Earth and Planetary Science Letters, 2008,
267 (3-4), p. 453-467 ISSN 0012-821X
We use two high-quality pre-stack depth-migrated multichannel seismic profiles acquired to quantify physical properties variations of underthrust sediments along the first similar to 30 km of subduction off the erosional southern Ecuadorian margin. Seismic data show three zones along the subduction channel (referred to as Zones I, II and III) characterized by distinct velocity and velocity-derived physical properties, which are in agreement with values estimated from experimental results of deformation in granular media. These three zones result from transformational changes of underthrust sediments governed by fundamentally different physical processes that control their mechanical behavior at increasing confining pressures. Based on our observations and its comparison with experimental results, we argue that the transformations undergone by underthrust sediments as they dip into the subduction zone are the following: within Zone I, progressively increasing velocity (and decreasing velocity-derived porosity) indicates continuous sediment compaction, which must be accompanied by effective fluid drainage along the decollement and/or across the accretionary wedge. The underthrust material is here unconsolidated from a mechanical point of view. Laboratory experiments indicate that the dominant processes at this range of pressures are grain rolling, particle rotation and frictional slip at grain contacts. Within Zone II, velocity (and porosity) remains constant for similar to 16 km (SIS-72) and similar to 12 km (SIS-18). This suggests undrained conditions resulting in growing fluid overpressure at the subduction channel. Grain deformation is similar to Zone I. Within Zone III, velocity increases and porosity falls rapidly, indicating sediment compaction and subsequent release of over-pressured fluids, where grain deformation is likely to be elastic. This might be the dominant process until the grains attain their crushing strength, resulting in granular cataclasis and, eventually, in the collapse of the system. We suggest that over-pressured fluid release may induce hydrofracturation and it is likely to increase inter-plate coupling down from Zone III.
