SEISMIC-REFRACTION MEASUREMENTS IN THE ATLANTIC OCEAN: PART VII: ATLANTIC OCEAN BASIN, WEST OF BERMUDA

摘要

Results of 25 seismic-refraction stations in the Atlantic Ocean west of Bermuda are incorporated with other available data into four seismic cross sections through the oceanic crust and continental margins along tracts from Bermuda to the continent. These provide additional evidence for the marked contrast between oceanic and continental crustal structure. Under the northeast American continent the velocity of near-surface rocks is close to 6.0 km/sec and may increase with depth to near 7.0 km/sec at about 35 km, with the mantle (velocity exceeding 7.7 km/sec) below. The upper continental crust (6.0–6.3 km/sec) thins seaward, disappearing near the foot of the continental slope. For the deep stations of this paper (mean depth 4.9 km) the mean thickness of sediments is 1.3 km. Mean thickness of crustal rocks underlying this thin sedimentary layer is 5.1 km, with velocities up to 7.1 km/sec, but significantly less near Bermuda. Arrivals from the mantle were observed on five of the longer stations, giving velocities of 7.7–8.5 km/sec at 9.4 to 13.4 km below sea level. Exclusive of the water column, this oceanic crust is less than one-fifth as thick as the continental crust. Toward Bermuda, the mean crustal velocity decreases; observed values range from 5.6 to 6.5 km/sec. In several areas near Bermuda velocities near 4.5 km/sec (volcanics and other consolidated sediments) are observed. A conspicuous, constant-frequency arrival, appearing at a range of 40–50 seconds (59–74 km) and at the travel time of the first reflection, is interpreted as refracted along the ocean floor, possibly within a layer of low-velocity sediment from 0 to about 150 feet thick. On the basis of reflection at the critical angle sound velocity in sediments near the ocean floor is calculated from the maximum number of bottom reflections recorded under favorable conditions. Recorded sub-bottom reflections are described, and two methods of interpretation presented. Resulting sediment thicknesses and velocities compare with those from refraction and vertical reflection data.