Neogene sediments in the northern Chilean forearc display a wide range of near syndepositional structures. Analysis of the origin and distribution of these structures in space and time offers new insights into the development of the forearc basins. The structures are described in detail and show many features associated with soft-sediment deformation, pseudo-diapirism and slope failure. Synsedimentary deformation reached a peak in the Late Miocene to Early Pliocene while the sediments were saturated in a largely plastic state, and many of the structures were probably triggered by seismic shock. Late-stage tilting of the forearc generated shear stresses in the sediments leading to slumping and sliding. Base-level revision and drainage incision led to sediment bypass and cessation of lacustrine sedimentation that was not necessarily linked to climate change. Compaction and dewatering of the basins caused transition of the sediments from a plastic to a brittle state. The age and distribution of structures associated with seismicity appears to correlate with increasing subduction erosion and westward drift of South America but not with basin subsidence, shortening rates or plate convergence. This suggests that upper crustal deformation is at least partly decoupled from plate movement.