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.
- 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.