Strain markers are objects in deformed rocks that can be used to measure strain. Good strain markers include reduction spots, pebbles, fossils, fold sets, and lineations. Spherical markers originally circular in cross-section become elliptical due to homogeneous deformation, with the ratio of major to minor axes indicating strain. Fossils with lines of symmetry like trilobites and brachiopods can indicate the principal strain directions. Fold sets allow comparing initial and final layered sequences to analyze strain. Schistosity and lineations in metamorphic rocks can also act as strain markers.

1. In naturally deformed rocks
Vivek Kalyan Bhore
M.Sc. Sem-1
Department of Earth and Environmental
Science
KSKV Kachchh University

2. What is Strain?
 Deformation caused by stress.
 Aspects of shape change measured in line length,
angular relationship between lines, or volume.
 Analysis of strain include :-
1. translation (change in position)
2. Rotation (change in orientation)
3. Dilation (change in size)
4. Distortion (change in shape)

3. Strain Markers
Many rocks contain evidences for strain.
To actually measure strain we need strain markers & is
measured using objects(strain markers) of known
initial shape.
Reduction Spots

4. Defination
A deformed feature in the rock that can be measured
to determine strain.
a) Have to know the original shape for comparison.
b) Should have the same mechanical characteristics as
the original rock.

5. Strain markers :-
Good strain markers include :
• Reduction spots
• Pebbles
• Fossils
• Fold sets
• Lineations

7. Spherical Markers
 These are approximately spherical in their initial stage.
 In a section through undeformed rocks these appear
circular.
 If the rock is homogeneously deformed the circles are
transformed into ellipses
 The ratio of the major and the minor axes of an
elliptical marker gives the ratio of principal axes of the
strain ellipse.
 Strain ratios are commonly referred to the three
principal strain axes : X=long axis, Y=intermediate,
Z=short axis.

8. Fossils
Most beautiful & hence, popular strain markers
are the trilobites & brachiopods with line of
symmetry.
If the line of symmetry lies parallel to a
principal strain direction, then the angular
shear strain of the principal lines in the fossil
are zero.
If the initial lines of symmetry are not parallel
to principal strain directions the lines of
symmetry appear to shear during deformation.
Final shape of the brachiopod or trilobite is an
oblique form.

9. Deformed Ordovician trilobite.

10. Fold sets
 Fold sets can be used as strain markers by comparing
the initial and final shapes of the layered sequence.
 Strain involved in folding may be analysed in 2-D only,
in the plane perpendicular to the fold axis, by
comparing the initial and final lengths of the folded
layer.

11. Schistosity
 Metamorphic rocks of metasedimentary origin may be
marked by schistosity parallel to original bedding of
the sedimentary rocks.

12. Lineations
 All linear structures, regardless of their mode of origin,
are referred to as ‘lineation’.
 There are many causes for the formation of these
structures.
 These structures are also formed due to strain.
 Hence, they may act as strain markers.