Spreading ridges and subduction zones

Here we have a spreading ridge
http://www.dkimages.com/discover/previews/851/20118402.JPG
with a great big split down the middle. And it's hot.

And here we have a subduction zone
http://www.geo.lsa.umich.edu/~ruff/geo107.F98/Drift.to.Plates/subXsect.gif
and it's cold ('coz it's away from the ridge. And going down.
Because it's cold.)

Question:- Why doesn't the bend in the subduction zone have a great
big fault down the middle? And blob off from the top?

Is this a daft question?
.

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