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| Titel |
Magma dynamics above the Karoo plume, South Africa |
| VerfasserIn |
Eric Ferre, John Geissman, Stephanie Maes, Gillum Aneesa, Julian Marsh |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250107378
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| Publikation (Nr.) |
 EGU/EGU2015-7077.pdf |
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| Zusammenfassung |
| Mantle plumes produce voluminous amounts of magma (106 km3) during a short period of
time (106 years). The heat input of such plumes into sedimentary basins has been proposed as
a significant factor in several global climatic crises. Indeed heat transfer through conductive
and advective processes is likely to bake organic matter-rich sediments, which in turn may
release greenhouse gases (CO2 and CH4). One of the yet poorly understood aspects of this
model is the regional pattern of magma flow. The objective of this study is to constrain
magma dynamics in the Karoo Large Igneous Province (LIP) intruded in a continental basin
of South Africa.
Magnetic fabrics provide an efficient and accurate mean to determine magma flow
direction in gabbroic rocks. The anisotropy of magnetic susceptibility (AMS) is particularly
suited for this type of study. A previous study had shown that the AMS fabric is a reliable
proxy for magma flow as long as samples are collected from the upper chilled margin
of a sill. The central part is more complex due to interference caused by thermal
convection.
Oriented core samples were collected from 30 different sills and yielded 1598 specimens
for AMS measurements. The low-field magnetic susceptibility Km ranges widely from about
100 to 20,000 . 10-6 [SI], while the degree of anisotropy P’ ranges from 1.01 to 1.10.
Thermomagnetic experiments reveal that the main magnetic carrier is titanomagnetite with
variable ulvöspinel content. This is confirmed by measurement of hysteresis properties
that also indicate that titanomagnetite in general has a pseudo-single domain grain
size.
The results of this study clearly indicate that magma flow followed a main NW-SE
direction in the studied area. The AMS directional data is consistent with the nearly
horizontal attitude of the sill in 23 out of 30 cases, with subvertical K3 axes. In 5 out of 30
sills, K3 axes are subhorizontal, characterized by scattered directional data and are considered
anomalous AMS fabrics. K1 axes are systematically subhorizontal and mark the magma flow
direction. This regional scale flow pattern indicates that the Karoo plume head was not
located under the Drakensberg basalts, the thickest part of the Karoo volcanic pile. Instead the
plume head might have been located to the NW of the Karoo Basin, in Namibia. Overall these
results show that magnetic fabrics are an efficient tool to analyze large-scale magma
dynamics. |
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