HR: 1340h
AN: T53B-1424    [Abstracts]
TI: Physical Properties of the Subduction Channel off the Gulf of Guayaquil (Ecuador) From Seismic Reflection Data.
AU: Calahorrano, A
EM: calahorr@geoazur.obs-vlfr.fr
AF: IRD-G‚osciences Azur, BP 48, Villefranche sur Mer, 06235 France
AU: Calahorrano, A
EM: calahorr@geoazur.obs-vlfr.fr
AF: IG-EPN, Ladron de Guevara s/n, Quito, 1 Ecuador
AU: Sallares, V
T53B-1424 AF: CSIC, Pg. Maritim de la Barceloneta, 37-49, Barcelona, 08003 Spain
AU: * Collot, J
T53B-1424 AF: IRD-G‚osciences Azur, BP 48, Villefranche sur Mer, 06235 France
AU: * Collot, J
T53B-1424 AF: IG-EPN, Ladron de Guevara s/n, Quito, 1 Ecuador
AU: Sage, F
T53B-1424 AF: IRD-G‚osciences Azur, BP 48, Villefranche sur Mer, 06235 France
AU: Ranero, C
T53B-1424 AF: CSIC, Pg. Maritim de la Barceloneta, 37-49, Barcelona, 08003 Spain
AB: We use two high quality MCS lines (SIS-72 and SIS-18) acquired during the SISTEUR survey to 1) image the subduction channel (SC) across the southern Ecuadorian convergent margin off the Gulf of Guayaquil, and 2) estimate its physical properties and related mechanical behaviour. P-wave velocity analysis performed during pre-stack depth migration provided accurate velocity models over the first ~32 km of subduction down to a 8-km-depth. The SIS-72 model revealed that the velocity within the SC (2800 m/s ñ 150 m/s) is significantly lower than that of the overriding plate basement (~3800 m/s). The low velocity uncertainties allowed calculating porosity and fluid pressure in the SC. The ~constant low velocity in the SC associated with the landward increase in overburden pressure, reflects fluid overpressures as high as ~40 MPa at 25 km from the trench, down to a 7-km-depth. The overpressure parameter (λ*), defined as the ratio of fluid overpressure over overburden pressure allowed to identify three zones with different mechanical behaviours in the SC: a) at the margin's toe (up to 9 km from the trench), λ* lower than 0.5 indicates an effective fluid drainage related to high permeability in the incipient accretionary prism; b) between ~9-25 km from the trench, λ* increases to ~0.8 suggesting highly undrained sediments due to a low permeability within the underthrust sediments, the d‚collement or the overriding-plate basement. Such high fluid overpressure may induce hydrofracturation, favouring basal erosion and subsequent thickening of the SC. It could also indicate low inter-plate friction and shear stress, preventing earthquake nucleation but favouring rupture propagation; c) Beyond 25 km from the trench, λ* drops down to ~0.6 indicating that fluids are expelled, probably across ancient margin faults. In this zone, a lower fluid pressure is likely to increase the inter-plate coupling. Consistently, this zone roughly coincides with the area in which the first diagenetic and low-grade metamorphic processes associated to the updip limit of the seismogenic zone occur. An analogous analysis was carried out on line SIS-18 situated 60 km northwards of SIS-72, showing a similar evolution of velocity along the SC. However, at similar distances from the trench, the velocity in the SC is higher (~2.8-3.6 km/s) along SIS-18 than along line SIS-72, and comparable to that of the margin's base. Calculated fluid overpressure <20 MPa and λ* values lower than 0.5, indicate relatively poor fluid content along the SIS-18 SC. Interestingly, the different SC velocities calculated at both lines, correlate with different trench fill seismic facies. On line SIS-72 well-stratified deposits suggest that pelagic and turbiditic sediments underthrust the margin, whereas on line SIS-18 chaotic facies are interpreted as coarse avalanche deposit feeding the SC. The difference in physical properties and sedimentary nature that stems from this comparison supports inter-plate mechanical variations along the margin strike.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3025 Marine seismics (0935, 7294)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005