Structural anatomy of the Ligurian accretionary wedge (Monferrato, NW Italy), and evolution of superposed melanges

Abstract

We document in this study the internal structure of the Late Cretaceous–late Oligocene Ligurian accretionary wedge in northwestern Italy, and the occurrence in this exhumed wedge of broken formation and three different types of melanges that formed sequentially through time. The broken formation is the oldest unit in the accretionary wedge and shows bedding-parallel boudinage structures, which developed as a result of layer-parallel extension at the toe of the internal part of the Alpine wedge front during the Late Cretaceous–middle Eocene. This broken formation experienced an overprint of tectonic, diapiric, and sedimentary processes as a result of continental collision in the late Oligocene. The NE-vergent thrusting and associated shortening produced a structurally ordered block-in-matrix fabric through mixing of both native and exotic blocks, forming the tectonic melange. The concentration of overpressurized fluids along the thrust fault planes triggered the upward rise of shaly material, producing the diapiric melange, which in turn provided the source material for the downslope emplacement of the youngest, late Oligocene sedimentary melange. The sedimentary melange units unconformably cover the collisional thrust faults, constraining the timing of both this episode of contractional deformation related to continental collision and the combination and overlap of tectonic, diapiric, and sedimentary processes. Our multiscale structural analysis of the Ligurian accretionary wedge shows that tectonic, diapiric, and sedimentary processes played a significant role in its evolution, and that the interplay between and the superposition of these different processes strongly controlled the dynamic equilibrium of the accretionary wedge in the NW Apennines–western Alps. This kind of polygenetic melange development may be common in many modern and ancient accretionary complexes, and the processes involved in their formation are likely to be responsible for major tsunamic events in convergent margins.