shear zone-mylonites

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Shear Zones-Mylonites

By Abzal Alpysbayev

BSc Petroleum Geology

Introduction

Shear zones and their importance

Shear zone analyse

The geometry of a mylonite zone

Microstructures of mylonites in terms of shear sense

Eriboll mylonite classification

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.

Shear zones

Shear zones

http://www.see.leeds.ac.uk/structure/shearzones/

Analyzing shear zones

1 step 2 step

http://www.see.leeds.ac.uk/structure/shearzones/

Analyzing shear zone

Step 3 Step 4

http://www.see.leeds.ac.uk/structure/shearzones/

Analyzing shear zone

Step 5 Step 6

http://www.see.leeds.ac.uk/structure/shearzones/

Mylonite

The word “mylonite” derives from the Greek word and translated as a mill. 1st described by (Lapworth 1885)

Foliated and lineated metamorphic rock-ductile deformation (ductile flow)

Contains fabric elements with monoclinic shape symmetry

Mylonites occur in high strain zones known as “mylonite zone” interpreted as exhumed, “fossil” ductile shear zone

Grain size in the mylonite is usually smaller than that in the wall rock

Dynamics of mylonite development – “softening” or strain softening”

Characteristic fabric elements - porphyroclasts

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)

Mylonites in thin section

Mylonite derived from pelitic gneiss Quarts-feldspar mylonite

Mylonite outcrop

Sheath fold and highly folded mylonite

Deformed mylonites

50cm

Eriboll Mylonites Classification

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

Eriboll Mylonite Classification

Oyster shell mylonite

Mylonitic chlorite-muscovite phyllonite, locally garnet bearing

Also described as a “Variegated schist” which could have the appearance of frilled schist, veined schist, mica schist-Phyllitic mylonites (Lapworth 1885a)

Protolith mainly Lewisian gneiss

30 cm

Eriboll Mylonites

Green Mylonite

- Mylonitic Lewisian complex: Green Mylonite

-Very rich in chlorites , hornblende, amphibole, feldspar and mica

-Medium metamorphic grade

-Pegmatite fragments (rich in feldspars)

LEWISIAN COMPLEX: Lewisian gneiss/Orthogneiss/AmphiboliteMeta-igneous rocks:

Orthogneiss: mostly metatonalite with enclaves of mafic and ultramafic and rare metasedimentary rocks, locally mylonitic within Moine thrust zone

Mylonitic : interlayered quartsofeldspathic and hornblende bearing orthogneisses with subordinate amphibolite

50cm

Amphibolite

http://www.earth.ox.ac.uk/~oesis/nws/nws-rocktypes.html

Summary

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;

Mylonites act as a “fossil indicator” to recognize a past presence of high strain plastic shear zones;

Eriboll mylonites are result of a complex deformation event

References

Micro-tectonics , Cees W.Passchier, Rudolph A.J.Trouw;

http://www.see.leeds.ac.uk/structure/shearzones/

http://www.earth.ox.ac.uk/~oesis/nws/nws-rocktypes.html

Haakon Fossen, Structural Geology;