Research paperFault systems at hyper-extended rifted margins and embryonic oceanic crust: Structural style, evolution and relation to magma

•Detachment faults develop out-of-sequence, leading to symmetric exhumed domains.•Pure shear and simple shear modes alternations are favoured by a decoupling level.•Initiation of detachment faults from listric faults influenced by magmatic supply.•Mechanic and magmatic processes compete for the onset of the lithospheric breakup.

The study of seismic sections derived from the magma-poor Australia-Antarctica rifted margins enables to investigate the structural style and evolution of fault systems and their relation to magmatic additions during final rifting and lithospheric breakup. The aim of this study is thus to better characterize fault systems, their organization in time and space as well as their relationship to magmatic systems. Our observations show that extensional detachment faults related to mantle exhumation form out-of-sequence, and initiate as normal upward concave faults. The observations can be best explained by an alternation between a pure shear mode (development of symmetric arrays of normal faults) and a simple shear mode (development of detachment faults), with a continuous delocalization and re-localization of the deformation in the footwall of the previous exhumation fault. This model suggests that the asthenospheric uplift is slow and decoupled from the deformation in the brittle and serpentinized upper basement layer. The presence of such a décollement level, possibly formed by the serpentinisation front, has a major control on the style and organization of faulting. The polyphase, out-of-sequence organization of faults can explain that despite mantle exhumation is accommodated along multiple asymmetric exhumation faults, the overall exhumed mantle domain is mostly symmetric at the scale of both margins.