Quick question: what happens to the new oceanic plate in this instance? The red line is the mid-ocean ridge, and the blue line is the subduction zone. The plate with green lines in being subducted underneath a continent above. As the mid-ocean ridge is subducted underneath the pink continent, what happens to the oceanic crust created in this timestep? I assume that the part of the ridge that isn’t subducted keeps producing crust (?) but what happens to the bit that is subducted? Any help/advice appreciated!
Many different things could happen here.
Up until the the ridge itself is subducted, the simplest one is that the hydrated oceanic crust is subducted under the edge of the zone, and the release of the water through slab dehydratrion lowers the melting point of the mantle material, which when combined with the asthenospheric convection happening in the mantle wedge between the two plates causes arc magmatism on the overriding plate. So, you’d get an island arc or a line of volcanoes on the existing continental crust. There would potentially be a little forearc of flexed crust on the overriding plate in front of the mountains and perhaps a bunch of accreted material between the subducting slab and the overriding plate if there was material getting scraped off of the plate on the bottom or a lot of sediment from the overriding plate.
If the convergence between the two was strong or highly orthogonal then the overriding plate could be shortened, which causes fold and thrust belts as the crust material thickens and sort of piles up and spills out over the top. This is happening in several parts of the Andes, for instance.
If the convergence was oblique or slow, it’s possible to still have the arc magmatism and accretionary wedge and the forearc bulge, but it can also start to pull apart the plate on the top. This causes extension and thinning of the plate on top, which can make it similar to the Basin and Range province in the Western US, which is like a bunch of little rock islands suspended in a stretched out “crust tarp.” If the pull is really strong it can cause back-arc rifting and a little ridge of continental crust gets pulled off the coast and can extend quite a long way away from its original position. (This is what happened to Japan.) Or, it can just pull the crust a little bit and you get fissures and magmatism/basalt floods behind the mountain ranges (like the Rio Frío or Las Máquinas basalts behind the Andes).
This formation of a back-arc basin can also happen when the subducting slab rolls back (“slab rollback” or “subduction hinge migration”). As the dense plate sort of collapses it basically takes the trench with it, along with whatever is attached to it. (Japan again, but also the same reason that several pieces of the Mediterranean rifted off of Iberia and went on a journey themselves or that the basin where the the Carpathians are are now filled in and surrounded with mountains). For what it’s worth, I think that tends to happen with really old and dense oceanic crust from what I’ve read.
If the crust is buoyant because it’s young or has less dense stuff on top like an extinct ridge or oceanic plateau it can also do flat slab subduction where its buoyancy makes it float up under the overriding plate and scrape along, staying coupled as it drags along under the bottom. This tends to cause shortening on the overriding plate, with the “Laramide” style orogenies, fold and thrust belts, and a lack of volcanism or a severe decrease in it. (This is seen in several areas of subducted ridges in the Andes.) Where there is volcanism it tends to be farther back where the slab itself is decoupling and the slab dehydration is allowing the melt to get through the crust.
When the ridge itself gets subducted a lot of different things can happen. First, the crust there is really buoyant and young, and tends towards a more collision interaction and flat slab subduction. If the ridge is subducted perpendicularly, the two sides of the ridge peel away from each other and the mantle flows upwards in the slab window that is formed. It can stop magmatism or make it anomalous, with things like the edges of the slab window creating different kinds of magma compositions and far deeper volcanism than the normal 100-150km you get from “normal” subduction.
If the ridge is subducted parallel or closer to it, the first part subducted just kind of falls away, and the center of that original spread causes melt above itself. The portion last to go in can continue to subduct and the center of the volcanism migrates around as the original middle gives way to a dehydrating slab. However, it’s also possible that it couples to the overriding plate to such a degree (owing to speed of convergence, buoyancy, topographic prominence, etc.) that it turns part of the subduction zone into a transform fault. It can even grab and take a piece of the overriding plate on it’s own journey. (This is what happened at the San Andreas fault, where the oblique convergence the subducted ridge let the Pacific plate tear a ribbon off of Laurentia and began to drag it NW and it’s also - from what I’ve read - what happened to various accreted Mesozoic materials along eastern Asia that dragged along and formed part of Japan before subduction resumed and led to the back-arc rifting.)
Everywhere that multiple plates interact you’re also going to have a triple junction. That’s a whole 'nother thing because there’s a lot more “potential” triple junctions than what are actually stable. So basically you’d also need to consider how each of those transition into more stable configurations, which can change how the whole border interacts.
Hope this helps. In case it matters; I’m also a worldbuilder, but I’ve spent the last two or three years reading just about every book and/or research paper you can find on the subject. There’s a lot more information and nuance out there than I can hope to cover in a post. Also, there’s a monstrous amount of content in the form of college lectures and stuff in video format on the web that can help.
In essence, the subduction process “recycles” the newly formed oceanic crust at that location by carrying it down into the mantle along with the older lithosphere as the ridge itself is dragged into the subduction zone. Any remaining seaward portion of the mid-ocean ridge, meanwhile, is still producing new oceanic crust as usual. In order to allow spreading to continue unhindered in the open ocean while the subducted section is being consumed, the ridge axis will eventually either jump (also known as “ridge relocation”) or be truncated by a transforming fault.