Assessing the conditions of continental breakup in magma-poor systems from the perspective of slow-spreading ridge systems. – Rift2Ridge

This project concerns the Ocean Continent Transition (OCT) at magma-poor rifted continental margins. OCTs represent the genetic and geographic link between the margin and the first oceanic crust. Because most present-day OCTs are thermally equilibrated, not tectonically active, and covered by thick sediments, the methods and concepts used to study OCT have so far been primarily those used in rifted margins rather than those developed by the ridge community. Since a successful rift system will, ultimately, evolve into a spreading system, the aim of this project is to test how far the concepts developed at mid-ocean ridges can be used to explain and quantify the thermal, magmatic, hydrothermal, isostatic, rheological and tectonic evolution of final rifting recorded in OCTs. We will thus develop a new approach that differs from most previous studies performed on OCT. Our belief is that this original approach has the potential to bring new light, particularly regarding the thermal and subsidence history of OCTs. We will focus on so-called « magma-poor continental margins » that are characterized by large domains of exhumed subcontinental mantle. We study and compare an « ancient » OCT belonging to the Iberia-Newfoundland rift-system, a 'young' OCT in the Gulf of Aden, and an « active » slow-spreading magma-poor ridge system located in the Southwest Indian Ridge (SWIR). In addition to these offshore sites, we have access to remnants of the « fossil » OCT of the Alpine Tethys exposed in the Alps. The choice of these sites enable us to study the system across a whole range of evolutionary stages (fossilized and equilibrated to active) as well as across an evolution from its initiation at a rifted margin to its final steady state functioning at a mid-ocean spreading system. We will also, through ongoing collaborations with TOTAL and ExxonMobil, have access to data sets from the southern Atlantic and other rifted margins showing evidence for magma-poor extension during continental break-up. We have identified two questions which we view as key to understand the rift to drift transition. A first question concerns the anomalous subsidence and deformation history observed in many magma-poor continental margins during final rifting and mantle exhumation (e.g. Lemoine et al. 1987 ; Karner et al. 2003 ; Manatschal 2004 ; as well as industry data from the southern Atlantic and the NW Australian shelf). The other question concerns the post-breakup subsidence and thermal evolution of the OCT. We are particularly interested to understand the causes of distributed deformation, magmatic activity and uplift of the OCT after continental breakup observed in both the Iberia-Newfoundland and Aden OCT. Via these key questions, we will address fundamental aspects of the OCT evolution, such as the extent of serpentinization, the distribution and extent of magmatism, and the source of early magnetic anomalies. This project benefits from the strong background of our three teams (Géosciences Marines IPGP, Géologie EOST-ULP Strasbourg, and Géologie Marine UPMC) in the fields of both ridge and continental rift studies, with a thorough knowledge of the proposed study areas. This project fosters collaborations between the ridge and margins communities, geologists and geophysicists and links offshore with onshore studies. It will reinforce the dynamic that is ongoing at the national (french) level with the « Action Marges » initiative, and is complementary to the project YOCMAL that is supported by the ANR. It is also connected with ongoing studies sponsored by TOTAL of early oceanic crust and distal margins and their alpine analogues. We anticipate that our results will enable us to get a better grasp at OCT processes, and in particular to better understand and quantify the isostasy and thermal evolution during final rifting and onset of seafloor spreading. This will contribute to the preparation of future IODP drilling at OCTs (2 proposals are currently active for the Newfoundland-Iberia system). Finally, the results of this study have not only implications for the understanding of how continents break apart and of how a new plate boundary forms, but are also of direct economic interest for the oil industry. This is because thermal conditions, which will be the main target of our study, have direct implications for the survivability of syn-to post-rift petroleum systems in deep margins and OCTs, which represent one of the last frontiers for the exploration industry.