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@shipwreck
Difficult to see without cleaning it up properly and having it in front of me but it certainly looks like a progressive failure in which the dark surface has been exposed to the atmosphere for some time. The final clean bit on the dirty side has some ratchet marks which would have formed when large loads pushed the cracks through the remaining material each time you pushed hard on the pedals.
The whole thing looks like it originated at the point where the pedal contacts the crank arm. As the pedal bites in to the alloy it will harden the material a little, making it more prone to cracking.
I've just been out to check on mine and noticed that they do have black steel washers on, which would almost certainly prevent this type of failure.