The South Ecuador subduction channel: Evidence for a dynamic mega-shear zone from 2D fine-scale seismic reflection imaging and implications for material transfer

Abstract

[1] Tectonic processes that control the transition from poorly consolidated sediment entering the subduction channel (SC) to the seismogenic zone are documented using seismic imaging. We applied pre-stack depth migration and a post-processing sequence to a seismic reflection line acquired across the Ecuador convergent margin to obtain a 2D-quantitative image of the first ∼24 km of the SC. Structural interpretation shows that the SC consists of a 630-1150-m-thick, low-velocity, continuous sheet of sediment that dips ∼6°landward and undergoes shear deformation. The long sheet is bounded at top and bottom by décollement thrusts, and developed over time Riedel shears and basal thrust faulting and folding downdip, pointing to a dynamic mega-shear zone. Modeling the strong uppermost and basal SC reflectors reveals that they are associated with 40-80-mthick, 50-350 m/s, low-velocity perturbations layers inferred to be fluid rich and mechanically weak. A fine-scale velocity model shows two anomalously low-Vp areas in the long sheet, advocating patches of over-pressured fluids. Evidence for Vp variations along the upper-plate foundation suggests either underplated bodies or a fluid-damage zone. A simple temporal reconstruction indicates that underthrusting the long sheet initiated >450 kyr ago and interrupted ∼54 ± 13 kyr ago, when frontal accretion resumed. During this transient evolution, the SC boundaries revealed highly unstable as most of the SC was underplated while down going plate material may have been sheared off and incorporated to the SC.

A revised budget for Cenozoic sedimentary carbon subduction

“…That some material reaching the trench does get subducted to some considerable depth is in little doubt. In addition to the impact on arc magmatic chemistry [ Morris et al ., ; Plank , ; Plank and Langmuir , ], seismic images reveal the existence of a clear “channel” between the plates spanning ~2–4 km thickness and which would accommodate the loss of sediment from the trench and fore arc [ Collot et al ., ], in both accretionary and erosive margins [ Vannucchi et al ., ].…”

Dynamics of interplate domain in subduction zones: influence of rheological parameters and subducting plate age

“…In nature this interface is probably made of pounded material mixing subducted sediments and slices of over-riding crust torn during underthrusting (e.g. Lallemand et al, 1992;Lallemand, 1995;Collot et al, 2011), therefore much weaker than the subducting oceanic crust. Hence, its rheological properties are assumed to be close to those of a continental crust to mimic the behaviour of a real subduction channel.…”

Structural anatomy of the Ligurian accretionary wedge (Monferrato, NW Italy), and evolution of superposed melanges

“…Highly sheared, disrupted and 68 fragmented rock units and tectonic mélanges are the products of tectonics occurring along the 69 basal décollements in accretionary wedges and out-of-sequence thrust-faults, and within the 70 subduction channels (e.g., Karig and Sharman, 1975;Cloos, 1982; Moore and Byrne, 1987; 71 Taira et al, 1992; Dileonardo et al, 2002;Collot et al, 2011). Mass-transport deposits and 72 sedimentary mélanges result from slope instability in the trench-inner slope and in the upper 73 parts of frontal wedges (e.g., Lallemand et al, 1990; Duperret et al, 1995; Goldfinger et al, 74 2000;von Huene et al, 2000;Collot et al, 2001; McAdoo et al, 2004;Sage et al, 2006; 75 Mosher et al, 2008; Ogawa et al, 2011; Strasser et al, 2009Strasser et al, , 2011.…”