I worked a little more on the basic map. Here’s the result:
As you can see, I rearranged the plates a little, and the map is more detailed now. Light blue is continental shelf. Dark blue are trenches. The brown spots are mountainous areas. The names of the plates are quite arbitrary, I could have just as well numbered them.
Basically, there’s one major divergent plate boundary, between the Northern Plate and the Eastern Plate. It’s the most active zone on Arnâron, especially the south, where the New Plate is being born from violent volcanism. The Western Plate is being pushed up on both its western as well as eastern convergent boundaries. The Western Plate is the most stable of the plates of Arnâron, and has a good chance to stay in place for a long time, until dynamics of the planetary interior change.
I’ll actually have to change the graphic again – as I write this, I notice that the Northern Plate should be pushing northwards as well as eastwards, and this in turn should be what’s pushing the Polar Plate southwards.
This is a fairly simplified picture of plate tectonics, but it should suffice for my purposes.