Another Tectonic Mechamism Question

[HandwaviumAdAbsurdum]

Not crazy! This idea actually reminds me of a paper that was published last year, which found evidence of an episode of passive margin collapse on the western margin of India. The cause that the paper proposes is compression brought about by India's rotation at that time provided the impetus for sunduction initiation in the form of continental crust overthrusting oceanic lithosphere. So, what we need to look for in your project is a moment where the southern margin of green is undergoing compression. At some point the distance between green and the southern supercontinent needs to get smaller. This is made possible by the fact that it is moving due to slab pull, and is smaller than the southern supercontinent (less drag).

Regarding the unresolved mechanism for initial breakup, don't be afraid to invoke bottom up tectonics. It might actually make this second phase more plausible since the northern and southern supercontinents would only be drifting away from each other due to ridge push (quite slowly). This should ensure compression on green's southern margin once it splits off the northern supercontinent.

[MrBragg]

My gplates model is a bit of a mess right now (ignore the fact that the green and black plates never seem to touch), but here's a zoomed in animation of what's going on in the green/orange neighborhood around the time of the breakup. Basically, at ~100 Ma green comes under slab pull and starts moving southwest. At least some component of that vector seems like it should put compressive stress on green's southern margin, which in my model leads shortly thereafter to a subduction zone developing and orange starting its trek north.

100Ma_anim.gif

[HandwaviumAdAbsurdum]

This is coming on nicely! There seems a fair amount of compression there so I have no complaints. The only thing I can think of to improve the model at the moment is delaying the breakoff of orange somewhat to allow for the transition from subduction initiation to true subduction and then the transition from the onset of continental rifting to seafloor spreading. The former takes maybe between 2 to 3 million years. The duration of the latter, however, can vary immensely, and would depend on the structural inheritance of the rift zone between orange and the rest of the supercontinent. In other words if there is an obvious lithospheric weakness like a suture or shear zone. Adding on another couple of million years for this as a mininum should be fine but I'll leave it to your discretion.

I think it would be a good idea now to work on the exterior ocean, get an idea of where the ridge(s) are at the time of continental breakup, stuff like that.

[MrBragg]

Ok, yeah that all sounds reasonable. I currently have a delay of ~5 Ma built in between green's change in motion and orange breaking off; I can certainly add in a few million years of slow continental rifting before orange really gets going. Thanks for the quick feedback!

Here I have green changing direction at 100 Ma, true subduction beginning at 97 Ma, and full on seafloor spreading beginning at 92 Ma.
rifting2_100Ma.gif

As far as my equivalent of Panthalassa goes, the only event I currently have for that region--which isn't even added to my gplates model yet--is the ridge subduction that initiates breakup of the northern supercontinent from earlier in this thread. I guess what's going on with the "Antarctica" continent also is likely coupled there, too, but I'd be lying if I said I'd given the region any more thought than that

[HandwaviumAdAbsurdum]

That's looking fantastic! When does it collide with green?

In that case, maybe we ought to hold off just a bit before opening that can of worms. As for the Antarctica continent, I was looking at the image you posted of the south pole in the last page and now I'm a bit confused because magenta and yellow can be seen from that view when I thought they were part of the northern continent. If there was a continent in that location with a northward dipping subduction zone, or maybe even between it and purple, then I could definitely see grey breaking off due to slab pull at some point and drift over the south pole. The reason why I don't see purple being able to cause this is because the southern supercontinent is rotating counterclockwise (albeit very slowly) away from the south pole.

[MrBragg]

The collision between orange and green begins long enough before the present day to start pushing up a nice Himalaya-style range; the eastern margin of green is basically undefined (and more or less unconstrained) at this point so it can be placed wherever it needs to for the collision to produce some nice mountains. It certainly begins before the two pieces visually come together in the current gplates model.

Ok, regarding the south polar region, here are two animations showing 225 Ma - present day to give you a sense of what things look like, both regionally and globally.

Orthographic near south pole (aka most boring animation ever...)
southPole_anim2.gif

Mollweide

mollweide.gif

Right now the gray Antarctica continent breaks off around 25 Ma (pink comes under slab pull at 30 Ma in the current model), though my big bright disclaimer here is I really haven't thought much at all about the details of what's going on down there. Since there's already subduction happening to the south of pink, I've toyed around a bit with another iteration of ridge-subduction-leading-to-rifting, but I've admittedly not really tried to fit that into the broader scheme of what's happening elsewhere.

