Deep structure of the Namibia continental margin as derived from integrated geophysical studies

During the Geophysical Measurements Across the Continental Margin of Namibi a (MAMBA) experiments, offshore and onshore refraction and reflection seism ic as well as magnetic data were collected. Together with the existing free -air gravity data, these were used to derive two crustal sections across th e ocean-continent transition. The results show that the Early Cretaceous co ntinental breakup and the separation of South Africa and South America were accompanied by excessive igneous activity offshore. Off Namibia we found a 150-200 km wide zone of igneous crust up to 25 km thick. The upper part of this zone consists of an extrusive section comprising three units of basal tic composition: two distinct wedges of seaward dipping reflectors (SDRs) s eparated by flat-lying volcanic flows. The inner wedge of SDRs can be model ed as the source of a long-wavelength magnetic anomaly that borders long pa rts of both South Atlantic margins (anomaly G), The crust underneath these extrusives is characterized by high-velocity and high-density material (ave rage values 7 km s(-1), 3 x 10(3) kg m(-3)). Free-air gravity anomalies alo ng both sides of the high-density crust are interpreted as edge effects res ulting from juxtaposition with normal oceanic and continental crust on eith er side. We define the abrupt landward termination of this zone as the cont inent-ocean boundary, and consequently, the crust seaward is interpreted as exclusively igneous material and not intruded continental crust. Extrapola tion of the interpreted geophysical features along the southwest African ma rgin suggests a fast prograding narrow rift zone and sharp lithospheric rup ture leading to the formation of a margin-parallel magmatic belt south of t he Walvis Ridge. The influence of the Tristan da Cunha mantle plume may exp lain the widening of this thick igneous crust near the Walvis Ridge.