HR: 14:40h
AN: T33E-05 INVITED    [Abstracts]
TI: Subduction Erosion Processes Along the Northwestern Margin of South America
AU: * Collot, J
EM: collot@geoazur.obs-vlfr.fr
AF: IRD/UPMC UMR Geosciences Azur, B.P. 48, Villefranche s/mer, 06235, France
AU: Sage, F
EM: sage@geoazur.obs-vlfr.fr
AF: IRD/UPMC UMR Geosciences Azur, B.P. 48, Villefranche s/mer, 06235, France
AU: Calahorrano, A
EM: alcinoe@cmima.csic.es
AF: Institut de Cià¨ncies del Mar, CMIMA,CSIC, Pg. Marà­tim de la Barceloneta 37-49, Barcelona, 08003, Spain
AU: Agudelo, W
EM: william.agudelo@ecopetrol.com.co
AF: ICP, El Limonal km14 Autopista Piedecuesta, Bucarramanga, 1, Colombia
AU: Ribodetti, A
EM: ribodeti@geoazur.obs-vlfr.fr
AF: IRD/UPMC UMR Geosciences Azur, B.P. 48, Villefranche s/mer, 06235, France
AB: Subduction erosion is one of the dominant processes that shape convergent margins. Mechanisms favoring subduction erosion occur at both highly- and weakly-coupled margins. Multibeam bathymetry and MCS data collected along the Ecuador-SW Colombia trench show an erosional margin fronted by a narrow wedge of imbricated slope sediment. Ubiquitous arcuate slump scarps on the relatively steep inner trench slope denote frequent slope instabilities along a margin that consists of a trenchward-tilted oceanic basement and fore-arc basin, overlain by slope sediments. PSDM seismic reflection sections across three segments of the Ecuador-SW Colombia margin show that physical conditions enabling basal erosion at the plate interface vary along the margin. In southern Ecuador, seaward of the Gulf of Guayaquil, margin extensional deformation suggests a low- friction plate interface. There, the shallow segment of the subduction channel (SC) is roofed by a strong reflector, and dominated by high excess pore pressure that peaks to 40 MPa. Such pore pressure implies that a permeability barrier prevent fluids from migrating upward. Therefore, breaking the permeability barrier, likely during a megathrust slip, would release over-pressured fluids and allow basal erosion by hydrofracturation In central Ecuador, where the Carnegie Ridge enters subduction, margin extensional deformation indicates a low- friction plate interface. The deeper section of the margin basement gradually thins seaward and disappears ~13 km from the trench indicating basal erosion. PSDM sections image a 3D patchiness across the plate interface implying rapid variations in mechanical coupling and erosion processes. Basement weakening, which results from over pressured fluids in the SC, is marked by enhanced reflectivity at the base of the upper plate. Moreover, at the basement apex, basement breakup is caused by superposition of compressional and extensional fault systems. In northern Ecuador, the seaward section of the 2 km-thick fore-arc basin is sharply tilted trenchward at the inner-trench slope break, thus reflecting subduction erosion of the outer margin wedge. Compressive deformation suggests relatively high-friction plate interface. A strong reflector does not roof the thick, presumably water-rich subduction channel, suggesting that over-pressured fluids are not confined in the SC, but pervasively invade the overlaying outer wedge. Moreover, a crustal splay fault associated with low rock velocities is interpreted as a major conduit for fluid flow. The overall low velocity of outer wedge rocks suggest that they are altered by fluids, and therefore considered weak and easy to break up. Furthermore, diffuse shearing in relation with slip hardening is thought to occur along the upper segment of the megathrust. Therefore, faulting, rock alteration, hydrofracturation and diffuse shearing along the interplate fault would favor basal erosion of the outer wedge.
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting