University:
Massachusetts Institute of Technology
Course:
12.520 | Geodynamics
Academic year:

2023
Views:

239

Pages:

16
Author:

Steven Jennings

AFRICAN PLATE PLUME TRACKS Now you see them! Now you don’t Map largely based on Morgan 1971 which linked active hotspot volcanoes to as few as one igneous rock along track. For example: Sierra Leone norite (200 Ma) was linked to Ascension( which has no track). More upto-date ages and bathymetry from satellite altimetry show this practise is unjustified Reunion is now known to be trackless Tristan, (shown here as a pair of tracks) is the only active hotspot volcano with an established track. For all the rest activity is within a 300 km area and ages are < 30 My. Tristan hotspot activity within a 500km x 200km ellipse centered 800 km East of ridge axis over past 31 My The S.Atlantic spreading center axis has moved west at ca.25 mm/yr with respect to the Tristan hot spot in 31 My Ocean floor that has formed since 30 Ma around Africa to show location of Tristan Tristan hot-spot track Etendeka on land (134 Ma) to Tristan (30 Ma to 0 Ma). Wispiness of young track shows supply of energy and rock (?) from deep mantle has declined with time. 3 He is now low No mappable ridge jumps ( they are all on the S. American side) Rate of propagation has been uniform. Plume has stayed on spreading center till 30 Ma Reunion not Deccan Deccan plume hotspot track died at Nazareth bank/CC 31 Ma. Reunion, Mauritius and Rodrigues ridge volcanics are typical African plate style hotspot products showing no progression in age. Background green bathymetry is from Early Cenozoic fracture zones unrelated to hotspot volcanism. DOES THE PATTERN OF IGNEOUS ACTIVITY ON THE AFRICAN PLATE TELL MUCH ABOUT THE MANTLE CONVECTION PATTERN BELOW ? DEEP: AT CMB LIPS COME ONLY FROM EDGES OF LLVPS AT SHALLOW DEPTHS: THERE ARE A LOT OF CLUES BUT THE MESSAGE HAS HARDLY BEGUN TO BE READ Location of Afar plume on the edge of the African LLVP A large fraction of the shear wave velocity variatio CMB is concentrated close to the edges of the two Velocity Provinces (LLVPS of Lay 2005) . Plumes generating and hotspots have risen from t CMB at those edges. No volcanoes on cratons. There are swells but lithosphere is too thick for base (even where elevated under a swell) to cross basalt solidus An attempt to see if crests of swells are regularly spaced Thiessen’s spacing results for Hotspots And Highspots A possibly Relevant Model Richter & Parsons (1975) Intermittent igneous activity in small areas (~ 300 km diameter) shows that the African plate has not moved with respect to the underlying mantle convection pattern for ca.30 My. Four of many areas selected.Sites of rising plumes discerned ? THE CAMEROON LINE A SET OF 10 SMALL(d~ 100 km) VOLCANO-CAPPED SWELLS HALF ON CONTINENT AND HALF ON OCEAN FLOOR Cameroon Line