Ductile Shear Zones!

1. Ductile Shear Zones!

2. ductile shear zone “zone”: area with higher strain than surrounding rock This is heterogeneous strain. “shear”: simple shear dominates “ductile” deformation mechanisms similar to faults in that displacement occurs, but no fracture forms

3. Strain distribution in a Shear zone

7. potentially can determine: • sense of displacement • amount of displacement • amount of strain

8. shear zones: offset markers marker shows gradual deflection marker shows discrete offset from: Davis and Reynolds, 1996

9. deflection and offset of markers across shear zones--sense of shear similar terminology to faults

10. relationship of shear zones at depth to faults near surface thrust displacement normal displacement from: Davis and Reynolds, 1996

11. consider a fault from surface to depth… brittle (frictional) shallow; ductile (plastic) deep T°C depth 100° gouge frictional brittle-plastic transition cataclasite 10 km 300° quartz plasticity mylonite greenschist plastic 500° feldspar plasticity 20 km amphibolite sd relative crustal strength curve fault rock deformation mode

12. Mylonites

13. Mylonites often have lineations

14. L-S tectonites have both foliation and lineation

15. These feldspars are mostly brittlely deformed

16. Feldspars are not as deformed as quartz

17. Quartz is black, feldspars are elongated

18. a refresher on mylonite from: http://www.rci.rutgers.edu/~geolweb/slides.html from: http://www.geolab.unc.edu/Petunia/lgMetAtlas/meta-micro marble mylonite and quartz mylonite form at lower temperatures • dynamic recrystallization of calcite > 250°C • dynamic recrystallization of quartz > 300°C feldspar mylonites form at higher temperatures • dynamic recrystallization of feldspar > 450°C

19. http://www.geo.umn.edu/teaching/microstructure/images/079.htmlhttp://www.geo.umn.edu/teaching/microstructure/images/079.html types of mylonites protomylonite: matrix is < 50% of rock ultramylonite: matrix is 90-100% of rock rocks with 50-90% matrix simply called mylonites http://uts.cc.utexas.edu/~rmr/images protomylonite ultramylonite mylonite

20. main goal is to identify sense of shear: need shear-sense indicators where to look? optimal surfaces are those perpendicular to foliation or shear zone boundaries shear zone and foliation 1) determine orientation of shear zone 2) find perpendicular (profile) plane 3) identify line of transport …direction along which relative displacement occurred… (in perpendicular plane) SOS plane perpendicular plane is sense-of-shear plane (SOS) from van der Pluijm and Marshak, 1997

21. …now we know to look in SOS plane for indicators… what are they? • offset markers • foliations • S-C fabrics and shear bands • grain-tail complexes • disrupted grains (mica fish) • folds offset markers usually obvious …make sure similar features on both sides are same from: http://www.leeds.ac.uk/learnstructure/index.htm

22. foliations from: Davis and Reynolds, 1996

23. what does foliation subparallel to boundary in center imply? --either: • coaxial strain (normal to zone) • noncoaxial strain with very high shear • foliation represents very thin shear zones …this leads to S-C fabrics… from: Davis and Reynolds, 1996

24. S-C fabrics most shear zones have one foliation at angle < 45° to boundary; this foliation is s-foliation (schistosité from French); …crystal-plastic processes elongate crystals to extension another foliation parallels shear zone boundaries; this foliation is c-foliation (cisaillement from French); …shear direction is within c plane a third foliation may be oriented oblique to boundary; this foliation is c’-foliation and crenulates mylonitic foliation; …shear bands… from van der Pluijm and Marshak, 1997 s points in shear direction; displacement on c’ is same as shear zone

25. S-C pattern is similar to that for foliation in shear zone as a whole from: Davis and Reynolds, 1996

26. s-c fabrics • s points in direction of shear • c parallel to shear direction • c’ displacement same as that of shear zone from: http://www.earth.monash.edu.au/Teaching/mscourse

27. pressure shadows form on flanks of rigid inclusions in shear zones …rigid inclusion shields matrix on flanks from strain… …crystallization of quartz, calcite, chlorite, etc. most pressure shadows are microscopic--see in thin-section growth accompanies each increment of extension orientation of fibers depends on coaxial versus noncoaxial (rotational) strain from: Davis and Reynolds, 1996

28. different types of pressure shadows: pyrite: material mineralogically same as matrix but different from inclusion …fibers grow in crystallographic continuity with matrix crinoid: material similar to inclusion not matrix …fibers grow in crystallographic continuity with inclusion composite: aspects of both from: Davis and Reynolds, 1996

29. Impressive evidence for rotation of cleavage during its formation can sometimes be read from fibrous mineral growth in the strain shadows of resistant minerals such as pyrite (From Passchier and Trouw, 1996)

30. tail grain tail grain-tail complexes (inclusions; porphyroclasts; porphyroblasts) grains in matrix may have tails that form during deformation …tails are distinguishable from matrix …grains may be …inclusions …porphyroclasts (relics from protolith) …porphyroblasts (grow during deformation) …tails may be.. ..attenuated, preexisting minerals ..dynamic recrystallization at grain rim ..synkinematic metamorphic reactions grains are rigid bodies that rotate during deformation …tails give sense of displacement… to use grain-tail complexes to indicate shear-sense, need reference frame…relative to shear zone foliation.. …two “winged” types of tails: s-type and -type

31. two asymmetric types: s-type and -type wedge-shaped tails that do not cross reference plane when tracing tail away from grain; looks like s s right-lateral tails wrap around grain so they cross-cut reference plane when tracing tail away from grain; looks like   right-lateral from van der Pluijm and Marshak, 1997 reference plane is shear zone foliation right-lateral (dextral) shear: clockwise rotation left-lateral (sinistral) shear: counter-clockwise rotation

32. Sense of Shear Indicators

33. two-types related: relationship between rate of crystallization and rotation of grain …formation fast relative to rotation: s-type …rotation fast relative to formation: d-type (tail dragged and wrapped around grain) presence of both types indicates different: • rates of tail growth • initial grain shape • times of tail formation development of s from d http://www.geo.umn.edu/teaching/microstructure/images/079.html from van der Pluijm and Marshak, 1997

34. Shear Sense Indicators

35. other minerals, such as phyllosilicates, display useful geometry phyllosilicate grains (micas) connected by mylonitic foliation …basal planes oriented at oblique angle to foliation… …point in direction of instantaneous elongation grains have stair-step geometry in direction of shear when large enough to see in hand specimen, …look like scales on a fish (“mica fish”) from van der Pluijm and Marshak, 1997 from: Simpson, Microstructures CD-ROM

36. can determine asymmetry of mica fish by observing reflections in sunlight… …fish flash… • mark north arrow on sample • put back to sun and sample in front of you • view parallel to lineation • tilt sample • note if flashy or dull from: Davis and Reynolds, 1996

37. veins …veins commonly associated with shear zones… …form perpendicular to instantaneous extension… …initially form at 45° to shear zone… …subsequently rotate to steeper angle while new part of vein forms at 45° from: http://www.rci.rutgers.edu/~geolweb/slides.html from: http://www.science.ubc.ca/~eosweb/slidesets/keck

39. Evidence for Rotation during non-coaxial deformation Garnet in Qtz-mica schist