Dependence of Surface and Volumetric Strain on Basal Friction in Model Fold-Thrust-Belts Determined Using Laser-Scanner Data and Digital Images

The dynamic evolution of fold-thrust-belt depends on many factors including frictional behavior of the basal decollement. The basal friction in turn depends on mechanical properties of deformed materials, pressure, temperature, fluid pressure and the base roughness. Earlier studies have shown that frictional decollements produce wedges with steep taper, whereas weak decollements with lower basal friction (like salt in the SE Zagros fold-thrust-belt) spreads deformation over wider zones. In this study, we have quantified 3D deformation in a series of shortened sandbox models. We use sand with internal coefficient of friction of 0.57 in three identical sandbox models which had different basal friction; low (friction coefficient (μb) = 0.36), intermediate (μb = 0.5) and high (μb = 0.7). Surface strain (2D) was estimated using digital images and accurate (± 0.1mm) laser scans of the model surface were used for determining vertical strain. Control points in the digital images and laser scans of the top surface of the models were used to compare the topography, uplift rate and kinematics of the thrusts in these three models in order to outline the effect of basal friction. Model results show that,in addition to influencing the taper, and kinematics and geometry of the wedge, basal friction also governs both surface and volumetric strain in the models. Progressive volumetric strain for ~ 2500cm3 of sand is measured at each 1cm shortening increment. At final stage of shortening (after 8 cm or ~ 16% of bulk shortening), volumetric-strain of 5%, 9.5% and 12.5% were estimated for low, intermediate and high basal friction, respectively. This shows a direct correlation between volumetric-strain (dilatation) and basal friction. Similarly, penetrative strain, quantified for similar zones in each model, is highest in models with high basal friction. The volumetric reduction observed in our models is due to compaction of the sand layers by repacking of the sand grains during shortening. Since the individual sand grains do not deform under the model conditions, this volume decrease is taken up by reduction of the bulk porosity of the sand layer. Applied to nature, our models results show that more compaction and penetrative strain is expected in fold-thrust belts shortened above high-friction decollement relative to fold-thrust belts shortened above a weak horizon. This volume decrease may result in porosity reduction in the deformed lithologies with a negative impact on reservoir quality.