Møre volcanic rifted margin, sea-floor spreading and microcontinent: Insights from new high-resolution aeromagnetic surveys

The Continental Shelf Geophysics team at NGU has acquired and processed more than 88.000 km of new aeromagnetic data over the entire oceanic Norway Basin (NB) located between the Møre volcanic rifted margin (MVRM) and the Jan Mayen microcontinent (JMMC). The new compilation allows us to revisit the crustal structure of the conjugate rifted system and its spreading evolution from the Early Eocene breakup time (at Chron C24) to the Late Oligocene when the Aegir Ridge became extinct. Seismic data and potential field modelling suggest that the size and thickness of the continental blocks/rafts preserved beneath the Cretaceous Basin of the MVRM are larger and thicker compared to the allochthonous blocks often described in distal “hyper-extended” domains. The continental crust preserved on top of the outer lower crustal bodies in the outer part of the MVRM remains relatively thick (> 5-8 km) to favour a broad zone of exhumed and denudated serpentinized mantle adjacent to the oceanic domain. Modelled Vp velocities of 7.2-7.3 km/s for the underlying lower crustal bodies would possibly require an increased process of serpentinization that usually is symptomatic of more drastic crustal thinning (ßcrustal » 4) and/or complete mantle denudation. Such a crustal configuration is not so obvious in the outer part of the MVRM where a marginal plateau is suggested nearby the lava flows. To explain the magnetic anomalies observed in the central part of the MVRM, our potential field modelling suggests also the preservation of middle crustal material with high magnetic susceptibilities associated with the crustal rafts observed on seismic data. We favour a “super-extended” tectonic scenario where a large part of the crust observed underneath the sedimentary basin represents both preserved upper continental basement and mid to lower crustal lenses of inherited and intruded, high-grade metamorphic rocks also controlling the inner necking zone. We interpret the nature of the crust closer to the SDRs as a similar mixture of residual continental crust later affected by breakup-related intrusions. No zone of exhumed and serpentinized continental mantle has been clearly identified from magnetic chron C24n3n towards the continent. The long period of rifting (and intra-thinning cooling event ?), the large amount of pre-breakup sedimentation and the significant amount of breakup magmatism (SDRs) make the MVRM to appear quite different from (Iberian type) magma-poor margins. The spreading system is now fully covered by reliable data and reveals a more complex system of asymmetric oceanic segments locally affected by episodic ridge jumps. The new aeromagnetic compilation confirms that a controversial fan-shaped spreading evolution of the NB was clearly active before the cessation of seafloor spreading and extinction of the Aegir Ridge. An important Mid Eocene kinematic event at around magnetic chron C21r can be recognized in the NB. This event coincides with the onset of dyking and the increase of rift activity (and possible oceanic accretion?) between the proto-JMMC and the East Greenland margin. It led to a complete and second phase of breakup and microcontinent formation in the Norwegian-Greenland Sea ~26 Myrs later in the Oligocene.