Based off of the depression made by the Pillar of Illuin theory, I wonder if this might be true. Say that this theoretical depression was the "bowl' in which Helcar formed. Naturally, being depressed by a pillar (which would have to have a flat base to stand up-right), the Sea would probably be a relativly shallow one. Somehow, Helcar dried up, and virtually disappeared leaving its high shores and shallow basin. When the mega-volcano (the one that created the region of Ūdun at the northwest corner of Mordor*) erupted, its lava flows followed the ancient shores of Helcar, and also spread slowly out into the basin. Thus creating the low, ringing mountains and the plateau inside.
*I don't know of any info in Tolkien's work supporting the theory of Ūdun being the caldera of an old imploded mega-volcano, but I read in the Atlas of Middle Earth and it makes pretty good sense.
"A caldera is the remnant of a volcano that has exploded and/or collapsed. A ring-dike is a circular ridge of cooled igeneous rock surrounding a deep valley. It occurs when a round block subsides into an underlying magma chamber and the fluid magma is forced up around the edges. Often the upwelling is intermittent, leaving passes such as the Isenmouth and Cirith Gorgor. Either procees could have resluted in the feature shown by Tolkien, although compared with our world either would have been gigantic. Imagine the original height of a volcano with a forty-five-mile base. This colossus would have towered almost 29,000 feet! In contrast Mt. Doom was only seven miles across and stood 4500 feet" (Fonstad).
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'They say that the One will himself enter into Arda, and heal Men and all the Marring from the beginning to the end."
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