HR: 11:20h
AN: T32A-05    [Abstracts]
TI: Exploring grain deformation processes of underthurst sediments beneath the Ecuadorian margin based on multichannel seismic data
AU: * Sallares, V
EM: vsallares@cmima.csic.es
AF: Marine Technology Unit, CMIMA-CSIC, Passeig Maritim Barceloneta, 37-49, Barcelona, 08003, Spain
AU: Calahorrano, A
EM: alcinoe@cmima.csic.es
AF: Institute of Marine Sciences, CMIMA-CSIC, Passeig Maritim Barceloneta, 37-49, Barcelona, 08003, Spain
AU: Collot, J
EM: collot@geoazur.obs-vlfr.fr
AF: UMR - Geosciences Azur (IRD, UPMC, UNSA, CNRS), BP 48, Villfranche-sur-Mer, 06235, France
AU: Sage, F
EM: sage+33-493763763
AF: UMR - Geosciences Azur (IRD, UPMC, UNSA, CNRS), BP 48, Villfranche-sur-Mer, 06235, France
AB: In this work we quantify the physical properties variations of underthrust sediments along the first tens of km of subduction of the non-accretionary sourthern Ecuadorian margin using high-quality multichannel seismic data. Two pre-stack depth-migrated profiles (SIS-72 and SIS-18) have revealed the presence of three zones along the subduction channel (SC) characterized by contrasting velocity and velocity-derived physical properties. These variations agree with expected based on the different stages of deformation of granular media subjected to progressively increasing confining pressures observed in laboratory experiments. The three zones likely correspond to transformational steps of underthrust sediments governed by different physical processes that control its mechanical behaviour at increasing confining pressures. The seismic images show that the sediments feeding the SC are series of stratified layers related to turbiditic sequences that fill the trench in SIS-72, and chaotic seismic facies suggesting coarser mass wasting deposits in SIS-18. Within Zone I (0-9 km from the trench and 5-6 km depth in line SIS-72, 0-5 km and 4.5-5 km depth in line SIS-18), progressively increasing velocity and decreasing velocity-derived porosity indicate continuous compaction of unconsolidated (fluid-supported) incoming material, accompanied by effective fluid drainage along the décollement and/or across the highly porous accretionary wedge. Comparison with experimental results in granular media indicate that the dominant processes at this range of pressures (<30 MPa) are grain rolling, particle rotation and frictional slip at grain contacts, with no o little deformation. Within Zone II (9-25 km and 6-7 km depth in SIS-72, 5-12 km and 5-5.5 km depth in SIS-18), the underthrust material is under the critical porosity, becoming frame-supported. Velocity and porosity remain constant regardless the increasing confining pressure, suggesting undrained conditions due to a low permeability within either the underthurst sediments or the décollement, and resulting in overpressured fluids within the SC. Laboratory experiments show that, at pressures of 30-100 MPa, the dominant process is Hertzian-like, elastic deformation of individual grains with increasing grain damage. Within Zone III (25-30 km and 7-7.3 km depth in SIS-72, 12-16 km in and 5.5-6.5 km depth SIS-18) velocity increases and porosity falls rapidly, probably indicating sudden sediment compaction and subsequent release of over-pressured fluids. Experimental results agree with this behaviour showing pervasive grain cracking (cataclasis), acceleration of compactive strain rates and, eventually, a sudden collapse of the system. The highest pressures at which these transformations occur in SIS-72 with respect to SIS-18 could be due to the larger grain diameter and broader size distribution of underthurst sediments in SIS-18.
DE: 3025 Marine seismics (0935, 7294)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 5114 Permeability and porosity
SC: Tectonophysics [T]
MN: 2007 Joint Assembly