Chord - Yet Another tectonics/winds/climate/etc World Map Project

[Charerg]

Is a mid-ocean subduction that strange? Looking at Earth at least the Pacific and Australian plates seem to have this going on (on the non-Australian/large landmass side of that plate). Nonetheless I also note that subduction is a lot more common (because of crust type density) on oceanic/continental areas so I've reworked many of my convergent plate borders to fall more along the coastlines of major landmasses. Regarding Plate O in particular, I reworked that convergent boundary along with the boundary/directions of plates V2, E, and V although I still have some oceanic subduction where O hits the spreading zone between V and A.

Yes, or rather oceanic-oceanic (crust) subduction is relatively rare. Even in the Pacific/Australian border there is a lot of continental crust forming the submerged continent of Zealandia. Also note that this zone is the intersection of three major converging plates (Australian, Eurasian and Pacific), creating a rather special situation.

It's worth looking at the formational process behind the area in a bit more detail, here's Australia 65 Mya (blue=continental crust, the rest is oceanic):



Note the ancient orientation of the S. Pacific ridge and the fragments of Zealandia rifting off.

35 Mya:


Around this time the Tasman Sea ridge has become an extinct ridge and the South Pacific ridge has connected up with the Indian Ocean ridge. Note the formation of several island arcs to accommodate the convergent movements of the Eurasian, Australian and Pacific plates.

And the present day situation:


I hope this provides some insight into the matter. I'll try and post some extra feedback later.

[Pixie]

So let me describe what I see, based on the map and your words.

Plates C and M were the bulk of a very large continent. They rifted a long time ago opening up an ocean between them. Plate M, on its western boundary, has been consuming a lot of crust by subduction/collision, and that explains the movements of B, F and N. It also explains the proximity of O, which escaped collision because of a more recent rift separating it from F. As to why C and M broke away, that's because the huge bloc that is plate C got pulled by a subduction boundary on its SW side, where the mess that is V+E+A was subducting everything around.
Fast forward a bit, the ocean between C and M initiates subduction... That starts as an infection by the subduction under group V+A. These infections spread quickly (not kidding, that's the actual scientific wording), this time along the old oceanic crust near C's continental margin, and M ends up suffering the pull that is now pulling it back to C.
This would explain your map in rough, but there are so many angles/directions that should really be refined to support it. Polar projections would help too. And I didn't even consider untangling E+V.
M2 is fine with those boundaries. It was part of M, but it got loose (rifted) because it's not suffering the same subduction pull that its parent plate.

Hope this helps, sorry if it doesn't

[AzureWings]

Yes, or rather oceanic-oceanic (crust) subduction is relatively rare. Even in the Pacific/Australian border there is a lot of continental crust forming the submerged continent of Zealandia. Also note that this zone is the intersection of three major converging plates (Australian, Eurasian and Pacific), creating a rather special situation.

Hmm, so what would happen to say plate O, that presumably was subducting under V+A before they started rifting apart? Should O be separating along the same spreading ridge? As it stands I seem to have it being pulled under that ridge which doesn't seem right.

B also is going under M and A but has that oceanic convergence border with M as a result. Would it make more sense for B to be being pulled apart by the continental portions of A and M but then not being pulled under the eastern/oceanic section of M?

So let me describe what I see, based on the map and your words.

Plates C and M were the bulk of a very large continent. They rifted a long time ago opening up an ocean between them. Plate M, on its western boundary, has been consuming a lot of crust by subduction/collision, and that explains the movements of B, F and N. It also explains the proximity of O, which escaped collision because of a more recent rift separating it from F. As to why C and M broke away, that's because the huge bloc that is plate C got pulled by a subduction boundary on its SW side, where the mess that is V+E+A was subducting everything around.
Fast forward a bit, the ocean between C and M initiates subduction... That starts as an infection by the subduction under group V+A. These infections spread quickly (not kidding, that's the actual scientific wording), this time along the old oceanic crust near C's continental margin, and M ends up suffering the pull that is now pulling it back to C.
This would explain your map in rough, but there are so many angles/directions that should really be refined to support it. Polar projections would help too. And I didn't even consider untangling E+V.
M2 is fine with those boundaries. It was part of M, but it got loose (rifted) because it's not suffering the same subduction pull that its parent plate.

