Seismic Layer 2A: Evolution and Thickness From 0‐ to 70‐Ma Crust in the Slow‐Intermediate Spreading South Atlantic

Abstract

Layer 2A, the porous and permeable uppermost igneous oceanic crust, permits the circulation of fluid within the crust, the exchange of dissolved mineral species between the ocean and crust, and the convective dissipation of heat from the crust. We examine the presence, temporal extent, thickness, and evolution of layer 2A using multichannel seismic data collected at 30°S in the South Atlantic across crustal age ranges of 0–70 Ma and half spreading rates of 12–31 mm/year. We observe the layer 2A/2B boundary in 0–48 Myr old crust but not in crust older than ~48 Ma. The thickness of layer 2A in the South Atlantic has substantial variability, with a mean of 760 m and a standard deviation of 290 m. Layer 2A has no systematic change in thickness with age in the South Atlantic, and thickness does not correlate with spreading rate. The crust in the South Atlantic is never fully sealed by sediment cover, which implies that the fluid circulation system in the upper crust never becomes fully closed and the thickness of layer 2A can work as a proxy for the depth at which significant circulation can occur. The disappearance of the layer 2A/2B boundary in older crust implies that fluid circulation within the upper crust continues to occur for at least ~48 Myr after crustal formation in the South Atlantic, after which layer 2A becomes indistinguishable from layer 2B in reflection images.

Seismic Structure of the Upper Crust From 0–75 Ma in the Equatorial Atlantic Ocean on the African Plate Using Ultralong Offset Seismic Data

“…We have presented a seismic data analysis of 1,500 km long profile in the equatorial Atlantic Ocean with 560 km of high‐resolution velocity models spanning an age range of 0–75 Ma. Along with the recent results of Kardell et al () and Estep et al () on the South America plate, our results provide a global view of the upper crustal structure in the Central Atlantic Ocean. The major conclusions from our study are as follows: The sediment cover increases rapidly to 170 m within the first 4 Myr with an estimated rate of deposition of 47 mm/Kyr and then varies slightly until 30 Ma.…”

“…At the ridge axis, the thickness of Layer 2A is variable and lies in the range of 800-900 m ± 315 m with the maximum thickness being ∼900 m at ∼1 Ma. This is close to thickness estimated at the Lucky Strike segment of the MAR (Combier et al, 2015), the hot spot influenced Reykjanes segment (Navin et al, 1998), and near the Ascension islands (Minshull et al, 2003 Estep et al (2019) do not observe layer 2A event in the seismic beyond 48 Myr which they attribute to rough basement topography and/or the closure of pore spaces lying below the threshold to be seen in seismic records. These studies including our results suggest that thickness of Layer 2A is established at the ridge axis in slow spreading centers such as MAR.…”

“…Beyond 15 Myr, there is not much change in the Layer 2A thickness, and we estimate thickness in the range of 570–620 m with an uncertainty ranging from 365 to 165 m. These are similar to off‐axis thickness of Layer 2A observed elsewhere. However, Estep et al () do not observe layer 2A event in the seismic beyond 48 Myr which they attribute to rough basement topography and/or the closure of pore spaces lying below the threshold to be seen in seismic records. These studies including our results suggest that thickness of Layer 2A is established at the ridge axis in slow spreading centers such as MAR.…”

“…This is close to thickness estimated at the Lucky Strike segment of the MAR (Combier et al, ), the hot spot influenced Reykjanes segment (Navin et al, ), and near the Ascension islands (Minshull et al, ). Estep et al () estimate Layer 2A thickness of 1,000 m near the ridge axis and estimate a mean thickness of 759 326 m for crust aged 0–7 Myr in the South Atlantic. They do not observe Layer 2A thinning systematically with age and find an increase in Layer 2A thickness at 15–31 Ma, whereas we find a decrease in Layer 2A thickness from a maximum of 900 m near the ridge axis to 770 m at 4 Myr and estimate a thinning rate of 65 mm/Kyr.…”

“…Off‐axis thinning of Layer 2A at slow spreading centers to 350–600 m is observed (Hussenoeder et al, , which is similar to off‐axis thickness of Layer 2A at other spreading centers. Estep et al () have observed Layer 2A in the South Atlantic in crust aged 0–48 Ma with a mean thickness of 760 290 m.…”