Final Flashcards
(25 cards)
list and explain the main primary structures from this course
load structures: flame structures / ball and pillow
sand overlays mud. mud is more dense than mud so it wants to be on top. the massive sand deposit puts pressure put near preexisting weakness in the sand allows mud to inject into the weakness creating a flame shape. the sand sinks in as the mud shoots up, creating the pillow shape (load cast). sometimes the flame structure will curl in on itself creating the ball
scour and fill structures: flute casts
scour and fill structures occur when a current digs into the sediment in a swirly motion and creates a scoop. the scoop is biggest up current as this is where it scoops the hardest. these scoops then get filled by other material. flute casts are observed from the underside (looks like an upside down spoon) since the other side will have been covered and filled in.
scours are the same process but a side view
graded bedding
grading is a decrease in grain size across a sedimentary layer. normal graded bedding (fine on top, coarse at bottom) occurs because the heavier particles settle first, then are followed by finer and finer particles as current slows. reverse is where coarse is on top and fine is underneath, which is caused by other types of depositional systems
ripple marks
ripples can cause asymmetric or symmetric ripple marks. asymmetric are good paleocurrent indicators (lee side is down stream) but bad for younging. asymmmetric occurs from sinusoidal currents (swash) and are good younging indicators.
dewatering structures - sand volcanoes
sediments oversaturated w water and are under pressure. event (such as earthquake) disrupts & water wants out from underground so pushes up from under to create volcano shape
clastic dike
water moves through (prexisting?) cracks in a rock. minerals precipitate out and fill the cracks with quartz and things
compaction and diagenetic structures
stylolites
stylolites occur when pressure causes certain minerals to dissolve away, but the clay minerals remain since they are insoluble. the soluble grains preferentially dissolve along the faces at which stress is the greatest (Pressure solution) which creates the serrated pattern)
pinch-and-swell structures
Observed as a difference in thickness, pressure differences along a layer cause certain areas to pinch and others to swell (looks like sausage links)
Penecontemporaneous folds and faults (slump structures)
preexisting layers bedding slides down an incline (by some trigger, eg earthquake) and slump at the bottom. layers somewhat maintain cohesion. chaotic folds and faults
primary igneous structures
ignous intrusions: dikes, batholiths, etc
extrusive:
pillow basalt
pillow basalt occurs on the bottom of the ocean. lava comes out of a centre point. creating round bulbs of lava. new lava comes from the centre, older stuff is at the edges. the outer edge of the individual pillows will be rounded whereas the part closer to the source will be pointy. this is a good younging indicator.
columnar joints
columnar joints: changes in temperature cause rock formations to split vertically to create hexagonal joints. this happens because the top will cool first and expand and crack. the cracks propagate down in the direction of least strain.
flow foliation
what are the types of faults based on dip angle?
based on slip direction?
based on shear sense?
vertical (90º)
dipping (10-80º)
horizontal (0º)
lystric (dip decreases progressively with depth)
strike slip (net slip parallels strike of fault)
dip slip (net slip parallels dip of fault)
oblique (both strike slip & dip slip components) transcurrent/wrench fault (strike slip fault with very steep to vertical dips)
normal (hanging wall lower than foot)
reverse (hanging wall higher than foot)
left lateral (foot wall is to the left relative to the hanging wall) right lateral (foot wall to the right relative to the hanging wall)
scissor fault (rotates about a fixed point - amount of slip varies along the fault)
what is a thrust fault
allochthon vs autochthon vs window/fenster
nappe vs klippe
low angle reverse fault
an allochthon is a region where thrusted rock has been displaced
autochthon is the rock beneath the thrust which retains its original position
a window is an area of autochtonous rock surrounded by allochthonous
nappe is allochthonous rock that has moved more than 10km relative to the footwall
klippe is an isolated block of allochthonous rock
describe the 4 brittle fault rocks
describe Slickenslides, striations, and slickenfibre
Breccia:
>30% large angular fragments in fine- grained matrix; noncohesive; can be cemented by later deposits (e.g. calcite or quartz)
Cataclasite: with <30%
large angular fragments in fine-grained matrix;
cohesive
Fault gouge:
pulverized rock, very fine-grained (generally <0.1 mm in diameter);
non cohesive; clayey
Pseudotachylyte(pseudotachylite):
cohesive, glassy, very fine-grained
formed by local melting due to frictional heating during slip
on a fault.
slickensides:
smooth/shiny shear surfaces in rocks
commonly display striations (or striae).
