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| Titel |
Late-stage stretching and subsidence rates in the Danakil Depression, evidenced from borehole records and seismic reflection data |
| VerfasserIn |
Adam Booth, Ian Bastow, Craig Magee, Derek Keir, Giacomo Corti, Chris Jackson, Jason Wilkinson |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250123688
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| Publikation (Nr.) |
 EGU/EGU2016-2984.pdf |
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| Zusammenfassung |
| The Ethiopian and Afar Rift systems provide a globally unique opportunity to study the
incipient transition from continental rifting to sea-floor spreading. A consensus has emerged
that a considerable proportion of plate extension in Ethiopia is accommodated by dyke
intrusion, with smaller contributions from crustal thinning. However, observations of thinned
crust and a pulse in Quaternary-Recent basaltic volcanism within Ethiopia’s Danakil
Depression have been cited (Bastow and Keir, 2011) as evidence that localised plate
stretching may mark the final stages of continent-ocean transition. We explore this hypothesis
using an archive of five 2-D seismic reflection profiles, each between 7-10 km in length, and
∼120 borehole records distributed over an area of 225 km2. From depth and age
relationships of key marker horizons, we also suggest local subsidence and extension
rates.
The borehole archive reveals extensive evaporite sequences deposited in and around
an asymmetric basin, bounded to the west by a network of east-dipping normal
faults. West of the basin, the maximum observed thickness of evaporites is 150 m,
beneath which are deposits of clastic sediment, but a sequence of evaporites at
least 900 m thick is observed at the basin centre. The sedimentary architecture
of these sequences suggests deposition in a shallow salt-pan environment, with
seasonal – potentially diurnal – freshening of the brine supply (Warren, 2012). Isotopic
analysis of reef carbonates in the basin flank dates the last marine incursion into the
Danakil Depression at 24-230ka (Lalou et al., 1970; Bonatti et al., 1971; Bannert
et al., 1971), therefore the evaporite sequence must be younger than this. A key
marker horizon within the evaporites is the potash-bearing Houston Formation,
also distinct in borehole records given its high porosity (25-40%) and radioactivity
(50-250 API units). The elevation of the Houston Formation is ∼500 m deeper in the
centre of the basin than on the flank. This depth change corresponds to a plausible
vertical subsidence rate of between 2-20 mma−1 and, assuming a 60˚ fault dip, a
horizontal extension rate of 1-12 mma−1 during the deposition of the Houston
Formation, consistent with recent geodetic constraints offered by Ar Rajehi et al.
(2010).
The borehole archive shows no evidence of significant magmatism anywhere in
the survey area, and the characteristic reflectivity of igneous bodies is absent in
the seismic data. Extension of this basin is, therefore, not obviously explained by
dyke intrusion. We consider that the ∼500 m change in elevation of the Houston
Formation is instead diagnostic of rapid stretching, possibly indicating a late period of
non-magmatic extension in the transition to sea-floor spreading in the Danakil Depression. |
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