MODELING CONJUGATE FAULT SYSTEMS AND RELATED FOLDING WITH THE DISTINCT ELEMENT METHOD: NUCLEATION AND GROWTH EXPLAINED BY GEOMECHANICS

According to fundamental rock mechanics, faults are expected to develop as conjugate pairs. Thus, conjugate faulting is ubiquitous in compressional domains. Different spatial and temporal interactions of conjugate faults, however, can generate a wide variety of structural geometries including complex patterns of faulting and fault-related folding. Their study and comprehension are warranted in many academic and industrial applications.

We model the nucleation and growth of conjugate faults using the Distinct Element Method. This numerical technique performs a mechanical simulation on an assembly of particles which interact using contact laws. To mimic rock mechanical behavior, we assign appropriate parameters to the particles and contacts such as: density, friction, stiffness, and strength. This latter parameter allows to bond pairs of particles. Fracturing and faulting within the assembly happen when the stress is high enough to break the bonds, respecting a Mohr-Coulomb criterion. Deformation is triggered by forces acting on the particle ensemble: gravity and a displacement imposed on a system boundary.

We build a series of two-dimensional models composed of disks to represent several compressional structures observed in natural settings. We focus on five deformation styles. The first case corresponds to folding along a single fault. The second case is the simultaneous nucleation and growth of two conjugate faults. The last three models are intermediate of the two previous ones. These three models have a series of backthrusts sliding along a main forethrust. One case has the secondary backthrusts cutting and displacing the hanging wall layers without any significant folding; the layering remains tabular. The last two models have folding in the hanging wall, but differ by the role the backthrusts play in this deformation.

We show that the Distinct Element Method and the implied mechanics can efficiently model different conjugate faulting styles. In addition, we explore natural examples of conjugate fault systems from the Niger Delta and Qaidam basin (China) imaged in seismic reflection data, and highlight their similarities with these numerical models. We then illustrate how the models, in turn, help to understand and interpret these natural structures.