lecture 7 Foliations & lineations

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Kristallingeologie lecture 7 Foliations & lineations 28 participants Results test A, 2008 Maximum 70 points Pass!35 points (!

rock Foliations and lineations must be Penetrative - Not: slickensides fault striations What makes a foliation?

1 Kristallingeologie lecture 7 Foliations & lineations 28 participants Results test A, 2008 Maximum 70 points Pass!35 points (!50%) Best result 67 points Average result 54 points ("2.3) Worst result 30 points (2 of 28 failed, i.e. 7%) Foliations and lineations Foliation (Foliation) Any planar structure in a rock Lineation (Linear) Any linear structure in a rock Foliations and lineations must be Penetrative - Not: slickensides fault striations What makes a foliation? Alignment of mineral grains or grain aggregates Platy minerals, e.g. mica Flattened grains of normally not-platy minerals e.g. quartz or olivine Flattened mineral aggregates, e.g. pebbles in a deformed conglomerate Alignment of structures Stylolites & solution seams Microfractures Shear bands

Example Another example Foliation formed by different minerals Fibrous

formed by varying crystallographic orientation Foliation formed by

Primary foliations form as part of the original rock Sedimentary

shortly after sedimentation By compaction & rotation of clay minerals

lithified, by tectonic and metamorphic processes Magmatic layering

foliation (Alignment of platy minerals) Magmatic layering in

2 Example Another example Foliation formed by different minerals Fibrous fibrolite (sillimanite) Platy biotite Elongate quartz grains Foliation formed by varying crystallographic orientation Foliation formed by varying grain size varying mineral composition Primary foliations (S 0 ) Primary foliations form as part of the original rock Sedimentary layering Diagenetic & secondary foliations Diagenetic foliation forms shortly after sedimentation By compaction & rotation of clay minerals Normally parallel to bedding Secondary foliations form after a rock is lithified, by tectonic and metamorphic processes Magmatic layering Chemical/mineralogical layering (rhythmic settling of crystals) Flow foliation (Alignment of platy minerals) Magmatic layering in leucogranite, Elba Island Sedimentary layering (S 0 ) Diagenetic foliation First real tectonic foliation (S 1 )

Mechanisms of foliation formation Foliation classifications Mechanical rotation and bending

tectonic foliation: cleavage Coarse tectonic foliation: schistosity Continuous cleavage Spaced

Cleavage domains Oriented new-growth Foliation usually (initially) forms normal to the maximum

foliation forms // shear plane Spaced cleavage Use of cleavages Disjunctive cleavage No older

3 Mechanisms of foliation formation Foliation classifications Mechanical rotation and bending Solution transfer Crystal plastic flow Foliation = very general term for anything planar Fine tectonic foliation: cleavage Coarse tectonic foliation: schistosity Continuous cleavage Spaced cleavage A.Microlithons B.Cleavage domains Oriented new-growth Foliation usually (initially) forms normal to the maximum compression Parallel to XY-plane of strain ellipsoid In case of shear zones (C-plane), can foliation forms // shear plane Spaced cleavage Use of cleavages Disjunctive cleavage No older tectonic foliation between cleavage domains Crenulation cleavage Micro-folding of older foliation between cleavage domains Cleavages are extremely useful to determine sequence of deformation events S 2 S 0 S 1

4 Disjunctive cleavage Crenulations: small scale Cleavage formed by dissolution seams (stylolites) in low-grade limestones, Biure, Spain Crenulations in amphibolite-facies meta-pelites, Cap de Creus, Spain Crenulations: micro-scale Example of crenulation of a biotite foliation. Amphibolitefacies meta-pelites, Cap de Creus, Spain Crenulations in high-grade meta-pelites, Finland

