1989 Theses Doctoral

High resolution marine and airborne gravity surveys: applications to rifted margins

Bell, Robin E.

Analysis of carefully navigated marine gravity surveys over an eastern U.S. gravity test range and in the Bay of Naples documents the state of the art in marine gravity. The sub-mgal accuracies obtained with adequate navigation from an axially symmetric gravimeter, the BGM-3, are a significant improvement over the accuracies available from earlier asymmetric systems such as the GSS-1.

More than an order of magnitude faster than marine gravity, airborne gravity is a versatile tool for surveying inaccessible regions. Comparing a Lacoste-Rombergair sea meter and the BGM-3 revealed that the Lacoste system is currently better suited for airborne work with accuracies on the order of 4.5 mgal observed. Both airborne gravity and Geosat derived gravity resolve anomalies with amplitudes on the order of 10 mgal with widths of 15 - 20 km. OPS phase measurements provide velocities that can be used to correct the measured acceleration for the vertical motions of the aircraft.

Airborne gravity flights across the margins of the western Weddell Sea collected over 16000km of gravity data. The shelf edge mapped in the DBDB5 bathymetric compilation is misplaced up to 100 km based on the peak in the edge effect anomaly recorded in the airborne gravity. The southwestern Weddell margin has none of the complex rifting structures observed in the eastern Weddell suggesting a different rifting history for these two segments. The peninsula margin is divided into early and late rifting segments. The late rifting northern segment appears to have been affected by the subduction continuing beneath the peninsula through the Cenozoic.

Gravity across the Triassic rift basins of the eastern U.S. margin was used to constrain the distribution of extension associated with these asymmetric basins. The gravity high over the hanging wall can be modelled as extension transferred laterally into the lower crust by a low angle detachment. The fit to the model is improved if the detachment becomes the conduit for the basaltic material within the basin and the dikes in the hanging wall block.