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. 684-716)

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 labA 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" ProblemAny 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