Fossen pre reading- structural geology Flashcards
What are the 2 definitions of deformation?
difference in position of points before and after deformation
or
the strain history from undeformed to deformed stage
What components are included in deformation from an external co-ordinate system?
Translation
Rigid rotation
Internal deformation
What components can internal deformation be further split into?
Rigid rotation
Strain
What is Re?
rotation of a rigid body relative to an external co-ordinate system
What is Ri?
Rotation of axes of the strain ellipsoid (main strain axes)
What is non-coaxial deformation?
Process of internal rotation involved in internal deformation (axes of strain rotate)
What is strain?
Defines a change in size and or shape
What are some examples of what strain can be like?
Change in original shape
Change in volume (dilation)
Rotation of planes and lines
Change of original length of lines
What 2 types of strain are there?
Homogenous
Heterogenous
What is homogenous strain characterised by?
No strain gradient
Linear transformation (straight line straight, parallel stay parallel)
Will natural strains be more heterogenous or homogenous? (with example)
Exhibit some heterogenity
Shear zone show increasing strain from side wall to centre
What is isotropic dilation?
Equal lengthening or shortening in any direction
What is anisotropic dilation?
Shortening or lengthening in only 1 or 2 directions
What is positive and negative dilation?
Positive = increased volume
Negative = decreased volume
How is progressive strain described in terms of?
Infinitesimal or Instantaneous strain parameters
What are the infinitesimal strain parameters?
Infinitesimal stretching axes
Velocity field
Flow apophyses
Vorticity and Wk
Steady state/ non-steady state deformation
How are the infinitesimal stretching axes (ISA) aligned?
perpendicular to each other
What does ISA1 describe?
direction of max stretch rate
What will happen to physical lines on ISA1?
experience fastest stretching during deformation
What is ISA3?
minimum stretching rate
What are the two ISAs?
ISA1
ISA3
What will occur at ISA3?
slowest (usually) negative stretching rates (fastest shortening)
What is the velocity field?
the velocity and direction of motion of the particles as strain progresses
What is vorticity?
measures angular velocity
What are flow apophyses?
theoretical planes that compartmentalise the flow pattern in which particles cannot cross
What is simple shear?
Paralle lines remain parallel and maintain constant distance
What is pure shear?
planar coaxial deformation
What is subsimple shear?
non-coaxial constant area that combine with simple or pure shear
Does knowing the stress field during deformation reveal the progression of deformation?
ultimately no
What are some examples of strain markers?
Pebbles
mineral grains
ooids
vesicles
Pillows in pillow lava
Ammonites
Belemnites
Graptolites
Worm burrows
Brachiopods/ trilobites
Crinoid strems
Xenoliths
Reduction spots
What are passive strain markers?
markers with the same rheologic properties as the surroundings (no viscosity difference so deforms with rock entirely)
What is the perfect marker for deformation?
reduction spots
Why are reduction spots the “perfect” marker for deformation?
due to extremely small amounts of colour pigment
What are active deformation markers?
markers which will deform differently to the surrounding rock
When would you use the Breddin graph for deformation?
where we have pairs of lines in deformed rock where we know original angle
What does the Rf/(oval with line through) diagram handle? (deformation)
markers with initial elliptical shapes
or
markers with different original shape
What does the Fry diagram use? (deformation)
centerpoints of objects with similar initial size
What does the normalised Fry method handle? (deformation graphing)
objects with different initial sizes and involves object centerpoints + shapes and orientations
How is 3D strain presented?
Flinn or Hsu diagrams to express the geometry of the strained ellipsoid
What is force?
push or pull on an object that results from the interaction with another object (physical or in a force field)
What is Newtons first law?
An object remains at rest or moves at constant velocity when no net force is exerted in the object
What is Newtons second law?
the change in velocity (acceleration) of an object with mass is equally directed and proportional to the applied net force
What is Newtons third law?
for every force there is an equal and opposite force
What is the context of pressure?
hydrostatic stress field
materials with negligible shear strength
Fluid or gas
What does normal stress indicate for compression and tension?
Compression is positive
Tension is negative
What does shear stress indicate for counter-clockwise and clockwise rotation?