[HandwaviumAdAbsurdum]

It all makes so much sense seeing it like this. My incompetence in judging movement across a spherical object is second to none!

Yes, it does look like a subduction zone at the southern margin of pink could be a trigger for the rifting of grey, but what is a bit strange to me is how the southern supercontinent is barely while the northern supercontinent looks like its trying to beat a world record. Is there any reason for this?

[MrBragg]

Yeah, projections are fun

There definitely is a velocity discrepancy between the two supercontinents, but some of the apparent difference is an exaggeration from the projection (spheres messing everything up again). Depending on the fragment, the northern continent is traveling at between 0.5 - 3.5 cm/yr while the southern one is moving somewhat uniformly around 0.9 cm/yr, so certainly different but same order of magnitude. I was hoping a factor of 2 - 3 is something I could hand-wave away with an argument like the crust of the southern fragment just so happens to be thicker on average so it moves slower, but like everything else here, my intuition of what makes a plausible argument isn't something I place a lot of stock in.

[MrBragg]

First off, apologies for the upcoming novel, reading back over this I realized I may have gotten a bit carried away...

Anyway, I spent some time thinking about what's going on around the south pole and outer ocean when I realized that I should probably figure out what's going on in the more constrained inner ocean first, since that will potentially impact what happens at the south pole and will definitely need to be incorporated into whatever is happening at the eastern margin of the outer ocean. So, without further ado:

In its simplest incarnation, I started with the following sketch, where the inner ocean essentially consists of a single plate containing the black terrane and having spreading ridges in the west and south, subduction along the east and a transform fault in the north. The east, south, and west are pretty well constrained by the other things going on so those all seem reasonable, but the north looks troublesome to me. In the static images the transform looks ok if you don't look at it for too long, but when I animate things it's clear that as black gets close to hitting the green island arc the absolute motion of the region between yellow and blue is pulling away from black, which seems incompatible with this simple one plate sketch. The RTF triple junction in the northwest also kind of bothers me since it seems like there'd need to be a near-perfect velocity match for that to be stable. My first question is: Is it correct to think that this sketch is too simple?

interiorOcean_1plate_225Ma.png intOcAnimation_small.gif

Alternatively, it seems like I can get a lot more flexibility if I add another plate to the mix in between the supercontinent and the oceanic plate, like in the following sketches, though it's also entirely possible that this causes more problems than it solves. In this model, the inner ocean is still mostly made up of a large plate on which the black terane is riding, but there's another, smaller plate off to the northwest. This smaller plate is moving northwestward and subducting under the continental plate, which is itself moving more or less orthogonally to the small plate. Obviously the locations I've drawn the ridges aren't based on any actual modeling, but as a rough sketch and to my untrained eye, this arrangement appears to rectify the problem with the one plate model. As time goes on, this smaller plate grows a bit and rotates, which seems to help alleviate the strangeness I was seeing in the north when only using a single plate.

225 Ma interiorOcean_2plate_225Ma_2.png 150 Ma interiorOcean_2plate_150Ma_2.png 100 Ma interiorOcean_2plate_100Ma_2.png

My second question: How much worse did I make things by doing this?

[HandwaviumAdAbsurdum]

Yeah, projections are fun

There definitely is a velocity discrepancy between the two supercontinents, but some of the apparent difference is an exaggeration from the projection (spheres messing everything up again). Depending on the fragment, the northern continent is traveling at between 0.5 - 3.5 cm/yr while the southern one is moving somewhat uniformly around 0.9 cm/yr, so certainly different but same order of magnitude. I was hoping a factor of 2 - 3 is something I could hand-wave away with an argument like the crust of the southern fragment just so happens to be thicker on average so it moves slower, but like everything else here, my intuition of what makes a plausible argument isn't something I place a lot of stock in.

A thickened crust, eh? That doesn't sound too hand-wavey to me. It would increase drag as it turns out. The question is how do we thicken the crust? A laramide-type orogeny or two along its active margin could help a little.

Your second post looks really interesting. I'll take some time to read it tomorrow.