Problem 5.6 You are mapping metamorphic rocks and you notice a lineation within
ID: 115406 • Letter: P
Question
Problem 5.6 You are mapping metamorphic rocks and you notice a lineation within the rocks. At five different outcrops you measure the pitch of the lineation on an exposed planar rock surface (not necessarily the same struc- tural surface at each outcrop). The table below lists the attitude of each exposed surface and the pitch of the lineation on that surface. Attitude of Outcrop no. surface Pitch of lineation 300°, 84°NE 76°E 350°, 30°E 50°N 040°, 70°SE 63 SW 337°, 30°W 50 S 272°, 45°N 59° You hypothesize that this lineation represents a planar fabric within the rock. Test this idea by de- termining whether these five lineation orientations are coplanar. If so, what is the attitude of the plane?Explanation / Answer
lines or planes on the stereonet. Imagine three lines—A, B, and C—with plunges of 308, 608, and 908, respectively, all lying within a vertical, north–south-striking plane. Figure 5.14a is an oblique view of these lines intersecting a lower hemisphere, and Fig. 5.14b shows the same three lines projected on the equal-area net. As the originally vertical, north–south-oriented plane rotates around the horizontal north–south axis of the net, the projection points of lines within the plane move along the small circles. Figure 5.14c shows points A, B, and C moving in unison 408 to points A’, B’, and C’ as the plane rotates 408 to the east. As the plane rotates 908 to a horizontal position, the projected points A’’, B’’, and C’’ lie on the primitive circle. Notice on Fig. 5.14c that when the plane is horizontal each line is represented circle, one in the northeast quadrant and one in the southwest quadrant. As the plane continues to rotate, A and B leave the northern half of the hemisphere and reappear in the southern half. In other words, when you rotate lines beyond the primitive circle you change from plotting the ‘‘head’’ of the line to plotting its ‘‘tail.’’ This occurs because only the lower hemisphere of the stereonet is used for plotting data. Figure 5.14d shows the projection points of the three lines as the plane in which they lie rotates 1808 from its original orientation in