a valley
a ridge
This is a topographic contour base map we will use to explore structure
contours and the interplay of geologic structure contours and topography.
Click and some pertinent aspects of the map will be highlighted to remind
you about the basic nature of a topographic map like this.
The fact that
all the
constructed
lines are
parallel and
evenly
spaced
indicates the
contact is
planar. If the
lines curve it
is nonplanar.
S60W
The angle
between N
and the
strike
contour line
is the strike
angle (about
60 or
N60E/S60W
degrees
here).
= 100 m for spacing between
strike contours
Imagine that the red line represents a geologic contact of some sort. Given that it doesn’t parallel contours we
know it isn’t horizontal, and given that it doesn’t cut straight across contours we know it isn’t vertical. Can we tell
something about its orientation?
Note how we can find three points (red stars) where the geologic contact is at 200 m elevation by finding
where the contact crosses the 200 m contour interval. A line through those points represents points on the
contact at the same elevation, which is a strike line, specifically the 200 m strike contour line.
In a similar fashion we can construct additional structure contours at other elevations for the geologic contact.
Finally, using the map scale, the distance between the structure contours is about 100 m as measured
perpendicular to the strike contour lines (otherwise you are working with apparent dip). That distance lets
you to calculate the plane’s slope, i.e. dip angle. The dip angle is the arctan of the elevation difference
divided by the strike contour spacing ( here arctan(100m/100m)=45 degrees). Thus the strike and dip of
the planar contact can be obtained from its map expression. It has a 60 degree strike and 45 SW dip.
You can also start with the orientation, the strike and dip of a contact, and then map out what the map
pattern should look like. Imagine the contact is located at the red star and strikes 120 and dips 45 to SW.
First draw the strike line that passes through the red star, which is on the 100 m topographic contour (and
which therefore has to be the 100 m structure contour line. The line needs to be at the appropriate strike angle
from north. We are using the strike azimuth system so the line is at a 120 clockwise angle from North.
Then draw other parallel contour strike lines at the appropriate spacing and label them as to their elevation.
Since this contact dips 45 degrees, that same amount it dipped in the previous example (the strike is
different), the spacing is the same as in that example (chosen for convenience).
Finally, find the points where the the topographic and strike contours of the same value intersect and
mark them (here with the red stars), and then draw the line connecting the stars and you have the
expected map expression of the planar contact with the orientation 120 – 45SW
projection of 200 m strike
contour on surface
up
100 m scale bar
horizontal strike contour
spacing = 60 m
topographic profile
500 m
400 m
map distance
300 m
dip angle
elevation difference
200 m
100 m
This slide focuses on being able to compute the dip from strike contour spacing or compute the spacing from dip
value. We start out with a cross section perspective along a direction perpendicular to the layer strike (so that we
see the true dip), and with equal vertical and horizontal scales and a topographic profile. Blue lines show constant
elevation, and the red line is the cross section view of the plane of interest that is coming in and out of the board,
with the dip angle shown.
From where the geologic feature crosses the elevation contours we can identify strike contour points s at
different levels, representing horizontal strike contour lines that come in and out of the vertical plane. Then we
can vertically project their position (dashed lines) upwards to identify the map spacing between the strike
contours.
You can then use the horizontal map scale to figure out what the spacing is – 60 m in this case.
Finally, using the relationship that tan (dip angle) = elevation difference / map distance either the dip or
the strike contour spacing can be solved for. In this case it is 59 degrees.