L-Del Flashcards
Where does brittle deformation or rock failure dominate in the crust?
At shallow layers, but also been found to characterize zones in the mantle with high mechanical strength.
What is cataclasis?
Brittle deformation mechanisms, involves intrergranular deformation where the friction between the mineral grains is large enough to inhibit grain sliding, so that the grains themselves start to break.
Typical for consolidated sediments and crystalline rocks.
What is granular or particulate flow?
Brittle deformation mechanism that involves inter granular deformation where the mineral grains rotate and slide agains one another without being crushed.
Typical of unconsolidated (loose) and poorly consolidated sand at shallow burial depths.
What is frictional sliding?
Sliding or slip associated with brittle deformation is characterized by friction and is thus referred to as frictional sliding.
What is a fracture?
Surface discontinuities formed in response to external or internal stresses acting on the fractured object.
Three principal classes of fractures:
Faults, joints, veins.
What are faults?
Faults are characterized by relative movement parallel to the fracture surface.
What are joints?
Fractures with very small (commonly invisible) offset perpendicular to the fracture surface.
What are fissures?
Characterized by opening or aperture, i.e., movement normal to the fracture surface.
What are normal faults?
Dip-slip fault wit movement parallel to the dip direction. Hanging wall moves down relative to footwall, with missing section.
What are reverse faults?
Dip-slip faults with movement parallel to the dip direction. Hanging wall moves up relative to footwall, doubled section.
What is heave?
The horizontal component of the displacement.
What is throw?
The vertical component for the displacement.
What are strike-slip faults?
Movement parallel to the strike. If hanging moves right: dextral, if hanging moves left: sinistral.
What are listric faults?
Listric faults can be defined as curved normal faults in which the fault surface in concave upwards; its dip decreases with depth.
What is oblique slip fault?
Combination of dip slip and strike slip.
What are the main elements in faults?
Fault core and enveloping fault damage zone
What is the fault core?
The fault core absorbs most of the deformation and may range in width from that of a single dip surface up to several meters.
Contains various types of cataclastic material; gouge and breccias.
What is the fault damage zone?
The fault damage zone contains various types og fault-related structures, such as deformation bands and minor slip surfaces and mezo-scale shear fractures as well as joints.
Define stress.
Force per unit area.
Force that causes strain - invisible
Define strain.
Strain is the deformation that results from applied stress. Strain occurs when stress reaches a certain level.
Change in shape - visible.
Explain stress on a surface.
Stress perpendicular to a surface: normal stress (sigma_n)
Stress parallel to a surface: shear stress (sigma_s)
If a cube is oriented so that all of the shear stresses are 0, explain what you can see.
Each surface has one normal stress component, and two shear stress components.
The sides of the cube = principal planes of stress
Normal stress components = principal stress axes.
sigma_1 largest, sigma_3 smallest.
What is the mean stress?
The mean of the principal stresses.
Sigma_m =
(S_1 + S_2 + S_3)/3
Define deviatoric stress
Deviation from mean stress (avvik).
Total stress = mean + deviatoric
Describe the stress ellipsoid
The ellipsoid describes stress state at a single point.
3 perpendicular axes, where
1 > 2 > 3, which is the principal stresses.
How is the stress in thrust faults?
Vertical stress = sigma_3 and horizontal stress = sigma_1.
Horizontal stress in this case is greater than the vertical stress.
How is the stress in normal faults?
Vertical stress = sigma_1, horizontal stress = sigma_3
What is the equation for differential stress?
(sigma_1 - sigma_3) = diff.stress.
diameter of mohr circle
What is the equation for deviatoric stress?
(sigma1-sigma3)/2
radius of the circle
What is the equation for mean stress?
(sigma1+sigma3)/2
Why does rocks fail?
- Change in the failure envelope
- Change the stresses
- Increase the fluid pressure
Name paleostress indicators for contraction and extension.
Contraction: stylolites form perpendicular to sigma_1
Extension: joints, veins and dikes form perpendicular to simga_3
What is deformation?
