Strange Mesh Behaviour when coupling Laminar Flow and Moving Mesh (ALE)

Hi all,

I want to simulate the convergence of tectonic plates and the resulting uplift of material at the plate boundary. The model is set up as follows: The left side and the left half of the bottom move horizontally with velocity v0, the right half of the bottom and the right side stay put. The top is an open boundary and should bulge up above the middle.

After some tries I got to make the top behave as I wanted it to, i.e. deform with the mesh following. However, at the bottom the mesh is not following the material flow, but interpenetrating itself, although I chose an incompressible flow. At the bottom, the mesh is moving along, leading to cells with zero volume and inverted elements. The solution gets unstable after one timestep and sometimes singularities occur.
The boundary conditions for the mesh I used are as follows:
Left half of bottom and left side moving horizontally with v0 (prescribed mesh velocity),
right half of bottom and right side no displacement (prescribed mesh displacement)
top following material (prescribed mesh velocity u and v, respectively)
free deformation for the volume

I also attached the file in case that helps.

I'm a beginner at Comsol, so I might have missed something obvious, but I couldn't figure out a way to have the mesh following the path of the material. Can anyone help me on how to set this up correctly?

Thanks a lot,

Matthias