Some New Rheological Experiments on Igneous Rocks at Temperatures up to 1120 C

Abstract

The stress-strain relations and creep behaviour of a dolerite, a microgranodiorite, and a dunite have been determined in uniaxial compression at temperatures up to 1120°C. All three rocks remained brittle at temperatures up to 1050"C, with approximately linear stress-strain relations and small fracture strains. Creep tests were carried out at stresses (< 10' Nm-2)* which were low compared to the uniaxial compressive strength at the same temperature. At these stresses and in experiments with a duration of -1 d only transient creep was observed, with the creep strain proportional to a fractional power of the time (Andrade creep), as was observed in earlier experiments at lower temperatures, and the creep rate decreased with the elapse of time. The apparent viscosity, therefore, increases with time, having a value in the range 10I2 -10'' Nsm-2 after times of 10% and 10"-lo2' Nsm-' after times of -3y (by extrapolation)?. The activation energy for creep at a low stress of 130 x lo5 Nm-2 determined for microgranodiorite at temperatures greater than 800°C and for dolerite at temperatures greater than 940°C indicated that the creep was probably diffusion controlled. Using a theory due to Mott, the steady-state creep rate and the corresponding equivalent viscosity were calculated for the conditions of the experiments. The viscosities for pressures on the geotherms of Ringwood were then calculated, by making use of the proportionality between activation energy for diffusion and the absolute melting temperature. For stresses in the range lo6-10' Nm-' dolerite has a viscosity of 10"-lo2' Nsm-2 at at depths of 60-80 kin in oceanic areas, and at depths greater than 120 km in shield areas; and microgranodiorite at the same stresses has a similar viscosity at depths in the range 80-130 km in shield areas. These estimates are in satisfactory agreement with present concepts of the lithosphere. At temperatures above 1050°C the dolerite and microgranodiorite exhibited partial melting. The dolerite remained brittle, with the partial melt being extruded from microfractures, but the microgranodiorite showed some ductility. We can postulate that even rocks deep in the mantle may be brittle if their temperatures are not more than N 1OOO"C. A discussion is given of the apparent conflict between the experimental observation of hot brittleness and the geological observation that even some comparatively cool silicate rocks have apparently deformed in a ductile manner.lo5 Nm-2 = l b = lo6 dyn/cm2 = 14-50Lb/in2 t lo-' Nsm-2 = 1 poise. 1 212

Deformation of partially molten granite: a review and comparison of experimental and natural case studies

“…The first deformation experiments performed on partially molten granite (Murrel and Chakravarty 1973;Murrel and Ismail 1976) showed that incipient melting dramatically reduces the strength of the aggregate. The melt fraction in these experiments was poorly determined (<0.25) and it was only at the end of the 1970s that new experiments constrained the strength of the sample as a function of melt fraction (Arzi 1978;van der Molen and Paterson 1979).…”

Mechanics of tectogenesis in plate collision zones

“…Temperature and strain rate have only a small effect on the strength of the brittle and cataclastic crust. The strength under these conditions is much less in tension than in compression (Murrell 1965) (see Fig. 1).…”

“…The application of some of these concepts of rock mechanics to the crust and lithosphere has been discussed by Murrell ( , 1977Murrell ( , 1981Murrell ( , 1982 and Murrell & Tsang (1979) and we attempt to summarize them in Fig. 5.…”

“…(see Murrell & Chakravarty 1973), where g is the shear modulus, -~ is strain rate, ~'0 is a frequency factor, g is a constant characteristic of the crystal structure (g/m-10) and m is a constant determining the strain rate sensitivity of the strength (m has a value in the range 1-3 at the low stresses characteristic of the mantle). Thus temperature and strain rate have a marked effect on the strength of ductile crust and mantle.…”

“…Fluids associated with magmatism arising from the asthenosphere during lithospheric extension, or fluids released by metamorphism due to burial by sediments following lithospheric extension or to tectonic burial caused by lithospheric shortening (and thickening), will accentuate tensile weakness by increasing the effective tensile stresses and reducing the effective compressive stresses (Murrell 1977). The forces required to shorten and thicken continental crust and lithosphere are correspondingly much higher and it seems that, at least in Phanerozoic and Proterozoic times, orogenic belts did not form except on the margins of, or in previously stretched and thinned regions of, the continents or cratons.…”