Rotations about the line of strike only affects the dip of a plane, while the strike of the plane (which is the rotation axis) remains unchanged. Because the rotational axis is already horizontal (by definition the strike line is horizontal) the operation is accomplished with a simple rotation of the net under the overlay. To accomplish this, the overlay is rotated such that the strike of the plane (the rotation axis) coincides with the N-S line on the stereonet. The specified rotation (of a line, plane, or of the pole to the plane) about the rotational axis can be traced out on the small circle on which the line, plane, or pole falls

Use stereonets to determine the original orientations of geological features (bedding, slickenlines, etc.) that have been tilted/rotated

Keep in mind that the direction or sense of rotation (expressed as CW or CCW) is dependent on which direction you are viewing the axis of rotation. For example, a 20o clockwise (CW) rotation looking north along a horizontal N-S axis instead looks like a 20o counterclockwise (CCW) rotation when viewed south along the same axis. For this reason, the direction of viewing must always be specified.

Note:

- Complete each question on a separate overlay

- 3 Key criteria:

1) The axis of rotation

2) The amount of rotation

3) the direction or sense of rotation

4. A sequence of Triassic strata unconformably overlies Silurian strata. Both groups of rocks were subsequently tilted and eroded to produce their current attitudes as shown on the sketch map below, and are in an upright orientation. The Silurian beds have an attitude of 284/74NE and the Triassic beds have an attitude of 222/26NW. A fault (dashed line and labeled f: 158/56NE) crosscuts the Silurian beds and contains slickenlines, which now pitch 35NW on the fault surface. Sense of movement on the slickenlines is predominantly sinistral (i.e., left lateral) so that in its current orientation the fault is classified as a sinistral normal oblique slip fault. a. Determine the attitudes of the Silurian beds and the fault in the Triassic period during deposition of the Triassic sediments. b. Determine the pitch of the slip on the fault (i.e., orientation of the slickenlines) during the Triassic Period. Does the fault still appear to be a sinistral normal oblique slip fault in the Triassic? If not, what would the fault be classified as? c. If the fault developed when the Silurian beds were horizontal, what was the attitude of the fault, and pitch of the slickenlines on the fault, at the time of formation? What would the fault be classified as in the Silurian? N Triassic 260 Silurian 749 56 74 1 Silurian 4. A sequence of Triassic strata unconformably overlies Silurian strata. Both groups of rocks were subsequently tilted and eroded to produce their current attitudes as shown on the sketch map below, and are in an upright orientation. The Silurian beds have an attitude of 284/74NE and the Triassic beds have an attitude of 222/26NW. A fault (dashed line and labeled f: 158/56NE) crosscuts the Silurian beds and contains slickenlines, which now pitch 35NW on the fault surface. Sense of movement on the slickenlines is predominantly sinistral (i.e., left lateral) so that in its current orientation the fault is classified as a sinistral normal oblique slip fault. a. Determine the attitudes of the Silurian beds and the fault in the Triassic period during deposition of the Triassic sediments. b. Determine the pitch of the slip on the fault (i.e., orientation of the slickenlines) during the Triassic Period. Does the fault still appear to be a sinistral normal oblique slip fault in the Triassic? If not, what would the fault be classified as? c. If the fault developed when the Silurian beds were horizontal, what was the attitude of the fault, and pitch of the slickenlines on the fault, at the time of formation? What would the fault be classified as in the Silurian? N Triassic 260 Silurian 749 56 74 1 Silurian