->The striae are believed to be parallel to the movement
direction during their formation.
slickenfibre
Mineral fibres that grew during the fault movement. They show
the direction of displacement.
define the following:
synthetic vs antithetic faults
detachment fault
horst
graben
half graben
Thin vs thick skinned tectonics
what fault system do they belong to? briefly describe the system
Synthetic fault – subsidiary faults that parallel the major faults
Antithetic faults – subsidiary faults whose dip is opposite to that of the major
Detachment fault: low-angle, separating unfaulted rocks below from faulted rocks above
Horst: an up-faulted block between high-angle faults; the foot wall block
Graben: a relatively down-faulted trough between high angle normal faults that are dipping towards each other.
Half graben: bounded by fault only on one side, usually involves rotation of fault blocks.
Thin skinned tectonics: Deformation not involving the
basement, only the overlying sedimentary cover.
Thick skinned tectonics: Deformation involving the basement,
not only the overlying sedimentary cover.
normal fault system
- extensional (results in lengthening of a layer)
- divergent plates, rifts, passive margins, mid ocean ridges
explain the following:
flat ramp structure
imbricate fan
duplex system
what fault system do they belong to? briefly describe the system
flat ramp structure
flats are where bedding is parallel to the fault line
ramps are where bedding is oblique to the fault line
ramp structure that connects an upper and lower flat
Imbricate fan – a series of reverse faults dipping in the same direction and soling out on a floor thrust.
-no covering thrust plane
-thrust does not continue up onto the footwall, instead, a new ramp forms further down the floor thrust and a thrust forms underneath the first, steepening the first
Duplex system - system of thrust slices overlying the floor thrust separated by individual ramps, covered by major upper thrust sheet (roof thrust)
formation:
major thrust slides over a ramp on top of a stack of brittle layers, creates stress in stack of brittle layers, creates ramp further along through those layers, thrust now occurs along this ramp, repeat
reverse (thrust faults) system
- contractional (results in shortening of a layer)
- convergent boundaries, orogenies
define the following:
transpression
transtension
what fault system do they belong to? briefly describe the system
Transpression (positive flower structure)
Transtension (negative flower structure)
strike slip fault systems
- transform boundaries
explain what fabric, fabric element, and penetrative mean
what is foliation vs lineation?
what is a tectonite and the 3 types?
fabric describes the shapes and characteristics of individual part of a rock mass. there are tectonic fabrics, which form as a result of tectonic deformation (eg schistocity), and primary fabrics, which form during the formation of the rock (eg bedding)
fabric elements are features that contribute to the fabric. these must be penetrative, meaning they occur over and over from sample to sample. penetrative is scale dependent (a feature may be penetrative at one scale and not another)
Foliation (S): Planar fabric
Any planar feature in a rock, Primary or tectonic
generations: Sn……. S0, S1, S2
Lowest number is earliest event
S0 = bedding
Lineation
-all linear structures that occur penetratively in a rock.
Generations: L1, L2, L3 …..
a tectonite is a rock with a penetrative tectonic fabric (tectonically deformed rocks)
S-tectonite have only planar fabric (only foliations, no lineations)
L-tectonite have only linear fabric (only lineations, no foliations)
L-S tectonite have both
describe the 7 different types of foliation
- Disjunctive cleavage aka pressure-solution cleavage
-formed by low T & pressure solution
-cleavage domains separating microlithons.
cleavage domains: original fabric&composition changed by pressure solution
microlithons: original fabric&composition preserved - Pencil cleavage
-found in weakly deformed shale
-tendency to brake // to bedding = tendency to break at a high angle to bedding
-> thus breaks into pencil-like shards. - Slaty cleavage & 4. Phyllitic cleavage and Schistosity
-clay/phyllosilicate grains are reoriented/grow perpendicular to shortening direction
-strong preferred orientation of phyllosilicates (clay, mica etc.).
