Another Tectonic Mechamism Question

[MrBragg]

Everyone here has been super helpful so far, so I wanted to pick your brains about another stage of tectonics that I'm working on to see if what I'm envisioning makes sense as a mechanism. (Apologies if this should be combined with my older thread; if so let me know and I can put it there)

The overall course of the motions is shown in the animation, where the northern supercontinent is traveling south. At some point a change in the tectonics occurs and a triple junction develops between red / blue / yellow; the arm between red and blue either fails or progresses very slowly while the arms involving yellow succeed and lead to the yellow continent breaking off.


The way I'm currently envisioning the underlying tectonics is detailed in the following few images (standard color scheme of blue = subduction, red = divergent, green = tyransform; coastlines are very rough and island arcs aren't shown). Initially there are three oceanic plates involved, A, B, and C. The driving force for the movement of the supercontinent is the subduction of plate A to the south of the pink continent; B and C are spreading apart with B subducting under the western margins of the supercontinent. Eventually, the spreading ridge subducts under the western edge of the pink continent, resulting in a new westward pull (suction I guess) on pink, which is what initiates the continent breakup. Things then progress with B completely subducting under the continents.

1) 2)
3) 4)

My question here is could subduction of a ridge like this reasonably lead to continent breakup in this fashion? Obviously this would also have implications for the rest of C, but I wanted to make sure this part makes sense before worrying about that.

Thanks!

[HandwaviumAdAbsurdum]

Yes, provided the oceanic plate marked as "C" extends around the globe and ends on the other side of the supercontinent, then subduction of the ridge will cause slab pull to break up that supercontinent leading to the formation of a new spreading ridge. This would be a case of passive rifting so wouldn't necessarily have volcanics associated with the event, although this doesn't preclude the possibility of the emplacenent large igneous province during the post-rift stage like we saw with CAMP during the separation of North America from Pangaea.

[Charerg]

You're not too far off in thinking that a mid-oceanic ridge being subducted can lead to major changes in plate arrangements. However, I'm not so sure that a major continental rift would occur in quite this fashion (given that typically a plate falling under "slab pull" ie. being subducted is involved). I suppose the best real-life example of a mid-oceanic ridge having been subducted in recent history (geologically speaking) is the Farallon plate being subducted beneath the North American Plate. I made an extremely rough concept about the scenario:



So in essence the subduction of the Pacific-Farallon ridge has lead to a part of the North American landmass (Baja California) breaking off, as well as the western margins of the Plate shifting from compressional tectonics to extensional tectonics with some major crustal extension (the Basin and Range province). In the future this might develop further into pieces of North America drifting northwest (similar to Baja California) but not quite continental-scale rifting. It's also notable that in this case the subduction of the ridge has essentially led to cessation of subduction (where it actually has been subducted, that is, there are of course some remnants of Farallon still left like Juan de Fuca and Cocos). Though from what I've seen of tectonic reconstructions this by no means occurs always (depends on the situation and the relative movements, I guess).

A good example of "slab suction" playing a role in tectonics are back-arc basins. These generally form where slab rollback occurs as a result of the subducting plate moving in a similar direction as the subducted one (a retreating subduction zone):



A typical real-world example is the difference between the Asian margins of the Pacific (which has been "retreating" for most of its tectonic history, hence there are many back-arcs) and the American margins of the Pacific (which is an "advancing" subduction zone with barely any back-arcs). I suppose my main point is that slab suction is generally not a tectonic force powerful enough to cause major continental rifting. That said, the whole process of continental breakup and all the causes for it are by no means fully understood (and I certainly make no claim to absolute understanding of the process ), so there's definitely room for imagination when it comes to continents breaking apart.

[MrBragg]

Thanks to both of you!

@HandwaviumAdAbsurdum
Yeah, that was the mechanism I was going for. Though in my arrangement I guess it would have to be slab suction causing the continental rifting, which in light of charerg's comment might not be sufficient to fully break up a continent. Unless I'm misunderstanding slab suction, which is entirely possible.

@charerg
The Farallon plate subduction was actually the concept I was basing this on, so at least that much was recognizable :-) I was just naively hoping that slab suction was a stronger force than it seems to be. Ok, I'll reconsider what I have going on here and will probably be back shortly with even more questions ;-)

On a (maybe) somewhat related note, what is thought to be the driving force behind the S. America / Africa rift forming? It's entirely possible I'm missing something obvious, but from the reconstructions I've seen it looks like southern Gondwana was surrounded by subduction toward the continent and it isn't clear what changed to cause the breakup.

[Charerg]

Thanks to both of you!

@HandwaviumAdAbsurdum
Yeah, that was the mechanism I was going for. Though in my arrangement I guess it would have to be slab suction causing the continental rifting, which in light of charerg's comment might not be sufficient to fully break up a continent. Unless I'm misunderstanding slab suction, which is entirely possible.

