Mantle layer is necessary when overplate is thicker than slab?

[zxg348]

I found that when the overlying plate thickness is larger than slab, there is a area beneath slab near the trench is also defied as composition of overriding plate if I don't include mantle layer. In the following file code and figure, the trench is at x=2500km, so the anomaly area is x<2500 and x>slab and y above bottom of overlying plate. Anyone know how to adjust parameters?

Part of my .wb file is below:
{
"model":"oceanic plate", "name":"Overriding Plate", "max depth":100e3,
"coordinates":[[1500e3,2000e3],[1500e3,4000e3],[2500e3,4000e3],[2500e3,3000e3],[2500e3,2000e3]]

   "composition models":[{"model":"uniform", "compositions":[2], "max depth":10e3},
                         {"model":"uniform", "compositions":[3], "min depth":10e3}]
},


{
  "model":"oceanic plate", "name":"Subducting Oceanic plate", "max depth":50e3,
  "coordinates":[[4500e3,2000e3],[4500e3,4000e3],[2500e3,4000e3],[2500e3,3000e3],[2500e3,2000e3]],

   "composition models":[{"model":"uniform", "compositions":[0], "max depth":10e3},
                         {"model":"uniform", "compositions":[1], "min depth":10e3}]
},

{
  "model":"subducting plate", "name":"Slab", "dip point":[0,0],
  "coordinates":[[2500e3,4000e3],[2500e3,2000e3]],
  "segments":
    [
      {"length":250e3, "thickness":[50e3], "angle":[0,60],
       "composition models":[

[MFraters]

Hey, thanks for your question. There are a few ways to resolve this problem, in order of my preference:

1. make the thickness of the overriding plate variable. See here how to do that: https://gwb.readthedocs.io/en/latest/user_manual/basic_starter_tutorial/08_passive_margin_variable_depth.html
2. Make the subducting plate thicker so that it completely overlaps with the overriding plate below the slab, and then make a layer in the slab which overwrites the compositions and temperatures how you need them.
3. You could make mantle layers, and use them to overwrite those sections, but I would not recommend this method.

Let me know if this helps or if you have additional questions!