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| Titel |
Existence, morphology and persistence of intrusions as a probe for lithosphere rheology |
| VerfasserIn |
Anna Massmeyer, Anne Davaille, Erika Di Giuseppe |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250076601
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| Zusammenfassung |
| The development of intrusions in the lithosphere depends strongly on its rheology. Less
viscous mushroom-shaped plumes or more viscous finger-shaped diapirs, depending on the
viscosity ratio between the rising and the matrix materials, are known to migrate through
ductile, quasi-newtonian lithosphere; while dikes fracture and propagate through a solid
matrix. However, the lithosphere presents solid as well as viscous properties. To determine
what happens in this complex case, we performed a combined study of laboratory
experiments and numerical simulations on the development of thermal plumes in aqueous
solutions of Carbopol, a polymer gel suspension forming a continuous network of
micrometric sponges. This fluid is shear thinning and presents a yield-stress, whereby flow
occurs only if the local stress exceeds a critical value Ïă0. Below this value, the fluid acts as
an elastic solid. The rheological properties of the solutions can be systematically
varied by varying the Carbopol concentration. Our experimental setup consists of a
localized heat-source of constant power, placed in the center of a squared plexiglas
tank.
Two conditions must be fulfilled for an instability to develop and rise: 1) the Yield
number Ï0 comparing the buoyancy-induced stress to the yield stress, should be greater than
a critical value Ïc ~ 6; and 2) the Bingham number Bi, comparing the yield stress to the
viscous stresses, needs to be locally smaller than 1. Then, a plug flow develops inside the
plume thermal anomaly, producing a rising finger-shape with strong shear zones confined
along its edges. Moreover, the instability halts its ascent as soon as Ï0 < Ïc or
Bi > 1.
Those finger-shaped diapirs show strong similarities with an off-axis diapir in
Oman emplaced in a ridge context. This geological object, a few kilometers in
diameter, presents strong shear localization along its edges. Our fluid dynamical
analysis places constraints on the parameter range within which such an object may
be emplaced. It suggests that a purely thermal anomaly could only be emplaced
in a partially molten lithosphere (with Ïă0 ~100 kPa). But the yield stress of the
surrounding matrix might be much higher (up to Ïă0 ~ 50 MPa), if the instability also
exhibits a chemical density anomaly. On the other hand, this critical yield stress
constitutes a lower value of Ïă0 for an intrusion to remain trapped in the lithosphere. |
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