NUMERICAL DYNAMIC MODELLING OF THRUST-AND-FOLD BELT MECHANICS: INFLUENCE OF LAYER GEOMETRIES AND MATERIAL PROPERTIES ON STRUCTURAL STYLE

Although the broad-scale mechanics of thin-skinned thrust-and-fold belts have been addressed by analytical, numerical, and analog "critical wedge" models, and are relatively well understood, the internal mechanics of thrust-and-fold belt deformation are more difficult to examine. Analogue models have provided some important insights, but fully coupled numerical dynamic modelling probably affords the greatest potential for a deeper understanding, especially for more realistic cases involving the effects of surface processes (erosion and sedimentation).

We describe a series of numerical experiments of thin-skinned thrust-and-fold belt deformation using a 2-D finite-element continuum mechanics code. The SOPALE code (Serial Optimized Arbitrary Lagrangian-Eulerian, developed by Philippe Fullsack) is capable of accommodating very large strain, and is suitable for examining thin-skinned deformation styles. Here we discuss simple models with only a few layers of constant lateral thickness, each composed of a frictional (Coulomb plastic) material characterized by an effective angle of internal friction and a cohesion, both of which may depend on strain. During deformation, the upper surface of the thickening wedge can be exposed to surface processes, allowing removal and redistribution of material. Although faults are not modelled explicitly, narrow high-strain zones develop, forming structures very similar in style to those in thrust-and-fold belts. Even with very simple initial geometries, the experiments produce a wide variety of structural styles.

We divide model results into three basic structural styles: (1) a quasi-pure shear style, where there is relatively little vertical variation in the magnitude of horizontal shortening; (2) a detached-layer style, characterized by an internal detachment localized along a weak layer, dominantly foreland-vergent structures, and greater advance of the thrust front at a high structural level; and (3) a bi-vergent style characterized by the absence of a strong sense of foreland structural vergence. Models gradational between these styles can be produced, but our efforts to date suggest that these styles may represent mechanical "modes" adopted in part by the complex feedback relationships between internal wedge deformation and surface processes.