QUANTITATIVE-ANALYSIS OF MIOCENE TO RECENT FORE-ARC 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 Panam a 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 Charle s 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-di pping continental basement. Sedimentary sequences within the forearc b asins indicate successive landward migration of the basin depocentre a s the structural high bounding its oceanward edge is forced upward and landward by continued growth of the accretionary complex. Uplift bene ath the oceanward side of the basins has resulted in progressive landw ard rotation of the older sedimentary sequences. Prominent seismic ref lectors across the basins show a complex onlap-offlap relationship bet ween successive sequences that reflects the interplay between tectonic uplift, sediment supply, differential sediment compaction and basemen t subsidence due to loading. A numerical model has been devised to inv estigate how Miocene to Recent forearc basin stratigraphy is controlle d by progressive growth of the accretionary complex resulting in eleva tion of the outer-are high and landward motion of the rear of the comp lex up the seaward-dipping backstop formed by the leading edge of the continental lithosphere. The effects of sediment accretion are modelle d by treating the accretionary complex as a doubly vergent, noncohesiv e Coulomb wedge, where forces exerted by the proto-and retro-wedges mu st be balanced for the system to be in equilibrium. The model generate s synthetic basin-fill architecture over a series of steps, each of wh ich represents the deposition of individual sedimentary sequences and their subsequent deformation due to wedge growth. The model accounts f or differential sediment compaction and the flexural response of the u nderlying lithosphere to changes in load distribution over time. Forea rc basin evolution is simulated for various rates of sediment supply t o the forearc and accretionary complex growth until the synthetic basi n-fill geometry matches the observed geometry. The model enables eithe r the rate of accretionary wedge growth or the rate of sediment supply to the forearc basin to be established. The technique is generally ap plicable to those convergent margins with forearc basins that have dev eloped between an actively accreting wedge and a seaward-dipping backs top. Other examples include Peru, S. Chile, Sumatra and Barbados.