Linear Inversion of a Negative Gravity Anomaly in Se Rio Grande Cone: a Graben on Oceanic Crust?

We detect, for the first time, a negative free-air gravity anomaly of 23 mGal amplitude over a region in the South Atlantic Ocean centered at 48◦W and 35◦S. To this end, we used the integration of conventional shipborne gravity data and gravity data derived from GEOSAT/ERM satellite altimetry. The north bound of this anomaly coincides with the Chuı́ Lineament and the south bound indicates another lineament, which is the extension of the Meteor Fracture Zone. The anomaly trend is NE-SW, its width is 400 km and its length is 600 km. Two-dimensional linear inversion with relative and absolute equality constraints was used to calculate the density distribution along three profiles perpendicular to the main axis of the anomaly. The result suggests that the sediment thickness in the deepest part of the basin is at least 3.0 km where the ocean bathymetry is 4,800 m. This tectonic feature, an asymmetric half-graben formed between two lineaments, probably lies over an oceanic crust. The estimated volume of sediments in this basin is approximately 50% of the post-Miocene sediments volume deposited in the Rio Grande Cone where gas-hydrates were found.