Trench sediment heterogeneity controls accretion mechanisms in subduction zone

Trench basins related to an almost entirely subducted oceanic lithosphere are filled by deposits sourced from either the passive continental margin or the accretionary wedge. They also include large-scale submarine landslides often triggered by the inherited rough topography of the subducting plate. This mechanism provides the deposition of material by mass-transport processes from the wedge to the trench, resulting in highly heterogeneous trench sedimentation. The role of the mass-transport deposits in the accretion mechanisms, as well as their influence on seismic behaviour at the plate interface is difficult to assess in the active trenches, because of their limited accessibility. This study reports on a well-preserved fossil trench-fill sequence exposed in the Northern Apennines. A multidisciplinary survey including fieldwork, stratigraphy, biostratigraphy, geochemistry, and zircon provenance, allows reconstructing the anatomy of these deposits. The heterogeneities derived by distinct sediment packages made of mass-transport deposits and deep-sea turbidites reflect a setting with two feeding source areas: the accretionary wedge and the continental margin, respectively. Alongside the roughness of the subducting plate, this deposits association favours coherent underplating processes developed at depth. Such accretion mechanisms, linked to thrust faults at the base of the wedge, can generate both major earthquakes and new mass movements, reinforcing the complex role of trench-fill processes in subduction dynamics.