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Rotation

From technical point of view only lines can be rotated. A plane can be rotated as a line, which is its pole.

The sense of rotation must be specified as well as an angle. Te sense can be specfied as clockwise or anti-clockwise (at the picture clockwise rotation is marked). We rotate structural elements about a rotation axis, which orientation must be also specified.

To rotate the line, plot it on the net. Turn the net until the great circle "0" reaches the axis of rotation. Then move the point representing the line along the small circle by an amount of angle of roation.

Example:
The line 35-260 rotate clockwise by 80 degrees with axis of rotation with strike 225.
Firstly estabilish the line and the strike of axis of rotation. Turn the net until marked strike joins the great circle "0". Then move the line along the small circle by 40 degrees, amount of angle of rotation. Finally the line notation is: 50-225.

When the angle of rotation is high or the line is close to the primitive circle. It's possible that we cross the primitive circle. In that case continue rotation by the small circle with the strike increased by 180 degrees.

Rotation is used for restoration of i.e. tilt of beds, palaeocurrents.

The above figures shows beds before and after rotation. At first the rocks lying beneath the unconformity were alredy tilted when the jurassic were laid down. However, now jurassic beds is also tilted, so the dip of both beds is now different from their jurassic attitude. At second we restore the dips of both beds to their jurassic attitude by rotation.

The pole of an horizontal plane will occure at the center of the net.

Rotation is explained with an aid of an example:
We observe an unconformity where the younger limestone beds (124/50/NE) are underlined by older sandstones (304/50). Find the tilt of sandstone from the period when limestone were deposited .
On the overlay estabilish the planes as their poles. To restore the tilt of sandstone rotation the limestone to the horizontal position is needed, so the strike of this beds we choose as a rotation axis. Turn the net until an axis reaches the great circle "0". Move the pole of limestone bed clockwise along the small circle by angle of 50 degrees that the pole occures at the center of net. Move the pole of sandstone along the small circle by the same amount of angle. This is the projection of horizontal limestones and the sandstone lying beneath. Tilt of sandstone is 268/78.

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Autor: Kamil Wojtanowski[Contact]