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

Analogue for rock deformation & metamorphism

Coaxial
def path

Principal strain axes remain parallel to same material lines throughout strain

i.e. irrotational deformation

Pure Shear

1. Relative length of edges changes

2. Edges remain parallel to one another

i.e. irrotational

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)

Valley
Glacier

i.e. basal ice along glacier margins/ice streams

Along centreline

Base/sides

close to coaxial (non-rotational)

Strongly non-coaxial (rotational)

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

  1. 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

Deformation

= transformation from an initial to a final geometry

by means of

rigid body transformation

rotation

&/or volume change

strain

Strain

= any change in shape

with/without volume change

implies rock particles changed position wrt eachother

Strain
ellipsoid

visualisation of 3D deformation change

i.e. deformed shape of imaginery sphere with unit radius deformed along with rock