Synchronous and velocity‐partitioned thrusting and thrust polarity reversal in experimentally produced, doubly‐vergent thrust wedges: Implications for natural orogens

Abstract

A sandbox analog modeling research program was used to study the detailed evolution of doubly‐vergent thrust wedges. High‐resolution multilayers of thin alternating sand and mica layers were used in contractional Coulomb wedge experiments to simulate deformation of anisotropic, brittle upper crustal strata in doubly‐vergent orogens. Experiments incorporating syntectonic sedimentation in foreland basins and in piggyback basins were also carried out. Our laboratory models evolved in two main stages: (1) initial high‐velocity thrusting in the retrowedge and high‐frequency together with low displacement folding and thrusting in the prowedge; and (2) low‐frequency, high‐displacement synchronous thrusting in the prowedge and low‐velocity thrusting in the retrowedge. Transition from stage I to stage II occurred when the growing wedges reached the critical height at which they behaved as a backstop for further prowedge accretion. Addition of syntectonic sediments increased the persistence of stage I and triggered out‐of‐sequence thrusting in the axial zone of the experimental orogens. Thrust motion was stick‐slip. Retrovergent thrusting occurred along a long‐lived ramp whose lower tip was located at the subduction slot. Provergent kink bands nucleated at the subduction slot in stage I. In contrast, during the second stage of wedge evolution, kink bands nucleated in a piggyback fashion in the foreland far from the subduction slot and then evolved into high‐displacement faults that remained active up until the end of the experiments, at progressively decreasing rates of thrusting. The axial zones of the model wedges were characterized by fast uplift rates during stage I due to backward translation of the belt along the retrovergent, long‐lived ramp and due to the localization of deformation close to the subduction slot. Outward migration of the deformation front in the prowedge region during stage II caused the progressive decrease in the rate of the wedge uplift, until it eventually stopped. Analytical models to quantify the Coulomb behavior in the wedges validated the reversal of thrust polarity with increasing shortening, triggered by the buildup of the topographic load during deformation. This thrust polarity reversal highlights problems with the classic concept of back thrusting as a reflection of a significant change in the deformation regime. Our results compare well with the kinematic evolution of natural accretionary prisms and of thrust‐and‐fold belts such as the Lesser Antilles Arc System, the Mediterranean Ridge, and the Pyrenees.