Modelling complex geological structures byFORC2 implementation of the HCA numerical technique

We present results of the implementation of the HCA method to describe the evolution of complex geological cross-sections by the FORC2 software. The Hybrid Cellular Automata (HCA) is a numerical forward modelling algorithm that follows an hybrid methodology between the cellular automata (CA) and the finite element method (FEM) philosophies. Layered rock units are simulated by a mesh of a very large number of semi-independent cells. Cells are related by three types of links that simulate the natural behaviour of layered rocks: a) intralayer relations; b) interlayer relations, and c) discontinuity relations. Intralayer relations link cells belonging to the same layer. Interlayer relations regulate the relationships among adjacent layers. These relations take into account the weaker rheologies of interlayer material, physical boundary conditions, and volume preservation conditions, while partial freedom is given to surface variations. Discontinuity relations correspond to the presence of ruptures such as faults. Volume and surface preservation is accomplished by the large amount of cells (typically >100,000) and by preserving the average distance among adjacent cells. The forward modelling pace is selected small enough to ignore not-adjacent cells in the computation and to reduce the links to first order equations. Units are simulated by grouping elementary cells with identical link properties into a mechanical unit. A multilayer of different mechanical units constitutes the mechanical stratigraphy. The physical parameters and type of links within the mechanical stratigraphy can be modified or are modified by the local conditions at any step during the run, thus resulting a self-constraining algorithm. No other conditions are imposed to the model that “self-decide” its evolutionary pathway. The conceptual architecture of the modelling tool is independent of the fold kinematics and geometry, and can simulate also complex tectonic evolutionary paths. The HCA-FORC implementation used in the modelling allows to compute at each step the stress and (brittle) deformation conditions at each cell. Three kinematic effects were considered to compute the latter component: a) torsion-induced fibre stress; b) flexural slip (i.e. interlayer slip) induced stress; and c) slip along the discontinuities. By varying the rheology, the elastic parameters, and the bedding geometry for each cell, it is possible to replicate the mechanical stratigraphy on the model multilayer as well as along strike variations. Example applications include: fault-related folding either in thrusts and extensional environments, complex duplex structures, syntectonic sedimentation and erosion, and salt diapirs.