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| Titel |
Sedimentary Flux to Passive Margins From Inversion of Drainage Patterns: Examples from Africa |
| VerfasserIn |
Bhavik Harish Lodhia, Gareth G. Roberts, Alastair Fraser |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250143053
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| Publikation (Nr.) |
 EGU/EGU2017-6742.pdf |
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| Zusammenfassung |
| We show that inversion of more than 14000 rivers from the African continent provides
information about Cenozoic uplift and sedimentary flux to its passive margins. We test
predicted sedimentary flux using a dense two-dimensional seismic dataset offshore northwest
Africa. First, six biostratigraphically dated horizons were mapped (seabed, 5.6 Ma, 23.8
Ma, 58.40 Ma, 89.4 Ma and basement) across the Mauritanian margin and used
to construct isopachs. Check-shot data were used to convert time to depth and to
determine best-fitting compaction parameters. Observed solid sedimentary fluxes are
∼2x103 km3 /Ma between 58.4 and 23.8 Ma, ∼4x103 km3 /Ma between 23.8 and
5.6 Ma, and ∼28x103 km3 /Ma between 5.6 and 0 Ma. Compaction errors were
propagated into our history of sedimentary flux. Secondly, we inverted our drainage
inventory to explore the relationship between uplift and erosion onshore and our
measured flux. The stream power erosional model was calibrated using independent
observations of marine terrace elevations and ages. We integrate incision rates along
best-fitting theoretical river profiles to predict sedimentary flux at mouths of the rivers
draining northwest Africa (e.g. Senegal). Calculated Neogene uplift and erosion is
staged. Our predicted history of sedimentary flux increases in three stages towards
the present-day, which agrees with the offshore measurements. Finally, using our
inverse approach we systematically tested different erosional scenarios. We find that
sedimentary flux to Africa’s passive margins is controlled up the history of uplift and
erosional processes play a moderating role. Predicted fluxes are indistinguishable if
precipitation rate varies with a period less than ∼ 1 Ma or drainage area varies by less than
50%.
To investigate the geodynamic setting of the Mauritanian margin we backstripped eight
commercial wells that penetrate Neogene stratigraphy. Wells in the central part of
the Mauritania basin include 500-800 m of Neogene water-loaded subsidence that
cannot be attributed to extension, thermal subsidence, salt-tectonics or glacio-eustasy.
Stratigraphy mapped across the margin shows that this anomalous subsidence affected an
area larger than 500 by 500 km. We suggest that this anomalous subsidence was
caused by Neogene dynamic drawdown. Conversion of the Schaeffer & Lebedev
(2013) velocity model to temperature and simple isostatic calculations indicate that
negative buoyancy anomalies directly beneath the Mauritanian margin generate up to
500 m of drawdown today. Measured ocean-age depth residuals and calculated
subsidence histories suggest that dynamic uplift of the Cape Verde swell and dynamic
drawdown in the east generated a gradient in dynamic support during the last 25 Ma. |
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