Earth Mat Flashcards - Ch 16
___is a directed force of some magnitude applied over an area.
___is a change induced by stress
Stress; Deformation
The following conditions exist in uniform stress
The three perpendicular, principal stress axes can have any orientation as stress is equal in all directions.
No shear stresses occur.
No change in shape occurs.
Volume change can occur.
a 1 kbar (0.1 GPa) would correspond to what depth
3.3 km
The following conditions exist in non-uniform stress
Stress axes are not equal in all directions.
Shear stresses can occur on rock bodies, but not on a principal plane
Shape changes can occur.
Volume change can occur with corresponding changes in density.
Four components of deformation
distortion, dilation, translation and rotation
Homogeneous strain:
equal strain,
parallel lines remain parallel, perpendicular remains perpendicular
Heterogenous strain:
strain intensity varies,
Angular changes
Parallel or perpendicular arent the same no more :(
indicates a change in volume.
dilation
, also known as displacement , means that an object has moved from one point to another point.
Translation
infers that an object has moved in a circular arc about an axis, sort of like tightening a screw into a wall or the way in which a wheel rotates around an axle.
Rotation
like stress axes, are imaginary lines that are perpendicular to each and intersect planes of zero-shear strain
Principal strain axes,
How would stress axes and pricipal strain axes correspond to each other
σ3 , σ2 , σ1 = X, Y, Z
refers to one or more intermediate strain steps describing separate strain conditions
Incremental strain
Strain axes are denoted by the letters X, Y and Z as follows:
X = Max
Y = Mid
Z = Min
implies that no rotation of the incremental strain axes occurred from an initial to final strain state.
Coaxial strain
Coaxial strain conditions
Pure shear (irrotational strain)
Requires conditions such as
Uniform elongation at a single reaction
Uniform contraction in a perpendicular direction
Strain = stress
No volume change
is a rotational strain in which the strain axes rotate through time
Non-coaxial strain
Pure shear and simple shear are two idealized end members. ____________ is a combination of 2 shears; the most common one in the field
General shear
Non - Coaxial strain conditions
principal strain axes do not remain the same.
Simple Shear (rotational shear)
Requires conditions such as
Strain axes do not remain parallel
All strain axes rotate
Maximum elongation not parallel to direction of min compressive stress; same goes with minimum elongation, max compressive stress
refers to how materials respond to stress.
Rheology
Strain proportional to stress; linear relationship.
Hooke’s Law
elastic behavior is also referred to as
Hookean behavior
Elastic behavior can be described in terms of:
Length change (translation)
Shape change (Strain/Distortion)
Volume Change (Dilation)
Measure of resistance to shape change
AKA rigidity (G)
Ratio of shear stress to shear strain
Shear modulus
Resistance to a change in shape
Expressed as K = change in pressure/change in volume
Bulk Modulus (incompressibility)
Measure of fattening compared to lengthening
Response to compression
All earth materials increase in diameter, decrease in length
Poisson’s Ratio
Plastic behavior occurs through the
reorientation of the crystal structure
Plastic behavior is favored by
Favored by high T, high P, and low strain rate
Mechanisms of Plastic behavior
Cataclastic Flow
Diffusional Mass Transfer
Mesoscopic ductility facilitated by microscopic fracturing and frictional sliding
Low lithostatic pressure
Cataclastic flow
High PT; material flow through crystals
Diffusional Mass Transfer
Grain boundary or volume diffusion
High PT, translation w/in a mineral
Crystal lattice vacancies migrate to the greatest stress
Atoms relocate to minimal stress
Solid State Diffusion
is permanent, irreversible deformation characterized by the development of visible fractures and loss of cohesion between rock particles.
Brittle behavior
Rocks experience elastic deformation until a ________________ is attained
rupture point (rupture strength)
the material is the maximum stress level that can be achieved prior to the onset of brittle failure.
the ultimate strength
is the depth within Earth where rock behavior changes from brittle to ductile behavior
The brittle-ductile boundary (or brittle-ductile transition)
Brittle-Ductile boundary zone at
~10-20 km and ~300°C
refers to the rate at which rock is pulled apart, compressed, or sheared
Strain rate
Generally, Earth materials display the following behaviors:
Brittle behavior: shallow depth, low T, high strain rate
Ductile behavior: deeper depth, high T, low strain rate
temperatures at which change from brittle to ductile behavior.
