[CIG-LONG] Convergence issues with long-term extension models

Wed Mar 10 00:20:29 PST 2010

Hi John,

Have you tried adding a compressible layer at the surface of the model?

Nicolas Riel and I ran some extension models recently and found out that the only way to achieve large deformation is to use such compressible layer.

The initial geometry of the model is as follow with the compressible layer in red:

[cid:image003.jpg at 01CAC06D.9F330650]

so you can extend the model quite extensively (the compressible has been removed for visualisation)

[cid:image004.jpg at 01CAC06D.9F330650]

Give it a go!

Cheers

gilly

________________________________________________

Dr Guillaume Duclaux
CSIRO Earth Science and Resource Engineering
Visiting address: ARRC, 26 Dick Perry Av., Kensington WA 6151
Postal address: PO Box 1130, Bentley WA 6102, Australia
Ph: + 61 8 6436 8728    Fax: + 61 8 6436 8555    Web: www.csiro.au<http://www.csiro.au/>

________________________________
From: cig-long-bounces at geodynamics.org [mailto:cig-long-bounces at geodynamics.org] On Behalf Of John Naliboff
Sent: Wednesday, 10 March 2010 3:03 PM
To: cig-long at geodynamics.org
Subject: [CIG-LONG] Convergence issues with long-term extension models
Hello all,

I'm hoping to get some input on convergence problems I'm having with 2D long-term extension models.

The models are designed following Wijns et al. 2005 (EPSL), who modeled extension of a brittle upper crust overlying a viscous lower crust using Underworld.  The initial geometry is 160 km (width) x 60 km (depth) and the upper crust follows a yielding law similar to the DruckerPrager yield criterion.  The boundary conditions are free-slip on all sides, although a weak, low-density layer is present above the upper crust (i.e. top of lithosphere should behave in a similar fashion to a true free-surface).   Extension is driven by a fixed velocity of 3 cm/yr applied to the right side of the 2D box. The models in the paper are run up to roughly 80% extension (~ 4 Myr).

The simulations I'm running mimic the models above very closely, although I'm using an upper free-surface rather than placing a weak, low-density layer above the upper crust and specifying free-slip on the top boundary.

In short, the problem I'm encountering reproducing the long-term extension models is that my simulations stop converging  after ~ 1-2 Myr (~ 25%-40% extension).  In terms of specific error messages, the problem is almost always related to the Stokes Uzawa Solver (ex: uzawaRhsScale has illegal value 'inf', etc).

In terms of the model output, by 1-2 Myr very large strain rate gradients have developed at the model surface roughly following horst and grabben structures.  I think this may be the source of the convergence issue, but at this point I'm really not certain.

After initially having convergence issues with a resolution of 1 km and 30 particles per cell (ppc), I systematically increased the resolution up 0.25 km with 60 ppc, but all the models still stopped converging between 1-2 Myr.
I also tried decreasing the time step by 2 (dtFactor = 0.5) and turning off the pressure and velocity interpolation onto the new mesh, neither of which had any significant effect on the total run time before the models stopped converging.  In all of the cases above I used the MUMPS solver, which performed better than the default solver.

The convergence issue is also not likely related to the specific upper crustal yielding rheology, as I've tried both DruckerPrager, VonMises and the yielding law used in the Wijns et al. models.

Finally, I tried using a free-slip top boundary with a weak, low-density layer above the upper crust, but this produced a whole different set of issues.

Thanks in advance to anyone who has some ideas/input on the convergence issues and I've attached a sample input file (resolution = 0.5 km, ppc = 60) if that helps at all.

Cheers,
John Naliboff
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