Lab 5: Structural Analysis Using
Stereonets (2 weeks, focus on the Arbuckles) (30 pts.):
Background information on the use
of stereonets in structural analysis.
The above is an equal area stereonet
projection showing great circles as arcuate lines connecting the
North and South Points and small circles as arcuate lines in a
latitudinal type position. The great circles represent north-south striking
planes with dips in 10 degree increments. Those labeled with dip
amounts on the left side, dip to the west. If the same plane was
rotated about a vertical axis in the stereonet center, they would
then retain their dip, but have a different strike. The numbers
in the upper right quadrant represent potential strike line positions
from 10-80 degree, in 10 degree increments (see below diagram). Small circles represent half of a conical surfaces with the apex at hemisphere center. They are hemisphere surface paths from one line being rotated about another line (the pole of rotation), both passing through the hemisphere center.
- understand the geometry of the stereographic
- understand the projections uses in structural
- be able to plot by hand strike and dip and
trend and plunge on a stereonet.
- be able to use a program to plot and contour
- be able to interpret the patterns on such
Some stereonet facts:
The above diagram shows the same
plane in two positions. The blue plane position is where North
has been rotated so that the great circles all have a strike of
N45W (315). In this case the North position is designated in blue. In this
position it is easy to trace out the great circle with the appropriate
dip, here 50 degrees to the NE. The green represents the plane's
orientation when North is rotated back to its standard top-of-the-stereonet
The open and filled red stars represent
two lines (solid 124 58, open 214 37) and the dashed red great circle
represents their common plane with a strike of 70-60SE. The green
stars and great circle represent that line rotated 35 degrees
counter clockwise so that the filled star is on the equatorial
plane where you can count its plunge as about 58 degrees. The
blue represents the position where you can count the plunge of
the open star as about 37 degrees.
In the above diagram two planes
are plotted, one red, one blue. The strike and dips are given
to the left. The green point represents their common line, i.e.
the intersection between the two lines. The trend and plunge is
given as 89 32. Remember the convention is that the first number
represents the trend direction and the second represents the plunge
- In contrast to mineralogy, structural geologists
use lower hemisphere plots. Typically, it is as if you are looking
into a hemispheric bowl sitting on the ground, and oriented along
a N-S axis.
- There are an assortment of different types
of stereonets (equal angle, equal area, polar). You should know
which one you are using and why. One can recognize them by their
- Great circles look like lines of longitude,
and represent where planes with a north-south strike but incrementally
varying dips intersect the outer hemisphere surface.
- Small circles look like lines of latitude,
and represent where cones with a north-south horizontal axis
and incrementally increasing apical angles intersect the outer
- You can plot two types of features on a stereonet,
a line (e.g. trend and plunge) and a plane (e.g. a strike and
dip). A line plots as a point and a plane plots as a great circle.
- All elements plotted pass through the center
of the hemisphere, and their projection reflects where the plane
or line intersects the outer hemispheric surface. This means
that information about relative position is not represented in
the plot, only information about relative orientation.
- Planes are often represented by plotting
their pole, which is the line perpendicular to that plane.
- More complex structural features can be represented
by plots of multiple elements. Some examples are:
- a) a fault plane, with the direction (line)
of movement on it.
- b) a cylindrical fold, often viewed as a
great circle distribution of poles to the layering being folded.
- c) a conical fold, often expressed as a small
circle distribution of poles to the layering being folded.
- d) an angular unconformity, expressed as
the difference between the orientation of the bedding above and
- e) a population of joints or any other structural
feature, often expressed as a data cloud with dispersion around
some average value.
- You can also use a stereonet to:
- find the intersection between two planes
(e.g. the fold axis if folding is cylindrical).
- find the angle between two lines, two planes
or a line and a plane.
- to find the restored orientation of a geologic
feature such as a cross bed once it is rotated about some axis.
- bisect the angle between two planes (e.g.
if you are trying to model kinematic axes or principal stresses
associated with conjugate faults).
Some structural elements whose orientations
can be plotted on a stereonet are: bedding, cross beds, joints,
foliations, fault planes, veins, dikes, fold axes, fault striae, slickensides, boudin axes,
long axes of inequant objects such as fossils, parting lineations,
shatter cone axes and more.
The first part of your stereonet lab will explore
the mechanics of manually plotting elements on a stereonet, while
the second part will focus using computer programs to contour
data and make analysis. Remember it is always good to know what
the black box software program is doing for you.
You will need the following materials in order
to proceed with this and the subsequent stereonet exercises: a)
an equal-angle stereonet taped on to cardboard (or other stiff
backing), b) a tack, which can be pushed up through the center
of the stereonet so that the point is showing, c) onion skin paper,
which is used as an overlay for plotting the structural features,
d) colored pencils to help differentiate the various elements
plotted. The onion skin overlay permits you to rotate the points
being plotted with respect to the underlying, fixed reference
frame. As you start plotting points you will see why this is necessary.
Part 1 - Plotting and manipulating
elements on a stereonet.
This part needs to be done with pencil and tracing paper, with a stereonet projection underneath.
A) Plot the
following two planes: 310 - 55W and 65 -70E. Label each one clearly.
the trend and plunge of the intersection. To find the plunge rotate
the intersection point to the vertical equatorial plane and count
up from the intersection point to the nearest periphery point
in degrees along the equatorial plane - that is your plunge angle.
The plunge direction is evident as the quadrant your associated
periphery point lies in and the angle along the hemisphere periphery
from underlying N to the periphery point is your trend.
trend ___________ plunge _______________
C) Plotting the
poles to each of those planes and label them. To plot the pole
rotate the great circle representing the plane so that it's strike
line is oriented N-S, then count 90 degrees along the equator
passing through the middle point of the stereonet. The point you
arrive represents a line perpendicular to the plane you started
with, i.e. the pole.
D) Finding the
angle between the poles and thus between the two planes. In order
to do this, rotate the two pole points until they fall on the
same great circle. Then count along that great circle in degree
increments moving from one point (pole) to the other. That is
the angle desired. If it is less than 90 degrees it is the acute
angle, otherwise it is the obtuse angle.
acute angle ______________
E) Find the acute
bisector of the two plane. Along the common great circle containing
the two poles count in degree increments half of the angle found
in D above. Mark that point. This is the bisector. It could represent
a principal stress for a conjugate fault pair.
trend ___________ plunge _______________
F) Now rotate
the bisector point and the intersection point of the two lines
to a common great circle and draw and label that great circle..
That great circle is the bisecting plane. Its strike and dip is
G) On a new sheet
of paper plot the following two lines. 205 30 and 352 45. Label
each one clearly.
the strike and dip of the common plane those two lines define.
To do this rotate the two lines until they fall on one great circle.
The strike and dip of that great circle is that of the common
strike ______________________ dip __________________________
the angle between the two lines by counting along the common great
circle in degree increments from one to the other line.
acute angle ___________________
J) On a new page,
plot the following line 186 40 and then find the family of lines
(points on the stereonet) that is 20 degrees away.You can do this
by simply rotating the point representing the line on to any great
circle, and then count along that great circle 20 degrees in both
directions and mark those points (which will be two lines 20 degrees
either side of the first). Repeat this on another nearby great
circle. What is the form that results?
2 - Stereonet software, contouring data, and analysis.