Plate Reconstruction

Hi everyone,
i am quite the newbie to GPlates and currently have to reconstruct “plate tectonics” on Ganymede. Since I only have recent pictures, I have to reconstruct the plate movement backwards. I startet by creating continental fragments of the terranes and now I am moving them back in time. The problem I’m currently facing is that I don’t know how to merge to two continental fragments together. Say I have a continental fragment with the plate ID 203 and have reconstructed it to move to a fragment with the plate ID 204. In geological terms i now have a bigger continent that’s breaks apart moving along further in time to the present. I have to move the bigger continent even further back in time to another fragment, but I can’t figure out how to merge my two continental fragments as one.
I hope my question isn’t to confusing (I don’t really know how to put in in other words) and someone on this platform can help me.
Best regards,

Hi Greta,

I’d suggest to keep it simple - probably the easiest way to do this is to fix one plate with respect to the other to make them move together further back in time.

You have your plates 203 and 204 moving independently back until 203 has reconstructed to a pre-breakup position relative to 204. From then backwards you simply have to keep the rotation pole parameters of one plate constant with respect to the timestep when the plates joined (the fixed plate id is specified in the 6th column in the rotation pole specification below).

You have to decide on which plate id you keep as moving plate id with respect to whatever absolute reference frame (or hierarchy) you use (sounds as if this 204) . The stage poles for 204 simply stay the same for times earlier than your merge/breakup time. Note that this requires a hierarchical setup of the rotation tree like the ones for Earth in the GPlates sample data/rotation files.

On earth this is a case when (going back in time) we close the South Atlantic: Plate 201 (South America) is joined with Africa (701) before 145 Ma, with the poles for AFR (714) determining the rotations for both plates:

201 145.0   51.78  -34.96   53.08  714 !SAM-AFR 
201 250.0   51.78  -34.96   53.08  714 !SAM-AFR

This lets just the polygons move together, it does not alter the geometry of the two joined plates - they will be two independent polygons features moving together. To form one single geometry breaking in two/merging in one you’d have to look into topological geometries.

A simpler version for the “change feature geometry” problem would be to simply build a single “envelope” polygon by unionising the two independent shapes and let that disappear (age of disappearance/to age) at the time when your 204 and 203 plate are born and start to move independently.

Hope this helps,

Hey Christian,
Thank you so much! I was still a little confused at first, but I forgot to check the Highlight Children box. Now it works perfectly with 204 moving along smoothly with 203.

Thanks again and Best regards