Structural Geology 3443 Lab 6 Stereographic Projections

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Structural Geology (3443)Lab 6 Stereographic
Projections
Department of Geology University of Texas at
Arlington
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Poles to Planes
So far, we have represented a plane on the
stereographic projection as a great circle. If we
plot many planes, the projection soon gets
cluttered.
A plane, however, can be represented by its pole,
the unique line drawn perpendicular to the plane.
Because the pole is a line, it plots as a point
on the stereographic projection.
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Poles to Planes
Problem 6-1 A bed is 330, 50 (N30W, 50SW) Plot
the plane and its pole. Plot the plane as usual
(6-2b). Then plot the point 90o away from the
plane along the horizontal axis and that is the
pole. You get the same answer by measuring the
dip angle from the center of the projection
instead of the edge.
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Poles to Planes
Do exercise 6-1.
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Poles to Planes
Problem 6-2 Intersection line. The layers on the
limbs of a fold will intersect along a line that
is called the fold axis. One limb is 200, 60 and
the other is 060, 40. What is the trend (bearing
and plunge of the fold axis?
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Poles to Planes
Problem 6-2 Intersection line. The layers on the
limbs of a fold will intersect along a line that
is called the fold axis. Plot both the layers and
their poles labeling each. Put a dot where the
fold axis line plots.
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Poles to Planes
Problem 6-2 Intersection line. The layers on the
limbs of a fold will intersect along a line that
is called the fold axis. Now rotate the net so
that the 2 poles of the layers line up along a
great circle and dash in that great circle.
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Poles to Planes
Problem 6-2 Intersection line. The layers on the
limbs of a fold will intersect along a line that
is called the fold axis. What is the relationship
between the fold axis and the great circle that
passes through the poles of the layer?
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Poles to Planes
As a general rule, poles to beds folded
cylindrically will all plot on a great circle
perpendicular to the fold axis.
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Poles to Planes
Problem 6-3 Dihedral angle between two planes.
Limb A of a fold is 020,70. Limb B is 150,65.
What is the interlimb angle (dihedral angle
between the layers)?
Plot the planes and poles as before labeling
each. Dash the great circle passing through the
poles. The dihedral angle is the angle measured
in the plane perpendicular to both planes (the
dashed line) what is it?
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Poles to Planes
Problem 6-3 Dihedral angle between two planes.
Limb A of a fold is 020,70. Limb B is 150,65.
What is the interlimb angle (dihedral angle
between the layers)?
Now measure the angle between the two poles. What
is its relationship to the inter limb angle of
the layers? How would you get the inter limb
angle if you just plotted the poles and not the
planes?
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Poles to Planes
Problem 6-4 angle between a line and a plane
must be measured in a plane that passes through
the line and the perpendicular to the plane.
A fold hinge plunges 260,30. The fold is exposed
on a joint oriented at 150, 60 What is the angle
between the fold hinge and the joint?
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Poles to Planes
Problem 6-4 A fold hinge plunges 260,30. The
fold is exposed on a joint oriented at 150,
60 What is the angle between the fold hinge and
the joint?
Plot the hinge line, the joint, and the pole to
the joint. Rotate the overlay so the pole and the
hinge line lie on the same great circle and dash
it in. This is a plane that is perpendicular to
the joint containing the hinge.
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Poles to Planes
Problem 6-4 A fold hinge plunges 260,30. The
fold is exposed on a joint oriented at 150,
60 What is the angle between the fold hinge and
the joint?
Now measure the angle along the dashed great
circle between the hinge line and the plane. Now
measure the angle between the hinge and the pole
along the same great circle. What is the
relationship between the two angles you measured?
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Poles to Planes
Problem 6-5 Bisecting the dihedral angle
between two planes. E.g. The axial plane of a
fold commonly bisects the two limbs.
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Poles to Planes
Problem 6-5 Limb A of a fold is 240,40 and Limb
B is 100,50. What is the attitude of the axial
plane?
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Poles to Planes
Problem 6-5 Limb A of a fold is 240,40 and Limb
B is 100,50. What is the attitude of the axial
plane? Plot the 2 limbs, their poles, and dash in
the great circle containing the poles. Put a
heavy dot at the fold axis.
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Poles to Planes
Problem 6-5 Limb A of a fold is 240,40 and Limb
B is 100,50. What is the attitude of the axial
plane? Rotate the overlay so that the dashed line
containing the two poles lie along a great circle
and find the point the bisects the limbs.
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Poles to Planes
Problem 6-5 Limb A of a fold is 240,40 and Limb
B is 100,50. What is the attitude of the axial
plane? The axial plane contains the hinge line
and the point that bisects the limbs. Rotate the
overly so that those two points lie on a great
circle and draw it in that is the axial plane.
Now find its attitude (strike and dip).

• Do exercises 2, 3 4

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Exercises
Do the following exercises in lab 6 1, 2, 3, 4,
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Rotations
Rotation about a vertical axis A line is 230,
40. What is the orientation (trend plunge)
after a rotation of 40 clockwise about vertical
axis?
Also do problem 6-7
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Rotations
Rotation about a Horizontal axis A Mineral
lineation is 220,30. What is its orientation
after a rotation of 30 about a horizontal axis
trending 350. Assume a counterclockwise rotation
looking the that direction.
Also Do problem 6-9
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Rotations
Rotation about a Horizontal axis A layer is 200,
70. What is its orientation after a
counterclockwise rotation of 30 about a
horizontal line trending 350.