shear zone-mylonites
TRANSCRIPT
S
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;