Much of our work centres on modelling quantitatively the behaviour of mid-ocean ridges. In large areas of ocean, ridges are typically stable and predictably mid-way between conjugate margins (see Animation H). Where three ridges meet at a triple junction, such as the Bouvet triple junction, instability occurs rather more often, leading to a complex tectonic history. We offer here a working model with a limited number of ridge jumps to explain the observed complexity in the ocean floor topography and the origin of the three submarine edifices - the Agulhas plateau, the NE Georgia Rise and the Maud Rise - that were created at the triple junction during Cretaceous time. Six changes in ridge function and position, highlighted in yellow, merit special mention.

1. At the end of Jurassic time (142.3 Ma), the long offset separating East and West Gondwana off SE Africa turned into a rapid spreading axis in the Weddell Sea, shown by extensive M-series marine magnetic anomalies mapped there. Further north, the offset was initially a dextral strike-slip fault between the Maurice Ewing Bank (MEB - still fixed to Africa) and Limpopia (still fixed to Antarctica). Both attachments progressively ended in the following 20 My.

2. By 121.4 Ma (M0, start Aptian), MEB had moved SW along the Agulhas FZ by almost 300 km and Limpopia had escaped Antarctica and become firmly attached to the Africa plate. The velocity at which (a) Antarctica was leaving Africa and (b) MEB (not yet fully attached to South America) was moving along the Agulhas fault were closely similar for the next 40 My. The mid-ocean ridge to these systems were joined and increased in length to the NW and S respectively. Ocean growth between Antarctica and South America (strictly, the Agulhas plateau) slowed (vide Anomaly-T in the Weddell Sea) from this time.

3. At about 99 Ma, the trace of the earlier long-offset fault between Antarctica and Africa went into transtension, providing the active SAM-AFR ridge, immedaitely south of the Agulhas fault, a shorter path to the SE, stranding a triangular area of the 121-99 Ma ocean on the South America plate. This area included the NE Georgia Rise, erupted on the by now-extinct ridge at about 105 Ma. The new ridge location (note its central location above the Bouvet plume head) erupted the bulk of the Agulhas plateau around 95 +/- 5 Ma.

4. About 86 Ma, the SAM-AFR ridge jumped again, this time to a more south-directed path, leaving the Agulhas plateau stranded on the Africa plate. As with previous jumps, its position immediately south of the Agulhas FZ changed but little while its orientation further south changed markedly. The new ridge orientation is preserved in the C34 marine magnetic anomalies off Africa and Antarctica that mark the end of the Cretaceous Quiet Zone (121.4-83.64 Ma). Note that the new ridge orientation leaves only a triangular area of SAM-AFR ocean created in the interval 99-86 Ma attached to the South America plate, labelled 460 on the animation. Marine magnetic anomalies C34 and C33 are left on the eastern margin of fragment 460 early in this set up.

5. Towards 75 Ma the active ridge reorganised to a slightly more westerly position. The new ridge orientation was active for at least 10 My, even penetrating slightly N of the FAFZ before meeting its demise. This is well supported by marine magnetic anomalies with these dates.

6. At about 65 Ma the active ridge made its biggest jump, to west of fragment 460. The long (1100 km) offset on the FAFZ was finally abandoned after more than 60 My. The mid-Atlantic ridge south of the FAFZ now lay only 250 km NE of the NE Georgia Rise with only a short transform offest across the FAFZ. Remarkably, the initial trace of the new ridge appears to replicate the form and location on the South America plate of the new margin first adopted there at 99 Ma. This alignment is now preserved on the Africa plate as the NE 'escarpment' of the Agulhas plateau along with some 2000 km of ocean crust to its west, preserving there the tectonic complexity of ocean growth in the interval 99 to 65 Ma. Almost 2500 km of new ocean crust has been created between Fragment 460 and South America since the present-day mid-ocean ridge was established at 65 Ma.

Regional geophysical interpretation of an area has always been about honouring what is known of the geology and making credible predictions about what may exist where the geology is unknown. The ocean created by the Bouvet triple junction covers some 12 miilion km2, more than one third the area of Africa. There is precious little geological information to constrain an intepretation of its tectonic history. In practice, a conservative plate tectonic interpretation has proven exceedingly dificult to find, consistent with the record of ocean-floor topography and the rules of global tectonics. There is already enough data to paint the interpreter into a corner, as it were. The model presented here will certainly contain errors in detail but here is a working model that is worthy of consideration at a broad scale that may be refined. Refinement may prove more productive than trying to start over and eliminating again all the untenable possibilities I have tried while developing what is shown here. Making the supporting evidence explicit is beyond the scope of this web page but I am always pleased to hear of anything I may have missed that could impact the solution.