Gondwana’s Demise:
Exploration into the third and fourth dimensions
WEBINAR
1st November 2023 19:00 GMT
by Professor Colin Reeves
(formerly at ITC in Delft, The Netherlands)
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Abstract
Modelling continental movements involves working backwards in time. The more recent past, back to about 84 Ma (Santonian), is reasonably well-defined by marine magnetic anomalies worldwide. Reaching back further towards Gondwana reassembly involves extrapolation across another 100 Myr, the first 40 Myr of which is the Cretaceous Quiet Zone, devoid of such anomalies. Once mapped in detail by satellite gravimetry (released about 1997), the retracing of ocean fracture zones provided an important new constraint. Unresolved issues surround the ‘stopping-point’ of the process (i.e. reassembly) but consensus appears now to converge on a tight reassembly where only the Precambrian crust now lost below younger cover in rifting and crustal extension zones need be considered to have separated the present-day terranes that outcrop or sub-outcrop inboard of the continental margins. The continental margins themselves, of course, represent an enormous potential economic resource, not to mention the geological record of rifting and early drifting.
No continent is fixed to Earth’s axis of rotation. Africa, the largest Gondwana fragment, has been moving steadily NE throughout Gondwana dispersal. This movement can be measured directly today (by geodetic GPS) at 20-30 mm/yr or 20-30 km/My. Our model has its motion as 23 km/My averaged over the last 10 My. The movements of the other continents with respect to Africa have been determined by studying the ocean-floor data (topography and magnetic anomalies) and the whole dispersing system of continents may then be related to ongoing convection in the earth’s mantle by relating the magma output in the large igneous provinces (LIPs) to the fixed location of plumes. A perhaps surprising result is that the mid-ocean ridge system that now separates the Gondwana continents is essentially fixed to Earth’s rotation axis.
An often overlooked (or under-estimated) LIP is associated with the Bouvet plume that has been active since the ‘Karoo’ igneous episode (183 Ma) and is now close to the Bouvet triple junction, south of the Atlantic Ocean. Working out the behaviour of this triple junction during the Early Cretaceous reveals a central element of Gondwana dispersion. From only modest separation of East and West Gondwana by the end of Jurassic times (143 Ma), most of the present-day continental outlines had been established by the start of the Albian (113 Ma) with Antarctica, the second-biggest Gondwana fragment, undergoing a 15-degree rotation about a point in the southern South Atlantic, as recorded in the ocean off SE Africa. The working of the triple junction as a whole is illustrated in animation at https://www.reeves.nl/gondwana/aac-anim-1.
Biography
Colin Reeves studied at Cambridge, Birmingham and Leeds universities. He has had a long career in the geophysical mapping of unseen geology. His first professional position involved the initial exploration of the geology hidden below Botswana’s Kalahari Sand using gravity and aeromagnetic survey, 1970-78. Primary reconnaissance of such large areas is only practical using airborne methods and from his subsequent positions in the Canadian airborne survey industry he worked on a succession of national geophysical coverages in Africa and elsewhere (Cote d’Ivoire, Brazil, Burundi, Kenya, Tanzania), all designed to stimulate economic investment in resource development. He was appointed in 1983 as Professor in Exploration Geophysics at ITC in Delft, an institute dedicated to the making and interpretation of maps and images for international development objectives. After more than 20 years, a reorganisation led to his early retirement in 2004 when he continued his consulting role in international aid projects independently, taking on projects in Nigeria, Mozambique, Madagascar, Uganda and Malawi. He has also worked extensively in Australia, India and South Africa. These new insights into the unseen geology of so many parts of the southern hemisphere led to his interest in Gondwana as a whole and the tectonics of its dispersion. Interpretation of ocean-floor imagery, new in 1997, took him into geodynamic modelling and his attempt to understand more clearly the geometry of creation of the oceanic crust that now separates the Gondwana continents. Still living in Delft, he continues this work in his retirement.