Effect of Large Changes in Strain Rate in the Experimental Deformation of Yule Marble

A new adaptable deformation apparatus has been developed for constant strain-rate tests up to 5 kb. confining pressure and 500° C. at strain rates from 0.4 to 3· 10⁻⁸/sec. In most of the sixty-four tests reported here, jacketed cylinders of Yule marble (oriented parallel, at 45° and normal to the foliation) were extended 10 per cent at temperatures from 25° to 500° C., 5 kb. confining pressure. Several samples were deformed 60-80 per cent at temperatures ranging from 350° to 500° C. at a strain rate of 3· 10⁻⁷/sec for fabric study. Stress-strain curves are plotted and compared for a wide range of temperature and strain rates. Stress-strain curves at 25° C. show only slight decrease in strength with decreasing strain rate; at 500° C. strengths at 10 per cent strain are decreased 80 per cent from the fastest (0.25-second duration) to the slowest (35 days) tests. Strong strain hardening, characteristic in tests at the lower temperatures and higher strain rates, changes gradually to steady state flow as strain rates are decreased or temperatures increased. By applying Eyring's theory for steady-state flow, equations are presented relating strength, strain-rate, and temperature for extension normal, at 45° and parallel to the foliation. The data fit the Eyring equation well; extrapolation on the basis of this equation to a reasonable geologic strain rate of 10⁻¹⁴/sec gives strengths for the Yule marble ranging from 70 to 500 bars at 300° C. At 500° C., strengths would be near 10⁻³ bars regardless of orientation. Equivalent viscosities at this strain rate range from 10²³ poises at 25° C. to 10¹⁶ poises at 500° C. Thin sections show that syntectonic recrystallization and other effects previously found to be common at 600°-800° C. and strain rates near 10⁻³/sec are well developed at 350°-500° C. at 10⁻⁷/sec. This suggests that recrystallization in nature would occur at even lower temperatures. Measurements of c-axes in newly re-crystallized grains show strong preferential alignment parallel to the axis of maximum principal compressive stress, consistent with earlier observations.