Hello everyone,

I am trying to create a compressional environment. The deformation is initiated through consistent inward horizontal velocities at the model sides, without imposing any velocity limitations in the vertical directions. Outflow at the model base equalizes lateral inflow. The inward velocity is 1 cm yr-1. The bottom boundary has a free slip, the top boundary is a free surface. The balance calculations look okay to me. However, my velocity vectors look weird, they rapidly change direction in each timestep. I tried to increase free surface stabilization theta to 1, enlarged the model box to triple sizes, decreased CFL number to 0.1 and max time step to 1e4, and decreased velocity down to 0.05 cm yr-1. They did not affect anything.

outputs.pdf (1.1 MB)

I would appreciate any suggestions to fix this issue. Thanks in advance.

Best,

Acelya

```
type or paste cosubsection Initial temperature model
set Model name = function
subsection Function
set Variable names = x,y
set Function constants = h=660e3, ts1=273, ts2=441, ts3=723, ts4=1623, \
A1=1.8e-6, A2=0.25e-6, \
k1=2.5, k2=2.5, k3=2.5, \
qs1=0.0415, qs2=0.031, qs3=0.0125
set Function expression = if( (h-y)<=15.e3, \
ts1 + (qs1/k1)*(h-y) - (A1*(h-y)*(h-y))/(2.0*k1), \
if( (h-y)>15.e3 && (h-y)<=40.e3, \
ts2 + (qs2/k2)*(h-y-15.e3) - (A2*(h-y-15.e3)*(h-y-15.e3))/(2.0*k2), \
if( (h-y)>40.e3 && (h-y)<=220.e3,\
ts3 + (qs3/k3)*(h-y-40.e3), ts4)))
end
end
```

```
type or paste code subsection Boundary velocity model
set Tangential velocity boundary indicators = bottom
set Prescribed velocity boundary indicators = left x: function, right x:function
subsection Function
set Function constants = cm=100.0 #*3600.0*24.0*365.25
set Function expression = if(x<1650e3 && z<370.e3, -(230/430)/cm, \
if(x<1650e3 && z>360.e3, 1.0/cm, \
if( x>1650e3 && z<370.e3, (230/430)/cm, \
if(x>1650e3 && z>360.e3, -1.0/cm, 0) \
))); \
```

```
Constant viscosity prefactors = 0.1, 0.1
set Prefactors for dislocation creep = background:6.52e-15, C_1:6.52e-15
set Stress exponents for dislocation creep = background:3.5 , C_1:3.5
set Activation energies for dislocation creep = background:530.e3 , C_1:530.e3
set Activation volumes for dislocation creep = background:18.e-6, C_1:18.e-6
set Prefactors for diffusion creep = background:6.52e-14 , C_1:6.52e-14
set Grain size exponents for diffusion creep = background:0. , C_1:0
set Activation energies for diffusion creep = background:530.e3 C_1:530.e3
set Activation volumes for diffusion creep = background:3.e-6 , C_1:3.e-6
set Cohesions = background:0.e6 , C_1:0.e6
set Angles of internal friction = background:15.0 , C_1:15.0
set Grain size = 5.e-3
end
end
```