Counter-clockwise is positive
Clockwise is negative
What is normal stress?
stress vector orientated normal to a surface (i.e. a fault)
What is shear stress?
stress vector which parallels the surface
When is shear stress highest?
When at a 45* angle to the surface
What is an example of an area where the rock in the crust has been tectonically inactive for 10 or hundreds of millions of years?
Baltic shield
What is the state of stress in crustal rock that has been tectonically inactive?
lithostatic (equal in all directions) and increases with burial depth
What are wide joints called?
Veins
What are joints?
fractures without visible offset perpendicular to the fracture surface
What is a joint set?
composed of joints with similar orientation and morphology
What is a joint system?
when there is a combined pattern of 2 or more jointed rocks
What are the different types of joint intersections?
T- orthogonal (regular, rectangle)
X- Conjugate (diagonal)
Y- polygonal (many sides i.e. hexagon)
What are conjugate joints?
2 individual sets formed as result of deformation during different stress
Where are joints most common?
Brittle upper crust in stiff rocks like well-lithified sandstone and limestone, granitic rocks and lavas
How can joints be found within the environment?
scattered or in zone of fracture corridors
What stress is needed to form joints?
Tensile stress
How are joints and orogenic processes linked?
think either the orogeny itself causes joints or the orogenic stress is stored in the rock and controls joint orientation
What are orogenic processes?
linked to continental orogen formation (land and mountain building)
What happens to rocks as they cool?
volume reduction (contraction) which is taken up (grainscale) if flexible but if not extension fractures form
What 2 things happen during cooling of rock?
Decompaction and cooling
What happens during decompaction?
Any cement (in porous sedimentary rock) that formed at depth locked in some of the elastic strain at those conditions change as rock gets near surface can cause cement to break and joints to form
How does exfoliation occur?
during the last part of exhumation when there is a removal of overburden (vertical stress)
What is exfoliation?
where joints more or less parallel to the surface form, leading to slab erosion formation
What rocks is exfoliation common in?
massive rocks like granites and thick sandstones
What zone are exfoliation joints restricted to?
upper 100-200m of the crust
How does hydraulic fracturing occur?
Where fluid is trapped and over pressurised by overburden, tectonic stress or fluid pulses
What are most vein systems a result of?
Repeated hydraulic fracturing during periods of elevated pore fluid pressure
How do crack-seal veins form?
when the vein grows by repeated fracture events at random locations in vein material
What is seismic pumping?
sufficient pulses of overpressure which can cause large volumes of fluid to be expelled from fluid-filled fractures
How do dike intrusions typically occur?
hydraulic fracturing (magma is the over pressurised fluid)
What is artificial hydraulic fracturing called?
Fracking
Why is fracking done?
to acquire hydrocarbons from wells, especially gas from shales
How does fracking done?
horizontal part of wellbore is sealed and pressure increased until the minimum horizontal stress is exceeded
Why is fracking used to release hydrocarbons?
its hoped the extension fractures will increase rock permeability making it easier for fluid flow
Why is sand (propants) added to fracking fluid?
to stop the fractures from sealing and becoming joints when pressure normalises
What are wingcracks?
Faults, fractures and other weak structures that are reactivated to produce joints in their walls
How do wingcracks form?
in extensional quadrants around the reactivated structure
What controls the distance between joints?
the thickness of the stiff layer (thicker = larger)
How can Dikes form joints?
Joints increase with spacing away from dike and thought to form ahead of dike tip due to the tensile stress of the magma
What are joint corridors?
wide zones of joints that can extent 100’s of km
How can fracture corridors be recognised?
one dominant orientation (but might have different fracture populations)
What is the Colatina fracture system (Brazil)?
Fracture corridor with many populations (many mafic dikes)
How do mineral veins form from a single event?
