7t h Int ernat ional Symposium on Andean Geodynamics (ISAG 2008, Nice), Ext ended Abst ract s: 156-159 156 Coeval subduction erosion and underplating associated with a crustal splay fault at the Ecuador-Colombia convergent margin Jean-Yves Collot 1 , Alessandra Ribodetti 1 , Boris Marcaillou 2 , & William Agudelo 3 1 Géosciences Azur, Université de Nice Sophia-Antipolis, IRD, CNRS, Université Pierre et Marie Curie, Observatoire de la Côte d’Azur, BP 48, 06235 Villefranche-sur-mer, France (collot@geoazur.obs-vlfr.fr) 2 IFREE/JAMSTEC, 2.15 Natsushima-cho, Yokosuka, Kanagawa, 237-0061, Japan 3 ECOPETROL ICP.Km 7 Via Piedecuesta, Piedecuesta, Santander, Colombia KEYWORDS : subduction zone, seismic reflection, subduction channel, erosion, underplating Introduction Subduction erosion and underplating are major processes governing the structural evolution of convergent margins. Subduction erosion is required at many margins by large-scale, long-term margin subsidence and is likely to be driven by over pressured fluids that disaggregate the underside of the margin basement (von Huene et al., 2004). In contrast, underplating has been invoked to thicken accretionary margins by duplex formation at the base of the accretionary complex (Park et al., 2002; Bangs et al., 2004). Based on seismic reflection, refraction and swath bathymetric data, we show that both subduction erosion and underplating occur simultaneously at an erosive segment of the North-Ecuador-South Colombia margin. The margin consists of an accreted oceanic terrane overlain by thick fore-arc basin deposits (Jaillard et al., 1995), and underthrust eastward at 5.4 cm/yr by. the Neogene Nazca plate (Trenkamp et al., 2002)(Fig. 1). Data In 2000, the SISTEUR cruise onboard the French R.V. Nadir acquired deep marine multichannel seismic reflection (MCS) and wide angle seismic data using Ocean Bottom Seismometer across the Ecuador and south Colombia margin to investigate its yet poorly-known deep structures (Collot et al., 2002). In 2001, the Salieri cruise onboard the German R.V. Sonne acquired complementary wide-angle seismic data and multibeam bathymetric data to explore crustal and seafloor structures in the same region (Flueh et al., 2001). In 2005, the AMADEUS cruise onboard the French R.V. L’Atalante collected 55000 km 2 of contiguous swath bathymetry coverage and underway geophysics, sedimentary cores, dredged rocks, and heat flow data at 12 core- and heat probe-sites between 0° and 3°30N (Collot et al., 2005). Results and Interpretation These data have allowed discovering the seafloor trace of the trench-parallel, ~ 90 km-long, Ancon fault system, which extends across and north of the Esmeraldas canyon along the north Ecuador-south Colombia margin (Fig. 2). The fault system separates a shallow outer basement high from the Manglares fore-arc basin, and is segmented along strike. The N57°-trending, southern fault segment deforms the seafloor by extensional faulting associated with an anticline. The N25°-trending northern fault segment is characterized by a high-angle crustal reverse fault that fans out northward into a horsetail pattern. Reprocessing multichannel seismic reflection (MCS) line SIS-44 (Fig. 2) through Prestack Depth Migration, using vertical reflection and wide-angle data to construct a refined velocity model (Agudelo, 2005), shows the Retour au Sommaire Sommaire / Contents / Contenido