NEOTECTONIC ACTIVITY ON CONTINENTAL FRAGMENTS IN THE SOUTHWEST INDIAN-OCEAN - AGULHAS PLATEAU AND MOZAMBIQUE RIDGE

The Agulhas Plateau and Mozambique Ridge are composed in part of conti nental fragments embedded within oceanic crust of the southwest Indian Ocean. Recent studies of the plateaus and their vicinity have discove red significant evidence for neotectonic activity. In both areas, newl y obtained seismic reflection profiles indicate possibly young basalti c intrusions in the northern, oceanic parts of the plateaus. Small roc k fragments recovered from the southern Mozambique Ridge comprise meta morphic and volcanic lithologies. The volcanic rocks are made of extre mely fresh quenched glasses. Although no radiometric dates are availab le for the volcanic glasses, their lack of any significant alteration suggests that eruption took place in the last few tens of thousands of years, supporting the seismic reflection evidence for magmatic activi ty in this region. The active crustal stretching and tensional stresse s implied by this relatively recent tectonism probably cannot be gener ated by distantly applied plate-driving torques, such as ridge push, b ut appear to require buoyancy-related forces originating in the underl ying upper mantle.