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Titel |
Evolution of asthenospheric layers as a result of changing temperature and stress fields |
VerfasserIn |
Leszek Czechowski, Marek Grad |
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 |
250110272
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Publikation (Nr.) |
EGU/EGU2015-10252.pdf |
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Zusammenfassung |
The lithosphere is underlain by the asthenosphere. Traditionally, the boundary between the
lithosphere and the asthenosphere (LAB) is defined by a difference in response to stress: the
lithosphere remains elastic or brittle, while the asthenosphere deforms viscously and
accommodates strain through plastic deformation.
The reology of rocks depends on many factors: temperature, pressure, chemical
composition, size of grains, etc. However, the basic differences of lithosphere and
asthenosphere properties could be explained as a result of the temperature and pressure. The
effective viscosity of mantle is proportional to CÂexp(AÂq), where q is the ratio
(melting temperature/temperature), C and A are positive constants. The mantle is not
molten, so q >1. If the temperature is close to the melting temperature then q is close
to 1 and effective viscosity is low (e.g. 1018 Pa s). This situation is observed in
asthenosphere.
The lithosphere is a thermal boundary layer for the convection in the mantle. The
temperature of the upper part is low (q is high) but the temperature gradient in the lithosphere
is high and temperature is increasing fast. In the mantle below the lithosphere, the
temperature gradient is low (could be close to the adiabatic one). The melting temperature is
increasing with depth faster than true temperature. Hence, q and the viscosity reach minimum
value just below LAB and are increasing with depth in the mantle below. It is a typical
situation.
The tectonic processes in subduction zones could change this picture. The one
lithospheric plate could be placed in the mantle below another plate. Distribution
q in such a case could have two minima, so two asthenospheric layers could be
formed.
Another important factor determining rheological properties is a stress tensor T.
Generally viscosity is proportional to the power of the invariant of the stress tensor:
I(T)^(1-n). For n=1 the viscosity does not depend on stress (i.e. Newtonian rheology), for true
mantle n is probably in the range from 3 to 5.
We investigate the processes of formation and evolution of low viscosity layers
(“asthenospheric layers”) in the upper mantle. The time scale of the temperature changes is
of the order of 10 Myr. The characteristic time of stress changes could be much
shorter depending on tectonic processes. Eventually processes of formation and
vanishing of low viscosity layers is very dynamical. In a relatively short time (below
1 Myr) the pattern the viscosity distribution and velocity gradient could change
substantially.
Using results from deep seismic sounding and surface wave tomography we have
found that below some regions there are structures in the mantle that could be a
forming/vanishing low viscosity layers. Reflectors in the lower lithosphere are observed
beneath Trans-European suture zone between Precambrian and Palaeozoic platforms. In a
thick Baltic shield lithosphere (200 km or more) low velocity zones and seismic
reflectors are observed in the depth range 60-100 km, which could be interpreted as
mechanical low Vp velocity zones, in contrast to thermal velocity zone in deeper
asthenosphere.
Acknowledgments: This work was partially supported by the National Science Centre
(grant 2011/01/B/ST10/06653). |
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