Problem with Euler poles and small circles

Q&A
1

I made a simple test file with a few polygons and lines. In a reconstruction the polygons and lines do not move parallel to the small circles, but also have a component in the perpendicular direction. This is not supposed to happen as far as we know.

I use GPlates version 2.2.0 have tested in both Windows and Ubuntu.

2

70Ma 100Ma
This is an illustration of the problem. The end of the yellow line moves away from the small circle, while it should maintain the same distance.

3

65Ma 75Ma
Also a point that starts at the pole moves away from it as illustrated here.

4

Can you send your example files?

I guess it depends on how the rotations are specified in the rotation file. For example, if I use something like…

001 0.0 90.0 0.0 0.0 000 !
001 200.0 90.0 0.0 180.0 000 ! rotation about North pole

…then geometries (with plate ID 1) rotate in small circles about the North pole.

Are you creating the small circles with a non-zero plate ID?

Regards,
John

5

Thanks John!
I just sent you an email with the files.

6

Hi Thomas,

Thanks for sending the files.

The rotation file has a rotation for plate 200 relative to 300, and it looks like the small circle has a centre (pole) representing this relative rotation. And the green polygon is on plate 200, however it moves relative to plate 000 (via the plate circuit 000->300->200). So it will move along a different small circle than 300->200.

If you give the small circle a plate ID of 300 (instead of 0) then the green polygon (200) will rotate parallel to it (easier to see by also setting the anchor plate ID to 300). This is basically attaching the visual representation of the 300->200 rotation (ie, the small circle) to plate 300 since the green polygon is also moving relative to 300.

Regards,
John

7

Hi John,

Thanks for your advice!
I’ll have another look at what happens.

Regards,
Thomas