NSFGEO-NERC: Quantifying evolution of magmatism and serpentinisation during the onset of seafloor spreading

The breakup of continents involves a complex interplay of extensional tectonics and magmatism. The final stages of this process, resulting in the initiation of seafloor spreading, remain poorly understood. One common end-member involves hyper-extended continental crust and broad regions of exhumed and serpentinised (hydrated) mantle. Serpentinisation at mid-ocean ridges, rifted margins and subduction zones mediates geochemical exchange between the lithosphere and the hydrosphere. There is evidence that normal faulting plays a key role in supplying fluids driving serpentinisation beneath hyper-extended continental crust and possibly in regions of exhumed mantle. However, the seismic P-wave velocities of mafic crustal rocks and of serpentinised mantle rocks are similar, so interpretations of seismic data are uncertain. Serpentinised mantle rocks are generally more conductive, often by about an order of magnitude, than mafic crustal rocks, so controlled source electromagnetic (CSEM) and magnetotelluric (MT) techniques provide a promising route to resolve controversies around lithospheric structure.

These techniques have successfully imaged localised fluid-rich zones associated with normal faulting in a subduction zone setting, but have yet to be applied to continent-ocean transitions at rifted margins. We will conduct the first joint CSEM, MT and seismic experiment in the continent-ocean transition at a rifted margin, to determine how competing processes of magmatism and serpentinisation evolve during the initiation of seafloor spreading. We will focus our work at the sediment-starved Goban Spur rifted margin southwest of the UK, which is uniquely suited to such a study.