# Set up a plate velocity at the top boundary in a 2D model

Hi all,
I am a PhD student who just started using aspect to investigate plume-lithosphere interaction. I want to set up a 2D model with a plate velocity at the top where a plume upwells from the centre of the bottom.
My model setup is as follows.

subsection Boundary velocity model
set Prescribed velocity boundary indicators = left y: function, right y:function, top:function
subsection Function
set Variable names = x,z
set Function constants = u0=0.05
set Function expression = if (z=0,u0,0);0
end
end

subsection Boundary traction model
set Prescribed traction boundary indicators = bottom:initial lithostatic pressure,right x:initial lithostatic pressure, left x:initial lithostatic pressure
subsection Initial lithostatic pressure
set Representative point = 0,0
set Number of integration points = 1000
end
end

However, the result is that the model as a whole all has a velocity Vx, and however I changed the prescribed velocity, it doesn’t seem to make a difference.
Could anyone please give me some suggestions on how to achieve my model setup? Thank you in advance for your help!

Cheers
Ziqi

Hi Ziqi,

It looks like you prescribe a horizontal velocity at the surface; and the left, right, and bottom boundaries have a stress (the lithospheric pressure) prescribed. So there is nothing that resists the sidewards motion of the plate and your whole model moves towards the side.

How to change this depends on what exactly you want to achieve in your model. You could prescribe zero horizontal motion at the bottom, and in that case, the deformation would be distributed across the whole vertical extent of your model (i.e., if your viscosity was constant, you would get a constant strain rate across the model). If you want most of your model to be stationary and only the plate to move, you could use a different geometry model (Box with lithosphere boundary indicators) that allows you to prescribe the boundary conditions you have right now for the lithosphere, but then use something different below (for example, you could choose to have a zero horizontal velocity in the sublithospheric mantle).

Best,
Juliane

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