The Perisistence and Evolution of Seismic Layer 2A in a 70 MYR Continuous Transect in Slow-Intermediate Spreading South Atlantic Oceanic Crust
Abstract
Oceanic crust is often described in layers based on seismic velocities. Layer 2A is defined as the uppermost igneous portion of oceanic crust, characterized by low seismic velocities, overlying a steep velocity gradient transitioning to the higher seismic velocities of Layer 2B ( 5-6 km/s). Global studies show that velocities in Layer 2A vary with age, increasing from just more than 2 km/s to values more typical of Layer 2B over 10 Ma. Evidence exists that Layer 2A velocities may persist in old oceanic crust, but a lack of comprehensive data inhibits the study of Layer 2A in older crust. The persistence and evolution of Layer 2A remains poorly understood at crustal ages >10 Ma. The recent CREST (Crustal Reflectivity Experiment Southern Transect) expedition in the South Atlantic provides a 1500 km continuous 2D multichannel seismic (MCS) transect from the Mid Atlantic Ridge (0 Ma) west towards the Rio Grande Rise (70 Ma) of crust formed at the same ridge segment. By utilizing a 12.6 km, 1008 channel streamer, the CREST data provide a longer offset than that utilized for previous oceanic crust and layer 2A studies. Long-offset MCS data produce a seismic triplication, the Layer 2A event, in the steep gradient velocity transition from Layer 2A and 2B. Preliminary results from the CREST data show that we are able to image the Layer 2A event throughout the entire MCS transect. The presence of the 2A event in 70 Ma crust suggests that slower Layer 2A velocities persist into old crust, contrary to the thought that Layer 2A velocities mature quickly and become similar to Layer 2B. Further analysis will focus on characterizing the evolution of Layer 2A velocities with crustal age in the South Atlantic and whether these velocities mature at a young age or if detectable change occurs in crust older than previously thought. The persistence of lower 2A velocities in older oceanic crust has implications regarding hydrothermal circulation in lightly sedimented oceanic crust.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.T13B2704E
- Keywords:
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- 3025 Marine seismics;
- MARINE GEOLOGY AND GEOPHYSICSDE: 3036 Ocean drilling;
- MARINE GEOLOGY AND GEOPHYSICSDE: 3075 Submarine tectonics and volcanism;
- MARINE GEOLOGY AND GEOPHYSICSDE: 7220 Oceanic crust;
- SEISMOLOGY