CRYSTALLOGRAPHIC ANALYSIS OF EXPERIMENTALLY DEFORMED ICE

Ice streams are fast flowing sectors of ice imbedded within a slow-flowing ice sheet. The lateral margins of ice streams, where fast flow is separated from slow flow, are called shear margins and consist of highly deformed ice. The resistance to flow from shear margins depends on the rheological properties of ice that is potentially at the melting temperature, but whose properties are incompletely known. To better evaluate the rheological properties of ice at its pressure-melting temperature in shear with confined compression, a series of laboratory experiments were conducted.

This study focuses on the development of crystal texture and fabric with strain. Ice at the melting temperature is sheared in a large diameter ring-shear device over realistic strain rates until secondary or tertiary creep is achieved. The ice is removed from the device and cut down to an 8 x 8 cm square section. The section is then fixed to glass plates, and a microtome is used to plane the sample down to approximately 500 microns in thickness. Once the ice thickness is reduced sufficiently, the sample is viewed with cross-polarized filters, so that grain size, grain shape, and c-axis orientation can be analyzed with a universal Rigsby stage. These measurements are then used to characterize the ice-crystal texture and to map c-axis orientations, which depend on strain and potentially on water content.