Gondwana

Global Thoughts

The most recent and complete account in the literature, summarizing the work that has been undertaken over many years, is cotnained in Reeves et al., (2016): 

Reeves, C.V., Teasdale, J.P., & Mahanjane, E.S., 2016.  Insight into the Eastern Margin of Africa from a New Tectonic Model of the Indian Ocean.  Geological Society Special Publication No. 431:Transform Margins: Development, Controls and Petroleum Systems(eds NemĨok, M., Rybár, S., Sinha, S.T., Hermeston, S.A., & Ledvényiová, L.).  

Some of the considerations incorporated into that model in recent years and, in some cases, their more recent revisions are set out below. It is planned that further updates on our trains of thought will appear at the end of this page as they develop.

Updated 2016 July 22

 

Antarctica<>Africa  and India<>Anrtarctica better constrained

Key new constraints from conjugate M-series magnetic anomalies came in 2012-3 from the work of König and Jokat (2010) off Mozambique and Antarctica. The relative positions of Africa and Antarctica through the Jurassic and Early Cretaceous (until the KNS at ~125 Ma) became much less ambiguous. The ANT<>AFR relative movement over much of this period (~153-127 Ma) is parallel to the main or central part of South Africa’s Lebombo ‘monocline’ which I envisage as having the role of the continent-continent transform in this interval. This movement may be approximated to a single interval pole (~153-127 Ma) that fits well to the König and Jokat finite poles and has the central Lebombo lying on an Euler small circle.  Without changing the Reeves et al (2002) Gondwana fit substantially, the earlier ANT<>AFR movement (~183-153 Ma) may then be achieved with an interval pole that shows ANT<>AFR movement perpendicular to the strike of dykes from the Limpopo valley to the Angoche swarm along the coast of Mozambique east of the Zambezi estuary. The Konig & Jokat (2010) finite rotation pole at 153.19 Ma has been taken as a turning point in this oceanic evolution with the onset of strike-slip along the Davie Fracture Zone. This is the pattern of events incirporated into CR15GSCB (Reeves et al, 2016).

More recently (July 2016) the marine magnetic anomalies M22 have been adopted as a waypoint at 151.4 Ma.  These have been identified both in the Konig & Jokat (2010) data and in older data in the ocean between Madagascar and Africa. While we still adhere to the precise ANT-AFR fit of the youngest M-series anomalies (towards 126 Ma), we now note that the oldest fractures zones on the Antarctica plate, immediately west of the Gunnerus Ridge, show a distinctly different direction of growth from that predicted by CR15GSCB. A better fit with the FZ data is achieved by retaining the match of conjugate M22 anomalies ANT-AFR but rotationg the whole of ANT anti-clockwise by about 6 degrees about this fit.  This requires a rather more curved path in the AAC during the period defined by the anomalies which must be continued furtehr into the past to conform to these early Antarctic FZs. We estimate 165 Ma as a time when this pattern of ocean growth started and determined an earlier path for the whole of East Gondwana (including Madagascar) against Africa that achieved (very nearly) the CR15GSCB fit.  This new, earliest motion give rather more extension (in the transtensional sense) along the Lebombo and hence we have called it the 'Lebombo First' option.

Updated 2016 July 22

 

Africa - Madagascar - India

The relative movements of MAD<>AFR and MAD<>IND have been poorly constrained by data. Sandwell (2014) published more detailed ocean-floor topograhic data which has been imaged by Ian MacLeod at Geosoft for our use.  This resolves the trajectory of MAD against AFR more clearly and this new movement has fitted well into the remainder of our model.  A fit of all fragments of East Gondwana is then achieved at 140 Ma, including the closure of the plate-circuit in an elegant way between IND and MAD.  East Gondwana can thus remain intact throughout the early break-up of Gondwana, 183-140 Ma, in model CR16AAGE. 

The arcuate shape of the Davie Fracture Zone (DFZ) off Mozambique and Tanzania is now (CR16AAGE) used to define the movement of Madagascar+India against Africa in the interval 140 to 120 Ma.  By definition, then, the opening of IND away from ANT+AUS must start in the same interval - in agreement with marine magnetic anaomlies off Western Australia.

In this way, the issue of strike-slip between India and Madagascar before about 140 Ma has been eliminated and replaced by MAD-IND rifting (starting at about 135 Ma) followed by dextral transtension as India continued southward, even after Masgascar came to rest as part of the Africa plate.

An Aptian Side-step? 

In the interval (127-124 Ma approximately a temporary change of spreading direction between Antarctica and Africa is noted in the AAC fracture zones off Dronning Maud Land, Antarctica. This coincides with the period in which we expect teh demise of ocean-floor spreading between MAD and AFR and the instigation of rifting between MAD and IND that we see continuing until about 100 Ma. Even by the time India starts its rapid NE journey at 88 Ma, the opening between it and Madagscar is no more than the space necessary to create the Mascarene fragments by crustal stretching and/or igneous intrusion and extrusion. This process may be observed in the animation at the head of this section.  This dextral transtensional rifting direction between Madagascar and India (in the Aptian and Albian) is evident in the satellite gravimetry but still invites investigation on the ground in the passive margins concerned. The most recent adjustments to the model ensure that there is only ever extension (i.e. no compression) between India and Madagascar.

The more detailed study of the movements of Antarctica against Africa in the period 100 to 130 Ma approximately indicates a singular short-lived movement of Antarctica+Australia near the start of this period. This I see as part of the process of transition from East Gondwana and West Gondwana being the only plate-tectonic players before about 136 Ma to a situation where there is essentially only ‘North Gondwana’ (Africa, Madagascar, India) and ‘South Gondwana’ (South America, Antarctica, Australia) after about 120 Ma. North Gondwana is one plate for about 12 myr (100 to 88 Ma, enirely within the Cretaceous Quiet Zone that lacks magnetic anomalies) as South Gondwana is driven away from it by the actions of the three mantle plumes, Tristan, Bouvet and Kerguelen. The timing of the reorganisational event (127-124 Ma, Barremian to Aptian) also coincides with the breaking through of the Equatorial Atlantic and ocean spreading throughout the Atlantic becoming essentially coast-normal.

Updated 2016 July 22

 

Latest Research Updates

Research Update No.1 (June 2016) - [click here]

Research Update No.2 (July 2016) - [click here]

Research Update No.3 (October 2016) -  [click here]

Research Update No.4 (November 2016) - [click here]

Research Update No.5 (February 2017) - [click here]


Please get in touch with feedback!  
reeves.earth@planet.nl

CVR
2017 February 23.