Centrifuge modelling of thrust faulting: strain partitioning and sequence of thrusting in duplex structures

Abstract

Abstract The kinematic evolution of fold-thrust duplex structures has been investigated by analog scale modelling using the centrifuge technique. Plasticine and silicone putty simulate rocks such as limestone and shale, respectively. Model stratigraphic units consist of microlaminations (55–250 μm thick) of these two materials, different ratios of thickness of the constituent laminae simulating different limestone/shale ratios (bulk competence). The models involve six stratigraphic units totalling 4 mm in thickness, with alternating bulk competence (low at the base, high at the surface). They were subjected to layer-parallel compression from one end, and kinematic evolution was monitored by cutting profile sections at six deformation stages. Structural style varies with depth. The top competent unit develops upright buckle-folds which propagate from ‘hinterland’ to ‘foreland’. The middle competent unit also buckles, but the folds have pronounced foreland vergence. The bottom competent unit first develops gentle folds that propagate forwards, but the foreland-dipping limbs of these folds are soon cut by thrust faults. As shortening continues, this unit is telescoped into a classic blind duplex structure: thrust faults root in the underlying incompetent unit, ramp through the competent unit, and merge into a roof thrust in the overlying incompetent unit. The ‘wavelength’ of the duplex structure is inherited from the initial buckling instability. The duplex develops by serial nucleation of faults from hinterland to foreland as in natural thrust belts. However, all the faults remain active while the stratigraphic pile deforms as a tapered wedge. Palinspastic reconstruction by restoration of buckle folds and fault slip shows that layer-parallel shortening accounts for approximately half of the total shortening at all structural levels. Folding and faulting account for the remainder, with folding dominating at high levels and faulting at depth. The transfer is accommodated by decollement in the incompetent units. The models bear striking resemblance to fold-thrust structures of the Appalachian Valley and Ridge Province of West Virginia and Virginia, USA.