slate: individual grains too small to be seen with naked eye, dull appearance
phyllite: grains slightly coarser, silky appearance on cleaved surfaces
schist: grains coarse enough, usually clearly visible with the naked eye. - Crenulation cleavage
rocks have earlier foliation, earlier foliation gets folded again (crenulated) by microfolds
-seen parallel limbs of microfolds folding earlier foliation
-found in all layered silicate rocks of all metamorphic grades
-best developed in medium to high-grade rocks rich in mica - Gneissic layering and migmatization
Compositional banding characteristic of gneiss
Paragneiss (meta-sedimentary rocks) and orthogneiss (meta-igneous rocks) - Axial planar foliations
-parallel alignment of platy minerals resulting from folding
-approximately parallel to the axial plane of the folds
-create convergent and divergent fans
what is cleavage refraction and what does this mean for axial fans
Cleavage refraction is when the orientation of foliation varies across a contact depending on the competency of the rocks
The angle between bedding and foliation is larger in the
competent layer (forming convergent fans) than in less competent layer (which may fan divergently)
ie for a sandstone layer in shale, sandstone is the more competent layer so it would fan convergently
how does the origin of foliation vary for layer silicates vs non layer silicates
for layer silicates, there is new grain growth in a preferred orientation, or preexisting grains rotate to a preferred orientation
for non layer silicates,
- rotation of inequant grains
- grains are deformed (eg flattened) into a preferred orientation
- growth of elongated grains due to the given preferred dimensional orientation of a layer silicate
(e.g. growth of elongate quartz grains due to the orientation and distribution of micas).
Origin of gneissic layering
explain transposition and how to recognize it
- protolith had preexisting compositional contrast
-(ie sandstone alternating with shale, mica schist alternating with quartzite)
-lit par lit: magma intrudes parallel to bedding planes
- differentiation - Transposition
pre-existing foliation (e.g., bedding) is folded into an orientation approximately parallel to the axial plane of the folds
bedding no longer provides information concerning stratigraphy
-tight folding causes elimination of fold closure
How to recognize it in the field?
Presence of rootless/intrafolial folds (not connected to anything, within foliations)
Younging direction reversal
Parallelism of foliation and layering
Regular repetition of lithologies
3 types of lineations and tectonic implications
- Form lineations
Fold (and crenulation) hinges
Mullions
Cusplike shape of the surface of a competent unit in contact with a less competent unit. cusps point to competent layer
boudins
-segmentation of preexisting bodies that are more competent than the rock surrounding them. body (eg a given bed or dyke) is broken up into a series of elongate bodies (boudins) aligned parallel to one another.
Elongated clasts (stretching lineation) - Surface lineations
Intersection lineations (axial planar), Slickenslides and striations (striae), Slickenfiber, - Mineral lineations
-preferred dimensional orientaion of inequant grains or by elongate mineral aggregates)
Tectonic implications of lineation:
lineations parallel to the fold axis: fold hinge/crenulation
lineation, bedding-cleavage intersection lineation, boudins,
mullions
Lineations parallel to the direction of stretching (stretching
lineations)
Lineations parallel to the shear direction: slickenside striae
or slickenfibre
explain the 3 mechanisms and kinematic models of folding
mechanisms
1. Buckling
-> ——- <-
-a higher competency layer is shortened parallel to its length within lower competency layers
-neutral surface folding, flexural slip/flexural flow
-> -parallel folds
-Cuspate-lobate folds
- passive folding
->
——–
<-
-passive flow of rocks (flow in opposite directions on either side of a layer)
-no competency contrast
-shear folding, pure shear - bending
↓ ↓
————-
↑
occurs when forces act across the layering
-flexural slip, shear folding
kinematic models:
1. Neutral surface folding
-neutral surface separates outer arc extension from inner arc contraction
-parallel folds (1B)
- Flexural slip/flexual flow folding
-involves shear on surfaces parallel to the folding
-parallel folds
-no distortion in the plane of the folded layer - Shear folding
-shearing on closely spaced planes oblique to the layer being folded
-shearing planes are planes of no distortion
-similar folds
classes of folds
which are most common in nature?
