Mantle division in SW-Iceland: How accurately can it be pinpointed?

The difference between division line between plates and convection rolls can be understood quite easily in the south of Iceland.

The central point east of Hveragerði is one of the most appearent in the world, because the N-American and Eurasian plates meet there.

The main division line of lower mantle is oriented SW-NE. The lines between the upper most four layers form an “X”, due to the coincidence that the division lines all meet at 64°N.

Each line stands for one of the four convection rolls layers found between 120-410 kilometers deapth. Besides that, the lower mantle cells meet below the black line. For those new to this subject (that is everyone), the intersection zone adds some level of complexity. Everywhere else than at these latitudes, only two layers of upper more convection rolls are found, and only a single set of lower mantle convection rolls. North of 64°N, the layer 1 convection cells have a stronger effect than layer 3. The rolls of layer 1 only reach south to 60.7°N, so south of there layer 3 becomes the upper most layer. At 64°N the thickness of all layers is equal, but north of there layer 1 is thicker than layer 3 and becomes more dominant.