Observations and modelling of centimetre-scale folding in the NEEM ice core

Daniela.Jansen [ at ] awi.de


Disturbances on the centimeter scale in the layering of the NEEM ice core (North Greenland) can be mapped by means of visual stratigraphy as long as the ice has a visual layering, such as, for example, cloudy bands. Different focal depths of the visual stratigraphy method allow, to a certain extent, a three-dimensional view of the structures. In this study we present a structural analysis of the visible folds, discuss characteristics and frequency and present examples of typical fold structures. The structures evolve from gentle waves at about 1500 m to overturned z-folds with increasing depth. Occasionally, the folding causes significant thickening of layers. Their shape indicates that they are passive features and are probably not initiated by rheology differences between layers. Layering is heavily disturbed and tracing of single layers is no longer possible below a depth of 2160 m. Lattice orientation distributions for the corresponding core sections were analyzed where available in addition to visual stratigraphy. The data show axial-plane parallel strings of grains with c-axis orientations that deviate from that of the matrix, which shows a well developed single-maximum fabric at the depth where the folding occurs. We conclude from these data that folding is a consequence of deformation along localized shear planes and kink bands. The findings are compared with results from other deep ice cores. The observations presented are supported by microstructural modeling. We are using a crystal plasticity code that reproduces deformation, applying a fast Fourier transform method (FFT), coupled with the microstructural platform ELLE to include dynamic recrystallization processes. The model results reproduce the development of bands of grains with a tilted orientation relative to the single maximum fabric of the matrix and also the associated local deformation.

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Conference (Poster)
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International Symposium on contemporary ice sheet dynamics, 16 Aug 2015 - 21 Aug 2015, Cambridge, UK.
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Jansen, D. , Llorens, M. G. , Westhoff, J. , Steinbach, F. , Bons, P. D. , Griera, A. , Weikusat, I. and Kipfstuhl, S. (2015): Observations and modelling of centimetre-scale folding in the NEEM ice core , International Symposium on contemporary ice sheet dynamics, Cambridge, UK, 16 August 2015 - 21 August 2015 .

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