Quantitative analysis of Miocene to Recent forearc basin evolution along the Colombian convergent margin
The Colombian accretionary complex forms the active convergent margin of the North Andes block of South America beneath which the east Panama Basin of the Nazca plate is subducted at a rate of 50–64 km Myr−1. Multichannel seismic reflection data, collected as part of RRS Charles Darwin cruise CD40, image a series of well-developed forearc basins along the length of the margin, bounded on their oceanward side by an active accretionary complex and on their landward side by oceanward-dipping continental basement. Sedimentary sequences within the forearc basins indicate successive landward migration of the basin depocentre as the structural high bounding its oceanward edge is forced upward and landward by continued growth of the accretionary complex. Uplift beneath the oceanward side of the basins has resulted in progressive landward rotation of the older sedimentary sequences. Prominent seismic reflectors across the basins show a complex onlap–offlap relationship between successive sequences that reflects the interplay between tectonic uplift, sediment supply, differential sediment compaction and basement subsidence due to loading. A numerical model has been devised to investigate how Miocene to Recent forearc basin stratigraphy is controlled by progressive growth of the accretionary complex resulting in elevation of the outer-arc high and landward motion of the rear of the complex up the seaward-dipping backstop formed by the leading edge of the continental lithosphere. The effects of sediment accretion are modelled by treating the accretionary complex as a doubly vergent, noncohesive Coulomb wedge, where forces exerted by the proto- and retro-wedges must be balanced for the system to be in equilibrium. The model generates synthetic basin-fill architecture over a series of steps, each of which represents the deposition of individual sedimentary sequences and their subsequent deformation due to wedge growth. The model accounts for differential sediment compaction and the flexural response of the underlying lithosphere to changes in load distribution over time. Forearc basin evolution is simulated for various rates of sediment supply to the forearc and accretionary complex growth until the synthetic basin-fill geometry matches the observed geometry. The model enables either the rate of accretionary wedge growth or the rate of sediment supply to the forearc basin to be established. The technique is generally applicable to those convergent margins with forearc basins that have developed between an actively accreting wedge and a seaward-dipping backstop. Other examples include Peru, S. Chile, Sumatra and Barbados.