Hope this helps, sorry if it doesn't

That does help! I initially planned out some continental movements but it was rather eyeballed and rudimentary and focused only on the landmasses, leading to a lot of the initial directions I used (that didn't have much tectonic sense behind them). What you're describing is a lot more... coherent than any of my original ideas. As you can see from that start of this thread I originally decided what borders were divergent and convergent based on what directions I had the plates moving, rather than the other way around. I assume you mean that M was consuming crust on its eastern boundary (in the western hemisphere)?

In my original plans the major landmasses of plates S, C, and M were all originally part of the same supercontinent, with S and C breaking apart more recently than C and M (although I'm not sure now how plate C2 would have come about in the mix of that...).

Throwing the map back on a sphere again I did find my directions on the polar plates were a bit off from what I intended (not to say that what I intended necessarily made sense!):



The white arrows are the general direction that I'd intend - although I'm not sure if S should really be moving all that much, since it's almost certainly not being subducted by M2 (and M2 should probably be moving a bit more towards S). I mentioned above that the place O's oceanic crust is being subducted doesn't make a lot of sense at the moment, but I do like the idea of the V+A zone pulling O under. Maybe it should just be A still pulling O while both rift from V (so O would be moving in a bit more of an A-ward direction).

[Charerg]

Hmm, so what would happen to say plate O, that presumably was subducting under V+A before they started rifting apart? Should O be separating along the same spreading ridge? As it stands I seem to have it being pulled under that ridge which doesn't seem right.

B also is going under M and A but has that oceanic convergence border with M as a result. Would it make more sense for B to be being pulled apart by the continental portions of A and M but then not being pulled under the eastern/oceanic section of M?

Tbh I'd suggest making use of GPlates if you're willing to invest time into the tectonics. Because the answer in a lot of cases is *depends*, and that largely boils down to "what was subducting what way back when".

Here's a sketch I threw up about how the O Plate might look like, if we imagine that E and F rifted away from it, and it is being subducted under V and A:



Some orthogonal pics of the above:



Of course this is just one possibility. As an example, if you wanted you could say that a new rift is forming in the middle of O, and break it into two plates. Or say that the V/A spreading ridge does indeed connect up with say, the F/O ridge, which would be a different situtation too. Oh, and as you probably noted I changed the colour scheme (I'm way too used to seeing red as rifting, green as transform and blue as subducting), hopefully it's still readable.

[AzureWings]

You're probably right. I've gone back to your helpful GPlates tutorial armed with a better sense of why some of the more complex modeling features are important. I do have a few questions though...

I did a bit of looking around but couldn't find much information about spreading ridges going extinct, so I don't know what causes that or what happens with that boundary afterwards - is that something that's generally known? I liked some of Pixie's idea regarding the history of C and M, but I don't quite know what would happen to their spreading ridge/plate boundaries after C starts subducting M, especially since the boundary between the two being along the coast of C's landmass doesn't seem to line up with them having rifted apart a long time ago. It seems like the spreading center would put the boundary close to the middle of the ocean between them instead, and then I don't know why C's newer oceanic crust would subduct M's newer oceanic crust.

I've been trying to start out by scrapping my old estimate of the original supercontinent and instead working my way backwards in time in order - with the most recent significant changes first (the F/O rift followed shortly by the V/A rift) and then working back further, then tweaking the result of all of that to get things making more sense. Is that approach a decent idea to be using? When I tried to put together detailed movements that would produce my original supercontinent I had all kinds of trouble - plates under slab pull hardly moving, plates under none being among the fastest and moving straight out into the middle of a big ocean for no apparent reason, 'spreading' ridges where the oceanic basin actually narrows over time, and so on - so I thought working from one direction only would give me more freedom.

