Mantle wind, and why the Pacific is asymmetrical

[L1qu1dN1trog3n]

Hi all!

Not a map post (so apologies if this is better off elsewhere), but I wanted to share something for our small community of Plate Tectonic Hyper-realists that might be of interest!

For those of you who don't know (which is probably everyone as I've not posted as much since my last drawing tablet broke ) I'm currently studying Earth Sciences at university and am now in the midst of a 3rd year module on plate tectonics. One thing that was brought up by one of my lecturers yesterday was the concept of Mantle wind and the impact it has on subduction zones.

Mantle wind, in essence, is the effect of the Coriolis force on mantle convection, such that sinking packets of mantle material will accelerate to the east, as they are now closer to the axis of rotation and need to preserve angular momentum.
As subduction zones are sites of mantle downwelling, this has a surprisingly big influence.

In a paper I can't find for the life of me (I'll email one of my lecturers if people are curious) you can see a remarkable pattern to subduction zones, based on weather the subducting slab is dipping to the west or to the east. In short, westward dipping slabs like at the Mariana trench, Japan trench, Philippine trench etc are much steeper than eastward dipping slabs.

The steepness of these trenches means the underlying plate is much less coupled (i.e. transmits less lateral force) to the overlying plate, and instead tries to sink vertically down into the mantle. This causes the hinge of the subduction zone to retreat, pulling the overlying arc with it, creating back arc extension. There is sometimes enough extension to cause a new mid-ocean ridge to form on the other side of the arc, which is what I think happened to create the Philippine plate, though correct me on that if I'm wrong.

Steep Trench.png
(If you're confused why the point of subduction naturally wants to move backwards, look up the phenomenon of 'Slab rollback', or DM me and I'll try to explain)

By comparison, eastward dipping slabs, like that subducting at the Peru trench, are pulled even further east by the mantle wind, and thus dip very shallowly. The Peru slab is dipping at only 30 degrees, compared to the near vertical Mariana slab. This means that they transmit much more force into the overlying crust due to their greater coupling, and you get compressional tectonic features instead of extensional. This should make sense for the Peru trench, as it is just seaward of the Andes mountain chain!

Shallow trench.png

So what does this mean for us ultra-realistic mapmakers? Well if you study a map of the Pacific (where Earth's most recent superocean is being subducted on all sides, and thus where most of our subduction zones are) you'll see a weird asymmetry. The western pacific is characterised by many little island arcs, some much bigger than others, but no truly continental volcanic mountain chain (Japan is an intermediate step, you have a large chunk of built up continental crust but which has a large, extended, back-arc basin). In contrast, the Americas feature no island arcs at all on their west coast, instead we have the volcanic mountain ranges of the Andes, central America and Cascadia.

CAVEATS
Mantle wind is not the only factor at play here! Every system in the Earth Sciences is highly complex and subduction zones are no exception. The true determinant is weather the velocity of slab rollback is greater (for back arc extension) or lesser (for accretionary orogenesis) than the velocity of the advancing plate. Mantle wind just so happens to tip the scales depending on whether the slab is dipping east or west. In fact the Andes used to be dominated by back arc extension. Proximity to a mid ocean ridge will also likely play a big factor, as the lithosphere subducting off of South America is very close to the East Pacific Rise and therefore hotter and more buoyant than that subducting off the shores of Asia. It just so happens that the pattern on the modern-earth is held up very well.

Also, North or South dipping subduction isn't nearly as subjected to these effects, and will likely have other factors at play.


TL;DR
Subduction on the west side of an ocean should have lots of island arcs, subduction on the east side of an ocean should have lots of andes-style mountain chains. The pattern we see in the Pacific is not by pure chance. There are other factors at play, but I'd hesitate to have more than 50% be the 'unfavoured' type of subduction.

Please let me know if you've found this helpful or have any further questions, either about this or other areas of plate tectonics!

Ευχαριστώ πολύ

Liquid

[Tiluchi]

Wow, this is super helpful and answers some questions I'd had in my head about back-arc basins and how they form. FWIW as you say I don't think it's the *only* reason why back-arc basins form where they do; I think there used to be an Andes-style mountain range along the east coast of China until 80 million years ago or so before it got shut down by back-arc rifting, and the Andes used to have some back-arc basins. Thankfully it appears that my world mostly follows this rule, although I'm realizing now I do have a very high continental volcanic arc on the east coast of one continent that might make sense to turn into a back-arc basin. Will have to think about that one a little more...

[L1qu1dN1trog3n]

As far as I'm aware no-one's done a full mechanistic study of this phenomenon, just that there's a statistical significance to the dip angles of the various subduction zones around the world depending on if they are west or east facing, and my professor at least ascribes this to the influence of mantle wind. This is a very cutting edge area of geodynamics! You're also right to stress that it's fundamentally the balance of forces in the overlying wedge that determines the geomorphology, and therefore other factors can easily swamp the 'signal' of the mantle wind. That's why I think its better to think of it as a bias rather than a hard rule.

We're also in the somewhat odd position that the majority of subduction zones are oriented roughly NS, with the Aegean, Aleutian and Java trenches being the only ones I can think of that are oriented EW. This probably makes this phenomenon stand out a bit more on the modern Earth than, say, when the Tethys was in its main closure stage.