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| Titel |
The role of small-scale convection on the formation of volcanic passive margins |
| VerfasserIn |
Jeroen van Hunen, Jordan Phethean |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250091702
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| Publikation (Nr.) |
 EGU/EGU2014-6007.pdf |
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| Zusammenfassung |
| Volcanic passive margins (VPMs) are areas of continental rifting where the amount of newly
formed igneous crust is larger than normal, in some areas up to 30 km. In comparison,
magma-poor margins have initial oceanic crustal thicknesses of less than 7 km (Simon
et al., 2009; Franke, 2012). The mechanism for the formation of these different
types of margins is debated, and proposed mechanisms include: 1) variation in
rifting speed (van Wijk et al., 2001), variation in rifting history (Armitage et al.,
2010), enhanced melting from mantle plumes (e.g. White and McKenzie, 1989), and
enhanced movement of mantle material through the melting zone by sublithospheric
small-scale convection (SSC) driven by lithospheric detachments (Simon et al., 2009).
Understanding the mechanism is important to constrain the petroleum potential of
VPM.
In this study, we use a numerical modelling approach to further elaborate the effect of SSC on
the rate of crust production during continental rifting. Conceptually, SSC results in patterns of
upwelling (and downwelling) mantle material with a typical horizontal wavelength of a 100
to a few 100 km (van Hunen et al., 2005). If occurring shallowly enough, such upwellings
lead to decompression melting (Raddick et al., 2002). Subsequent mantle depletion has
multiple effects on buoyancy (from both latent heat consumption and compositional
changes), which, in turn, can affect mantle dynamics under the MOR, and can potentially
enhance SSC and melting further. We use two- and three-dimensional Cartesian flow models
to examine the mantle dynamics associated with continental rifting, using a linear viscous
rheology (in addition to a semi-brittle stress limiter to localize rifting) in which
melting (parameterized using (Katz et al., 2003)) leads to mantle depletion and crust
accumulation at the surface. The newly formed crust is advected away with the diverging
plates.
A parameter sensitivity study of the effects of mantle viscosity, spreading rate, mantle
temperature, and a range material parameters have indicated the following results.
Decompression melting leads to a colder (from consumption of latent heat of melting) and
therefore thermally denser, but compositionally more buoyant residue. The competition
between thermal and compositional buoyancy determines the mantle dynamics after rifting
initiation. For a mantle viscosity > ~ 1022 Pa s, no SSC occurs, and a uniform 7-8 km-thick
oceanic crust forms. For mantle viscosity < ~ 1021 Pa s, SSC might be vigorous and can
form passive margins with a crustal thickness > 10-20 km. If thermal density effects
dominate, a convection inversion may occur for low mantle viscosities, and mantle
downwellings underneath the rift/ridge area can result in a significant upwelling return flow
that enhances further decompression melting, and can create VPMs. Such dynamics could
also explain the continent-dipping normal faults that are commonly observed at VPMs.
After the initial rifting phase, the crustal thickness reduces significantly, but not
always to a uniformly thick 7-8 km, as would be appropriate for mature oceanic
basins. |
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