Form as the mineral grows from fluids in an already open fracture cavity
How do mineral grains form from a multi-event growth?
form periodically as the veins open
What is syntaxial mineral growth?
growth along centre fracture
Vein youngest in centre
Same mineral as wall rock
Crystallographic continuity with wall
What is Antitaxial mineral growth?
growth along 2 walls
central part of vein is oldest
fibrous veins
crystallographically different from wall
What is crack-seal growth veins?
veins that grow by repeated cracking of existing vein material
What are sigmodial veins?
form at shear zones in echelon style and rotate systematically as shear accumulates
What is the oldest of the sigmodial vein?
the central part as a result it is more rotated due to the longer strain exposure
How do joints have a positive influence on fluid flow?
the joints are like pathways allowing increased permeability
Where are joints particularly important for fluid flow?
non- to low-porosity rocks (basement rocks) and high porosity limestone reservoirs with poor permeability
What is apeture?
the ‘openness’ of veins at depth (distance between the 2 walls)
where are faults found they found?
brittle deformation structures commonly forming in the upper 10-15km of earths crust
What are faults?
fracture zones of localised deformation that accommodate movement parallel to fracture surface.
What is the common movement of faults?
1 meter +
What are smaller movement fault-like structure called?
shear fractures
What is a fissure?
fracture where movement normal to fracture surface is easily recognised
What are the most common vein materials?
Quartz and Calcite
How are non-vertical faults defined?
hanging wall defines the overlying fault block (footwall is underlying fault block)
What are the 3 main classes of faults based on the movement of the hanging wall relative to footwall?
Normal faults
Reverse faults
Strike-slip faults
What are normal faults?
Down dip (dip-slip) displacement of hanging wall relative to footwall
What are reverse faults?
Up dip (dip-slip) displacement of hanging wall relative to footwall
What are strike-slip faults?
Strikes-parallel displacement of hanging wall relative to footwall
What are oblique-slip faults?
Combination of reverse or normal-slip with strike slip
What are wrench faults?
sub-vertical
steeper than normal or reverse
What are low and high angle faults?
Low angle- <30 degrees
High- steeper than 60 degrees
What are synthetic faults?
dip in the same direction as primary fault
What are antithetic faults?
dip in the opposite direction to the primary fault
What do oppositely forming faults produce?
Graben (lower area)
and
Horsts (raised area)
What is displacement (net slip)?
the distance between 2 originally contigous points on fault surface
What defines displacement?
Displacement vector (split into dip-slip or strike-slip component)
What is the Pitch (rake)?
angle between the displacement vector and the strike of the fault
What is separation? (faults)
apparent displacement observed in any given section (only if with true displacement vector)
What is strike separation?
apparent displacement as measured parallel to strike of the fault
What is Dip separation?
dip-slip component of actual displacement observed in vertical section perpendicular to fault
What is heave?
horizontal component of dip separation
What is throw?
vertical component of dip separation
What is stratigraphic separation?
the isopach thickness of strata between 2 bedding horizons bought into contact at a fault
What is isopach thickness?
Thickness in direction of normal bedding
What 2 architectural elements do most faults contain?
interior fault core
enveloping fault damage zone
What material do fault cores contain?
cataclastic material (gouge and breccias)
What are lenses (horses) in the fault core?
bodies of host rock ripped from sidewalls and incorporated into the fault
How does faulting initiate in non-porous rocks?
development of a isolated shear fracture
What does Griffths model say about shear fractures?
they nucleate by growth and linkage of pre-existing micro-discontinuities
How do faults grow in porous rocks?
fractures develop as deformation bands
with the pores collapsing in shear
How will material change with faulting of sandstone?
Non-cataclastic bands at shallow burial
Cataclastic bands for deeply buried
What are the 4 main types of fault intersection?
Intersection (overprinting)
Mutual interaction
Single-tip interaction
Double tip interaction
What is the function of overlap zones?
to accommodate transfer of displacement between overlapping structures
What are unlinked/ isolated faults?
faults that do not mechanically interact with other faults
What are soft-linked faults
Faults that form an overlap zone but do not physically connect but instead mechanical or geometric coherence obtained by elastic/ ductile strain between faults
What are hard linked faults?
