Alan K. Cooper — Page 4

USGS1976

Preliminary residual magnetic map of the eastern Bering Shelf and parts of western Alaska

Residual magnetic anomalies for the offshore data were calculated by subtracting the Earth's main dipole field, adjusted for secular variations (based on the International Geomagnetic Reference Field (IGRF), epoch 1965, Fabiano and Peddie, 1969), from the observed values of the surveys listed below. The effects of diurnal variations and magnetic storms have been ignored. Individual marine magnetic profiles have been upward continued to an elevation of 1 km (Robinson, 1970; Henderson, 1970) on the assumption that the magnetic sources are two-dimensional in a direction normal to the ship's track. The upward continued profiles were computed by convolving each profile with an optimum (7, 15 or 30 points) upward continuation operator. Additional errors of unknown magnitude result from the use of two-dimensional analytic techniques in a three-dimensional field. However, on the average, values at crossings differ by only 10 percent.

Alaska↗

USGS1974

Tectonic transition zone in the northeastern Caribbean

Seismic reflection data indicate that the Atlantic plate has been underthrust beneath the Caribbean plate east of the Lesser Antilles. The data further reveal that the transition from underthrust to strike-slip plate motion occurs near lat 19.3° N. and long 62° W. in alinement with the Anegada Trough. Oceanic basement and reflectors above basement have not been detected beneath the landward wall of the Puerto Rico Trench west of the transition zone. Southeast of the zone, the horizontal distance over which the reflectors and basement can be traced beneath the landward wall (outer part of the Lesser Antilles rise) systematically increases from a few kilometers in the north to as much as 37 km near lat 16.5° N. If a thrust fault emerges at the landward trench wall, the data suggest that a decollement exists between reflectors above basement and an overlying acoustically transparent zone. Gravity and seismic refraction data indicate a thick accumulation of low-density, low-velocity material beneath the Lesser Antilles rise, possibly sediments scraped off the underthrusting Atlantic plate, although massive slumping might cause a similar low-density configuration. The magmatic history of the Lesser Antilles arc suggests two periods of underthrusting, one in the Mesozoic or early Cenozoic and a second in the late Cenozoic. Although the magmatic history of the Lesser Antilles apparently began in the Mesozoic, its Cenozoic history is remarkably similar to the Cenozoic histories of circum-Pacific arcs. Magmatic histories in both areas suggest that island arc evolution in the Cenozoic may have worldwide synchroneity.

Journal of Research of the U.S. Geological Survey↗