3D numerical modelling of upper crustal extensional systems
paper submitted in February - full version to appear here when accepted
To date few 3D models exist that follow the evolution of
tectonic processes into large deformation modes with sufficient
resolution to resolve individual faults and shear zones. We use an
Arbitrary Lagrangian Eulerian (ALE) fully parallel finite element
code which solves for visco-plastic flows in 3D. Plastic materials
weaken with accumulating strain. To localize deformation, a weak
seed region is introduced at the base of a plastic model extended
by velocity boundary conditions. Controls on the geometry and
spacing of three-dimensional frictional-plastic shear zones are
investigated. The sensitivity of varying the offset between weak
seeds and of strain weakening parameters on the linkage between
offset rift zones and on the efficiency of rift propagation are
tested.
The model results indicate primary controls of strain
dependent frictional plastic rheology and rift offset on
efficiency of rift propagation and style of rift segment
interaction.
The three dimensional models indicate three main rift
modes for linkage between two upper crustal rift segments in 3
dimensions: 1) small offset grabens with a single relay zone; 2)
intermediate offset grabens with one or more secondary step-over
graben segments; and 3) large offset graben with no significant
segment interaction.
Setup
The simulation domain is a box of dimension 210 km x 210 km x 15
km representing the brittle lithosphere. Extensional boundary
conditions of 0.5 cm/yr are applied on 2 opposite sides of the
box. In models C0-C6, two weak regions each dimensioned
2.63 km x 32 km x 1.25 km are placed at two ends of the box. In
this set of experiments, the offset Δ, between the 2 weak
seeds, is increased by a multiple of h, from 0 to 6h.
Numerical considerations
The numerical grid is composed of 160x160x12=307,200 elements.
Trilinear velocity - constant pressure elements are used, along with a penalised formulation.
This allows to substantially reduce the size of the algebraic system to solve.
In the present case, the symmetric sparse matrix is of size N~1,000,000 and the massively parallel IBM sparse matrix solver
WSMP is used on 64 threads in order to obtain its solution.
For all the following simulations, 1000 timesteps (δt=10kyr) were carried out. On average a simulation took about a day to run.
The model results were visualised with MayaVi.
Simulation Results
Comparison with natural systems
Comparison of models with natural systems. a) Northern
part of the right-stepping Viking Graben system, b) Model C3
(offset Δ= 3h) with
σsw=20 reflected along the y-axis at t= 7 Ma. c)
Devil's lane stepover in Canyonlands, SE Utah d)
Model C2 (offset Δ = 2h reflected along the y-axis at t=5.8 Ma.
