Revisiting the Seismic Structure of Atlantic Oceanic Crust

Much of our current knowledge on the seismic structure of mature oceanic crust comes from studies older than 25 years, which show limited lateral resolution. These studies suggest that away from fracture zones, the crustal structure varies little, with an average thickness of 7.6 ± 0.5 km. Whilst these results may still be valid for crust formed at fast-to intermediate spreading rates they may not hold for that formed at slow rates. Recent studies along the Mid-Atlantic Ridge have revealed significant along axis variation in crustal accretion, with low melt supply at segment ends resulting in extension being accommodated tectonically, exhuming mantle peridotites along large detachment faults. This motivates the re-examination of the structure of mature oceanic crust formed at slow-spreading ridges at a comparable along-strike resolution.

We present results from a multichannel and wide-angle experiment conducted over 80 Ma central Atlantic crust in 2017. The 225 km long profile crosses the medium-offset Marathon fracture zone. We have developed 2D-compressional velocity models by forward and inverse modelling of wide-angle seismic data from 51 ocean-bottom seismometers. The oceanic crust, away from the influence of the Marathon fracture zone, has a mean thickness of 6.3 ± 0.4 km, with crustal velocities of 4.7 ± 0.2 kms^-1 increasing to 6.2 ± 0.2 kms^-1 in layer 2 (gradient of 0.8 s^-1), and 6.4 ± 0.1 kms^-1 increasing to 7.3 ± 0.2 kms^-1 in layer 3 (gradient of 0.3 s^-1). This is thinner than that of the highly referenced models of mature Atlantic oceanic crustal structure, but also presents a more homogeneous velocity structure than what might be expected given the recent studies at the Mid-Atlantic Ridge.

In contrast, the velocity structure within the Marathon fracture zone is like that of layer 2 in neighbouring oceanic crust but exhibits no distinct PmP reflections or velocity structure indicative of layer 3. Instead velocities below layer 2 rapidly increase to that of the mantle (gradient of 1.1 s^-1). These results are consistent with an interpretation that this structure formed through the pervasive hydration and serpentinization of shallow mantle peridotites during active transform faulting.