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It is claimed by some that the reason for gaps in chains of volcanoes generated by plate subduction is that some parts of the plate dive deep into the mantle and generate magma plumes, while other parts of the plate continue in an almost horizontal direction and therefore don't create magma plumes, hence the lack of volcanoes. Where one part of a plate continues almost horizontally while an adjacent section is driven down into the mantle, wouldn't that cause a violent fracture of the plate, and if not, why not?

Push a sheet of cardboard along a table top so that the leading edge of the cardboard overlaps the edge of the table, then grab a bit of cardboard and force it to descend. It will either carry the rest of the cardboard down with it, or it will break. I know that plates are not made of cardboard, but that's the best analogy I could think of to illustrate the principle. A similar fracture in an oceanic plate would surely be quite violent.

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    $\begingroup$ A subduction zone is a plate boundary. Only one plate is subducted (on a ball, not a flat table). Are you referring to the Nasca plate we recently discussed ? There parts don't dive down because they are too buoyant, so it breaks and parts subduct, while others stay afloat and push under the crust of the adjacent plate. $\endgroup$
    – user18607
    Commented Jan 14, 2020 at 12:45
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    $\begingroup$ If you bother to read the second paper in my answer here (earthscience.stackexchange.com/a/18877), you'll have your answer in figure 1: the Nazca plate is divided into several segments moving at different velocities and separated by fracture zones. Some segments sink, other experience flat subduction. $\endgroup$
    – Jean-Marie Prival
    Commented Jan 14, 2020 at 13:16
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    $\begingroup$ Yes it needs reference as allwaya $\endgroup$
    – user18590
    Commented Jan 14, 2020 at 17:02
  • $\begingroup$ Well, you have a reference given on Jan 4th. See below. $\endgroup$
    – Michael Walsby
    Commented Jan 14, 2020 at 17:26
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    $\begingroup$ Then please add it to your question. Click "edit", then third button from the left, paste the url, click "add link". $\endgroup$
    – Jean-Marie Prival
    Commented Jan 14, 2020 at 18:00

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Where one part of a plate continues almost horizontally while an adjacent section is driven down into the mantle, wouldn't that cause a violent fracture of the plate, and if not, why not?

In case of the Nazca plate probably not because that is still warm and partly ductile (see the linked work in the other thread). Also, these cracks and deformations happen as the plate subducts, they don't displace a huge pile of material in one single event, like does a movement along a transform crack (e.g. San Andreas), or a "megathrust" earthquake along a subduction zone. A whole plate breaking along the bend line otoh can trigger catastrophic earthquakes.

I could not find a specific work that deals with the interface between a steep and a shallow subduction, but a work discussing the dynamics of the Nazca plate subduction. They state that the ridges (being warm and buoyant) are subducted because of the movement of the adjacent slabs (dragged along so to say), and becaue of global plate motion, meaning the earth can't change volume so some parts just must give way for emerging parts elsewhere (Atlantic opening is mentioned). There is a map of epicenters of (generally low mag) earthquakes (and links to other work) along the Nazca subduction as well as above the flat segments, suggesting that the cause there is the greater friction. The oceanic crust, being quite topographic, is flattened while being pushed or drawn under the continent.

Earthquakes because of segmenting as asked in the OP aren't mentioned specifically.

I find it interesting, maybe someone else knows more.

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