)
Death ValleyDeath Valley
Ductile Flow FabricCoarse BrecciaFractured Footwall
![Page 29: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/29.jpg)
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
![Page 30: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/30.jpg)
Weakly Deformed QuartziteWeakly Deformed Quartzite
Deformation bands within quartz clasts (G)
![Page 31: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/31.jpg)
Moderately Deformed QuartziteModerately Deformed Quartzite
Quartz grains begin to form a foliation
![Page 32: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/32.jpg)
Foliation DevelopmentFoliation Development
Continued development of foliation due mostly to
flattening of quartz crystals and some rotation
![Page 33: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/33.jpg)
MyloniteMylonite
Elongated quartz crystals and mylonitic foliation
![Page 34: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/34.jpg)
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)
![Page 35: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/35.jpg)
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
![Page 36: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/36.jpg)
Ultramylonite fromSilver Plume graniteBig Elk Meadowsshear zone
![Page 37: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/37.jpg)
Ultramylonite
Mylonite
Ultramylonite fromSilver Plume graniteBig Elk Meadowsshear zone
![Page 38: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/38.jpg)
Mylonitic orthogneiss - Nash Fork Shear Zone, Medicine Bow Mtns, So. Wyoming.
K-sparBio+Qtz
K-spar
![Page 39: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/39.jpg)
Ultramylonite Rocky Mtn Natl Park, orthogneiss protolith. Note brittle overprinting (randomly oriented clasts in breccia layer) on mylonitic foliation
![Page 40: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/40.jpg)
Ultramylonite Rocky Mtn Natl Park, orthogneiss protolith. Note brittle overprinting (randomly oriented clasts in breccia layer) on mylonitic foliation
![Page 41: Geology 3120: Fault Rocksgeode.colorado.edu/.../coursenotes/3120lect_8.pdf · Brittle Fault Zone Processes • Stress exceeds rock strength • Fracturing • Grinding and crushing](https://reader034.fdocuments.net/reader034/viewer/2022050213/5f5fc25f614cb91b6953c5b3/html5/thumbnails/41.jpg)
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