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Ice Deformation (Glacial deformation (Geometrical reference frame = fixed,…
Ice Deformation
Glacial deformation
Geometrical reference frame =
fixed
Deformation structures develop progressively
as glacier moves over bedrock
∴ Space = proxy for time
Walking along glacier = moving through timeline
Pure Shear
1. Relative length of edges changes
2. Edges remain parallel to one another
i.e. irrotational
Simple
Shear
Vertical sides
of square
Rotate as sheared
Remain parallel
wrt eachother
Progressively lengthen
Horizontal
lines-unchanged
do not change length
remain parallel
Square ---> Parallelogram
deck of cards sheared wrt eachother
Finite vs Infinite Strain Ellipse
Two main ellipses at any stage of deformation
Finite strain ellipse
represents
cumulative, total deformation
2.
Infinitismal ellipse
represents strain for an
instant in time
Finite
strain
= sum of incremental strains
= total strain from progressive deformation
variety of strain paths may lead to same finite strain ellipse
Strain
= any change in shape
with/without volume change
implies rock particles changed position wrt eachother
Coaxial
def path
Principal strain axes remain
parallel
to same material lines throughout strain
i.e. irrotational deformation
Strain
Change
Ice Cap/Sheets
Ice divide -
pure shear
Dome centres -
uniaxial shortening
With increasing depth/ distance from divide
Strain is increasingly non-coaxial (rotational)
i.e. basal ice along glacier margins/ice streams
Valley
Glacier
Along
centreline
close to
coaxial
(non-rotational)
Base/sides
Strongly non-coaxial (rotational)
Deformation
= transformation from an initial to a final geometry
by means of
rigid body transformation
rotation
&/or volume change
strain
Strain
ellipsoid
visualisation of 3D deformation change
i.e. deformed shape of imaginery sphere with unit radius deformed along with rock
Analogue for rock deformation & metamorphism