(WIP) Tamanak Plate Tectonics + Tectonics Questions

[CTA]

I've been working on the plate tectonics for my world (Tamanak) recently, and this is what I have so far.

Current time (1 mya, due to some gplates quirks)
Blue: subduction, arrows pointing to upper plate
Red: spreading center
Green: transform boundary


50 mya-1 mya: https://www.youtube.com/playlist?lis...4OGwY0rooL-q22

First off, this is still very much a work in progress--I should probably revise the upper left corner of Phaena, for example. I also haven't drawn in the island arcs at subduction zones yet. Regardless, do these tectonic maps look plausible? Is there anything I should change?

I also have a few questions about plate tectonics in general. In no particular order:
I think I have a decent idea of when subduction zones disappear (it seems to require serious continental collision), but how do subduction zones arise?
What happens when continental crust reaches a subduction zone and there's no continental crust on the other side to stop it?
How does continental crust form?

Thanks so much!

[MrBragg]

This is really cool and looks like a great start! Out of curiosity, how far back in time does your model go? And are you trying to model in reasons for why the plates move the way they do, or are you not that foolish?
There are a few places where I think your boundaries are mislabeled, e.g. the boundary between Rarenarena and Utha D looks like it should be subduction instead of transform and the boundary between Rarenarena and Rhandas B seems like it should be divergent instead of transform. How much this kind of thing matters to you probably depends on how far down the tectonic rabbit hole you want to go...

I think I have a decent idea of when subduction zones disappear (it seems to require serious continental collision), but how do subduction zones arise?

There are a few ideas for this which fall into two main categories: spontaneous and induced. This figure summarizes them pretty well:

What happens when continental crust reaches a subduction zone and there's no continental crust on the other side to stop it?

This is somewhat covered in the above figure, but the most common results are 1) the subduction zone jumping over the continent and resuming on the other side, and 2) the polarity of the subduction zone reversing and the ocean beginning top subduct under the continent.

[CTA]

Oh, I absolutely want to model the reasons the plates move the way they do! I couldn't figure out a way to label the videos, but I'm trying to follow Scotese's Twelve Rules of Plate Tectonics (plates only move because of subduction or ridge push, oceanic plates move faster than continental plates, etc.). To answer your other question, my reconstruction goes from 50 mya to 1 mya.

Why should those boundaries be subduction and divergent? I'm pretty new to this, so I'd love an explanation of your reasoning.

That subduction chart is awesome; thank you so much! That's just what I was looking for. Looking at Lykenel (the reason I asked my second question), the subduction zone seems unlikely to reverse direction since there's an opposing subduction zone on the Rarenarena plate; what do you think?

[CTA]

EDIT: It appears I posted two replies by mistake. Feel free to disregard this one.

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Thank you! It goes from 50 mya to 1 mya, and I'm definitely trying to model reasons for plate motion. I couldn't figure out how to draw on my videos, but I placed subduction zones and mid-ocean ridges where I thought made sense and tried to derive the movement from those. If you have questions about any specific era, I'm sure I could draw up a diagram. I'd love to hear your reasoning for why those boundaries should be subduction and divergent--I'm pretty new to this, so I'm trying to learn as I go.

Thanks for the subduction diagram! That's exactly what I wanted. In the case of the Lykenel plate, I'm wondering which of the two outcomes is more likely. Does the fact that the Rarenarena plate is already being subducted make the creation of a subduction zone going the opposite direction less likely?

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[MrBragg]

Alright, I'll see if I can't explain my rationale here; if anything isn't clear, let me know.

Using snapshots from 50 Ma and 25 Ma I think helps to highlight both of the points I made above. In those 25 million years, the Rarenarena continent has moved to the southeast the distance indicated by the white arrows, creating new oceanic crust to its north and west. You already have a divergent boundary in the north, so that looks fine, but in the west that transform boundary seems to also need a divergent component in order to created the needed oceanic crust. (Note that the movement between Rarenarena and Rhandas B is oblique, so there's surely also a transform component needed.)

Explaining why the Rarenarena-Utha interactions need to be subduction follows similar logic. The eastern "wing" of the Rarenarena plate is moving together with the continent since the plate is a single entity, so it would have movement (roughly) indicated by the white arrow. Because the Utha boundary seems to stay ~ fixed in place on an absolute coordinate system and Rarenarena is moving southeast against that boundary, there would seem to need to be subduction of that eastern wing in order to allow the movement to happen. Again the interaction here is oblique, but oblique subduction is fine.

[CTA]

Ah, that makes sense. I'll make those changes.

[CTA]

So the more I thought about the Rarenarena plate, the less its movements made sense--if the plate is moving south, for example, shouldn't the subduction zone at Lykenel A also be moving south? Therefore, I smoothed out the western boundary of the plate and rotated the continent slightly (so now it's moving almost directly south) and made the eastern wing its own plate (Kidder Plate). This is the resulting map (prior boundaries still apply, with Rarenarena-Kidder a transform boundary):


(disregard the weirdness in the northwest corner of the Rhandas B Plate)

I can update the videos, but I think the movement makes sense now.

[CTA]

Unrelated question: what happens when you have a supercontinent breakup and two adjacent plates move away from a third? How do the mid-ocean ridges behave?
To illustrate what I mean, here are the three possible scenarios I thought up (black = plate boundary, red = mid-ocean ridge)





In case it's unclear, the two lower plates are moving southwest and southeast, respectively, and the upper plate is not moving.

Which of these scenarios (if any) is closest to reality?

[MrBragg]

Splitting Rarenarena into two would definitely give you some more flexibility in terms of what’s going on. Is the Kidder plate moving? It seems plausible for it to be obliquely rifting off Rarenarena and subducting under the Uthas, but I’m not sure if that’s what you had in mind. Also, one thing that jumps out at me is that in quite a few places in your tectonics you have very long transform faults. Similar features don’t really exist on earth (https://commons.wikimedia.org/wiki/F...entered-en.svg) where most transform faults are broken up by divergent faults. I’m not a geologist or geophysicist, so I don’t know if this is an unavoidable result or simply an idiosyncrasy of earth, but it might be something to keep in mind given how many huge transform faults you have.

Regarding your three plates question, in a general sense the interaction can be any triple junction where one “arm” is a ridge where the continent is breaking apart (https://en.wikipedia.org/wiki/Triple_junction). Unfortunately, I can’t get your attachments to display, but it sounds like you’re describing an “RRR” style junction where the boundaries between all of the plates are divergent (e.g. a supercontinent breaking into two pieces and each of those pieces moving away from a third); if so, that junction would almost certainly be stable, so its plausibility would depend mostly on the history leading up to its formation.

[CTA]

Hm, you're right. I suppose I can rework my plate movements, set things in motion, et cetera. I hadn't planned for the Kidder plate to move, but it probably will be once I'm done revising.

See if the attachments work now. The question is largely a moot point (there's another plate involved with the Syvr-Rarenarena-Lykenel split), but I'm still curious.