Amphibolite-facies metapelites, Cap de Creus, Spain

crenulation and solution transfer can lead to a

quartzitic Q-domains Mica-rich layers: pelitic

biotite, garnet and elongate quartz grains Q P Q

opposed to sedimentary layers Very coarse PQ-fabric

perpendicular to maximum shortening Cleavage

5 Amphibolite-facies metapelites, Cap de Creus, Spain PQ-fabric Crenulations are barely visible in section! crenulation lineation P Q P Combination of crenulation and solution transfer can lead to a tectonic layering with Quartz-rich layers: quartzitic Q-domains Mica-rich layers: pelitic P-domains A clear foliation formed by alignment of biotite, garnet and elongate quartz grains Q P Q Tectonic layering is generally very regular, as opposed to sedimentary layers Very coarse PQ-fabric Axial planar cleavage Cleavage normally develops perpendicular to maximum shortening Cleavage therefore usually develops parallel to the axial plane Therefore normally in XY-plane Looking from the side Looking down on layers S 0 S 0 S 1

$refraction (Brechung)$ cleavage in

$refraction vergence Cleavage$ has different orientation in

6 Axial planar cleavage Continuous gneissic cleavage in migmatite, Masku "Riviera", Finland Cleavage refraction (Brechung) Axial planar cleavage Axial-planar crenulation cleavage in amphibolite-facies rocks, Cap de Creus, Spain Cleavage refraction vergence Cleavage has different orientation in different layers Cleavage in competent layers makes angle with axial plane This produces cleavage fans Competent: larger angle with layering Incompetent: smaller angle with layering

7 Lineations - 1. Trace lineations Lineation = any linear element that occurs pervasively in a rock Trace lineations are formed by intersecting foliations Intersection lineation (e.g. L 23 ) or "-lineation for lineation on bedding A crenulation lineation is a special type of intersection lineation

older foliation gives you the fold axis Here two

lineation formed by aligned objects with a linear

regates (e.g. pebbles) Grain lineation Formed by

grains that are normally linear in amphiboles)

Oriented growth of elongate minerals (minor

Stretching of minerals and mineral aggregates -->

8 Intersection lineations and fold axis Intersection of an axial planar cleavage with an older foliation gives you the fold axis Here two examples of intersection (") lineations Lineations - 2. Object lineations Object lineation = any lineation formed by aligned objects with a linear shape " 1 S 1 S 0 S 0 " 1 Aggregate lineation Formed by linear grain aggregates (e.g. pebbles) Grain lineation Formed by elongate grains that are normally not linear in shape (e.g. quartz) Mineral lineation Formed by elongate grains that are normally linear in shape (e.g. amphiboles) Formation of object lineations 1. Oriented growth of elongate minerals (minor importance) 2. Stretching of minerals and mineral aggregates --> stretching lineation (Streckungslinear) Formation of object lineations 3. Passive rotation of rigid minerals or mineral aggregates Common in high strain shear zones ± parallel to X-direction of strain ellipse Magmatic mineral lineation of aligned feldspars in granite, Elba Island

2. Stretching lineation L, S, LS tectonites Tectonite = strongly deformed rock S-tectonite: foliation dominates L-tectonite: lineation dominates LS-tectonite: both important Stretched

$But, refraction changes orientation of cleavage Important lineations: Intersection lineation -> often indicates fold axes Stretching lineation -> indicates shear direction (X-axis of$

9 2. Stretching lineation L, S, LS tectonites Tectonite = strongly deformed rock S-tectonite: foliation dominates L-tectonite: lineation dominates LS-tectonite: both important Stretched conglomerate, South Australian coast The lineation indicates the direction of maximum stretching: the tectonic transport direction The shear direction in shear zones Summary Rocks can contain foliations and lineations Some important foliations Primary foliation (S 0 ) Axial planar cleavage Crenulation cleavage Cleavage // axial plane // XY-plane of finite strain But, refraction changes orientation of cleavage Important lineations: Intersection lineation -> often indicates fold axes Stretching lineation -> indicates shear direction (X-axis of strain)

GY403 Structural Geology. Tectonite Fabrics

GY403 Structural Geology. Tectonite Fabrics GY403 Structural Geology Tectonite Fabrics Tectonite Fabric A tectonite is a rock that possess a penetrative fabric consisting of cleavage, foliation and/or lineation Tectonite fabrics are associated with