Varying frontal thrust spacing in mono-vergent wedges: An insight from analogue models

Abstract

Sandbox experiments are used to study frontal thrust fault spacing, which is a function of physical properties within the thrust wedge. We consider three styles of thrust progression in mono-vergent wedges: Style I, II and III. In Style I, frontal thrusts progress forelandward, maintaining a constant spacing, whereas Style II and Style III progression show increasing and decreasing spacing, respectively. The three styles are shown as a function of the following factors: basal friction (μ b ), initial surface slope (α) and basal slopes (β), and surface erosion. For high μ b (~0.46), thrust progression occurs in Style II when α 2° and β > 0.5°). Style II transforms to Style I when the wedge undergoes syn-thrusting surface erosion. In contrast, low-basal friction (μ b = 0.36) gives rise to either Style I or III, depending on the magnitudes of α and β. Conditions with α = β = 0 developed Style I, whereas Style III in conditions with any non-zero values of α and β. In this case, surface erosion caused the process of thrust progression unsteady, and prompted out-of-sequence thrusting in the wedge. This study finally presents an analysis of the three styles, taking into account the following two parameters: (1) instantaneous increase of hinterland thickness (ΔH e /H e ) and (2) forelandward gradient of wedge thickness (δH/δx). Experimental data suggest that thrust sequences develop in Style II for low δH/δx and large δH e /H e values and, in Style III as either δH/δx increases or ΔH e /H e drops.