IMPACT OF THE SUBDUCTION OF THE CARNEGIE RIDGE ON THE ECUADORIAN SHELF AND SLOPE BASINS DURING THE LAST 1MA
Résumé
The subduction of the Nazca Plate below the South American Plate shows the subduction of
aseismic ridges of different origins and morphologies. One of them is the Carnegie Ridge (400km
wide, 2km high) produced by the relationship between the Galapagos Hotspot and the Galapagos
Spreading Center. The ages and the effects of the Carnegie Ridge subduction on the Ecuadorian
margin are still a matter of debate. Newly acquired offshore data complementing the onshore
record help to highlight part of this history.
The goal of this study is to describe the last 1Ma sedimentary record of Ecuadorian continental
shelf to improve our understanding of the impact of the Carnegie ridge subduction on sea floor and
shoreline morphologies. For this aim, we acquired high-resolution (50-450Hz frequency) multichannel
(72 channels) and single channel (Sparker) seismic data, piston cores, sediment profiles
(3.5khz) and high-resolution multibeam bathymetry, during the ATACAMES cruise of the RV
L’Atalante, in 2012.
In the offshore, we identified two types of Quaternary basins: the platform basins located
at northern limit of the Carnegie Ridge, and the slope basins located along the margin in front of
Carnegie Ridge subduction. All of them exhibit three major angular unconformities, which
encompass some minor unconformities that limit transgressive-regressive sequences (T-R se- quences),
which correlate to 100 to 40kyrs Milankovitch cycles. The E-W (dip) seismic profiles, perpendicular
to the coastline, show that these angular unconformities mark the progressive mi- gration of successive
depocenters in a landward direction. This landward stepping started by 1.4 Ma in agreement with the
age postulated by some author for the beginning of the Carnegie ridge subduction. The highly
subsiding platform basins bordering the northern side of the Carnegie Ridge are bounded by a set of
regional faults, which includes the Jama fault zone. The subsidence of these basins might have
accompanied the relative movement between the EW convergence of the Nazca Plate and the NE
dextral escape tectonic along the Jama Faults of the North Andean Block