Centrifuge modelling of thrust faulting: structural variation along strike in fold-thrust belts

Abstract

The three-dimensional geometry and the kinematic evolution of fold-thrust structures are investigated by analog modelling using the centrifuge technique. We focus on along-strike structural variation and transfer of displacement between faults and folds. The total horizontal shortening across the model fold-thrust belts is constant, yet there are systematic variations in shortening by the mechanisms of thrusting, folding and layer-parallel shortening. These along-strike changes in structural geometry and strain mechanism are manifestations of displacement transfer. The model thrust faults have different amounts of thrust displacement at different points along strike. The variation is accommodated by transfer of displacement to over- or underlying faults or to en-echelon faults, by transfer of shortening into folding, or by transfer of displacement into layer-parallel shortening. In the models it is clear that faults nucleate at points that are localized by earlier folding of competent units. Faults nucleated at different points along a single fold may propagate along the fold and link together to form major thrusts; the thrust displacement in such cases varies from smaller values at linkage points to larger values at nucleation points. The models have a stratigraphie succession like that of the central Appalachian Valley and Ridge province. After an equivalent amount of structural shortening the models exhibit structural features analogous to those of the Valley and Ridge. The lowest competent unit (equivalent to the Cambro-Ordovician lithotectonic unit) forms a large-scale duplex like that of the Waynesboro sheet). The middle competent unit (equivalent to the Siluro-Devonian clastic units) forms anticlinoria and synclinoria like those of the Martinsburg sheet). These structures are spatially associated with underlying culminations and depressions that reflect, respectively, large and small displacements on thrusts that ramp through the lowest competent unit. However, in the models it appears that the amplitude of buckling of the middle unit controls the amount of displacement on underlying thrusts, whereas in the Appalachians the structural culminations of the middle unit are interpreted to have been caused by large displacement on underlying faults.