Lab 10: Kinematic Indicators and Strain Analysis - Geophysics at ...

Lab 10: Kinematic Indicators
and Strain Analysis

Fault Rocks and Crustal Depth

Mylonite Zones
• mylonite zones are formed
by ductile deformation in the
mid and lower crust
• there are many features
formed in mylonites that give
us clues as to the sense of
shear during deformation

Mylonite Zones Cont…

S-C C Fabrics
S-surfaces =
foliation planes
(from French word
for schistosity)
C-surfaces = shear
bands (French
word for shear is
cisaillement)

S-C C Fabrics Cont…

Mantled Porphyroclasts

Mantled Porphyroclasts Cont…

Mica-Fish

Mica-Fish Cont…

Brittle Shear Sense Indicators
• brittle fracturing can produce features like tension
gashes and chatter marks

Microfaults and Imbrication

Fibrous Slip Lineations

Slickenlines
• linear features,
known as slickenlines,
are formed by the
rocks on two sides of
a fault sliding past
each other
• slickenlines indicate
the general sense of
motion on a fault, but
they cannot tell you
absolute motion by
themselves

Tectonic Lineations

Pure and Simple Shear
•in simple shear, the finite stretching
axes (S1 and S3) rotate during
deformation
• in pure shear, the finite stretching axes
(S1 and S3) do not rotate, but rather
shorten in one direction and lengthen in
the other direction

Angular Shear and Shear Strain
• angular shear (ψ) describes the change in angle between two lines
that were originally perpendicular to each other
• shear strain (γ) is another way to describe this change in angular
relationship as the horizontal translation of a line

The Strain Ellipse
• the strain ellipse allows us to
describe the amount of
deformation that has occurred in
an originally circular body
•S 1 = the direction and
magnitude of maximum finite
stretch (long axis, e 1 )
•S 3 = the direction and
magnitude of minimum finite
stretch (short axis, e 3 )

Stretching and Elongation
l
l
−
l
S =
f
e
=
f
o
l
l
o
o

Wellmann Method of Strain Analysis
• uses bilaterally symmetric objects, such as shell fossils to
determine a strain ellipse for a sample

Wellman’s s Method Cont…
1. number each fossil
2. draw arbitrary reference line
3. draw reference line/pts. on tracing paper
4. draw hinge and median lines at each
reference point for each fossil
5. make a point for line intersections 6. draw best fit ellipse through points