@charerg
The Farallon plate subduction was actually the concept I was basing this on, so at least that much was recognizable :-) I was just naively hoping that slab suction was a stronger force than it seems to be. Ok, I'll reconsider what I have going on here and will probably be back shortly with even more questions ;-)

On a (maybe) somewhat related note, what is thought to be the driving force behind the S. America / Africa rift forming? It's entirely possible I'm missing something obvious, but from the reconstructions I've seen it looks like southern Gondwana was surrounded by subduction toward the continent and it isn't clear what changed to cause the breakup.

A more tricky question to answer is what caused the opening of the N. Atlantic ca. 180 Mya? Slab pull seems to have played a very minor role in the rifting, apart form perhaps a very brief occurrence when Stikinia collided with Laurentia. The opening of the S. Atlantic can be largely attributed to slab pull forces. As can be seen from the vid, Africa falls under slab pull around 160 Mya.

[MrBragg]

Alright, so incorporating everyone's feedback I've modified the model to hopefully be more reasonable. I've maintained ridge subduction as the main driving force, but basically flipped the directionality so that the pink/yellow continent falls under slab pull.

So, in essence, a rough sketch of what I have now is that there initially is an ocean basin separating the western purple continent and the eastern supercontinent; the western margin of that ocean is being subducted by purple while the eastern margin is a passive margin. As such, the ridge steadily marches toward the subduction zone and is eventually subducted. This is the event that causes the breakup of the eastern supercontinent, where a new rift opens north of yellow and the pink/yellow continent, now under slab pull, changes its vector to be more westward.

1) 2)

3) 4)

This seems more reasonable based on feedback, but if I've opened up a whole new can of worms definitely let me know

[HandwaviumAdAbsurdum]

This version is a big improvement on the last given that the continental fragment which has broken of due to slab pull is now moving towards the subduction zone rather than away from it. I didn't notice that until after I had posted. From the looks of it, the purple continent broke off from the supercontinent at an earlier time and drifted west, right? What about grey, antarctica-like continent to the south?

A more tricky question to answer is what caused the opening of the N. Atlantic ca. 180 Mya? Slab pull seems to have played a very minor role in the rifting, apart form perhaps a very brief occurrence when Stikinia collided with Laurentia. The opening of the S. Atlantic can be largely attributed to slab pull forces. As can be seen from the vid, Africa falls under slab pull around 160 Mya.

Thanks for linking the Muller et al. reconstructions. I'm working on an alternative tectonic history of Earth in which the point of divergence is in the Late Triassic, so these are really going to come in handy!

[MrBragg]

Improvement is the direction I'm shooting for

You're right about purple having broken off earlier; my current conception of things has an initial supercontinent beginning to break apart ~225 Ma with the purple-based fragment drifting southeastish and the green-based fragment going northwestish. Eventually the black terrane collides with the island arc on green's plate, resulting in subduction polrity reversal + green breaking off the northern supercontinent + orange breaking off purple. The rest of the major action is covered above, except for Antarctica, which I still need to play with to 1) justify why this happens and 2) fix the funky kink in the flowlines.

225 Ma


South Pole Weirdness

[HandwaviumAdAbsurdum]

I think it's worth me summarising what my understanding of what's going on so we can see if we're on the same page.

225 Ma - supercontinent breaks in two due to an as yet unresolved mechanism.

Later - a terrane, possibly an oceanic plateau, collides with the island arc off the coast of green leading to subduction polarity reversal. We now have slab pull as you wanted but its effects will be different to how envisaged them. It will not affect orange in any way since there is already a ridge between it a green. It would affect green in one of two ways, either a slither breaks off from its west coast and is pulled towards the newly formed west-dipping subduction zones, or the whole of green moves towards it. The latter would involve the opening of an ocean along green's northern margin with green undergoing clockwise rotation. It won't be earth shattering in the grand scheme of things. Just a minor rearrangement.

[MrBragg]

225 Ma - supercontinent breaks in two due to an as yet unresolved mechanism.

Yep, pretty much. At this point I'm willing to hand-wave the unresolved mechanism as something happening in the empty, hemisphere-wide ocean that causes the northern supercontinent to (even just briefly) fall under slab pull and initiate rifting. Based on charerg's post from a few days, maybe this could be a Stikinia-like collision that provides just enough of a nudge to get things moving.

It will not affect orange in any way since there is already a ridge between it a green. It would affect green in one of two ways, either a slither breaks off from its west coast and is pulled towards the newly formed west-dipping subduction zones, or the whole of green moves towards it. The latter would involve the opening of an ocean along green's northern margin with green undergoing clockwise rotation. It won't be earth shattering in the grand scheme of things. Just a minor rearrangement.

The mechanism I was going for here was essentially that yes, the terrane-island arc collision puts green under slab pull and, instead of just a piece breaking off, the entire green fragment rifts from the rest of the northern continent. In order for green to start being subducted by black, the eastern end of green almost has to start swinging south, which necessarily causes a change in the green-orange ridge system. Development of a new subduction zone to the south of green seemed like a reasonable way of accommodating this change in motion, which in turn seemed like a reasonable enough impetus to cause rifting within the southern supercontinent to send orange on its way north. There's a lot more detail / diagrams in a thread from a few weeks ago here, where charerg was generous enough to provide this idea: https://www.cartographersguild.com/s...ad.php?t=46417

If this still seems crazy, let me know