600 Million Years of African Geology

600 Million Years of African Geology • Effects of Continental Collisions dominant from 600 Ma to 200 Ma: Panafrican, Laurussian and Pangean assemblies • Those of Mantle Plumes dominant since 200 Ma. TRISTAN 131 Ma CAMP 201 Ma eo Bas fL AFAR 31 Ma KARROO 183 Ma er e h p s itho DECCAN 65 Ma MANTLE 2 very low velocity zone 1 CORE 2 1 0 THOUSANDS OF KM FIVE GIANT PLUMES THAT IMPINGED ON THE BASE OF THE LITHOSPHERE IN THE AFRICAN REGION. KARROO PINNED THE LITHOSPHERE FROM 180-130 Ma AND AFAR FROM 30-5 Ma. BASINS, SWELLS AND RIFTS THAT FORMED DURING THE TWO EPISODES OF ARREST WERE CRITICAL TO OIL AND GAS DEVELOPMENT. OLDEST SOUTH ATLANTIC OCEAN FLOOR: •M11 (131Ma ?) Off Durban *M4 (126 Ma ? ) Off Namibia 3 anomalies strike obliquely into the shore between the Cape and the Walvis Ridge. *Oldest Marine faunas Ca.126 Ma. *Subaerial ocean floor formed north of the Walvis ridge: No Magnetic anomalies and No marine faunas Africa and South America began to separate ca 126 Ma (end Barremian) about 7 My after Tristan Plume erupted and 14 My after rift initiation on the ocean site. Strike-slip motion began at ca 126 Ma (Neves data) SOUTH Potiguar AMERICA AFRICA Tristan Plume Erupted ca 133 Ma (Hawkesworth) S. Atlantic magnetics ca 126 Ma Durban Basin magnetics ca 126 Ma (Karen Marks) End of a quiet time for Afro-Arabia Igneous activity integrated for the 65-35 Ma interval CONTROLS ON AFRICAN CHANGE DURING THE PAST 34 MILLION YEARS East Antarctic Ice Sheet: 34 Ma to now West Antarctic and Greenland Ice Sheets: 14 Ma to now Sea Level lowered at 34 Ma again at 14 Ma Many Submarine Canyons initiated at 34 Ma Pinning by Afar plume 31 Ma Shallow Mantle convection set up 31 Ma). Persists today Basins and Swells developing over shallow convection pattern from 31 Ma till now Parts of Eastern Rift active 31 Ma till now Zagros collision at 15 Ma Western Rift active from 15 Ma till now Arabia-Somali-Nubia plates distinct beginning ca 15 Ma Climate changes: 34 Ma, 14 Ma. Indian ocean monsoon. Oscillations since 3 Ma (Sahara) linked to northern hemisphere glaciations Published ages of igneous rocks on the African Continent showing an increase in activity at ca. 30 million years ago Image courtesy of The Geological Society of South Africa. Image courtesy of The Geological Society of South Africa. Volcanic rock ages from four places showing that activity has persisted in the same ca 200 km diameter areas for ca. 30 million years. More could have been plotted.Significant progression with a consistent azimuth has not been demonstrated. Gripp & Gordon’s 1990 sketch showing slowly moving Africa in a hot-spot reference frame over ca.3.7 Ma. Since then slow relative motion of Nubian, Arabian and Somali plates over the same interval has been discerned . Numerous hot-spot tracks have been reported from the African plate. Only the track of Tristan forming the Walvis Ridge stands up to scrutiny.Reunion is trackless. At Dakar a shallow-sourced Mantle plume has been erupting for ca 25 My. basement is elevated but the “Mammelles” volcanoes are at sea level because the Senegal and Casamance Rivers have eroded the rising dome. Thick sediments eroded from what is now the Sahara are in deep water Offshore. In the Ahaggar in the center of the Sahara a swell ca.1 M sq km in area has raised basement to ca.3 km asl in the past 30 My. Volcanic rock volume, as is typical for the shallow-sourced mantle plume derived rocks of the African Plate, is very small. Erosion of this swell carried a lot of sediment to the Niger delta before the Sahara desert first formed at ca.3 Ma. The Jos plateau in Nigeria is a swell with 12 young crestal basaltic cones The Chad Basin is surrounded by 11 volcano-capped swells arranged in an ellipse. <-Dakar Possibly this relates to a shallow-mantle convection pattern.