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Notes Structural geology & tectonics
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Summary of the lectures and reading material for the Structural Geology and Tectonics course
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Lecture 1, 08-02-2023Deformation in the Earth’s crust and mantle:
- Geometry: description of structure
- Kinematics: motions that resulted in
development of structures
- Dynamics: forces, responses of rock
(mechanical properties)
Strain à change in shape, with or without
volume change.
Deformation is also possible without strain. The
deformation can then be found with an internal
and external reference frame. Internal à
changing in shape, external à with respect to.
Strain can be homogeneous or heterogeneous.
Homogeneous:
- Straight lines remain straight (2D)
- Parallel lines remain straight (2D)
- Planes remain straight (3D)
- Rigid rotation and translation
Heterogeneous: the state of strain (strain ellipse or ellipsoid)
varies within the area or volume in question. Strain and
volume/area change.
Scale is important, a deformation that is homogeneous on one
scale can be heterogeneous on a different scale.
Different measures of strain:
- Linear strain DL
o Difference between final and initial
o (L-L0)/L0 = elongation
o L/L0 = stretch
o (L/L0)2 = quadratic elongation
o Ln(S) = natural strain = related to strain
rate
- Volumetric strain DV
o (V-V0)/V0
o V/V0
- Shear (angular) strain Dy
o g = tany
o Angular shear describes the change in angle between two originally
perpendicular lines in a deformed medium.
Strain ellipse:
- Axes S1, S2 (or x, y)
- Ratio S1/S2
- 2 lines of no finite longitudinal strain
,Lecture 2, 10-02-2023
FLINN diagram and logarithmic FLINN diagram. Logarithmic describes volume change, to the
right is decrease to the left is increase. When you’re looking at 1 line, to the left is the
constriction field and to the right the flattening field.
Rf-f’ method:
The effect of deformation on a series of elliptical
objects. The original objects are ellipses Ri. The ellipses
deform into ellipses of a new axial ratio Rf. The final
ratio Rf depends on Ri, strain ratio Rs and orientation f’.
A. Pebbles in different orientations, different
shapes. The initial ratio is 2 so both plot on the Ri
2 line.
B. Shortened in vertical direction, ratio goes down,
angle decreases of the green one.
C. Strain ratio is more à more shortening.
When Rs<Ri à 𝑅𝑠 = $𝑅𝑓𝑚𝑎𝑥/𝑅𝑓𝑚𝑖𝑛 and
𝑅𝑖 = $𝑅𝑓𝑚𝑎𝑥 ∗ 𝑅𝑓𝑚𝑖𝑛
When Rs>Ri à 𝑅𝑠 = $𝑅𝑓𝑚𝑎𝑥 ∗ 𝑅𝑓𝑚𝑖𝑛 and
𝑅𝑖 = $𝑅𝑓𝑚𝑎𝑥/𝑅𝑓𝑚𝑖𝑛
Angular method:
- In 2D
- Uses distortion/shearing of objects that were originally symmetrical.
- Strain ratio only
- Formula’s: g= f(f’, f, R)
f’ = final orientation
f = initial orientation
R = ratio s1 / s2
- With the Breddin curve
, Bulk method (Fry):
- In 2D
- Uses relative positions of centres of grains, ooids etc.
- Assumes spheres/circles before straining.
- Best fit ellipse through ‘band’ of highest data density: strain ratio
- Improves through ‘normalization’ for size.
- With the dots moving
- Zooms in on grain aggregates à elliptic shape forms à strain ellipse.
Lecture 3, 15-02-2023
Different fold mechanisms:
- Buckling: shortening parallel to the layering
- Bending: equal but opposite torques (intrusion)
- Passive/shear folding: flow
Passive shear folding:
- Contrast is material properties are small
- Markers indicate bulk strain
Reduction spots à reduction of ferric oxide (Fe2O3) to ferrous oxide.
Bending Buckling
Initiation Pairs of forces: equal but Instability needs (parallel to
Compression opposite torques layer)
Net shearing (perpendicular to layer)
Orthogonal flexure: lines originally perpendicular and remain so
Flexural shear: internal shearing of layers
Solution flexure: volume loss (notably in inner arc)
Different position of the neutral surface
No neutral surface, different orientation long axis in hinge
No strain in outer arc in case of bending
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