# The topography has abnormally high value when simulating continental extension

Hi all,

I tried to use the lateral variation of lithospheric structure on the basis of continental_extension.prm. And according to Airy model for isostatic compensation, I set up the corresponding initial topography. The topography changes from 600 m to 1200 m from left to right. But when I started running simulations, I found that the surface on the right side had unusually high values of topography. I suspect that the problem may come from the velocity boundary conditions.

``````subsection Boundary velocity model
set Tangential velocity boundary indicators = 0, 2
set Prescribed velocity boundary indicators = 1 x:function
subsection Function
set Variable names      = x,y
set Function constants  = v=0.004, h=110.e3, H=270.e3, d=20.e3
set Function expression =   if(y>=H+d, v, \
if(y<=H, -v*(h+d/2)/(H+d/2), ((-v*(h+d/2)/(H+d/2)-v)/d)*(H+d-y)+v)); \
0
end
end

``````

I only set the extension velocity in the right lithosphere and the inflow velocity in the asthenosphere, with a linear transition zone of 20km in the middle to ensure the volume of material is balanced. I tried lithospheric thickness with and without the initial topographic height (h=111.2e3/110.e3), and both have this problem. Are there any methods to eliminate this anomalous value? And the program terminates after running for a period of time with no error.

ref_model.prm (13.3 KB)
log.txt (2.8 MB)

Hi @egg_wang,

Thank you for posting the question to the forum.

I think what is happening is that the top right corner of the model is fixed, but the flow field is producing an overall decrease in topography that creates the sharp gradient you observe.

To confirm this, can you send plots of the following properties:

1. Topography over time (there should be a .txt file with topography in the output folder)
2. The same image as posted, but with strain rate for the background color and velocity vectors added? As with 1, please plots this for a few time steps.

A solution for the sharp topography gradient is to allow the mesh to move tangentially along the right boundary. This can be done with with the `Additional tangential mesh velocity boundary indicators` parameter, which can be added as follows in the Mesh deformation subsection:

``````subsection Mesh deformation
set Mesh deformation boundary indicators = 3: free surface, 3: diffusion
set Additional tangential mesh velocity boundary indicators = right
``````

Can you give this a try and see if it helps?

Cheers,
John

Hi @jbnaliboff ,

Thanks for your suggestion. I’m sorry for not replying to you in time. There are some problems with our server these days. I tried the `Additional tangential mesh velocity boundary indicators` parameter, which did eliminate sharp topography gradient. But the evolution of the topography still doesn’t seem to make sense. The initial topography, which was low on the left(600m) and high on the right(1200m), was completely reversed after simulation. It doesn’t seem to fit with the velocity vectors.
Here are strain rate and velocity distributions at different times,along with the corresponding topography files.

topography01000.txt (21.3 KB)
topography02000.txt (21.3 KB)
topography03000.txt (21.3 KB)
original.prm (13.4 KB)

Sorry for the slow reply on my end as well. What I think is happening is that the thicker lithosphere (lower density) on the right produces an overall upwelling effect, which is not compensated by the higher topography. At the start of the model, is the pressure equal at the maximum depth of the lithosphere (i.e., compensated)? Even if the model is isostatically compensated at some depth, you are almost certain to get significant deformation patterns as the stresses that partially support lateral variations in lithospheric structure are missing (I actually just mentioned this in a similar post).

The next steps will depend on exactly what hypothesis, processes, and observations you are trying to investigate, but here are a few ideas to consider:

1. Try making the top no-slip/free-slip and see what flow patterns arise
2. In combination with point 1, make the side or bottom “open” traction boundaries with the lithostatic pressure applied. In this scenario, at least one boundary (left) should be no slip.

Cheers,
John