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Death Valley Faultfault Rocksrocks Andand Strengthstrength Brittlebrittle Faultfault Zonezone Processesprocesses

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Death Valley Faultfault Rocksrocks Andand Strengthstrength Brittlebrittle Faultfault Zonezone Processesprocesses GeologyGeology 3120:3120: FaultFault RocksRocks Brittle shear zone, CO Natl Mon OutlineOutline • Fault rocks and strength • Fault zone processes • Fault rocks • Example from Death Valley FaultFault RocksRocks andand StrengthStrength BrittleBrittle FaultFault ZoneZone ProcessesProcesses • Stress exceeds rock strength • Fracturing • Grinding and crushing • Friction • Fluid flow PlasticPlastic ShearShear ZoneZone ProcessesProcesses • Crystal plastic processes (dislocation climb and glide) • Fluid flow • Neomineralization FaultFault RocksRocks BrittleBrittle • Breccia • Cataclasite • Pseudotachylite DuctileDuctile • Mylonite CataclasticCataclastic ProcessesProcesses BrittleBrittle • Grain size reduction by brittle fracture • Randomly oriented fabrics • Fluid flow + cementation/silicification • Frictional melting (pseudotachyllite) DuctileDuctile • Grain size reduction by crystal plastic strain • Systematically oriented fabrics • Development of foliation • Development of stretching lineation 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 FaultFault RockRock ClastClast SizeSize MegabrecciaMegabreccia >> 0.50.5 mm BrecciaBreccia 0.50.5 mm -- 11 mmmm MicrobrecciaMicrobreccia 11 -- 0.10.1 mmmm GougeGouge << 0.10.1 mmmm MegabrecciaMegabreccia -- TitusTitus Canyon,Canyon, DeathDeath ValleyValley BrecciaBreccia andand GougeGouge atat microscopicmicroscopic scalesscales Breccia Granular Gouge Clay Gouge 0.5 mm • Angular clasts in a finer matrix • Generally no preferred orientation, random fabric Limestone fault breccia Calcite veins (composition similar to wallrock) How much rotation of clasts has occurred? Limestone fault breccia Calcite veins (composition similar to wallrock) How much rotation of clasts has occurred? Highly silicified fault breccia Death Valley Clast rotation? Weakly silicified fault breccia (siltstone), Basin & Range, NV Not 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 FaultFault RockRock ClastClast SizeSize CataclasiteCataclasite 0.10.1 -- 1010 mmmm UltracataclasiteUltracataclasite << 0.10.1 mmmm CataclasiteCataclasite -- QuartzQuartz SandstoneSandstone (micro(micro scale)scale) 1 mm Cataclasite, Maxwell Fault, Fourmile Canyon, Granite protolith How 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 PseudoPseudotachylitetachylite • 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 Vredefort South Africa, an asteroid impact ejecta sheet of granitic clasts and glass Pseudotachyllite vein in a tonalitic protolith deformed in a brittle low- angle normal fault (West Salton Detachment) 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 tonalitic protolith deformed in a brittle low- angle normal fault (West Salton Detachment) 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? FaultFault RocksRocks andand StrengthStrength DeathDeath ValleyValley DeathDeath ValleyValley Badwater normal detachment fault DeathDeath ValleyValley Ductile Flow Fabric Coarse Breccia Fractured Footwall MetamorphicMetamorphic CoreCore ComplexesComplexes Deeply penetrating faults span the brittle to plastic regimes Brittle faults are typically narrower than plastic shear zones Deeper fault rocks are overprinted by shallower ones WeaklyWeakly DeformedDeformed QuartziteQuartzite Deformation bands within quartz clasts (G) ModeratelyModerately DeformedDeformed QuartziteQuartzite Quartz grains begin to form a foliation FoliationFoliation DevelopmentDevelopment Continued development of foliation due mostly to flattening of quartz crystals and some rotation MyloniteMylonite Elongated quartz crystals and mylonitic foliation Mylonitic Silver Plume granite from Big Elk Meadows shear zone near Pinewood Sprgs CO (near RMNP) K-spars brittlely deformed Quartz is highly strained by crystal plastic processes Shear Zone Temp = 300C (qtz), < 450C (K-spar) Mylonitic Silver Plume granite from Big Elk Meadows shear zone near Pinewood Sprgs CO (near RMNP) K-spar K-spars brittlely deformed Quartz is highly strained by crystal plastic processes Black = Qtz K-spar Shear Zone Temp = 300C (qtz), < 450C K-spar How is foliation oriented? Ultramylonite from Silver Plume granite Big Elk Meadows shear zone Ultramylonite from Silver Plume granite Big Elk Meadows shear zone Mylonite Ultramylonite Mylonitic orthogneiss - Nash Fork Shear Zone, Medicine Bow Mtns, So. Wyoming. K-spar Bio+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 ReferencesReferences http://www.geology.washington.edu/~cowan/faultrocks.html#photo Scholz, C. H., The Mechanics of Earthquakes and Faulting, 2nd. ed., Cambridge University Press, 471 p., 2002 Coney, 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.htm http://earth.leeds.ac.uk/assynt/quartzmfr.htm.
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