Few Questions Regarding (Super)Continent Breakup

[MrBragg]

Alright, I've been trying to read up a bit on tectonics and figured the best way to see if I'm actually learning anything would be to try something and see how messed up you all think my model looks. At this point I'm at the point where I'm breaking up a few continents and wanted to solicit some feedback on what I'm doing and whether my underlying rationale makes sense; the boundaries between fragments are still a work in progress and subject to change, so try to ignore places where they don't fit together well. Here is an overview of the continent movements so far:

tectonics.gif

Starting at 225 Ma (years are basically arbitrary; I picked the start date to give reasonable velocities between the start and end points), I have the supercontinent begin to break apart. I have a triple junction develop at the red star that results in the green fragment cleaving off the orange/purple/teal fragment. The third leg of the junction fails, but will later become active when orange breaks off from purple.

triple225.png

After the rift, nothing really happens for a while while the two chunks drift apart. In my current model I simply have the two fragments drifting apart and generating oceanic crust at essentially a constant rate--age is shaded in 25 Ma intervals--which, to my super untrained eye, looks ok, though opinions on this would be appreciated.

100MaCrust.png

Where I become much less sure is at 100 Ma where I have a bunch of things happening at the same time. First, I have a new triple junction develop between the green / red / blue pieces that eventually causes the green to cleave off and change direction to now gyrate off to the southwest. The snapshot of oceanic crust age on the right is at 20 Ma. I'm unsure if 1) green abruptly changing its direction of movement is plausible, or 2) if the hinging movement between green and blue makes a lot of sense (the flowlines look better to me than the animation does).

triple100.png20 MaCrust.png

At the same time, I also have orange break free from purple and begin heading northwest. The speed of this movement is pretty high, so presumably it's being subducted by something (green's plate??), but I'm unsure about what the driving force could be to get orange moving and what the interaction would be between the moving orange plate the the ridge created between green and purple.

So yeah, looking for any input anyone is willing to provide, even if that input is that I'm going about this in entirely the wrong way. Thanks in advance!

[Charerg]

So first off, this already looks very interesting, great job . Though one thing I'd advise against is spending the time and effort on drawing the slabs of oceanic crust before you've finalised plate movements (since if you change the movements you'll have to redraw the crust as well). I think the ages sound fairly plausible, although it's worth noting that plate velocity varies a lot based on the tectonic forces acting on a particular plate, and also based on the size and composition of a plate to some degree (continental plates have a "deep keel" and tend to be far more sluggish than plates mostly made of oceanic crust). If you want to go the extra mile you could use the tools in GPlates to monitor plate velocity and see how it compares with averaged-out velocities of earthly plates (though this is not a necessary step for fantasy tectonics, of course).

Regarding your questions, hinging movements between plates are actually fairly common, so there's nothing wrong in that. What does rise some eyebrows is why would the "big plate" develop that triple junction and break up in the first place. I guess it's not entirely impossible that there might just be a spontaneous breakup, but usually that is caused by a shift in the tectonic forces acting on the plate (a portion of the plate being subducted being the #1 reason why it would break apart).

But if we assume that the green plate does in fact break apart from the rest this could/would initiate subduction on the eastward margins of the green plate, which could easily explain the breakup of the eastern continent and the orange plate colliding with green at a high velocity. In that case the interaction would be a continuous subduction zone at the border between the "green plate" and the "orange plate".

[MrBragg]

Thanks for the feedback! I agree wholeheartedly about not drawing in the oceanic crust at this early of a stage, I mostly did it here to get some practice in gplates so I have some idea of what I'm doing when the inevitable time comes that I'll want to do it for real.

So yeah, regarding the northern supercontinent breaking up, my initial, superficial thought was that at 100 Ma something off to green's southwest would have to start subducting green's plate in order to begin pulling it apart from the rest of the northern supercontinent and that the resulting tectonic rearrangement would also - somehow - initiate orange's platre to start being subducted by green's plate, explaining its rapid voyage toward green. I hadn't thought beyond that point, so if you'll indulge some "thinking out loud" here, here goes.

If I color in some rough outlines of what some plates *could* look like in the vicinity of green's action, I can make an oceanic red plate that is more or less an observer until 100 Ma. At this point, red begins subducting green's plate, initiating its cleavage from the northern supercontinent. Whether there is a potential force that could cause this subduction to begin isn't super clear to me; assuming spontaneous subduction is reasonable here I suppose I could wave my hands and say the oceanic crust on the southwest corner of green is somehow older and denser than red and so starts getting drawn into the depths because of its age. Though, looking at earth, I can't find an example of a plate having both oceanic and continental crust is being subducted by a purely ocean plate, so maybe this isn't likely, idk.

