DOES THE MOZAMBIQUE RIDGE BELONG TO MOZAMBIQUE

Field: Earth and Planetary Sciences (Physical Sciences)

The confirmation of the presence of continental fragments in the ocean basins has now moved from a matter of scientific interest to that of political importance as countries seek to expand their boundaries in terms of the new proposals concerning the Exclusive Economic Zone. One such fragment is the Mozambique Ridge that has the potential of being claimed by Mozambique and the Republic of South Africa. The ridge extends from the coast of central Mozambique southwards to 35oS, separating the Natal Valley to the west from the Mozambique Channel to the east, rising up to 3 km above the flanking oceanic crust on either side. Up to now the continental origin of this ridge has been generally accepted, despite the problems that such a fragment imposed on a tight refit of SE Africa and Antarctica. This interpretation was based on dedging results which yielded rocks such as kinzingite that were correlated with similar lithologies in the Kibaran belts of KwaZulu-Natal, South Africa and Drönning Maud Land, Antarctica. Dredging also brought up basalts, which were considered to be overlying this continental basement. These constraints were used for the geophysical modelling of gravity over the ridge.As part of a programme aimed at establishing the break-up events between Africa and Antarctica, a marine geophysical survey was undertaken in region from May to July 2005. The survey commenced just south of the Mozambique Ridge and extended along its full length before covering the Mozambique Channel. Lines were aligned NE-SW and spaced at 12 nautical miles, and the survey involved measuring bathymetry, gravity and magnetics as well as dredging. It revealed that a magnetic reversal pattern occurs along the whole length of the Mozambique Ridge, so the ridge must be oceanic in origin. This pattern could be confidently correlated with the Mesozoic magnetostratigraphic timescale and also matched the magnetics reported on the ocean floor off Drönning Maud Land, Antarctica which was the conjugate margin during Gondwana break-up. This confirmed that the initiation of sea-floor spreading related to Gondwana break-up did not commence in this region during the ca. 180 Ma Karoo igneous event but during the late Jurassic. This interpretation of an oceanic origin for the Mozambique Ridge is at a variance to the gravity models and the direct evidence continental rocks from dredging. The latter may be slivers left along strike-slip faults scarps as the continents separated while the former needs to be revisited in light of these results.As the Mozambique Channel is oceanic crust, its development can be modelled by well-established age-depth equations that allow depths (or heights) of formation to be calculated. Aseismic ridges on oceanic crust can also be modelled in the same way so it is also possible to undertake these calculations for the Mozambique Ridge. These calculations showed that the oceanic crust of Mozambique Channel formed well below sea-level at typical depths for mid-ocean ridges although slightly elevated. In contrast, the results for the Mozambique Ridge revealed that, prior to 132 Ma, the present top of the ridge formed above sea-level, at elevations of up to at least 2.6 km. It is important to note that these values are calculated for the present surface of the Mozambique Ridge and do not take into account any crust that may have been eroded away since 132 Ma.The Mozambique Ridge, therefore, developed seawards from the ocean-continent boundary in central Mozambique into a widening ocean during the late Jurassic break-up of Gondwana. At the very least it formed a volcanic archipelago and may possibly have been a subaerial volcanic plateau, so it is a natural oceanic extension of Mozambique. However this interpretation of an oceanic origin, based on observed magnetics and age-depth modelling, needs to be reconciled with the interpretation of a continental origin, based on observed rocks and gravity modelling, before the claims on the ridge by adjacent countries can be fully assessed.