Wide-Angle Seismic Imaging of Two Modes of Crustal Accretion in Mature Atlantic Ocean Crust

R. G. Davy , J. S. Collier , T. J. Henstock , Saskia Goes , Jon Blundy , Nick Harmon , Catherine Rychert , Colin G. Macpherson , Jeroen van Hunen , Mike Kendall , Jamie Wilkinson , Jon Davidson , Marjorie Wilson , George Cooper , Benjamin Maunder , Lidong Bie , Stephen Hicks , Ben Chichester , Stephen Tait , Richie Robertson , Joan Latchman (1) , Frank Krüger (1) , Jenny Collier , Tim Henstock , Robert Allen , Sophie Butcher , Gabriella Castiello , Chen Chen , Caroline Harkin , Dan Posse , Ben Roche , Anna Bird , Andy Clegg , Ben Pitcairn , Martin Weeks , Henning Kirk , Erik Labahn

1 IPGP (UMR_7154) - Institut de Physique du Globe de Paris

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 nontransform 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 the 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. While 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 midcrustal seismic velocity inversions as alteration and rock-type assemblage contrasts across crustal-scale detachment faults.

Mots clés: slow-spread oceanic crust; active-source seismic; tomography; oceanic core complexes; fracture zone; nontransform offset

Domaines: Planète et Univers [physics]

R. G. Davy, J. S. Collier, T. J. Henstock, Saskia Goes, Jon Blundy, et al.. Wide-Angle Seismic Imaging of Two Modes of Crustal Accretion in Mature Atlantic Ocean Crust. Journal of Geophysical Research : Solid Earth, 2020, 125, pp.3011-3053. ⟨10.1029/2019JB019100⟩. ⟨insu-03584765⟩