Small-scale disturbances in the stratigraphy of the NEEM ice core: observations and numerical model simulations
Disturbances on the centimetre scale in the stratigraphy of the North Greenland Eemian Ice Drilling (NEEM) ice core (North Greenland) can be mapped by an optical line scanner as long as the ice has visual layering, such as, for example, cloudy bands. Different focal depths allow, to a certain extent...
Main Authors: | , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2016-02-01
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Series: | The Cryosphere |
Online Access: | http://www.the-cryosphere.net/10/359/2016/tc-10-359-2016.pdf |
Summary: | Disturbances on the centimetre scale in the stratigraphy of the North Greenland Eemian Ice Drilling (NEEM) ice
core (North Greenland) can be mapped by an optical line scanner as long as
the ice has visual layering, such as, for example, cloudy bands.
Different focal depths allow, to a certain extent, a three-dimensional view
of the structures. In this study we present a detailed analysis of the
visible folds, discuss their characteristics and frequency, and present
examples of typical fold structures. We also analyse the structures with
regard to the deformation boundary conditions under which they formed. The
structures evolve from gentle waves at about 1500 m to overturned <i>z</i> folds
with increasing depth. Occasionally, the folding causes significant
thickening of layers. Their similar fold shape indicates that they are
passive features and are probably not initiated by rheology differences
between alternating layers. Layering is heavily disturbed and tracing of
single layers is no longer possible below a depth of 2160 m. <i>C</i> axes
orientation distributions for the corresponding core sections were analysed,
where available, in addition to visual stratigraphy. The data show
axial-plane parallel strings of grains with <i>c</i> axis orientations that deviate
from that of the matrix, which shows a single maximum fabric at the depth
where the folding occurs.
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Numerical modelling of crystal viscoplastic deformation and dynamic
recrystallisation was used to improve the understanding of the formation of
the observed structures during deformation. The modelling reproduces the
development of bands of grains with a tilted-lattice orientation relative to
the single maximum fabric of the matrix, and also the associated local
deformation. We conclude from these results that the observed folding can be
explained by formation of these tilted-lattice bands. |
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ISSN: | 1994-0416 1994-0424 |