Note that every parallel unit vector (blue) points following a hypothetical flow from left to right, and every perpendicular unit vector (red) is oriented in such a way that the vector for the third dimension would point to you following the right-hand rule. Easy, right? (Rewording it: if parallel unit vector points 3 o'clock, perpendicular unit vector points 12 o'clock.) This criteria will be quite relevant for next steps and actually simplifies the algorithm: Indeed, once I have defined these basis vectors, I just need to follow:

1) For every boundary, I got the relative velocity as a single vector defined as the difference between the velocity vector of the plate at the 'top' (the one that the red unit vector is pointing) and the velocity vector of the plate at the 'bottom' (the one that the red unit vector is not pointing).

2) I project the relative velocity into the basis of the boundary (remember, red a blue unit vectors) just by the dot product between the velocity and each of the unit vectors. Transform forces are always given by the dot product with the parallel unit vector (blue) and sign or direction (left or right) doesn't really matters. However:

If the dot product with the perpendicular unit vector (red) is positive , we deal with a divergent boundary.

, we deal with a boundary. If the dot product with the perpendicular unit vector (red) is negative, we deal with a convergent boundary.

I'm showing below the convergent boundaries in red and the divergent in blue following this method. Cool. With this information I know where to put mountains, where to put valleys, and the scale of it according to the value obtained from the dot products.