100Ma_draft1.png

Switching over to the orange-green interaction, I initially wasn't sure how green moving off to the southwest would initiate subduction on green's east, but when I draw it out what you're saying makes perfect sense! I also feel ok about rationalizing why orange would break off by saying the failed rift from 225 Ma sufficiently weakened the plate to make it predisposed to fracture. What I'm much less sure about how to rationalize is how the green-orange boundary could go from an active ridge to a subduction zone; something about that just seems strange and potentially impossible to me, so do you think that is something that could possibly occur?

I'm obviously making all of this up as I go, so if anything I've said here sounds outrageous, let me know before I convince myself that it's not completely crazy ;-)

[Charerg]

I made a little scenario about how the subduction of the green plate might come about (and how that would in turn cause the orange plate to end up under slab pull). Although this involves changing the continental layout fairly drastically, namely one of the continents (yellow in this case) would have to collide with the northern supercontinent. So, keep in mind that all this is completely optional, just to give you some ideas about possible scenarios .

Anyway, here's the concept:
Concepts.png

So, the idea here is that to begin with, the northern and southern supercontinents are diverging, while the yellow continent is being pulled to collide with the blue island arcs (possibly including a micro-continent like drawn here) between the two supercontinents. After the microcontinent and island arcs eventually colllide with yellow, the subduction zone switches its polarity, causing the northern supercontinent to fall under slab pull. This causes the red and green continents to rift apart and a new subduction zone eventually forms near the eastern margins of green (the mid-oceanic ridge either becomes inactive or is eventually subducted by green). Eventually, this sequence leads to orange falling under slab pull and drifting apart from purple.

Once again, these are just some ideas about possible scenarios, hope you find the feedback useful.

[MrBragg]

Pretty much all of your feedback is useful and appreciated

I think I'm understanding what you're saying, but to make sure I went ahead and did a quick sketch of this in gplates. Basically what I did was reduce the starting size of green and create another black (sub)continent in the western ocean. I have the black continent's plate being subducted by the northern supercontinent where it will eventually crash into the island arc in the southwest of green to initiate this chain reaction. Basically what I have is:

150 Ma
150Ma.png
At ~150 Ma, I have the black subcontinent in the middle of the western ocean happily being subducted by and traveling toward the northern supercontinent.

100 Ma
100Ma.png
At around 100 Ma the black continent finally collides with the island arc and, since the continent itself resists subduction, the polarity reverses and begins subducting the oceanic crust on green's plate. This sudden reversal causes green to begin rifting off red/blue and, at the same time, forces a new subduction zone to form between green and orange. I'll need to play around a bit to figure out exactly how that plays out between green/orange/purple, but conceptually it makes sense.

15 Ma
15Ma.png
By ~15 Ma, green has gotten pulled sufficiently far toward black that a continent-continent collision ensues and starts pushing up a bunch of mountains. Orange is well on its way toward green where it will soon initiate another continent-continent collision to make green a pretty mountainous place.

Sorry if this largely repeats what you wrote before, I'm partially writing this out to get things straight in my own head (and to have a record of what I'm think for when I forget in a few days...). I think this captures the major gist of your proposal, and also doesn't appear to force a complete rearrangement of the other continents.

[MrBragg]

Apologies in advance for the double post...

Nothing super profound here, but I fleshed things out a little and, likely to no one's surprise, ended up with something that looks very close to Charerg's sketch.

225 Ma: nothing exciting, black subcontinent subducting toward supercontinent prior to breakup
a_225Ma.png

175 Ma: again nothing exciting, supercontinent has split in half and black subcontinent continues to subduct toward northern supercontinent fragment
a_175Ma.png

100 Ma: black collides with the island arc at the subduction zone (left); subduction zone reverses polarity, rift opens between green and red/blue, ridge between green and orange/purple goes extinct, rift opens between orange and purple, new subduction zone opens near the passive margin on green's plate (right)
a_100Ma.png a_100Ma2.png

50 Ma: green continues subducting toward black, orange subducts toward green, rift between green and red/blue continues spreading
a_50Ma.png

0 Ma: black and orange have both collided with green
a_0Ma.png

Most of this feels ok, though I'm not sure about how easy it would be to open a new subduction zone near the southern passive margin of green's plate. Also not entirely sue what is going on in the north between blue/black/green, but that's a battle for another day.