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Titel |
Seismic observations of large-scale deformation at the bottom of fast-moving plates |
VerfasserIn |
Eric Debayle, Yanick Ricard |
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 |
250089268
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Publikation (Nr.) |
EGU/EGU2014-3465.pdf |
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Zusammenfassung |
We present a new tomographic model of azimuthal anisotropy in the upper mantle and
discuss in details the geodynamical causes of this anisotropy. Our model improves upon
DKP2005 seismic model (Debayle et al., 2005) through a larger dataset (expanded by a factor
~ 4) and a new approach which allows us to better extract fundamental and higher mode
information. Our results confirm that on average, azimuthal anisotropy is only significant in
the uppermost 200-250 km of the upper mantle where it decreases regularly with depth. We
do not see a significant difference in the amplitude of anisotropy beneath fast oceanic plates,
slow oceanic plates or continents.
The anisotropy projected onto the direction of present plate motion shows a very specific
relation with the plate velocity; it peaks in the asthenosphere around 150 km depth, it
is very weak for plate velocities smaller than 3 cm yr-1, increases significantly
between 3 and 5 cm yr-1, and saturates for plate velocities larger than 5 cm yr-1.
Plate-scale present-day deformation is remarkably well and uniformly recorded
beneath the fastest moving plates (India, Coco, Nazca, Australia, Philippine Sea
and Pacific plates). Beneath slower plates, plate-motion parallel anisotropy is only
observed locally, which suggests that the mantle flow below these plates is not
controlled by the lithospheric motion (a minimum plate velocity of around 4 cm yr-1 is
necessary for a plate to organize the flow in its underlying asthenosphere). The
correlation of oceanic anisotropy with the actual plate motion in the shallow lithosphere
is very weak. A better correlation is obtained with the fossil accretion velocity
recorded by the gradient of local seafloor age. The transition between frozen-in and
active anisotropy occurs across the typical -age- isotherm that defines the bottom
of the thermal lithosphere around 1100 °C. Under fast continents (mostly under
Australia and India), the present day velocity orients also the anisotropy in a depth
range around 150-200 km depth which is not deeper than what is observed under
oceans. |
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