f Defining the geometry of the subducting Caribbean slab beneath northwestern Colombia and its controls on the overriding Lower Magdalena forearc basin

We have integrated 800 earthquake events, 7000 km of 2D seismic lines, 13 exploration wells, and regional gravity and magnetic data in order to test the idea that the Lower Magdalena basin (LMb) formed as a forearc basin above the subducting Caribbean slab, in NW Colombia. Earthquake events combined with deep-penetration seismic reflection data show that subduction is occurring at a shallow angle (~12-19°), attributed to the subduction of late Cretaceous Caribbean oceanic plateau crust previously determined to be 10-15 km thick. Earthquake focal mechanisms show normal events occurring as a result of bending of the Caribbean oceanic plateau along with thrust and strike-slip events. Gravity and magnetic models combined with seismic reflection profiles show the presence of a lower, more dense sedimentary unit, inferred to be an older and highly deformed Cretaceous to Eocene accretionary prism underlying the Oligocene to Recent forearc basin units of LMb and overlying the subducting Caribbean slab. The forearc basin fill varies in age from Oligocene to Holocene and can be seen onlapping the upper sedimentary basement unit. These onlaps are consistent with the upper basement unit forming a ridge since Oligocene. The directions of depocenter migration shift in both landward and seaward directions from Early Miocene to Holocene time likely reflecting infill, faulting and uplift events forming the forearc ridges and perhaps dip changes in the subducting Caribbean slab. The forearc basin was overfilled in Pleistocene time and resulted in formation of the 68000 km2 Magdalena fan offshore, in the Colombian basin.