Help needed fixing/building my plate tectonics

[rbnvgl]

Hey together,

i started a big map project and expanded what was once a continental map (the southern continent) to a world map. I got a bit too ambitious and began thinking about correct plate tectonics and climatic influences.
The system needs to make the southern continent work somehow, the northern part is not so important/open to changes. All i know is, that i want a big desert there. The northwestern round patch is a dried out, partly dammed sea (was once a comet impact crater, the poor little, world was kinda shot at a lot in its younger days - could alos explain some out of place mountains).
There is magic in the world, so i can handwave some things ... but I don`t want to rely on that too much.

Here is the tectonic map i did so far:


I would be really grateful if some people who have less dangerous half-knowledge about these things took a look at it and gave some tips.

[rbnvgl]

Ok, i noticed there is stil la lot of work to do to understand that correctly.
What i did so far was "drawing an arrow on a plate" and arbitrarily choose a direction. If i understand correctly i need to take an euler point, around which the general movement around the globe takes place (for all plates, for each plate different?).

I`m not sure if even my southern continent (main place of action) stands up to that/if its geology/climate is feasible at all. I think i will give it another try, but would still be glad about some recommendations/hints while I`m at that.

I also got that impact craters of comets on earth are a lot smaller than i drew them on my (vaguely earth sized) map. Perhaps it was still right, to have them impact plate tectonics if they were so large that they threw up mountains/broke the crust?
I`ll have to read/think about that and its effects further... maybe its a think allowing for some handwaving too, geology is not that comparable to earths... maybe I am making up illogical BS here.

[rbnvgl]

Another thing I`m rather clueless about right now is climate dynamics. If i understand correct, mountains can shield landscape from rain, so i placed a desert in the northern crater of the continent which is surrounded by mountains. The area below, farther south, might be a lot wetter, due to being open to rainfall from the sea from two sides and no mountains there. I imagine something monsoon-like in the area, at least once a year... the world i thought to have been through an anthropogenic global warming, so this zone is still pretty hot... im not sure if i should include permafrost regions at all, perhaps on very high lying mountain areas.
Here to, i would like to know if this is even feasible at all.

[Tiluchi]

This is certainly an interesting world you've got started on! I like the idea of multiple huge impact craters as a topography idea. Obviously this is already quite a departure from Earth as we have no extant structures anywhere near that size (if I remember my planetary history correctly we may have in the very early part of Earth's formation, but any craters that size would have been erased by plate tectonics and magmatism that effectively resurfaced the planet before life existed), so it's unclear to what extent Earth-like plate tectonics are even applicable here. Large impacts can be responsible for local magmatism and may even result in the creations of hotspots at the antipodes, but as a mere enthusiast without any good modeling software it's hard to predict what impacts this size would have on a planet where plate tectonics are already happening.

I suppose the big world-building question to answer, assuming you're going to be trying for some degree of realism, would how long ago these impacts were. The fact they're more or less intact now seems to suggest that they were fairly recent in history, in which case you'll need a hand-wavy answer for why they didn't result in the destruction of essentially anything that makes the planet habitable. Perhaps a good excuse for some magic-building! If they were formed many billions of years ago, before life was present, we'd need an explanation for why they haven't been distorted and erased through plate tectonics, as one would expect on an Earth-ish planet. Perhaps plate tectonics began only recently, which would suggest a very different geologic history than what we're used to on our own planet. But it's your own world, so the extent you choose to get into these questions is entirely up to you!

[rbnvgl]

This is certainly an interesting world you've got started on!

Thanks a lot. It was a huge amount of work already, so far I am doing it completely in illustrator, so everything you see there is also zoomable to some extent. But thats also time-consuming painting all the height maps as vectors... so they need to be more rough until now...

Concerning the craters, I am thinking about an unclear cataclysm several million years ago. Might be explained as a war of ancient gods (there are gods actually present, but they are mostly much to weild the necessary power) with some geoengineering and re-terraforming afterwards, at least in the mythology or (pseudo-)scientific attempts to explain it by the worlds denizens. I might as well leave it open... Maybe there has been prior civilizations. I thought it might be cool to let them dig out their remains, like we dig out dinosaurs... to there might be some extent of knowledge about it. Just lookd up that they went extinct 65 Million years ago. The Yucatan Crater, supposed to have caused that in some theories is nearly the same age. Compared to the huge craters i have here, its really actually small with a diameter of 180 km to 300 km... while supposedly killing 70 percent of all species. Oo
My biggest crater as far as i can roughly estimate with an equator length of 30.000 km (if its 1/4 smaller than earth) is up to 2.000 km. SO more than tenfold that. There are also several craters of a size near to that... I think it might take a lot longer to smooth these crater sizes with tectonis than the yucatan one... but it would also have all possibility of a habitable planet, lets say a 100-200 million years ago. So an earthlike development of life/evolution is not possible here. There must have been terraforming and "intelligent design" (brr) of some kind. Or a bridge to another world importing "already developed life"... or this giant holes in the ground have other, less harmful causes than giant rocks slamming the planet from space. Like being levitated away by some force, whatever. I have to think more about that... but it think its doable somehow with handwaving 100 millions of years backwards. Which is ok in my book.
What i will erase again are craters as deep as trenches/breaking the mantle. Even if the did, magma would have filled them up to some extent again.

