Cape Verde Threshold

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The Cape Verde Sill is a submarine threshold region in the eastern North Atlantic . Their emergence is likely to be related to rising mantle convection .

geography

The position of the Cape Verde sill in the north-east Atlantic, outlined in yellow

The Cape Verde threshold, engl. Cape Verde Rise , is a pronounced bathymetric anomaly of the eastern North Atlantic. Starting from the African continental margin of Mauritania and Senegal at 17 ° West, it moves in a generally westerly direction towards the Mid-Atlantic Ridge . This separates the Canary Basin in the north from the Cape Verde Basin in the south. Its extension in north-south direction is around 900 kilometers between 14 and 22 ° north latitude.

In the area of ​​the Cape Verde Islands and the associated seamounts (such as the Cabo-Verde-Seamount , Cadamosto-Seamount , Nola-Seamount , Senghor-Seamount, etc.), all of which sit on the threshold of the same name, the depth anomaly reaches a maximum value of + 2200 meters, one of the largest known values ​​for deep sea thresholds worldwide. The anomaly then continues in a weakened form to about 36 ° west.

Fracture zones

The threshold is roughly bounded by two large east-southeast trending (N 105) fracture zones , in the north by the Kane fault zone and in the south by the Fifteen Twenty fault zone and the Jacksonville fault zone . A total of 13 fracture zones have so far been identified in the central part of the Cape Verde Threshold. However, a direct connection between the alignment of the fracture zones and the arrangement of the individual islands is not evident.

Magnetic anomalies

In the area of ​​the Cape Verde Islands, the polarity zones M 0 to M 16 can be identified on the threshold , further towards the continent up to M 25 . The anomalies are roughly perpendicular to the fracture zones (N 015). This means that the islands of 142 to 124 million years BP old, under Cretaceous oceanic crust is underlain (period Berriasian to Aptian ). In the direction of the mainland, the Upper Jurassic crust joins, which reaches back to the Kimmeridgian (155 million years ago BP). The average spreading rate was relatively low at around 1 centimeter / year.

Geoid anomaly

A positive anomaly of the geoid was also found on the Cape Verde Threshold . The deviation has a maximum of + 8 meters between the islands arranged in the shape of a horseshoe, on the rest of the threshold mostly values ​​of + 2 to + 4 meters are achieved.

Heat flow

Heat flow measurements carried out by RRS Discovery on the Cape Verde sill showed increased values ​​of 61 ± mW / m² (oceanic crust of comparable age normally only has 45 mW / m²).

Seismics

Seismic refraction studies on board the Meteor in the central area of ​​the Cape Verde Islands yielded the following results

  • The oceanic crust has a normal thickness of 7160 meters.
  • On its very rough surface, 2000 to 1000 meters thick sediments have been deposited, which lose their thickness towards the ocean.
  • A lower crust of increased density or an upper mantle with reduced density could not be determined.
  • The elastic effective crust thickness T e is 30 kilometers.
  • The Mohorovičić discontinuity, and thus the crust / mantle boundary, averages 12 kilometers deep.

The sediment cover resting on the Cape Verde sill within the Cape Verde archipelago could be divided into four units by Ali among others. Units I and II were deposited between the Upper Cretaceous and the Lower Miocene before the onset of island volcanism. The sediments originate from the African continental margin and thin out towards the interior of the basin. Due to their load, the continental margin experienced a slight flexure. Units III and IV, on the other hand, are submiocene to recent ages and can be traced back to the neogene volcanism of Cape Verde. They were deposited in a concentric flexure around the islands, their thickness increasing towards the islands. The flexure itself is due to the load on the volcanic islands. In addition, the sediments are bent up near the islands, which suggests that the islands are tectonically raised.

Deep sea drilling

Schematized, highly elevated profile along the Cape Verde threshold

Two research wells, DSDP 368 and ODP 659, have so far been drilled on the Cape Verde Threshold as part of the Deep Sea Drilling Project and the Ocean Drilling Program . DSDP 368 reached the Cape Verde Threshold in 3367 meters water depth and passed through 985 meters of sediment, ODP 659 in 3071 meters water depth and drilled 275 meters of sediment. The following stratigraphy resulted (from young to old):

geology

The oldest exposed rocks on the Cape Verde Threshold are 131 to 128 million year old pillow lavas in the interior of May and in the north of Santiago . The lavas have been raised up to 4000 meters and partly tectonically shifted. On Maio, lavas of the submarine substructure are concordantly overlaid by a pelagic carbonate series that reaches back at least as far as the Valanginian .

The actual, neogene volcanism began in the east of the archipelago in the Burdigalium 20 million years ago. It then reached its peak in the period between 15 and 7 million years in the Middle and Upper Miocene - the islands experienced their most significant growth spurt during this period. After a strong leveling phase during the Pliocene, which accumulated up to 2000 meters of sediment concentrically around the islands, the volcanic activity is currently limited to Fogo . Fogo, Brava and the Cadamosto Seamount are currently seismically active .

causes

The increased heat flow and the positive geoid anomaly in the area of ​​the Cape Verde Threshold as well as the neogene volcanism of the Cape Verde Islands and the Seamounts suggest dynamically rising mantle rock (as mantle plume , mantle diapir or convection cell ) below the threshold and possibly indicate a hotspot . So-called underplating ( pushing a crust / mantle segment with anomalous density) and thermal re - heating of the lithosphere. as alternative explanatory mechanisms are unlikely due to the normal crust structure.

Individual evidence

  1. ^ Williams, CAU a .: Fracture zones across the Cape Verde Rise, NE Atlantic . In: Journal of the Geological Society . tape 147 . London 1990, p. 851-857 .
  2. ↑ Monnereau, M. and Cazenave, A .: Depth and geoid anomalies over oceanic hot spot swells: a global survey . In: J. Geophys. Res. Band 95 , 2000, pp. 15429-15438 .
  3. ^ Courtney., C. and White, R. S .: Anomalous heat flow and geoid across the Cape Verde Rise: evidence for dynamic support from a thermal plume in the mantle . In: Geophysical Journal of the Royal Astronomical Society . tape 87 , 1986, pp. 815-867 .
  4. ↑ Pim, J. et al .: Crustal structure and origin of the Cape Verde Rise . In: Earth and Planetary Science Letters . tape 272 , 2008, p. 422-428 .
  5. ^ Ali, MY, Watts, AB and Hill, I .: A seismic reflection profile study of lithospheric flexure in the vicinity of the Cape Verde islands . In: J. Geophys. Res. Band 108 , 2003, p. 2239 , doi : 10.1029 / 2002JB002155 .
  6. ↑ Mitchell, JG, Le Bas, MJ, Zielonka, J. and Furnes, H .: On dating the magmatism of Maio, Cape Verde Islands . In: Earth Planet. Sci. Lett. tape 64 , 1983, pp. 61-76 .
  7. Jump up ↑ Heleno, SIN and Fonseca, JFBD: A seismological investigation of the Fogo volcano, Cape Verde Islands: Preliminary results . In: Volcanol. Seismol. tape 20 , 1999, p. 199-217 .
  8. ^ Sleep, NH: Geophysics - a wayward plume . In: Nature . tape 378 , 1995, pp. 19-20 .
  9. ↑ Phipps Morgan, J., Morgan, WJ and Price, E .: Hot spot melting generates both hot spot volcanism and a hot spot swell? In: J. Geophys. Res. Band 100 , 1995, pp. 8045-8062 .
  10. ↑ Detrick, RS and Crough, ST: Island subsidence, hot spots, and lithospheric thinning . In: J. Geophys. Res. Band 83 , 1978, pp. 1236-1244 .