Strain_rate anomalies in lithosphere

Attached is a model based on Aspect 1.4 I had created a few years ago now I need to answer a few questions I had missed back then. The top surface is a free surface, the left lithospheric boundary is pined, but the mantle boundary has outflux, and the front and back boundaries are free-slip. The model was visco-plastic. The right lithosphere was a subducting lithosphere with a convergence rate of 3cm/yr for this question. I couldn’t upload a second image to demonstrate the model geometry, but the right-hand side has a subducting slab that rolls back and causes the trench retreat and the strain rate curvature you see.
Can anyone figure out what causes the high strain rate localizations especially in corners?
Thank you

Dear Payman,

Thank you for posting to the ASPECT forum!

I think the high strain rates at the top corners on the left side is likely due to that boundary being pinned (i.e., the left lithospheric boundary is pined). Is the “mantle boundary” below also constrained to have zero velocity in the y-direction?

What I have seen happen in other models with a free surface is that the velocities adjacent to pinned boundaries are sufficiently large to create high strain rates, with associated significant deformation of the mesh over time in those regions also likely feeding back into that process.

A simple test for this would be to make the left “lithosphere” boundary free slip and also potentially allowing the mesh to deform on that boundary. The latter option is achieved through the Additional tangential mesh velocity boundary indicators parameter within subjection Mesh deformation.

Another option would be to extend the width of your domain, such the boundaries are farther away from the subducting slab. My prediction would be that the magnitude of the corner strain rate anomalies would decrease with increasing model width.


Thanks, John. I agree the increase in the model dimensions would reduce the effects of the anomalies on the rest of the model, however, that is not feasible since the models are 5 years old and I’m just trying to respond to the questions as to what and why they are created.
The left mantle boundary below the lithosphere has -x flux same as the right side to account for the convergence of material from the right lithosphere.
As you say it has to do with how the mesh deforms when cornered with a fixed boundary to its left and a free boundary to its top. Now, how velocity field changes in those cells affect the strain rate tensor defined as 0.5*(gad U + transpose(grad U)).
Thanks again John