The continent-ocean transition at the Deep Galicia Margin: insights from wide-angle seismic data

Magma-poor rift margins provide the ideal environment in which to study the processes responsible for continental breakup and the transition to seafloor spreading. The Iberia-Newfoundland rifted margin in the North Atlantic Ocean has long been an archetype of such rifted margins, with previous studies of this margin having shaped the current understanding of how rifted margins evolve. The Deep Galicia margin is situated on the Iberia margin and is characterised by continental hyperextension, observed as a complex pattern of faulting, thin continental fault blocks, and the serpentinisation, with local exhumation of mantle peridotites along an interpreted detachment surface known as the S reflector. West of these features, the enigmatic Peridotite Ridge has previously been inferred to delimit the western extent of this continent-ocean transition. This thesis presents a variety of analyses applied to new wide-angle seismic data collected at the Deep Galicia margin in 2013. Travel-time tomography modelling of a 160-km-long wide-angle seismic profile provides new insights into the transition from continental thinning to the onset of oceanic crust. West of the Peridotite Ridge, mantle exhumation is seen to continue over a short distance (< 25 km), before shallow and sparse Moho reflections indicate the earliest formation (< 122 Ma) of an anomalously thin (0.5 – 1.5 km) oceanic crustal layer. This thin oceanic crust is inferred to be underlain by serpentinised mantle peridotite, indicated by low velocity gradients and a smooth transition to mantle velocities. East of the Peridotite Ridge, a combination of travel-time tomography and time-domain full-waveform inversion were used to produce a high resolution P-wave velocity model of the hyperextended continental crust and the underlying mantle, separated by the S reflector. This model is used to interpret previously unidentified faults and crustal blocks in seismic reflection imaging, giving an increased understanding of the fine-scale patterns of deformation. Velocities below the S reflector are shown to vary between 5.5 and 8.0 km s-1, corresponding to peridotite serpentinisation of 70% to 0% (unaltered), respectively. Lower seismic velocities and higher degrees of serpentinisation are coincident with the terminus of normal faulting, and is interpreted to be the result of preferential mantle hydration along such faults. Additionally, analysis of ambient noise recorded during this seismic experiment has been used to determine the phase velocity dispersion of fundamental Rayleigh waves, which were inverted to give estimates of the shear velocity within the hyperextended domain. Shear wave velocities are seen to be 0.94 ± 0.12 km s-1 in the sediments, 2.21 ± 0.36 km s-1 in the upper crust, 3.48 ± 0.36 km s-1 in the lower crust, and 4.25 ± 0.35 km s-1 in the uppermost mantle