Exhumed mantle-forming transitional crust in the Newfoundland-Iberia rift and associated magnetic anomalies

[1] Transitional zones located between Iberia and North America formed during continental rifting and mostly consist of exhumed mantle. In this study we show that ages of exhumed mantle at Ocean Drilling Program (ODP) sites 1068 and 1070 in the Iberia Abyssal Plain and site 1277 in the Newfoundland Basin are similar to ages determined from magnetic lineations created by serpentinization during mantle exhumation. On the basis of paleomagnetic and geological data and a comparison with a fossil ocean-continent transition in the Alps, we envisage a first episode of mantle serpentinization during which a strong component of magnetization was acquired followed by a second episode occurring at the contact with cold seawater, and which only affects the upper tens of meters of the exhumed mantle. The inversion of magnetic data ( Euler deconvolution) shows that magnetic sources are N-S trending horizontal cylindrical bodies located within the highly serpentinized upper crust. Therefore the serpentinization process is able to produce magnetic lineations formed in a similar way to those formed by seafloor spreading. Within transition zones, sequences of magnetic anomalies can provide information concerning the timing of the emplacement of crust, but not on its nature ( oceanic versus exhumed mantle). This discovery enables us to date the exhumation of mantle rocks in transition zones and allows kinematic reconstructions of the final stages of continental rifting. During rifting the deep distal continental margins and the adjacent transitional zones in the Newfoundland-Iberia rift system were formed by ultraslow extension from early Berriasian to late Valanginian - early Hauterivian and by slow extension from early Hauterivian to the late Aptian - early Albian boundary. Therefore transitional zones share many similarities with slow and ultraslow spreading midoceanic ridges.