Coupled DEM and FEM Models: an Approach to Bridge the gap Between Large-Scale Geodynamic and High-Resolution Tectonic Modeling

During the past decade, improved modeling techniques have heralded an era of visualization and quantification in tectonophysics. However, there is still a broad gap between lithosphere-scale geodynamic models mainly addressing ductile deformation and high-resolution models of distinct tectonic structures picturing the brittle domain. Since the first is best managed by numerical continuum methods and the second is a figurehead of analogue (sand-box) modeling, an interconnection of both domains is difficult to handle. Therefore, a numerical approach suited to simulate brittle fracturing and, thus, to act as a ?virtual sand-box?, represents an essential tool to study the interplay of shallow tectonics and deeper-seated geodynamic processes. This can be achieved using PFC2D, a special implementation of the discrete-element method (DEM) based on circular particles. High-resolution DE models offer a number of advantages over sand-box models: material properties can be determined more adequately; lithostatic pressure influences frictional sliding at different crustal levels; the evolution of stress and strain through time can be monitored at any point of the model. Additionally, DE models can easily be coupled with computer simulations of exogenic processes. However, the principal advantage of high-resolution DE models is their capability to be coupled with continuum models describing the lower, ductile part of the lithosphere. Thermomechanically coupled finite-element models (FEM, ANSYS) allow to consider temperature- and strain rate-dependent material behavior. Therefore, they are well suited to simulate ductile kinematics and, thus, to determine boundary conditions, that can applied to the base of the DE model. Vice versa, current reaction forces of the DE model can be used as input data for the FE model. Half-grabens forming above detachment faults are used as an example to illustrate the capabilities of high-resolution DE models. A lot of analogue modeling has been done on this topic, providing the chance to compare the results and to validate the new method. Thereby, special focus is laid on the fault pattern which evolves in the pre- and syn-rift strata of the hanging wall block. As a second step, the mechanically limited boundary conditions of sand-box models can be left behind by using the coupled DEM-FEM technique. Thereby, the asymmetry induced by the upper crustal detachment results in an asymmetric distribution of strain in the lower part of the lithosphere, which, in turn, leads to vertical movements along the detachment plane.