Class 1 - convergent; Dip isogons converge downward towards axial surface, the curvature of the outer arc is less than that of the inner arc.
1A - Limbs thicker than hinges. orthogonal thickness increases from hinge to limb.
Class 1B - Layer thickness constant; parallel fold, dip isogons perpendicular to surface
Class 1C - Limbs thinner than hinges
Class 2 - parallel; the curvature of the outer arc exactly matches the curvature of the inner arc; similar fold - dip isogons parallel to each other
Class 3 - divergent; Dip isogons diverge downward towards axial surface, curvature of the outer arc is greater than that of the inner arc.
1c and 3 are most common in nature because
The strong (competent) layers form class 1B to 1C folds. They control the geometry of the weaker (less competent) layers which fill in the space between the competent layers. If the competent layers are closely spaced, class 3 folds are formed, and if widely spaced, class 2 folds form
parallel
- constant centre
- thickness of layers constant throughout
- dip isogons are perpendicular to the folded surface throughout the fold
similar
- shape of the fold varies
- beds are thinner at the limbs and thicker at the hinge
- dip isogons are parallel to each other
Intrafolial folds
Parasitic folds
pumpellys rule
Intrafolial folds: Isolated, tight fold closures in rocks that are
otherwise not obviously folded. They occur is areas of intensive
folding which may not be obvious
Parasitic folds: small folds occurring on the limbs or in the fold
closure of larger folds
towards centre of symmetric fold (cw on left, ccw on right)
(Using asymmetry of parasitic folds to predict the location of
the closures of larger scale fold)
Pumpelly’s rule: The orientation of hinge lines and axial
surfaces of small folds is representative of that of regional folds
types of fold interference patterns
type 0 - squished in 1 direction, squished again in the same direction
Type 1 - “dome and basin”
-squished in 1 direction, squished again in the opposite direction
type 2 - mushroom
- fold is sideways then gets squished parallel to hinge line
type 3 - refolded fold
what is a shear zone
what is mylonite
what types of shearing are there
explain Progressive folding in a shear zone
describe lineations in shear zones with simple shear
what are the 5 shear sense indicators and examples
a concentrated zone of high deformation
mylonite is a fine grained metamorphic rock formed by deformation dynamic recrystallization, resulting in a reduction in grain size
types of shearing:
boundary parallel shearing -> simple shear
boundary normal shearing -> pure shear
transpression -> has both components
progressive folding:
newly formed folds are less tight and hinges highly oblique to movement direction
folds get progressively tighter and their hinges rotate towards movement direction
In shear zones with simple shear (boundary parallel), lineations are oblique to the shear direction at low strain and closer to it at high strain. In transpressional and transtensional shear zones, the relationship is more complicated
- Grain tail complexes
-Snowball garnet - Disrupted grains
- Mica Fish - Foliations
-S-C fabric - Textures (LPO fabrics)
-Shape Preferred orientation - Folds
what are the types of homogeneous strain
(a) General strain (S1>S2>S3)
(b) Axial symmetrical extension (cigar-shaped strain ellipsoid; S1>S2=S3)
(c) Axial symmetrical shortening (pancake- shaped strain ellipsoid; S1=S1>S3)
(d) Plane strain (S2=1)
(e) Simple shortening (volume loss; S1=S2=1; S3<1))
Coaxial vs non-coaxial deformation
Pure shear & Simple shear
Coaxial deformation:
The principal axes of the incremental strain ellipsoids always have the same orientation as those of the total strain ellipsoid during deformation. the principal directions of total and incremental strain are fixed to the same lines of material particles throughout deformation.
Noncoaxial deformation: The principal axes of the incremental strain ellipsoids and those of the total strain ellipsoid rotated relative to each other during deformation. Consequently, two or more axes of the principal directions of total strain lie along different lines of particles at different times in the deformation.
Pure shear:
-homogeneous deformation
-plane strain or a general strain,
-lines of particles that are parallel to the principal axes of the strain ellipsoid have the same orientation throughout deformation. It is the same as coaxial deformation.
