Continental Rise off East Coast of North America: Shallow Structure: ABSTRACT

Abstract

End_Page 707------------------------------Gravity and magnetic measurements together with continuous seismic profiling have provided new information about the structure beneath the sediments of the continental shelf, slope, and rise off the east coast of North America. Free-air and simple Bouguer gravity anomaly charts were prepared for this region from data obtained on three cruises of the R/V Chain in 1968 and published submarine pendulum observations by Vening Meinesz and Worzel. The zero free-air gravity anomaly contour parallels the strike of the continental slope and is located over water depths ranging from 1,000 to 3,000 m. A continuous band of positive free-air anomaly values occurs over the landward side of the continental shelf. The maximum measured anomaly ranges from about +10 to +85 mgal along the band, and maxi a occur near Cape Hatteras, off southern New Jersey, near 66° W long., and near 60° W long. Structure-model studies suggest that the positive anomaly band is caused mainly by a basement ridge beneath the edge of the continental shelf. The same structure models also suggest that the continental shelf and rise are largely in isostatic equilibrium, and that only crustal segments near the continental slope with widths of about 40-50 km are not in equilibrium. Magnetic measurements made on various cruises of WHOI ships show that the continental rise and slope have a smooth magnetic anomaly field about 400 km wide. The anomaly amplitudes here are generally less than 200 ^ggr except above isolated seamounts. West of this low-amplitude region, a belt of strong positive anomalies trends along the slope. East of the smooth anomaly region, an abrupt transition takes place to high-amplitude anomalies, commonly greater than 500 ^ggr. Tentative correlation of these anomaly peaks suggests that the anomaly trend north of the New England seamount chain is toward the east and, south of the chain, toward the south-southwest. Model studies suggest that the top of the magnetic material that produces the anomalies observed across the abyssal plains could be the rough opaque seismic reflector beneath this area. Magnetic susceptibility contrasts within the basement material, rather than topographic effects, are required if the basement produces the observed anomalies. Sea-floor-spreading-type models were constructed using simple two-dimensional blocks of alternately positive and negative magnetized material and a spreading rate of 1 cm/yr. The region of low-amplitude anomalies is inferred to have a uniform negative magnetic polarity and to have formed during the Kiaman Magnetic Interval: a 50-million year period during late Paleozoic time when the geomagnetic field polarity was reversed. It is suggested that the belt of strong magnetic anomalies beneath the slope, but west of the low-amplitude anomalies, was formed during Early Permian time p ior to the Kiaman Magnetic Interval. End_of_Article - Last_Page 708------------