Download Chapter 11 Summary

Chapter 11 Summary
A foliation if a planar structure in a
rock that is homogeneously distributed
throughout the volume.
A lineation is a homogenously
distributed linear structure that can
be: surficial, if present along discrete
surfaces, and penetrative, if occur
throughout volume of a rock.
Both foliations and lineations can
be primary or secondary depending on
the type of processes they originate
from.
On orders of tens of centimeters.
11.1 Tectonites
Tectonites: rocks whose structure is a
product of deformation and are commonly
metmorphosed.
-characteristic of orogenic belts and fault
zones
-develop as a result of deformation;
mostly secondary features
S-tectonite- dominated by foliation
L-tectonite- dominated by lineation
Rock Cleavage- tendency of rock to
break/cleave along surfaces of specific
orientation
Layer/Banding- describes planar tabular
features in rocks distinguished by
differences in composition or possibly
texture from adjacent rock
Figure
11.1
classification
provides
11.3 Disjunctive Foliations
-contain thin domains, cleavage domains/
seams, marked by concentrations of
oxides and strongly aligned platy
minerals.
Subdivided into:
• styloliticlong
continuous,
irregular cleavage domains with
tooth-like geometry geometry in
X-section
• anastomosing- long continuous,
wavy cleavage domains that
form irregular network outlining
lenticular microliths
• rough- in rocks with abundant
sand-size
material;
sort
discontinuous cleavage domains
of highly oriented platy materials
that bound the coarse grains
• smooth- cleavage domains long,
continuous,
smooth
with
concentrations of highly oriented
platy minerals
11.4 Crenulation Foliations
-formed by harmonic wrinkles/chevron
folds that develop in pre-existing foliation
-orientation pattern of platy minerals in
cleavage domain provides further
subdivision
foliation
zonal- platy minerals in new cleavage
domain oriented at small angle to domain
& form continuous variation of
orientations from minerals in microlithons
-marked by layers/laminae of different
mineralogical composition
discrete- orientation of platy minerals in
new cleavage domains is parallel to
domains and sharply discordant
11.2 Compositional Foliations
Diffuse foliations- widely spaced weak
concentrations of mineral in a rock of
mostly 1 lithology.
Banded foliations- closely spaced
compositional
layers
that
are
mineralogically
distinct
and
of
comparable abundance
microcontinuous- parallel alignment of all
platy or inequant grains in a rock, and
lacks any domainal structure
11.6 The Relationship of Foliations to
Other Structures
i. Relationship with Folds
• axial surface foliations and plane
cleavages
-secondary foliations commonly parallel/
subparallel to axial surfaces of folds that
association is almost axiomatic
-orientations fan across folds and can be
convergent or diergent
refracted foliation/cleavage- orientation
changes at lithologic contact; strongly
convergent in less platy materials and
most divergent in platy-mineral-rich rocks
• relationship of foliations subparalllel to
axial plane of folds so consistent that it
can determine geometry of fold in field
mapping
ii. Relationship with Ductile Shear
Zones
S-C tectonites- rocks from ductile shear
zones with S/C foliations from a single
deformation
S-foliation- continuous coarse foliation
defined by preferred orientation of mica
grains, commonly by elongate quartz
grains
C-foliation- set of shear bands in rock
that develop subparallel to boundaries of
shear zone, may have fibrous crystals
lying on and subparallel to foliation
surfaces
11.5 Continuous Foliations
transposition foliation- superposition of
tectonite
foliation
on
earlier
compositional layering
-defined by domains with spacing less
than 10 micrometers or by nondomainal
structure
11.7 Special types of Foliation and
Nomenclature
-divisible by grain size into fine and
coarse continuous foliations
Platy Cleavage - fine continuous
foliations characteristic of slate
EPS116 Chapter Summary 2011
Anisa Ahmadzai
Phyllitic Cleavage - resembles slaty
cleavage but grain size of rock slightly
coarser
Schistosity - foliation found in coarsegrained, mica-rich, medium- to highgrade metamorphic rocks with chlorite,
biotite, or muscovite defining the foliation
Gneissic Foliation- develop in gneisses
in which platy minerals are sparse or
absent
Flow Cleavage- applied to continuous
axial surface foliations interpreted to be
the result of large amount of ductile
deformation in rocks.
-grain lineations are formed by the
parallel alignment of individual acicular
mineral grains such as amphibole, by
grains of minerals that have been
stretched into an elongate shape, or by
mineral fibers that have grown in a
preferred orientation
Polycrystalline mineral lineations formed by preferred orientation of
elongate clusters of grains of a particular
mineral measuring at least a few grains in
diameter
(More descript details of specific types in
section 11.9 pgs 314-316)
Fracture Cleavage- variety of disjunctive
foliations or discrete crenulation foliations
11.10 Associations of Lineations With
Other Structures
Shear, solution, and strain slip
cleavages describe spaced foliations.
i. Lineations and Foliations
11.8 Structural Lineations
-Lineations may develop on surfaces other
than foliations like fault surfaces
-defined by preferred orientation of linear
structure contained within a rock
-Some lineations are defined at least in
part by foliations, and must be parallel
ii. Lineations and Folds
discrete
lineationsformed
by
deformation of discrete objects such as
ooids, pebbles, fossils, and alteration
spots
-This relationship can decipher structural
geometry of an area and in interpreting
conditions under which the structures
formed
constructed lineations- formed from
planar features constructed or deformed
during
deformation
and
include
intersection of 2 foliations, crenulation
hinge lines, boudin lines, structural
slickenlines, and mullions
-Because lineations are usually smaller
structures than folds it may be easier to
map geometry of fold hinges through
these structures
boudins- linear segments of a layer
formed when the layer has been pulled
apart along periodically spaced lines of
separation called boudin lines
pinch-and-swell structures are periodic
oscillations in the thickness of a bed with
pinches becoming thinner as the amount
of lengthening of the bed increases
mullionslinear
fluted
developed within a rock
-Lineations may be found parallel or
perpendicular to fold aces and also as
arbitrary angles to fold axes
References & Resources
Robert J. Twiss, Eldridge M. Moores,
Structural Geology 2nd edition, (W. H.
Freeman), p. 297-317, 2006
structures
Chevron/Kink Folds: cylindrical, al layers
of sedimentary or metamorphic rock
11.9 Mineral Lineations
-consist of preferred orientation of either
single elongate mineral grains or elongate
polycrystalline aggregates
EPS116 Chapter Summary 2011
Anisa Ahmadzai