Simple shear:
-homogeneous deformation
-plane strain
-a single family of parallel material planes remain undistorted and parallel throughout deformation
- It involves both strain and rotation,
- It is a noncoaxial deformation, but not all noncoaxial deformation is simple shear.
what is stress and its components?
what is lithostatic pressure?
mean stress and deviatoric stress?
what is andersons classification of tectonic stress and what does that mean for the 3 main fault regimes?
σ = F/A
vectors:
Normal stress
Shear stress
Stress on each surface (plane) can be resolved into normal
(σN) and shear stress (σs) components
Lithostatic pressure (= ρgh): generated at a depth h below the ground surface due solely to the weight of rocks, of mean density ρ, in that interval.
Mean stress = (σ1 + σ2 + σ3)/3
Deviatoric stress = Total stress – Mean Stress
Stress and fault regimes
Anderson’s classification of tectonic stress: Assuming one of the principle stresses is vertical, the other two must be horizontal.
Normal Fault regime: σv=σ1
Strike-slip regime: σv=σ2 (σ1 horizontal)
Reverse-fault regime: σv=σ3 (σ1 horizontal)
Isotropic – anisotropic material
Elastic and anelastic behaviour vs Viscous and plastic behaviour
Strain hardening vs softening
Factors affecting brittle or ductile behaviour:
An isotropic material has the same mechanical
properties in all directions, so that it reacts to
stress identically regardless of the directions
anisotropic: More stress is required to deform parallel to
foliation
Elastic and anelastic behaviour (no permanent distortion, strain recoverable)
Elastic behavior: Strain takes place instantaneously once the stress is applied or removed, i.e. the response is instantaneous and the strain is recoverable
Anelastic behavior: Although strain is recoverable,
the complete response is not instantaneous
Viscous and plastic behaviour
-strains are permanent and nonrecoverable
plastic: strain only takes place in localized regions where the critical value of stress is reached.
viscous: deformation throughout wherever a deviatoric stress is present.
Strain hardening: gradual rise in the stress required to produce further deformation at the same strain rate.
Strain softening: gradual decrease in the stress required to produce further deformation at the same strain rate.
Factors affecting brittle or ductile behaviour:
Temperature
confining pressure
material (quartz vs. feldspar etc.)
strain rate (which has an effect opposite to that of temperature)
pore-fluid pressure (which has an effect opposite to that of confining pressure).
3 Brittle deformation Mechanisms
types of fractures
3 modes of fractures
- Frictional grain boundary sliding
- Grain rotation
- Fracturing: intragranular and intergranular
Granular and cataclastic flow
Cataclasis: The fracture and crushing of grains, coupled with frictional sliding along grain contacts and grain rotation.
Granular or particulate flow: achieved by grain translation and rotation
Cataclastic flow: due to cataclasis
Types of fractures
Shear fractures: relative movement parallel to the
fracture surfaces
Extension fractures: extension perpendicular to the
fracture surfaces, including joints (narrow) and fissures (wide, filled with air/fluids).
Veins: fractures filled with minerals.
Dykes/sills: fractures filled with magmatic rocks
Contractional planar fractures: Stylolites
Mode I – Opening mode (a tensile stress normal to the plane of the crack), <–>
Mode II – Sliding mode (a shear stress acting parallel to the plane of the crack and perpendicular to the crack front), backward/forward
Mode III – Tearing mode (a shear stress acting parallel to the plane of the crack and parallel to the crack front). up/down
joints
orthagonal vs conjugate
origin of joints
joints
-planar
- tensile strength of stressed rocks is exceeded
- no shear displacement
form parallel to σ1 and σ2, perpendicular to σ3
Orthogonal system:
two sets of joints that are perpendicular to each other
Conjugate system:
two sets of joints with a dihedral angle of significantly less than
90º (e.g., about 30º-60º)
Origin of joints
1. Differential volume change
- Columnar joints
2. Uplift/unroofing
-sheeting joints
3. Hydraulic fracturing
- Pf > σ3 + T
4. Regional Deformation
- pinnate fractures occurring in the vicinity of a fault
plane, intersect the fault in an acute angle pointing in the direction of relative movement
