# Problem with velocity and Strain localization in a continental extension model

Hi,
I’m running a continental extension model, with 1000km by 660 km, with Lithosphere 200 km, using box with lithosphere boundary indicator. The first problem is that negative and positive x velocities have two different values.How can i solve this

and this is the corresponding function i wrote

The second problem is that strain is localized at the boundary between velocity boundary condition and traction boundary condition.

I could not figure out why strain localize at exactly the node between velocity and traction. is there a way around it

Thank you,
Chameera

Chameera:
I don’t think the velocity field should be expected to be symmetric. You prescribe symmetric horizontal velocities at the top boundaries, but then you have uplift in one half and subsidence in the other half for the vertical velocity. So the prescribed velocity field is not left-right symmetric.

As for the corner singularity where you switch from prescribed velocity to prescribed normal stress (=traction): This is a question that has been asked before (within 2023, I think) on this forum, and you might find a longer answer in the archives. Fundamentally, what it boils down to is that above that point, you prescribe a velocity but the traction prescribed below just happens to lead to a different velocity. Or alternatively, the traction necessary to achieve the prescribed velocity above the point is different from the traction prescribed below. The mismatch is what causes the circular flow and consequent strain locatization.

Best
Wolfgang

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Hi Wolfgang,
Thank you. I ran couple of models without constraining y values and did not get the symmetry. I will check it again. Also, I’m planing to run a continental extension restart model, where i stretch the lithosphere with velocity and hold it to study the rift. What would be the ideal restart setup. v= 0, initial lithostatic pressure or extract stress from last step and calculate traction and apply to the boundary of Lithosphere ?

Best,
Chameera

Chameera:
What is right depends on what kind of situation you are trying to model. If you have a fluid model (i.e., no elasticity), then if you deform the model and then set the velocity to zero, you will end up with a model where nothing moves (not at the boundary, and not in the volume either). If you prescribe tractions, then these tractions will drive some kind of motion unless they are purely hydrostatic. Which of these is right depends on what it is you want to do.

Best
W.

Hi Chameera,

1. Please upload your prm files rather than screenshotting subsections; this will help us troubleshoot.
2. What are the Vx values in the top left and top right corners? I’m not sure your colour bar has been renormalised to the min/max values of your data at t=0. The paraview standard greyshading can make it difficult to compare colour bar with domain colour.

Best wishes,
Bob

P.S. Here’s a solution for making the colour of 2D images in Paraview look the same as the colour bar: Color legend does not agree with color displayed in data - #4 by bastian - ParaView Support - ParaView

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Hi Wolfgang and Bob,

Thank you for the suggestions. I solved the V_x problem. It was because i had null-space removal in my example.prm file where i was removing net x translation. It was affecting applied horizontal velocities at the boundaries. @bobmyhill Thanks for the Paraview suggestion

@bangerth I was trying to setup a model using visco-plastic material model to study multi-phase rift evolution by controlling the boundary conditions. I want to check what happen to the rift after I replace Velocity BC with traction, whether it progress (further extension or rift axis migration) or will it cease the extension. I want to check the force balance of the rifted lithosphere, when i remove the extensional force. Asthenosphere is hold using initial lithostactic pressure.What would be the Boundary conditions that i should apply to the lithosphere at a restart model ?

Thank you,
Chameera

Chameera:
I think you will have to play around with different options a bit. Each choice for the boundary conditions corresponds to what is likely a valid modeling assumption. Which one you choose depends on the situation you want to describe, and I’d just play with them to see what makes sense to you.

Best
W.

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Hi Wolfgang,
I restart my model with traction calculated from stress tensor (1st phase model ran with velocity, restart with traction for lithosphere (original.prm, restart prm file and 2 ASCII files are attached here) . It does not show any error, but its stuck. any suggestions

H1_T6_3_5mm_yr_10M_T200km_Adjust_T_Adjust_Visco_1.prm (10.3 KB)
original.prm (12.0 KB)
box_2d_left lithosphere.0.txt (18.1 KB)
box_2d_right lithosphere.0.txt (18.2 KB)

Thank you,
Chameera

Chameera:
well, this might be the time to learn how to use a debugger and figure out what “it is stuck” actually means!

I would like to give a better answer, but not knowing whether it’s just taking a long time, nor what the details of your simulation are, means the best we can do here is to point you to learning about the tools that help you figure these sorts of things out.

Best
W.

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