The change in shape or volume, translation and/or rotation of an object or rock volume.
What is deformation?
The change in shape or volume, translation and/or rotation of an object or rock volume.
How can we recognize deformation?
Compare deformed to initial state
What is volume change?
Dilation - shrinkage or expansion.
Can be considered as a special type of strain - volumetric strain
What is homogeneous strain?
Deformation identical throughout the rock volume.
Straight and parallel lines stay straight and parallel.
Identical object keep identical shape and orientation.
Scale dependent!
What is coaxial deformation?
Lines that are parallel with principal strain directions have the same orientation as before - they do not rotate.
What is non-coaxial deformation?
Strain ellipse progressively rotates; lines that are parallel to the Instantaneous Stretching Axes (ISA) will rotate during deformation.
Geometric deformation
Brittle and ductile
Stress-strain relationships
Elastic, viscous and plastic
Deformation processes
- Brittle (frict. sliding, granular flow, cataclasis)
- Crystal-plastic
- Diffusive mass transfer
Define elastic
Strain proportional to applied stress. Recoverable strain - eg. rubber band
Define plastic
- Constant force (stress) regardless of rate of def., eg. sand.
- Permanent deformation obtained by crystal-plastic deformation mech. (dislocation creep, diffusion)
Define “viscous”.
Strain rate proportional to applied stress. Permanent deformation, e.g., honey, evaporites and rocks at high T.
Ductile vs. brittle deformation
Ductile deformation preserves continuity of originally continuous layers and structures.
Brittle material deforms by fracturing.
elastic deformation
Strain is proportional to applied stress
Strain is recoverable - material returns to original shape once the stress stops
Analog: rubber band or spring
Plastic defromation
Deforms at constant stress once the yield stress is achieved
Constant stress regardless of strain rate
Permanent strain (not recoverable)
Viscous deformation
Stress depends on strain rate
Delayed response to stress
Permanent strain
Viscous deformation
Stress depends on strain rate
Delayed response to stress
Permanent strain
Competency
Relative viscosity (relative to surroundings)
Resistance of layers or objects to flow
What does rheology of rocks depend on?
- Temperature
- Pressure
- Strain rate
- H2O (more fluids, weaker rock)
- Inherited weaknesses (foliations, fractures)
What is an extensional fault?
Normal fault, where heave is the horizontal component of the displacement and throw is the vertical component of the displacement.
Domino model vs. horst-and-graben systems
Domino:
- Asymmetric
- Simple shear
- Synthetic faults
- Weak (ductile) layer at base
H-a-G:
- Symmetric
- Pure shear
- Conjugate faults
What does a ramp-flat-ramp consist of?
Imbrication fan, ramp with riders, a flat, another ramp with roof faults and floor fault and horses in the middle.
What does “Basin and Range” province consist of?
Listric (normal) faults
Active vs. passive rifting
Passive:
- Driven by far-field stresses related to plate tectonics
- Rift initiation along zones of weakness
Active:
- A rising plume causes doming of the overlying lithosphere -> gravitational collapse
- Dominated by magmatism, relatively little extension
Rift development
- Doming of the crust, formation of steep fractures, pre-rift sediments
- Stretching: crust is thinned and extended, main phase of faulting, syn-rift sedis.
- Subsidence: crust cools, deposition og post-rift sedis.
Symmetric vs. asymmetric rift geometries
McKenzie model:
- Symmetric thinning
- Pure shear
- Horizontal extension balanced by vertical thinning
- Plastic deformation in lower crust, brittle deformation in upper crust
Wernicke model:
- Asymmetric rift
- Localized simple shear
- Controlled by detachment fault or shear zone that transect crust/lithos
What is accommodation zone?
Area of overlap between half-grabens
Wide rifts
Basin and Range, extension distributed over a wide area, multiple horsts and grabens
Narrow rifts
Red Sea/Gulf of Suez, Upper Rhine Graben, North Sea
Extension is localized in a narrow rift system.
