Searching for the Onset of Seafloor Spreading West of Galicia: Wide-Angle Seismic Constraints

Abstract:

Rifting and the subsequent breakup of continental lithosphere has given rise to the magma-poor Galicia Bank rifted margin in the North Atlantic Ocean. Hyperextension of continental crust is observed at the deep Galicia margin (west of Spain) 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 serpentinized 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.

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. First-arrival travel time tomography models of the crustal velocity structure were produced using two algorithms, with the best fit model having a RMS travel time misfit of 38ms, a χ² of 0.99 and strong correlation with the structure observed in seismic reflection images. East of the PR, the 3.0-3.5 kms^-1 velocity contours match top of crust and the S-reflector generally lies between the 6.0-6.5 kms^-1 velocity contours, giving a crustal thickness of 1.5-3.5 km and an average velocity gradient of 0.75 s^-1. Similarly, west of the PR we observe a basement layer which is 2.0-4.0 km thick and has an average velocity gradient of 0.72 s^-1. High velocity gradients, an absence of velocities typical of oceanic layer 3 and no clear mantle reflections suggest the continued presence of exhumed, serpentinized 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.