I've moved on to starting to think about what's going on in the outer ocean and think I've got a rough outline of justifying how the northern (eastern now) supercontinent fragments. The animation captures the important features, but basically I've set up the outer ocean basin to be something like Tethys, with subduction in the east, a ridge in the ocean (probably also one off to the north, too), and a passive margin at the far western purple continent. At 200 Ma, the ridge jumps within the purple continent and tears a piece off, which subsequently starts a long journey across the ocean toward the subduction zone. While that chunk heads east, a rift forms within the pink eastern continent (~120 Ma) and begins breaking a piece off; depending on the geometry, this could be a back arc basin, or justified by some other yet undetermined mechanism. In any case, that rift eventually goes extinct (~70 Ma) and the rifted fragment hangs out off the coast of pink until it collides with the piece of purple that finally arrives around 30 Ma. This collision reverses the polarity of subduction, putting pink under slab pull and initiating the breakup of the supercontinent. At 16 Ma, pink collides with the offshore fragments and the subduction polarity again reverses, essentially restoring the initial subduction zone with the fragments accreted onto the margin of pink. No longer under slab pull, pink loses most of its westward vector and travels largely south from the northern pieces of the supercontinent.

200 Ma - present
OuterOceanAnim.gif

Eventually I'll need to try and work out the details, especially what's going on off to the north/south, but for the section in the middle I don't think I've violated too many tectonic rules