The magnetic basement topography and associated magnetic anomaly lineation pattern in an area north of the New England seamounts lying between anomaly 31 and the continental margin were examined in light of the Mesozoic sea floor spreading history of the northwest Atlantic. Within the magnetic quiet zone the crust is typically oceanic as far landward as the slope anomaly and exhibits a normally polarized magnetism in the inner quiet zone. Magnetic lineations have been correlated with the Keathley sequence near Bermuda, and, based upon best estimates of the reversal time scale, cross-strike spreading rates are found to be about 1.7 cm yr -x from -172 m.y. to -136 m.y. and between 0.9 and 1.0 cm yr-x from -136 m.y. to -72 m.y. Landward of the Keathley sequence, magnetic anomalies decrease in amplitude, although the intensity of magnetization of the normally polarized crust exhibits average values. Magnetic observations within a large area of the quiet zone surveyed in detail can be fitted to the basement topography if a Jurassic pole position of normal polarity is assumed. However, three zones trending subparallel to the Keathley lineations require a weak reversely polarized magnetism. These low values of remanent magnetization are attributed to either viscous remanence or contamination of the original crust by widespread volcanic flows or intrusives during periods of predominantly normal magnetic polarity. The normal and reversed polarity sequence resulting from this study is correlated with the sequence obtained from paleomagnetic measurements on land. Trends of isochrons within this relatively limited oceanic area necessitate an alteration in the shape of the ridge axis. A difference in spreading rate and direction is obvious on either side of the New England seamount chain and is discussed with respect to the history of early plate motions in the northwest Atlantic. 1973. Vogt, P. R., and G. L. Johnson, Cretaceous seafloor spreading in the western North Atlantic, Nature, 234, 22-25, 1971.

Sonobuoy and tape recording buoy seismic refraction measurements were carried out in Baffin Bay and Davis Strait to study the extent and tectonic development of the oceanic region and the structure of some of the major features of the surrounding continental shelves. Both the oceanic and continental shelf areas are occupied by thick sequences of sediment, 3-7 km, the sediment pile being thicker in the north. A refraction profile in Davis Strait shows that it is underlain by a crust similar to that beneath Iceland, with a total crustal thickness of over 20 km. The main difference is that a normal mantle velocity is measured beneath the Davis Strait. The results, supported by a seismic reflection profile in the region, suggest that Davis Strait may once have been the site of a hot spot or upwelling mantle plume. The main oceanic crustal layers, layers 2 and 3, under the central basin are thin. A total main crustal thickness of 4 km was measured (omitting the sediment, that is) compared with a mean thickness of 7 km for the major ocean basins. The crust is underlain by mantle rocks exhibiting normal mantle velocities, the mean being 8.0 km s-'. Seven refraction lines distributed over the east-west extent of oceanic crust show no detectable median ridge. This accords well with models of the decrease of ridge topography after spreading has ceased as the thermal anomaly beneath decays and supports the hypothesis that the area has not been spreading for about 50 My. The anomalously thin crust can also be related to its age. Thin crust is found near the active mid-ocean ridges at distances corresponding to ages between 50 and 80 My. We postulate that the crust in Baffin Bay is not fully developed and in time a thicker crust will form. These results support the hypothesis that at least the lower part of layer 3 of the ocean basins is composed of altered mantle material.

A seismic refraction experiment was conducted in the Pacific Ocean basin, off the coast of British Columbia, Canada. The purpose of these measurements was to obtain an estimate of the anisotropy of the mantle P-wave velocity in the area and to relate this parameter to the direction of sea floor spreading. The results show that the crustal structure is similar to that measured elsewhere in the Pacific basin. Significant anisotropy of the mantle rocks is observed; the direction in which the maximum velocity occurs being 107° and the change of velocity, about 8% of the mean value, 8.07 km/s. The direction of maximum velocity does not coincide exactly with the direction of sea floor spreading, 090°, inferred from magnetic lineations.

Seismic refraction measurements were carried out using ocean bottom seismometers over foundered continental crust in Orphan Basin and Flemish Pass, on the continental margin north-east of Newfoundland. Tau-p travel time inversion, synthetic seismogram analysis and conventional layered model calculations were applied to the data. The results show that these outer regions of the margin are underlain by thinned continental crust, with a total depth to the M discontinuity of about 22km. There are two main crustal layers with P-velocities of about 6.1 and 7.0 km s-' which appear to be homogenous, and separated by sharp interfaces. These are overlain by a layer in which P-velocities are about 5.5 km s-'; this is interpreted to be Precambrian or Palaeozoic basement. Mesozoic and Cenozoic sediments cover the basement rocks, and are over 4 km thick.The results imply that crustal thinning to about 50 per cent of the original crustal thickness occurred. The gravity anomaly data show that the thin crust has a maximum horizontal extent of about 450 km, from the ocean-continent boundary near Orphan Knoll landward to the outer continental shelf. The subsidence history and thermal evolution of the region was computed, assuming that the observed thinning is produced by horizontal extension of the lithosphere. It is suggested that extension can only satisfy the observed crustal structure and elevation of the margin during the rift phase if more extension took place in the lower lithosphere than in the upper lithosphere. The computed subsidence is compared to the observed subsidence and the total amounts of subsidence are similar. However, the shape of the observed subsidence curves measured in deep exploratory wells differs significantly from the predicted subsidence, assuming cooling began when final continental breakup occurred in the Late Cretaceous. The temperature distribution within the lithosphere due to extension may be related to the flexural rigidity of the plate as a function of both time and position across the margin. Therefore, it is suggested that the response of the lithosphere to sediment loading, and the large amplitude of the gravity 'shelf-edge' anomalies are directly related to the thermal history of the region.