Modeling two sequential coaxial phases of shortening in a foreland thrust belt

Analog sandbox models are used to simulate two sequential coaxial phases of shortening in a foreland thrust belt. A sufficient hiatus is considered so that erosion and sedimentation after the first phase create an angular unconformity that is subsequently deformed. The effect of variation in thickness of post-erosional sediment package and presence of a weak layer at the unconformity level are analyzed. During the second phase, some first phase thrusts are reactivated and new thrusts are also initiated. Thrust reactivation results in a structure spacing that is smaller than the expected spacing for a thicker sediment package. Reactivation of pre-existing structures prevents the weak layer from acting as an intermediate décollement. An increase in thickness ratio tends to weaken reactivation of pre-existing thrusts. Model results also show that total displacement along individual reactivated thrusts generally increases downwards across the unconformity, which could be used to distinguish thrust reactivation in the field. Two regional examples from the northern Eastern Cordillera in Colombia and from the Variscan frontal zone in Western Europe, respectively, where multiphase coaxial shortening occurred, are compared with model results. Both natural cases show features, such as partially eroded first-generation folds and truncated first-generation thrusts that are indicators for two sequential phases of deformation as observed in the models.