Now tectonics. I liked the idea to have the impacts change them somehow, or that maybe my weird mountains could be the result of much smaller plates like for example around indonesia in our world. Another point iam thinking about... the more vulcanism the more naturally generated CO2... which would make anthopogenic climate change/destabilizing an halfway balanced "natural" climate system even easier than in our world. I have a sea level rise of 60 meters and a complete glacier melt (maybe except the highest mountain regions) in like 300 years ago, preceded by a now mostly extinct industrial civilization which was massively burning fossil fules (gas, coal, oil, other?) also over 2 to three centuries. So there was some a kind of climate acpocalypse in the near past from which the world is slowly regenerating. I`ll add an actual w.i.p. with some deserts, forest and flooded areas sketched in to get the point across.



Next i think I`ll remake my crude tectonics and see if I can make sense of them somehow.

[rdanhenry]

If the gods were active when the craters were formed, life could have been moved to safety for awhile. Could be a dimension bubble, a temporal stasis effect, a space ark, or something wilder, but there's a number of ways to handwave survivals while the mess gets cleaned up. Depending on how advanced the ancient civilizations were, might not even need the gods.

[rbnvgl]

If the gods were active when the craters were formed, life could have been moved to safety for awhile. Could be a dimension bubble, a temporal stasis effect, a space ark, or something wilder, but there's a number of ways to handwave survivals while the mess gets cleaned up. Depending on how advanced the ancient civilizations were, might not even need the gods.

Sure... but I don`t think I need to go into detailed solutions for a period of 100 Million years ago. I think it will be in the fossils somehow as a global extinction event and people who get this, will make something up to explain that life did go on.

At the moment I am still pondering more about the cartographic problems than the narrative ones emerging from them.
Need to update this plate model in a way that it actually makes sense.

[rbnvgl]

Here a sketch for updated plate tectonis... still not completely happy, but got a feeling its going into the right direction.

[rbnvgl]

Here are also some ocean currents. Do they look feasible?

[Tiluchi]

Your currents look fine overall, but just a note that warm currents become cold currents as soon as they start heading back towards the equator, taking colder water with them. Thinking specifically about the southbound current in the upper left and the southbound part of the large gyre in the southern hemisphere, but I'm sure there are others. The reverse is also true (although less common); cold currents become warm currents once they start heading back toward the poles.

Regarding the tectonics, I'm a little less convinced unfortunately; I see lots of subduction zones within continental zones and continental collisions in oceanic areas or near to coasts, rifts that don't really follow lines of the geography, etc... Looking at the map without the hypothesized plate boundaries I can see a number of spots where I could theoretically imagine rifting or convergence, but it's not necessarily where it is now. I recommend this post as a good place to start figuring out geology things- your world doesn't really lend itself to the whole "tectonics from Square One" model that the author recommends in the second half of the post, but the first part is a pretty good rundown of how to figure out geology in a conworld.

Honestly though, given that you've already got a world that departs significantly from Earth in some major ways (including but not limited to big honkin' craters), my recommendation might be to not try too hard to match plate tectonics to how they work in the real world. You've got a good start on your world with some great-looking land forms and you've clearly put a lot of thought into your coastlines and the way everything works so far, and the further you go into building plate tectonics, the more you'll feel the need to start messing with things to make it "plausible" (trust me, this is where I am now). Given you've already got a pretty good excuse for why tectonics don't work as expected (again, big honkin' craters), I think the best idea might be to just take that and run with it. This post might give you some ideas on how to think about plate tectonics on worlds that are generally... weird.
If you still want volcanism, keep in mind that craters tend to flood with basalt immediately after impact, although this doesn't last for particularly long. On the other hand, there's also the controversial antipodal volcanism theory; essentially the idea that the seismic waves from asteroid impacts concentrate on the antipodal point and start large-scale volcanism such as large igneous provinces and hotspots. I'd expect there to be some heavy volcanism at the antipodes of all your big craters there, which would lead to some interesting geography. Something to consider...