ALBERT

Description: Topographic, magnetic, and earthquake epicenter data from the wholly submerged Central Indian Ridge were interpreted, using the Theory of Plate Tectonics. The pole of relative motion between the Indian and Somalian plates, lying at 16.0° N., 48.3° E. and with opening at 6.2 × 10−7 deg/yr, was obtained from the strike of fracture zones taken as transform faults and the spreading rates based on magnetic anomaly patterns. Since this pole appears to have moved little since the Miocene, the plate positions at that past time can be obtained by finite rotation about the present rotation pole. Such a reconstruction shows that the complicated nature of the present plate margins results from Miocene to Recent opening along a north-south fracture zone that existed in this area during an interval of rapid spreading in the late Cretaceous and early Tertiary.

Description: Since Paleozoic time, the development of sedimentary basins on the continental margin of southern Africa has been controlled by the structures formed during the breakup of Gondwanaland. In Mozambique, the earliest rift (∼180 m.y. B.P.), between East and West Gondwana, produced a north-south–trending series of large horsts and grabens which were buried beneath detritus from the Limpopo and Zambezi river systems. Oceanward sediment dispersion was controlled by the Mozambique Ridge. This stage of continental breakup coincided with the establishment of marine conditions in the older, epicontinental basins which lay over the present-day Agulhas Bank and off the Transkei and Natal coasts (Outeniqua, St. Johns, and Durban basins). When West Gondwana broke up (125 to 130 m.y. B.P.), a large sediment wedge (Orange Basin) was initiated on the west coast of southern Africa by discharge from the Orange River and associated rivers onto a downfaulted, tensional-formed margin. At the same time, a large transform fault (Agulhas fracture zone) truncated the Outeniqua to Durban Basins as the Falkland Plateau separated from south and east Africa. These movements resulted in the formation of new ocean basins and the enlargement of older adjacent ones. Subsequent major sea-level movements are attributable to epeirogenic/eustatic events which are possibly related to variations in world-wide ocean-ridge spreading rates. Most variations in sediment accumulation rates are related to the distribution of marginal traps rather than differences in detrital discharge rates from the major river systems.

Description: The Jotun nappe is composed of relatively mafic allochthonous rocks that may have been emplaced by: (1) overthrusting of a rigid plate from a great distance, (2) igneous intrusion, (3) formation of recumbent folds from a near or distant source. A source area for the Jotun nappe has never been demonstrated. A gravity profile shows a positive Bouguer gravity anomaly of about 34 mgal over the Jotun nappe. Gravity interpretation yields models with maximum thickness from 8 to 15 km for a trough-shaped area of the Jotun nappe. The gravity model can be interpreted in terms of an overthrust plate or of locally derived folded nappes similar to the Swiss Alps, but locally derived folded nappes seem to be most consistent with available geological and geophysical data.