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| Titel |
Morphological modifications of the Kerguelen Islands (South Indian Ocean) in response to Neogene climate change: evidence from 40Ar/39Ar and (U-Th)/He thermochronology |
| VerfasserIn |
Floriane Ahadi, Guillaume Delpech, Cécile Gautheron, Sebastien Nomade, Rosella Pinna-Jamme, Léandre Ponthus, Damien Guillaume |
| 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 |
250132264
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| Publikation (Nr.) |
 EGU/EGU2016-12755.pdf |
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| Zusammenfassung |
| The processes driving erosion in geodynamic contexts in which regional tectonics is of minor
importance, such as in oceanic islands, can be seen as a combination of positive/negative
retroactions between climate change, isostasy or dynamic topography. The Kerguelen Islands
(48-50˚ S, 68.5-70.5˚ E) are of particular interest to understand the impact of
Cenozoïc climatic variations on the long-term geomorphological evolution of emerged
reliefs at mid-latitudes. The Kerguelen Islands (6700 km2) are the emerged part of
the vast Kerguelen oceanic plateau and reach a maximum height of 1852m asl.
The archipelago is mostly made up of Oligocene basaltic traps (≈25 Ma) up to
1000m asl that are cross-cut by a dense network of large and deep valleys. The
impact of glacial erosion during the last Quaternary glaciations on the landscape
morphology is attested by the occurrence of U-shaped valleys, abundant moraines,
erratic blocs and glacial lakes, as well as remnants of glaciers. Numerous plutonic
complexes of various age (25-4.5 Ma) locally intrude theses traps and cover about
15% of the main island’s surface; the largest being located in the Rallier du Baty
peninsula (800 km2). This plutonic complex is mainly constituted of syenites with
minor occurrence of gabbros and monzonites. The southern part of this complex
has a laccolith structure with satellites plutons and formed between 13.7 and 8.0
Ma.
The cooling history of syenites from the Rallier du Baty plutonic complex was
investigated in order to identify one or several denudation periods and to understand the
potential role of climate change on the geomorphological evolution of the islands since the
Oligocene. We conducted the first thermochronological study on the Kerguelen Islands using
the biotite 40Ar/39Ar thermochronometer and the apatite (U-Th)/He thermochronometer
(AHe). The 40Ar/39Ar ages range from 9.44 ± 0.13 Ma to 13.84 ± 0.07 Ma for the various
parts of the southern complex. These ages are identical to high-temperature crystallisation
ages (U-Pb on zircon) and suggest an extremely rapid cooling between ≈700 and ≈300˚ C
and that these rocks were emplaced at shallow depth. The mean AHe ages range between 4.4
± 0.3 Ma and 7.4 ± 0.7 Ma. Theses ages agree with a model implying a rapid erosion of
the upper volcanic rock pile that occurred several My after emplacement of these
plutonic rocks at the Miocene-Pliocene transition. This transition coincides with a
global climatic cooling which is particularly strong at these mid-latitudes. It is
suggested that the morphological evolution of this part of the main island results from
global climate changes at this period, with a possible contribution from positive
retroaction between climate cooling and local isostatic accommodation. This study
further strengthens the link between climatic variations and increase in erosion
rates at mid latitudes since 6 Ma as recently demonstrated for Patagonian Andes. |
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