Soutenance de thèse de Miguel GONZALEZ

Nature et origine des dépôts sédimentaires de la fosse de subduction d’Equateur : implications paléo-sismologiques

Abstract

Recent deep marine sedimentation in subduction trenches is characterized by the inter-stratification of hemipelagic and turbidite sediments locally interbedded with debris flow, which can result from continental slope shaking triggered by large earthquakes. The active margin of Ecuador is comprised by strong tectonic erosion that contributes to the formation of a deep trench filled by a complex suite of sedimentary facies. Gravity flow sedimentation is ubiquitous along the margin and facies range from laterally continuous m-thick mass transport deposits to isolated cm-thick turbidites intercalated with hemipelagite, volcanoclastics and tephra layers. However, the nature and architecture of those deposits remain not clear.
In this study interpretation of detailed swath bathymetry, high-resolution seismic profiles and petrophysical data from sedimentary cores are shown. Most of the information was collected during the scientific campaign “ATACAMES”, onboard the R/V L’Atalante (2012). 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, located northward of the Carnegie Ridge, is characterized by a wide (5-10 km) and deep trench (3800 – 4000 m), a gentler gullied continental slope and a shelf (10-40 km wide) with active subsidence. The central segment facing the Carnegie Ridge, is 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 steep 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, located southward of the Carnegie Ridge, presents a wide (5–10 km) and deep (4000–4700 m) trench, a starved continental slope with well-defined canyon systems and a wide subsiding shelf (20–50 km)
The sedimentary dynamics along the margin is evaluated by the analysis of 15 sediment cores from which visual description, high-resolution colored photographs, X-Ray imagery, XRF data and measurement of petrophysical properties (gamma density, magnetic susceptibility, resistivity) led to the identification of 11 sedimentary facies that characterize seven sedimentary processes: turbidite beds (turbidity currents), hemipelagites (continuous marine sedimentation), tephras (airfall ash layers consecutive to volcanic eruptions), debris flow deposits (cohesive debris flows), homogenite (large-scale and/or hybrid gravity flows), slump deposits (mass wasting), and ooze carbonate deposits (bio-clastic marine sedimentation). The chronostratigraphy of the deposits is defined by radiocarbon age dating of well-identified hemipelagite sediments. Ages range from 500 to 48,000 years BP over the topmost 10m of the trench deposits.
High-resolution seismic profiles allow definition of three echo-facies: transparent, layered and chaotic. The transparent echo-facies is mainly associated to homogenite deposits, the layered echo-facies is associated to the turbiditic-hemipelagic interbedded deposits and the chaotic echofacies is associated to reworked gravity flow deposits.
The trench fill represents a lacunar but important record of the history of the subduction margin. Large eastward stepping debris flows in the lower two sequences of the trench fill are provided by the outer wall of the trench as a results of slope failures along large normal faults due to the downward bending of the subducting oceanic plate. The sediment of the upper sequence of the trench fill draping the trench floor, are more largely provided by the inner trench wall with a strong control of the Carnegie Ridge. As a result, the depth, frequency, thickness, composition and lateral disposition of the sedimentary deposits vary greatly from those in the north and south. The large, simple mega-beds like slump, debris flows and homogenites are located in the northern and in the southern segments. They are triggered by large regional fault scarps, in the North and enhanced by the activity of sets of splay faults in the South overhanging the seafloor, at the toe of the slope. Small-size, fluid rich (slump) events are triggered by the subduction of isolated seamounts at the edges of the Carnegie Ridge due to frequent but small destabilizations of an inner trench wall “preconditioned” by the repeated impacts of the successive seamounts. Sets of thick partly volcanoclastic turbidites in the central segment might have been triggered by the complex interaction of slope and continental platform deformation by seamount subduction and large plinian eruption. These results show the complexity of the sedimentation in the trench of subduction margin that can only be clarified by a precise age dating of the sedimentary events.