LaMEM: a Massively Parallel Staggered-Grid Finite-Difference Code for Thermo-Mechanical Modeling of Lithospheric Deformation with Visco-Elasto-Plastic Rheologies

Abstract:

The complexity of lithospheric rheology and the necessity to resolve the deformation patterns near the free surface (faults and folds) sufficiently well places a great demand on a stable and scalable modeling tool that is capable of efficiently handling nonlinearities. Our code LaMEM (Lithosphere and Mantle Evolution Model) is an attempt to satisfy this demand.
The code utilizes a stable and numerically inexpensive finite difference discretization with the spatial staggering of velocity, pressure, and temperature unknowns (a so-called staggered grid). As a time discretization method the forward Euler, or a combination of the predictor-corrector and the fourth-order Runge-Kutta can be chosen. Elastic stresses are rotated on the markers, which are also used to track all relevant material properties and solution history fields. The Newtonian nonlinear iteration, however, is handled at the level of the grid points to avoid spurious averaging between markers and grid. Such an arrangement required us to develop a non-standard discretization of the effective strain-rate second invariant. Important feature of the code is its ability to handle stress-free and open-box boundary conditions, in which empty cells are simply eliminated from the discretization, which also solves the biggest problem of the sticky-air approach – namely large viscosity jumps near the free surface. We currently support an arbitrary combination of linear elastic, nonlinear viscous with multiple creep mechanisms, and plastic rheologies based on either a depth-dependent von Mises or pressure-dependent Drucker-Prager yield criteria.
LaMEM is being developed as an inherently parallel code. Structurally all its parts are based on the building blocks provided by PETSc library. These include Jacobian-Free Newton-Krylov nonlinear solvers with convergence globalization techniques (line search), equipped with different linear preconditioners. We have also implemented the coupled velocity-pressure multigrid prolongation and restriction operators specific for staggered grid discretization. The capabilities of the code are demonstrated with a set of geodynamically-relevant benchmarks and example problems on parallel computers.

This project is funded by ERC Starting Grant 258830