# Convergence Failure with velocity conditions to produce lithospheric thickening

**URL:** <https://community.geodynamics.org/t/convergence-failure-with-velocity-conditions-to-produce-lithospheric-thickening/3900>\
**Category:** ASPECT\
**Created:** [January 30, 2025, 2:26pm UTC](https://community.geodynamics.org/t/convergence-failure-with-velocity-conditions-to-produce-lithospheric-thickening/3900 "2025-01-30T14:26:40Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![arijitchkc](https://avatars.discourse-cdn.com/v4/letter/a/dec6dc/32.png) [@arijitchkc](https://community.geodynamics.org/u/arijitchkc)\
**Post date:** [January 30, 2025, 2:26pm UTC](https://community.geodynamics.org/t/convergence-failure-with-velocity-conditions-to-produce-lithospheric-thickening/3900/1 "2025-01-30T14:26:40Z")

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Hello Everyone,

Hope you’re doing well!  
I am currently working with ASPECT and trying to model a scenario of lithospheric thickening. My goal is to thicken a lithosphere by putting velocity conditions at the boundary of my model. I must say that I am pretty new to ASPECT.

My model consists of a 2D box of 800 km in depth and 2400 km in width. I have 2 compositions: crust and mantle. Crust is top 40km, while the rest of the model is mantle. I also have a thermal anomaly at the bottom boundary to produce plumes. Finally, I am trying to impose an inflow condition at the top 200 km and an equally balanced outflow condition below it. I would Ideally like to start the convergence after the lithosphere has been affected by multiple plumes, but currently, only for debugging, I am starting the convergence from 5 million years.

The problem I face is that the stokes solver never converges. I have tried multiple velocity conditions, but the same problem persists. The velocity conditions, along with the implementations, are:

1. Inflow of 1 cm/yr at both left and right boundary at the top 200 km and an equal outflow below it.  

2. Inflow of 1 cm/yr at both left and right boundary at the top 200 km(but with a sin function) and an equal outflow below it.  

3. Inflow of 1 cm/yr at both left and right boundary at the top 200 km(but with a tanh function) and an equal outflow below it.  
This model runs for 5.18 million years

I have also tried removing the crust and keeping only one mantle composition to make it simpler. But that did not solve the issue.

I have attached the output error message of the linear velocity(1) boundary condition and also a picture of the first timestep of the model when the convergence starts, along with the log.txt file.

[output.e12043736.txt](https://community.geodynamics.org/uploads/short-url/sj3yzLU2bInslJr3ukZqOTGg8Xe.txt) (6.1 KB)  
[log.txt](https://community.geodynamics.org/uploads/short-url/4Yblc01I4MdM5ljnUY9zndGwfrg.txt) (73.7 KB)

 ![velocity_ss](https://canada1.discourse-cdn.com/flex036/uploads/geodynamics/original/2X/0/01993456f418cd676ed43cde9e673f89099b1cea.png)

Really appreciate any help with this issue.

Thanks,  
Arijit

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**Author:** ![jbnaliboff](https://avatars.discourse-cdn.com/v4/letter/j/58956e/32.png) [@jbnaliboff](https://community.geodynamics.org/u/jbnaliboff)\
**Post date:** [January 31, 2025, 4:07pm UTC](https://community.geodynamics.org/t/convergence-failure-with-velocity-conditions-to-produce-lithospheric-thickening/3900/2 "2025-01-31T16:07:51Z")

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Hi @arijitchkc,

Welcome and thanks for posting to the forum!

The way I have seen most groups implement this type of boundary conditions on the side walls is to have an intermediate section where there is a linear transition from inflow to outflow.

For example, you could have inflow in the upper 300 km, a linear transition to outflow over the next 200 km (300-500 km depth) and then outflow from 500-800 km depth.

I think this will produce a much smoother flow field than what you currently have (something looks off in the images).

My suggestion would be to try this approach first in a simplified setup with a constant viscosity and no initial thermal perturbations. If the flow field at time 0 looks correct and the solver converges in a stable fashion, then start re-introducing complexity back into the model.

This will be farther down the line in your workflows, but I also suggest having the imposed flow patterns at 5 Myr gradually ramp up over some period of time. I’ve found many instances where instantaneous changes in the boundary velocity produce solver convergence issues.

I hope this helps and let’s keep the discussion going as you test various scenarios.

Cheers,  
John

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**Author:** ![cedrict](https://yyz2.discourse-cdn.com/flex036/user_avatar/community.geodynamics.org/cedrict/32/344_2.png) [@cedrict](https://community.geodynamics.org/u/cedrict)\
**Post date:** [January 31, 2025, 7:16pm UTC](https://community.geodynamics.org/t/convergence-failure-with-velocity-conditions-to-produce-lithospheric-thickening/3900/3 "2025-01-31T19:16:54Z")

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Here are my notes with regards to the boundary conditions that John describes above.  
I hope this will help.

 ![image](https://canada1.discourse-cdn.com/flex036/uploads/geodynamics/original/2X/4/421a824de9d72eb706590f217142bd212a733999.png)  
 ![image](https://canada1.discourse-cdn.com/flex036/uploads/geodynamics/original/2X/e/eab91fddb6a46e4742c665180c013504c2221b96.png)

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**Author:** ![arijitchkc](https://avatars.discourse-cdn.com/v4/letter/a/dec6dc/32.png) [@arijitchkc](https://community.geodynamics.org/u/arijitchkc)\
**Post date:** [February 10, 2025, 5:02pm UTC](https://community.geodynamics.org/t/convergence-failure-with-velocity-conditions-to-produce-lithospheric-thickening/3900/4 "2025-02-10T17:02:52Z")

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Hello John,

Thanks for your help. I could solve the issue, with the suggestions. Also thanks to @cedrict , those notes are really helpful. I have used those formulas in the models and I have attached a picture below.

I initially tried with no thermal perturbation in the model and gradually introduced complexity. The total duration of the models I am currently running are 500 Mya.

Then I tried varying the onset and duration of convergence, and this works quite well now. I also varied the size of the box models a bit, to make sure everything works for a larger setup.

Here’s an example of the model where I started convergence from 140 Mya and continued till 240 Mya, and they ran without any convergence issue. I have attached the velocity(X direction) and Temperature plot for the starting time step and the one just before it.

 ![T_138](https://canada1.discourse-cdn.com/flex036/uploads/geodynamics/original/2X/a/aab727f2fdd653b702794a86d5877cf4e7046865.jpeg)  
 ![T_140](https://canada1.discourse-cdn.com/flex036/uploads/geodynamics/original/2X/f/f065d7950eee17cd024d00733df551c70a34a8c6.jpeg)

Regards,  
Arijit

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<div class="post-metadata">

**Author:** ![jbnaliboff](https://avatars.discourse-cdn.com/v4/letter/j/58956e/32.png) [@jbnaliboff](https://community.geodynamics.org/u/jbnaliboff)\
**Post date:** [February 10, 2025, 7:06pm UTC](https://community.geodynamics.org/t/convergence-failure-with-velocity-conditions-to-produce-lithospheric-thickening/3900/5 "2025-02-10T19:06:48Z")

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@arijitchkc - Thanks for the update and glad the models are working now!

Cheers,  
John