Long‐Lasting Evolution of Layer 2A in the Western South Atlantic: Evidence for Low‐Temperature Hydrothermal Circulation in Old Oceanic Crust. Issue 3 (2nd March 2019)

• Record Type: Journal Article
• Authors: Kardell, Dominik A.; Christeson, Gail L.; Estep, Justin D.; Reece, Robert S.; Carlson, Richard L.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 3(2019)
• Page Start: 2252
• Page End: 2273
• Publication Date: 2019-03-02
• Subjects: Ocean crust -- Slow‐spreading centers -- Hydrothermal circulation -- Streamer tomography; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2018JB016925
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Previous seismic studies suggest that hydrothermal processes are active only within young oceanic crust (<10–16 Ma). However, differences between measured and predicted heat flow at the ocean floor indicate that hydrothermal fluids may be transporting heat advectively in crust up to ages of 65 Ma. We report on seismic velocities of 0–71 Ma slow to intermediate spreading rate upper crust in the western South Atlantic. Thirteen high‐resolution 2‐D velocity models were built using traveltime tomography on downward continued streamer data acquired during the Crustal Reflectivity Experiment Southern Transect. In the Crustal Reflectivity Experiment Southern Transect area, velocities at the top of seismic layer 2A increase rapidly from ~2.4 km/s at 0 Ma to ~4.2 km/s at 6 Ma and then undergo a more gradual increase to ~4.9 km/s at 58 Ma. These new results resolve the long‐standing debate about the duration of interaction between ocean crust and seawater, providing seismic evidence for hydrothermal circulation continuing to crustal ages predicted by heat flow studies. Seismic layer 2B does not show a systematic off‐axis velocity trend but has an average value of 5.15 km/s. We interpret this result to indicate that the hydrothermal system becomes too shallow to affect the physical properties of layer 2B shortly after crustal accretion. Upper crustal heterogeneity in ridge‐parallel profile orientation is more pronounced for crust accreted at slow spreading rates, compared to intermediate rates. This result is consistent with shorter magmatic segments at slower spreading rates, increasing the frequency of tectonic and magmatic accretion alternately occurring along the ridge. Plain Language Summary: Previous studies of seismic velocities have suggested that hydrothermal processes are active only within young oceanic crust up to ages of ~10 million years, where the heat supply from underneath the spreading center is large enough to fuel them. However, differences between measured and predicted heat flow at the ocean floor indicate that hydrothermal fluids may be transporting heat from the crust into the overlying oceans up to crustal ages of 65 Ma. We show that seismic velocities of shallow crust in the western South Atlantic, between the Mid‐Atlantic Ridge and the Rio Grande Rise, increase significantly in relatively old oceanic crust. These results nicely match the heat flow observations and suggest that hydrothermal fluids may indeed circulate through the upper part of mature oceanic crust. We also estimate the rate at which oceanic crust has been formed in the past and investigate the continuity of upper oceanic crust at different spreading rates. Parallel to the spreading center, we find that slow spreading rates produce upper crust whose seismic velocities are less laterally homogeneous than at intermediate rates. This is likely because the rate at which magmatic and fault‐aided extension alternate is generally higher at slower spreading rates. Key Points: Seismic layer 2A increases in velocity in crust aged 0‐58 Ma, suggesting ongoing hydrothermal activity in the uppermost oceanic crust Seismic layer 2B shows no systematic velocity trend with crustal age Intermediate spreading rate upper crust parallel to the ridge is more laterally homogeneous than slow spreading rate crust