The MARCONI reflection seismic data: A view into the eastern part of the Bay of Biscay

Seismic data acquisition for the MARCONI project took place in 2003 in the eastern half of the Bay of Biscay. The overall goal of the multichannel experiment was to collect seismic data to elucidate the structure and evolution of the southern margin of the Bay where an along-the-margin transition between oceanic subduction in the west and continental convergence in the east occurs, and its relationship to the Pyrenean realm. We describe the multichannel experiment and present an interpretation of the abyssal plain sector of the profiles. Altogether, eleven regional seismic reflection profiles were collected with a total length of 1800 km, between the longitudes 6° and 2°W and latitudes 43.5° and 46°N. The seismic profiles show first images of the thick sedimentary pile that covers the eastern abyssal plain of the Bay. The interpretation includes three sedimentary sequences separated by unconformities that correspond to the pre-, syn- and post-Alpine sequences. The post-orogenic deposits lay unconformably over the previous compressional structures in the eastern part of the Bay, floored by transitional crust. These compressional features are evident in the presence of a young accretionary prism sealed at its first stages of development. The overall structure and shape is similar to those of active subduction prisms worldwide, but evidencing differences in width and sediment wedge thickness from west to east along the margin. Buoyancy of the transitional crust and resistance to subduction led to stronger plate coupling towards the east, where oceanic crust is absent, and may be the main cause for these morphological changes along the east–west traverse of the wedge. The MARCONI multichannel deep seismic dataset provides the first estimation of the thickness of the sediment load on the 5000 m deep abyssal plain of Bay of Biscay, where no direct data were available.

Research highlights
► New seismics in Eastern Biscay show Mesozoic syn-rift basins in the abyssal plain.
► The character of continental crust in this area is confirmed based on the sections.
► There is pre, syn and post alpine deformation sedimentary sequences.
► The main compressional structure belongs to a fossil accretionary wedge.
► The interpreted data provides new clues on the Cantabrian-Pyrenean orogeny.