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Tectono-magmatic evolution at distal magma-poor rifted margins: from crustal breakup to lithospheric breakup

Author(s): M. Gillard IPGS-CNRS-EOST, Université de Strasbourg, 1 rue Blessig, F-67084 Strasbourg, France
Julia Autin IPGS-CNRS-EOST, Université de Strasbourg, Strasbourg, France
Gianreto Manatschal IPGS-CNRS-EOST, Université de Strasbourg, Strasbourg, France

The discovery of large domains of hyper-extended continental crust and exhumed mantle along many present-day magma-poor rifted margins questions the processes leading to lithospheric breakup and onset of seafloor spreading. In particular, the evolution of faults, the amount of magma and its relation to tectonic structures are yet little understood. Trying to find answers to these questions asks to work at the most distal parts of rifted margins where the transition from rifting to steady state seafloor spreading is recorded.

The deformation recorded in the sedimentary units along several magma-poor rifted margins, such as along the Australia-Antarctica and the Iberia-Newfoundland Ocean Continent Transitions (OCT's), highlights a migration of the deformation toward the future ocean. In particular, the observation that each tectono-sedimentary unit downlaps oceanwards onto the basement suggests that final rifting is associated with the creation of “new real estate” by a process that is not yet steady state seafloor spreading. These observations lead to a new model for the evolution of exhumed mantle domains implying the development of multiple, polyphase extensional detachment faults organizing out-of-sequence, i.e. stepping backwards into the previously exhumed footwall. This spatial and temporal organization of faults may explain the final symmetry of exhumed mantle domains generally observed at several magma-poor OCT. Indeed, it appears that the current models implying in-sequence detachment faults to explain the presence of exhumed serpentinized mantle cannot account for the observations made at OCT's.

We propose that this fault evolution observed in exhumed mantle domains at magma-poor rifted margins is linked to cycles of delocalisation/re-localisation of the deformation. These cycles of deformation appear to be strongly influenced by the magmatic budget, but also by the thermal state and hydrothermalism during mantle exhumation, resulting in alternations between pure shear and simple shear modes. This evolution implies the presence of a decoupling interface between an upper brittle layer made of serpentinized mantle and a yet poorly-defined underlying level. The complex interaction between out-of-sequence detachment systems and the successive rise of the asthenosphere may explain the observed transition from a fault- to a magma-controlled strain accommodation and the transition to more symmetric and localized accretion associated with the formation of a stable and localized spreading centre.

However this model of evolution for exhumed mantle domains raises the question of the nature and dating of magnetic anomalies in such OCT's and their value for the interpretation of the kinematic evolution of conjugates rifted margins.

Tectono-magmatic evolution at distal magma-poor rifted margins: from crustal breakup to lithospheric breakup
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IPGS-CNRS-EOST, Université de Strasbourg, 1 rue Blessig, F-67084 Strasbourg
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