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Mozambique Plains, Ridge and the role of Limpopia

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Separation of East and West Gondwana had been in a NW-SE direction (Africa coordinates) from onset until about 155 Ma (start Kimmeridgian) when a N-S direction took over. Extension east of the Zambezi delta would have been about 400 km, well into the creation of new ocean crust. Off the Lebombo, however, it would have been only about 200 km, so extended continental crust with continued magmatic activity is expected to predominate there. Limpopia followed Antarctica in this period and continued to do so until much later.
Post-155 Ma N-S movement necessitated a long-offset (1070 km) dextral transform outboard of the Lebombo. We have drawn it 160 km east, continuing to the south to separate the Maurice Ewing Bank and Antarctica. At about 151 Ma the axis of extension (ridge) west of the Beira High jumped about 400 km to the south, reducing the length of this offset by a similar amount. New conjugate margins are to be seen off the northern edge of present-day Limpopia and close the present-day coastline of southernmost Mozambique. A regular ‘staircase’ configuration of the active mid-ocean ridge then extended all the way from the off Lebombo FZ, through the Africa-Antarctica Corridor (AAC), to the Davie FZ.
By the start of the Cretaceous (142.3 Ma) almost 400 km of extension had occurred between Limpopia and onshore southern Mozambique. At this time rifting between Limpopia and Antarctica started and was eventually to replace the growth of ocean north of Limpopia.
In the Hauterivian (about 134 Ma), pressure from Antarctica moved Limpopia slightly westward, displaced Maurice Ewing Bank (MEB) from its starting position off the KwaZulu-Natal coast and opened rifts in southern onshore Mozambique. A small block of crust from the Limpopia-Mozambique extension was moved westward along the KwaZulu-Natal coast.
By about 128 Ma the long offset transform west of Limpopia had become extinct and had been replaced by a 880 km dextral transform offset immediately to its east (an eastward jump of about 300 km) and Limpopia had become a fixed part of the Africa plate. An extension from Mozambique of about 690 km had been achieved since 151 Ma, an average growth rate of 31 km/My. This compares with 945 km within the Africa-Antarctica Corridor (AAC) in the same period (43 km/My).
By 124 Ma the fragments off SE Africa had all stabilised onto the Africa plate, the MEB was similarly becoming fixed to the South America plate and a ridge-ridge-ridge triple junction was established off SW Limpopia. This was to prove long-lived. By this time Antarctica was pursuing a new, arcuate path towards the SW while the mid-ocean ridge between it and Africa was still well north of Limpopia. This created the observed curvature on the Africa side of the AAC that is absent from the Antarctic side, Limpopia (Mozambique Rise) providing an obstacle to direct westward movement of the Antarctica plate.
A consequence of the model is that onshore southern Mozambique – the Mozambique Plains – is predicted to consist of extended continental crust hosting a failed ridge system and a copious supply of Early Jurassic magma. The offshore crust, south of Maputo, meanwhile, is predominantly oceanic, post-dating 151 Ma (Kimmeridgian). Remarkable is the absence of magmatic activity east of the N-S line outboard of the Mozambique Ridge and its extension to west of the Biera High. Ocean growth in the AAC proceeded steadily about a central mid-ocean ridge throughout the period under discussion.
Last updated: 2023 October 12