North Atlantic Transect: A wide‐aperture, two‐ship multichannel seismic investigation of the oceanic crust

Abstract

The North Atlantic Transect experiment collected a unique set of two‐ship, multichannel seismic data across oceanic crust spanning almost 200 m.y. in age from the Blake‐Bahama Plateau, off the U.S. East Coast, to the Mid‐Atlantic Ridge near 23°N. A large, synthetic receiving array was simulated by having two ships, each equipped with a large air gun source array and a multichannel streamer, steam in line maintaining a fixed separation while alternately firing their air guns and recording arrivals generated by shots from both ships. More than 3800 km of wide‐aperture common depth point (CDP) data with a total synthetic aperture of more than 10 km were collected along the transect. At intervals along this wide‐aperture profile, 11 expanding spread profiles were also acquired on oceanic crust of various ages: in the Jurassic and Cretaceous quiet zones, on magnetic anomalies MO, 34, 26, and 5, in the Mid‐Atlantic Ridge rift valley, on the Blake‐Bahama Plateau, and north of the Puerto Rico Trench on the Antilles Outer Ridge. These data have provided some remarkable new images of the oceanic crust. A very continuous Moho reflection is observed along most of the western half of the transect. The character of the Moho reflection and its approximate depth appear to vary only slightly over distances of several tens of kilometers, although major changes in crustal thickness and seismic stratigraphy are observed along the profile. Near the Blake‐Spur fracture zone southwest of Bermuda the Moho gradually shallows over a distance of several tens of kilometers. Beneath the fracture zone itself the Moho abruptly rises to less than half its normal depth, providing the most convincing evidence yet for anomalously thin crust beneath even small‐offset fracture zones. An intracrustal reflection, called horizon “R,” is identified about 1.8–2.0 s beneath the sediment‐basement interface but well above the Moho reflection. Although this event is extremely variable laterally, the total thickness of the crust and the thickness of the unit between horizon “R” and the Moho appear to thin progressively toward fracture zones. An expanding spread profile on 118‐m.y.‐old crust southwest of Bermuda indicates a surprisingly thick (∼8 km) crustal section and the presence of a low‐velocity zone near the base of the crust. Although the analysis and interpretation of the transect data will continue, these results convincingly demonstrate the ability of modern, multichannel seismic techniques to map variations in oceanic crustal structure with a high degree of spatial resolution over thousands of kilometers.