Abstract: A Comparison of Offshore Namibia Gravity Data With Onshore Magnetic and Gravity Data

Corner, Branko - Corner Geophysics; Roger Swart - NAMCOR

Offshore Namibia satellite gravity are juxtaposed, in co-registered images, against the aeromagnetic data and Bouguer gravity data of southern Africa. The objectives of this study are to elucidate any new structural and lithological features within the offshore magnetic basement, and to compare these to known major onshore structures. In so doing a more comprehensive and cohesive structural map of the onshore-offshore area is facilitated, allowing conclusions to be drawn with respect to potential locations of major drainages and sources of sedimentation into the offshore domain.

The satellite data, compiled by GETECH comprise satellite altimetry from the closely spaced Geodetic missions of both GEOSAT and ERS-1, combined with the available bathymetry data in order to achieve the best possible resolution. Bouguer, Free Air, bathymetry and isostatic residual anomaly data were provided. The onshore aeromagnetic data and Bouguer gravity data are derived from the Geological Survey of Namibia and Geodass (Pty) Ltd.

A number of features may be highlighted from the combined data sets:

- A large circular gravity high is centered some 50 km southwest of Walvis Bay. It lies on the direct offshore extension of the Omaruru lineament. Offshore seismic reflection data indicate a thickened sequence of Palaeozoic-Mesozoic basalts and sediments. We interpret these to overlie an intrusion of similar origin to the Erongoberg which also correlates with a deep seated gravity high and is capped by basalts and sediments.

- A major linear northeast-trending gravity high is seen to flank the Walvis Ridge in the north, and is interpreted to be associated with transcurrent faulting.

- At least three major coast-parallel gravity highs are seen in the offshore data. The first lies approximately 100 km offshore and continues along the entire length of the Namibian coastline. The other two, occurring up to 250 km offshore, are not as continuous, being more obvious south of the Walvis Ridge. Light et al (1992) first reported on the former anomaly, detected at that time by relatively sparse marine gravity profiles. They favour a model in which the anomaly arises from a continuous mantle wedge displacing less dense continental material. They do not however discount the possibility of an intrusion as the source of the anomaly. We favor the latter model for these relatively narrow, semi-linear features, interpreting them to be Mesozoic intrusions associated with extension during the breakup of Gondwana.

Perhaps the most striking result of this study is that these Mesozoic anomalies, irrespective of their origin, have been clearly faulted along the offshore continuations of the Pan-African Omaruru lineament and two Namaqua age lineaments seen in the onshore gravity and magnetic data, including the Pofadder lineament. This indicates that these early Palaeozoic-Late Proterozoic structures were active until Late Mesozoic times. An important implication arising from this conclusion is that these structures provided potential pathways for major drainage systems and hence the focus of major offshore sedimentation.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil