New World (Help with Tectonics?)

[davoush]

Aha of course! Thanks! Is there an easier way to set a continent to 0 present-day rotation if you've dragged it to another location? So far, I have just been redrawing exactly where I want present-day locations - which is a bit tedious when I keep changing it around.

Anyway, that worked nicely, so here are the main rifts and flow lines with present-day subduction zones. I'd be interested to know if there is anything which seems very unlikely, or difficult to justify?

I'm also wondering: The ocean which has mostly closed with the collision of two continents still has a piece left in the middle. Must this subduct somewhere, or could it be absorbed into a plate with the semi-new oceanic crust around it? I remember reading that once subduction begins, the plate is doomed, but is this always 100% the case?



Also, a view centred on 180 - I'm wondering about this big piece of ocean. I think the most likely scenario would be that after 'Purple' continent has moved westwards towards the rift, it left a lot of oceanic crust in its trail. This crust then splits, and starts subducting, or would it still be forming a mid-oceanic ridge? Or maybe the split should first occur closer to the continents?

[Charerg]

Aha of course! Thanks! Is there an easier way to set a continent to 0 present-day rotation if you've dragged it to another location? So far, I have just been redrawing exactly where I want present-day locations - which is a bit tedious when I keep changing it around.

It's been a few months since I opened GPlates so I'm not entirely sure. But I "think" it might be possible to select a feature and then copy the points into the clipboard, though I don't know if that will work in this case (since it might just copy the original locations of the points, rather than the rotated coordinates).

[davoush]

Quick question regarding subduction:

It seems that the south polar ocean is being subducted to the west and the east, although it might have started just being subducted to the north. Is it possible for an oceanic plate to (eventually) split and start being subducted in different directions as in the image, or does it generally only take place on one margin? If so, I'm going to have to rethink the Southern Hemisphere...

[Charerg]

Quick question regarding subduction:

It seems that the south polar ocean is being subducted to the west and the east, although it might have started just being subducted to the north. Is it possible for an oceanic plate to (eventually) split and start being subducted in different directions as in the image, or does it generally only take place on one margin? If so, I'm going to have to rethink the Southern Hemisphere...

Yes it's normal for an oceanic plate to have a spreading ridge (or split in two or even three as a result of subduction). Again see the Pacific Plate as an example (which formed at a triple junction between the now-subducted Izanagi, Farallon and Phoenix plates). In fact, it would be unusual to see an oceanic plate without a spreading ridge. Though there could also be a similar case to the modern North American margin of the Pacific, which has a transform boundary (the San Andreas fault), since the Farallon plate was subducted relatively recently (Nazca, Cocos, Rivera and Juan de Fuca are remnants of the Farallon Plate).

Edit:

To comment on the tectonics:

Aha of course! Thanks! Is there an easier way to set a continent to 0 present-day rotation if you've dragged it to another location? So far, I have just been redrawing exactly where I want present-day locations - which is a bit tedious when I keep changing it around.

Anyway, that worked nicely, so here are the main rifts and flow lines with present-day subduction zones. I'd be interested to know if there is anything which seems very unlikely, or difficult to justify?

I'm also wondering: The ocean which has mostly closed with the collision of two continents still has a piece left in the middle. Must this subduct somewhere, or could it be absorbed into a plate with the semi-new oceanic crust around it? I remember reading that once subduction begins, the plate is doomed, but is this always 100% the case?

Both the Black Sea and the Caspian Sea have some oceanic crust, so there can occasionally be some pieces that are left as remnants. Though I think those are former back-arc-basins, so they were probably never under subduction (don't quote me on that though). But in a case like yours where there's a full-on collision, I don't see how there could be leftover pieces of oceanic crust there, if it's similar to Arabia-Eurasia or India-Eurasia. That doesn't necessarily rule out shallow seas or lakes though, since much of continental crust is actually submerged on Earth as well.

The big problem as I see it is the purple continent: that's subducting oceanic crust of both pink and yellow, which should put those continents under "slab pull" and pull them towards the trench.

[davoush]

Yes it's normal for an oceanic plate to have a spreading ridge (or split in two or even three as a result of subduction). Again see the Pacific Plate as an example (which formed at a triple junction between the now-subducted Izanagi, Farallon and Phoenix plates). In fact, it would be unusual to see an oceanic plate without a spreading ridge. Though there could also be a similar case to the modern North American margin of the Pacific, which has a transform boundary (the San Andreas fault), since the Farallon plate was subducted relatively recently (Nazca, Cocos, Rivera and Juan de Fuca are remnants of the Farallon Plate).

Edit:

To comment on the tectonics:



Both the Black Sea and the Caspian Sea have some oceanic crust, so there can occasionally be some pieces that are left as remnants. Though I think those are former back-arc-basins, so they were probably never under subduction (don't quote me on that though). But in a case like yours where there's a full-on collision, I don't see how there could be leftover pieces of oceanic crust there, if it's similar to Arabia-Eurasia or India-Eurasia. That doesn't necessarily rule out shallow seas or lakes though, since much of continental crust is actually submerged on Earth as well.

The big problem as I see it is the purple continent: that's subducting oceanic crust of both pink and yellow, which should put those continents under "slab pull" and pull them towards the trench.

Thanks Charerg. The subduction near the purple continent did occur to me as a possible problem, but I was going to explain it as: The collision to the east has only just 'finished', so part of that starts moving back towards the subduction. The continent moving south is being pulled towards a large, faster moving subduction. I guess at 'present-day', the southern continent might be on the verge of rifting towards purple? I suppose any configuration is always going to be only a snapshot of what's happening.

Regarding the inland seas/lakes - I either envisioned them as existing due to ongoing collision which hasn't fully 'closed' the sea yet, or, the yellow continent starts to get pulled back towards the purple subduction zone, making a rift between a mountainous region. Do either of these seem plausible? For example, when India was in early stages of colliding with Eurasia, would there have been large areas of 'trapped' sea (which eventually closed over?)

[Charerg]

Regarding the inland seas/lakes - I either envisioned them as existing due to ongoing collision which hasn't fully 'closed' the sea yet, or, the yellow continent starts to get pulled back towards the purple subduction zone, making a rift between a mountainous region. Do either of these seem plausible? For example, when India was in early stages of colliding with Eurasia, would there have been large areas of 'trapped' sea (which eventually closed over?)

Yeah either explanation works.

[davoush]

Quick question(s):

1) Where is an oceanic plate most likely to 'crack' into two pieces after a large amount of spreading? Would it split more or less along the ridge, or closer to coastlines at first? Assuming it is being subducted on both sides.

2) I am unsure how a back-arc basin can come to form an inland ocean. Back-arc basins confuse me in general. Am I right in understanding that it occurs when: subduction happens a little away from the coastline, pulling part of the continent towards the subduction zone? How big can the piece of 'pulled' land be?

Apologies for my endless questions! Hopefully I will be able to start updating the present day tectonics map once I've got a few minor features sorted out. I definitely want a back-arc basin somewhere though...

[Charerg]

Quick question(s):

1) Where is an oceanic plate most likely to 'crack' into two pieces after a large amount of spreading? Would it split more or less along the ridge, or closer to coastlines at first? Assuming it is being subducted on both sides.

I'm not entirely sure how to answer this. In general, rifts are more likely to form at continental crust than oceanic crust, but if a plate is 100% oceanic crust it's a tough call. I'd guess the rifting would occur where the crust is thinnest (so perhaps the younger crust would be more likely to crack?), but it also depends a lot on the direction of stress. For fantasy tectonics it's probably sufficient to consider the main direction the plate is being subducted in, and then place a ridge opposite to it somewhere.

2) I am unsure how a back-arc basin can come to form an inland ocean. Back-arc basins confuse me in general. Am I right in understanding that it occurs when: subduction happens a little away from the coastline, pulling part of the continent towards the subduction zone? How big can the piece of 'pulled' land be?

Apologies for my endless questions! Hopefully I will be able to start updating the present day tectonics map once I've got a few minor features sorted out. I definitely want a back-arc basin somewhere though...

Back-arc basins usually form when a subducting plate is moving away from the subduction zone (a retreating subduction zone). The Asian margins of the Pacific are a good example: Eurasia has been moving southwards towards the Indian Ocean, leaving many back-arcs along the eastern margins. So I guess you could say that they form because of "slab suction" causing extensional stress towards the subduction zone while the main landmass moves away from it. As to how big can the landmasses pulled be? Something like Kamchatka or Japan seems to be about as large as they get on Earth.

[davoush]

I'm not entirely sure how to answer this. In general, rifts are more likely to form at continental crust than oceanic crust, but if a plate is 100% oceanic crust it's a tough call. I'd guess the rifting would occur where the crust is thinnest (so perhaps the younger crust would be more likely to crack?), but it also depends a lot on the direction of stress. For fantasy tectonics it's probably sufficient to consider the main direction the plate is being subducted in, and then place a ridge opposite to it somewhere.



Back-arc basins usually form when a subducting plate is moving away from the subduction zone (a retreating subduction zone). The Asian margins of the Pacific are a good example: Eurasia has been moving southwards towards the Indian Ocean, leaving many back-arcs along the eastern margins. So I guess you could say that they form because of "slab suction" causing extensional stress towards the subduction zone while the main landmass moves away from it. As to how big can the landmasses pulled be? Something like Kamchatka or Japan seems to be about as large as they get on Earth.

Thanks Charerg. Tectonics is really like an addictive puzzle...

The main problem I have right now is the South Polar Ocean. From the Plates model, it looks like it was once very big, but is now being subducted by the landmasses moving south, however the movement of E2 in the image doesn't seem to make sense (or I'm misunderstanding).

In the image, South Polar Ocean (O1) starts first being subducted under W2 and the smaller continent, this pulls E2 down in that direction - but what happens at the boundary of E2 and O1? Can O1 be subducted on all sides, or would something else happen? Or, can a continent be dragged towards a subducting plate without it actually happening on its own boundary?

Thanks once again - your knowledge and generosity in sharing it is very much appreciated.

[Charerg]

Thanks Charerg. Tectonics is really like an addictive puzzle...

The main problem I have right now is the South Polar Ocean. From the Plates model, it looks like it was once very big, but is now being subducted by the landmasses moving south, however the movement of E2 in the image doesn't seem to make sense (or I'm misunderstanding).

In the image, South Polar Ocean (O1) starts first being subducted under W2 and the smaller continent, this pulls E2 down in that direction - but what happens at the boundary of E2 and O1? Can O1 be subducted on all sides, or would something else happen? Or, can a continent be dragged towards a subducting plate without it actually happening on its own boundary?

Thanks once again - your knowledge and generosity in sharing it is very much appreciated.

It's not really plausible for a single plate to be subducted in multiple directions, you'd have to break it apart into at least two separate plates. You could also make E2 more-or-less surrounded by divergent boundaries like Antarctica, maybe something like this: