Mass-Transport Complexes as Markers of Deep-Water Fold-and-Thrust Belt Evolution: Insights from the Southern Magdalena Fan, Offshore Colombia

Mass wasting is an important process in the degradation of deep-water fold-and-thrust belts. However, the relationship between mass-transport complex (MTC) emplacement and the timing and spatial progression of contractional deformation of the seabed 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 recorded in the source, distribution and size of MTCs. More than nine distinct, but coalesced MTCs overlie a major composite basal erosion surface. This surface formed by multiple syn- and post-tectonic mass-wasting events and is thus highly diachronous, thereby recording a protracted period of tectonism, seascape degradation and associated sedimentation. The size and source location of these MTCs changed through time: the oldest ‘detached’ MTCs are relatively small (over 9–100 km² in area) and sourced from the flanks of growing anticlines, whereas the younger ‘shelf-attached’ MTCs are considerably larger (more than 200–300 km²), are sourced from the shelf, and post-date the main phase of active folding and thrusting. 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 contractional deformation, and seascape evolution in time and space.