Field trip this Saturday! We will leave from the loading dock at 7:30 AM sharp! HYDRATE... or else! BRING >1 GALLON OF WATER!

Techniques for projecting and plotting structural data (p )

Structural measurements involve 2 angles: (1) Compass direction (azimuth) in horizontal plane (2) Angle down from horizontal

Stereographic projection plotting 3D structural data on a hemisphere (usually the lower), which is projected onto a horizontal plane bring tracing paper to lab next week

All lines and planes are imagined to pass through the center of the hemisphere Lines intersect the lower hemisphere as points. Shallow lines plot near the perimeter Steep lines plot near the center Let's plot the following lines: (1) plunge = 70, trend = 60 (2) plunge = 10, trend = 340

Planes intersect the lower hemisphere in the form of great circles. Let's plot the following planes: (1) strike = N-S, dip = 90 (2) strike = 30, dip = 60 (3) Strike = 120, dip = 10

A plane can also be represented by its pole, which is a line perpendicular to the plane. Let's plot the pole of the following plane: (1) strike = 340, dip = 60

Stereonets are useful for presenting a large amount of structural data and for statistical analysis. Stereographic projection is a powerful tool to solve a number of interesting structural problems

Example: Problem from lab A fault plane is oriented 250, 68. Which of the following lineations cannot lie within it? a) plunge = 68, trend = 250 b) plunge = 68, trend = 340 c) plunge = 0, trend = 70 d) plunge = 80, trend = 310

Rake = The acute angle between the horizontal (strike line) and a line in the plane, MEASURED IN THE PLANE

Orthographic projection: 2D line drawings that are used to determine angular and spatial relationships in 3D

Determining the true thickness of a bed For a dipping bed, the map-view thickness is an "apparent" as opposed to "true" thickness! 1. Draw a structural profile (X- section) perpendicular to strike 2. Plot the true dip of the beds and project them to depth 3. Use trigonometry to calculate the true thickness

Constructing structure contour maps project surface data to depth plot points along bed at various depths project points vertically to surface make structure contour map

"3-point" Problem Any three points on a plane (not in a straight line) can be used to determine the attitude of that plane 350m strike invtan (200/350) = 30 degrees

Stereographic Projection (Summary) Imagine planes or lines being projected onto a hemisphere Lower hemisphere projection (always look from above) - convention