Mass-wasting of tectonically active margins is an important process in the degradation of deep-water fold-and-thrust belts. However, tectono-stratigraphic links between mass-transport complexes (MTCs), the evolution of MTC basal surfaces, and the timing, and spatial progression of deformation have not been extensively studied. This study uses high-quality, 3D seismic reflection data from the southern Magdalena Fan, offshore Colombia to investigate how the growth of a deep-water fold-and-thrust belt (the southern Sinú Fold Belt) is reflected in the source, distribution and size of MTCs. At least 11 distinct, but now-coalesced MTCs, overlie this surface. Their size and source location changed through time: the oldest, ‘detached’ MTCs are relatively small (10-160 km2) and sourced from the flanks of growing anticlines; the younger, ‘shelf-attached’ MTCs are considerably larger (200-400 km2), are sourced from the shelf and post-date the main phase of active thrusting and folding. Changes in the source, distribution and size of MTCs are tied to the sequential nucleation, amplification and along-strike propagation of individual structures showing that MTCs can be used to constrain the timing and style of deformation, and seascape evolution in time and space. The basal surface of the largest MTC was created by multiple syn-tectonic and post-tectonic mass-wasting events, is highly diachronous and represents an extended period of slope instability. Thus, the geometry and extent of MTC basal surfaces can evolve through time, and the deposits that overlie them do not necessarily record the processes that led to their creation. These insights complicate assessments of the anatomy and genesis of MTC basal surfaces and could be applied at deeper burial depths where seismic resolution may be poor.