S-C Mylonites

1. S-C Mylonites G.S. Lister and A.W. Snoke Presented by: Justin Sperry and David Wheeler

2. Outline • Properties of S-C mylonites • Kinematic indicators • S-C tectonites

3. S-C Mylonites • General mylonite properties • Description of Type I and II S-C fabrics • Kinematic indicators

4. Mylonites • Crystal-plastically deformed rock • Typically locus of intense non-coaxial deformation • Generally develop strong crystallographic fabric

5. Mylonites • Extensive dynamic recrystallization • Typically reduced grain size • Cataclastic processes possible in non-matrix minerals

6. Type I S-C Fabrics • Two oblique planes of foliation • S-surfaces formed by accumulation of finite strain • C surfaces formed by localized high shear strains

7. Type I S-C Fabrics From Lister and Snoke (1984)

8. Type I S-C Fabrics • S and C fabrics do not necessarily develop simultaneously • Can be used to determine sense of shear • Common in mylonites from granites, granodiorites and gneisses

9. Type I S-C Fabrics • Draw attention to areas of non-coaxial laminar flow • Can form synchronously or S-surfaces can be cut by late stage shear bands • Shear bands can cut pre-existing foliation in separate event

10. Type II S-C Fabrics • Found in quartz-mica rocks • White mica ‘fish’ produced by boudinage and microfaulting define S-surfaces • Grain scale shearing and discontinuities linked by phyllosilicate trails define C-surfaces

11. Type II S-C Fabric • C-surfaces primary mesoscopic foliation • S-surfaces not always discernible without microscope • S-surfaces appear to form synchronously by dynamic recrystallization of quartz

12. Type II S-C Fabrics From Lister and Snoke(1984)

13. Type II S-C Fabrics • S-surfaces in Type II useful indicators of shear sense • Widespread occurrence as a useful kinematic indicator

14. Kinematic Indicators • Mica ‘fish’ • Oblique lineations in dynamically recrystallized quartz aggregates • Quartz C-axis fabrics

15. Mica ‘Fish’ • Mica fish are boudinaged, microfaulted or folded mica grains • Deformed by both brittle and crystal plastic processes • Micaceous cleavage is 30° to subparallel with C-surfaces

16. Mica ‘fish’ • Microfaults and microshears can be synthetic or antithetic to bulk flow field • Based on orientation of micaceouscleavage • Generally either subparallel or tilted back against bulk shear sense

17. Mica ‘fish’ From Lister and Snoke (1984)

18. Mica ‘fish’ • One common type of mica ‘fish’ is a consistent kinematic indicator • Originates as listric normal microfaults antithetic to shear sense • Drawn apart by displacement continuity and separated from the initial grain

19. Mica ‘fish’ From Lister and Snoke (1984)

20. Oblique Foliations in Quartz • Foliations in quartz adjacent to trails of mica ‘fish’ • Defined by elongate grains or zones of grains, and grain boundary alignment • Can be a sign of steady state foliation

21. Oblique Foliations in Quartz • Ribbons of recrystallized grains can be used as a shear sense indicator • Orientation of oblique foliation determined by deformation processes • Dynamic recrystallization does not allow for direct strain observations from the grains

22. Oblique Foliations in Quartz • Crystal-plastic extension causes low angle foliation to the C-axis • Subgrain boundary rotation causes GBM recrystallization at high angles • GBM consumes elongate grains

23. Oblique Foliations in Quartz From Lister and Snoke(1984)

24. Quartz c-axis fabrics • Asymmetry in quartz fabrics may serve as an indicator of past deformational environments • Skeletal analysis • Asymmetry of the c-axis or a-axis orientation distribution

25. Quartz c-axis fabric indicators • Asymmetry of fabric gives a sense of shear • Consistent with other indicators • Under extremely high shear strain fabric skeletons should be aligned with C-surfaces • Estimates of magnitude of shear often low

26. Mica ‘Fish’ • Easily recognized where the original microstructure is preserved • More complex histories are common • Recrystallization is often responsible for the elimination of past microstructures

27. Advantages of Mica ‘Fish’ • Quartz fabrics are highly susceptible to alteration in subsequent deformations • Mica Fish are generally more resistant to modification

28. Resistance of Mica ‘Fish’ • Recrystallization often occurs without diffusion of material across grain boundaries • Accommodated by changes in crystal orientation • Under this mechanism grain boundaries between different mineral species are not mobile • Under this mechanism mica crystals in quartz fabrics can’t change morphologically

30. Resistance of Mica ‘Fish’ • Recrystallization with diffusion across grain boundaries • Happens when conditions, such as temperature, favor diffusion • Trails of small mica grains often link larger crystals • Mica ‘Fish’ are often not preserved

32. S-C Tectonites • Use fabrics to determine methods of formation • Can be used to describe a broad range of rocks with multiple fabrics • Should be used contextually with the macrostructure

34. S-C Tectonites • Fabrics may form at the same time or in separate periods of deformation • Alternatively a slight rotation of the bulk flow can cause the deformation of previous fabrics

35. Shear bands and their rheologic importance • Commonly form after foliations • Strain accumulates in narrow zones • Softening of sheared material perpetuates the concentration of deformation

36. Shear bands • Formation • Penetrative plastic deformation (significant non-coaxial component) • Plastic deformation concentrates into narrow zones • Brecciation and cataclasis • Formation of discrete brittle faults

37. Conclusions • Mica ‘fish’ serve as a consistent kinematic indicator • S-C mylonite fabrics contain many useful kinematic indicators • Using S-C tectonites we can move away from standard geometric analysis of foliations