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| Titel |
Magnetostratigraphy and dating of the Lesotho lava pile (Karoo traps) : an attempt to constrain the timing of the eruptive sequence in relation with the end-Pliensbachian extinction event |
| VerfasserIn |
Maud Moulin, Frédéric Fluteau, Vincent Courtillot  , Julian S. Marsh, Guillaume Delpech, Xavier Quidelleur, Martine Gerard |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250037314
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| Zusammenfassung |
| The correlation between ages of eruptions of large igneous provinces (LIP) and mass extinctions is now generally accepted and a causal connection seems unavoidable. But details of this causal connection remain incomplete and there is a need for comparative restudy of several cases. Such a restudy has recently been completed by Chenet et al in the case of the Deccan traps/KT extinction. These authors have combined geochronology (K-Ar), paleomagnetism (secular variation), volcanology (flow types) and analysis of alteration levels between flows, and have shown that emplacement occurred as a small number of discrete, very large and short-lived volcanic pulses which would have led (mainly because of associated SO2 emission into the atmosphere) to major environmental change, resulting in mass extinction. But other similarly large LIPs have not led to major mass extinctions, and one needs to understand the reasons for such differences. For instance, the (originally huge) Karoo traps of South Africa are likely linked to the relatively small end-Pliensbachian extinction. Successful modeling of the environmental effects of LIP eruptions requires high-resolution timing of volcanism, i.e. knowledge of numbers, volumes and durations of peak episodes. We have therefore undertaken in the Karoo the same kind of analysis previously accomplished for the Deccan. We focus here on the Lesotho lava pile, the main remnant of the Karoo traps. We have begun this study with a section (the lower 800 m of the traps) located at Naude’s Nek in South Africa, near the southern border of Lesotho. We have also investigated other sections (Moteng Pass and Oxbow) further North in Lesotho, where the Karoo lavas are the thickest (almost 1500 m). Our first age determinations (40K-40Ar Cassignol-Gillot technique) yield ages of 181.3 ± 1.8 Ma, in good agreement with previous studies (Jourdan et al., 2007). Detailed flow by flow magnetostratigraphy (site-mean directions based on thermal demagnetization) shows that the eruptive sequences can be divided into several volcanic pulses having each likely lasted less than ~100 years. Particular attention has been focused on the remarkably detailed record of the single reversal first identified by van Zijl 50 years ago: up to 300 m of lava erupted in less than a few thousand years over much of the area. We are attempting to correlate volcanic pulses of the southern and northern parts in order to reconstruct the full Lesotho eruptive sequence. |
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