[WIP] Building a world from tectonics onward

[MrBragg]

So I've been fiddling around with tectonics some more (because masochism is fun!) and, since misery is much better with company, I wanted to solicit opinions on hypothetical scenarios.

Basically, I'm experimenting with having a "Tethys like" scenario early on during the supercontinent stage (~250 Mya), with a ridge that is destined to be subducted between the northern margin of the ocean between two halves of the supercontinent. When that subduction happens (~225 Mya), the northern margin of the southern continent should rift off and begin heading north. At around the same time--the precise timing doesn't appear to matter all that much--I also have the northern and southern halves of the supercontinent splitting, so a rift exists the entire way between them. Up until now, things seem fine.

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After some time, there should be an oceanic-oceanic margin developing between the "A" and "B" fragments (~175 Mya). My inclination is that B should be subducting under A here as basically a continuation of the margin under the continent, though it also seems feasible that this could be more of a region of diffuse deformation and be more akin to a transform boundary. If either of those options seem wildly incorrect (or wildly correct!), I'd appreciate opinions. Lastly then, when the continental fragment on B collides with A (150 Mya), the question remains with what to do with that same pesky margin; if it's subducting, does subduction cease, or does it instead spread along the continent-ocean passive margin? If it's not, would the two plates basically fuse together? Again, opinions are appreciated

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[Tiluchi]

Ah, nothing like the rabbithole of plate tectonics, where you can always make things more complicated!

It's always tough to suss things out without playing around on GPlates myself, but I think my instinct here would be to have the southern A/B margin as more or less a transform boundary, and have it running northwest to southeast, somewhat parallel to the direction of microcontinent B. The other option might be to have something like what you have at 150 Ma, with the subductive margin extending southwards beyond the continent; this is perfectly normal in plate tectonics as far as I know, though I might expect the subduction zone to keep moving to the west as there's no continental margin to stop it; this is more or less what happened with the Scotia Plate, and I think maybe the Greater and Lesser Antilles.

But these are all just guesses and I'm by no means an expert- getting a realistic tectonic history is a cycle of constant frustration for me, even though I find myself steadily happier with my results as I go on...

[MrBragg]

getting a realistic tectonic history is a cycle of constant frustration for me, even though I find myself steadily happier with my results as I go on...

One of the truest things I've read in a long time

@Charerg, yep, we're definitely on the same page, you just said it better than me!

Appreciate both of your feedback! I actually tweaked things a bit and made a smaller Cimmeria-like block break off first rather than the bigger microcontinent; "Cimmeria" now collides with the northern continent before the cycle more or less repeats itself with the ridge subducting and then the rift jumping south and breaking off the microcontinent later (it will still eventually hit the north).

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I've also followed the evolution a bit farther in time to the next big event happening in this region, which is a microcontinent-arc collision that happens around 100 Mya on the southwestern flank of the northern supercontinent (A). Given that the oceanic crust to the east of that arc is quite old, I have the subduction polarity reversing, placing A under slab pull and thus initiates breakup of the northern supercontinent. This part is more of a reach, but it also seemed reasonable that, which this change in direction of A, the subduction zone to the east of A might extend itself around to the west, consuming that old oceanic crust from both sides.

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[Charerg]

You could also use the subduction of the ridge ca. 250 Mya as the driver for the breakup. I'm guessing this is more-or-less the scenario you were envisioning, with a hinge-type rotation between the separating landmasses. In essence the, eastern portion of the "southern plate" is subducting, while the western portion is diverging. After initial break-up, the rift would spread further east, with a piece of the southern plate breaking off. I guess the end result is largely the same, but I thought it was worth mentioning.

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After the separating piece collides with the northern continent, it would be expected for the subduction zone to "jump over", and subduction to continue until the mid-oceanic ridge has been subducted. What happens beyond this point gets a bit complicated.

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The subduction might cease (depending on the relative montions of the plates), since slab-pull would no longer be a factor. However, unless the two continents merged into a single plate (with a mass of oceanic crust in the middle), a new rift would have to form. This would likely form over continental crust. Like you mentioned, another possibility is that the subduction continues, in which case the continents could return to the initial hinge-type movement, eventually colliding. It's also possible that another portion of the southern continent would break off, repeating the cycle.

[Charerg]

Looks good, though I'd drop the subduction zone extending westwards. This scenario would place the adjacent oceanic crust under slab pull in both directions, forcing a new ridge in the middle of that sea. That would also conflict with continent A shifting direction and breaking apart (as it would no longer be under slab pull in this scenario).

[MrBragg]

That would also conflict with continent A shifting direction and breaking apart (as it would no longer be under slab pull in this scenario).

Doh! It's so obvious after you point it out...

Ok, so with the offending subduction zone removed, at 100 Mya I end up with the following scenario. Moving forward, since C has lost most of its northeastern slab pull forces, it changes direction to a more northerly vector. This, in turn, changes the nature of the C-B interaction from being simple convergent subduction to a retreating subduction zone. My current thought is to treat that as something of a Kermadec-Tonga situation where a piece of B--likely including a chunk of the continent in the south--gets ripped off and a new spreading center opens. That will evolve for a while, until I can think of what else to do with it.

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[MrBragg]

Alright, so continuing to hack away at this and (hopefully!) make some progress. This is the current whole-world state of things at the "present" age; all of the boundaries in the saturated colors seem to check out as reasonable from gplates while the lighter boundaries are places not fully fleshed out and are more just rough ideas (dashed red are incipient or recently failed rifts). In particular I don't really like all of the intra-oceanic subduction happening in the far north and far west of the map as those types of boundaries don't really seem occur so extensively on earth. The northern subduction arises from when the supercontinent over the north pole breaks up, so I'm not quite sure what to do there other than keep extending the subductrion zone as the fragments drift apart. I'll keep thinking about this region, but if anyone has any brilliant ideas for how to handle it--or to tell me it's fine and to stop obsessing --I'm all ears!

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[MrBragg]

I've finally gotten around to cleaning up the last tectonics iteration and have something that I like a bit better than my previous attempt. The history at least is more consistent and I've gotten rid of some of my all-land-is-surrounded-by-subduction problems. The boundaries here are still roughly depicted, but if anything seems grossly implausible please tell me before I settle on this as "final" and then discover something off in a year

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Attachment 131668

[Tiluchi]

Looks great overall! I'm assuming that tectonically complex bit around the equator will turn into an Indonesia-style archipelago of microcontinents, yes? I've also struggled with random oceanic subduction zones going off the edge of continents in my own world, and it can be frustrating, but I think what you have there is fine. Perhaps an Alaska-style peninsula on the northwest of that western continent would help it look a bit less incongruous? Otherwise hard to say without seeing the GPlates animation.

[MrBragg]

Thanks Tiluchi! You're right about that messy area being an Indonesia / Oceania style archipelago, I just haven't actually added the land there yet.

I like the idea of trying to add in a peninsula to the northwestern continent. I can imagine that kind of thing being consistent with either an original part of the continent that split off during the breakup (rough gplates sketch) or as being something that accumulated over time as a series of terrane collisions a la real Alaska. Added to the to-do list!

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