Can Someone Help Me Determine Plate Divisions?

Question

I have a world designed called Ser. I want to place tectonic plates so that they can explain how the current landmasses formed.

Below shows the 6 major continents of the world. As continents are commonly defined by their tectonic plates, I want this to be a guideline to how to determine where the tectonic plates should be.

Below shows how the landmasses have fit together in the past. The color brown indicates regions of mountain ranges. The colors on the shorelines represent how other landmass fit together during the last supercontinent.

Based on these two images, I want to decide where tectonic plates and their movement could have split the landmasses apart the way they are currently represented.

My previous research has helped me determine that divergent boundaries are usually between landmasses they are separating (forming mid-ocean ridges), and convergent boundaries are commonly at the location of mountain ranges. Although, these facts do not have to be a restriction.

Constraints: I don't want to change the continent division. I don't want to change the locations of mountain ranges. I also don't want to alter the landmasses at all.

Good answers will provide tips and/or resources that will help me decide where my tectonic plates should be. Please be descriptive and explain where plate boundaries should be and where they should not be, based on the data in these pictures.

EDIT: The planet depicted is an Earth-sized planet, and the landmasses shown in the images are all of the landmasses of the planet.

Answer 1

You have a few problems

1. Starting on the left, purple, you can't have a land mass between two places that were once joined,otherwise if you wind time back they can't join up because there is a land mass in the way.

2. Pale blue, you can't form spikes like that,not as a plate to plate break. You can't have a sharp V shaped spreading center, crust breaks in 120 degree angles. The only way to get a narrow seaway is to have a spreading center in the center of it. and the only way to get a large acute peninsula is a subduction zone uplift, or multiple sequential spreading events in close proximity.

3. You have way to many scattered large islands, if a landmass moved through someplace there can't be a bunch of large islands where it moved, it would have scooped them up. Islands are either so small they should barely show up on the map or they are chunks of continental crust, which means they need a whole plate pushing them along, they can't move over other plates moving other islands. You can have a few but not a whole mess of them and definitely not in a shotgun scattering, you can get a chain of islands on a subduction zone but that s what shape they will take a subduction arc (Alaska or Japan), or two subduction arcs coming from two directions for earth (Oceania). And you will not have many between two landmass boundaries that were once joined, there is no way to get them there.

4. On the right the purple spiral is a mess, you can't have a divergent boundary between two mountain ranges, a spreading center will not form on thickened crust, spreading centers form where the crust is weak and thin, they are literally breaks in the crust and a material is not going to break at its strongest point.

5. Looking at the map overall you also can't have nothing but divergent boundaries, every sea you have is bordered by land masses that were joined, that can't happen unless your planet is actually getting bigger. crustal material has to go somewhere, if it is being forming in one place it has to be getting subducted somewhere else an the same axis, otherwise the diameter of the planet along that axis is getting bigger. You can get a away with some minor ones because your looking a two dimensional representation of a spherical surface but they need to be more hinge like (dead ended) so rotation is happening. I considered running a continent to continent convergent boundary down hte middle of your central continent but it is too small to support that, one of the oceanic plates would just give way and start subducting, the Himalaya only exist because Asia is huge and India is small yet close to its spreading center.

I have done my best with your map while trying to change as little as possible. Spreading centers are marked in red and subductions zones marked in green. If two places can't be areas that were once joined I have erased the matching colors. Islands that I just couldn't justify no matter how much a stretched I just erased. I added a mountain range near the yellow coastline because subduction zones create mountain ranges, and putting the subduction zone there disrupted less of the map than anywhere else.

I have just ignored your first image because it makes no sense, they look like political borders not tectonic ones.

Really you would be better off starting over from scratch, it will look a lot more convincing. Or at least you have to do some serious editing of your map to make something possible. You have too much land in the mid & low latitudes and too many random islands.ideally you could even take a bug chunk out of a continent to open up some room for things to move.

The best way to build a map is to start with rough idea of what you want and work progressively towards it instead of taking a finished map and asking how to fix it (and that is what you are asking whether you intend to or not). I know you don't want to change anything but you just can't make something believable from what you have without changing it. Sometimes the answer is "you can't get there from here". You can't add in techtonics after the fact, techtonics are what creates landform.

The quick and dirty way to map spreading centers is just make a line in the center of the ocean that roughly matches the coastline wherever you have two coastlines that match. Spreading centers are not always centered but they most often are. You need large subduction zones to move them away from center. if you have two parallel spreading centers you either need a subduction zone between them or one on each side, again crustal material has to go somewhere. Mountains are formed by convergent plate boundaries, sometimes they are old plate boundaries but if mountains are near the coast that is usually due to an active subduction zone, hence why I put your subduction zones near existing mountains whenever possible.