Continental hyper-extension and mantle exhumation at the ocean-continent transition, West Iberia: new insights from wide angle seismic.

Rifting and the subsequent breakup of continental lithosphere has given rise to the magma-poor Deep Galicia rifted margin in the North Atlantic Ocean. Here, hyperextension of continental crust and has been accommodated by the rotation of continental fault blocks, which are underlain by the S-reflector, an interpreted detachment fault, which has exhumed serpentinised mantle peridotite. West of these features is the enigmatic Peridotite Ridge (PR) which has been suggested to delimit the western extent of the ocean-continent transition. An outstanding question at this margin is where unequivocal oceanic crust begins, with little existing data to constrain this boundary and a lack of magnetic seafloor spreading anomalies. We present results from a 160-km-long wide-angle seismic profile, which encompasses the S-reflector to the east, the PR, and the unidentified basement west of the PR. This profile consists of 32 OBS/H recording wide angle seismic data from coincident multichannel seismic surveying. Forward and inverse travel time tomography models of the crustal velocity structure were produced using a range of algorithms, with the best fit model having a RMS travel time misfit of 47 ms, a χ2 of 0.98 and strong correlation with the structure observed in seismic reflection images. East of the PR, highly thinned and rotated crustal blocks overlie the S-reflector, which generally correlates with the 6.0-6.5 kms-1 velocity contours, giving a crustal thickness of 1.0-1.5 km and an average velocity gradient of 0.75 s-1. Similarly, west of the PR we observe a basement layer which is 3.0-4.0 km thick and has an average velocity gradient of 0.95 s-1. High velocity gradients, and an absence of velocities typical of oceanic layer 3 and of clear mantle reflections suggest the presence of exhumed, serpentinised mantle peridotite west of the PR, which could be analogous to the large expanses of mantle peridotite exposed at the seafloor on the flanks of the ultra-slow Southwest Indian ridge. Additionally, we present the initial results of full waveform inversion performed on this wide angle seismic dataset, showing an increased resolution in the seismic velocity structure associated with the S-reflector and the patterns of mantle serpentinisation beneath the detachment fault.