(Bt, Qtz, Felds, Amp, Gar)
250
300
400
650-700
600-800
is a term that describes the resistance of rocks to flow. Rocks that Flow easily are less competent, or incompetent
Competency
commonly display ductile behavior, mohs scale less than 3
Incompetent
commonly brittle ductile behavior, mohs scale more than 3
Competent
refers to the amount of stress necessary to induce failure.
Strength
are isolated remnants of competent rock that once formed a continuous bed surrounded by less competent rocks; formed by rupture
Boudins
Brittle behavior commonly occurs at depths less than _________ because of upper crustal low temperature/low lithostatic pressure conditions, which allows for the development of fractures.
10 km
_______vein arrays are produced by high strain rate events that blast rock apart due to high pressures.
Non - systematic
__________ vein arrays consist of veins that display orientations suggesting a common origin in response to directed stress.
Systematic
Compressive vertical stress produces horizontal tensions; brittle fracturing ____________
forms tension veins parallel to max compression
vein array consists of a series of offset, parallel veins that formed in response to sinistral shear
En echelon quartz
Blocky/Sparry/Equant minerals indicates
growth within an unimpeded open space
displays a linear, acicular character, vein growth was incremental in response to fracture width increases.
Fibrous veins
Fibrous veins develops by a repearted cycle of crack and seal mechanism. Explain the process
Fluid pores crack a vein, seals it by precipitation
are essentially ductile faults in which displacement is dominated by plastic deformation processes rather than brittle rupture
Shear zones
are relatively straight (low curvature) layers separated by a high curvature region of the hinge
Limbs
is a point of maximum curvature separating two limbs.
Hinge
is an imaginary line connecting a series of hinge points along the strike of the fold
Hinge line
is the point at which the sense of curvature changes from one fold to another.
Inflection point
Elongate folds with a convex - upward structure are called
antiforms
Elongate folds with a concave - upward shapes are referred to as synforms.
synforms
Limbs dip toward hinge
Young at hinge
Synclines
Limbs dip away hinge
Old at hinge
Anticlines
Rock layers dip away from center
Old at center
Domes
Rock layers dip towards center
Young at center
Basins
Different folding episodes causes
interference patterns
Younger folds superimposing on older folds
Superposed Folds
consist of folds in which the limbs and hinges have been pulled apart due to extension
Occur with multiple fold generations; involves replacement of ealier tectonic fabric by a recent ductility
Transposed folds
are small folds occurring in the limbs and hinges of larger scale folds.
Parasitic folds
Characterisitcs of a parasitic fold
Upper bed displaces toward hinge, lower bed away hinge
Right rotation produces z-shape clockwise rotation
Left rotation produces s-shape counterclockwise rotation
Minimal rotation at hinge
fabric developed during lithification
Primary Fabric
fabric; Deformation process after initial lithification
Tectonic Fabric
fabric; mm scale continuous structures; no remaining undeformed parts
Continuous Fabric
fabric; some undeformed space exists between fabrics; visible deformed and undeformed parts
Spaced Fabric
fabric; No orientation
Random
fabric; arrangement in a predictable manner; 2 main classes:
Folations (planar)
Lineations (linear)
Preferred Fabric
are sheet - like structures that include joints, veins, faults, axial surfaces of folds, shear zones and cleavage.
Planar features
Parallel folations near perpendicular to maximum compressive stress; converges towards inner arc of hinge
Axial planar cleavage
Where S1 = Axial plane cleavage and S0 = Bedding angle
S1 > S0 would be
upright fold
Where S1 = Axial plane cleavage and S0 = Bedding angle
S1 < S0 would imply
complex folding patterns occured
form by the intersection of two planar fabrics
Intersection lineations
is an intersection lineation marked by the development of elongate, pencil - like shards
Pencil cleavage
are linear features that occur as a result of a secondary cleavage imposed upon a fine - grained rock (slate or phyllite) that experienced an earlier cleavage
Crenulation lineations
refer to vein mineral fibers that precipitate on rock surfaces via crack – seal processes
Slip or fibre lineations
are fibre lineations produced during displacement in faults and shear zones
Slickenlines
