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Deformation modes and geometries in the EPICA-DML ice core, Antarctica

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Hamann, I. , Kipfstuhl, S. , Lambrecht, A. and Azuma, N. (2008): Deformation modes and geometries in the EPICA-DML ice core, Antarctica , EPICA Open Science Conference, Venice, ItalyNovember 2008.. .
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Combination of physical-properties methods (crystal-orientation-fabrics, grain-elongation-data, line-scan stratigraphy-documentation) reveal evidences for five deformation geometry regimes:1. Random c-axes distributions and crystal elongation directions (<~450 m depth) indicate that deformation is not strong yet.2. Orientation tensor eigenvalues of the c-axes distributions start separating into three levels indicating progressive evolution of girdle fabrics (~450 to 1700 m depth). Alignment of the crystal elongation directions starts due to an increasing horizontal uni-axial tension deformation, typical for an ice-divide or flank drilling site (e.g. NorthGRIP). 3. Eigenvalues below ~1700 m decrease (middle value) and increase (largest value), due to a rewidening of girdle fabrics and a slight tendency of concentration of c-axes within the girdle. Variable mean crystal elongation direction and slight buckling of stratigraphic layers are observed. This indicates a destabilization of the horizontal uni-axial tension, probably due to local inclination of tension direction and starting changeover to the next deformation geometry. 4. Same level of lower eigenvalues and high level of the largest eigenvalue (single maximum fabric along the vertical core axis) show that the change of deformation geometry is completed (>~2020 m depth). Here bed-parallel simple shear deforms the ice causing folding and inclination of stratigraphic layers.5. A last change of geometries is observed at ~2370 m depth, with a locally very restricted (~10 m) backslide to girdle fabric, isoclinal z-folding and borehole closure. Below that an inclined single maximum fabric reoccurs.Simple shear can easily produce the observed small-scale folding of layers which however may belong to disturbances on a larger scale with possible overturning and thus age reversal of layers. Below ~2020 m the EDML climate record has to be interpreted with great care.

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