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| Titel |
Creep of phyllosilicates at the onset of plate tectonics |
| VerfasserIn |
E. Amiguet, B. Reynard, R. Caracas, B. Van De Moortèle, N. Hilairet, Y. Wang |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063891
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| Zusammenfassung |
| Plate tectonics occur only after the onset of subduction at the Earth’s surface. Subduction
initiates if a material with low yield strength is present in the otherwise stiff cold lithosphere
at the thermal boundary layer with hydrosphere. Models suggest that plate tectonics initiates
when the strength of lithosphere is lower than 20-200 MPa, below the frictional strength of
lithospheric rocks (>700 MPa).
Previous deformation experiments on serpentine minerals for pressure up to 4 GPa
suggested plastic behavior and flow strength of antigorite an order of magnitude lower
than dry lithospheric rocks. However, extrapolations from laboratory strain rates
(10-4-10-6 s-1) to tectonic strain rate (as low as 10-15 s-1) have large uncertainties
because they are based on empirical rheological laws and the structural differences
in serpentine variety may influence the deformation mechanisms and rheological
behavior.
We carried out high-pressure deformation experiments with in-situ stress and strain
measurements on polycrystalline Elba Island lizardite - the low temperature variety
of serpentine - using a Deformation-DIA apparatus coupled with monochromatic
synchrotron light source (APS), at the GeoSoilEnviroCARS, Argonne National
Laboratory. Fourteen deformation cycles were performed on four specimens at
pressures (P) and temperatures (T) ranging from 1 to 8 GPa and 150 to 400Ë C,
with strain rates (ËÉ) between 10-4 and 10-6 s-1. Thin sections were extracted
parallel to the compression direction using Focused Ion Beam. TEM observations of
recovered samples show plastic deformation features and no evidence of brittle
failure.
Lizardite has a large rheological anisotropy, comparable to that observed in the micas.The
alignment of the basal (001) planes normal to the compression axis leads to high stresses
corresponding to the elastic response of crystals whose orientation precludes glide on (001).
Crystals where basal planes are oblique to the compression, allowing glide on (001)
show yield stress in the range 50-200 MPa with no systematic dependence with
strain rate, grain size, pressure or temperature within experimental uncertainties.
First-principles calculations confirmed easy gliding on lizardite basal plane and show
that the flow stress of phyllosilicate is in the range of the critical value of 20-200
MPa down to depths of about 200 km. Thus, foliated serpentine or chlorite-bearing
rocks are sufficiently weak to account for plate tectonics initiation, and aseismic
sliding on the plate interface below the seismogenic zone. Serpentinisation easing the
deformation of the early crust and shallow mantle reinforces the idea of a close link
between the occurrence of plate tectonics and water at the surface of the Earth. |
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