Seismic structure of mature Atlantic Ocean crust

Our current knowledge of the structure of mature Atlantic Ocean crust relies on studies older than 25 years which suffer from limited lateral resolution. Here we present results from an active seismic experiment, with 54 ocean-bottom seismometers spaced every 4 km, conducted over ~65 Ma central Atlantic crust in 2017. The profile crosses four ridge segments separated by a transform (Marathon) and two non-transform offsets. The data have been analysed using conventional forward and inverse modelling to produce a 2D p-wave velocity model.

We identify two types of crustal segment. The first type displays the classic two-layer structure with a high velocity gradient layer 2 (~1.0 s^-1) above a lower gradient layer 3 (0.2 s^-1). Here the PmP reflector coincides with the 7.5 kms^-1 contour and an increase to >7.8 kms^-1 less than 1 km below. We interpret these segments as having been dominated by magmatic processes, with PmP representing a petrological boundary between crust and mantle. The second type has a much reduced velocity gradient contrast between upper and lower crust with a PmP reflector that is shallower than the 7.5 kms^-1 contour. We interpret these segments as being tectonically controlled, with PmP representing a serpentinised (alteration) front. Across our 225 km long profile the two segment types are present in approximately equal proportions, consistent with mapping of the modern MAR.

All three discontinuities share the same primary characteristics, with no apparent correlation with size of the ridge-offset. Within them the PmP reflector shallows by up to 3.1 km over distances of about 15 km and the velocity gradient becomes linear (0.5-0.7 s^-1). We conclude that both orders of discontinuity present major boundaries to magmatic and tectonic processes at the MAR. We will also show what we believe to be the first identifications of Oceanic Core Complexes in mature oceanic crust. These features present as ~20 km wide "domal" morphology, with shallow basement and increased upper-crustal velocities. Internally they show seismic velocity inversions, which we interpret as due to alteration and rock-type assemblage contrasts across a crustal-scale detachment fault. Overall, our results question the validity of assigning a single 1D velocity-depth envelope to slow-spread oceanic crust.