2. Introduction
S Shear zones and their importance
S Shear zone analyse
S The geometry of a mylonite zone
S Microstructures of mylonites in terms of shear sense
S Eriboll mylonite classification
3. Shear zones
- Shear Zones are tabular zones of strain localization in the crust;
-Shear zones occur on all scales;
-Like faults, shear zones tend to involve a considerable amount of wall-parallel movement
(Simple shear)
-Just like faults, shear zones may be reverse, normal, strike slip or show oblique
displacements.
7. Analyzing shear zone
Step 3 Step 4
http://www.see.leeds.ac.uk/structure/shearzones/
8. Analyzing shear zone
Step 5 Step 6
http://www.see.leeds.ac.uk/structure/shearzones/
9. Mylonite
S The word “mylonite” derives from the Greek word and translated as a mill. 1st
described by (Lapworth 1885)
S Foliated and lineated metamorphic rock-ductile deformation (ductile flow)
S Contains fabric elements with monoclinic shape symmetry
S Mylonites occur in high strain zones known as “mylonite zone” interpreted as
exhumed, “fossil” ductile shear zone
S Grain size in the mylonite is usually smaller than that in the wall rock
S Dynamics of mylonite development – “softening” or strain softening”
S Characteristic fabric elements - porphyroclasts
10. The geometry of a mylonite zone and the nomenclature
used
Parallel to the aggregate lineation, the most common
types of shear sense indicators (For thin section)
11. Mylonites in thin section
Mylonite derived from pelitic gneiss
Quarts-feldspar mylonite
15. Eriboll Mylonites Classification
S Quarts Mylonite
-The rock has a very well developed platy
texture, and splits into thin flags
-It shows the intense fine- banding
-surface has a striation on it that shows the
direction of movement of the thrust sheet
-Probable protolith: Eriboll Sandstone
Formation
16. Eriboll Mylonite Classification
Oyster shell mylonite
S Mylonitic chlorite-muscovite
phyllonite, locally garnet
bearing
S Also described as a “Variegated
schist” which could have the
appearance of frilled schist,
veined schist, mica schist-
Phyllitic mylonites (Lapworth
1885a)
30 cm
S Protolith mainly Lewisian gneiss
17. S Green Mylonite
Eriboll
Mylonites
- Mylonitic Lewisian complex:
Green Mylonite
-Very rich in chlorites ,
hornblende, amphibole, feldspar
and mica
-Medium metamorphic grade
-Pegmatite fragments (rich in
feldspars)
20. Summary
S Deformation in a shear zone causes development of
characteristic fabrics and mineral assemblages that reflect
P-T conditions, flow type, movement sense and
deformation history in the shear zone;
S Mylonites act as a “fossil indicator” to recognize a past
presence of high strain plastic shear zones;
S Eriboll mylonites are result of a complex deformation event
21. References
S Micro-tectonics , Cees W.Passchier, Rudolph A.J.Trouw;
S http://www.see.leeds.ac.uk/structure/shearzones/
S http://www.earth.ox.ac.uk/~oesis/nws/nws-rocktypes.html
S Haakon Fossen, Structural Geology;
Editor's Notes
Shear zones can be subdivided into brittle zones or faults, and ductile zones. Ductile shear zones are usually active at higher metamorphic conditions than brittle shear zones. The depth of the transition between brittle and ductile behavior depends on factors such as bulk strain rate, geothermal gradient, grain size, lithotype, fluid pressure, orientation of the stress field and pre-existing fabrics
Distribution of the main types of fault rocks with depth in the crust. A) Schematic cross section through a transcurrent shear zone;
