Wide-angle seismic imaging of two modes of crustal accretion in mature Atlantic Ocean crust

Abstract

We present a high-resolution 2-D P-wave velocity model from a 225 km long active seismic profile, collected over ~60-75 Ma central Atlantic crust. The profile crosses five ridge segments separated by a transform and three non-transform offsets. All ridge discontinuities share similar primary characteristics, independent of the offset. We identify two types of crustal segment. The first displays a classic two-layer velocity structure with a high gradient layer 2 (~0.9 s-1) above a lower gradient layer 3 (0.2 s-1). Here PmP coincides with the 7.5 km s-1 contour and velocity increases to >7.8 km s-1 within 1 km below. We interpret these segments as magmatically-robust, with PmP representing a petrological boundary between crust and mantle. The second has a reduced contrast in velocity gradient between upper and lower crust, and PmP shallower than the 7.5 km s-1 contour. We interpret these segments as tectonically dominated, with PmP representing a serpentinized (alteration) front. Whilst velocity depth profiles fit within previous envelopes for slow-spreading crust, our results suggest that such generalizations give a misleading impression of uniformity. We estimate that the two crustal styles are present in equal proportions on the floor of the Atlantic. Within two tectonically dominated segments we make the first wide-angle seismic identifications of buried oceanic core complexes in mature (> 20 Ma) Atlantic Ocean crust. They have a ~20 km wide “domal” morphology with shallow basement and increased upper-crustal velocities. We interpret their mid-crustal seismic velocity inversions as alteration and rock-type assemblage contrasts across crustal-scale detachment faults.

More information

Identifiers

ORCID for T.J. Henstock: orcid.org/0000-0002-2132-2514

ORCID for Nicholas Harmon: orcid.org/0000-0002-0731-768X

Catalogue record

Export record

Altmetrics