THRUST FAULT GEOMETRY AND EVOLUTION DURING CRITICAL COULOMB WEDGE GROWTH: INSIGHTS FROM DEM SIMULATIONS

The relationship between structural geometry and mechanical state of accretionary wedges is well explained by critical Coulomb wedge theory, and the predictive value of this model for homogeneous clastic systems has been demonstrated in numerous analog sandbox experiments. Numerical sandbox models created using the distinct element method (DEM) provide a versatile and powerful tool for further exploring the evolution of accretionary wedges and for testing theoretical models. A set of simple DEM simulations, designed to reproduce dry, non-cohesive sandbox experiments, were conducted in order to examine effects of basal dip and internal and basal friction coefficients on decollement and thrust fault activity. At each strain increment, active thrust surfaces were digitized and used to quantify horizontal range of thrust activity, total length of active thrust fault surface, mean thrust fault dip, and position of the decollement tip through time. Wedges generally accreted self-similarly with tapers in agreement with critical Coulomb wedge theory. Thrust activity was confined to zones of fairly constant width near the decollement tip, which advanced and retreated during wedge growth. In wedges with a low ratio of basal to internal strength, the positions of the decollement and thrust front were nearly coincident and fluctuated greatly; decollement extent and wedge width decreased as basal dip increased from 0° to 20°. In simulations with a high ratio of basal to internal strength, the wedge was narrower, and the decollement extent was more stable and less sensitive to basal dip. Reduced decollement extent was compensated by increased forethrust activity which extended well forward of the decollement tip. Backthrust activity was most reduced in high basal friction wedges, and greatest in low internal friction wedges. Thrust fault dips evolved with strain, likely due to decreasing boundary effects. High basal friction led to low forethrust dips, and high internal friction produced low backthrust dips, consistent with critical Coulomb wedge theory. In general, simulated deformation patterns successfully replicated those of analog sandbox models.