Mass‐transport complexes as markers of deep‐water fold‐and‐thrust belt evolution: insights from the southern M agdalena fan, offshore C olombia

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 MTC s. More than nine distinct, but coalesced MTC s 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 MTC s changed through time: the oldest ‘detached’ MTC s are relatively small (over 9–100 km 2 in area) and sourced from the flanks of growing anticlines, whereas the younger ‘shelf‐attached’ MTC s are considerably larger (more than 200–300 km 2 ), are sourced from the shelf, and post‐date the main phase of active folding and thrusting. Changes in the source, distribution and size of MTC s are tied to the sequential nucleation, amplification and along‐strike propagation of individual structures, showing that MTC s can be used to constrain the timing and style of contractional deformation, and seascape evolution in time and space.