A lithospheric seismic refraction profile in the western North Atlantic Ocean

• Published in: Geophysical Journal International
• Main Authors: The LADLE Study Group, Whitmarsh, R. B., Keen, C. E., Steinmetz, L., Tomblin, J., Donegan, M., Lilwall, R. C., Loncarevic, B. D., Nichols, B., Shepherd, J.
• Format: Text
• Language: English
• Published: Oxford University Press 1983
• Subjects: Articles; North Atlantic
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/75/1/23; https://doi.org/10.1111/j.1365-246X.1983.tb01912.x

A study of the seismic velocity structure of the subcrustal oceanic lithosphere was conducted in the western North Atlantic north of the Lesser Antilles, where the age of the lithosphere is 80–95 Ma. A 1000km long north-south array of 18 ocean-bottom seismographs (OBS), spaced at 60 km intervals, recorded arrivals from explosions at ranges up to 800km and arrivals from earthquakes in the northern Caribbean region at ranges up to 1400 km. Analysis of the travel-time and amplitude data from the explosions provided the velocity structure down to about 65 km below sea-level. The earthquake data were intended to extend the travel times to ranges over 1100 km, and thus sample the deeper part of the lithosphere. However, the geometry and velocity structure of the downdipping slab beneath the Puerto Rico Trench deflected the ray paths between the Caribbean hypocentres and the OBS array such that no rays travelling through the lower lithosphere were detected by the array. Only rays through the upper lithosphere reached the OBS array. The results show that the subcrustal oceanic lithosphere consists of two main regions. The uppermost region starts just below the M discontinuity and exhibits velocities of 8.2–8.4 km s−1. This region is characterized by three alternating zones of low (8.2 km s−1), high (8.4 km s−1) and low (8.2 km s−1) velocity. The second region lies below 53 km and exhibits high velocities of 8.5-8.6 km s−1. A sharp boundary separates these two regions, suggesting that a major change in composition or phase occurs there. The velocity structure within the upper region shows broad agreement with petrological models for the generation of oceanic lithosphere starting from a pyrolite composition. The alternating zones of low and high velocity correspond to zones depleted of, and enriched in, the basaltic fraction of the primary mantle material, respectively. This is a consequence of the processes of migration and differentiation of the upwelling asthenosphere near the mid-ocean ridge axis. However, none of the ...