Centrifuge modelling of the propagation of thrust faults

Abstract

Three-dimensional propagation of thrust faults in duplex structures and the mechanism of formation of large thrusts have been investigated by analogue modelling using the centrifuge technique. Plasticine and silicone putty are used to simulate rocks such as limestone and shale, respectively. The models contain a stratigraphic succession composed of six units with alternating bulk competency (beginning with low competency at the base).

The models are subjected to horizontal, layer-parallel compression from one end. They exhibit three mechanisms of shortening: layer-parallel shortening (LPS), buckling and thrust faulting. LPS is homogeneously distributed and accumulates throughout the deformation. Folds nucleate as isolated periclines which propagate and link together along strike.

Sections cut through the models after successive stages of compression show that in the lowest competent unit folds propagate from the hinterland to foreland, and the front (foreland-dipping) limbs of these low-amplitude folds are soon cut by foreland-verging thrust faults. As shortening continues, this unit is thrusted into a duplex structure with floor thrust in the underlying (lowest) incompetent unit and roof thrust in the overlying (middle) incompetent unit. Thrusts develop, one by one, in time and space from hinterland to foreland, but not a single thrust dies during the deformation.

Observation of the model deformation sequence shows that the thrust ramps are localized solely by earlier-stage low-amplitude folds. Therefore, the pronounced systematic spacing of thrust ramps is inherited from the equal-wavelength buckling instability in competent units. Fault-related folds nucleate as detachment folds, develop into fault-propagation folds as thrust ramps propagate, and finally are modified by fault-bend folding as thrust displacement increases. Thrusts nucleate at different points along a single fold which was formed by along-strike propagation of several doubly-plunging small folds. In the same way that the folds join together, the thrusts localized by them also join to form a major thrust. The thrust displacement in such cases varies from smaller values at linkage points to larger values at nucleation points.