1. Numerical Models of the Formation of Extensional Sedimentary Basins:
Sensitivity to Rheology and Surface Processes
Susanne Buiter, Ritske Huismans
and Christopher Beaumont

2. Model set-up We solve the mechanical equilibrium equations using a
The model is on crustal scale in order to obtain a high resolution. To achieve extension, a velocity is applied on both sides, 0.5 cm/yr each. The seed is formed by a few elements of weaker (fully strain softened) material and is used to localise extension. At the base there is a roller boundary condition, which means that the crust is completely decoupled from the mantle. Isostatic compensation of mass changes is taken into account through an elastic beam which underlies the model. Faults are not specified beforehand, but develop as shear zones in a dynamic manner as the model evolves.
We solve the mechanical equilibrium equations using a
2D finite-element method (sopale, Dalhousie University)

3. Viscous-plastic rheology
Strain softening
The model is thermomechanical. Frictional-plastic strain weakening is included through a reduction in the angle of internal friction with increasing strain.
Coulomb failure criterion: J2 = C cos(f) + p (1-l)sin(f)
Wet quartz: Gleason and Tullis, 1995
The temperature evolution is determined by solving the heat equation

4. Extension of Continental Crust
Full sedimentation, with strain softening
30 km
The figure shows the centre 200 km out of the 400 km wide modelling domain. The grid is a tracking grid, shown is a subset of around 1/6 of the total tracking grid. The calculation (Eulerian) grid is completely regular (not shown here). This model has full sedimentation, which means that every depression that forms is immediately filled with sediments. The sediments have the same properties as the crust, only their density is lower (2400 kg/m3 instead of 2800).
50 km
70 km

5. Effects of softening and sedimentation
No sedimentation
With sedimentation
no soft.
soft.
80 km extension in all models

6. Effect of viscous strength

7. Effect of viscous strength - comparison
All models after 70 km of extension

8. Susanne Buiter, Adrian Pfiffner, Ritske Huismans
Numerical Models of the Formation and Inversion of Extensional Sedimentary Basins
Susanne Buiter, Adrian Pfiffner, Ritske Huismans

9. Inversion PR e 60 km c 10 km c 30 km
Shortening of the ‘reference’ extension experiment. During inversion, I applied diffusion erosion at the surface. The syn-inversion sediments can be seen in red.
c 10 km
c 30 km