Centrifuge modelling of fold—thrust structures in a tripartite stratigraphic succession

Abstract

Analog models measuring 127 × 76 mm in plan were deformed at 2500–4000 g in a centrifuge. Scaled stratigraphic sequences were constructed of anisotropic multilayers with individual layers of Plasticine and silicone putty as thin as 40 μm. The plasticine—silicone putty multilayers are analogs for interbedded competent carbonates and clastics, and incompetent pelites, given the model ratios of acceleration, 2500 g; length, 5 × 10 −6; specific gravity, 0.6; time 10 −10. Modelling of fold—thrust tectonics emphasizes the influence of stratigraphic succession on structural evolution. The models are constructed with a tripartite stratigraphic succession comprising basal and upper, well-laminated and incompetent units, and a middle, somewhat more isotropic and competent unit. The models deform by three mechanisms: layer-parallel shortening, folding and thrust faulting. They reproduce a number of fold—thrust relationships that have been observed in nature. Folds are typically periclinal, in en échelon arrays. Folds and thrusts are arcuate in plan, reflecting differential shortening. Fold attitudes grade from upright at high levels to overturned at deeper levels within a structural panel, reflecting drag against the basal décollement; fold axial surfaces and thrust faults are listric. While competent units may be offset by localized displacement on thrust faults, the discrete faults may die out both upwards and downwards into regions of ductile strain in less-competent units. Thrust faults appear to follow staircase trajectories through the strata, transecting incompetent units at shallow angles to bedding and competent units at steeper angles. However, the apparent staircase pattern results from propagation of a fault along a relatively straight trajectory through previously-folded strata. Foreland-verging thrusts are more common than back thrusts; the latter have steeper dips. The models suggest a mechanism of thrust-ramp nucleation following detachment folding: long-wavelength buckling of a competent unit can initiate localized strain (folding and layer-parallel shear) in an underlying incompetent unit, beneath the anticlines of the competent unit; thrust faults propagate up-section from these high-strain zones through the foreland-dipping limbs of buckle-folds in the competent unit. This mechanism may explain the commonly-observed spatial periodicity of thrust ramps. The model results bear similarities to natural fold—thrust belts in which the stratigraphic succession consists of three mechanical units, for example, the Asiak Foreland and Bear Creek Hills fold—thrust belts of the Slave Province, Northwest Territories, Canada.