Nature and distribution of recent sediments in the trench of the subduction margin of Ecuador
Résumé
Recent deep marine sedimentation in subduction trenches is
characterized by the inter-stratification of hemipelagic and turbidite
sediments locally interbedded with debris flows, which can
result from continental slope shaking by large earthquakes. The
objective of this study is to describe the morphologic complexity
on the Ecuadorian border of the Nazca tectonic plate where a set
of deep marine asperities is subducting at different scales, and its
consequences on the lateral distribution of the sediments in the
different sub-basins.
The Ecuadorian active margin comprises three geomorphological
segments: The northern segment characterized by a wide (5-10
km) and deep trench (3800 – 4000 m), a rough gullied continental
slope and a shelf (10-40 km wide) with active subsidence.
The central segment strongly influenced by the subduction of the
Carnegie ridge which induced a narrow (0–5 km wide) and relatively
shallow trench (3100 – 3700 m depth), a smooth and
gullied slope with no canyons and a 15–40 km wide shelf characterized
by areas with active subsidence and uplift. Finally the
southern segment presents a wide (5–10 km) and deep (4000–
4700 m) trench, an irregular and starved continental slope with
canyon systems and a wide subsiding shelf (20–50 km).
Bathymetry, high-resolution seismic profiles and sedimentary cores
collected during the scientific campaign ”ATACAMES” along the
active margin of Ecuador show that since the last glacial maximum,
the trench is filled by turbidites, hemipelagites, volcaniclastic
deposits, homogenites as well as slumps, debris flow and
other mass transport deposits. Hemipelagites, turbidites and homogenites
are found in all segments. Hemipelagite beds range in
thickness 5cm to 1m reaching over 3m in the north. Turbidites
are coarser and sandier showing a higher frequency in the central
segment. Mass transport deposits are mainly found in the
northern and southern segments when volcaniclastic deposits are
mainly present in the central segment.
The distribution of sedimentary facies along the trench could be
related to the subduction of the Carnegie Ridge. The ridge exerts a
strong influence and control over the individualization of the three
segments as well as on the nature and the lateral distribution of the
sediments in the trench. Due to shallower deposition conditions,
the central segment facing the Carnegie Ridge is full of sandy and
silty turbidites in a proportion of 2:1 with respect to the other segments
and its location on the main ash pathways might explain
the concentration of the volcaniclastic deposits. The southern
and northern segments, which are fed by large canyons and affected
by isolated seamounts, show more slump, debris flows and
mass-transport deposits.