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

A sandbox analog modeling research program was used to study the detailed e volution of doubly-vergent thrust wedges. High-resolution multilayers of th in alternating sand and mica layers were used in contractional Coulomb wedg e experiments to simulate deformation of anisotropic, brittle upper crustal strata in doubly-vergent orogens. Experiments incorporating syntectonic se dimentation in foreland basins and in piggyback basins were also carried ou t. Our laboratory models evolved in two main stages: (I) initial high-veloc ity thrusting in the retrowedge and high-frequency together with low displa cement folding and thrusting in the prowedge; and (2) low-frequency, high-d isplacement synchronous thrusting in the prowedge and low-velocity thrustin g 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 backs top for further prowedge accretion. Addition of syntectonic sediments incre ased the persistence of stage I and triggered out-of-sequence thrusting in the axial zone of the experimental orogens. Thrust motion was stick-slip. R etrovergent thrusting occurred along a long-lived ramp whose lower tip was located at the subduction slot. Provergent kink bands nucleated at the subd uction slot in stage I. In contrast, during the second stage of wedge evolu tion, kink bands nucleated in a piggyback fashion in the foreland far from the subduction slot and then evolved into high-displacement faults that rem ained active up until the end of the experiments, at progressively decreasi ng rates of thrusting. The axial zones of the model wedges were characteriz ed by fast uplift rates during stage I due to backward translation of the b elt along the retrovergent, long-lived ramp and due to the localization of deformation close to the subduction slot. Outward migration of the deformat ion front in the prowedge region during stage II caused the progressive dec rease in the rate of the wedge uplift, until it eventually stopped. Analyti cal models to quantify the Coulomb behavior in the wedges validated the rev ersal of thrust polarity with increasing shortening, triggered by the build up of the topographic load during deformation. This thrust polarity reversa l highlights problems with the classic concept of back thrusting as a refle ction of a significant change in the deformation regime. Our results compar e well with the kinematic evolution of natural accretionary prisms and of t hrust-and-fold belts such as the Lesser Antilles Are System, the Mediterran ean Ridge, and the Pyrenees.