SCOTIA SEA TECTONICS FROM HIGH-RESOLUTION SATELLITE GRAVITY

The release of altimetric data from the Geosat Geodetic Mission by the US Navy [1] is leading to a much-improved understanding of tectonics in the Southern Ocean, a region in which remoteness and adverse physic al conditions have limited the acquisition of geophysical data by rese arch ships. The Scotia Sea is an outstanding example of back-arc sprea ding, which is revealed in some detail by free-air gravity anomaly map s derived from the latest release of data acquired south of 30-degrees -S [2]. Sea surface height data for this region have been reduced to a grid of free-air gravity anomalies, and are illustrated here by means of colour shaded relief and contour maps. The new data confirm the ex istence of a number of inactive spreading ridges within the Scotia Sea and surrounding small basins. The amplitudes and wavelengths of gravi ty anomalies over these ridges conform, in general, to the expected re lationship with spreading rate, except in the central Scotia Sea, wher e a proposed Miocene slow-spreading ridge appears to have left no clea r signature. The spreading ridge axis in the east Scotia Sea comprises seven or more segments, separated by small, mainly sinistral, offsets and exhibits a median valley with depths of 200-1000 m that is reflec ted in free-air lows of 10-40 mGal. Near both its northern and souther n termini, the gravity signature of the ridge becomes less distinct, w ith a less pronounced axial low. The northernmost segments of the ridg e are displaced in a right-lateral sense by a feature which appears to represent a southward migrating non-transform offset. Whereas the pro cess of spreading in Drake Passage and the east Scotia Sea was compara ble to mid-ocean ridges, that in the central Scotia Sea may have been disorganized, as observed in some western Pacific back-arc basins.