Reply to comment by P. Duval and M. Montagnat on “Superplastic deformation of ice: Experimental observations”

Abstract

[1] In a recent paper, we reported the results of a detailed experimental investigation of the rheological properties of ice [Goldsby and Kohlstedt, 2001]. One of the innovative aspects of our investigation was the use of fine-grained samples that enabled us to explore deformation not only by grain-size insensitive mechanisms involving grain matrix dislocation processes but also by grain-size sensitive flow mechanisms involving grain boundary sliding (GBS). An exciting discovery in our research was an extensive creep regime in which GBS contributes substantially to the flow of ice. Most importantly, this deformation regime in which basal slip is accommodated by GBS, referred to in our paper as the ‘‘superplastic flow’’ regime, governs the flow of ice over a wide range of temperature, grain size and stress conditions found in glaciers, ice sheets and icy planetary interiors. [2] In their comment on our paper, Duval and Montagnat [2002] repeat the objection that they had raised previously [Montagnat and Duval, 2000] concerning our conclusion that superplastic deformation controls the flow of glaciers and ice sheets. They assert ‘‘that GBS as a significant creep mechanism is not compatible with observations on the development of fabrics and microstructures in ice sheets. Special emphasis is placed [instead] on the role of grain boundary migration as a recovery process in glacier ice.’’ They then continue by arguing that basal slip accommodated by grain boundary migration (GBM) rather than by GBS is the dominant deformation mechanism in ice. [3] In this reply, we extend the results presented in detail by Goldsby and Kohlstedt [1997] and Goldsby and Kohlstedt [2001] to further demonstrate that GBS is a very important deformation process in natural ice bodies. Our reasoning is based on three points: (1) The microstructures reported for samples from large natural ice bodies are remarkably similar to those observed in samples deformed in the laboratory in the superplastic flow regime. (2) Our constitutive equation for ice is in excellent agreement with the flow behavior determined from field measurements on glaciers and ice sheets. (3) Contrary to the assertion of Montagnat and Duval [2000] and Duval and Montagnat [2001], fabric development is fully compatible with deformation in the superplastic regime described in our paper. In addition, we note that, while GBM associated with dynamic recrystallization is an important recovery mechanism in ice, it is not a deformation mechanism and hence cannot accommodate basal slip.