Phytophthora spp. causes the most serious soil born diseases of citrus. Your tree certainly looks to have a disease called foot rot. Although your tree is a Navel orange, a variety that is very susceptible to Phytophthora induced diseases, the damage to your tree is not to the Navel orange scion, but to the root stock, and most commercial rootstocks , but not all, have a tolerance to foot rot.. Do you know what the rootstock is? Whatever, you need to get the tree looked at quickly.

Looks like phytophthora to me too. There's not a whole lot you can do; maybe apply a copper fungicide? It's a bit of a long shot, but it may help somewhat.

-Tom

Any possible chance that this is mechanical damage from long ago? Damage that was not noticed until now? The reason I ask is that I have a Volk trunk that is actually more severe... but is for certain freeze damage from a few years ago [19F]. The new growth has that same rounded edge/margin and is slowly closing in/up.

If phytophthora, I'd treat as per earlier posts/suggestion and add a good soaking dose of fungicide. I have used AgriFos and / or SubdueMax in the past .... good luck.