[Charerg]

You're probably right. I've gone back to your helpful GPlates tutorial armed with a better sense of why some of the more complex modeling features are important. I do have a few questions though...

I did a bit of looking around but couldn't find much information about spreading ridges going extinct, so I don't know what causes that or what happens with that boundary afterwards - is that something that's generally known? I liked some of Pixie's idea regarding the history of C and M, but I don't quite know what would happen to their spreading ridge/plate boundaries after C starts subducting M, especially since the boundary between the two being along the coast of C's landmass doesn't seem to line up with them having rifted apart a long time ago. It seems like the spreading center would put the boundary close to the middle of the ocean between them instead, and then I don't know why C's newer oceanic crust would subduct M's newer oceanic crust.

Well, if the crust is being stretched, it has to break somewhere. If the crust is no longer stretched, or it's stretched in a different direction, then the old rift will go extinct (and is no longer a boundary between plates). Here's a scribble I made about what Pixie probably meant, a subduction virus spreading into the ocean between two continents that were previously divergent:



In this case the mid-ocean ridge would likely be subducted rather than going extinct. Which would put the other continent under slab pull, and the ocean between them would eventually close.

I've been trying to start out by scrapping my old estimate of the original supercontinent and instead working my way backwards in time in order - with the most recent significant changes first (the F/O rift followed shortly by the V/A rift) and then working back further, then tweaking the result of all of that to get things making more sense. Is that approach a decent idea to be using? When I tried to put together detailed movements that would produce my original supercontinent I had all kinds of trouble - plates under slab pull hardly moving, plates under none being among the fastest and moving straight out into the middle of a big ocean for no apparent reason, 'spreading' ridges where the oceanic basin actually narrows over time, and so on - so I thought working from one direction only would give me more freedom.

It helps to start out small, start out with just a few continents and adjust their movements until you're happy. Then you can start adding pieces to the whole. It's probably not wise to try to get all the movements right at once, it's a lot easier to just figure out the movements of a few key pieces first.

[Pixie]

Like Charerg said, small steps. Not only in terms of number of plates, but small time steps. Go back, say, 10 million years, adjust and evaluate. Then another 10, and so on.

Changes in the direction of a plate movement can be abrupt, just have a look at the Pacific ocean floor west of hawaii and notice the turn in the hotshot trail.

... sent from mobile ...

[Charerg]

Like Charerg said, small steps. Not only in terms of number of plates, but small time steps. Go back, say, 10 million years, adjust and evaluate. Then another 10, and so on.

Changes in the direction of a plate movement can be abrupt, just have a look at the Pacific ocean floor west of hawaii and notice the turn in the hotshot trail.

... sent from mobile ...

That's one way to do it, though personally I don't use that approach. For me, dropping a reconstruction pole at select critical points is enough. Say, if I have Plate X breaking from Plate Y at 100 Mya, then colliding with Plate Z at 50 Mya, I'd just put poles at 100, 50 and 0, and then maybe some extras if necessary for the movements to work out. Then again, I don't necessarily aim for quite 100% realistic-level detail, so I'm willing to cut some corners to reduce the amount of work involved. To each his own method though, there's no "right way" or "wrong way" to go about it. That said, some methods are definitely more time consuming than others .

[Pixie]

That said, some methods are definitely more time consuming than others

You're right, I should stop making recommendations - I'm working on Maward for years and still have next to nothing to show.

[OldEarth]

Tbh I'd suggest making use of GPlates if you're willing to invest time into the tectonics. Because the answer in a lot of cases is *depends*, and that largely boils down to "what was subducting what way back when".

Oh wow! How did I not know that GPlates existed? Thanks for the rec.

I love that there is so much plate-tectonics nerdery that exists here.