An introduction to stereographic projections
Complete the following exercises in Marshak and Mitra by hand, using the equal angle stereonet provided to you in the back of M&M.
Marshak and Mitra problems |
Page # |
5-1: plotting planes |
102 |
5-2: plotting lines 5-3: plane from lines, rake, angle between lineations |
103 |
5-6: apparent dip from true dip 5-8: true dip from apparent dip |
104 |
Now, solve these problems using one of the computer programs, found at http://home.earthlink.net/~schimmrich/structure/structure.html.
Marshak and Mitra problems |
Page # |
6-1: plotting planes and their poles 6-2: fold geometry |
124 |
And the following problem…
In areas where structural measurements have been made it is often convenient to enter the data on equal-area (see pp. 157-62, 164-66, etc…in M&M) nets. The method of plotting is identical to that used with the equal-angle net, but the rectangular areas in the net occupy the same percentage of space so that points in the net can be counted and contoured for statistical purposes.
If the beds or foliations in a region have undergone cylindrical folding (pp. 157-59, 269 in M&M) and we plot their attitudes on the net, the planes tend to intersect at a point (or several points), called the b-axis; it coincides with the trend and plunge of the fold axis in the region.
This diagram, called a b-diagram (pp. 157-58, M&M), becomes very crowded when there are a large number of planar data. Hence it is generally more convenient to plot the poles to the planes, rather than the planes themselves. This is called a p-diagram (pp. 158-62 in M&M). If the fold is cylindrical, the points plotted will be distributed along a great circle on the stereonet. This arc, called a girdle (pg. 156 in M&M), has a pole normal to it, which is the fold axis.
Use the following array of data to construct b and p-diagrams (on different traces) and answer the following questions:
N45W, 80NE |
N27E, 41SE |
N50E, 44SE |
310, 40SW |
N3W, 55E |
S75E, 60SW |
277, 60SW |
0, 50W |
292, 70SW |
N68E, 60SE |
N60E, 70SE |
N20E, 70NW |
N25W, 60NE |
335, 48SW |
N82W, 44SW |
S40W, 90 |
18, 42SE |
N7E, 60NE |
N65E, 44SE |
S82W, 50SE |