Faults which are physically connected
Where do eye structures form in faults?
form between doubled linked faults
What are transfer faults?
ones orientated normal to overlapping faults and exhibit substantial stroke-slip movement
What are branch lines?
define lines along which 2 faults are hard linked
What is the Wasatch fault?
major extensional fault marking boundary between Wasatch mountain range to the east and basin range system to the west this area is seismically active with several minor Earthquakes a year
What are foliations?
planar structures that penetrate metamorphic rocks
What is tectonic foliation?
general term about penetrative and cohesive that involve shortening across the structure
What are primary foliations?
non-tectonic penetrative planar structures in rock
What are some examples of primary foliations?
Layering or lamination (sedimentary)
Flow banding (volcanic rocks)
Cumulate (intrusive rocks)
What is cleavage?
foliation in very low grade metamorphic rocks (rock easily split or cleaved)
What is schiostocity?
tectonic foliations in more coarse grained recrystallised rocks (quartz schist)
What are cleavage domains?
domain concentrated in insoluble materials (mostly phyllosilicates)
What is spaced cleavage?
cleavage domains spaced at cm-scale
What is continuous cleavage?
cleavage domain at mm scale or less
What is crenulation cleavage?
cleavage formed by microfolding
What is disjunctive cleavage?
no evidence of microfolding (cleavage cut straight across earlier planar structures)
What are M-domains? (cleavage)
Mica-rich domain
What are QF-domains? (cleavage)
Quartz-feldspar rich domains
How does a pencil cleavage form?
when tectonic strain is added to the compaction strain so that the 2 strains are approx equal to magnitude at a high angle
Where is pencil cleavage usually found?
foreland of orogenic belts
What is slaty cleavage?
First forming cleavage occurring at onset of metamorphic conditions around 200*c
What is phyllitic cleavage?
continuous larger mineral then slaty
What does it mean if folds form actively?
instability generated by mechanical and rheological properties of layering versus the layers of surrounding matrix
What is buckling?
when the active folds nucleate and grow as the competent layer shortens
What is passive folding?
folding which occurs during flow in rocks without internal viscosity contrasts
What do the layers function as in passive folding?
like markers that “go along for the ride”
Where do passive folds form?
Mylonite zones (shear zones)
What is bending/ forced folding?
when layers competent or not bend as forces act across layering
When does buckling occur?
when a layer is shortened parallel to its length
What does buckling require?
A layer with higher competence (viscosity) then surroundings
Layer-parallel to shortening
Plastic deformation
How will shortening occur id there is no competence contrast?
By thickening
What might shortening lead to in a brittle regime?
Brittle failure
What happens when shortening direction is somewhat oblique compared to layering?
Formation of asymmetric folds (potentially reflecting obliquity)
How can closeness of layers affect buckling?
Far apart will act as individual layers
Close together will act as singular layer which the thickness is the sum of the individual layers
What is an example of multilayer buckling?
Finnmark NE Norway
What happens when buckling occurs on alternating thick and thin layers?
Thin layers will start to fold first, while thick layers go through longer thickening period
What will fold be like in a thin layer?
small folds that later get folded by the thicker layer
What are first and second order folds?
Largest fold is first order
Smaller folds related to the same fold are second order
Why do smaller folds become asymmetric?
Occurs along the limbs because of shear and vergence reflects position of origin
When do flexural flow and flexural slip occur?
During multilayer buckling combined with other processes
What is flexural slip similar to?
Bending a paper back book
What might flexural slip leave evidence for?
Bedding parallel to slip (slicklines, shear fractures or shear deformation) on bedding plane
What is the difference between flexural slip and flexural flow?
There is a discrete slip layering parallel surfaces are replaced by disturbed shear
What properties are required for flexural slip and flow?
Constant layer thickness
Zero strain at hinge point
Orthogonal lines don’t typically remain orthogonal
What occurs with orthogonal flexure?
Line initially orthogonal stay so
Neutral surface separates (outer arc extension and inner arc contraction)
Parallel folds
When does large scale bending occur?
when salt or magma rise towards the surface
What are laccoliths?
Shallow extrusions that force the overlying rocks and sediments upwards
What are kink folds?
Deformation of pre-existing foliation
What are the characteristics of kink folds?
Typically small
Band thickness of a few cm
Angular hinges
Straight limbs
Asymmetric fold geometry
What might kink folds grow into?
chevron (angular) fold (but usually stop developing before that level of strain required is met)
