Geology 3120: Fault Rocks. Brittle shear zone, CO Natl Mon

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1 Geology 3120: Fault Rocks Brittle shear zone, CO Natl Mon

2 Outline Fault rocks and strength Fault zone processes Fault rocks Example from Death Valley

3 Fault Rocks and Strength

4 Brittle Fault Zone Processes Stress exceeds rock strength Fracturing Grinding and crushing Friction Fluid flow Plastic Shear Zone Processes Crystal plastic processes (dislocation climb and glide) Fluid flow Neomineralization

5 Fault Rocks Brittle Breccia Cataclasite Pseudotachylite Ductile Mylonite

6 Cataclastic Processes Brittle Grain size reduction by brittle fracture Randomly oriented fabrics Fluid flow + cementation/silicification Frictional melting (pseudotachyllite) Ductile Grain size reduction by crystal plastic strain Systematically oriented fabrics Development of foliation Development of stretching lineation

7 Breccia 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 Rock Megabreccia Breccia Microbreccia Gouge Clast Size > 0.5 m 0.5 m - 1 mm mm < 0.1 mm

8 Megabreccia - Titus Canyon, Death Valley

9 Breccia and Gouge at microscopic scales Breccia Granular Gouge Clay Gouge 0.5 mm Angular clasts in a finer matrix Generally no preferred orientation, random fabric

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

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

12 Highly silicified fault breccia Death Valley Clast rotation?

13 Weakly silicified fault breccia (siltstone), Basin & Range, NV Not much vein infilling. How much rotation of clasts has occurred?

14 Clay gouge, Maxwell Fault, Fourmile Canyon, Granite protolith How weak/strong is this fault rock?

15 Cataclasite 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 Rock Clast Size Cataclasite mm Ultracataclasite < 0.1 mm

16 Cataclasite - Quartz Sandstone (micro scale) 1 mm

17 Cataclasite, Maxwell Fault, Fourmile Canyon, Granite protolith How weak/strong is this fault rock?

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

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

20 Pseudotachylite Tachylite - basaltic volcanic glass Pseudotachylite - fault rock produced by frictional melting Cryptocrystalline - < 1 µm crystals in an isotropic groundmass (e.g. glass) This example is from Vredefort South Africa, an asteroid impact ejecta sheet of granitic clasts and glass

21 Pseudotachyllite vein in a tonalitic protolith deformed in a brittle lowangle 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?

22 Pseudotachyllite vein in a tonalitic protolith deformed in a brittle lowangle 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

23 Mylonite Shear-induced foliation Grain size reduction by shear Ductile regime

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

25 Fault Rocks and Strength

26 Death Valley

27 Death Valley Badwater normal detachment fault

28 Death Valley Ductile Flow Fabric Coarse Breccia Fractured Footwall

29 Metamorphic Core Complexes 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

30 Weakly Deformed Quartzite Deformation bands within quartz clasts (G)

31 Moderately Deformed Quartzite Quartz grains begin to form a foliation

32 Foliation Development Continued development of foliation due mostly to flattening of quartz crystals and some rotation

33 Mylonite Elongated quartz crystals and mylonitic foliation

34 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)

35 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 How is foliation oriented? K-spar Black = Qtz K-spar

36 Ultramylonite from Silver Plume granite Big Elk Meadows shear zone

37 Ultramylonite from Silver Plume granite Big Elk Meadows shear zone Mylonite Ultramylonite

38 Mylonitic orthogneiss - Nash Fork Shear Zone, Medicine Bow Mtns, So. Wyoming. K-spar Bio+Qtz K-spar

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

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

41 References 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,