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MARGATS, a mission to reveale the secrets of Demerara plateau ?

Off the coasts of Guyana and Suriname, the south american continental shelf looks like a large peninsula : the Demerara Plateau, 300 km wide and immersed by nearly 2,000 meters. The deep internal structure of this enigmatic shelf, born when the Gondwana supercontinent was formed nearly 100 million years ago, is the subject of an oceanographic exploration campaign : the MARGATS mission *.

With David Graindorge (IUEM, University of Bretagne Occidentale) at its head, this mission took place from October 20 to November 16, 2016 aboard the Ifremer ship “L’Atalante”. It used seismic refraction and heavy multitrace to see the deep structure of the plateau. MARGATS is the last part of a program created to understand the specificities, origin and formation of the Demerara Plateau.

MARGATS followed IGUANES, a first exploratory scientific campaign (28 April-22 May 2013, Oceanographic Ship “L’Atalante”, Lies Loncke), which studied the superficial structure of the shelf by fine mapping of underwater reliefs and by high-resolution seismic imaging. This campaign revealed the existence of massive submarine slides which progressively destabilize the important relieves of the continental slope. Landslide scars can be traced along the 300 km of the shelf edge. The slides supply a vast area of more than 10 000 km2 which is among the largest sliding complex of the world.

A recent dredging campaign on the northeastern edge of the Demerara Plateau (DRADEM : 8-20 July 2016, Oceanographic Ship Pourquoi pas ? C. Basile) allowed to sample the oldest outcrops on the shelf, revealing an unknown until now magmatic basement of mafic acid rocks. Series of beach sandstones, sampled at a depth of nearly 4000 meters, were also discovered, raising the question of the history of the vertical movements of the Guyanese margin.

Passive continental margins

The work on Demerara Plateau provided to acquire a better understanding of dynamic and evolution of the passive continental margins, in particular transforming. The aim is to characterize the earth’s crust of the marginal shelfes, deep underwater reliefs associated with 30% of the transforming margins in the world and whose Demerara Plateau is one of their. Located at the junction of oceans of different ages, they constitute real geodynamic nodes. Their mode of structuration and their nature are poorly understood and they constitute extremely large and close to the continental submarine domains.

An intermediate depth domain between the continental shelf and the abyssal domain

Demerrara Plateau builds up when the Atlantic Ocean opening cuts Gondwana supercontinent to separate South America from Africa. It is located at the junction between the Central Atlantic, opened during the Jurassic and the Equatorial Atlantic, opened during the Cretaceous. It represents an intermediate depth domain between the continental shelf and the abyssal domain. This plateau has an intermediate thickness (about 25 km) between oceanic and continental crust but its formation is not yet solved.

The first results of MARGATS campaign were presented at the EGU 2017 General Assembly. The observations of the deep structure shows that it could be a thick accumulation of lava formed near a hot spot, a sort of volcanic shelf with a structure similar to Plateau des Kerguelen, but probably linked to the Central Atlantic Magmatic Province (CAMP), which is the beginning of the Atlantic opening and whose numerous indices are recognized throughout the Atlantic.

* MARGATS campaign results of collaboration between :
- Université de Bretagne Occidentale (laboratoire Domaines Océaniques de l’IUEM)
- Ifremer (Géosciences Marines)
- Université de Perpignan (laboratoire CEFREM)
- Université de Grenoble Alpes (laboratoire ISTerre)
- Université du Suriname (Anton de Kom Universiteit van Suriname)
- Géosciences Montpellier (teams of Université des Antilles and Université de Guyane)