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A New Color Image Available from the National Geophysical Data Center (now the National Centers for Environmental Information)
World Data Service for Geophysics

by K.M. MARKS, D.C. McAdoo, and W.H.F. Smith

NOAA/National Ocean Service/Office of Ocean and Earth Science/Geosciences Laboratory

view larger gif of Southern Ocean Gravity from GEOSAT.

Seafloor topography beneath the Southern Atlantic, Indian, and Pacific Oceans, as well as the Scotia and Tasman Seas and the smaller seas around Antarctica, is detailed in this marine gravity field. It is possible to "see" the seafloor in image because gravity and topography are highly correlated in the 20- 200 km waveband. The complex history of seafloor spreading that formed these ocean basins is recorded in the tectonic fabric of the ocean floor. Seafloor spreading occurred along the extensive mid-ocean ridge system that bisects these oceans; positive gravity anomalies are associated with these spreading centers. Fracture zones that offset the spreading ridges are delineated by long and sinuous gravity lows, of which the Heezen, Tharp, and Udinstev Fracture Zones, extending southeastward across the South Pacific Ocean, are among the largest. Slow-spreading sections of the mid-ocean ridge, such as the South Atlantic and Southwest Indian Ocean Ridges, are crossed by numerous fracture zones, whereas the fast-spreading East Pacific rise has fewer. Spreading ridges that were active in the Cenozoic, but are now extinct, may be seen in the Scotia and Tasman Seas. Fracture zones generated at these extinct spreading ridges are evident in the Scotia, Tasman, and Weddell Seas. The largest amplitude anomalies coincide in location with trenches near the Scotia and Western Pacific Island Arcs, where seafloor is, or has been, subducting. Aseismic ridges and plateaus such as those in the Southern Indian Ocean coincide with high-amplitude positive gravity anomalies.

This gravity field was computed from sea-surface height measurements collected by the US Navy GEOSAT altimeter between March, 1985, and January, 1990. The high density GEOSAT Geodetic Mission (GM) data that lie south of 30 deg. S were declassified by the Navy in May of 1992. These GM data contribute most of the fine-scale gravity information.

GM data are available from the National Oceanographic Data Center, NOAA (NODC Env. Bull. 93-1); the digital gravity data shown in this image are available from the National Geophysical Data Center (now the National Centers for Environmental Information)on the Global Relief CD-ROM (NGDC Data Announcement 93-MGG-01).

Mercator Projection Scale approximately 1:22,400,000 at 30 deg. S

This colorful new image, published as World Data Service for Geophysics, Boulder Report MGG-8, is available from National Geophysical Data Center (now the National Centers for Environmental Information). Orders can be made by mailing or faxing the enclosed order form. Questions and credit card phone orders can be directed to our office by mail, phone, fax, or e-mail.

The size of the map is 30" x 40" which includes a color shaded scale for mGals. The digital data used for this map are available on the Global Relief CD-ROM.