Satellite altimetry reveals seafloor tectonic fabric



During the 1980s satellite radar altimetry matured as a geophysical technique, quickly becoming the best way to map the gravity field of the world's oceans. Maps derived from SEASAT altimeter observations of sea surface topography (or geoid) revealed much that was previously unknown about the marine gravity field [Haxby, 1987] and underlying seafloor, particularly in the vast areas that are poorly charted by traditional shipboard observations.

Seafloor topography, and to a lesser extent, crustal/upper mantle density variations, produce the short wavelength (<200 km) undulations in the marine gravity field. This is simply due to the gravitational attraction of the mass in the seamount or similar structure emplaced on the seafloor. For topographic structures beneath the sea that are 20-200 km in spatial extent, this attraction causes the gravity anomalies at the sea surface to vary in proportion to the relative height of the underlying structures. (This ignores isostasy or internal deformation of the Earth's crust and mantle, which acts to reduce this gravitational effect particularly over long-wavelength—greater than 200 km—topography.) The ratio between gravity anomalies and height variations is slightly less than 0.07 milligals per meter (1 mGal equals 10-5 m/sec2). Therefore, the fine structure in these marine gravity fields reflects the tectonic fabric imprinted in the seafloor. This fabric includes the mid-ocean ridge axes (where tectonic plates form), fracture zones, which record the directional history of seafloor spreading, seamount chains, rifted continental margins, and other features that chronicle the history of relative motion among the tectonic plates over the past 200 million years.