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 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.

Seismic Crustal Structure and Morphotectonic Features Associated With the Chain Fracture Zone and Their Role in the Evolution of the Equatorial Atlantic Region

“…In contrast to clear crustal thickness differences between TF and FZ domains, crustal thickness variability associated with the NTOs presented in our compilation ( Figure 11) does not show any clear pattern. Mapped crustal thicknesses vary from 2 to 7.2 km (Canales et al, 2000;Dannowski et al, 2018;Davy et al, 2020;Detrick et al, 1993;Dunn et al, 2017;Kahle et al, 2016), including the results from this study. Although the pseudofault crossed by our profile could be considered related to an NTO, we need to emphasize that the crustal thickness variations that are observed across the pseudofaults may not be directly comparable to the global signature of the off-axis trace of an NTO.…”

“…In addition to the studies at the MAR, using eight OBH instruments, Muller et al (2000) examined crustal structure across TFs at 57°E and 66°E offsetting the slow-spreading Southwest Indian Ridge. More recently, a modern OBS survey using 54 instruments along a 225 km long seismic line was carried out across the Marathon FZ and three NTOs (NTO-1, NTO-2, and NTO-3) over a ∼65 Ma central Atlantic lithosphere (Davy et al, 2020). This recent results for the Marathon FZ are falling within the crustal thickness range from 5.2 to 7 km reported by Peirce et al (2019) for the same FZ examined closer to the ridge axis (∼1 Ma crust).…”

Evolution and properties of young oceanic crust: constraints from Poisson's ratio

“…At greater distances (ages) than that sampled by SAP_B, the differences between magmatic and tectonic-end member oceanic crust may become indistinguishable due to the infilling of fractures. However, tomographic imaging of Atlantic oceanic crust indicates that clear seismological differences remain even at ~60-75 Myr (Davy et al 2020).…”

“…Bratt & Solomon 1984;Collier & Singh 1998), two-dimensional profiles (e.g. Van Avendonk et al 1998;Audhkhasi & Singh 2019;Peirce et al 2019a,b;Wilson et al, 2019;Christeson et al 2020;Davy et al 2020), and three-dimensional tomographic grids (e.g. Toomey et al 1990;Gregory 2018;Robinson et al 2020;Simao et al 2020), that have become more detailed through the use of greater numbers of instrumentation deployed at smaller spatial intervals and able to sample at higher temporal rates.…”

“…The vertical velocity gradient, when used coincidently with depth-contextualised absolute values of Vp, has been demonstrated to differentiate between localised regions of basaltic crustal accretion, localised magmatism, and mantle exhumation processes at slow and ultraslow spreading centres (Harding et al 2017;Grevemeyer et al 2018b;Peirce et al 2019a,b;Davy et al 2020). However, Grevemeyer et al (2018b) highlight that this approach fails to determine the extent of crust formed through magma-poor processes, as might be expected from observations of lower crustal thickness.…”